A POSSIBLE LATE PLEISTOCENE IMPACT CRATER IN CENTRAL NORTH

AMERICA AND ITS RELATION TO THE YOUNGER DRYAS STADIAL

PLAN B PAPER SUBMITTED TO THE FACULTY OF THE UNIVERSITY OF MINNESOTA

BY

David Tovar Rodriguez

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE

Howard Mooers, Advisor

August 2020

2020 David Tovar

All Rights Reserved

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ACKNOWLEDGEMENTS

I would like to thank my advisor, my family, and my friends.

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Abstract The trigger of the Younger Dryas (YD) Stadial 12.9 cal ka BP remains hotly debated.

Studies indicate that the drainage of Lake Agassiz into the North Atlantic Ocean and

south through the Mississippi River caused a considerable change in oceanic thermal

currents, thus producing a decrease in global temperature. Other studies indicate that

perhaps the impact of an extraterrestrial body (asteroid fragment) could have impacted

the Earth 12.9 ky BP ago, triggering a series of events that caused global temperature

drop. The presence of high concentrations of iridium, charcoal, fullerenes, and molten

glass, considered by-products of extraterrestrial impacts, have been reported in

sediments of the same age; however, there is no impact structure identified so far. In

this work, the Roseau Structure's geomorphological features are analyzed to

determine if impacted layers with plastic deformation located between hard rocks and

a thin layer of water might explain the particular shape of this structure. Geophysical

data of the study area do not show gravimetric anomalies that are typical of impact

structures. One hypothesis developed on this works is related to the structure's shape

might be explained by atmospheric explosions (airburst) dynamics due to the

disintegration of material when it comes into contact with the atmosphere.

Relationship between structure's diameter (D) and deformed strata thickness (h) as

well as the relationship between the diameter of the projectile (d) and the depth of the

water column (H ), which is considered in this study due to the geographical

considerations of the area 12.9 ky ago and BP, are consistent with an extraterrestrial

event. Other hypotheses, such as lake processes and glacial processes, are difficult

to reconcile with the reported observations, so the impact hypothesis and its

relationship with the formation of the Roseau structure are viable.

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TABLE OF CONTENTS

LIST OF ILLUSTRATIONS

1. INTRODUCTION

2. BACKGROUND

2.1. CONTEXTUALIZING THE YD EVENT: TIMING AND POSSIBLE

TRIGGERS

2.2. THE IMPACT HYPOTHESIS AND THE CONTROVERSY

2.2.1. THE PHYSICAL AND GEOCHEMICAL EVIDENCE

2.2.2. ONSET OF THE YD AND TIMING OF CULTURAL

TRANSITIONS AND EXTINCTIONS

2.2.3. THE GEOMORPHOLOGICAL EVIDENCE

2.3 ARGUMENTS AGAINST ASTEROID IMPACT HYPOTHESIS

2.4 GENERAL RIDICULE

3. GLACIAL AND POSTGLACIAL HISTORY WITH EMPHASIS ON LAKE

AGASSIZ

3.1 GLACIAL HISTORY

3.2 A BRIEF HISTORY OF LAKE AGASSIZ AND THE YD

3.3 STAGES OF LAKE AGASSIZ

4. DESCRIPTION AND SETTING OF THE ANOMALOUS STRUCTURE

4.1 GLACIAL ORIGIN

4.2 LACUSTRINE ORIGIN

4.3 IMPACT ORIGIN

5. DISCUSSION

6. CONCLUSIONS

REFERENCES

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LIST OF TABLES

Table 1 Timing of the main stadial and interstadial periods ---------------------------- 8

Table 2 Shock-produced deformation effects: diagnostic and non-diagnostic 16

Table 3 Summary of the main characteristics of an impact crater --------------- 33

Table 4 Summary of the effects in the formation of impact craters -------------- 35

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LIST OF FIGURES

Figure 1 Lake Agassiz drainage routes ---------------------------------------------------- 44

Figure 2. Lake Agassiz strandline point elevation data --------------------------------- 45

Figure 3 Lake Agassiz lake-level hydrograph -------------------------------------------- 46

Figure 4 Roseau Structure -------------------------------------------------------------------- 47

Figure 5 Gravity data --------------------------------------------------------------------------- 48

Figure 6 Carolina Bays ------------------------------------------------------------------------- 49

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1. Introduction

Few climate events have received as much attention from the scientific community as the

Younger Dryas. As climate was rapidly warming at the end of the last Ice Age, this climatic

anomaly plunged the Earth back into glacial conditions from 12.9 - 11.7 cal ka BP. The

abrupt end of the Younger Dryas marks the beginning of the Greenlandian Age and the

transition from the Late Pleistocene to the Holocene. The causes of this climatic event

have been the subject of heated debate, and hypotheses have evolved rapidly.

Mangerud (1987) argues that an event of this magnitude requires a global trigger such as

changes in ocean circulation. In addition, given the importance of the North Atlantic

currents in terms of thermal equilibrium of climate, it is sensible to think that the YD event

did not exclusively affect Europe; therefore it had an impact in other parts of the world,

particularly in North America. This idea is reinforced by Broecker et al. (1989) suggests,

through analysis of marine oxygen isotope records, that Lake Agassiz in North America

had a flow of melted water to the north, which considerably reduced the salinity of the

North Atlantic, resulting in a variation in its circulation pattern and consequently a variation

in the climate of the northern hemisphere. Teller et al. (2002) show that not only large

amounts of water were released from Lake Agassiz into the North Atlantic once Laurentide

Ice Sheet (LIS) began its recession. Perhaps one of the most controversial hypotheses is

that raised by Firestone et al. (2007a) in which it is stated that the start of the YD is due to

the impact of an asteroid with the Earth's surface. This hypothesis is based on the

observation of charcoals, nanodiamonds, fullerenes, spherules, iridium anomalies, among

others, in the sediments associated with Lake Agassiz.

The research reported by Firestone et al. (2007a) summarized evidence from 50+ Clovis-

age archaeological sites in North America that is consistent with an extraterrestrial impact.

Common to all of these sites is an organic layer dating to 12.9 cal ka BP that corresponds

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to the extinction of Pleistocene megafauna and marks the end of Clovis culture. Contained

within this layer are magnetic grains with elevated iridium concentrations, magnetic

microspherules, charcoal, carbon spherules, nanodiamonds, and fullerenes with

extraterrestrial (ET) helium signatures. They suggest that all of these lines of evidence

are consistent with an ET impact and associated biomass burning (Firestone et al.,

2007a). This assertion has been met with cautious support (Israde-Alcantara et al. 2012;

Haynes, 2008; Kennett et al., 2009), arguments against (Paquay et al. 2009; Boslough et

al. 2012; Buchanan et al. 2008; Wu et al. 2013; LeCompte et al. 2012), and general ridicule

(Surovell et al. 2009; Pinter et al. 2011).

We herein describe a circular feature in northern Minnesota that has characteristics of an

impact crater. In addition, the location of this feature constrains its formation to within error

of 12.9 cal ka BP, and we evaluate possible sets of conditions that would allow a feature

of this size and characteristics to result from a bolide impact. The feature is located a few

km north of the Campbell Beach of Lake Agassiz in Roseau County, Minnesota. Because

of its location, the feature is hereafter referred to as The Roseau Structure, or simply the

Structure. The Structure is a circular shallow depression with concentric rings on the NE

side. The concentric rings have been wave washed during rising and falling stages of

Glacial Lake Agassiz. The location of the Structure constrains its age to after ice retreat

from this area but before the drainage of Lake Agassiz. Lake Agassiz occupied the Tintah

Beach at an elevation of 359 meters until 12.9 cal ka BP at the beginning of the Younger

Dryas. At that time, the lake level dropped as the outlet of Lake Agassiz switched from

the southern outlet to a lower outlet to the east (Teller, 2013) or northwest (Fisher et al.,

2009) or neither (Lowell et al., 2013). By 11.7 cal ka BP Lake Agassiz rose again to the

Campbell Beach.

2. Background

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2.1. Contextualizing the YD event: Timing and possible triggers

The Last Glacial Maximum refers to the time of maximum extension of the ice sheets

during the last glacial period, approximately 21,000 years BP (Table 1). Global climate

was generally warming and environmental conditions in Western Europe began to

resemble current conditions ca. 15,000 cal yr BP. The Younger Dryas (YD) was a brief

phase (~1,300 ± 70 years long) of climate cooling in the late Pleistocene, between 12.9

and 11.7 cal ka BP (Broecker, 2010; Carlson, 2010; Lowell, 2013 ). In Blytt-Sernander

climate theory, the Younger Dryas succeeds the Bölling / Allerød interstade, and

precedes the Lower Holocene Preboreal period. This event takes its name from the

alpine flower Dryas octopetala, an arctic-alpine flower plant in the Rosaceae family.

Few other climate anomalies have been as widely studied as the YD, as this event marked

the last stadial at the end of the most recent glacial period. Numerous investigations

around the proposal of hypotheses that allow us to account for the event or events that

could have contributed to the termination of the YD.

Timing last glacial maximum Date (ka cal BP)

Oldest Dryas (stadial) 18.5- 14.7

Bølling (interstadial) 14.7–14.1

Older Dryas (stadial) 14.1 – 13.9

Allerød (interstadial) 13.9 – 12.9

Younger Dryas (stadial) 12.9 – 11.7

Pre Boreal (interstadial) 10.2 - 8

Table 1. Timing of the main stadial and interstadial periods during the Last Glacial

Period.

In order to contextualize the Young Dryas, here we outline the hypotheses put forward for

this event. The YD has received great attention from the scientific community, and even

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within the press and broader public. The causes that produced this event are hotly debated

and the hypotheses related to its origin are diverse and controversial.

Late Glacial climate oscillations, including the Allerød and Younger Dryas, were identified

in the Paleoenvironmental record as early as the investigation of Hartz and Milthers

(1901), and Flohn (1979) explores the time scales and causes of abrupt paleoclimate

events. In an effort to explain YD cooling, Mangerud (1987) summarized the possible

causes of the Younger Dryas based on evidence collected in Norway, the North Sea, and

Svalbard. He tabulates these hypotheses as: 1) Changes in the North Atlantic Ocean

Current and atmospheric circulation that are dynamically related, 2 ) changes in the

directions of the North Atlantic Ocean Current, 3) interactions of the North Atlantic Current

with the Norwegian Sea, resulting in cooling of surface and intermediate depth water, 4)

major climatic changes in Europe and the Atlantic North added to small climatic changes

in other parts of the world, 5) and geographical factors that contributed to a greater

amplitude of climate change.

However, thoughts on the cause of YD cooling evolved rapidly, and one of the hypotheses

with the greatest amount of glaciological, geomorphological and sedimentary evidence,

postulates that the North Atlantic region during the YD, is related to the diversion of glacial

meltwater from the Mississippi River/Gulf of Mexico to the St. Lawrence drainage, which

flows into the North Atlantic Ocean (Rooth 1982). This hypothesis, fully developed by

Broecker et al. (1989), maintains that the great discharge of fresh water contributed to the

North Atlantic Ocean, resulting in a low-salinity surface layer over the North Atlantic. The

cold, largely freshwater cap suppressed the formation of North Atlantic Deep Water

producing a decrease in temperature and consequently a cooling of the North Atlantic

region (Teller et al., 2002). Lake Agassiz, the largest glacial lake in North America during

the last Ice Age, is known to have dropped below the level of its southern outlet to the

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Mississippi River at the beginning of the Younger Dryas (Fisher, 2003). Although the

causes remain a subject of study, it is clear that not only the cooling of the North Atlantic

region was affected by changing ocean circulation patterns, but also by large-scale

atmospheric changes (Bakke, 2009).

