199Journal of Paleolimnology 24: 199–211, 2000.© 2000 Kluwer Academic Publishers. Printed in the Netherlands.

Relationship of Mn-carbonates in varved lake-sediments to catchmentvegetation in Big Watab Lake, MN, USA

Lora R. Stevens1, Emi Ito & David E.L. OlsonLimnological Research Center, School of Earth Sciences, University of Minnesota, Minneapolis, MN 55455,USA1Present address: Department of Geosciences, University of Nebraska-Lincoln, Lincoln, NE 68588, USA

Received 11 August 1998; accepted 9 September 1999

Key words: Mn-carbonate, oxygen-18, varves, anoxia, diagenesis, mixing, pollen, wind

Abstract

Mn-carbonates are documented in the late-glacial varved sediments from Big Watab Lake, Minnesota, USA. TheMn-carbonate is authigenic and forms rims around contemporaneous epilimnetic calcite. Although such carbonatesare found in minor amounts throughout the entire late-glacial sequence, significant quantities of Mn-carbonateare associated mainly with laminated intervals.

Because of the suspected difference in isotopic fractionation between different carbonate minerals, thestable-isotopic compositions of bulk carbonate samples are used as a proxy for relative amounts of the Mn-carbonatein the sediment. High δ18O and low δ13 C values are associated with abundant Mn-carbonates. Low δ18O and highδ13C values are associated with only minor concentrations of Mn-carbonates.

The oxygen-18 record is correlated with fluctuations in the vegetation assemblage based on pollen spectrausing a multiple regression model with backward elimination. The proposed link between the sedimentary archiveand local vegetation is the mediation of advective mixing in the lake by forest composition. In this model, periodsof forest closure resulted in a well-stratified water column that was anoxic at the sediment/water interface, permittingthe formation of authigenic Mn-carbonates. Openings of Artemisia in the forest allowed wind shear to mix oxygento depth, causing bioturbation of the laminations and preventing the formation of Mn-carbonate.

Introduction

The occurrence of Mn-carbonates in sediments, onceconsidered relatively rare, is now known to be morecommon than previously thought. Several studies overthe last 30 yrs have identified endogenic Mn-carbonatesin deep profundal sediments of both oceanic (Zen,1959; Lynn & Bonatti, 1965; Calvert & Price, 1970;Suess, 1979; Pedersen & Price, 1982) and lacustrinebasins (Shterenberg et al., 1966 as cited by Calvert &Price, 1970; Callender et al., 1974; Dean, 1993). In oceanicsediments, Mn-carbonates are described as nodules orconcretions at discrete levels in the sedimentaryprofiles (Callender et al, 1974; Suess, 1979; Pedersen &Price, 1982). In Elk Lake, Clearwater County, Minnesota,rhodochrosite (MnCO

3) is distributed in small con-

centrations throughout much of the Holocene sediment

(Dean & Megard, 1993) and is found in modern sed-iment traps (Nuhfer et al., 1993).

The formation of Mn-carbonates is related to redoxreactions that occur in the pore waters of sedimentssubjected to anoxia (see Wetzel, 1983). In most casesthe manganese is derived from recycled manganeseoxides, which dissolve upon burial (Lynn & Bonatti,1965). Investigations of Mn-carbonate phases in youngsediments has been hampered in part because of in-sufficient data on the solubility of the solid solutionphases of Mn carbonates (Pedersen & Price, 1982;Sawlan & Murray, 1983). However, recent laboratoryexperiments at 20 °C have succeeded in transformingaragonite to Mn-carbonate phases (Boettcher, 1997).

We report the occurrence of Mn-carbonate with anaverage stoichiometry between kutnahorite (Mn

0.5

Ca0.5

CO3) and rhodochrosite in the laminated sedi-

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ment of a deep lake in central Minnesota. In this study,the formation of the Mn-carbonate in laminated zonesis related to internal lake dynamics and to the hypoth-esized role of catchment vegetation in mediating lakeprocesses.

Setting

Modern environment

Big Watab Lake, Stearns County (45 ° 33′ 05′′ N, 94 ° 27′07′′ W) is a small deep lake located in the St. Croixmoraine of central Minnesota (Figure 1). The lakeoccupies an ice-thrust depression, formed when a layerof till and gypsiferous Cretaceous clay froze to the baseof the advancing Superior lobe of the Laurentide icesheet (Mooers, 1988). The resulting ridge on thewestern side of the basin (approximately 60 m of relief)acts as a regional ground water divide (Helgesen et al.,1975) for shallow aquifers. The surrounding till isdominated by iron-rich sediment scoured from theSuperior basin. Stagnant ice remained buried in thebasin until it melted, creating the lake.

