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Sediment magnetic signature of climate in modern loessicsoils from the Great Plains

Christoph E. Geissa,�, C. William Zannerb

aDepartment of Physics, Trinity College, McCook Hall 105, 300 Summit St, Hartford, CT 06106, USAbDepartment of Soil, Water and Climate, 70 Borlaug Hall, 1991 Upper Buford Circle, University of Minnesota, St. Paul MN 55108, USA

Abstract

Our magnetic analysis of 40 modern loessic soil profiles from the midwestern United States shows that the upper soil horizons of all

sites are characterized by an increase in the concentration of fine grained (do0.1 mm) ferrimagnetic minerals (magnetite and/or

maghemite). Our sites were selected from stable upland positions along a precipitation gradient extending from SWNebraska (mean ann.

precip. o500mm/yr) to central Missouri (mean ann. precip. 41000mm/yr). Changes in magnetic remanence parameters (IRM, ARM)

and magnetic susceptibility (w) between soil and parent material vary systematically along the transect and are likely to reflect changes in

source area and transport distance. Magnetic enhancement is calculated for several magnetic parameters by taking the ratio Menhanced/

Mparent material, where M stands for either ARM, IRM or w. Our analyses show that magnetic enhancement of modern soils based on

IRM and w correlates with changes in mean annual precipitation. The best correlation, however, is observed when ARM is used as a

proxy of magnetic enhancement, which might reflect the bias of ARM towards small, single-domain grains of likely pedogenic origin.

Our study shows that magnetic soil properties, in combination with non-magnetic proxies, such as solum depth, B-horizon color, depth

to carbonates or other weathering indices have the potential to yield reliable reconstructions of paleoclimate in parts of the Great Plains

where well preserved loessic paleosols exist.

r 2006 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction

Scattered evidence (e.g., Laird et al, 1996; Muhs et al,1997; Forman et al., 2001; Clarke and Rendell, 2003) fromdune fields and lake sediments suggests that during theHolocene the Great Plains region of North America hasexperienced climate variability and droughts more severethan seen in the 400 yr since humans-Native Americans andEuropean pioneers-established permanent settlements.However, farming and ranching at the threshold limits ofneeded rainfall and dependency on regional rivers as wellas the Ogallala Aquifer for irrigation and drinking watermeans that human activities in this region would beseriously threatened by any significant change in climate.Given the very real potential for drier conditions inmidcontinental areas associated with documented globalwarming (Dai et al., 2004), we would be significantly better

e front matter r 2006 Elsevier Ltd and INQUA. All rights re

aint.2006.10.035

ing author.

esses: [email protected] (C.E. Geiss),

edu (C.W. Zanner).

s article as: Geiss, C.E., Zanner, C.W., Sediment magnetic signa

(2006), doi:10.1016/j.quaint.2006.10.035

prepared for such an eventuality if we were to betterunderstand the long-term climate history of the GreatPlains. Establishing a baseline of past climate variabilitycould then be used to assess the potential impact of futureclimate change.Unlike ocean basins, from which researchers have

collected large numbers of sediment cores, or recentlyglaciated regions for which an abundance of lacustrinerecords is available, the Great Plains lack suitable high-resolution archives that reflect past climatic conditions.Knowledge of long-term paleoclimate change is thereforelimited to a few loess-paleosol sequences, a small number oflong lake records, and geographically scattered microfossilfinds and calcite deposits in caves. These sites allow fortemporal reconstruction of only local climate variability,but, in general, spatial reconstructions of paleoclimatechange across midcontinents (with the exception of sites onthe Chinese loess plateau, e.g., Maher and Thompson,1995; Porter, 2000; Yang and Ding, 2003) are lacking.The last decade has seen the application of rock-

magnetic techniques to quantitative climate reconstruction

served.

ture of climate in modern loessic soils from the Great Plains. Quaternary

dx.doi.org/10.1016/j.quaint.2006.10.035

mailto:[email protected]

mailto:[email protected]


ARTICLE IN PRESS

Fig. 1. Map of the study area showing the location of our study sites as

well as the approximate limits of loess deposition (after Bettis et al., 2003).

Solid symbols denote sites used for magnetic analyses, open symbols show

sites that were rejected for reasons outlined in the text. Sites mentioned in

the text: BAR—Barn Bluff, MN; DAV—Davisdale Conservation Area,

MO; MIR—Miriam Cemetery, NE; MTC—Mount Calvary Cemetery;

PRA—Prairie Pines, NE.

C.E. Geiss, C.W. Zanner / Quaternary International ] (]]]]) ]]]–]]]2

efforts worldwide (e.g., Beget et al., 1990; Verosub et al.,1993; Banerjee, 1994; Maher et al., 2002; Tang et al., 2003).Many magnetic studies, however, rely on magneticsusceptibility as the main proxy used for climate recon-struction. Such an approach, though promising in areas ofrelatively homogenous loess deposits or where the magneticcomponent is well characterized by other studies, issimplified and may not lead to reliable interpretations ofpaleoclimatic conditions in other settings. Changes inmagnetic susceptibility can reflect the presence of a widerange of magnetic minerals as well as changes in theirgrain-size (Dunlop and Ozdemir, 1997). As a result, theirinterpretation in terms of paleoenvironmental change orpaleoclimate is seldom straightforward and requires addi-tional information from magnetic or non-magnetic studies.

Many modern and buried soils contain higher concen-trations of ferrimagnetic minerals in their upper soilhorizons (generally the A- and upper B-horizons) than inthe underlying parent material. This magnetic enhance-ment of soils was observed early on (e.g., Le Borgne, 1955;Mullins, 1977) and has been used to delineate paleosolhorizons (Kukla et al., 1988) and, more recently, toreconstruct paleoclimatic conditions of soil formation(e.g., Verosub et al., 1993). In most instances, the magneticproperties of the enhanced soil horizons are characterizedby an increase in fine-grained superparamagnetic (SP) andsingle-domain (SD) magnetite and maghemite (do0.1 mm)(e.g., Maher, 1986; Heller and Evans, 1995; Hunt et al.,1995). Our recent study of a modern soil profile fromNebraska, however, has shown that the magneticallyenhanced soil horizons contain increased quantities ofboth ferrimagnetic (magnetite, maghemite) and antiferro-magnetic (hematite or goethite) minerals (Geiss et al.,2004). The processes causing magnetic enhancement arenot entirely clear, but are likely caused by the conversion ofweakly magnetic antiferromagnetic or paramagnetic miner-als to strongly magnetic ferrimagnetic particles. Severalstudies (e.g., Heller et al., 1993; Maher, 1998) have shownthat the degree of magnetic enhancement correlates wellwith modern precipitation values and can be used toreconstruct past climates by analyzing the magneticproperties of paleosols.

