Varves in Elk Lake are composed of seasonally deposited laminations of diatoms, calcite, aragonite, and layers enriched in Mn, Fe, organic matter, and clay and/or silt. The proportions of these components and the character of varve laminations changed systematically over the past 10,000+ years and define a post-glacial lake rich in calcium and manganese, a mid-Holocene prairie lake with sediments enriched in clay and silt, and a modern lake rich in Fe. Sequential changes in varve composition during the three phases of lake development are the result of a maturing lake and drainage basin and a systematic shift in airstream movements, accompanied by changes in precipitation, vegetation, and sedimentational responses to climatic forcing. Changes in varve thickness between 3.8 and 8.0 ka are attributed to eolian processes and are believed to reflect changes in regional surface winds. Within this ~4000 year time window is a 2000 year interval when time series for Δ 14 C (from tree rings) and varve thickness cross correlate and when both express periodicity at ~200 years, 40–50 years, and 20–25 years, supporting meteorological evidence that solar-geomagnetic events lead to changes in both cosmic particle flux and tropospheric winds. The Earth’s magnetic dipole moment reached its lowest value in the Holocene during the same 2000 year interval, suggesting that solar-geomagnetic events had a greater effect on the wind field when the Earth’s magnetic field was weak. A period of ~100+ years is weakly expressed in iron-rich varves during the past 3800 years, with a shift from cyclicity of ~100 years to 40–50 years between the Medieval Warm Epoch and the Little Ice Age.

Multiple varve counts over common intervals in four parallel cores permit estimation of the precision of the Elk Lake varve chronology. At the 95% confidence level, the imprecision of the counts averages 12%. External evidence and comparison of independent varve counts suggest that the accuracy of the varve chronology is well within this limit. Magnetic susceptibility is shown to be an exceptional tool for determining stratigraphic correlations, allowing unambiguous matching of all cores used.

Elk Lake is near the present forest-prairie border in northwestern Minnesota, and is also located on the boundary between hard-water lakes that are typical of once-glaciated parts of the north-central United States and more saline prairie lakes of western Minnesota and the Dakotas. The sediments of the prairie lakes just west of Elk Lake are unusual in that they commonly contain high-Mg calcite and dolomite in addition to low-Mg calcite, which is the dominant carbonate mineral in most marl lakes. During the mid-Holocene dry period, prairie conditions expanded eastward into the forested regions of Minnesota. Variations in types and abundances of carbonate minerals in the Holocene sediments of Elk Lake recorded this climatic change. Studies of primary productivity, carbonate saturation, water chemistry, and sediment-trap samples show that low-Mg calcite precipitates during the summer, triggered by algal photosynthesis. The epilimnion of Elk Lake is always oversaturated with calcite, and the degree of oversaturation increases progressively during the summer. The pH of the epilimnion increases from <8.0 after spring overturn to almost 9.0 in late summer in response to photosynthetic removal of CO 2 during the summer months. The rate of calcium depletion from the epilimnion is proportional to the increase in pH and the rate of photosynthetic carbon fixation. Today the only carbonate minerals that are accumulating in the sediments of Elk Lake are low-Mg calcite and manganese carbonate (rhodochrosite). Rhodochrosite, and probably manganese oxyhydroxide, precipitates when manganese-rich anoxic bottom waters come in contact with carbonate-rich oxic surface waters. During the arid mid-Holocene prairie period, however, low-Mg calcite, dolomite, aragonite, and rhodochrosite all accumulated in the sediments of Elk Lake. Dolomite formed in Elk Lake during this period in response to a higher Mg:Ca ratio in the water, just as it is forming today in lakes of the prairie regions of western Minnesota. The coincident occurrence of aragonite and biological indicators of high salinity suggests that the salinity of Elk Lake and the Mg:Ca ratio were higher than in any of the present prairie lakes of western Minnesota.