The modern lake has a surface area of 0.9 km2 andfour sub-basins, one of which is 36 m deep (Figure 1).The lake receives a small input from a wetland to thenorth and discharges to the southeast by way of theWatab River. Analyses confirm that the lake is freshwaterand dimictic. The dominant anion is HCO

3–, with minor

amounts of SO42–; the dominant cation is Ca2+. Summer

and winter stratification result in seasonal anoxia inthe hypolimnion, which prevents bioturbation. Themodern sediment is well-laminated and comprised oflow-Mg calcite, with Mn-carbonates occuring atdiscrete intervals.

Local climate is typical of the northern MidwesternUnited States, with cold winters (average = –11 °C ) andmild summers (average = 20 °C). Annual precipitationand evapotranspiration are approximately equal (Winter& Woo, 1990); however, the long-term precipitationaverage of 677 mm yr–1 is sufficient to support a mixeddeciduous forest around the lake.

Late-glacial environment

The late-glacial environment of western and centralMinnesota differed significantly from that of thepresent. Despite maximum insolation values, it ispostulated that seasonality was reduced (Bryson &Wendland, 1967), because the presence of the ice sheetsto the north decreased summer temperatures and

blocked frigid Arctic air during the winter (Wright,1987). The cool summer temperatures and decreasedevapotranspiration resulted in high lake-standsregionally (Kennedy, 1994; Schwalb et al., 1995; Lairdet al., 1996) and a vegetation assemblage that has nomodern analogue (Cushing, 1967). This assemblage,recorded in the late-glacial sediment of Big Watab, isdescribed as an open Picea (spruce) parkland in whichlarge groves of spruce and larch were separated byopenings dominated by Artemisia (a heliophiloustaxon) (Cushing, 1967). The lake at this time wasnewly formed, and the deepest basin had at least 12 mless sediment, making it approximately 48 m deeprelative to modern lake levels.

Methods

Core retrieval and preparation

Three cores (A, B, C) were retrieved from the deep basin(Figure 1) during the winter of 1992, with a square-rodpiston corer (Wright, 1967). Individual drives, 1 meterlong, were overlapped between adjacent cores to insurerecovery of the entire sedimentary sequence. The coreswere split longitudinally and stored in a refrigeratedfacility (3 °C), where they were allowed to oxidizewithout drying. After oxidation the entire sedimentarysequence was photographed in 10-cm sections to aidin sampling and archiving. The laminated structure ofthe sediment allowed unambiguous correlation betweencores A and C, which were located at least 15 metersapart.

Only the bottom 70–90 cm of the core were preparedfor thin section, mineralogical, stable-isotopic, andpollen analyses. This stratigraphic focus was necessaryto establish a high-resolution record of the spruce/pinetransition. Smear slides for pollen analysis were usedto identity the depth of this transition, and the subsequentsampling was based on these results.

Thin sections

Standard petrographic and polished thin sections weremade of the bottom 90 cm of core following the techniqueof Clark (1988). Sections of sediment, six-cm long, weredehydrated with acetone (Card, 1994) and embedded inSpurr’s® epoxy (refractive index = 1.54). The embeddedblocks were cut into thin-sections of standard thickness.A duplicate set of polished sections was made forelectron microscopy.

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Mineralogy

The mineralogy of the bottom 70 cm of sediment wasdetermined by both standard x-ray diffraction (XRD)and electron microscopy. XRD was carried out on 13samples (approximately every 5 cm) of bulk wet sedimentto reduce preferential orientation of minerals duringdrying. The x-ray sweeps were made from 5 to 65 ° witha 0.05 ° step interval using CuKα radiation. The relativeconcentrations of each mineral are based on peakheights as the data were not standardized. Scanningelectron microscopy, energy dispersive x-ray analyses,and electron microprobe analyses were also conductedon randomly selected grains in the polished thin

sections of the core. The electron microprobe work wasdone on a JEOL 8900 electron microprobe with 15 Waccelerating voltage and 20 ηA probe current. Quanti-tative elemental chemistries are based on standardizationto Smithsonian Institute reference minerals: calcite(#136321), dolomite (#10057), and siderite (#R2460).

Oxygen and carbon isotopes

Twenty-five samples were collected at irregular intervalsover the bottom 70 cm using a square-base pollensampler with a volume of 0.5 ml. The sediment wasfreeze-dried and treated with dilute H

2O

2 to remove

organics. CO2 gas was extracted following the technique

Figure 1. Map of Minnesota showing the modern distribution of vegetation (after Marschner, 1930). White dot with ‘X’ indicateslocation of Big Watab Lake. Inset: Bathymetric map of Big Watab Lake, Stearns Co. Grey dot indicates location where cores A, B,C were extracted.

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outlined by McCrea (1950) and analyzed on a FinniganMAT delta E multi-collector mass spectrometer. Analysesare reported as ‰ relative to the PeeDee Belemnite(PDB) standard.