However, the link between magnetic enhancement andclimate is not universal. The technique appears to workwell in areas like the Chinese loess plateau (Maher andThompson, 1995) or the steppe regions of Russia (Maher etal., 2002), whereas loess-paleosol sequences in Alaska(Beget, 1990; Beget et al., 1990), Illinois (Grimley et al.,2003) and Europe (Oches and Banerjee, 1996) show a morecomplicated picture. Several authors have investigated theeffects of climate on loessic soils in the midwest, usingphytolith analyses (Fredlund and Tieszen, 1997) and soilgeochemistry (Muhs et al., 2001). However, only multi-proxy studies combining several of these approaches willallow us to obtain robust paleoclimate reconstructionsfrom paleosols. These studies might also shed light on theunderlying processes of magnetic enhancement and their

Please cite this article as: Geiss, C.E., Zanner, C.W., Sediment magnetic signa

International (2006), doi:10.1016/j.quaint.2006.10.035

empirical relation to climatic conditions during pedogen-esis. This study investigates changes in magnetic propertiesas they occur along a rainfall gradient from westernNebraska to central Missouri. The magnetic properties ofsoils are controlled by the abundance, mineralogy andgrain-size distribution of iron-bearing minerals, whichdepend, among others, on factors like redox conditions,biogenic activity and initial abundance of iron minerals inthe parent material.Our sampling sites follow a strong east-west precipita-

tion gradient (400–1000mm/yr) from Nebraska eastwardto the Missouri river, then following the river souththrough western Iowa and into Missouri (Fig. 1). It ispossible to develop a climosequence across this region,because we can control for the other factors that influencesoil formation and the development of rock-magneticsignatures. Upland vegetation consisted of short to tall-grass prairie, which can be considered a biological responseto climatic conditions, thus complementing abiotic climate-dependent processes. Topographic and aspect effects canbe controlled by selecting sites on well-drained, stablebroad upland summits. All sites used in this studydeveloped in Peoria loess, but some may have experiencedminor additions of loess during the Holocene. Theduration of soil formation (o10,000 yr) since the end ofmajor Wisconsinan loess deposition has been similar acrossthe region.The presence of a strong precipitation gradient coupled

with our ability to control the other factors influencing soilgenesis offers a promising opportunity to quantify theeffect of regional climate gradients on rock-magneticparameters. In this paper, we show that magneticparameters beyond simple magnetic susceptibility measure-ments are promising for developing a model for climatevariability from the Great Plains into midcontinental



ARTICLE IN PRESSC.E. Geiss, C.W. Zanner / Quaternary International ] (]]]]) ]]]–]]] 3

North America. This study is a first step in developing amulti-proxy transfer function that can be applied topaleosols with the intent of recovering past climaticinformation. This will lead to a more robust understandingof past Great Plains climates and the potential forproblems associated with future change.

2. Methods

2.1. Site selection and sampling

Loess is widespread in the central United States inFenneman’s eastern Great Plains and Central Lowlandsphysiographic provinces (Fenneman, 1931; Bettis et al.,2003) (Fig. 1). Concentrating on areas of Nebraska, Iowa,and Missouri that have been shown to have deep (42m)loess deposits, we preselected sites situated on public landsor cemeteries with broad, flat and well drained uplands.County soil surveys were used to determine regionaldifferences in mollic A-horizon thickness and we thenlocated the area with the broadest upland for initialsampling. Using a push probe, we determined whether thesoil at that site matched the local concept of a well-drainedMollisol, the typical grassland soil that we expected to findacross our study area. Our basic reference point fordetermining whether a site was disturbed and/or erodedwas thickness of the dark A-horizon (Munsell colorsp3/3). From west to east across Nebraska, depth tocarbonate increases. Thus, we used depth to the firstoccurrence of carbonates as a second indicator of potentialdisturbance. Push probe samples that showed evidence ofdisturbance using either indicator were rejected. Once thebest (defined as the most Mollisol-like) area was deter-mined, we collected a 7.6 cm diameter soil core up to 4m inlength using a pick-up truck mounted Giddings soil probe.In a few cases, where truck access was not possible, wecollected 2m deep auger cores. Soils were described in thefield using standard Natural Resources ConservationService terminology (Soil Survey Division Staff, 1993).We subsampled in 5 cm increments to the depths thatindicated soil development and then 10 cm increments tothe bottom of the core. To test for the influence of changesin parent material on soil magnetic properties we alsosampled several sandy and alluvial soils, mostly in north-eastern Nebraska. Magnetic enhancement for these sandysoils is very low and these sites were not included in ouranalyses.

2.2. Precipitation estimates

Mean annual precipitation was estimated for each site byusing an average value based on precipitation data from upto 5 weather stations within 50 km of each site. The climateinformation is based on data publicly available from theNational Climatic Data Center (NDCD), which wasaccessed via (Worldclimate, 2004).