Variations in the ratios of 18 O: 16 O and 13 C: 12 C in calcite throughout the Holocene in Elk Lake, Minnesota, are recorded in three varve-calibrated carbonate cores. Marl in a varved deep-basin (29.6 m) core consists mainly of calcite precipitated from surface waters during the summer and probably provides the least complicated isotope record. Marl in a sublittoral (10 m) core consists of calcite contributed from several inorganic and organic sources and probably is the most complicated of the three isotope records. Calcite from shells of the ostracod Candona ohioensis in the sublittoral core provides a record of shallow-water conditions in Elk Lake for the period between 10,500 and 5500 varve yr. Variations in the 13 C: 12 C ratio of organic carbon deposited in Elk Lake during the Holocene are recorded in organic matter in the deep-basin core. All three oxygen isotope records show that, in general, the 18 O: 16 O ratio in carbonate was enriched in 18 O by several parts per mil during the mid-Holocene relative to the past few thousand years. This pattern of oxygen isotope variation is similar to that observed for carbonate materials from other lakes in the northeastern and north-central United States. Oxygen isotope records from these other lakes also show that the 18 O: 16 O ratio during the early Holocene was lower than during the mid-Holocene, and this pattern has been interpreted as representing a response to a generally warmer and drier climate during the mid-Holocene beginning about 8000 varve yr (the so-called hypsithermal). Ostracod and diatom assemblages from Elk Lake cores show, however, that the lake was colder and more saline than at present until at least 6700 varve yr, with conditions similar to those that exist today in cold prairie lakes of Canada. It may be more appropriate, therefore, to refer to the mid-Holocene in northwestern Minnesota as the “prairie period” rather than the hypsithermal, indicating that the climate was drier, but with no connotation regarding temperature. The oxygen isotope data from the three Elk Lake records for this period are somewhat equivocal. Values of δ 18 O in the marl from the sublittoral core and shells of Candona increase from 10,000 to about 6800 varve yr. However, values of δ 18 O in the marl that accumulated in the deepest part of the lake over the same interval (10,000–6800 varve yr) are more or less constant and enriched in 18 O; this probably reflects the cold, saline prairie-lake conditions predicted from the ostracod and diatom assemblage data. All three oxygen isotope records show decreases in 18 O: 16 O ratios after about 6800 varve yr in response to an increase in temperature and decrease in salinity of the lake. The 13 C: 12 C ratios in carbonates from all three Elk Lake records show a distinct pattern; the ratio increased gradually from 10,000 to 8000 varve yr going into the mid-Holocene prairie period and then decreased gradually coming out of the prairie period between about 5500 and 2500 varve yr. These changes in the 13 C: 12 C ratio could have been related to temperature through its effect on solubility of carbon dioxide; however, this interpretation is not supported by the oxygen isotope data. Another possibility is that changes in the 13 C: 12 C ratio are related to organic productivity that removes 13 C-depleted organic carbon and results in 13 C-enriched surface waters. This interpretation implies that organic productivity was higher in Elk Lake during the mid-Holocene prairie period. Support for the high-productivity, 13 C-enriched surface-water model for the mid-Holocene prairie period in Elk Lake is provided by changes in the 13 C: 12 C ratio of organic carbon in the deep-basin core. These changes parallel almost exactly those in the 13 C: 12 C ratio of carbonate carbon, but are about 2% larger (about 6% as opposed to about 4% for carbonate carbon). The difference of about 2% may represent 13 C depletion due to CO 2 limitation. The percentage of organic carbon in the sediment did not increase during the prairie period because it was diluted by an increased flux of detrital clastic material. The ultimate burial rate of organic carbon increased considerably, however, indicating that organic productivity was higher and/or the degree of preservation increased. Diatom assemblages and plant-pigment concentrations indicate that productivity was higher during the prairie period. Pyrolysis hydrogen and oxygen indices show that the 13 C-enriched organic matter that accumulated during the prairie period was hydrogen rich and oxygen poor relative to organic matter that accumulated before and after. These two indices demonstrate that the organic matter that accumulated during the prairie period was much better preserved.

Fossil pigments were examined in a 22 m core of varved sediment from the deep basin of Elk Lake, Minnesota. The lake appears to have evolved gradually from oligotrophic mesotrophic conditions in the earliest period (ca. 10,000+ years ago), to mesotrophic eutrophic conditions at present. Variations in productivity, species diversity, and relative importance of individual plant groups are related to changing climate and water level. The cyanobacteria gradually became more important in the planktonic flora; the least biomass and greatest variation occurred during the first 1000 years of the lake’s postglacial history, and again during the mid-Holocene prairie interval. The ratio of chlorophyll derivatives to carotenoids indicates that there were no periods of large-scale influx of allochthonous detritus, nor were there major slumps of littoral detritus into the deep profundal zone. In addition, there is no evidence to indicate that excessive drying of the lake occurred during the mid-Holocene prairie interval.