Pollen

Samples of sediment, 0.5 ml in volume, collected at thesame intervals as the isotopic samples, were preparedfor pollen analysis following the procedures outlinedby Faegri et al. (1989), including the addition of 0.5–1.0ml of a Eucalyptus pollen suspension of known con-centration (6.45 × 104 grains ml-1). Picea pollen wasidentified qualitatively as either P. mariana (blackspruce) or P. glauca (white spruce) following thetechnique of Hansen & Engstrom (1985). Pollen zona-tion was accomplished by stratigraphically constrainedcluster analyses (Grimm, 1987) of all pollen taxa ex-ceeding 2% of any one sample, using Edwards andCavalli-Sforza chord distance as the measure of dis-similarity. Pollen data are reported as percentages andnot as absolute fluxes.

Multiple regression model

A multiple regression model using the statistics program,XLISP-STAT 2.0 (Tierney, 1989), was created to determinethe degree of correlation between pollen types and theδ18Ο of the sediment. By choosing different taxonomiccomponents we were able to estimate the simplest forestmodel required to give us the highest coefficient ofdetermination (r2) with a 95% confidence interval. Wethen did a backward elimination test to remove taxa thathad the least effect on the r2 value.

Chronology

The sediment discussed in this paper is late-glacial toearly Holocene in age. A basal age of 9910 ± 70 14C BP(11,010 cal yrs BP) (CAMS # 17042) is derived from acharred spruce needle collected at 1366 cm. Thegeneral timing of this period can also be based uponthe abrupt (< 500 yrs) decline of Picea, which occurredin Minnesota between 10.5–10.0 ka 14C yr BP (seeWhitlock et al., 1993).

Results

The late-glacial sediment consists of alternating bandsof laminated and massive sediment with an organic

carbon content greater than 10%. The laminations areconsidered to be annual on the basis of the highconcentrations of chrysophycean cysts at the transitionfrom the light summer layers to dark winter layers(Olson, 1993). Lack of diatoms, ostracodes, or othermicrofossils used by Peglar et al. (1984) preventedstronger confirmation of the seasonality of the layers,but the presence of distinct, repeated carbonate lam-inations is suggestive of annual carbonate depositionduring spring and/or summer (Saarnisto, 1986). Thelaminated sections are orange to brown in color (colorvaried with the degree of oxidation) and are interpretedas varves composed of summer calcite layers, falllayers of crysophyte cysts, and winter layers of fineorganic matter. The laminations vary in quality andhave been assigned descriptors based on lateralcontinuity in each core. In general, the laminations arewell defined but only 0. 1 to 0.3 mm thick. Laminatedsections represent intervals 20 to 50 yrs in duration.Core sections that have massive layers 1–2 mm thickinterspersed among well-defined varves are considered‘moderately laminated’. The ‘poorly laminated tomassive sections’ are typically greyish brown andinclude lenticular or partially disturbed laminations andmassive sediments. These sections range in thicknessfrom 0.4 to 5 cm. Contacts between the laminated andmassive sections are often sharp, although transitionalsections of poorly defined laminations may occurbetween these two main units.

The carbonate mineralogies of the laminated andmassive sections differ significantly. With only oneexception, samples with abundant calcite and minoramounts of Mn-carbonate are ‘poorly laminated tomassive.’ Moderate to well laminated sections havehigh concentrations of Mn-carbonates, moderate tominor amounts of calcite, and gypsum. The Mn-car-bonates have a d-spacing consistent with kutnahorite[(Ca

0.5 Mn

0.5) CO

3], although ordering of the minerals

was not established.Because kutnahorite is a constituent of local meta-

morphic complexes (McSwiggen et al., 1995), it wasnecessary to determine if the Mn-carbonates in thelaminated sections originated from endogenic or de-trital sources. Electron microscopy of polished thinsections determined the precise composition andrelationship of the Mn-carbonate to the other mineralphases (i.e., was it preferentially concentrated atcertain levels). Backscatter images indicated that thesediment was composed mainly of small euhedralcalcite crystals, ranging in size from 5–15 µm. In welllaminated sections the calcite crystals may be more

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Figure 2. Back-scatter electron image of authigenic calcitecrystal (dark core) with a Mn-carbonate overgrowth (brightrim). Scale bar = 1 micron.

irregular in shape and have secondary overgrowths ofa refractive mineral (Figure 2) identified as a Mn, Cacarbonate. The overgrowths are similar to ‘crusts’described by Pedersen & Price (1982) but occur in morediscrete units.