Firestone et al. (2007a) suggest that a large bolide broke into multiple pieces creating

multiple airbursts, possibly with some surface cratering. This event at 12.9 cal ka BP is

hypothesized to have caused the Younger Dryas cooling Moore et al. (2020) has reported

the presence of meltglass, nanodiamonds, charcoal, and microspherules in Abu Hureyra

(AH), Syria, which coincides with the Younger Dryas Boundary (YDB). This evidence

correlates quite well with the possible impact of an asteroid with Earth 12.9 k and B.P.,

which would be in agreement with the hypothesis by Firestone et al. (2007a).

2.2. The Impact Hypothesis and the Controversy

The onset of the YD is correlated in time with a significant climate change, but also with

the collapse of the Pleistocene megafauna and major changes in human behavior such

as the origins of agriculture (Harris, 2003; Munro, 2003; Wright et al., 2003) and dramatic

changes in cultural traditions (Waters and Stafford, 2007; Haynes, 2008; Haynes et al.,

2007; Sweatman and Tsikritsis, 2017). Therefore, the “Impact Hypothesis” (Firestone et

al., 2007a), has received a great deal of scrutiny, which has come in the form of both

support and dismissal.

2.2.1. The physical and geochemical evidence

More than 50 locations in North America have been reported in which a layer of dark clays

with organic matter content and carbonaceous silts are observed that coincides

stratigraphically with the onset of the YD at 12,900 cal ka BP (Firestone et al., 2007a).

Haynes (2008) reports that two-thirds of the geoarchaeological sites in North America that

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have records spanning the Pleistocene/Holocene transition are associated with “black

mats” that coincide with the beginning of the YD. The mats are interpreted to relate to

rapid cooling and mark the end of the Rancholabrean land mammal age (Haynes, 2008).

Kennett et al. (2009) report the presence of shocked nanodiamonds in the YD boundary

sediments, which are associated with charcoal, carbon spherules and soot consistent with

biomass burning. The presence of microdiamonds can be correlated with events in which

high pressures and high temperatures occur. These types of events correlate well with

impact metamorphism with pressures ranging from 80-1000 kilobars and temperature

ranges reaching 3000° C. Similar evidence supporting a YD impact has been reported

from central Mexico (Israde-Alcantara, 2012), southern Chile (Pino et al., 2019), and Abu

Hureyra, Syria (Moore et al., 2020).

Gabrielli et al. (2004) report large concentrations of iridium and platinum have been found

in the GRIP (Greenland) ice core at the YDB. Similar evidence has been reported by

Moore et al. (2020) in Abu Hureyra (Syria), among which the presence of glass with a low

content of magnetite and a very low percentage of water, similar to that of tectites and

inconsistent with volcanic (obsidian) or anthropogenic events, stands out (French and

Koeberl, 2010). Many places in North America, Europe, and particularly in Syria (Abu

Hureyra), the presence of nanodiamonds, deformed and melted quartz grains, carbon and

high contents of iridium, platinum, nickel and cobalt, elements that are present, have been

reported in asteroids and meteorites in high concentrations. In particular, Moore et al.

(2020) reports that about 40% of the glass found in Abu Hureyra, shows traces of siliceous

plants as well as carbon infusions, which indicates that they were formed at temperatures

ranging between 1200 ° C - 1300 ° C, based on laboratory tests. In addition, the presence

of magnetite, melted quartz, and chromferide in the glass found in Abu Hureyra, indicate

minimum formation temperatures of 1720° ˚C, possibly reaching 2200° C and higher.

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The suggestion by Firestone et al. (2007) of an extraterrestrial YD impact has renewed

interest and reinterpretation of the Carolina Bays (Melton and Schriever, 1933) and high

plains playas as possible impact structures (Firestone, 2009). As reported by Firestone et

al. (2007a) the presence of microspherules at various sites in North America, which are

enriched with titanomagnetite, at the YD boundary at 10 sites corresponding in age to

Clovis. In addition, similar titanomagnetite anomalies, with modest enrichment of nickel as

well as Iridium, in sediments in 15 Carolina Bays at the YD boundary support the impact

hypothesis (Firestone et al., 2007a). Firestone et al. (2007) indicates that in the 15

Carolina Bays, as well as in 8 of 9 Clovis age sites, there are peaks of presence of

charcoal. Similarly, in 13 of the 15 Bays and in 6 of 9 Clovis-age sites that correspond to

the YDB, spherules with the presence of charcoal were found. Additionally, they report

that these spherules contain nanodiamonds and fullerenes, which are extremely rare on

Earth but are found in some meteorites and impact craters (Guilmour et al., 1992; Kennett

et al., 2009a, 2009b; French and Koeberl, 2010). Firestone et al. (2007a) reports

sediments that belong to the YDB, have fullerenes with the presence of ET helium, which

are commonly associated with meteorites and impact structures resulting from the collision

of comets or asteroids with the Earth's surface (French and Koeberl, 2010). At the Carolina

Bay and other YDB study sites mentioned by Firestone et al. (2007a), several fragments

with glassy textures were found, which suggests melting of the material. Furthermore,

some nanodiamonds were found at these locations, being these associated to asteroid

impacts as reported by Gilmour et al. (1992).

2.2.2. Onset of the YD and timing of cultural transitions and extinctions

The YD onset is correlated in time with the Clovis/Folsom cultural transition. The youngest

Clovis sites correlate to the beginning of the YD, however, Surovell et al. (2016) find that

the Folsom complex dates to at least 100 years after the beginning of the YD; they

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acknowledge their chronology is complicated by small sample size. Therefore, Surovell et

al. (2016) conclude that there is little correlation between the start and end of the Folsom

complex and the onset and termination of the YD. According to Faith and Surovell (2009),

the cause or cause/causes of Pleistocene megafauna collapse has/have been debated,

however, the timing is correlated closely with onset of YD cooling. One hypothesis

establishes that Pleistocene megafaunal extinction may be attributed to global climatic

changes associated to rising CO2 levels as well as elevated temperatures and

precipitation, prompted a transition from a nutrient accelerating mode to a nutrient

decelerating mode in North America.

Although, Gill et al. (2009) present evidence that the collapse of the Pleistocene

Megafauna occurred several hundred years before the onset of the YD, which is in

agreement with the idea that causal relationships between novel plant communities

associated with the last deglaciation in North America and megafaunal extinctions are still

unclear.

Based on studies held by McDonald et al. (2006) and Schulte et al. (2010) other extinction

events have been correlated with the impact of asteroids. Such is the case of the extinction

that occurred in the late Cretaceous period in which approximately 75% of all the species

on the planet became extinct. Additionally, the wide disappearance of some mammalian

genera suggests that the cause of their extinction was abrupt and not gradual, as

suggested by the hypotheses of climate cooling or human overkill (Firestone et al., 2007a).

2.2.3. The geomorphological evidence

Geological and geomorphological features have been cited as supporting evidence for a

YD impact. Characteristics of impact structures may resemble other geological structures;

the most common of all is the circular or circular-like shape, circular deformation patterns,

magnetic anomalies, among others. For example, the Carolina Bays (Firestone et al.,

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2007a) and the playas of the southern High Plaines (Firestone, 2009; Firestone et al.,

2010) are interpreted to be of possible impact origin.

The description made by Prouti (1952) about the Carolina Bays, serves to have a general

panorama about the structures displayed on the area, and I quote: “The terms "bay" and

"pocosin" as generally used in the Atlantic Coastal Plain refer to an area usually overgrown

with swamp vegetation and underlain by peat deposits. In some areas shallow lakes, with

peat-filled border swamps, are also classified as bays”

According to Firestone et al. (2010) the Carolina Bays are made up of approximately

500,000 depressions with elliptical shapes that are distributed in North America, more

specifically from the Atlantic coast in New Jersey to the state of Alabama. They are

characterized by having depths of up to 15 meters and dimensions ranging from 50 m to

10 km in length. In the description made by Firestone (2009) and Firestone et al. (2010),

the major axes of these elliptical structures are oriented to the northwest and the beach

rim had the typical marks associated with YDB, including fullerenes, nanodiamonds,

carbon spherules, charcoal, Soot, iridium, and glass-like carbon. As it is reported by

Melton and Schriever (1933) the formation of the Carolina Bays was originally ascribed to

asteroid impacts because its uniqueness. The bays in this area are characterized by

having a wide distribution and presenting elliptical shapes, parallel alignment of their major

axes and high edges. This leads Melton and Schriever (1933) to suggest that rare

geological processes, such as the impact of various asteroids on the Earth's surface, as

a result of the disintegration of a larger asteroid in the atmosphere, could have been

responsible for the formation of these structures Nonetheless, the lack of evidence

regarding to meteorite material associated to these structures, suggests that other

geological processes, such as eolian, marine, dissolving action of ground water,

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volcanism, glaciation, and submarine, might be the main cause (Melton and Schriever,

1933) .

Similar shaped basins have tentatively been identified in Texas, New Mexico, Kansas,

and Nebraska (Kuzilla, 1988), but are far less abundant than the Carolina Bays. Analysis

of the long axes of these oval-shaped basing indicated they are preferentially aligned

toward the Great Lake region, suggesting a potential impact in that region (Firestone et

al., 2009).

Larger impact structures are known to be associated with other geological processes such

as diapirs, tectonic deformation due to magma rise, or volcanic activity (French and

Koeberl, 2010). In such a way, in order to correctly identify and classify an impact

structure, the main data related to the impact is metamorphism such as described by

Wolbach et al. (2018a, 2018b) and French and Koeberl (2010). Such characteristics are

summarized in table 2.

A. Characteristic regarding to shock metamorphism and impact cratering

1. Meteorite fragments preserved

2. Chemical and isotopic projectile signatures

3. Mesoscopic features - Shatter cones

4. High-pressure (diaplectic) mineral glasses

5. High-pressure mineral phases

6. High-temperature glasses and melts

7. Planar fractures (PFs) in quartz

8. Planar deformation features (PDFs) in quartz

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B. Non-diagnostic features produced by meteorite impact and by other

geological processes.

1. Circular / circular-like morphology

2. Circular structural deformation

3. Circular geophysical anomalies

4. Fracturing and brecciation

5. Kink banding in micas

6. Mosaicism in crystals

7. Pseudotachylite and pseudotachylitic breccias

8. Igneous rocks and glasses

9. Spherules and microspherules

10. Stratigraphic criteria – inverted layers

11. Other problematic criteria

Table 2. Shock-produced deformation effects: diagnostic and non-diagnostic. Modified

from Holocombe et al. (2001) and French and Koeberl (2010).

2.3 Arguments against asteroid impact hypothesis.

Evidence for an extraterrestrial impact at the lower boundary of the YD still being highly

debated. There are a number of studies that cast doubt on the impact hypothesis.

Regarding the "YD event", Firestone et al. (2007a) present an unusual variety of evidence

outside the criteria accepted by the impact community. Reimold, 2007 points out that

impact structure report should be reviewed in detail, as it could be compromised,

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incomplete or incorrect. Eventually, this information may be widely disseminated to the

scientific community and the general public, which could impair reliability in the affected

databases where other identified impact structures are recorded.

Firestone et al. (2007a), reports iridium concentration corresponding to 117 ppb and

another of 51 ppb were from separate magnetic beads and spherules, fully compatible

with a non-catastrophic source of micrometeorites (Pinter and Ishman, 2008; Boslough et

al., 2013).

Despite the broad acceptance of iridium anomalies because extraterrestrial impact after

Alvarez et al. (1980), authors such as Dyer et al. (1989) have found further evidence to

explain iridium anomalies on sedimentary rocks, including claystones and shales. Actually,

the authors found that a variety of common microorganisms found worldwide, which

include heterotrophs, photoautotrophs, and chemoautotrophs, might affect the chemistry

of iridium by concentrating or dissolving it. Additionally, the geochemistry of iridium must

be evaluated from a microbial perspective given the permanent biological activity on

sedimentary rocks that might erase, enhance or create iridium anomalies.

