PALEOCLIMATIC INFERENCES FROM THE D/H RATIO OF BULK ORGANIC MATTER IN LAKE SEDIMENTS

We have carried out multiple isotope measurements on organic matter extracted from a sediment core from Lake Winnebago in Wisconsin, USA. Based on radiocarbon dates of the organic fraction, the record spans the past 12k years. The C/N ratio average around 10 indicating that the sediments are predominantly lacustrine in origin. Although a lack of correlation between d¹³C and d¹⁸O values may be taken to rule out carbonate precipitation via evaporative processes, the d¹⁸O values could not be translated to any meaningful climatic information. On the other hand the dD values of the organic fraction, extracted by cycles of HF-HCl digestion of the bulk sediment could be used to document interesting paleotemperature information for the region. The dD values ranged from -129‰ to -142‰ with the higher values corresponding the previously documented Mid Holocene Warm between 7k and 4k radiocarbon years. Paleotemperatures were estimated by converting the dD values of the organic fraction to the dD values of the lake water and by implication to that of local precipitation.  The temperature estimates show that MHW was warmer by about 3⁰C, in agreement with speleothem data from the Midwestern United States (see figure). Our study shows the usefulness of bulk sediment D/H ratios as a climate proxy that can compliment compound specific hydrogen isotope studies.