Late Quaternary paleohydrology in the North American Great Plains inferred from the geochemistry of endogenic carbonate and fossil ostracodes from Devils Lake, North Dakota, USA

PALAEO ELSEVIER

Palaeogeography,Palaeoclimatology,Palaeoecology124 ( 1996) 179-193

Late Quaternary paleohydrology in the North American Great Plains inferred from the geochemistry of endogenic carbonate and fossil ostracodes from Devils Lake, North Dakota, USA Brian J. Haskell, Daniel R. Engstrom, Sherilyn C. Fritz Lirnnologieal Research Center, University of Minnesota, 310 Pillsbury Drive S.E., Minneapolis, MN 55455, USA Received 28 February 1995; revised and accepted 22 January 1996

Abstract

Devils Lake, North Dakota is a closed-basin lake in which water chemistry is responsive to hydrological changes, making it a valuable resource for documenting paleoclimate conditions. Ostracode trace-element and bulk-carbonate geochemistry in a core are used to generate a 12,000-year record of salinity fluctuations. The relative amounts of Sr, Ca and Ba in the bulk-carbonate fraction can be linked mechanistically to changes in sediment mineralogy (calcite/ aragonite) and/or solute chemistry, whereas the elemental geochemistry of ostracode valves reflect solute chemistry. Associated minima in Sr/Ca, and maxima in Mg/Ca, in ostracode valves from the Devils Lake core may be due to aragonite mediation of Sr chemistry during the precipitation of multiple mineral phases during more saline intervals. The Devils Lake record can be divided into four hydrological periods on the basis of the geochemical proxies: (a) 12.5-9 k.y.: Low salinity; (b) 9.5-4.5 k.y.: Fluctuating, saline conditions with a maximum at about 8 k.y.; (c) 4.5-3.5 k.y.: Low salinity phase, but probably not as fresh as at 12.5-9 k.y.; (d) 3.5 k.y.-Present: Saline phase, although generally less so than 9.5-4.5 k.y. However, a large degree of variability characterizes the entire record, suggesting the absence of long-term climatic stability in this region.

1. Introduction

Devils Lake, North Dakota (48°05'N, 98°56'W) lies along the eastern margin of the northern Great Plains, a physiographic region in the north central USA and southern Canada. The interaction of warm moist air from the Gulf of Mexico, cold Arctic air, and dry Pacific air (Bryson, 1966) produces a sub humid-semi arid regional climate. The net negative moisture balance, in combination with saline groundwater sources, results in the presence of shallow saline lakes in which the hydrogeochemistry is strongly influenced by climatic control of the evaporative concentration of 0031-0182/96/$15.00© 1996ElsevierScienceB.V. All rights reserved SSDI 0031-0182(96)00002-8

lake water solutes. Hence, sediment components generated within the lake environment serve as sensitive proxies to climate change. The agricultural importance of the northern Great Plains emphasizes the need to understand regional hydrological response to both natural and anthropogenic influence upon the climate system. The dominance of prairie vegetation during most of the Holocene period has made interpretation of the paleoclimatic conditions difficult using palynological approaches (Barnosky et al., 1987), which have been the mainstay of climate research in areas to the east (e.g. Webb III et al., 1987; COHMAP, 1988). This and other studies (e.g. Engstrom and Nelson, 1991;

180

B.J. Haskell et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 124 (1996) 179-193

Fritz et al., 1991; Chivas et al., 1993; Fritz et al., 1994) focus instead on using a variety of paleolimnological indicators to document hydrochemical changes in saline lakes. Devils Lake is one of a confluent series of lakes in a 9800 km 2 closed drainage basin in northeastern North Dakota (Fig. 1). Present-day Devils Lake is a relatively shallow (< 10 m), flat-bottomed lake that occupies a depression produced by glacial thrusting of underlying Cretaceous shale (Aronow, 1957; Clayton and Moran, 1982). Brine evolution along the flow path through the series of lakes (Jones and Van Denburgh, 1966) results in Devils Lake water being dominated by Na, Mg and SO4. Historical records from the last ca. 50 years document salinity varying between 1%o and >30%0, and Mg/Ca from 2.9 to > 19, largely driven by climate influenced changes in hydrological balance (Engstrom and Nelson, 1991). This paper explores paleoenvironmental information gained from the elemental geochemistry of Devils Lake sediments, particularly that associated with endogenic and biogenic carbonate minerals. Recent studies have demonstrated the potential of ostracode trace-element and stable isotope geochemistry in reconstructing hydrological changes in saline lakes (Engstrom and Nelson, 1991; Chivas et al., 1993). The study of bulk carbonate geochemistry complements the ostracode record, because it provides an assessment of solutes being precipitated from the lake versus solutes remaining