Iridium anomalies have also reported by Coveney et al. (1992) and Sawlowicz (1993) on

shales and other sedimentary environments. According to their results, precious metals in

shales, particularly on Cambrian shales in China, are likely not of extraterrestrial origin

given the abundances of PGE and volatile elements (Mo and Se), which differ greatly from

calculated values on metallic meteorites. In fact, Sawlowicz (1993) indicates that it is

possible to use the abnormal antibody values of PGE and associated elements in order to

identify contributions of specific processes such as 1) extraterrestrial, 2) volcanogenic, 3)

precipitation from seawater, 4) microbial, 5) exhalative-hydrothermal, 6) leaching, and 7)

transport and precipitation at a redox boundary.

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Studies about major cultural changes and Paleoindian population decline are reported by

Buchanan (2008), showing that simulation experiments detect that, even at high site

destruction rates, if Paleoindians had suffered a population bottleneck, they would be

visible in the summed probability distribution of the calibrated Paleoindian radiocarbon

dates. Therefore, their results indicate neither Firestone et al. (2007a) hypothesis is valid

nor migration to south by Paleoindians because of an asteroid impact.

The set of high-quality radiocarbon dates in the Folsom and Clovis complexes used by

Surovell et al. (2016) show a revised age range for the Folsom complex and examine its

correlation with Younger Dryas, the end of Clovis, and megafauna extinction. Surovell et

al. (2016) show that to refine the ages of the Folsom complex, it is best to reduce the

complete set of radiocarbon dating and thus obtain more accurate dates of Folsom

occupation. With the data collected and its subsequent analysis, Surovell et al. (2016)

reveal that in addition to the temporal correlation between the YD and the Folsom complex,

the duration of the cooling of the North Atlantic occurred between 12,850 and 11,650

years, with which in these 1200 years it is difficult to reconcile times of disappearance of

Folsom and Clovis complex; additionally, the authors show that the oldest cultural

evidence dated from the Folsom complex, corresponds to 240 years after the start of YD.

Despite what was shown by Steffensen et al. (2008), which establishes a clear analysis

on the survival of hunter-gatherers to the changes associated with YD, there is no clear

correlation between climate change and strong cultural changes in the Folsom complex.

Waters and Stafford (2007) published a revised timeline for the complex showing that

dates in Clovis appeared to be highly clustered in time, and that the duration of the Clovis

complex was considerably shorter than previously believed, results that do not differ from

those obtained by Surovell et al. (2016)

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Paquay et al. (2009) report the presence of elements of the platinum group (PGE: Os,

iridium, Ru, Rh, Pt, Pd), which together with the presence of gold particles (Au) and the

proportions of 187Os / 188Os in terrestrial, lacustrine sections and marine results indicate

that the iridium values reported by Firestone et al. (2007a) do not coincide. Furthermore,

the isotopic ratios of 187Os / 188Os in the sediments are similar to the average cortical

values, indicating a non-extraterrestrial origin.

Boslough et al. (2012), propose that despite the existence of evidence that supports the

asteroid impact hypothesis and its relation to the YD onset, there are not only one but

several impact hypotheses that are difficult to reconcile between them. As the authors

mention, the main reasons against the impact hypothesis include the lack of solid evidence

showing an impact structure, as pointed out by French and Koeberl (2010), lack of

temporal correlation between the supposed impact and the climatic, paleontological and

archaeological changes and the failure to reproduce any of the results obtained by the

authors of these hypotheses.

Reimold et al. (2007, 2014) emphasize the relevance of reviewing existing databases

reporting impact craters around the world, as some of these structures do not appear to

meet the basic requirements to be classified as impact structures. Likewise, the authors

attribute to the lack of careful review of the structures associated with impact craters,

misunderstandings not only in the academic community but in the media. Specifically in

the case of the YD event, the lack of evidence of a structure that supports the reports

described by Firestone et al. (2007a), makes the impact hypothesis less and less solid.

LeCompte et al. (2012) find that in three of the places reported by Firestone et al. (2007a),

there are spherules deposits with chemical compositions similar to those also reported by

Firestone et al. (2007a). Despite reproducing similar results, LeCompte et al. (2012) do

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not assure that it is indeed material produced by the impact of an asteroid 12,900 years

ago.

One of the most relevant evidences provided by Firestone et al. (2007a) to support the

impact hypothesis is related to the presence of spherules found in YDB together with the

iridium anomaly. Wu et al. (2013) interpret the Pt/iridium relationship data found in the

Greenland Ice Sheet Project (GISP-2) and Greenland Ice Core Project (GRIP) as of

terrestrial origin. Likewise, the low isotopic ratios of 187Os/188Os could be attributed to

volcanic processes, although the authors do not rule out a possible extraterrestrial origin.

Despite findings reported by Wu et al. (2013) supports Firestone et al. (2007a) hypothesis,

it is hard to conceal how impacts were part of a number of other impacts worldwide

associated to spherules with low enrichment in PGEs.

In their analysis of platinum group elements (PGEs) and gold from continental YD

sections, Paquay et al. (2009) find no evidence of elevated iridium concentrations. In

addition, osmium isotopic ratios of the sediments are reported to be similar to Earth

average continental crust (Paquay et al., 2009). The iridium mass concentration reported

by Firestone et al. (2007a) indicates a peak of just 2.3 - 3.8 ppb and even maximum iridium

values were below this amount at 10 different locations. This forces the authors to argue

that the impactor had abnormally low initial Go values.

2.4 General ridicule

Discussion held by Pinter et al. (2011), describes the differences between the types of

physical evidence that has been associated with the impact process. This can be divided

into two large groups: the first argues that given the evidence widely rejected by the

scientific community, it should not even be a matter of discussion (high concentrations of

iridium, fullerenes enriched in 3He, among others). The second group is characterized by

21

the extensive study of evidence that has been the subject of recent research, including

magnetic grains, nanodiamonds, among others.

Pinter et al. (2011) point out that under the criteria specified by Koeberl (2007) and French

and Koeberl (2010), it is considered as unequivocal evidence of impact craters the

presence of shock-metamorphic effects and, in some cases, the discovery of meteorites,

or traces of them. At the macroscopic scale, the presence of shatter cones and at the

microscopic scale, characteristics of quartz fracturing called Planar Deformation Features

(PDF), are also evidence that corroborate the impact of an asteroid on the Earth's surface.

However, as Pinter et al. (2011) reports, none of these characteristics has been reported

by Firestone et al. (2007).

Despite showing a marked difference between the evidence that has been shown has no

relation to the impact hypothesis, there is another considerable amount of evidence that

is worth reviewing, as it could constitute an advance in the confirmation or discard of the

hypothesis proposed by Firestone et al. (2007a). There is no doubt that future works

should focus on verifying whether such evidence, which according to Pinter et al. (2011)

would belong to the second group of tests, they could have a temporal and causal

relationship of the beginning of the YD, with the impact of an asteroid 12,900 years ago.

Regarding the catastrophic event of the impact and its relationship with the YD, evidence

is presented related to the presence of foraminifera in chevron-like folds and as splashes

in oceanic cores associated with impact gaps. As reported by Abbott et al. (2008) CaCO3

melts are documented for the Chicxulub impact site, but these are a far cry from the "well-

preserved carbonate microfossils" encased in silicate melt. Other authors specify that the

presence of silica and the associated carbonate in Chicxulub, occur as "molten carbonate

particles", recrystallized and decomposed calcite.

22

Pinter and Ishman (2008) show that recent experiments prove that it is possible to find

intact grasses in silica melts as a result of impacts. However, the summary by Harris and

Schultz (2007) suggests only one mechanism in which potential grass debris is preserved

in impact gaps. Fully charred plant remains are documented in the geological record (e.g.

Scott, 2008), but few researchers strongly accept that unaltered plant material or the

presence of foraminifera have been in direct contact with impact molten material. As

mentioned by Pinter and Ishman (2008), the comments made by Firestone et al. (2007)

reiterates the relationship between an impact event in North America 12.9 ka B.P. based

on evidence found at 50 Clovis age sites. Due to the absence of a surface impact crater,

Firestone et al. (2007a) and Firestone et al. (2007b) report the presence of carbon such

as glass, nanodiamonds and spherulites that may be associated as by-products of the

impact. Furthermore, they report the presence of micrometeorites lodged in mammoth

tusks (Firestone et al., 2007b).

Granulometric parameters indicate that at the end of the YD event, there was a gradual

retreat of the glacier. This added to the distribution of lake sediments and water content,

show an immediate response to the absence or presence of a glacial mass (Bakke, 2009),

so an extraterrestrial impact would not explain the retreat of said glacier in such an

extensive time.

3. Glacial and Postglacial History with Emphasis on Lake Agassiz

3.1. Glacial History

Northern Minnesota and adjacent Canada were extensively glaciated during the

Pleistocene, however, the modern landscape was the result of Wisconsin Glaciation with

the Last Glacial Maximum culminating about 21,000 cal years BP. The general glacial

history of northwestern Minnesota is summarized by Wright (1972) and detailed glacial

history as it pertains to this study is summarized by Fenton et al. (1983), Mickelson et al.

23

(1983), Clayton et al. (1985), and Harris (1995). Ice advanced to its maximum limit along

the southwestern margin of the Laurentide Ice Sheet by about 27-30,000 cal years BP

(Mooers and Lehr, 1997). The glacial history is complicated by repeated surging of the

southwestern margin (Clayton et al., 1985; Lusardi et al., 2011), however, the final ice

advance into northwestern Minnesota culminated at 11,700 yrs BP (13,500 cal yrs BP)

(Fenton et al., 1983; Fisher, 2003). Ice then retreated to the north of the continental

divide in western Minnesota that separates drainage to the Mississippi River from

drainage to the north to Hudson Bay. At this point Lake Agassiz was formed between

retreating ice and the divide.

3.2. A brief history of Lake Agassiz and the YD

One of the largest lakes that has ever existed in North America was Agassiz Glacial

Lake. The lake initially formed about 13,500 years ago and expanded as ice retreated

north of the continental divide in western Minnesota (Fenton et al., 1983). At that time,

meltwater was ponded between the retreating Laurentide Ice Sheet and the divide.

Throughout its existence, the lake varied in size depending on the position of the

glaciers and the availability of drainage outlets. At its greatest extension, the lake was

1127 km long and 402 km wide, extending from South Dakota to Canada. Most of the

glacial ice had completely melted 8,000 years ago and the giant lake drained north into

the newly opened Hudson Bay. Lake Agassiz drained south through Glacial River

Warren along the course of the Minnesota River to the Mississippi and to the Gulf of

Mexico (Teller and Clayton, 1983).

One of the explanations that has been given to Late Pleistocene cooling of the YD

approximately 12,900 - 11,700 cal yrs BP, is a change in the North Atlantic meridional

overturning circulation (MOC) caused by the discharge of large amounts of fresh water

from Lake Agassiz. There is broad support that the YD resulted from a weakened MOC,

24

which in turn reduced heat transport from the north and warmed the southern

hemisphere, although the relationship between causality and temporality remains a hot

debate (Broecker et al., 1989; Teller et al., 2002; Breckenridge, 2015). According to

Breckenridge (2015), the main source of doubt is the uncertainty regarding the routing of

the Lake Agassiz overflow at the start of the YD. During the YD, it has been proposed

that Lake Agassiz drained both to the Arctic Ocean (Fisher et al., 2009; Murton et al.,

2010) through a northwest outlet, and to the Atlantic Ocean through an eastern outlet

(Fenton et al., 1983; Broecker et al., 1989; Teller et al., 2002) (Figure 1 from Murton et

al., 2010). Other evidence suggests that the northwest and east outlets remained

covered by the Laurentide Ice Sheet at the beginning of the YD, and that the increased

flow of fresh water into the Atlantic Ocean was absent (Lowell et al., 2013).