The overgrowths of 15 grains were analyzed for el-emental composition with the JEOL electron micro-probe. The grains were randomly selected from 5laminations between 1370 and 1390 cm depth. Manyovergrowths had diameters less than 1 µm thick (the

width of the electron beam), which made it difficult toobtain discrete analyses of the overgrowth com-positions. Of the 15 analyses, one was discarded be-cause of overlap onto the calcite grain by the beam,and a second was rejected as a result of an unac-ceptably low cation to anion ratio of 0.64. The molefractions of Ca, Mg, Mn, and Fe for the remainingthirteen grains are reported in Table 1. The elementalcompositions of these rims varied by nearly 10%, withan average composition of [(Ca

0.31 Mg

0.007 Mn

0.671

Fe0.01

)].The pollen data are divided into four assemblage

zones, BWP1-BWP4 (Figure 3). The basal zone, BWP1,is dominated by Picea mariana (black spruce), Fraxinus(ash), and Artemisia pollen. An increase in Picea pollenpercentages coincides with a decrease in Artemisia andAmbrosia (ragweed). The low values (10%) of Betula(birch) pollen may be due to long-distance transport(Bradshaw & Webb, 1985). BWP2 is interpreted as aPicea-Larix (larch)-Artemisia assemblage. There is adrop to 20% Picea pollen towards the top of BWP2,driven largely by the decline in P. mariana pollenvalues. P. glauca (white spruce) pollen abundanceremains nearly constant. Pollen values of the prairieforbs (Artemisia and Ambrosia) remain high, whileBetula pollen reaches values of 27%. Zone BWP3 isinterpreted as a Betula-Fraxinus-Quercus (oak) as-semblage and represents a brief transitional periodbetween the late-glacial spruce forest and early Holo-cene pine forest. During this period Picea values declinebelow 10%. The hardwood component consists of highpercentages of Quercus, Fraxinus, and Ulmus (elm).Zone BWP4 is a Pinus (pine) – Betula-Ulmus assem-blage. Pinus pollen rises abruptly in this zone to valuesbetween 30 and 45%. The continued high levels ofBetula indicate that pine and birch were probablyregional co-dominants when sediments of BWP4 weredeposited.

Twenty-five samples taken between 1318 and 1390cm depth (Figure 3) were analyzed for stable-isotopiccontent. The δ18O values show variations of nearly 5 ‰over stratigraphic intervals as small as 2 centimeters.The δ18O fluctuations are mirrored by even greater (8 ‰variations in δ13C. The δ18O and δ13C values also exhibita slight negative covariance (Figure 4). A comparisonbetween the isotopic values and the sedimentarystructure (laminated versus massive) of each sampleshows a general correlation between high δ18O (lowδ13C ) values and the well-laminated sections of core.Low δ18O values (high δ13C) are associated with massiveto poorly laminated intervals.

Table 1. Molar values of Ca, Mg, Mn, and Fe for 13 of 15measured calcite overgrowths

Cation/Grain Mole Ca Mole Mg Mole Mn Mole Fe Anion

1 0.389 0.006 0.599 0.005 1.102 0.226 0.006 0.756 0.013 0.903 0.253 0.005 0.729 0.013 0.834 0.319 0.006 0.667 0.008 0.976 0.305 0.005 0.682 0.008 0.807 0.226 0.002 0.762 0.010 0.788 0.509 0.014 0.471 0.005 0.959 0.411 0.009 0.570 0.009 0.8411 0.256 0.004 0.728 0.012 0.8112 0.267 0.005 0.720 0.008 0.7913 0.446 0.009 0.535 0.010 0.8614 0.220 0.019 0.742 0.020 0.8015 0.229 0.001 0.763 0.007 0.90

Average 0.312 0.007 0.671 0.010Value

Standard 0.096 0.005 0.096 0.004Deviation

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Figure 3. Depth profiles of percentage abundance for selected pollen taxa, δ18O, and δ13C for the late-glacial period (basal age =9910 ± 70 14C yr BP) . Pollen values are reported as relative percentages and are divided into four zones (see text).

Discussion

We base our discussion of the relationship betweencatchment vegetation and lake stratification on twoassumptions. The first is that the presence of Mn-car-bonate in the sediment is a good proxy for lake strat-ification. The second assumption is that the δ18O valueof the sediment corresponds to relative Mn-carbonateconcentration at that level. This latter assumption issupported by Figure 5, which shows the correlation (r2

= 0.78 and 0.68) between the δ18O and δ13C values and

the relative amounts of calcite to Mn-carbonate. Therelative amounts of each mineral were determined bytaking the ratio of the relative peak heights. The methodis only semi-quantitative.

The following discussion begins with a review ofthe constraints on Mn-carbonate formation. Theseconditions are then linked to lake processes that canproduce a laminated stratigraphy. Finally, the internallake dynamics are examined with respect to changesin upland vegetation.