108ow:

96°Wi

Cana : Fig. 1. Location of Devils Lake in the northern Great Plains.

at the time of ostracode shell formation. Bulk carbonates thus provide mechanistic linkages between ostracode shell chemistry and brine evolution during the early stages in lake evaporation when carbonate precipitation can play a key role in driving changes in water chemistry. Carbonate mineralogy also may change in response to the environmental conditions, thus acting as an independent indicator of changes in the lake environment. Our recent study of a short core from Devils Lake encompassing the past 500 years indicated strong coherence between bulk carbonate geochemistry and other paleoenvironmental proxies (Fritz et al., 1994).

2. Methods

The 24 m long core used in this study was recovered in 1985 from 7.5 m of water in Creel Bay at the north end of the main basin of Devils Lake. Coring was conducted from the winter ice surface with a modified Livingston corer supplemented by casing, chain-hoists, and other equipment required to penetrate a thick sequence of stiff organic sediments (Wright, 1980, 1991). Diatom analyses (Fritz et al., 1991) were performed on point samples, and the bulk-carbonate geochemistry samples integrated 2-cm sections of core overlapping with the diatom samples. Ostracode samples came from 8-cm sections adjacent to the diatom and bulk carbonate samples. The determination of elemental composition in bulk carbonate used the following procedure: Dried sediments were ground to a fine uniformsized powder with a mortar and pestle, and 0.1 g samples were placed in 50 mL acid-washed, plastic centrifuge vials. The samples were leached with 40 mL of 0. l 5 M acetic acid at room temperature for 10 minutes, and centrifuged at 2700 gravities for 5 minutes. This mild extraction technique was used to minimize element leaching from dolomite and non-carbonate mineral phases. Aliquots of 20 mL were removed and stored under refrigeration in acid-washed plastic bottles until analysis by DCPAES (direct current plasma atomic emission spectrometry). Carbonate mineralogy was analyzed on selected samples by x-ray diffractometry to

B.J. Haskell et al./Palaeogeography, Palaeoclimatology, Palaeoecology 124 (1996) 179-193

examine major associations between sediment mineralogy and chemistry. Fossil ostracodes were processed according to methods outlined in Engstrom and Nelson (1990). Individual valves of Candona rawsoni, a ubiquitous species in Devils Lake (Van Alstine, 1980), were picked under magnification from sieved sediment samples and cleaned for 15 minutes in hot (90°C) 6% H 2 0 2. A subsample of 5-16 cleaned adult valves was selected under magnification and dissolved in 10 ml of 0.5 M ultrapure HC1 in acidwashed plastic vials. Only those shells that were thoroughly clean and without discoloration or evidence of dissolution were used for analysis. Analytical determinations of Ca and trace elements Mg, Sr, and Ba were made by DCP-AES. The amount of water, organic material, carbonate and non-combustible (terrigenous) material in the samples was calculated by loss-on-ignition (LOI). Samples were heated to 100°C, 500°C and 1000°C, respectively, and the amount of each fraction calculated from the weight loss. Carbonate content is expressed as calcium carbonate. Core chronology was based on four AMS radiocarbon ages from terrestrial macrofossils as presented in Fritz et al. (1991). Sediment ages were interpolated between dated intervals according to a second order regression curve fit to the radiocarbon dates, and are reported as radiocarbon years.

3. Results

Core lithology is predominantly grey silts, except the bottom 250 cm where there is a transition to coarse sand at the base of the core. Sediments overlying the basal sands are black in color and laminations are present from 1645 to 1890cm. Loss-on-ignition shows that organic material generally comprises about 7 wt.% of the dry sediment, carbonate about 23 wt.%, and non-combustible material (terrigenous and biogenic silica) about 70 wt.% (Fig. 2). 3.1. Bulk carbonate geochemistry Calcium, magnesium, strontium and barium The resemblance of the Ca, Sr, Ba (Fig. 3), and to a lesser extent, the Mg concentration curves, to