3.3. Stages of Lake Agassiz

Multiple stages, or lake levels, are recorded by strath terraces along River Warren and

associated beaches in the southern part of Lake Agassiz (Matsch, 1983; Breckenridge,

2015). However, concurrent isostatic rebound of the lake basin resulted in the formation

of extensive beach complexes in the northern part of the Agassiz basin (Fenton et al.,

1983; Breckenridge, 2015). Near the southern outlet, four major strandlines are

recognized. These four major lake levels also define four major stages of Lake Agassiz

(Fenton et al., 1983). The earliest phase of Lake Agassiz is the Cass Phase and is

associated with the highest major strandline of Lake Agassiz, the Herman level (Figure 2

from Breckenridge, 2015).

As the lake expanded in area Lake Agassiz merged with Glacial Lake Koochiching

(Nikiforoff, 1947) ushering in the Lockhart Phase of Lake Agassiz (Fenton et al., 1983).

From the high stand at the Herman level, the Lake level dropped in stages, first to the

Norcross level and then to the Tintah level between about 13,500 cal BP to about 13,000

25

cal BP (Fisher, 2003; Breckenridge, 2015). Water then dropped from the Tintah level to

the Moorhead low-water phase (Teller, 1983), and ceased to drain through the southern

outlet and on to the Gulf of Mexico. The timing of this drop is constrained by numerous

radiocarbon and optically-stimulated luminescence (OSL) dates, however, associated

error places the beginning of the Moorhead phase about 13,400 – 12, 600 cal BP. This

drop has been correlated to the beginning of the Younger Dryas at 12,900 cal yrs BP.

At this time, it is suggested that Lake Agassiz either drained eastward through the Great

Lakes to the Atlantic Ocean (Broecker et al., 1989; Teller et al., 2002), NW to the Arctic

Ocean (Fisher et al., 2009), or neither (Lowell et al., 2013). Investigations by Fisher et al.

(2009) and Lowell et al. (2009), involving extensive radiocarbon dating of the northwestern

and eastern outlets of Lake Agassiz, respectively, indicate that both potential outlets are

too young. Breckenridge (2015) also concludes that the NW outlet could not have been

open at that time. In the absence of a well-documented eastern or northwestern outlet for

Lake Agassiz, Lowell et al. (2013) suggest that the basin may have become hydrologically

closed, although radiation balance and meteorological calculations over the retreating

Laurentide Ice Sheet suggest hydrologic closure is unlikely (Mooers, unpublished data).

Geochemical analysis of sediment cores near Fargo, North Dakota, did not show evidence

of enrichment by evaporation at the top of the Brenna Formation associated with the

Moorhead low-water phase (Lowell et al. 2013; Liu, et. Al. 2014).

Although new and reanalyzed data from the Agassiz basin has emerged to restrict lake

paleogeography and meltwater routing history (Fisher, 2003, 2007; Lowell et al., 2005;

Teller et al., 2005; Lepper et al., 2007), there is uncertainly of the absolute dates of the

drop of Lake Agassiz to the Moorhead low-water phase, within error the Moorhead phase

coincides with the onset and termination of the YD.

26

Given the coincident timing of the beginning of the Moorhead low-water phase and the

YD, Broecker et al. (1989) and Teller et al. (2002) suggest that the diversion of LIS

meltwater from the Mississippi and Gulf of Mexico to the St. Lawrence and the North

Atlantic resulted in the changes in ocean circulation that triggered YD cooling. This

conclusion is supported by the work of Levac et al. (2015) among others.

The Moorhead phase ended when the level of Lake Agassiz rose to the Campbell level

(Liu et al., 2014; Breckenridge, 2015) by about 11,500 cal yrs BP (Liuk et al., 2014; Fisher,

2003; Lowell et al., 1999). It has been speculated that the closure of the eastern outlet

and the end of the Moorhead low-water phase was related to the advance of the Superior

Lobe in the Lake Superior Basin at 11,500 cal yrs BP (9900 BP (Hughes et al., 1978;

Lowell et al., 1999). Liu et al. (2014) (Figure 3) show the range of radiocarbon dates and

their errors associated with the Moorhead low-water phase.

4. Description and setting of the Roseau Structure

The Roseau Structure is located in northern Minnesota, at 48 ° 55’02.22” N, 95°

52’14.08” W close to US-Canada border (Figure 4). The structure is located 8 km north

of the Campbell beach (average elevation 331 meters) and 34 km north of the Tintah

beach (average elevation of 361 meters). This circular-like structure has a central basin

lying 1-2 meters below the surrounding landscape. It is oval shaped, 10.7 km (major axis

oriented predominately to the northeast) by 8.12 km (minor axis) with an average

elevation of 313 m.a.s.l. The structure would be located 48 meters below the Tintah

beach and 18 meters below the Campbell beach. The northeastern margin of the

Structure is characterized by a series of ridges with a spacing ranging from 72 to 155

meters; these ridges display a concentric pattern as shown on Figure 4. The ridges are 1

to a maximum of 2 meters high and are composed of silt loam to fine sandy loam soils

27

with organic soils (peat and muck) in low swales between ridges (Web Soil Survey,

USDA).

The anomalous Structure described above is the only one of its kind in the region. It is

important to now address hypotheses for its origin. Given its setting and the geological

history of the landscape, the Structure must be of glacial or lacustrine origin, with the

only alternative being of extraterrestrial origin.

4.1. Glacial origin

The circular-like structure may be interpreted as a result of glacial processes. These

processes include subglacial meltwater flow, erosion by ice scour, formation of a kettle

lake related to melt out of buried ice, or ice marginal stagnation processes (Benn and

Evans, 2010). The presence of subglacial channels that can reach the surface through

incisions, have cuts to the glacial bed or a combination of both has been reported by

Röthlisberger (1972). There are several examples of reaches of incised channels up and

down in glacier beds, sometimes within the same system. There is evidence that

branched channel networks occur beneath glaciers (Kamb, 1987). Subglacial channels

would have circular to semicircular shapes in cross section; however, the studied

circular-like structure does not correspond to a cross section of a subglacial channel first

because it is not associated with a glacier and second because the circular shape is in

plain view and not in cross section as described by Röthlisberger (1972).

Another possibility is the presence of pits or pockmarks whose formation may be

associated either by expulsion of fluid through the lake floor (pockmarks), or by the

temporary grounding of an iceberg through tidally-influenced changes in lake level (pits)

(Brown et al. 2017). According to Brown et al. (2017), the pits associated with icebergs

can be found in different types of sediments exposed on the surface either during glacial

28

or post-glacial conditions. Its formation’ occurs when, in the case of sea, icebergs

eventually hit the substrate either to changes in water depth or buoyancy. In the case of

Lake Agassiz, the circular-like structure, could be the fall of a fragment of the glacier of

considerable size that left an imprint on the substrate. Other authors such as Bass and

Woodworth-Lynas (1988) and Newton et al. (2016) mention that both pits and structures

associated with ice scour with semicircular geomorphologies have been reported on the

seabed and whose causes have been reported by Brown et al. (2017). Based on

satellite images near to the location of the structure, it is possible to determine that the

studied structure is isolated from other circular-like structures. Spectral bands

combination is suggested in order to have a better approach to identifying similar

structures as well as GPR. Subglacial erosion can create overdeepenings on the glacier

bed. Moran et al. (1980) describe the erosion of blocks of the glacier bed on a scale

similar to the anomalous Structure. In their case, blocks of the glacier bed were

detached and displaced some distance downglacier. The elongation is in the direction of

ice flow, and the raised ridges are downglacier from excavated depression. In the case

of the anomalous Structure, the ridges are to the northeast, or upglacier and are of a

very different morphology.

Another example of structures with a similar geomorphology are kettle lakes, such as

Kettle Lake, which lies within the Missouri Coteau (Clayton,1967) or Kettle Moraine in

Wisconsin (Carlson et al., 2005). Kettle lakes form when blocks of ice melt from beneath

a sediment cover. Typically, kettle lakes form in groups such as most of the lakes of

Minnesota. For the Structure to be an ice block depression, it would be one of the largest

in the region. This structure is characterized as resembling a bowl, reaching depths of up

to 10 m (Grimm et al. 2011). For example, Lake Mille Lacs in central Minnesota is a

broad, shallow ice scoured basin with raised shorelines around its margins (Wright,

29

1972). However, Lake Mille Lacs has a prominent moraine on the down-ice side of the

basin; no such moraine exists anywhere in the vicinity of the Structure. Kettle Lake

(North Dakota) is not the only structure whose geomorphology resembles a bowl, in fact,

the images show similar structures in North Dakota and Canada, which is contrasted

with the null presence of structures similar to the one in northern Minnesota.

4.2. Lacustrine Origin

Lacustrine processes associated with Lake Agassiz include wave dominated shorelines,

strong currents related to glacial meltwater throughput to the Lake, and sedimentation

associated with shallow-water and deep-water environments. None of these lacustrine

sedimentary environments are known to be associated with circular depressions. The

ridges surrounding the anomalous Structure do indeed appear to be wave washed,

however, likely related to the fall and rise of the water level of Lake Agassiz.

A possibility raised by Ken Harris (written communication, 2020), retired from the

Minnesota Geological Survey, is that the anomalous Structure is an ice-walled lake

plain. Ice-walled lake plains can be of just about any size. Larson et al. (2016) describe

ice-walled lake plains formed in thin stagnant ice in northeastern Minnesota. They are

characterized by relative flat areas with slightly raised rims, but none are associated with

multiple concentric ridges. Ice-walled lake plains require underlying stagnant ice, and it

is unclear if there are any mechanisms for the preservation of stagnant blocks of ice in

Lake Agassiz. In addition, this is the only feature of its kind. Ice-walled lake plains are

commonly found in groups.

4.3. Impact Origin

Impact craters are classified according to their size as follows: 1) simple craters

(approximately 15 km in diameter), complex craters (15 - 150 km), and impact basins

30

with mound rings (150 - 1200 km) (Melosh, 1980). There are three stages in the

formation process of these structures: 1) the compression stage, impact of the

asteroid/comet with the surface of the rocky body (moon or planet). At this stage, kinetic

energy transfers to the surface, and the shock waves affect both the projectile (asteroid)

and the surface. 2) Excavation: the removal of the material that gives way to the crater

occurs and the production of the ejection material and 3) the modification stage in which

the structure collapses and changes as a result of elastic and gravitational rebounds

(Melosh, 1980).

The compression stage ends once all the material released by the impact covers the

meteorite. At this instant, the meteorite penetrates a certain distance into the substrate,

ranging from 10-3 to 10-1 s for projectile sizes ranging from 1m - 10m. The substrate

material is at rest until the shock wave hits it. Similarly, the projectile continues to

penetrate the substrate until the blast wave slows down. In terms of the implicit variables

in the compression stage, it is possible to arrive at a solution of Hugoniot's equations for

pressure, particle velocity, shock velocity and internal energies of the projectile and the

substrate (Melosh, 1980; Shomaeker, 1963 ). It is essential to mention that

approximately half or a little more, either of the kinetic or internal energy of the projectile,

is transferred to the target, this being one of the main results of the compression

process.

Experimentally, Oberbeck (1971) reports that there is a close relationship between the

size and shape of the crater as a function of depth. The same explosive charge can

create a small crater if it is placed very close to the surface or very deep. However, there

is an ideal depth at which such loading can produce the largest craters. This relationship

can be expressed as:

31

D = L (ρj / ρt )1/2 ,

(1)

and indicates explosion effective depth (D) and the diameter of the projectile (L), in

equation (1) and the relationship between the density of the impactor (ρj) and the density

of the substrate (ρt). If the projectile has a high density, for example a metallic asteroid

(7.21 - 7.71 g cm-3) (McCasuland, 2011), D will be higher if the substrate has a low

density. If, on the contrary, a rocky asteroid (3 - 3.30 g cm-3) (McCasuland, 2011)

impacts a high-density substrate, then D will be smaller (Melosh, 1980).