2.3. Magnetic measurements

Samples were air dried, gently crushed, sieved through a2-mm screen to remove root fragments, and tightly packedinto weakly diamagnetic plastic boxes of 5.28 cm3 volume.We used a combination of magnetic parameters to rapidlyassess changes in the concentration and grain size of themagnetic component. Changes in mass-normalized mag-netic susceptibility (w) and isothermal remanent magnetiza-tion (IRM) were used to track changes in the concentrationof ferrimagnetic minerals, such as (titano)magnetite ormaghemite. The ratio of anhysteretic remanent magnetiza-tion (ARM) over IRM was used to estimate the relativeabundance of fine-grained (0.01–0.1 mm) single domain(SD) particles (e.g., Hunt et al., 1995), while frequency-dependent susceptibility (wfd) was used as a proxy for thepresence of ultra-fine grained (o0.01 mm) magnetic miner-als (e.g., Worm, 1998). wfd measurements were limited to afew sites across the profile, and the measurements wereconducted using a Bartington susceptibility meter equippedwith a MS2B dual-frequency sensor. Low-field suscept-ibility (w) was measured using a AGICO KappabridgeKLY-2 for samples collected in 2002 and a AGICOKappabridge KLY-4S for samples collected in 2003. ARMwas acquired with a D-Tech 2000 AF demagnetizer usingpeak AF fields of 100mT combined with a bias field of50 mT. IRM was acquired in a 100mT DC field of anelectromagnet. All remanence parameters were measuredusing a cryogenic magnetometer (2G Corporation, Model760R). Both Kappabridges have a sensitivity better than4� 10�8 SI (m3/kg), while the sensitivity of the magnet-ometer is approximately 2� 10�11 Am2, leading to negli-gible measurement errors (o0.01%) for all magneticparameters. Measurement errors shown in Figs. 7, 9 and10 are due to variations within the soil profile, which leadsto errors in the average magnetic values for the enhancedand unaltered soil horizons.Magnetic enhancement was calculated using changes in

w, ARM, IRM and the ratio of ARM/IRM. The positionand extent of the magnetically enhanced horizon as well asthe unaltered parent material were determined from a plotof magnetic parameters vs. depth (e.g., Figs. 3–6) and theoriginal soil descriptions obtained in the field. Themagnetically enhanced horizon contains at least threesamples centered around the peak in the magneticparameter measured and excludes the top 5 cm of theprofile to avoid contamination through highly magnetic flyash from coal burning powerplants. The exact depth of the‘‘magnetically enhanced horizon’’ varies between the fourmagnetic parameters (w, IRM, ARM and ARM/IRM)because these parameters measure different components ofthe magnetic fraction which may peak at different depths inthe soil profile. Based on our magnetic measurements weselected samples well below the magnetically enhancedhorizon as ‘‘magnetically unaltered parent material’’,avoiding horizons with excessive redoximorphic featureswherever possible. Figs. 3–6 show the location of the



ARTICLE IN PRESSC.E. Geiss, C.W. Zanner / Quaternary International ] (]]]]) ]]]–]]]4

samples used in the calculation of magnetic enhancementfor mass-normalized magnetic susceptibility. Relativemagnetic enhancement is calculated as the ratio betweenthe averages for the magnetically enhanced horizon and the

Fig. 2. Normalized magnetic variations of unaltered parent material

across study area in percent of the maximum value (e.g.,

ARM ¼ ARMsite/ARMmax� 100). Concentration-dependent parameters

ARM and IRM show general increase from east to west across Missouri

and Nebraska.

Fig. 3. Soil profile and magnetic properties for Miriam Cemetery (MIR 04-A)

at the site is mapped as Holdrege silt loam: (a) mass normalized susceptibilit

remanent magnetization (IRM). All three parameters are proxies for the con

ARM/IRM is a proxy for the relative abundance of fine (dE0.01–0.1mm) sin

abundance of SD grains, (e) frequency-dependent susceptibility wfd is a proxy o

whf is a proxy for the presence of paramagnetic minerals, including clays. Profile

(Soil Survey Division Staff, 1993).

Abbreviations used in profile description (Figs. 3–6): Texture: SiL—silt loam, S

poor); boundaries: abr—abrupt (0–2 cm), clr—clear (2–5 cm), grd—gradual (5

medium, c—coarse, gr—granular, sbk—subangular blocky, pr—prismatic, ma—

profile indicates extent of magnetically enhanced (enh.) horizon as well as unalt

the extent of these two horizons is very similar for the various magnetic paramet

easier comparison between the sites.



unaltered parent material. Standard deviations of thecalculated averages were used to determine the absoluteerror of the enhancement parameter. Our enhancementratios are dimensionless numbers and may, in our opinion,be better suited to quantify magnetic changes in transectswith (slightly) varying parent material than absolutepedogenic susceptibility (wped ¼ wmax�wC-horizon) favoredby Maher and Thompson (1995) and Maher et al. (2002).

3. Results

3.1. Magnetic properties of the parent material

Values of concentration-dependent parameters IRM,and ARM for the C-horizon are generally high in south-western Nebraska and are gradually decreasing towardsthe east (Fig. 2a and b). These variations likely reflectvariations in the concentration of magnetic minerals due tovariations in source area and transport distance among thestudied sites. As loess is blown across the landscape ordeposited along stream channels it undergoes fractionationin terms of grain-size and heavy mineral content. Ingeneral, the iron content of Peoria loess increases fromIowa and Missouri towards the western part of ourtransect (Bettis et al., 2003), which is reflected in themagnetic properties of the parent material. Changes in the

in southwestern Nebraska (mean annual precipitation 500mm/a). The soil

y w, (b) anhysteretic REMANENT magnetization (ARM), (c) isothermal

centration of magnetic minerals in the soil, (d) the dimensionless ratio of

gle-domain (SD) magnetic particles with higher ratios indicating a greater

f ultrafine (do0.01mm) magnetic particles, and (f) high-field susceptibility

description is based on field notes following Soil Survey Manual guidelines

iCL—silty clay loam, SiC—silty clay, h—heavy (clay-rich), l—light (clay-

–15 cm); structure: wk—weak, mod—moderate, st—strong, f—fine, m—

massive. Colors are given in Munsell notation. Shaded bars to the right of

ered parent material for mass normalized susceptibility. For most horizons

ers. The vertical and horizontal scales of Figs. 3–6 are identical to allow for



ARTICLE IN PRESS

Fig. 4. Soil profile and magnetic properties for Prairie Pines (PRA 02-A), near Lincoln, NE (mean annual precipitation 720mm/a). The soil at the site is

mapped as Sharpsburg silt loam. For more information see caption for Fig. 3.