181

the carbonate abundance curve (Fig. 2) suggests that these elements were mainly incorporated in the carbonate minerals. Fig. 3 shows a good degree of congruence between Ca, Sr and Ba but a very poor relationship between these three elements and Mg. The use of acetic acid should minimize the extraction of Mg from dolomite and clay minerals, but the dissolution of even a small amount of dolomite will affect the Mg signal due to the large amount of Mg present in dolomite compared to calcite or aragonite. Because of uncertainty regarding the source of Mg leached by the acid, the element distribution patterns are discussed largely in the context of Ca, Sr and Ba. Element composition is also expressed as molar ratios to eliminate the effect of changes in the flux of individual sediment components on concentration profiles (Fig. 4). Ba/Ca ratios are in phase with Sr/Ca variations throughout the core including the development of a very pronounced peak between the bottom of the core and 2100 cm. The peak is a feature common to all the M/Ca profiles (where M =Mg, Sr or Ba) and appears to be the result of particularly low Ca values rather than very high trace element values. The Sr/Ba ratio shares the same major inflection points with Sr/Ca, but there are some significant differences in the development of trends within the core. Sodium and potassium Sodium concentrations strongly resemble the water content of the sediment when calculated on a dry weight basis (Fig. 5), suggesting that Na is present largely in the dissolved phase. Using the water content of the sediment from loss on ignition, the Na distribution was recalculated in an attempt to reconstruct the original concentration in the pore water at each sample depth in the core. Although potassium concentrations also decrease down core, details of the K distribution do not follow the water content as closely as Na, suggesting multiple sources of K. Iron and manganese Fe and Mn show relatively little variability except near the base of the core (Fig. 6). Relatively low levels of Fe and Mn are present in most of the core, except between 2000-2300 cm, with peaks

182

B.J. Haskell et aL /Palaeogeography, Palaeoclimatology, Palaeoecology 124 (1996) 179-193 Water Content (g/cc wet sediment) 0 10 20 30 40 50 60 70 80 90 100 ,,,IL,,,I

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profiles showing the general composition

in Fe and Mn occurring at 2300 cm and 2150 cm, respectively.

3.2. Ostracode geochemistry Both Sr/Ca and Mg/Ca ratios fluctuate throughout the profile and many of the individual fluctuations show corresponding events in both ratios, although the sense and the magnitude of the changes is not always the same (Fig. 7).

4. Discussion

4.1. Geochemical interpretation The evaporative concentration of lake water solutes leading to increases in salinity typically

of the Devils Lake core.

results in conditions favoring the precipitation of carbonate minerals. The selective removal of different elements by the carbonate minerals will change ionic ratios in the lake water and in the mineral chemistry of the carbonate precipitate. In general, ostracodes incorporate trace elements into their calcite shells as a direct funtion of the ratio of each element to calcium in the host water, and as geochemical "fossils" serve to document historic changes in lake water chemistry (Chivas et al., 1986; Engstrom and Nelson, 1991). Because ostracode shells represent a discrete source of low-Mg calcite, interpretations are not confounded by problems of mixed mineralogy as might be encountered in bulk sediment samples. The final ratio of Sr/Ca, Ba/Ca or Mg/Ca in the carbonate depends upon the distribution coefficient:

KD [M ] = ( M/Caminer,1)/(M/Caw,te0

183

B.J. H a s k e l l et al./Palaeogeography, Palaeoclimatology, Palaeoecology 124 (1996) 179-193

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Fig. 3. Weight percentage of Ca, Sr, Ba and Mg from a weak acid leach of bulk sediment. Covariance between Ca, Sr and Ba, and carbonate (Fig. 2) suggests these three elements are associated with the major carbonate minerals. The Mg curve differs and may be controlled by minor carbonate or non-carbonate phases. Age scale is calculated from AMS 14C ages (O) from Fritz et al. (1991).

(where M is a cation substituting for Ca in the carbonate mineral) for the element pair for each mineral phase, and the ratio of the elements in the water from which the carbonate is precipitated. Published distribution coefficient data for endogenic carbonates (Table 1) indicate that Mg/Ca, Sr/Ca and Ba/Ca ratios in calcite are all approximately an order of magnitude lower than the ratios in the solution from which the calcite precipitates (Veizer, 1983). Aragonite, on the other hand, favors the uptake of Sr and Ba over Ca (KD > 1) but still discriminates against Mg. In general the precipitation of either calcite or aragonite will result in an increase in Mg/Ca ratios in the water and in subsequent minerals precipitated from the

water. Sr/Ca and Ba/Ca ratios will also increase during the precipitation of calcite but may actually decrease when there is substantial aragonite precipitation. According to published distribution coefficients, Sr/Ba ratios in the water should increase slightly during carbonate precipitation, with either calcite or aragonite having about the same relative effect• Changes in the Sr/Ba ratio are therefore more likely to reflect changes in water chemistry irrespective of the carbonate species actually precipitated. Thus the Sr/Ba ratio should show similar patterns to Mg/Ca ratios in ostracode shells, which are also mineralogically insensitive. Lake water chemistry may in turn affect the