Regarding the excavation stage, Melosh (1980) highlights that, depending on the

mechanical behavior of the substrate, it deforms in plastic or fragile way (ballistic

fragments). Based on the calculations of Melosh (1980), the relationship between the

diameter of the impact crater, measured from ring to ring (D) and the thickness of the

substrate that tends to deform plastically (h), it is possible to associate the

corresponding morphology and structures of the crater as shown in Table 3. Additionally,

the crater size is more extensive in terms of depth and diameter than the projectile that

creates such a crater at a typical speed of 15 km s-1.

D/h ratio Main characteristics

< 4 Bowl Shaped crater. Simple carter with a depth/diameter ratio of about 0.2

< 4 , > 8 The crater displays a central mound

> 9 The crater displays a set of benches of the top of fragile layers

>> 10 Formation of Concentric craters when the bench occurs high on the crater

wall

32

Table 3. Summary of the main characteristics of an impact crater in terms of the D / h

ratio, according to Melosh (1980).

Ormö and Lindström (2000) outline the morphology of land-target craters. Craters less

that about 2-4 km wide have depths about one third to one fifth of the width and rim

heights of about 4% of the diameter (Ormö and Lindström, 2000). Land target craters

greater that about 4 km are generally more complex because of gravitational collapse

and rebound that forms a central uplift (Ormö and Lindström, 2000).

At the Roseau Structure, the water would have been about 50 meters deep and the

lacustrine and glacial sediment about 65 meters (about 200 feet) thick over Precambrian

igneous and metamorphic bedrock. In terms of deformation, this yields a plastic layer

about 115 meters thick over crystalline bedrock, which provides an ideal scenario for

cratering with concentric rims (Melosh, 1980).

However, to evaluate cratering for application to the Roseau Structure there needs to be

emphasis on impacts in water, even though the global cratering record is heavily biased

toward terrestrial structures. An important exception is the Mjølnir crater in the Bering

Sea. The Mjølnir crater is 40 km in diameter formed by a projectile estimated to have

been 1-3 km diameter, which impacted in approximately 400 meters water depth

(Shuvalov et al., 2002). Shuvalov et al. (2002) developed numerical models to evaluate

the mechanical response, and the role of the unique geological setting in the cratering

process. Some results of the numerical models on impact processes obtained by

Shualov et al. (2002) and described in Osinski and Piezzaro (2013) associated with

strata that deform plastic and fragile indicate collapse structures, blocks delimited by

normal low-angle faults and concentric rings towards the periphery of the impact craters.

33

Through studies such as Shuvalov (2002) and Shuvalov et al. (2002), knowledge of the

impact crater formation processes underwater columns has increased significantly.

Experiments and numerical models carried out by Gault and Sonett (1982) and

Artemieva and Shuvalov, (2002) show that it is possible to have a better idea about the

role of the water column and its relation to the projectile. According to Shuvalov et al.

(2002), the d / H ratio, where d is the diameter of the projectile and H is the depth of the

water column, has high relevance in the process of formation of impact craters as

summarized in Table 4.

d/H ratio Result

< 0.1 No crater is formed at seafloor

> 0.1 , < 1 Relevant results in morphology and size of the structure

If d > H only a minor direct influence of the water column

In any case, significant differences are observed on submarine impact craters

compared to their continental counterparts.

Table 4. Summary of the effects in the formation of impact craters and their relationship

with the d / H ratio.

If the impactor is small compared to water depth (d/H < 0.1), there will be no disturbance

of the sediment below the water column; However, if d/H > 1, the water column will have

no influence on the impactor, and the crater will develop as if it were terrestrial. Impact

craters, their morphology, size, and formation process are widely documented in the

literature. As mentioned by Shuvalov et al. (2002), the morphology of the Mjølnir crater,

34

located 400 meters deep under the sea, differs considerably from subaerial impact

structures without a significant water column. This structure displays low topography

towards the periphery, contrary to that observed in impact craters such as Meteor

Crater. Additionally, due to the plastic deformation of the weak sediments located in the

first 3 km of the ocean floor, a collapse occurred at the same time as an unusual filling of

the structure. A similar process could have occurred in the study area since the glacial

and lacustrine sediments located deep underwater (approximately 115 m) at the time of

possible impact structure formation, would also have a plastic deformation.

According to Sonett et al. (1991), once the impact of the extraterrestrial body with the

body of water occurs, two types of waves occur, the first being related to the excavation

of the body of water and the second associated to crater slosh. Assuming that the

Structure is an impact crater and considering the dimensions of the structure, a bolide

with a size equivalent to 0.5 km in diameter traveling at an average speed of 25 km / s

would be needed (Schmidt and Holsapple, 1982; Sonett, Pearce, and Gault, 1991).

However, according to Shuvalov (2002) and Shuvalov et al. (2002) and I quote “the

processes accompanying impacts into the ocean strongly depend on the sea depth. If

water depth exceeds two impactor diameters, the projectile is totally disrupted and

decelerated as it passes through the water column, and only a small depression in the

sea bottom is formed under the action of shock compressed and downwards accelerated

water.” Additionally, Shuralov (2002) indicates: “the formation of concentric craters is

not directly related to the depth of the water column. Such craters appear to form

because of special underwater target lithology (layered targets). In particular, such

craters are created in low strength sediments or regolith layer overlying the crystalline

basement.”

35

Ormö and Lindström (2000) describe several impact craters in Baltoscandia that were

formed in shallow-water marine environments with water depths of a few 10s of meters.

Several are on the order of the diameter of the Roseau Structure including the Lockne,

Kaluga, Granby, Kardla, Flynn Creek, and Brent craters; of these, Granby, Kardla, Flynn

Creek, and Brent craters were formed where the water depth was estimated to be less

than 75 meters (Ormö and Lindström, 2000). All of these shallow-water craters are

associated with slump blocks but no resurge deposits. All of these impacts, however, are

suggested to have been associated with basement rock fracturing and cratering, depths

of a few 100s of meters.

Another possibility that must be considered is that a rocky or cometary fragment

detonated above Lake Agassiz in an airburst. Information on airbursts, particularly over

water is difficult to find. Robertson and Mathias (2019) conduct ALE3D Hydrocode

(https://wci.llnl.gov/simulation/computer-codes/ale3d) simulations of asteroid airbursts in

relation to the Tunguska event of 1908. The tree fall zone at Tunguska was over 2000

km2 in area, however, no cratering was present at the site. Robertson and Mathias

(2019) estimate the Tunguska meteor at between 50 and 100 meters diameter with

airburst altitudes of 5-15 km depending on the velocity, angle, and density. Collins et al.

(2017) conduct a numerical assessment of airbursts of rocky and cometary materials in

the 10-100 meter diameter range. Their simulation show that airbursts can occur at any

altitude with the energy release increasing with decreasing altitude (Collins et al., 2017).

However, it is very difficult to predict the nature of the impact or cratering associated with

airburst. In addition, they emphasize the process of airburst to crater formation is not

discontinuous, and there is an intermediate range where both can occur (Collins et al.,

2017).

36

Huss et al. (1966), reports the presence of a metallic meteorite fragment (Octahedrite) in

Anok, Minnesota (45 ° 12 'N, 93 ° 26' W), at a distance of 465 kilometers from the study

area. Compositionally and genetically, this fragment is associated with a group of

meteorite fragments found in Havana, Illinois, in layers whose age is estimated to be

2336 ± 250 BP (Arnold and Libby, 1951; McCoy et al., 2017) attribute its origin to the fall

of meteorites in a broad region, which, due to its large vegetation cover, made it difficult

to find (Carr and Sears, 1985). These fragments could be related to asteroid break-up in

the atmosphere that may have formed the crater.

5. Discussion

The Roseau Structure in northern Minnesota has characteristics that resemble those of

an impact crater. It’s nearly circular shape, bordered by ridges in a northeast direction,

resembles the descriptions made by Shuvalov et al. (2002) for impact craters formed by

impacting weak strata that are plastically deformed. In the study area, these strata are of

lacustrine and glacial type; in turn, these lie on strong rocks that tend to deform fragilely.

Furthermore, structure formation is not consistent with the presence of glaciers due to it

displays ridges as a product of washing waves. However, by the time these structures

formed, there was a body of water with an average depth of 50 meters with

approximately 60 m of low-density sediment overlying solid rock, which constitutes a

scenario similar to Mjolnir crater in the shallow Barents Sea.

The circular-like structure displays a set of ridges that are visible at the eastern side of

the structure itself. They might be associated with 1) an impact crater, which due to the

high temperatures and pressures present during the crash against the earth's surface,

would produce a partial melting of the substrate, which at cooling down would produce

these ridges; another possibility is that the eastern side of the studied structure might

suffered wind erosion, displaying similar features as has been observed in other impact

37

craters in rocky bodies in the solar system, specifically on Mars (Raitala and Kauhanen,

1992; Zimbelman, 2010).

Firestone et al. (2007) suggest the Carolina Bays may be evidence of multiple impacts

associated with the breakup of a rocky or cometary body. Melton and Schriever (1937)

ruled out other “ordinary geological processes” and concluded that the Carolina Bays

were most likely impact related. Although most of the Carolina Bays are smaller than the

Roseau Structure, their morphologies are strikingly similar including multiple concentric

ovoid rings. The largest of the Carolina Bays are similar in size to the Roseau Structure.

Given the study area's geographical and geological context, it is sensible to think that the

structure is the result of glacial or lacustrine processes. However, the delicacy of the

structure suggests that once the ice sheet readvances in the region, the structure would

disappear, making it difficult to reconcile this process with the formation of such structure

due to the type of weak material. Besides, the ice flow was in the NE - SW direction

throughout most of the Wisconsin Glaciation, and the ridges are in the NE direction of

the study structure. At the start of the YD, Lake Agassiz was at the Tintah Level,

covering the structure with a water column of at least 50 meters. Subsequently, the

water level dropped and returned to reach the Campbell level at the end of the YD. On

the other hand, the wave processes due to Lake Agassiz's presence in this area indicate

that the structure is postglacial and, therefore, it is possible to associate it with Lake

Agassiz.

The work of Melosh (1980, 1989), Shuvalov (2002), Shuvalov et al. (2002), and Ormö

and Lindström (2000) suggest that a solid impactor producing a crater the size of the

Roseau Structure would have resulted in cratering and fracturing of the basement rock.

However, gravity surveys suggest there has been no disruption of the crystalline

38

basement (Figure 5). Little information is available in the literature regarding the impact

of icy bodies.

There are no other reported similar structures in the region. This structure is the only one

with this circular shape, size, and ridges on its periphery in northern Minnesota and

surrounding areas. Consequently, glacial or lacustrine processes, typical of this region,

are unlikely, and are ruled out to explain their origin. In terms of time and for the reasons

explained above, the structure must have formed after the retreat of the ice sheet

covering this area and before the drainage of Lake Agassiz.

Perhaps the structure's formation is temporarily related to the start of the YD, but the

quasi-quality is unclear. The reasoning mentioned above proposes that structure formed,

either by the impact of an asteroid or meteorite airburst. As pointed out by Melosh et al.

(1980) and Shuralov et al. (2002), both the D/h ratio and the type of structures resulting

from the plastic deformation of sedimentary material covered by a shallow column of

water lying on strong rocks, are very well related to the structure present in the study

area. The absence of a gravimetric anomaly in the geophysical data suggests that a

meteorite airburst is a viable option, given the lithological conditions in the area, with 50+

meters of water overlying low-density sediment.

On the contrary, if it is considered that the studied structure was formed after 12.9 k BP,

the impact would be subaerial, since by this time Lake Agassiz would have retreated.