C.E. Geiss, C.W. Zanner / Quaternary International ] (]]]]) ]]]–]]] 5

grain-size-dependent ratio ARM/IRM are generally muchsmaller and less systematic (Fig. 2c), reflecting themagnetically coarse-grained, multi-domain (MD) characterof unaltered Peoria loess.

These changes are intrinsic to Peoria loess and not asignal of postdepositional alteration of the iron phase. Allour sites are well drained and none are gleyed in theinvestigated horizons. It is interesting to note that slight(few to common) redoximorphic often do not significantlyaffect the magnetic properties of the parent material (e.g.,Fig. 4, C2 horizon, 5 BC-C1 horizons). This is due to thecoarse grained nature of the magnetic particles present inthe parent loess, whose magnetic properties are littleaffected by the formation and destruction of weaklymagnetic antiferromagnetic minerals generally responsiblefor changes in soil color (note the almost identicalsusceptibility values for hydric and non-hydric soils shownin Grimley and Vepraskas, 2000). The magnetic propertiesof the upper soil, however, are characterized by theaddition of much finer grained magnetic minerals, andredoximorphic processes are likely to affect these poorlycrystalline iron phases. For this reason our study is limitedto well-drained soils.

Figs. 3–6 show the magnetic properties for four selectedsoil profiles across our transect and demonstrate thechanges in soil development and magnetic properties withprecipitation. Miriam cemetery (Fig. 3) is located in SWNebraska (N41.0141, W100.6591), approximately five milessouth of the Platte river and the thick loess deposits ofBignell Hill. The cemetery was established in 1911 and



disturbance through agriculture is minimal. Prairie Pines(Fig. 4) is located a few miles NW of Lincoln, Nebraska(N40.8421, W96.5591) near the center of our transect onproperty owned by the University of Nebraska. The corewas taken in a part of the property which, according to itsprevious owner, has never been farmed. Mount Calvarycemetery (Fig. 5) is located in the loess hills of westernIowa (N40.8721, W95.4201). The core was taken in theoldest part of the cemetery which has been establishedaround 1880. Davisdale Conservation Area (Fig. 6) islocated in central Missouri (N39.0351, W92.6301) on thenorthern bluffs of the Missouri river. The core was takenin the upland regions at the north end of the conserva-tion area. The mean annual precipitation values for thefour sites are 540, 700, 840 and 970mm/a, respectively.Table 1 shows the location, precipitation data, soil seriesand slope angle, and lists the magnetic properties for themagnetically enhanced horizons as well as the parentmaterial.

3.2. Magnetic enhancement signal in modern soils

All loessic sites examined in this study show magneticenhancement in the uppermost soil horizons, which isexpressed in higher concentrations of ferrimagnetic (mag-netite and maghemite) and possibly antiferromagneticminerals (goethite, hematite Geiss et al., 2004) minerals.Asides from these mineralogical changes, the magneticallyenhanced horizons are also characterized by increasedabundances of fine- to ultra-fine grained ferrimagnets.



ARTICLE IN PRESS

Fig. 5. Soil profile and magnetic properties for Mount Calvary Cemetery (MTC 03-A) in the loess hills of western Iowa (mean annual precipitation

840mm/a). The soil at the site is mapped as Marshall silty clay loam. For more information see caption for Fig. 3.


Figs. 3–6 demonstrate the general pattern of soildevelopment and magnetic enhancement found along ourtransect, which is consistent with loessic soils thatdeveloped under temperate climatic conditions elsewhere(Nawrocki et al., 1996; Oches and Banerjee, 1996; Maheret al., 2002). To facilitate comparison between the sites, alldata are plotted on the same vertical and horizontal scales.Concentration-dependent magnetic parameters (magneticsusceptibility w, ARM, IRM, saturation magnetizationJS—not shown) all increase in the upper soil horizons withmost of the magnetic enhancement occurring in theuppermost 50 cm of the soil profile. The grain-size-dependent parameters ARM/IRM and frequency-depen-dent susceptibility (wfd) also increase in the A- and AB-horizons of the shown profiles, indicating that at least partof the pedogenically produced magnetic fraction is in thesingle-domain (0.01–0.1 mm) and SP (o0.01 mm) grain sizerange. A comparison of Figs. 3–6 shows that the zone ofmagnetic enhancement is limited to the biologically activeA-horizon and does not grow downward significantly with



B-horizon development, which is consistent with a largelybiomediated origin of pedogenic magnetite. Bt-horizondevelopment is tracked poorly by changes in paramagneticsusceptibility (whf), which is a measure of paramagnetic, Fe-bearing minerals including clays, but whf is generally low inthe magnetically enhanced horizons. These variations in whfsuggest that paramagnetic minerals act as a possible sourceof iron for the formation of pedogenic ferri- andantiferromagnetic minerals.

3.3. Degree of magnetic enhancement

For all four magnetic parameters, the enhancementparameter shows a positive correlation with mean averageprecipitation (Fig. 7). Between the various parameters thecorrelations differ in the degree of scatter as well as theslope of the correlation between magnetic and climaticparameters. For w (Fig. 7a), which has been previously usedfor paleo precipitation reconstructions (e.g., Maher andThompson, 1995; Maher et al., 2002), the observed scatter



ARTICLE IN PRESS

Fig. 6. Soil profile and magnetic properties for Davisdale Conservation Area (DAV 03-A) in central Missouri (mean annual precipitation 970mm/a). The

soil at the site is mapped as Winfield silt loam. For more information see caption for Fig. 3.


is quite large and changes in magnetic enhancement acrossthe transect are small, ranging between 1.0 (no enhance-ment) and 3.0. The correlation between magnetic enhance-ment and climate is better if the remanence parameter,ARM (Fig. 7b) is used. ARM might be best suited forpaleoprecipitation reconstructions because, as Fig. 7(b)shows, it displays relatively little scatter and a large changein magnetic enhancement ranging between 1.5 and 10across the entire transect. Magnetic enhancement valuesbased on IRM (Fig. 7c) and the ratio of ARM/IRM(Fig. 7d) display a greater amount of scatter, especiallytowards the wet end of the transect.

