POST-GLACIAL LACUSTRINE ORGANIC MATTER RECORDS: CLIMATIC OR BIOTIC PROXIES?

Lake sediments provide natural archives that record the response of the Earth's biota to climatic and environmental perturbations. Organic remains preserved in lake sediments allow us to reconstruct the environmental histories of lakes and their watersheds because they reflect the biological communities found in and around lakes. The last glacial-postglacial climatic transition presents an opportunity to investigate how, and at what rates, watershed and lake ecosystems were established on once glaciated, carbon and nutrient-poor landscapes. Perhaps as importantly, the analysis of sedimentary records from these newly created post-glacial lakes forces a critical evaluation of the limits of geochemical and biological indicators as reliable proxies for climate change.

Detailed records of isotopic (delta-13C) and elemental composition (%C, %N, C/N ratios) produced for nine post-glacial lakes in northern New England have provided remarkable insights into the response of lacustrine ecosystems to deglaciation. A sharp increase in organic matter content and a decline in bulk sediment delta-13C values following deglaciation characterize these records. This pattern is interpreted as the combined result of landscape revegetation and the onset of primary productivity within the lakes. Our investigations have been extended on selected cores by analyzing oxygen and hydrogen indices (OI and HI), molecular bio-markers, and diatom assemblages. The addition of these proxies has enhanced our capability to identify the dominant sources of sedimentary organic matter (i.e., lake- vs. land-derived) and thus track the evolution of land and lake biota following deglaciation. The new results have allowed refinement of previous interpretations based solely on the isotopic and elemental data and support the hypothesis that even on a local scale the response of different lakes to climate change is not identical. We suggest that records of geochemical proxies such as delta-13C and %C, often used for regional and global correlation, may in fact be the result of biological rather than contemporaneous climatic factors.