This is well marked by continental evidence and in the ocean basins of a route east

through the Great Lakes - St. Lawrence to the North Atlantic and along a northwest route

through the Clearwater - Athabasca - Mackenzie system to the Arctic Ocean (Teller,

2013; Murton et al., 2010).

Whether a potential impact occurred before or after 12.9 k BP, the stratigraphic,

sedimentary, geomorphological, and geochronological evidence indicates that the ice

39

cap present at the rocks had already retreated north of the Structure location by 13,500

cal BP and therefore the location established for the possible impact structure would be

ice-free. Likewise, at 12.9 ka BP the surface would not be free of water either, since it is

recorded that for approximately 11,000 cal BP Lake Agassiz would have risen to reach

the Campbell level (Breckenridge, 2015). Based on Shuvalov (2002), impactors can be

deformed, fragmented and even decelerated during the flight through the Earth's

atmosphere. This effect, in turn, can influence the cratering process. This process could

explain the shape of the study Structure.

According to data provided by George H. Shaw (personal communication, 2020),

gravimetric anomalies are not significant in the study area (Figure 5). Although the

geophysical evidence associated with an impact crater indicates variations in the

gravitational field close to these structures, as a result of the thinning of the crust

(Osisnski and Pierazzo, 2013), it is crucial to indicate that this region has glacial and

lacustrine sediments lying on metamorphic and igneous rocks of the Precambrian. Both

types of lithologies have different rheologies, so the mechanical response to impact

would leave structures whose geophysical signature is different from typical subaerial

craters.

If the impact structure hypothesis is plausible, it is prudent to point out that an asteroid

fragment has impacted northern Minnesota as a result of the main asteroid break up

once go through the atmosphere. As pointed out by Passey and Melosh (1980),

meteoroids (fragments of asteroids that vary in size from dust particles to blocks) once

pass through Earth's atmosphere at speeds of several kilometers per second, are

subject to such stresses and pressures thus producing their fragmentation. Some of

these fragments survive the descent and can reach the surface while others disintegrate

in the atmosphere at heights that vary between 70 and 100 kilometers in height for

40

fragments between 0.1 - 10 cm in diameter and between 4 to 40 kilometers in height for

larger fragments (Passey and Melosh, 1980; Popova, 2004; Gritsevich, 2008; Silber,

2018). Passey and Melosh (1980) indicate that, if the size of the impact crater is greater

than 1 kilometer in diameter, the separation of the meteoroids is only a fraction of its

diameter, so the impact crater formed by the meteoroid would be indistinguishable from

that generated by the main impactor.

Considering that the crater has an approximate diameter of 6 km, based on the

equations of Melosh (1980) and under the lithological and rheological parameters

explained by Shuvalov (2002) and Shuvalov et al. (2002), the necessary dimensions of

the projectile to form an impact crater with the diameter mentioned

D = 8 km, h = 100 m

D/h = 80

Consequently, this value exceeds the relationship established by Melosh (1980) whose

main characteristic is the generation of concentric rings such as they are observed

towards the northeast of the structure.

According to the parameters established by Shuvalov et al. (2002), the water column of

approximately 60 meters deep and a small fragment, has the following relationship:

Water column (H) = 60 m

Diameter of the projectile (d) = 10 m

If d/H = 0.16

d/H > 0.1

With these parameters, it could be said that an asteroid fragment could have impacted in

this region generating the Roseau Structure.

41

6. Conclusions

Based on the geological, geophysical, and geographic observations of the study area

and the characteristics of the structure in northern Minnesota, it is possible that under

the circumstances given approximately 12,900 years ago, a meteorite impact was the

process that formed the Roseau Structure. The geophysical data (Figure 5), no gravity

anomaly, is inconsistent with an impact of a rocky or metallic body, as a solid body

would need to be of substantial size to generate a 6+ km diameter impact structure.

However, the response of an icy body is more difficult to predict; there is a paucity of

studies that analyze the impact of a low-density body. Similarly, there is little information

on how an airburst over water might manifest itself in the lake bottom. Analysis of the

Tunguska Event (Robertson and Mathias, 2019). The Tunguska Event is accepted to be

caused by an airburst of a stony meteoroid at an altitude of about 5-10 km. It flattened

trees over an area of approximately 2000 km2. The Roseau Structure is on the order of

30 km2 in area, but would have been beneath 50+ meters of water. Therefore, it is not

possible to determine with any degree of confidence if this is an impact structure that

related to that reported by Firestone et al. (2007). However, a glacial or lacustrine origin

in difficult to conceive. Further impact-related evidence from nearby environmental

archives would help in this Younger Dryas event reconstruction.

42

Figure 1. Large proglacial lakes along retreating Laurentide Ice Sheet at 12.65–12.75

cal. kyr BP, near the start of Younger Dryas. Three outlets are indicated: (1) to the

northwest along the Mackenzie River to the Arctic Ocean, (2) to the east along the St

Lawrence River to the North Atlantic Ocean, and (3) to the south along the river

Mississippi to the Gulf of Mexico. The distribution of glacial ice (white), proglacial lakes

(dark blue), and land (gray) is for reference. Modified from Murton, J. et al., 2010.

43

Figure 2. Summary graph of strandline point elevation data. Outlet sills (#1-14) are shown as vertical bars and best fit

lines for the lowest Tintah and Campbell levels are included (Breckenridge, 2015).

44

Figure 3. Lake Agassiz lake-level hydrograph. Gray bars represent radiocarbon-dated peat

deposits from Liu et al. 2014.

45

Figure 4. The Roseau Structure in Northern Minnesota. The shape of the structure is mainly elliptical and some ridges are located

towards Northeast of the structure (thin white solid lines).

46

Figure 5. Inverse-distance-weighted contour map of gravity data for portions of Roseau County, MN. Bold black ellipse indicates the

location of the Structure. Light blue dots represent locations of gravity measurements. Data are from the Minnesota Geological

Survey Geophysical Database (https://mngs-umn.opendata.arcgis.com/app/1a79082164d74c1f89f0fb31217fa5b7).

47

Figure 6. Satellite image form some of the "Carolina Bays" in Horry County, SC. The elliptical shape and the orientation of such structures are clearly observed from an aerial view.

48

References:

1. Abbott, D. H., Bryant, E. A., Gusiakov, V., Masse, W. B., & Breger, D. 2008. Impacts, mega-tsunami, and other extraordinary claims. Comment. GSA Today, 18(6), 12. DOI: 10.1130/GSATG9C.1

2. Alvarez, L. W., Alvarez, W., Asaro, F., and Michel, H. V. (1980). Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science, 208(4448), 1095-1108. DOI: 10.1126/science.208.4448.1095

3. Arnold, J. R., & Libby, W. F. (1951). Radiocarbon dates. Science, 113(2927),

111-120.

4. Bakke, J., Lie, Ø., Heegaard, E., Dokken, T., Haug, G.H., Birks, H.H., Dulski, P., and Nilsen, T. 2009. Rapid oceanic and atmospheric changes during the Younger Dryas cold period. Nature Geoscience. v. 2. p. 202-205; doi:10.1038/ngeo439.

5. Bass, D.W. and Woodworth-Lynas, C., 1988. Iceberg crater marks on the sea floor, Labrador Shelf. Marine Geology, 79(3-4), pp.243-260. https://doi.org/10.1016/0025-3227(88)90041-2

6. Benn, D. I., & Evans, D. J. A. (2010). Glaciers and Glaciation. Hodder Education. London, UK, 802 pp.

7. Boslough, M., and other 15 authors. 2012. Arguments and Evidence Against a Younger Dryas Impact Event. Climates, Landscapes, and Civilizations. AGU. doi:10.1029/2012GM001209.

8. Breckenridge, A. 2007. The Lake Superior varve stratigraphy and implications for eastern Lake Agassiz outflow from 10,700 to 8900 cal ybp (9.5–8.0 14C ka). Palaeogeography, Palaeoclimatology, Palaeoecology. v. 246. p. 45-61; doi.org/10.1016/j.palaeo.2006.10.026.

9. Breckenridge, A. 2015. The Tintah-Campbell gap and implications for glacial Lake Agassiz drainage during the Younger Dryas cold interval. Quaternary Science Reviews. v. 117. p. 124-134, doi.org/10.1016/j.quascirev.2015.04.009.

10. Broecker, W. S., Kennett, J. P., Flower, B. P., Teller, J. T., Trumbore, S., Bonani, G., & Wolfli, W. (1989). Routing of meltwater from the Laurentide Ice Sheet during the Younger Dryas cold episode. Nature, 341(6240), 318.

11. Broecker, W.S., Denton, G.H, Edwards, R.L., Cheng, H., Alley, R.B., and Putnam, A.E. 2010. Putting the Younger Dryas cold event into context. Quaternary Science Reviews. v. 29, Issues 9–10. p. 1078-1081; doi.org/10.1016/j.quascirev.2010.02.019.

12. Brown, C.S., Newton, A.M., Huuse, M. and Buckley, F., 2017. Iceberg scours, pits, and pockmarks in the North Falkland Basin. Marine Geology, 386, pp.140-152. https://doi.org/10.1016/j.margeo.2017.03.001.

13. Buchanan, B., Collard, M. and Edinborough, K. 2008. Paleoindian demography and the extraterrestrial impact hypothesis. PNAS. v.105 (33). p. 11651-11654; doi:10.1073/pnas.0803762105.

49

14. Carlson, A. E., Mickelson, D. M., Principato, S. M., & Chapel, D. M. (2005). The genesis of the northern Kettle Moraine, Wisconsin. Geomorphology, 67(3-4), 365-374.

15. Carlson, A. E., Clark, P.U., and Hostetler, S.W. 2009. Comment: Radiocarbon deglaciation chronology of the Thunder Bay, Ontario area and implications for ice sheet retreat patterns. Quaternary Science Reviews. v. 28. p. 2546-2547.

16. Carlson, A.E. 2010. What caused the Younger Dryas Cold Event? Geology. v. 38, p. 383-384; doi:10.1130/focus042010.1.

17. Carr, C., & Sears, D. W. (1985). Toward an analysis of the exchange of meteoritic iron in the middle woodland. Southeastern Archaeology, 79-92.

18. Clayton, L. (1967). Stagnant-glacier features of the Missouri Coteau in North Dakota. North Dakota Geological Survey Miscellaneous Series, 30, 25-46.

19. Clayton, L., TELLER, J. T., & ATTIG, J. W. (1985). Surging of the southwestern part of the Laurentide Ice Sheet. Boreas, 14(3), 235-241.

20. Collins, G. S., Lynch, E., McAdam, R., & Davison, T. M. (2017). A numerical assessment of simple airblast models of impact airbursts. Meteoritics & Planetary Science, 52(8), 1542-1560.

21. Collins, G. S., Artemieva, N., Wünnemann, K., Bland, P. A., Reimold, W. U. and Koeberl, C. 2008. Evidence that Lake Cheko is not an impact crater. Terra Nova, 20: 165–168. doi:10.1111/j.1365-3121.2008.00791.x

22. Coveney Jr, R. M., Murowchick, J. B., Grauch, R. I., Glascock, M. D., & Denison, J. R. 1992. Gold and platinum in shales with evidence against extraterrestrial sources of metals. Chemical Geology, 99(1-3), 101-114. https://doi.org/10.1016/0009-2541(92)90033-2.

23. Daulton, T.L., Pinter, N., and Scott, A. 2010. No evidence of nanodiamonds in Younger–Dryas sediments to support an impact event. PNAS. v. 107 (37). p. 16043-16047; doi:10.1073/pnas.1003904107.

24. Dyer, B. D., Lyalikova, N. N., Murray, D., Doyle, M., Kolesov, G. M., & Krumbein, W. E. (1989). Role for microorganisms in the formation of iridium anomalies. Geology, 17(11), 1036-1039. doi.org/10.1130/0091-7613(1989)017<1036:RFMITF>2.3.CO;2.