For comparison, we also calculate pedogenic suscept-ibility wped defined and used by Maher and Thompson(1995) and subsequent studies, and plot their values as afunction of current precipitation (Fig. 8). Pedogenicsusceptibility is defined as the difference between themaximum susceptibility value found in the B-horizon(wmax) and the average susceptibility of the C-horizon(wC-horizon), wped ¼ wmax�wC-horizon. Data shown in gray arewped values from the Chinese loess plateau (Maher andThompson, 1995; Porter et al., 2001) and Russian steppe



soils (Maher et al., 2002). Maher et al. (2002) use a semi-logarithmic scale to present their data (Fig. 8a), whichminimizes data scatter for sites that receive high precipita-tion. To facilitate comparison with Fig. 7 the data areredrawn using a linear scale (Fig. 8b). wped data for themidwestern US (shown in open circles) display littlecorrelation with mean annual precipitation and level offfor soils that receive more than 700mm of precipitation peryear. Midwestern sites tend to receive more rain then thesites studied by Porter et al. (2001) and Maher et al. (2002),but even when these humid sites are excluded thecorrelation between wp and modern climate is not improvedfor sites from the midwestern US. A linear fit through ourdata (not shown in Fig. 8b) yields a r2 value of 0.3.

4. Discussion

4.1. Magnetic enhancement as a precipitation proxy

Almost all soil profiles show magnetically enhancedupper soil horizons (A- and upper B-horizons) regardlessof the parameter used to calculate magnetic enhancement.



ARTICLE IN PRESSTable

1

Summarizedrock-m

agnetic

data

forallsoilsitesdiscussed

inthetext

Location

Lat

Long

Mapped

soil

series

Slope

(%)

Precip.

(mm/a)

w e(10�6SI)

w p(10�6SI)

ARM

e

(mAm

2/kg)

ARM

p

(mAm

2/kg)

IRM

e

(mAm

2/kg)

IRM

p

(mAm

2/kg)