Formation of Mn-carbonates

The chemical conditions necessary for the formationof Mn-carbonates include several variables. The firstis a sufficient concentration of reduced manganese.Manganese can originate from three sources: recyclingof Mn-oxides in the sediment column, humic complexes(Jones & Bowser, 1978), and from dissolution of Mn-richbedrock. Of these, the recycling of Mn-oxides is themost common source of manganese and was originallydescribed by Lynn & Bonatti (1965) for marinesediments. Recycling of Mn-oxides involves low redoxpotentials from anoxia, typically caused by theconsumption of oxygen during degradation of organicmatter. The resulting anoxia reduces Mn4+ to Mn2+, whichis mobile in aqueous environments. Upward diffusionconcentrates manganese in the surficial sediments,where higher redox potentials frequently cause theprecipitation of Mn oxides. Upon burial, the Mn-oxidesredissolve, and the process begins anew. In certaininstances Mn-carbonate will precipitate. This does notnecessarily require the Mn to diffuse to the sediment/

Figure 4. Inverse covariance of δ13C versus δ18O for thelate-glacial period. Filled squares represent well-laminatedsediment; stars, poorly laminated sediment; filled dots, massivesediment; and half-filled squares, half laminated/half massivesediment.

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water interface, but only to a level with high bicarbonateconcentrations. However, it is frequently the case thathigh bicarbonate concentrations coincide with thesediment/water interface, where microbial activity is high.

Deep lakes in temperate climates differ slightly fromocean basins in that anoxia is common twice each year,during summer and winter stratification. Dean & Megard(1993) suggest that rhodochrosite in Elk Lake, ClearwaterCounty, Minnesota precipitates during spring and fall

overturn, when Mn-rich bottom waters mix withoxygenated and alkaline surface waters. In fact theEh-pH stability field for rhodochrosite (Mn,Ca)CO

3) is

commonly reached in lakes with sufficient Mn (Jones& Bowser, 1978), and several other studies havereported pore waters saturated with respect torhodochrosite (Robbins & Callendar, 1975; Pedersen &Price, 1982; Sawlan & Murray, 1983; Dean, 1993).However, impure Mn-carbonates are more commonlyreported than pure rhodochrosite. Unfortunately thecalculation of saturation coefficients for these mixedphases requires corrections for the solid solutions,which are not currently available (Pedersen & Price,1982; Sawlan & Murray, 1983).

The second criterion for the precipitation ofMn-carbonates is sufficient alkalinity. The degradationof organic matter, which lowers redox potentials, alsotends to increase CO

2 levels, which are supported by

the cold temperatures at the lake bottom. If CO2 levels

become too high, the decrease in pH will be unfavorablefor the preservation of authigenic calcite and forformation of secondary carbonates. This effect may becounterbalanced by an increase in bicarbonate in thesediment, resulting from several common reactionsinitiated by the decrease in redox potential. At Eh ofless than 0.35 volts, NO

2 will reduce to NH4+, which

combines to form ammonium bicarbonate. Some authorsconsider this reaction to be one of the most importantmeans of generating significant bicarbonate levels inthe sediment (Sawlan & Murray, 1983). At even lowerredox potentials (Eh = 0.06) and no free oxygen, sulfateis reduced to sulfide by common bacteria, such asDesulfovibrio desulfuricans (Cole, 1983).

These reactions have two major consequences thataffect the formation and characteristics of. Mn-carbonates.First, each reaction produces HCO

3–, which acts to raise

the pH. If significant alkalinity is produced, the porewaters can become supersaturated with respect tocalcite and other carbonates. Second, the carbonincorporated into the HCO

3– molecule is derived from

the decay of isotopically light organic matter in thesediment. Organic matter in lakes generally is com-posed of algae, which preferentially remove 12C fromthe epilimnion. Thus carbonates precipitating insediment pore-waters will reflect the lower δ13C valuesof the algal matter.

The need for excessive bicarbonate has beenquestioned by Pedersen & Price (1982), who report noincrease in alkalinity in Mn-carbonate-bearingsediments from the Panama basin and Loch Fyne,Scotland. Both sites also had little organic carbon that

Figure 5. Plots of a) δ18O against the ratio of XRD peakintensities for calcite (Ic) and Mn-carbonate (Im) δ18O and b)δ13C against the log(Ic/Im). Symbols are the same as in Figure 4.

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could serve as a source for CO32- . Big Watab differs

from these and many ocean sites, in that it hasorganic-rich sediment (approximately 10% by weight).Thus, we postulate that Big Watab is similar to theLandsort Deep in the Baltic Sea, where Suess (1979)showed that the Mn-carbonate phase had lower δ13Cthan the foraminifera and thus that the carbon resultedfrom bacteria-generated alkalinity. The absence ofdiatom frustules in the late glacial sediment of BigWatab is also consistent with high alkalinity, becausediatoms often dissolve in alkaline solutions (Battarbee,1986).