Elk Lake is located in the forested region of north-central Minnesota at the headwaters of the Mississippi River and occupies one of countless basins left behind as the last great Pleistocene ice sheet retreated northward into Canada. In this respect it resembles many other moderately deep, dimictic, hard-water lakes in the north-central United States, the sediments of which contain a history of postglacial and Holocene climatic and environmental change. Elk Lake is different, however, because the Holocene sediments in the deeper part of the lake form an uninterrupted sequence of annual laminations or varves. The varves are a chronometer for timing precisely the biologic, geochemical, and sedimentological responses in the lake to cyclic and progressive changes in climate. The varves also, through profound changes in their composition, divide the history of Elk Lake into three, sharply defined episodes; a postglacial lake, a prairie lake, and a modern, mesic-forest lake. We use these episodes and the character of the varves as a framework to guide the reader to the chapters and discussions found in this volume.

The natural landscape of Minnesota includes the readily visible aspects of the scenery—primarily the landforms, the vegetation, and the lakes. The landforms, including the basins in which the lakes and bogs are located, owe their origin to glaciation. The ice affected the area in one way or another for many thousands of years, up until about 11,000 years ago. Subsequently the lakes were filled with sediment as climate and vegetation gradually changed. The history of the glacial period is recorded by the topographic features of the region as well as by the composition and structure of the glacial drift. The postglacial environment history is recorded by the fossils in the lake sediments. Elk Lake is a typical glacial lake, except that it has the distinction of containing annually laminated sediments, which permit a precise chronology. Herein the sequence of glaciation and the development of the glacial landforms are considered First, then the regional climatic and vegetational history since the time of glaciation, and finally the characteristics of the lakes themselves, to provide a background for the detailed chapters on the stratigraphy of the various components of the Elk Lake sediments.

Elk Lake is located on the Itasca moraine near the source of the Mississippi River in northwestern Minnesota. The basin is in calcareous glacial drift, and the lake water is a dilute solution of calcium and magnesium bicarbonate. Low-magnesian calcite formed by precipitation from the lake water has been a major component of the sediment throughout the lake’s history. The sediment also is laminated with alternating light and dark, millimeter-thick layers containing diatoms, organic matter, Fe(OH) 3 , and CaCO 3 . The sediment microstratigraphy has been preserved because the lake is unusually deep (maximum depth is 30 m) for its size (surface area is 1.01 km 2 ). Oxygen is present in low concentrations or absent in the deepest water during summer and winter. Water movements in the deepest part of the lake are insufficient some years for the complete aeration of the deepst water during spring and autumn circulation periods. Phytoplankton photosynthesis, which occurs mostly in the surficial 6 m of water, typically removes 0.5 g C m −2 day −1 from the epilimnion, which becomes strongly oversaturated with calcite during late spring and summer as the pH increases above the equilibrium pH (7.73) for calcite saturation. Most of the CaCO 3 that makes up the light-colored layers of sediments probably is formed during the late summer, when concentrations of calcium in the epilimnion decrease most rapidly. The silica and organic matter that form the darker sediment laminae are deposited earlier in the year, during a period extending from April to late June, when silica decreases fastest in the epilimnion.

A 22 m series of cores from a continuously laminated sequence of postglacial sediment was recovered from 29.6 m of water from the deepest part of Elk Lake, Clearwater County, Minnesota, by piston and freeze-coring methods during the winters of 1978 and 1982. A varve time series constructed and used as a basis for subsampling the cores and samples, based on the varve chronology, allows precise determination of fluxes of geochemical and biological sediment components. Chronological and petrographic studies have shown that the laminations are varves and their measurement and enumeration has produced a 10,400 year time series that estimates the rates and timing of paleolimnologic and paleoenvironmental changes in Elk Lake and its drainage. A radiocarbon date from surface sediment is 850 years. The difference between radiocarbon and varve dates continues down core; varve dates are older than radiocarbon dates, probably because of systematic incorporation of dead carbon (as bicarbonate) in organic matter in the sediment. Varve-dated boundaries of pollen zones in the Elk Lake cores compare closely with the ages of the same zones in cores from nearby lakes that have been radiocarbon dated.

