COMPOUND SPECIFIC D/H ANALYSIS OF LATE HOLOCENE LACUSTRINE SEDIMENTS IN SUB-TROPICAL NORTH AMERICA: IMPLICATIONS FOR RECONSTRUCTING ATMOSPHERIC CIRCULATION PATTERNS AND HYDROLOGIC CONDITIONS

The hydrogen (D/H) isotopic composition of individual organic compounds has the potential to provide high-resolution reconstructions of paleoclimatic and hydrologic conditions (e.g. atmospheric circulation patterns, relative humidity) in continental environments. Modern calibration studies have shown that specific fatty acids are unique to algae (C-16) and terrestrial (C-28) material, and that dD_C16 is more depleted than dD_C28. In lacustrine systems, the dD_C16 is a reflection of the isotopic composition of the source water it was synthesized in, which is determined by atmospheric circulation patterns that govern latitudinal sources and seasonal amounts of precipitation. The dD_C28 reflects an isotopic enrichment of source water resulting from evaporative processes unique to terrestrial vegetation. The magnitude of isotopic offset between the algae and terrestrial markers, (DdD_(C16-C28) ), takes into consideration variations in the source water and thus reflects only the relative intensity of evaporative processes.  Together, dD_C16 and dD_(C16-C28)from lacustrine archives can serve as proxies for the reconstruction of climatic conditions in continental settings.

Lake Tulane, FL, is a subtropical, groundwater-fed acidic lake with a high sedimentation rate and well-preserved organic matter. Its small catchment area allows for synchronous deposition of terrestrial and algal materials into the sedimentary record.  Variability in the dD_{C16 ,} reflecting source water changes, and variability in the DdD_(C16-C28)  has been observed in a sedimentary record from Lake Tulane representing the last 80kyrs. On glacial-interglacial time scales sea level is the dominant control on relative humidity and the long-term record of DdD_(C16-C28). This study focuses on reconstructing higher resolution continental climate change on decadal to interannual time scales with emphasis on specific climatic events such as the Little Ice Age and Medieval Warm Period. Over this interval sea level is largely invariant and we can attribute changes in dD_C16 to changing atmospheric circulation patterns (source waters), and the variance in DdD_(C16-C28)  to fluctuations in the P/E ratios (relative humidity). These parameters provide a new set of proxies that can reconstruct hydrologic and atmospheric conditions in the geologic past.