Although we suggest that high alkalinity plays acritical role in the precipitation of Mn-carbonates inlake systems, we do not agree with the model proposedby Dean and Megard (1993) in which the alkalinityoriginates in the epilimnion and is brought to depthduring overturn. We believe microbial processes are amore likely source, and that Mn-carbonates describedin lake sediments are the results of bacterial mediation.This assumption is based in part on the relationship ofthe Mn-carbonate to the calcite in the Watab sediments.The hummocky coatings around epilimnetic calcitegrains (Figure 2) suggest that microbes are using themas substrates on which to grow. The development ofMn-carbonate crusts on calcite grains can be likened tothe sheaths of calcite that develop around littoral vege-tation, such as Chara (green alga). However, the bestevidence for the importance of bacterial reduction comesfrom the sediment-trap study at Elk Lake, ClearwaterCounty, Minnesota (Nuhfer et al., 1993), where rhodo-chrosite was found in both deep and shallow-water trapsbut not in the trap treated with formalin. We suggest thatthis is because the formalin inhibits the bacterial growthnecessary for the production of HCO

3– and the subsequent

precipitation of rhodochrosite.

Occurrence of Mn-carbonates and mixing of thelake

The lower abundance of Mn-carbonates in the massivesections of sediment is likely a result of periods of oxianear the sediment/water interface that are sufficientlylong for the establishment of a benthic community andthus bioturbation. Higher redox levels associated withoxygen-rich periods would result in the oxidation ofMn2+ to Mn4+ forming manganese oxides, whichtypically lack a distinct x-ray pattern (Dean & Megard,1993). Higher redox also may inhibit nitrate and/orsulfate reduction. The laminated sections require theabsence of bioturbation and thus indicate seasonal

oxygen-deficiencies. This anoxia also initiates theprecipitation of Mn-carbonate by lowering the redoxpotentials. Thus, the formation of Mn-carbonates islinked to the stratification of the lake.

The stability of the water column can be linked tomany factors in the lake and catchment. It is temptingto interpret periods of varve preservation as being lesswindy or perhaps as having cooler summers thatresulted in the establishment of a weak thermocline.We suggest that the stratification of the water columnis linked to the type and distribution of the localvegetation, which may or may not be linked to climate.

Relationship of vegetation cover to Mn-carbonateformation

Using the δ18O composition (determined from samplescollected at the same depths as the pollen) as a proxyMn-carbonate concentration, the δ18O values werecorrelated with selected taxa to determine if a correlationbetween forest composition and lake stratification exists.Mn-carbonates are not restricted to a specific vegetationassemblage but occur within three different vegetationzones: the open spruce parkland, the deciduous forest,and finally the mixed pine-deciduous forest. Withinthese zones the stable-isotopic values tend to vary withchanges in the relative percentages of Artemisia (Figure3). Periods with high Artemisia values are accompaniedby massive to poorly laminated sediment, very littleor no Mn-carbonate, and relatively low δ18O values(high δ13C values). Periods with lower Artemisia per-centages and greater amounts of arboreal pollen areaccompanied by a well laminated stratigraphy, abundantMn-carbonate, and high δ18O values (low δ13C values).Significantly, no lag is evident between the vegetationand isotopic changes, suggesting that the Mn-carbonateprecipitated out in the upper few millimeters of sedimentand is not a later diagenetic feature of the core.Furthermore, it indicates that the connection betweenthe external disturbance and the response of the basinis rapid.

The correlation between vegetation compositionand Mn-carbonate production (Table 2) shows that allstatistically relevant taxa have a linear relationshipwith δ18O, except Artemisia, which was transformedto the inverse square-root of the pollen percentage. TheArtemisia alone accounted for 47% of the variance inδ18O values. The addition of Betula increased the r2 to0.63 and Fraxinus to 0.68. Each of these is negativelycorrelated with the δ18O values. The next two com-ponents, Larix and Picea mariana, raised the r2 to 0.85

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and are positively correlated with the δ18O values. Theaddition of Picea glauca, Pinus, and Populus com-ponents did not raise the r2 significantly and weredropped from the model assemblage.

Two possible scenarios may explain the correlationbetween vegetation and δ18O values. The first is thatboth the catchment vegetation and formation ofMn-carbonate were driven by the same external forcing,such as climate, but that the limnological changesnecessary for the Mn-carbonates were independent ofthe changes in vegetation. The second scenario suggestsa causal relationship between the changes in vegetationand the formation of Mn-carbonate. Both possibilitiesare examined in detail.

Within the first scenario, several sub-sets of conditionscan be invoked to explain the correlation. For example,long cold winters could extend the period of ice coveron the lake, prolonging the period of winter anoxia anddecreasing the amount and duration of spring mixing.This same temperature change could alter the vegetationin the catchment. But, the presence of Mn-carbonatesin multiple vegetation zones, which are characteristicof different temperature regimes, argues against this.Furthermore, it is not likely that long cold winterswould favor spruce growth over that of Artemisia.Artemisia has a broad geographical range, and growthis likely limited by sunlight, not temperature (West,1988). Another factor that could affect stratification isan increased flux of saline ground water causingchemical stratification. However, there is no evidencefor this in the geochemical composition of the sediment.