ARM/IRM

eARM/IRM

p

OughCem

.40.321

�101.522

BlackwoodL

0–1

49075

1.2770.03

1.0770.01

189711

13274

108967254

96067346

0.01770.001

0.01470.001

Miriam

Cem

.41.014

�100.659

Hord

SiL

149575

0.0870.00

0.0570.00

14373

6572

6640729

44007597

0.02270.001

0.01570.002

GroveCem

.40.122

�100.815

Keith

SiL

1�2

52075

1.2970.04

0.9370.02

216712

12576

106427447

80437849

0.02070.002

0.01670.002

SwansonLakeSWMA

40.176

�101.086

Keith

SiL

152575

1.6470.02

0.9870.04

17675

10975

117247401

79827553

0.01570.001

0.01470.002

MoorefieldCem

.40.679

�100.400

HallSil

252575

0.9370.01

0.6470.04

19073

97714

7475760

517871551

0.02570.001

0.01970.008

MedicineCreek

SWMA

40.422

�100.212

HoldregeSiL

353075

1.0470.02

0.5870.03

12876

6376

7993778

44767728

0.01670.001

0.01470.004

GarfieldCem

.41.385

�100.388

HoldregeSiL

4530715

0.5570.02

0.4670.01

8873

5471

41567185

35257456

0.02170.002

0.01570.002

CurtisCem

.40.652

�100.520

HallSiL

154075

0.9770.01

0.6170.02

17571

7574

7826781

49157528

0.02270.000

0.01570.002

StoweCem

.40.490

�100.075

HoldredgeSiL

2540715

0.8170.03

0.5670.01

14274

6974

60237362

45067681

0.02470.002

0.01570.003

WalnutGroveCem

.40.976

�99.994

Hord

SiL

154075

0.7070.02

0.4970.01

13073

5573

53987112

39887483

0.02470.001

0.01470.002

ArborCem

.40.549

�100.402

HallSiL

0�1

54075

0.8270.01

0.6370.02

14976

8276

64427119

51157901

0.02370.001

0.01670.004

Custer

CenterCem

.41.437

�99.704

HoldregeSiL

456075

0.6470.02

0.5270.01

9974

6871

49637212

44387230

0.02070.002

0.01570.001

Deaver

Cem

.40.103

�99.649

HallSiL

0�1

560720

1.1170.03

0.6070.01

20575

8072

87117292

51367314

0.02470.001

0.01670.001

BoleszynChapel

Cem

.41.669

�99.132

HoldregeSiL

0�1

570740

0.7970.01

0.5470.01

16173

7473

5907761

43497581

0.02770.001

0.01770.003

Fairview

Cem

.41.335

�99.922

HallSiL

0570740

0.7470.01

0.4970.02

13774

6173

56507107

39147383

0.02470.001

0.01670.002

Dry

Valley

Cem

.41.497

�99.395

HoldregeSiL

3570715

0.7370.01

0.5570.02

15973

7676

5602754

46427906

0.02870.001

0.01670.005

W.Sacramento

SWMA

40.359

�99.307

HoldregeSiL

4605720

0.6770.02

0.5570.06

11579

7578

47067105

377771781

0.02470.002

0.02070.011

St.WencelslausCem

.41.205

�98.571

HastingsSiL

0�1

60575

0.6370.02

0.5070.01

145711

6877

4323736

386472437

0.03370.003

0.01870.013

Warsaw

Cem

.41.185

�98.558

HastingsSiL

0�1

60575

0.6870.02

0.4770.02

16776

6676

48327269

345572968

0.03470.003

0.01970.018

Freew

aterCem

.40.308

�99.207

HoldregeSiL

261075

0.8670.01

0.5870.01

15875

7673

6605735

47597561

0.02470.001

0.01670.003

Ponca

SP

42.596

�96.711

CroftonSiL

3645710

0.7170.04

0.4070.02

12676

4072

4507727

26187889

0.02870.002

0.01570.006

Rem

senCem

.42.806

�95.978

GalvaSiL

3�4

715755

0.4570.02

0.1970.01

22577

3675

5163785

225971519

0.04470.002

0.01670.013

UnionTwpCem

.42.692

�96.029

—2

715735

0.3470.02

0.1970.01

16076

4074

39107133

230571269

0.04170.003

0.01770.011

Prairie

Pines

40.842

�96.559

Sharpsburg

SiCL

172075

0.7470.02

0.4670.02

16776

5774

5037791

31857590

0.03370.002

0.01870.004

Deloit(M

ilford)Cem

.42.108

�95.314

MononaSiL

5750725

0.8070.03

0.3870.01

20776

4574

5260732

247471607

0.03970.001

0.01870.013

KironCem

.42.181

�95.314

MarshallSiCL

2750720

0.3370.01

0.2070.01

13778

3974

40297163

25867917

0.03470.003

0.01570.007

Barn

Bluff

44.570

�92.527

Tim

ula

SiL

676075

1.6770.04

0.9670.15

25176

4876

104327675

53487861

0.02470.002

0.00970.002

MagnoliaCem

.41.690

�95.881

MononaSiL

4775730

0.5470.00

0.2270.01

252710

4277

66927185

256571967

0.03870.003

0.01670.015

CarsonCem

.41.240

�95.406

MarshallSiCL

3815740

0.5070.00

0.2170.00

24576

4972

64177246

27497425

0.03870.002

0.01870.004

Mt.Calvary

Cem

.40.872

�95.420

MarshallSiCL

2835735

0.4570.01

0.2170.01

22673

4772

5507729

27237877

0.04170.001

0.01770.006

Utterback

Cem

.40.628

�95.569

—3

855720

0.4970.01

0.2470.01

23275

4873

62687140

33087547

0.03770.002

0.01570.003

Brickyard

HillsCA

40.460

�95.557

Tim

ula

SiL

2870710

0.5670.01

0.2070.01

25470

5573

66667182

19897715

0.03870.001

0.02770.011

Honey

Creek

CA

39.957

�94.974

KnoxSiL

1�2

870725

0.9570.02

0.4870.02

23977

6078

69977101

356672249

0.03470.001

0.01770.013

River

BreaksCA

39.930

�95.116

KnoxSiL

3870730

0.5570.01

0.2570.01

24572

5072

69387215

35637617

0.03570.001

0.01470.003

BilbyRanch

CA

40.339

�95.178

Sharpsburg

SiCL

287575

0.4770.01

0.1770.00

23971

4172

65477291

18877608

0.03770.002

0.02270.008

CenterGroveCem

.40.492

�95.337

MarshallSiCL

387575

0.4870.00

0.2270.02

23473

4777

5897770

290171057

0.04070.001

0.01670.008

NodawayValley

CA

40.112

�95.023

MarshallSiL

1�2

920745

0.4670.02

0.1970.01

265712

4473

47097263

241371495

0.05670.006

0.01870.013

Davisdale

CA

39.035

�92.630

WinfieldSiL

3970710

0.3070.01

0.1070.03

26872

2974

3704736

116271523

0.07270.001

0.02570.036

Turner

Cem

.39.555

�94.816

MarshallSiCL

4970760

0.4970.01

0.2670.01

24078

5874

67497276

37367882

0.03670.003

0.01670.005

DanielBooneCA

38.767

�91.392

Blase

SiCL

299075

0.2870.02

0.1270.02

212712

3278

3898744

113174164

0.05570.004

0.02870.112

w e,averagemass

norm

alizedmagnetic

susceptibilityoftheenhancedhorizon;w p,averagemass

norm

alizedmagnetic

susceptibilityfortheparentmaterial;ARM

e,averageanhysteretic

remanent

magnetizationfortheenhancedhorizon;ARM

p,averageARM

fortheparentmaterial;IR

Me,averageisothermalremanentmagnetizationfortheenhancedhorizon;IR

Mp,averageIR

Mfortheparent

material;ARM/IRM

e,averageARM/IRM

ratiosfortheenhancedhorizon;ARM/IRM

p,averageARM/IRM

ratiosfortheparentmaterial.


Please cite this article as: Geiss, C.E., Zanner, C.W., Sediment magnetic signature of climate in modern loessic soils from the Great Plains. Quaternary



ARTICLE IN PRESS

Fig. 7. Scatter plots of magnetic enhancement of the upper soil horizons vs. present-day mean annual precipitation. Several magnetic parameters were

used to estimate magnetic enhancement as indicated on the plots, and magnetic enhancement is expressed as Menhanced soil horizon/Mparent material, where M

represents one of the following magnetic parameters: mass-normalized magnetic susceptibility (w), ARM, IRM or ARM/IRM. The enhancement

parameter is a dimensionless number regardless of the magnetic parameter used. Magnetic enhancement for Barn Bluff (BAR) is indicated in all figures.


All relative enhancement parameters show a clear correla-tion with mean annual precipitation estimates for each site(Fig. 7). As shown earlier (Geiss et al., 2004), magneticenhancement in loessic soils in Nebraska is due to theconversion of weakly magnetic iron-bearing minerals intostrongly magnetic ferrimagnetic minerals, such as magne-tite and/or maghemite. The enhancement process is not dueto soil compaction (all our measurements are massnormalized) nor leaching of non-magnetic carbonateminerals (in many sites the depth of carbonate leachingexceeds our sampling depth and both the magneticallyenhanced horizon as well as the parent material arecarbonate free). Several abiotic processes of magneticenhancement have been proposed (e.g., Taylor et al.,1987; Maher and Taylor, 1988), which are indirectly linkedto climatic conditions as they depend on the extent of



wetting and drying cycles in otherwise well drained soils.Activity of iron-reducing bacteria (Munch and Ottow,1980; Kostka et al., 1996) during humid periods reducesiron contained in clays or other minerals, which is thenoxidized to magnetite or hematite, depending on pH andlength of dry season (Maher, 1998; Maher et al., 2002).Even though it may not be possible to link changes inmagnetic enhancement solely to variations in climate suchas precipitation or temperature, the processes that influencethe magnetic properties of the soil profile, such asvegetation and soil biota do covary with climate.It is possible, though unlikely, that the observed

variations in magnetic enhancement are at least partlydriven by changes in parent material. The highestconcentrations of ferrimagnetic minerals occur in loesssamples from southwestern Nebraska, which is at the dry