B .L Haskell et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 124 (1996) 179-193

184

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F i g 4 Bulk carbonate chemistry results expressed as molar ratios to eliminate the effects of flux variations on down core geochemical profiles Lines are 3-point running averages

mineralogy of the carbonate precipitate, providing another indicator of changes in the lake environment. An increase in lake-water Mg/Ca caused by carbonate precipitation during periods of higher salinity will favor the precipitation of aragonite over that of calcite (Mtiller, 1972). Because aragonite incorporates almost an order of magnitude more Sr and Ba than does calcite precipitating from the same water, Sr/Ca and Ba/Ca ratios in the bulk carbonate will reflect salinity both in terms mineral chemistry and in terms of the mineral ratios. However, the order of magnitude difference in KDrsr] and KDtBa] between aragonite and calcite suggests that Sr/Ca and Ba/Ca signals in the bulk carbonate are likely to be dominated by changes in mineralogy rather than mineral chemistry. Mg/Ca ratios in the water, on the other

hand, will continue to increase to approximately the same degree irrespective of the mineralogy of the carbonate precipitate, unless dolomite precipitation is involved. Devils Lake sediments contain a mixture of carbonate minerals, primarily aragonite, calcite and dolomite (Callender, 1968). Because of this, Sr/Ca and Ba/Ca variations in this study are interpreted to indicate primarily changes in carbonate mineralogy (aragonite/calcite ratios). These mineralogical inferences are substantiated by X-ray diffraction of bulk sediment samples from stratigraphic levels with high and low Sr/Ca ratios, which indicate that those levels with high Sr/Ca generally have high ratios of aragonite to calcite. The presence of aragonite needles in the sediment indicates that the bulk of the aragonite is

B.J. Haskell et al./Palaeogeography, Palaeoclimatology, Palaeoecology 124 (1996) 179 193 Wt.% K Bulk Carbonate 0.05 0.1 0.15 0.2

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Fig. 5. Weight percentage of K and Na normalized to dry sediment. Na content appears to be closely linked to the water content of the sediment (Fig. 2). Normalizing the Na concentrations to water content yields a good comparison with diatom-inferred salinity from Fritz et al. (1991).

precipitating from the lake water. Mg/Ca variations between 2.9 and 19 have been documented for Devils Lake water (Engstom and Nelson, 1991), which encompasses the Mg/Ca range of 2-12 suggested to favor aragonite precipitation over calcite (MUller et al., 1972). Although there is biogenic calcite in the sediment, ostracodes are not sufficiently abundant to account for much of the calcite, and their abundance in the core does not correspond to the geochemical signals. Some detrital calcite may get washed into the lake although a comparison of calcite Mg/Ca ratios in the lake sediments and in the surrounding tills indicates that most of the carbonate is precipitated from the lake itself (Callender, 1968).

4.2. Comparison of bulk carbonate geochemistry with ostracode geochem&try Mineralogical control of strontium in Devils Lake water The general distribution of Sr/Ca in the ostracodes does not closely resemble that of Sr/Ca in the bulk carbonates, except for the broad maximum at 4-9 k.y. (Fig. 7). Likewise there is no overall correspondence between the Mg/Ca ratios in ostracodes and either of the Sr/Ca curves, except below 8 k.y. where all show a sharp decline. However, there are some consistent patterns on a finer scale, with maxima in Sr/Ca ratios in the bulk carbonate and Mg/Ca in ostracodes

186

B.J. Haskell et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 124 (1996) 179-193

0.02 0

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corresponding to minima in the Sr/Ca ratios in the ostracode shells. This is particularly noticeable at 2.5 3.5 k.y., 6.2 k.y. and 7.5 k.y., although it may also be seen in other parts of the cores. The most likely explanation for the finer scale Sr/Ca and Mg/Ca relationships is that lake chemistry is buffered over the short term by aragonite precipitation. Peaks in the Mg/Ca curve indicate intervals of high salinity, with high Mg/Ca favoring the precipitation of aragonite over calcite as indicated by increases in Sr/Ca in the bulk carbonate. The onset of aragonite precipitation during high salinity events would have the effect of removing proportionately more Sr than the precipitation of calcite alone, and would cause Sr/Ca in the water to decrease. The reduction in Sr/Ca in the

lake waters would produce minima in the Sr/Ca in ostracodes corresponding to maxima in the other two curves. Actual low salinity intervals are indicated by periods when all three signals are in phase, as occurs at 3.5-4.2 k.y., and prior to 8 k.y. Although the timing of events is fairly consistent throughout the core, there are discrepancies in the magnitude and long-term trends among the different M/Ca ratios. It is quite likely that the relationship between mineralogy and lake chemistry is modified by a number of factors, and that carbonate mineralogy in Devils Lake is not a simple function of absolute Mg/Ca ratios of the water. Differences in core sampling intervals in the study of the various proxies, and spatial and temporal differences in the formation of endogenic

