ISOTOPIC RECORDS IN POST-GLACIAL LAKE SEDIMENTS: IMPLICATIONS FOR BIOTA AND LANDSCAPE EVOLUTION

Assessing the effects of natural climate variability on the Earth’s biosphere is a key issue in the current efforts to predict the consequences of present and future global change. Because climatic perturbations affect the biological communities in and around lakes, evidence of local and regional environmental histories can be found in the elemental and isotopic compositions of sedimentary organic matter. The extreme climatic changes associated to glacial-interglacial transitions had profound ecological impacts. Lake sediments provide natural archives that record the response of the Earth system to such perturbations. In particular, new lakes formed after the glaciers retreated present a unique opportunity to investigate how, and at what rate, watershed and lake ecosystems were established on once glaciated landscapes. A few previous studies have examined the response of lake ecosystems to glacial-postglacial climate change. However, most of the previously studied lakes are located in regions that were not glaciated during the late Pleistocene. Thus, their sediments document the response of pre-existing lake ecosystems to climate change rather than the development of new ecosystems on a barren, carbon and nutrient-poor landscape. Detailed records of isotopic and elemental composition, produced for four post-glacial lakes in Vermont, provide remarkable insights into the individualistic response of lacustrine ecosystems to extreme climatic events and suggest that even on a local scale the response of different lakes to environmental change might not be identical. If, as our data suggest, geochemical records such as those provided by carbon isotopes reflect the individualistic response of the developing lake systems to the local geology, hydrology, and biota, these factors must be considered before attempting correlation of environmental climate proxies on a local, regional, or global scale. Our results also provide constraints on how much time is needed for lake ecosystems to recover from extreme natural or anthropogenic disturbances.