Varved Sediment Records of Recent Seasonal to Millennial-scale Environmental Variability

Abstract

The application of annually-laminated (varved) sediments in paleoenvironmental research has expanded significantly with the growing need for millennia-long records of interannual to century-scale climatic variability. In cases where the annual nature of the varves can be confirmed by radiometric or other methods, the varves provide annually-resolved chronostratigraphies that can be coupled with a wide range of paleoenvironmental proxies to provide time series with accurate seasonal- to centennial- scale sample resolution. In some cases, variations in the thickness and composition of the varves themselves may provide a useful paleoenvironmental signal. More commonly, a varve-based chronology is coupled with geochemical or paleontological measurements. Proper interpretations require that the climatic response time of the signal be taken into account. New and published data from eastern North America suggest that changes in relative fossil pollen abundance must lag climatic changes by a significant number of years, and that this lag is likely to be longer in forested regions than in regions dominated by non-arboreal vegetation. Vegetation model results illustrate the non-climatic factors (i.e., succession and disturbance) that can influence this lag. The climatic response times of some fossil plankton or geochemical variations are likely to involve less of a temporal lag, but the nature of the lag should be documented nonetheless. Paleoenvironmental interpretations based on multiple proxies and sites are most secure. One such interpretation suggests that the eastern Canadian Arctic experienced a dramatic warming over the last 150 years.