B.J. Haskell et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 124 (1996) 179-193

Sr/Ca Ostracodes (mole ratio) 0.0003 0.0008 0.0013 0 . . . . I . . . . I i

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Fig. 7. Comparison of ostracode and bulk carbonate element ratios. Maxima in ostracode Mg/Ca often have associated maxima in bulk St/Ca and minima in ostracode Sr/Ca (grey arrows) that could be explained by the mediation of lake Sr chemistry by aragonite. High ostracode Mg/Ca and bulk carbonate Sr/Ca are indicative of arid events. Lines are 3-point running averages. Table 1 Distribution coefficients (M/Ca, where M is a metal ion) for elements in calcite and aragonite (ranges shown in parentheses) (values from Veizer, 1983). Ko Aragonite/KD Calcite shows that both carbonate minerals remove Mg to the same extent, but aragonite is almost an order of magnitude more effective at removing Sr and Ba relative to calcite. The degree of enrichment of Sr/Ca and Ba/Ca in the water is therefore dependent upon mineralogy. Furthermore, KD> 1 will reduce Me/Ca in water, so aragonite precipitation may decrease St/Ca and Ba/Ca in the water but there will still be an increase in Mg/Ca. K D Sr/K D Ba shows that the precipitation of either mineral leaves the water enriched in Sr relative to Ba, and that the removal of Sr relative to Ba is also about the same KD Calcite

KD Aragonite

KD Arag./KD Calcite

Mg Sr Ba

0.013 0.06 0A3 0.25 (0.1-0.4)

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~ 1 ~10 ~ 10 (2.5-20)

Ko Sr/Ko Ba

0.52 (0.3-•.3)

0.7 (0.5-1.2)

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B.J. Haskell et al./Palaeogeography, Palaeoclimatology, Palaeoecology 124 (1996) 179 193

carbonate and calcification of ostracodes may account for some of the differences. Changes in the mineral ratios may therefore influence the short-term lake geochemistry that is then superimposed on the longer term changes of brine evolution, lake morphometry and groundwater sources and sinks.

half of the core and the uniquely low ratios at the bottom of the core. Excursions in bulk carbonate Sr/Ba are in phase with the ostracode Mg/Ca peaks, suggesting that unlike bulk carbonate Sr/Ca and Ba/Ca, bulk carbonate Sr/Ba is tracking water chemistry rather than carbonate mineralogy. Although in the upper half of the core the ostracode Sr/Ba trend does not conform with the Sr/Ba bulk carbonate trend which reflects lake geochemistry, both curves record the low salinity interval at 3.5-4.5 k.y. Barium geochemistry in natural waters is not well understood. It is possible that the mediation of Sr/Ba in the water over the past 4 k.y. was controlled by a non-carbonate phase (e.g. barite), or alternatively the uptake of Ba by

Bulk Sr/Ba as a m&eralogically independent indicator of hydrochemistry The Sr/Ba ratios in the bulk carbonate most closely resembles the patterns observed in the ostracode shell geochemistry, in particular the Mg/Ca ratios (Fig. 8). This coherence is most noticeable in the down core decrease in the upper

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Fig. 8. Three lake geochemistry proxies thought to be relatively independent of changes in carbonate mineralogy. There is general concensus between these 3 proxies and bulk carbonate Sr/Ca for a transition from fresh to saline at 12.5-8 k.y., and for a fresh interval at about 3.7 k.y. Interpretation of more arid intervals is also supported by the presence of the halophyte Ruppia maritima (Jacobson, unpublished data). Lines are 3-point running averages.

