A paleoclimatic study of the midwestern United States from the stable isotope records in lake sediments

Stable isotope records from both the organic and inorganic components of lake sediments have been obtained from radiocarbon dated cores raised from Ladd Lake in Ohio and Lake Winnebago in Wisconsin. The hydrogen ({dollar}delta{dollar}D) and carbon ({dollar}deltasp{lcub}13{rcub}{dollar}C) isotope records extracted from sediment bulk organic matter and the oxygen ({dollar}deltasp{lcub}18{rcub}{dollar}O) and carbon ({dollar}deltasp{lcub}13{rcub}{dollar}C) taken from calcium carbonate provide a unique means to obtain a continuous fine resolution record of environmental change on a regional scale. Sediment organic matter was concentrated by successive digestion in HCl followed by HF:HCl using a newly developed microwave acid digestion procedure. Hydrogen was extracted using a newly developed pyrolysis technique that provides a simple and reliable method for collecting hydrogen isotopic data from sediment organic matter with an overall reproducibility of better than 3{dollar}perthous.{dollar} Carbon to nitrogen ratios (C/N) were used to constrain the organic carbon data. Oxygen and carbon were extracted from sediment carbonate material using the septum phosphoric technique of Krishnamurthy et al. (1997).; The isotopic hydrogen and carbon record from Ladd Lake indicate rapid transitions between cool and warm phases over a 22,000 calendar year period. These data suggest that the so-called Mid-Holocene warm phase from {dollar}sim{dollar}8.5ka BP to 3ka BP (thousand calendar years before present) experienced oscillatory cooling periods throughout this {dollar}sim{dollar}6ka year interval. This may result from changes in relative dominance of precipitating air masses or changes in the seasonality (longer summers-shorter winters, etc.). In a world first, by combining the oxygen isotope ratio ({dollar}deltasp{lcub}18{rcub}{dollar}O) of calcium carbonate and the hydrogen isotope ratio ({dollar}delta{dollar}D) of coexisting organic matter from the Lake Winnebago core, an inferred paleotemperature record was generated. These results show that during the Mid-Holocene warm period {dollar}sim{dollar}6-2ka BP, the spring-summer lake temperatures were as much as 6-10{dollar}spcirc{dollar}C higher than present. This might result from changes in the length of season's i.e., longer summers and shorter winters. The isotopic records from both lakes show evidence for cooler temperatures around 8ka and 2ka BP and significant climatic instability during this interval. Evidence for cooler temperatures in the Midwestern United States at 8ka BP is in agreement with recent observations from ice core studies and may thus have been a global event.