The second scenario calls for a causal relationship,of which there are several possibilities. The firstpossibility is that changing vegetation cover increasedbiological productivity in the lake, which in turncaused anoxia. Although we have no direct data onbiological productivity for Big Watab, we can drawcomparisons with Elk Lake, Clearwater County,

Minnesota. During the late-glacial, Elk Lake wassimilar to Big Watab in geographical location, depth,substrate and vegetation cover. Pigment analyses of thesediment of Elk Lake indicate that the lake wasoligotrophic during the late-glacial with the lowestpigment concentrations of its history (Sanger & Hay,1993). A second possibility is that an increase in rainfallresulted in the expansion of boreal conifers and greaterinflux of humic matter, which consumes abundantoxygen during degradation (Wetzel, 1983). Thispossibility is supported by the work at Elk Lake, wherehigh values of chlorophyll derivatives to carotenoidsare interpreted as a significant influx of detrital organicmaterial. However, laminations also coincide withintervals of deciduous vegetation, during which therewould be decreased humic influx. Mn-carbonates havealso been described during the prairie period at LakeMina, Minnesota, 90 km northwest of Big Watab(Stevens, 1997). Without further evidence, however, weare unable to completely dismiss this hypothesis.

We suggest that the most likely hypothesis is thatthe density of vegetation cover affected the degree ofmixing and stratification of the lake (Figure 6). In deeplakes with large aspect ratios (~0.04), if the epilimnionis small relative to the hypolimnion (i.e., the thermoclineis shallow), density-driven mixing from thermalinstability may not extend to the sediment/waterinterface. This may be alleviated by strong surfacewinds, which provide additional kinetic energy to mixthe lake (Imboden & Wüest, 1995). Wind can alsoinitiate internal waves in the water column by pilingwater on the leeward end of the lake (Wetzel, 1985). Inaddition, cooler summer temperatures, 2�4 °C lowerthan today, (Bartlein & Whitlock, 1993) would havereduced the thermal gradient between hypolimnionand epilimnion, resulting in a shallow and unstablethermocline. Therefore, stratification would providelittle resistance to wind energy during periods when

Table 2. List of least squares estimates (pollen values are predictors, δ18O is response) and the resulting r2 with sequential removalof certain taxa. Taxa critical to the correlation are in bold face

Populus Picea undiff Pinus dipl. P. glauca P. mariana Larix Fraxinus Betula Artemisia r2

Estimates 0.013 �0.038 �0.029 �0.058 �0.066 0.554 �0.219 �0.168 �0.587 0.868�0.039 �0.031 �0.059 �0.067 0.546 �0.223 �0.169 �0.594 0.868

�0.022 �0.059 �0.061 0.555 �0.218 �0.162 �0.598 0.862�0.038 �0.042 0.585 �0.188 �0.146 �0.555 0.855

�0.050 0.520 �0.172 �0.134 �0.593 0.8450.341 �0.122 �0.085 �0.581 0.777

�0.086 �0.064 �0.585 0.687�0.076 �0.577 0.631

�0.471 0.470

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the lake was exposed to strong wind shear. Withoutstrong surface winds, density-driven circulation and ashallow thermocline may have been insufficient tocompletely mix the lake.

Mixing, stratification, and thus the formation ofMn-carbonates would then correspond to the compositionof the forest in the surrounding uplands. During thelate-glacial, the landscape of central Minnesota was

dominated by an open spruce parkland, an assemblagewith no modern analog (Cushing, 1967; Wright, 1987).The spruce parkland is probably comparable to modernaspen parklands, with groves of trees separated by openground that supported heliotrophs, such as Artemisia.Artemisia cannot survive in the understory of spruceforests because of the closed canopy. Because Artemisiais a low-lying herb, its expansion in the catchment and

Figure 6. Cartoon showing the effects of catchment vegetation on lake mixing.(a) Open vegetation with abundant Artemisia resultsin greater windshear, mixing to depth, and an unstable thermocline. Redox boundary is below the sediment/water interface: little orno Mn-carbonate formation; (b) Closed-canopy forest results in less windshear, a stable thermocline, and summer mixing only inthe epilimnion. Redox boundary is above sediment/water interface; abundant Mn-carbonate formation.

209

along the shore would result in greater exposure of thelake to wind energy. This would have two effects. Thefirst would be to promote a shorter period of ice-cover,which typically results in anoxia, as wind contributesto the break up of ice. The second would be that theresultant mixing within the lake would have advectedoxygen to depth and allowed the establishment of abenthic community. A drop in the Artemisia pollenpercentages signals the closure of the forest or grovearound the lake. In comparison to Artemisia, Picea hasa closed canopy, which would create a robust windbreak that would reduce wind speeds over a portion ofthe lake (Oke, 1987) and thus the kinetic energy bynearly a cube root. Protected from high winds by boththe steep western hill and the dense tree cover, the lakelikely remained ice covered for longer periods duringthe spring (i.e., late ice out), with possible weak thermalmixing during spring. These types of conditions(partial meromixis) have been reported for normallydimictic lakes, which occasionally skip a springcirculatory period (Wetzel, 1985).