B.J. Haskell et al./Palaeogeography, Palaeoclimatology, Palaeoecology 124 (1996) 179 193

ostracodes is influenced by environmental parameters other than water chemistry. 4.3. Other geochemical indicators Sodium and potassium Several features of the sodium curve appear to correspond with events in both the carbonate geochemistry and biotic indicators (Fig. 5). This is particularly noticeable with the event at 8 ky which is a well defined period of arid, saline conditions in all the proxies. The Na record also shows close agreement with the diatom curve from Fritz et al. (1991) in the timing and, to some degree, the magnitude of saline events. One possible interpretation is that the diatom signal is influenced by differential preservation of key taxa, which is linked to pore water geochemical conditions that also drive the Na variations. However, the carbonate geochemical proxies also indicate high salinity at 8 k.y., suggesting that the Na concentration does to some extent represent a paleosalinity curve. It is clear, however, that a significant Na concentration gradient has been established in the top meter of the core (also observed by Callendar, 1968) and that there is also a distinct trend of increasing Na down the entire core. A sharp break in the Na gradient below the top meter suggests that below this point the permeability of the sediment has decreased sufficiently such that the signal is locked into the sediment. The upper "diffusion zone" will smooth short term excursions in salinity, whereas the more gentle increase below the top meter suggests that some of the Na is "supported" from minerals in the sediment (e.g. breakdown of feldspars, or release from clays). If the Na profile does indeed represent a "locked-in" salinity signal, then the similarity in patterns between the diatom and Na signals would be clear, because both would relate to absolute changes in lake paleosalinity. Iron and manganese The Fe and Mn peaks could mark the transition from a deep, thermally-stratified lake with an anoxic hypolimnion containing relatively high levels of dissolved Fe and Mn, to a shallower lake with oxygenated bottom waters. Iron is more

189

readily oxidized and precipitated than manganese (Froelich et al., 1979; Lyle, 1983), and the occurrence of the Mn peak about 100 cm above the Fe suggests a temporal change in redox conditions. Increased mixing would occur as the lake shoaled, with Fe minerals precipitating first, followed by Mn minerals. Alternatively, Fe and Mn may be responding to changing vegetational cover in the surrounding drainage basin. The iron and manganese rich zone in the core coincides with shifts in pollen composition indicating the replacement of late glacial spruce woodland by early Holocene deciduous forest (Jacobson, unpublished data). The vegetational transition was probably accompanied by decreased leaching by organic acids of iron and manganese from forest soil (Engstrom and Wright, 1984). 4.4. Paleosalinity and climate Ostracode and bulk-carbonate data from the lowest part of the core representing 12-10.5 k.y. suggest low salinity at this time, corroborating inferences from sedimentary diatom assemblages (Fritz et al., 1991). Pollen and geochemical results indicate that the region was covered by spruce forest, with soil conditions that favored the leaching of iron and manganese into drainage waters entering the lake. During this period of cool/moist climate, a fresh and more extensive Devils Lake overflowed its now-abandoned spillway into the Sheyenne River (Aronow, 1957). Between 10.5 and 8 k.y. increases in the ostracode and bulkcarbonate elemental ratios suggest the development of a hydrologically closed basin. This interpretation is supported by diatom-inferred salinity and pore-water Na concentrations which indicate that maximum salinity of the lake occurred around 8 k.y. The rapid change in climate is also indicated by the replacement of spruce pollen by prairie taxa (H.A. Jacobson, unpublished data). After 8 k.y. there was an overall trend toward lower salinity, although this period was interspersed with large salinity fluctuations. This interval of fluctuating but generally lower salinity from 8 to 3.5 k.y. is indicated by diatom-inferred salinity trends, pore-water Na, Sr/Ba and Sr/Ca in the bulk carbonates, and Sr/Ba in ostracodes. A

190

B.J. Haskell et al./Palaeogeography, Palaeoclimatology, Palaeoecology 124 (1996) 179 193

particularly low salinity interval from 3.5-4.5 k.y. is apparent in the data and is also supported by the near absence of pollen of the halophytic Ruppia maritima, which was otherwise prevalent during most of the last 8000 years (H.A. Jacobson, unpublished data). Although short-term excursions in ostracode Mg/Ca correlate well with similar events in bulk-carbonate chemistry, Mg/Ca in ostracodes shows a general upward decrease only until 5.5 k.y. The less saline conditions from about 4.5-3.5 k.y. were followed by a return to higher salinities during the latter part of the Holocene, although maximum salinity values and the magnitude of salinity fluctuations were not so great as during the earlier Holocene. This increase in salinity is most strongly reflected in the carbonate and ostracode geochemistries, which indicate fluctuating ratios in bulk Sr/Ca about a relatively constant mean, or an increase in bulk Sr/Ba and ostracode Mg/Ca during the late Holocene. The diatom record shows higher mean salinity values during the last 2.5 k.y., but pore-water Na only shows high concentrations from 0.5-1.3 k.y. These data, coupled with a high-resolution record of the last 500 years (Fritz et al., 1994), suggest that the salinity of Devils Lake during the past 50 years has been considerably lower than it was throughout most of the Holocene (after 8.5 k.y.). Some of the differences among the various proxy records may be a result of the different sampling strategies that were employed in sampling the core. However, it is more likely that the individual proxies are responding to slightly different environmental factors, each of which reflect changes in the system as a whole. For example, the ostracodes used in this study lived on or in the substrate, whereas the endogenic carbonate was formed in the surface waters and the dominant diatom taxa are planktonic. The ostracodes may also have calcified at a different time of year in comparison with the precipitation of the endogenic carbonates, which most likely occurred during midsummer. Insufficient data are available about the autecology of saline diatoms to know whether they reflect spring, mid-summer, or autumnal water chemistry. In addition, carbonate mineralogy may be affected by kinetic factors not directly related to lake hydrology or geochemistry, and physical and