25. Faith, J. T. (2011). Late Pleistocene climate change, nutrient cycling, and the megafaunal extinctions in North America. Quaternary Science Reviews, 30(13-14), 1675-1680.

26. Faith, J. T., & Surovell, T. A. (2009). Synchronous extinction of North America's Pleistocene mammals. Proceedings of the National Academy of Sciences, 106(49), 20641-20645.

27. Fay, H., 2002. Formation of kettle holes following a glacial outburst flood (jökulhlaup), Skeiðarársandur. The extremes of the extremes: extraordinary floods. (271), p.205.

28. Fenton, M. M., Moran, S. R., Teller, J. T., & Clayton, L. (1983). Quaternary stratigraphy and history in the southern part of the Lake Agassiz basin. In Glacial Lake Agassiz (pp. 49-74). Geological Association of Canada Special Publication 26.

50

29. Firestone, R.B. and other 25 authors. 2007a. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. PNAS. v. 104 no. 41 p.16016–16021; doi: 10.1073/pnas.0706977104.

30. Firestone, R.B., West, A., Zsolt, S., Zsolt, R., and Hagstrum, J.T., 2007b, Micrometeorite impacts in Beringian mammoth tusks and a bison skull: Eos (Transactions, American Geophysical Union) v. 88, no. 52, Abstract U23A-0865, http://ie.lbl.gov/Mammoth/Impact.html.

31. Firestone, R. B. 2009. The case for the Younger Dryas extraterrestrial impact event: mammoth, megafauna and Clovis extinction. Journal of Cosmology. 2, 256-285. https://escholarship.org/uc/item/8fj3d8mc

32. Firestone, R.B., West, A., Revay, Z., Hagstrum, J.T., Belgya, T., Que Hee, S.S., Smith, A.R., 2010. Analysis of the Younger Dryas impact layer. Journal of Siberian Federal University, Engineering & Technologies. 3, 30–62. https://escholarship.org/uc/item/03q2r98x

33. Fisher, T.G. 2003. Chronology of glacial Lake Agassiz meltwater routed to the Gulf of Mexico. Quaternary Research. v. 59. p. 271-276; doi.org/10.1016/S0033-5894(03)00011-5.

34. Fisher, T.G. 2005. Strandline analysis in the southern basin of glacial Lake Agassiz, Minnesota and North and South Dakota, USA. GSA Bulletin. v. 117 no. 11-12 p. 1481-1496; doi: 10.1130/B25752.1.

35. Fisher, TG., and Lowell, T.V. 2006. Questioning the age of the Moorhead Phase in the glacial Lake Agassiz basin. Quaternary Science Reviews. v. 25. p. 2688-2691; doi.org/10.1016/j.quascirev.2006.05.007.

36. Fisher, T. G., Yansa, C. H., Lowell, T. V., Lepper, K., Hajdas, I., and Ashworth, A. 2008. The chronology, climate, and confusion of the Moorhead Phase of glacial Lake Agassiz: new results from the Ojata Beach, North Dakota, USA. Quaternary Science Reviews. v. 27(11-12). p. 1124-1135; doi: 10.1016/j.quascirev.2008.02.010.

37. Fisher, T.G., Waterson, N., Lowell, T.V., and Hajdas, I. 2009. Deglaciation ages and meltwater routing in the Fort McMurray region, northeastern Alberta and northwestern Saskatchewan, Canada. Quaternary Science Reviews. v. 28. p. 1608-1624; doi.org/10.1016/j.quascirev.2009.02.003.

38. Fisher, T.G. and Lowell, T.V., 2012. Testing northwest drainage from Lake Agassiz using extant ice margin and strandline data. Quaternary International, 260, pp.106-114. https://doi.org/10.1016/j.quaint.2011.09.018.

39. Flohn, H. (1979). On time scales and causes of abrupt paleoclimatic events. Quaternary Research, 12(1), 135-149.

40. French, B.M., and Koeberl, C. 2010. The convincing identification of terrestrial meteorite impact structures: What works, what doesn't, and why. Earth-Science Reviews. v. 98. p. 123-170; doi.org/10.1016/j.earscirev.2009.10.009.

41. Gabrielli, P., and other 12 authors. 2004. Meteoric smoke fallout over the Holocene epoch revealed by iridium and platinum in Greenland ice. Nature, 432(7020), 1011-1014. https://doi.org/10.1038/nature03137.

51

42. Gilmour, I., Russell, S.S., Arden, J.W., Lee, M.R., Franchi, I.A. and Pillinger, C.T. 1992. Terrestrial carbon and nitrogen isotopic ratios from Cretaceous-Tertiary boundary nanodiamonds. Science, 258(5088), pp.1624-1626.

43. Grimm, E.C., Donovan, J.J. and Brown, K.J., 2011. A high-resolution record of climate variability and landscape response from Kettle Lake, northern Great Plains, North America. Quaternary Science Reviews, 30(19-20), pp.2626-2650. https://doi.org/10.1016/j.quascirev.2011.05.015

44. Gritsevich, M.I., 2008. The Pribram, Lost City, Innisfree, and Neuschwanstein falls: an analysis of the atmospheric trajectories. Solar System Research, 42(5), pp.372-390. https://doi.org/10.1134/S003809460805002X

45. Harris, D. R. (2003). Climatic change and the beginnings of agriculture: the case of the Younger Dryas. In Evolution on Planet Earth (pp. 379-394). Academic Press.

46. Harris, K.L. (1995). Quaternary geology-Southern Red River Valley, Minnesota [Part A]. Minnesota Geological Survey. Retrieved from the University of Minnesota Digital Conservancy, http://hdl.handle.net/11299/59764.

47. Haynes, C.V., Jr. 2008. Younger Dryas “black mats” and the Rancholabrean termination in North America. PNAS. v. 105 no. 18. p. 6520–6525; doi: 10.1073/pnas.0800560105.

48. Hills, J.G. and Goda, P. 1993. The fragmentation of small asteroids in the atmosphere. Astronomical Journal. v. 105, no. 3, p. 1114-1144.

49. Hobbs, H.C., 1983. Drainage relationship of Glacial Lake Aitkin and Upham and early Lake Agassiz in northeastern Minnesota. In: Teller, J.T., Clayton, L. (Eds.), Glacial Lake Agassiz, pp. 245e259. Geological Association of Canada Special Paper 26.

50. Holcombe, T. L., Warren, J. S., Reid, D. F., Virden, W. T., & Divins, D. L. 2001. Small rimmed depression in Lake Ontario: An impact crater? Journal of Great Lakes Research, 27(4), 510-517. https://doi.org/10.1016/S0380-1330(01)70664-8

51. Hughes, J.D., and Merry, W.J. 1978. Marquette Buried Forest9,850 years old. American Association for the Advancement ofScience, Abstract for Annual Meeting, 12–14 February 1978.

52. Israde-Alcántara, I., and other 15 authors. 2012. Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis. PNAS. v. 109 (13) E738-E747; doi:10.1073/pnas.1110614109.

53. James T. Teller, J.T. 2013. Lake Agassiz during the Younger Dryas. Quaternary Research. v. 80. p. 361-369; doi.org/10.1016/j.yqres.2013.06.011.

54. Kamb, B. (1987). Glacier surge mechanism based on linked cavity configuration of the basal water conduit system. Journal of Geophysical Research: Solid Earth, 92(B9), 9083-9100.

55. Kennet, D.J., and other 16 authors. 2009a. Shock-synthesized hexagonal diamonds in Younger Dryas boundary sediments. PNAS. v. 106 (31). p. 12623-12628; doi:10.1073/pnas.0906374106

52

56. Kennet, D.J., Kennet, J.P., West, A., Mercer, C., Que Hee, S.S., Bennet, L., Bunch, T.E., Sellers, M., and Wolbach, W.S. 2009b. Nanodiamonds in the Younger Dryas boundary sediment layer. Science. v. 323. Issue 5910. p. 94; doi: 10.1126/science.1162819.

57. Koeberl, C., 2007. The geochemistry and cosmochemistry of impacts, In: Davis, A. (Ed.), online edition. Treatise of Geochemistry, vol. 1. Elsevier, New York, NY, pp. 1.28.1–1.28.52. doi:10.1016/B978-008043751-4/00228-5.

58. Kuzilla M.S. (1988) Genesis and morphology of soils in and around large depressions in Clay County, Nebraska, Thesis, University of Nebraska, Lincoln.

59. Larson, P.C., Mooers, H.D., and Meyer, M.S. (2016) Glacial Geology of the Laurentian Uplands. In: Institute of Lake Superior Geology, Field Trip Guidebook, Vol. 62, Part 2, 1-13.

60. LeCompte, M.A., Goodyear, A.C., Demitroff, M.N., Batchelor, D., Vogel, E.K., Mooney, C., Rock, B.N., and Seidel, A.W. 2012. Independent evaluation of conflicting microspherule results from different investigations of the Younger Dryas impact hypothesis. PNAS. v. 109 (44), E2960-E2969. doi:10.1073/pnas.1208603109.

61. Lepper, K., Fisher, T.G., Hajdas, I., and Lowell, T.V. 2007. Ages for the Big Stone Moraine and the oldest beaches of glacial Lake Agassiz: Implications for deglaciation chronology. Geology. v. 35 no. 7 p. 667-670; doi: 10.1130/G23665A.1.

62. Lepper, K., Buell, A.W., Fisher, T.G., and Lowell, T.V. 2013. A chronology for glacial Lake Agassiz shorelines along Upham's namesake transect. Quaternary Research. v. 80. p. 88-98; doi.org/10.1016/j.yqres.2013.02.002.

63. Liu, X., Fisher, T. G., Lepper, K., & Lowell, T. V. (2014). Geochemical characteristics of glacial Lake Agassiz sediments and new ages for the Moorhead Phase at Fargo, North Dakota, USA. Canadian Journal of Earth Sciences, 51(9), 850-861.

64. Lowell, T.V., Fisher, T.G., Hajdas, I., Glover, K., Loope, H., and Henry, T. 2009. Radiocarbon deglaciation chronology of the Thunder Bay, Ontario area and implications for ice sheet retreat patterns. Quaternary Science Reviews. v. 28. p. 1597-1607; doi.org/10.1016/j.quascirev.2009.02.025.

65. Lowell, T.V., and Fisher, T.G. 2009. Reply to comments by Carlson et al., (2009) on Radiocarbon glaciation chronology of the Thunder Bay, Ontario area and implications for ice sheet retreat patterns. Quaternary Science Reviews. v. 28. p. 254-2550.

66. Lowell, T.V., Applegate, P.J., Fisher, T.G., and Lepper, K. 2013. What caused the low-water phase of glacial Lake Agassiz? Quaternary Research. v. 80. p. 370-382; doi.org/10.1016/j.yqres.2013.06.002.

67. Lusardi, B. A., Jennings, C. E., & Harris, K. L. (2011). Provenance of Des Moines lobe till records ice‐stream catchment evolution during Laurentide deglaciation. Boreas, 40(4), 585-597.

68. Mangerud, J. (1987). The Allerød/Younger Dryas boundary. In Abrupt Climatic Change (pp. 163-171). Springer, Dordrecht. https://doi.org/10.1007/978-94-009-3993-6_16.

53

69. Melosh, H. J. (1980). Cratering mechanics—Observational, experimental, and theoretical. Annual Review of Earth and Planetary Sciences, 8(1), 65-93. doi:10.1146/annurev.ea.08.050180.000433

70. Melton, F.A. and Schriever, W., 1933. The Carolina" Bays": Are They Meteorite Scars?. The Journal of Geology, 41(1), pp.52-66.

