Evidence of mid-Holocene climate instability from variations in carbon burial in Seneca Lake, New York

The amounts and types of carbon delivered to the sediments Seneca Lake, New York, have varied since the middle Holocene. Concentrations of CaCO first fluctuate between 14 and 6% around 7 ka before decreasing erratically 3 until about 5 ka and then remain 2% in younger sediments. Because the amount of calcite that precipitates in hard-water lakes is related to summertime thermal stratification, the carbonate fluctuations suggest that cyclic strengthening and weakening of seasonality at intervals of about three centuries accompanied the end of the Holocene Hypsithermal in northeast North America. Organic C/ total N values record short, decade-long intervals of enhanced delivery of land-plant material during episodes of wetter climate that are independent of the 13 temperature variations. Higher organic d C values indicate that recent fertilization of lake waters from soil disturbance and land-derived runoff has increased aquatic productivity. Introduction al. 1996; Silliman et al. 1996). It is succeeded by 14 period of warm and dry climate from 4.8 to 2 C ka Evidence for a variable Holocene climate is widethat is evident in lake sediment records from the spread (e.g., Overpeck (1996)). In particular, sedicentral Great Lakes region (Yu et al. 1997). These ment records from North American lakes provide a changes are local and regional examples of the global continental perspective of local and regional repattern of Holocene climate instability evident in ice sponses to climate variations. A brief interlude of cores (e.g., Meese et al. (1994), Alley et al. (1997)). post-Younger-Dryas cold climate (the Preboreal OsPast environmental changes that have occurred 14 cillation, 9.6 C ka) is evident in lake sediments from within a lake and in its surrounding catchment comthe Great Lakes region (Anderson et al. 1997; Yu and monly appear as distinctive differences in the comEicher 1998). This period was closely followed by an positions of sediments that were deposited at succesearly-middle Holocene period of warmth and altered sive times. Climate changes are a major environmen14 precipitation from |9 to 5 C ka that is variously tal variable. Lakes in temperate latitudes can be called the ‘Altithermal’, the ‘Atlantic Warm Period’, especially sensitive to climate changes because their the ‘Little Climatic Optimum’, and the ‘Hypsitherseasonal stratification and primary production respond mal’ (Cronin 1999). Because the latter term is one of strongly to annual cycles. In particular, the calcite that long standing (e.g., Deevey and Flint (1957)), I will precipitates from the surface waters of hard-water use it in this contribution. The Hypsithermal is relakes offers a potentially valuable, high-resolution corded as a period of warm and dry climate in much sedimentary record of environmental changes and of central North America (Krishnamurthy et al. 1995; particularly of those caused by climate changes (Kelts Schwalbe et al. 1995; Dean et al. 1996; Haskell et al. and Talbot 1990; Anderson et al. 1997; Hodell et al. 1996; Hassan et al. 1997), but as a time of warm and 1998; Mullins 1998). wet climate in the eastern Great Lakes area (Dwyer et Summertime calcite precipitation events (whitings)