Stable carbon and oxygen isotope studies of the sediments of Elk Lake, Minnesota

Variations in the ratios of ¹⁸O:¹⁶O and ¹³C:¹²C 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 ¹³C:¹²C 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 ¹⁸O:¹⁶O ratio in carbonate was enriched in ¹⁸O 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 ¹⁸O:¹⁶O 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 ¹⁸O in the marl from the sublittoral core and shells of Candona increase from 10,000 to about 6800 varve yr. However, values of δ ¹⁸O 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 ¹⁸O; 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 ¹⁸O:¹⁶O ratios after about 6800 varve yr in response to an increase in temperature and decrease in salinity of the lake.

The ¹³C:¹²C 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 ¹³C:¹²C 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 ¹³C:¹²C ratio are related to organic productivity that removes ¹³C-depleted organic carbon and results in ¹³C-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, ¹³C-enriched surface-water model for the mid-Holocene prairie period in Elk Lake is provided by changes in the ¹³C:¹²C ratio of organic carbon in the deep-basin core. These changes parallel almost exactly those in the ¹³C:¹²C 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 ¹³C depletion due to CO₂ 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 ¹³C-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.