chemical factors may also change the distribution coefficients for cation pairs under conditions different from those used in laboratory calibrations (Engstrom and Nelson, 1991; Carpenter and Lohmann, 1992). The occurrence of the highest salinity interval about 8 k.y. at Devils Lake corresponds well with documentation by pollen analysis at other sites in the northern Great Plains of an early Holocene warm/dry period when forest gave way to prairie, although in regions to the south the transition may have occurred slightly earlier (Barnosky et al., 1987; Radle et al., 1989). High-resolution analyses across the forest/prairie transition at Medicine Lake, South Dakota (Radle et al., 1989) indicate that this change in both vegetation and lake salinity occurred quite rapidly, and thus suggest that the driving changes in moisture availability were similarly rapid. In regions to the east, pollen data indicate that by 8 k.y. prairie vegetation dominated much of Minnesota and Iowa and extended to central Illinois (Webb et al., 1983). In the western U.S. the timing of early Holocene aridity corresponds to the Northern Hemisphere summer insolation maximum at 12-9 k.y. and is apparently considerably earlier than in north-central regions (Barnosky et al., 1987). However, the climate of the northern Plains appears to be influenced by the waning ice sheet, as is the case in areas to the east (Jacobson et al., 1987). The Devils Lake record suggests that the intensity of saline (arid) intervals decreased after the early Holocene (8 k.y.). Similarly, Last and Sauchyn (1993) found that Harris Lake in southwestern Saskatchewan freshened from 7.5 to 5.5 k.y., although hypersaline events were still frequent during this period. In contrast, vegetation records from Minnesota and Wisconsin suggest a gradual warming of climate, indicated by the migration of the prairie/forest border to its maximum eastward extension at ca 6 k.y. (Webb et al., 1983), and pollen records from Iowa suggest drier conditions did not spread to some regions until 5.5-3.5 k.y. B.P. (Baker et al., 1992). The apparent asynchroneity in the intensity of drought within the various areas of north-central North America suggests time-transgressive movement in the placement of the convergence zones between dry westerly and

B.J. Haskell et al./Palaeogeography, Palaeoclimatology, Palaeoecology 124 (1996) 179 193

Arctic flow and moist tropical Gulf air, a pattern also apparent in data spanning the last few centuries (Fritz et al., 1994). Regional climate is a product of these three interacting air masses, and variability in their movement, moisture content, and the intensity of convergence produces considerable modern-day variability in the distribution of precipitation (Rosenberg, 1986). It is also possible that salinity records may in part reflect processes other than climate and thus do not accurately reflect the magnitude and duration of aridity. However, the coherence of Holocene patterns in salinity change amongst a suite of sites within the northern Plains (e.g. Radle et al., 1989; Fritz et al., 1991; Fritz, Engstrom, Ito, and Xia, unpublished data) suggest that the influence of non-climatic factors is minimal. The Devils Lake data suggest an interval of wetter conditions from ca. 4.5 to 3.5 k.y. Several paleoenvironmental records from the Midwest show either a unidirectional transition or a short interval of different conditions at at around 4 k.y., although the sense of the change is not clearly the same as at Devils Lake (e.g. Pickerel Lake, South Dakota; Stuiver, 1970; Manitoba: Ritchie and Koivo, 1975; Canadian prairie lakes: Teller and Last, 1990; Cold Water Cave, N.E. Iowa: Dorale et al., 1992; Elk Lake, N.W. Minnesota: Dean, 1993). The near synchroneity of change amongst these sites suggests that they may be recording a broad-scale switch in the climate system. The paleolimnological data from Devils Lake document an apparent increase in the frequency of saline (arid) conditions in the late Holocene (post-3 k.y.), although with the exception of ostracode Mg/Ca, the saline events do not appear to have been as extreme as those during the early Holocene. This interval of increased aridity differs from patterns in the Midwestern climate records to the east, again suggesting that the configuration of air masses must have varied regionally throughout this interval. In general the Devils Lake record shows considerable variability in salinity and inferred moisture availability (this study and Fritz et al., 1991), in contrast to more unidirectional trends suggested by eastern pollen records (Webb et al., 1983). In fact these moisture variations may not be resolvable with the time lags and threshold