ARTICLE IN PRESS

Fig. 8. Variations of pedogenic susceptibility wped ¼ wmax�wparent material for loessic soils from the Chinese loess plateau (Maher and Thompson, 1995;

Porter et al., 2001), the Russian steppe (Maher et al., 2002) and the midwestern United States (this study).


end of our sampling region, and concentration-dependentmagnetic parameters (ARM, IRM, w-not shown) arehighest for these sites (Fig. 2). These variations in themagnetic properties of the parent material are likely due tochanges in sediment source and transport mode and ordistance, which affects the abundance of magnetite andmaghemite as well as the total abundance of iron in thesediment. Since magnetic enhancement processes utilizeiron that is initially present in the parent material andconvert it into highly magnetic ferrimagnetic minerals, asystematic change in the source material could lead to asystematic magnetic enhancement signal. Fig. 9 shows theaverage magnetic properties of the C-horizon as a functionof mean annual precipitation. There is little correlationbetween any of the concentration-dependent magneticparameters (w, ARM, IRM) and mean annual precipita-tion. An apparent negative correlation for ARM and IRMis mainly driven by four sites (circled in Fig. 9c) in thesouthwest corner of Nebraska. The poor correlationbetween mean annual precipitation and C-horizon mag-netic properties is in stark contrast to the strong correlationbetween mean annual precipitation and magnetic enhance-ment (Fig. 7), making it unlikely that the changes inmagnetic enhancement across our transect are driven bysystematic variations in parent material.

Harder to quantify is the role of Holocene loess inputinto our sites. Holocene loess deposition is most importantin western Nebraska, where early Holocene Bignell loesscan reach a thickness of several meters (Pye et al., 1995).Deposition of Holocene loess, however, has been shown toextend through central Nebraska (Kuzila, 1995; Masonand Kuzila, 2000) and probably occurred throughoutNebraska. Episodic loess deposition may have led to adilution of magnetically enhanced material and theaddition of coarse grained ferrimagnetic minerals, causingoverall lower magnetic enhancement parameters. The



addition of Holocene loess has been invoked as anexplanation for the magnetic variations observed inmodern Chinese soils (e.g., Porter et al., 2001), but Maheret al. (2002) show that similar magnetic enhancementcharacteristics can be found in loessic soils from Russia,where Holocene loess deposition was absent. Based on thecombined records from China and Russia, Maher and co-workers argue that the development of the magneticenhancement signal occurs rapidly, and modern climate isthe main influence on the properties of these modern soils.Site BAR-02A (Barn Bluff near Red Wing) in southern

Minnesota can shed light on some of these questions. Thesite is located on a high bluff along the Mississippi, and itsloess is likely derived from the Mississippi floodplain orother local sources (Mason et al., 1994). At this site, theunaltered loess is strongly magnetic, showing relativelyhigh values of IRM and w comparable to sites in westernNebraska (Fig. 9a and c). Its magnetic enhancement,however, is consistent with the mean annual precipitationfor the site and falls onto the trends displayed in (Fig. 7). Ingeneral, systematic changes in parent material cannotexplain the observed trends in magnetic enhancementacross our transect.Some dispute exists about the rate of magnetic enhance-

ment. Limited evidence from the midwestern US andwestern China suggests that the magnetic signal developsover a relatively short time period (centuries) and rapidlyapproaches steady-state conditions (Maher and Thomp-son, 1995; Maher et al., 2003). A study of a California soilchronosequence performed by Singer et al. (1992) suggeststhat the magnetic susceptibility signal is time dependentand can even be used to estimate soil age. Vidic and Lobnik(1997) and Vidic and Verosub (1999), who studied achronosequence in Slowenia, as well as Grimley et al.(2003), who analyzed paleosols in Illinois, arrived at similarconclusions. Vidic et al. (2004) also investigated a



ARTICLE IN PRESS

Fig. 9. Scatter plots of C-horizon magnetic properties vs. mean annual precipitation. Data for Barn Bluff, which are further discussed in the text, are

indicated in all four panels.


loess–paleosol sequence from China and suggested meth-ods to correct the degree of magnetic enhancement for theduration of soil development in order to separate theclimate- and time-dependence of the magnetic suscept-ibility signal. Regarding this study, the ages of our modernsoils are sufficiently similar to ignore the effects of time(modern soils developed in Peoria loess with major soildevelopment limited to the Holocene), but interpretationsof paleosols will have to be corrected for the duration ofsoil formation, either through the methods suggested byVidic et al. (2004) or through comparison with othernearby quantitative records of paleoclimate (e.g., Doraleet al., 1998).

4.2. Preferred enhancement parameters for paleoclimate

reconstruction

As shown previously (e.g., Maher, 1998; Geiss et al.,2004) the magnetic properties of the pedogenically



produced magnetic component is dominated by fine-grained (0.01o0.1 mm) SD and even finer (do0.01 mm)SP ferrimagnetic minerals (magnetite or maghemite). Thesesmall, highly magnetic minerals dominate most magneticproperties of the upper soil horizons even though weaklymagnetic antiferromagnetic minerals (goethite or hematite)are a likely by-product of pedogenic enhancement. Sincethe initial parent material is dominated by coarse(d45 mm), MD ferrimagnetic grains, magnetic parameterssuch as ARM that are sensitive to the presence of fine SDand SP particles are likely to yield the best proxy ofpedogenic enhancement. Indeed, magnetic enhancementvalues based on ARM variations (Fig. 7b) show the largestchange between dry and wet sites and the highest degree ofcorrelation between magnetic enhancement and meanannual precipitation. IRM and magnetic susceptibility aremainly controlled by variations in coarse-grained ferri-magnetic particles and possibly paramagnetic clays. Thismagnetic component is more a reflection of the initial



ARTICLE IN PRESS

Fig. 10. Correlation of relative magnetic enhancement (wenhanced/wparentmaterial) with mean annual precipitation for Russian steppe soils (data from

Fig. 2, Maher et al., 2002) and soils from the midwestern United States

(redrawn from Fig. 3a). With the exception of three samples the both data

sets correlate well with climate, but the difference in slopes indicates that

relative magnetic enhancement parameters are not perfect in compensat-

ing for different parent materials.


parent material, and magnetic enhancement based on w andIRM tend to reflect pedogenic changes in ferrimagneticminerals to a lesser degree.

