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Abstract
Sediment cores recovered from Swan Lake included a sandy silt layer between an upper interval of lake gyttja and a lower interval of marsh sapropel. The gyttja contained 3.9 to 40% calcite and from 5.9 to 32.1% organic matter (OM). Calcite (marl), magnesium-free CaCO3, is present mainly as very fine grains and some aggregates, with ostracod shells in the upper half of the gyttja sections. The sapropel is characterized by up to 60.8% OM with an absence of carbonate. OM in the sandy silt layer provided the radiocarbon age of about 3,900 years before present (YBP) and was characterized by an increase in clastic particles that were enriched in clastic-borne elements relative to sediment above and below. These clastics and their elemental enrichments were most likely deposited during a period of windy and dry climate.
The sediment organic nitrogen-15 to nitrogen-14 ratios ranged from +0.55 to +3.54 per mil (‰) and are consistent with minimal fraction from an atmospheric source (0.0‰). Ratios of carbon-13 to carbon-12 (δ 13C) in bulk sediment OM ranged from 9.7 to −27.2‰ relative to the Peede belemnite (PDB) standard. The productivity in the lake was initially dominated by C3 plants such as sedges and cattails, followed by floating macrophytes from 3,950 to 3,830 ± 100 YBP, and later by plankton. Calcite from marl and the ostracod Candona lactea contained high δ13C values (+4.56 to +20.3‰PDB), which are indicative of diagenetic carbonate formed as a by-product of a CO 2 reduction reaction. Insoluble OM (kerogen) in post-3,900 ± 60 YBP sediment had δ13C values from −10.7 to −22.5‰ PDB, suggesting that the Swan Lake surface biomass and surface biocarbonate were low at 3,900 ± 60–2,810 ± 70 YBP, high at 2,810 ± 70–1,230 ± 40 YBP, and intermediate since 1,230 ± 40 YBP.
Values of oxygen-18:oxygen-16 (δ18O) in marl and Candona lactea changed from −2.2 to −7.19‰ PDB. Vertical changes in δ18O are similar to marl and kerogen δ13C profiles, suggesting that δ 18O may have been influenced by changes within a carbon reservoir that include temperature, evaporation, and other factors.
The deuterium to hydrogen ratio (δD) in kerogen varied from −143 to −179‰ relative to the Standard Mean Ocean Water (SMOW). These δD variations suggest that the climate was likely warmer from about 5,000 to 3,800 YBP than in later years.