The varved sediments of Elk Lake, Clearwater County, Minnesota, contain a 10,000 year record of climatic and limnologic events. Sediment traps deployed in the lake’s water column from 1979 to 1981 and from 1983 to 1984 collected samples that permitted us to identify materials, to see the timing of sedimentation events, and to deduce processes that form the microlaminae within varves. Fall and spring microlaminae consist mainly of sequential accumulations of biogenic silica and resuspended calcific and siliceous materials. Precipitates of iron, manganese, and organic detritus dominate the thin winter microlaminae. Calcific microlaminae are deposited in summer. Concentrated iron and manganese precipitates form when the onset of seasonal circulation (especially in autumn) oxygenates the lower water column, but precipitation of these metals also continues throughout periods of seasonal stratification, when these dissolved elements migrate upward and are converted to particles that rain back to the bottom. Mineraloids dominate the sediment; minerals compose only a minor part and include quartz, calcite, rhodochrosite, and rockbridgeite (iron phosphate). The bulk of the bottom accumulation occurs during the longer, calmer summer and winter periods, but important contributions are also made during spring and autumn overturn events. Sediment resuspended from the shallows accumulates together with newly formed endogenic sediment, and can even briefly dominate the seston in autumn and spring. Vigor and duration of seasonal circulations in the upper water column dictate the amount of resuspended sediment contributed annually to a varve. When abrupt warming within days after ice-out stratifies the lake, sedimentation in that year is diminished by resultant suppression of plankton blooms and lack of vernal resuspension that would normally move sediment from the littoral areas into the deep parts of Elk Lake. Thin sections of varves confirm that resuspension during autumn and spring is a varve-forming process that has probably varied in importance as a function of climate and changing morphometry due to infilling. Through an entire year, sediment traps catch a greater proportion of material from spring and autumn overturns than accumulates on the bottom. Lake morphometry is the most important factor governing sediment resuspension and associated annual accumulation rates in traps.

Elk Lake in northwestern Minnesota is situated close to a climatically sensitive ecotone, the forest-prairie border, that migrated back and forth over the drainage basin of the lake during the Holocene. The entire postglacial (Holocene) sediment record in the deepest part of Elk Lake is composed of annual layers (varves) that record the seasonal pulses of many sediment components, and, most important, provide high-resolution (seasonal) time calibration of rates and timing of environmental change. These varved sediments contain many allochthonous and autochthonous components that are sensitive to changing environmental conditions in the drainage basin and the lake. The mineral components of Elk Lake sediments consist mainly of authigenic calcium, magnesium, and manganese carbonate minerals (low-Mg calcite, high-Mg calcite, dolomite, and rhodochrosite), opaline silica (from diatoms), X-ray amorphous iron and manganese oxyhydroxides, and an iron phosphate mineral tentatively identified as rock-bridgeite [(Fe, Mn)Fe 4 (PO 4 ) 3 (OH) 5 ], plus minor contributions from fine-grained detrital quartz, feldspar, illite, and kaolinite. The most notable characteristic of the sediments in Elk Lake is that most of the components were formed in the lake. Q-mode factor analysis of sediment geochemistry reduced 23 observed compositional variables, expressed as percent or parts per million of elements, to three composite variables (factor loadings) whose “concentrations” are expressed on a scale of −1.0 to 1.0. Factor 1 expresses the composition of the inorganic clastic fraction based on concentrations of Mg, Na, Al, Cr, V, Y, Sc, Ni, Sr, Co, and Cu. Factor 2 expresses the similarities in variations of Fe, Mn, P, organic carbon, and Mo. Factor 3 loadings are a synthesis of concentrations of Mn, S, Ca, La, and Ba. Geochemical characteristics define three distinct chemical stages in the development of Elk lake: (1) a carbonate-, manganese-, iron-, sulfur-rich early-lake stage that lasted from 10,400 to 8200 varve yr; (2) a clastic- and diatom-rich mid-Holocene prairie-lake stage that lasted from 8200 to 4000 varve yr; and (3) a final iron-, manganese-, phosphate-, and organic-rich modern-lake stage that developed over the past 4000 yr. The chemical characteristics of the sediments deposited during these three lake phases can be represented by the average compositions of three groups of samples: (1) the average composition of sediments deposited over the past 2000 yr, representing the modern-lake stage; (2) the composition of a 50 varve sample centered on 5700 varve yr that represents the maximum clastic influx into the lake during the prairie period; and (3) the average composition of sediments deposited over the initial 2000 yr of the lake’s existence (10,400 to 8400 varve yr). The amplitudes of climatic oscillations, as reflected by changes in concentration of many climatically sensitive elements, were greatest during the prairie period, pronounced cycles having periodicities of several hundred years. The extremes of these oscillations occurred within several centuries or less, which suggests that significant changes in climate may occur abruptly and rapidly. Most notably, the end of the mid-Holocene prairie period, marked by a sudden decrease in the influx of detrital clastic material, occurred within a few decades.