Magnetic enhancement estimates based on the stronglygrain-size-dependent ratio ARM/IRM, which is diagnosticfor the relative abundance of fine SD-particles, correlatewell with precipitation for samples from the dry end of ourtransect but show little systematic change for soilsdeveloped under mean annual precipitation values largerthan 750mm/yr. This change in trends may reflect a changefrom prairie to forest soils or is due to increased diagenesisof fine-grained magnetic particles under more humidconditions. A similar result was observed by Ochesand Banerjee (1996) for the Eemian paleosol (PK3)exposed at Dolnı Vestonice in the Czech Republic. Atthis site, interstadial and glacial soils are magneticallyenhanced while the well-developed paleosol horizonassociated with MIS 5e shows little increase in magneticparameters with respect to the late glacial loess. Ochesand Banerjee (1996) interpret this apparent lack ofmagnetic enhancement with a loss of fine-grained magneticminerals due to weathering or pedogenesis under humidconditions.

All magnetic enhancement parameters shown in Fig. 7display a larger degree of scatter towards the humidend of the transect (mean annual precipitation 4750mm/yr). This increase may be due to higher variability of thepedogenic processes that cause magnetic enhancementunder these humid conditions, may be caused by the lossof iron minerals because of local redoximorphic processes,or may reflect climatic influences other than precipita-tion. In our transect sites experiencing mean annualprecipitation values larger than 750mm/yr are mainlylocated along a north-south trending line, following theloess hills of Iowa and northern Missouri (Fig. 1). In thispart of the transect, both precipitation and temperaturecovary, which is likely to influence the magnetic propertiesof the topsoil.

Statistical analysis that use a combination of severalmagnetic and non-magnetic parameters and incorporateseveral climatic parameters might be able to discriminatebetween these factors. However, such analyses will requirea larger amount of modern sites to cover a wider range oftemperature and moisture regimes.

In our study, we favor the use of relative enhancementparameters (ratios or normalized differences, which aremathematically equivalent) over absolute enhancementparameters (pedogenic susceptibility wp). This choice isbased on the assumption that magnetic enhancement isultimately limited by the amount of useable iron in theparent material. Soils developing in sites with low ironconcentrations will likely undergo less magnetic enhance-ment than soils that develop in parent material. A relativeenhancement parameter will be better suited to accom-modate these variations in parent material. In general,lower iron concentrations in the parent material willtranslate into lower values of w, ARM or IRM; therefore,



the normalized difference between the magnetically en-hanced horizon and the unaltered parent material will becompensated for variations in iron concentrations. Fig. 8,which shows the correlation between pedogenic suscept-ibility and mean annual precipitation suggests that this isthe case for samples from the midwestern United States.wped, an absolute enhancement parameter shows a muchweaker correlation with climate than any of the relativeenhancement parameters shown in Fig. 7 (r2E0.3, com-pared to r2 ¼ 0.6–0.8).Using Fig. 2 of Maher et al. (2002) it is possible to

calculate relative enhancement ratios for a subset of Maheret al.’s sites. Fig. 10 shows relative enhancement ratios(wenhanced/wparent material) for 10 of Maher et al.’s soil profilesand compares them to the equivalent data from themidwestern US (Fig. 7a, this paper). Except for three sites,which display unusually C-horizon susceptibility, theRussian data (closed circles) show a good correlation withmean annual precipitation, though the trend is muchsteeper than what is observed for samples from themidwestern US (closed squares). The differences in slopebetween the two data sets shows that relative magneticenhancement parameters are not perfect in compensatingfor changes in parent material. The sites studied by Maherand co-workers tend to have much lower C-horizonsusceptibility values (�20� 10�8m3/kg) compared totheir midwestern counterparts (�60� 10�8m3/kg), andrelative enhancement ratios might overcompensate forthe low initial abundance of magnetic minerals in theparent material, but such interpretations are rather specu-lative without supporting pedologic and rock-magneticanalyses.



ARTICLE IN PRESSC.E. Geiss, C.W. Zanner / Quaternary International ] (]]]]) ]]]–]]] 13

5. Conclusions

Our study of approximately 75 soil sites from themidwestern United States shows that the magnetic proper-ties of modern loessic soils reflect the present precipitationgradient of less than 500mm/yr in southwestern Nebraskato nearly 1000mm/yr in central Missouri. Our resultsinclude:

�

P

In

The upper horizons of loessic soils in the midwesternUnited States are enriched in fine-grained magneticminerals. This magnetic enhancement leads to highervalues of concentration-dependent parameters w, ARM,IRM in uppermost 30–50 cm of the soil profile.
� Magnetic enhancement can be estimated using a variety
of rapidly measurable parameters, such as IRM, ARMor magnetic susceptibility w. It is best expressed as theratio of the chosen magnetic parameter of enhancedhorizon and the unaltered parent material. Theserelative enhancement ratios correlate well with modernprecipitation gradients.
� Pedogenic susceptibility wp, an absolute measure of
pedogenically produced magnetic minerals appears to beinfluenced by variations in parent material rather thanmodern climate and its correlation with modernprecipitation data is poor.
� The observed systematic change in magnetic properties
reflects changes in present day precipitation.
� Applying our set of magnetic enhancement parameters
to buried soil horizons may enable us to reconstructspatial variations in midcontinental paleoclimate forselected time periods.

Acknowledgments

We are grateful to the numerous land owners and landmanagers who allowed us access to their land. We wouldalso like to thank Jim Bisbee and Dan Scollan for their helpin the field and for preparing samples for paleomagneticanalyses. Parts of the analyses were performed at theInstitute for Rock Magnetism at the University ofMinnesota which is funded by the W.M. Keck Foundation,the National Science Foundation’s Earth Science Divi-sion’s Instrumentation and Facilities Program and theUniversity of Minnesota. Field and student support camefrom two Trinity College Faculty Research Grants to CEGand from the Conservation and Survey Division of theUniversity of Nebraska.

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