71. McCausland, P. J., Samson, C., and McLeod, T. (2011). Determination of bulk density for small meteorite fragments via visible light 3‐D laser imaging. Meteoritics & Planetary Science, 46(8), 1097-1109. https://doi.org/10.1111/j.1945-5100.2011.01217.x

72. McCoy, T. J., Marquardt, A. E., Wasson, J. T., Ash, R. D., & Vicenzi, E. P. (2017). The Anoka, Minnesota iron meteorite as parent to Hopewell meteoritic metal beads from Havana, Illinois. Journal of Archaeological Science, 81, 13-22. https://doi.org/10.1016/j.jas.2017.03.003

73. McDonald, I., Irvine, G J., de Vos, E., Gale, A.S. and Reimold, W.U., 2006. Geochemical search for impact signatures in possible impact-generated units associated with the Jurassic-Cretaceous Boundary in southern England and northern France. Biological Processes Associated with Impact Events. C. Cockell, I. Gilmour and C. Koeberl, Springer Berlin Heidelberg, 257-286

74. Melosh, H.J. 1989. Impact Cratering. New York: Oxford University Press, 245 pp. 75. Moore, A.M.T., Kennett, J.P., Napier, W.M. et al. Evidence of Cosmic Impact at

Abu Hureyra, Syria at the Younger Dryas Onset (~12.8 ka): High-temperature melting at >2200 °C. Sci Rep 10, 4185 (2020). https://doi.org/10.1038/s41598-020-60867-w

76. Munro, N. D. (2003). Small game, the Younger Dryas, and the transition to agriculture in the southern Levant. Mitteilungen der Gesellschaft für Urgeschichte, 12(4), 47-71.

77. Murton, J.B., Bateman, M.D., Dallimore, S.R., Teller, J.T., and Yang, Z. 2010. Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean.

78. Newton, A.M., Huuse, M. and Brocklehurst, S.H., 2016. Buried iceberg scours reveal reduced North Atlantic Current during the stage 12 deglacial. Nature communications, 7(1), pp.1-7. doi: 10.1038/ncomms10927.

79. Oberbeck, V. R. (1971). Laboratory simulation of impact cratering with high explosives. Journal of Geophysical Research, 76(23), 5732-5749. https://doi.org/10.1029/JB076i023p05732.

80. Osinski, G. R., & Pierazzo, E. (2013). Impact cratering: Processes and products. Impact Cratering, Wiley-Blacwell.

81. Paquay, F., Goderis, S., Ravizza, G., Vanhaeck, F., Boyd, M., Surovell, T.A., Holiday, V.T., Haynes, C.V. Jr., and Claeys, P. 2009. Absence of geochemical evidence for an impact event at the Bølling–Allerød/Younger Dryas transition. PNAS. v. 106 (51). p. 21505-21510; doi:10.1073/pnas.0908874106.

82. Passey, Q., and Melosh, H.J. 1980. Effects of atmospheric breakup on crater field formation. Icarus. v. 42. p. 211-233.

83. Popova, O., 2004. Meteoroid ablation models. Earth, Moon, and Planets, 95(1-4), pp.303-319. https://doi.org/10.1007/s11038-005-9026-x

54

84. Pinter, N., and Ishman, S.E. 2008. Impacts, mega-tsunamis, and other extraordinary claims. GSA Today. Reply. p. e14. doi: 10.1130/GSATG13Y.1

85. Pinter, N., Scott, A.C., Daulton, T.L., Podoll, A., Koeberl, C., Anderson, S.R., and Ishman, S.E. 2011. The Younger Dryas impact hypothesis: A requiem. Earth-Science Reviews. v.106, Issues 3–4. p. 247-264; doi.org/10.1016/j.earscirev.2011.02.005.

86. Prouty, W. F. 1952. Carolina bays and their origin. Geological Society of America Bulletin, 63(2), 167-224.

87. Raitala, J. and Kauhanen, K., 1992. Ridge systems related to Martian impact craters. Earth, Moon, and Planets, 58(1), pp.65-78. https://doi.org/10.1007/BF00058074.

88. Rampino, M. R., & Caldeira, K. 2017. Correlation of the largest craters, stratigraphic impact signatures, and extinction events over the past 250 Myr. Geoscience Frontiers, 8(6), 1241-1245. https://doi.org/10.1016/j.gsf.2017.03.002

89. Reimold, W. U. 2007. The Impact Crater Bandwagon (Some problems with the terrestrial impact cratering record). Meteoritics & Planetary Science, 42: 1467–1472. doi:10.1111/j.1945-5100.2007.tb00585.x

90. Reimold, W. U., Ferrière, L., Deutsch, A. and Koeberl, C. 2014. Impact controversies: Impact recognition criteria and related issues. Meteorit Planet Sci, 49: 723–731. doi:10.1111/maps.12284

91. Rooth, C., 1982. Hydrology and Ocean Circulation. Progress in Oceanography. 11: 131-145. doi.org/10.1016/0079-6611(82)90006-4.

92. Röthlisberger, H. 1972. Water pressure in intra- and subglacial channels. Journal of Glaciology. 11, 177–203. DOI: https://doi.org/10.3189/S0022143000022188.

93. Sawlowicz, Z. 1993. Iridium and other platinum-group elements as geochemical markers in sedimentary environments. Palaeogeography, Palaeoclimatology, Palaeoecology, 104(1-4), 253-270. https://doi.org/10.1016/0031-0182(93)90136-7

94. Schmidt, R.M. and Holsapple, K.A., 1982. Estimates of crater size for large-body impact: Gravity-scaling results. In Geological Implications of Impacts of Large Asteroids and Comets on the Earth (Vol. 190, pp. 93-102). The Geological Society of America Boulder.

95. Schmieder, M., and Buchner, E. 2008. Dating impact craters: Palaeogeographic versus isotopic and stratigraphic methods – a brief case study. Geological Magazine. v.145(4). p. 586-590. doi:10.1017/S0016756808005049.

96. Shoemaker, E. M. 1963. Impact mechanics at Meteor Crater, Arizona. In The Moon, Meteorites, and Comets, ed. B. M. Middlehurst, G. P. Kuiper, 4: 301-36. Chicago & London. Univ. Chicago Press. 810 pp

97. Schulte, P., Alegret, L., Arenillas, I., Arz, J.A., Barton, P.J., Bown, P.R., Bralower, T.J., Christeson, G.L., Claeys, P., Cockell, C.S. and Collins, G.S. 2010. The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary. Science, 327(5970), pp.1214-1218.

98. Scott, A.C., Pinter, N., Collinson, M.E., Hardiman, M., Anderson, R.S., Brain, A.P.R., Smith, S., Marone, F., and Stampanoni, M. 2010. Fungus, not comet or

55

catastrophe, accounts for carbonaceous spherules in the Younger Dryas “impact layer.” Geoph. Res. Lett., v. 37, L14302, doi:10.1029/2010GL043345.

99. Shakun, J.D., and Carlson, A.E. 2010. A global perspective on Last Glacial Maximum to Holocene climate change. Quaternary Science Reviews. v. 29. Issues 15–16. p. 1801-1816; doi.org/10.1016/j.quascirev.2010.03.016.

100. Shuvalov, V., Dypvik, H., & Tsikalas, F. (2002). Numerical simulations of the Mjølnir marine impact crater. Journal of Geophysical Research: Planets, 107(E7), 1-1. doi:10.1029/2001je001698

101. Silber, E.A., Boslough, M., Hocking, W.K., Gritsevich, M. and Whitaker, R.W., 2018. Physics of meteor generated shock waves in the earth’s atmosphere–a review. Advances in Space Research, 62(3), pp.489-532. https://doi.org/10.1016/j.asr.2018.05.010

102. Sonett, C.P., Pearce, S.J. and Gault, D.E., 1991. The oceanic impact of large objects. Advances in Space Research, 11(6), pp.77-86. https://doi.org/10.1016/0273-1177(91)90234-B

103. Steffensen, J. P., Andersen, K. K., Bigler, M., Clausen, H. B., Dahl-Jensen, D., Fischer, H., ... & Masson-Delmotte, V. 2008. High-resolution Greenland ice core data show abrupt climate change happens in few years. Science. 321(5889), 680-684. DOI: 10.1126/science.1157707

104. Surovell, T.A, Holliday, V.T., Gingerich, J.A.M., Ketron, C., Haynes, C.V., Jr., Hilman, I., Wagner, D., Johnson, E., and Claeys, P. 2009. An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis. PNAS. v. 106 (43). p. 18155-18158. doi:10.1073/pnas.0907857106.

105. Surovell, T.A., Boyd, J.R., Haynes Jr, C.V. and Hodgins, G.W., 2016. On the dating of the Folsom Complex and its correlation with the Younger Dryas, the end of Clovis, and megafaunal extinction. PaleoAmerica, 2(2), pp.81-89. doi: 10.1080/20555563.2016.1174559

106. Svetsov, V.V., Nemtchinov, I.V., and Teterev, A.V. 1995. Disintegration of Large Meteoroids in Earth's Atmosphere: Theoretical Models. Icarus. v. 116. p. 131-153; doi.org/10.1006/icar.1995.1116.

107. Sweatman, M. B., & Tsikritsis, D. (2017). Decoding Göbekli Tepe with archaeoastronomy: What does the fox say? Mediterranean Archaeology & Archaeometry, 17(1).

108. Tarasov, L., and Peltier, W.R. 2006. A calibrated deglacial drainage chronology for the North American continent: evidence of an Arctic trigger for the Younger Dryas. Quaternary Science Reviews. v. 25. p. 659-688, doi.org/10.1016/j.quascirev.2005.12.006.

109. Teller, J. T., & Clayton, L. (1983). Glacial Lake Agassiz: Geological Association of Canada Special Publication No. 26.

110. Teller, J.T., Leverington, D.W., and Mann, J.D. 2002. Freshwater outbursts to the oceans from glacial Lake Agassiz and their role in climate change during the last deglaciation. Quaternary Science Reviews. v. 21. p. 879-887; doi.org/10.1016/S0277-3791(01)00145-7.

111. Teller, J.T., 2013. Lake Agassiz during the Younger Dryas. Quaternary Research, 80(3), pp.361-369. https://doi.org/10.1016/j.yqres.2013.06.011.

56

112. Tian, H., Scryves, D., and Claeys, Ph. 2011. Nanodiamonds do not provide unique evidence for a Younger Dryas impact. PNAS. v. 108 (1). p. 40-44; doi:10.1073/pnas.1007695108.

113. Waters, M. R., & Stafford, T. W. (2007). Redefining the age of Clovis: implications for the peopling of the Americas. Science, 315(5815), 1122-1126. DOI: 10.1126/science.1137166

114. Wolbach, W. S., and oher 26 authors. 2018a. Extraordinary biomass-burning episode and impact winter triggered by the Younger Dryas cosmic impact∼ 12,800 years ago. 1. Ice cores and glaciers. The Journal of Geology, 126(2), 165-184.

115. Wolbach, W. S., and other 30 authors. 2018b. Extraordinary biomass-burning episode and impact winter triggered by the Younger Dryas cosmic impact∼ 12,800 years ago. 2. Lake, marine, and terrestrial sediments. The Journal of Geology, 126(2), 185-205. https://doi.org/10.1086/695704

116. Wright, H. E., Thorpe, J. L., Mackay, A., Battarbee, R., Birks, J., & Oldfield, F. (2003). Climatic change and the origin of agriculture in the Near East. Global change in the Holocene, 49-62.

117. Wu, Y., Sharma, M., Lecompte, M.A., Demitroff, M.N., and Landis, J. 2013. Origin and provenance of spherules and magnetic grins at the Younger Dryas boundary. PNAS. v. 110 (38). E3557-66. doi: 10.1073/pnas.1304059110.

118. Zimbelman, J.R., 2010. Transverse aeolian ridges on Mars: First results from HiRISE images. Geomorphology, 121(1-2), pp.22-29. https://doi.org/10.1016/j.geomorph.2009.05.012.