Comparison of the bulk magnetic properties of the Elk Lake sediments with varve thickness and sediment density suggests that most of the magnetic carrier in the sediments is allochthonous to the lake. An exception may be the fine-grained magnetite in the upper three meters of section. Correlation of the Elk Lake directional record with the records from Lake St. Croix and Kylen Lake demonstrates the accuracy of the Elk Lake varve chronology.

Abundance maps of diatom percentages from 174 Minnesota lakes sediment surface samples show that many diatom species have centers of abundance in lake types from particular regions of the state. Small Stephanodiscus species characterize lakes in the southwestern prairies and in urbanized areas where trophic status is high. Aulacoseira granulata and Stephanodiscus niagarae are most abundant in the shallow, eutrophic lakes of southwestern Minnesota. These geographic associations result from environmental optima for species. Although correlations between particular species and environmental factors show high variance, clear relations can be demonstrated. In particular, the DECORANA program for ordination analysis shows that many species have clearly defined optima either in low- or high-alkalinity lakes. The relations discovered from the surface sample data set can be used to understand the fossil assemblages from Elk Lake, Minnesota. The dominance of small Stephanodiscus species in Elk Lake suggests that the lake has been somewhat eutrophic for most of its history. The appearance of Aulacoseira ambigua and Aulacoseira granulata during the prairie period at Elk Lake implies that the lake was shallower or more turbulent at that time. DECORANA ordination shows that the fossil diatom assemblages of Elk Lake have not changed much since the lake was formed. Thus, environmental changes at Elk Lake were probably very subtle.

Planktonic diatoms dominate the Holocene varved-sediment record of Elk Lake, Minnesota. For the past ~10,400 yr, the lake never became shallow enough to allow large numbers of benthic and epiphytic diatoms to become deposited in the center of the lake. The relatively great depth of Elk Lake throughout this time is consistent with the continuous presence of varves in the record and the predominantly autochthonous character of sediment in the profundal part of the lake. The planktonic diatom assemblages are dominated by two species, Fragilaria crotonensis and Stephanodiscus minutulus. They alternate in dominance on scales of hundreds to thousands of years and indicate shifting limnological conditions under subtle climatic control. Fragilaria crotonensis is typically a summer and early-fall diatom that prospers when supplies of silicon are high compared to those of phosphorus. Stephanodiscus minutulus blooms in the early spring when circulation provides abundant phosphorus. The alternation of these two diatoms reflects principally the climatic conditions that drive spring circulation and summer stagnation and thereby control the fluxes of silicon and phosphorus to and within the lake. Cold and dry climates in late spring and early summer promote blooms of S. minutulus, and hot summers with some frontal storm activity provide conditions suitable for F. crotonensis. The mid-Holocene prairie period (8.2 to 4.0 ka) is characterized by a greatly increased diatom accumulation rate and a general dominance of S. minutulus. Between 6.4 and 4.0 ka Aulacoseira ambigua became prominent, implying increased nutrient fluxes and summer turbulence, probably related to winds and storms in that season. Lake levels were probably lower at times during this period. This part of the Holocene, however, was interrupted by a 600 yr interval of moister and warmer climates (5.4 to 4.8 ka), with low diatom influx and a dominance of F. crotonensis. After 4.0 ka diatom productivity fell, and F. crotonensis tended to dominate in response to reduced spring circulation and probably increased precipitation. The Little Ice Age, between A.D. 1450 and 1850, is documented by increased abundances of S. minutulus, indicating cooler late spring conditions. Logging activities in the vicinity of Elk Lake in the early twentieth century allowed Aulacoseira ambigua to return by increasing turbulence and nutrient fluxes to the lake.

We have identified a total of 56 chrysophycean stomatocyst morphotypes from the postglacial sediments of Elk Lake, Minnesota. Cysts were well preserved and abundant throughout the lake’s history. Stratigraphic changes in the 22 dominant cysts were correlated with other paleoecological information available for this core. In general, shifts in chrysophyte assemblages coincided with inferred changes in past climate. The most striking change in chrysophytes occurred about 8.5 ka, and coincided with the shift to prairie-vegetation dominance in the lake’s drainage basin. The relative proportion of chrysophycean stomatocysts to diatom frustules also decreased at that time. A second major shift occurred about 5.3 ka, with the return of a chrysophyte assemblage resembling, in some respects, the early postglacial flora. Stratigraphic analyses at decade resolution recorded more recent changes in cyst assemblages that presumably track climatic variables. The relative abundance of chrysophytes appeared to be exceptionally high (values of about 40%–80%) during the Little Ice Age.