With the decline in Picea in pollen zones 3 and 4(Figure 3), the importance of other boreal trees, such asLarix, increased. For example, the sharp increase in δ18Oat 1356 cm (BWP3) corresponds to a relatively largeincrease in Larix, which likely explains its importancein the statistical model (Table 2). Larix, like Picea, has aclosed canopy compared to Artemisia and Ambrosia,which would reduce near-surface wind speeds. The roleof the deciduous trees, Betula and Fraxinus, in themodel calculation is less clear. In general, these taxacorrespond with low values of δ18O. Which species ofFraxinus is present might be important as F. penn-sylvanica (green ash) is an upland species.

The arrival of Pinus and increase in Ulmus andQuercus at the site represents the establishment of themixed-deciduous forest, with the accompanying closedcanopy. Within this zone δ18O values are generally highand Mn-carbonate present, suggestive of stablestratification.

Effects of Mn-carbonates on paleoenvironmentalindicators

In the late-glacial record of Big Watab, the extreme andclosely spaced variations in δ18O values are inconsistentwith other records produced for the same time periodin the Midwest (Dean & Stuiver, 1993; Schwalb et al.,1995; Hu et al., 1997; Stevens, 1997; Schwalb & Dean,1998). The variability in the Big Watab isotopic recordis due to changes in the carbonate mineralogy. Al-

though no experiments have been conducted to deter-mine the fractionation factors for oxygen and carbonin Mn-carbonates at low temperatures, our data suggestthat the δ18O value of the Mn-carbonate in Big Watabis approximately 3 ‰ higher than that of calcite formedin the same water. However, controlled experimentsare necessary to determine the exact fractionationdifference. The implication for other isotopic recordsis that a shift in δ18O could be mistaken for rapidfluctuations in temperature or evaporative concentrationrather than simply mineralogy. This study illustratesthe need to check all bulk samples for mineralogy.

Summary and conclusions

During the late-glacial period, the vegetation of centralMinnesota was dominated by a spruce parkland, whichwas succeeded by a closed-canopy mixed-deciduousforest. In the catchment of Big Watab Lake, the periodicexpansion of heliophilous vegetation, such as Artemisia,opened the lake to wind shear that enhanced turbulentmixing. This mixing brought oxygen to the deep basinand permitted a benthic community to becomeestablished. The related bioturbation of the sedimentdestroyed the seasonal layering (varves). During theseperiods, the δ18O values for calcite precipitated in theepilimnion are relatively low, –6 to –4 ‰, and areconsistent with isotopic values reported for otherlate-glacial records in the Midwest (Dean & Stuiver,1993; Hu et al., 1997; Stevens, 1997; Schwalb & Dean,1998). The δ13C values are relatively high, reflectingenrichment of the dissolved inorganic carbon pool dueto preferential uptake of 12C by algae.

Wind shear was reduced whenever Picea or Larixexpanded to close the canopy around the lake. Withoutthe added energy, differences in water density due tothermal gradients were insufficient to mix the lakecompletely or did so for only short periods of time. Lowoxygen levels prevented bioturbation, and annualcalcite/organic couplets were preserved. Anoxia alsopromoted nitrate and/or sulfate reduction by bacteriain the pore water and a lowering in redox potential. Theredox potential resulted in the reduction of Mn4+ andsubsequent migration of Mn2+ to the sediment/waterinterface. Anaerobic activity increased HCO

3– con-

centrations and supersaturated the pore water withrespect to carbonate minerals. Mn-carbonate precipitatedout as a result, coating the epilimnetic calcite grainsthat had settled out of the water column. The isotopicfractionation factor for Mn-carbonates is believed to

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be greater than calcite. Thus the δ18O signature reflectsthe presence of Mn-carbonate as isotopically higher(–3 to –1 ‰ PDB) values. The decomposition ofisotopically low organic matter resulted in pore waterwith extremely negative δ13C. This carbon was usedduring the precipitation of Mn-carbonates and isreflected in samples that are isotopically lower (–6 to–9 ‰) than those in the massive sections of the core.Detailed mineralogical analysis is suggested for allresearch that relies on stable-isotopic data.

Acknowledgments

We kindly thank A. Itkonen and C. Yansa for theircareful and thoughtful reviews. We also thank S. Fritz,H. E. Wright, Jr., and W. M. Last for their comments. R.McEwan, P. McSwiggen, and D. Slawinski providedassistance with the isotopic measurements, the electronmicroscopy, and statistical analyses, respectively. 14Cdating was provided by Lawrence Livermore. Partialfunding for this research came from NSF-Research inTraining Grant: Paleorecords of global change:understanding the dynamics of ecosystem response.This manuscript is LRC contribution #522.

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