191

responses inherent in prairie pollen records, and in addition variability may be masked by the sampling resolution in many of these pollen studies.

5. Conclusions

Patterns of change in bulk-carbonate geochemistry and ostracode geochemistry can be linked in a coherent fashion to provide a mechanistic explanation for the hydrochemical evolution of Devils Lake under a fluctuating climate. Major increases in Mg/Ca in the lake water are recorded in ostracode valves, and are related to increases in salinity and increased precipitation of aragonite as indicated by increased Sr/Ca in the bulk carbonate. Precipitation of aragonite appears to result in the depletion of Sr relative to Ca in lake waters during periods of elevated Mg/Ca and thus lower Sr/Ca in ostracode valves, contrary to the commonlyheld view that Sr/Ca increases with salinity. The trends from Devils Lake emphasize the need for understanding the processes driving lake water chemistry before using ostracode trace-element ratios as paleosalinity indicators. The various proxies investigated in this study exhibit slightly different behavior, but all share a degree of commonality indicating generalized periods of change in the character of the lake environment. Although some of the discrepancies may result from the slightly different sampling approaches taken in the study of the core, most of these differences probably reflect the way the individual proxies respond to various components of the lake environment. Late Glacial freshwater conditions in Devils Lake were superceded by more saline conditions between 10.5 k.y. and 8 k.y. The frequency and magnitude of high salinity events decreased from 7.5 k.y. to 4.5 k.y., although there were still major fluctuations in lake salinity during this period. After a relatively fresh interval from 3.5 to 4.5 k.y., the lake returned to a mode of fluctuating salinity that continues to the present day; with more frequent high salinity periods, although the salinity of these more recent fluctuations was not as great as during the early Holocene. An impor-

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B.J. Haskell et al./Palaeogeography, Palaeoclimatology, Palaeoecology 124 (1996) 179-193

tant feature of the Devils Lake record is the amount of variability in the temporal geochemical signals, suggesting that apparent long-term trends in salinity instead represent different degrees (frequency and intensity) of short-term aridity, and thus these data point to the lack of even relatively short-term hydrological stability in the region.

Acknowledgments This research was funded by NSF grant BSR 84-15866 and ATM 90-5875. B.J.H. was supported by a fellowship under the NSF Research Training Grant "Paleorecords of Global Change: Understanding the Dynamics of Ecosystem Response" (BIR 9014277). We thank E. Almgren, D. Haviland, H. Jacobson, J. Teller, J. Wolin, and H. Wright for coring assistance, H.A. Jacobson for permission to cite unpublished pollen data, and R. Forrester, E. Ito and J. Teller for comments on the manuscript. This is Limnological Research Center contribution number 483.

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B.J. Haskell et al./Palaeogeography, Palaeoclimatology, Palaeoecology 124 (1996) 179-193 diagenesis of inorganic CaMg carbonates in the lacustrine environment. Naturwissenschaften, 59: 158-164. Radle, N., Keister, C.M. and Batterbee, R.W., 1989. Diatom, pollen, and geochemical evidence for the palaeosalinity of Medicine Lake, S. Dakota, during the Late Wisconsin and early Holocene. J. Paleolimnol., 2: 159-172. Ritchie, J.C. and Koivo, L.K., 1975. Postglacial diatom stratigraphy in relation to the recession of Glacial Lake Agassiz. Quat. Res., 5: 529-540. Rosenberg, N.J., 1986. Climate of the Great Plains region of the U.S. Great Plains Q., 7:22 32. Stuiver, M., 1970. Oxygen and carbon isotope ratios of freshwater carbonates as climatic indicators. J. Geophys. Res., 75: 5247-5257. Teller, J.T. and Last, W.M., 1990. Paleohydrological indicators in playas and salt lakes, with examples from Canada, Australia, and Africa. Palaeogeogr. Palaeoclimatol. Palaeoecol., 76: 215-240. Van Alstine, J.B., 1980. Postglacial ostracod distribution and

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