COMPARISON OF DOWNCORE VARIATIONS IN CHARCOAL WITH CLIMATE AND ANTHROPOGENIC DISTURBANCES, FOURTH LAKE OF THE FULTON CHAIN, ADIRONDACK MOUNTAINS, NORTHERN NEW YORK

Previous research demonstrated multidecadal and centennial variations in a 350-year long record of sediment characteristics of Fourth Lake including grain size, magnetic susceptibility, organic content, and carbonate content. Comparison with North American pollen-inferred temperature and precipitation and other climate records suggested the lake sediment variations were responding to the North Atlantic Oscillation (NAO), the Atlantic Multidecadal Oscillation (AMO), and solar variability.

The precise mechanisms by which these external phenomena lead to changes in the sediments, however, remained poorly understood. For example, many maxima in % coarse-grained (>250 micron) sediment coincided with positive AMO phases and inferred warm, dry climate. One potential explanation is that decreased precipitation (droughts) increased the frequency of forest fires. Resulting loss of vegetation, and thus loss of the flow impedance and anchoring effect of plant roots, would then have facilitated increased erosion and delivery of coarse terrigenous material to the lake. Charcoal fragments >250 microns were quantified to test this proposed explanation.

Results show that coarse-grained charcoal fragments do not occur in association with coarse lithic material and positive (and potentially warm, dry) NAO events as anticipated. Rather, results show two prominent peaks in charcoal that occur in association with finer sediments and occur near in time to pronounced maxima in % carbonate. The charcoal peaks also coincide with two of the strongest negative AMO phases of the past 350 years. Abundant charcoal at times when the AMO causes much of North America to experience cool, wet climate would be consistent with enhanced fuel build-up during the longer recurrence interval associated with less frequent fires. Increased runoff during such intervals could also have increased delivery of fine sediments and nutrients, triggering increased carbonate production. Alternatively the region may have experienced warm, dry climate in negative AMO phases, highlighting the possibility that climate forcing may result in spatially variable responses. The controls on downcore variations in charcoal remain uncertain. Nonetheless, the timing of the peaks in charcoal suggests a climatic rather than anthropogenic influence.