A 1500-YEAR RECORD OF ENVIRONMENTAL CHANGE FROM WALDEN POND, MA

Four sediment cores were collected from Walden Pond, MA, in August, 2015, in order to examine their microfossil signals of environmental change during the last millennium. Previous paleolimnological studies have focused on the deepest basin (ca. 30 m), but these cores were taken from a shallower mid-lake basin to better capture fluctuations in the relative proportions of planktonic and benthic diatoms, and to yield more recent insights into lake conditions since the last cores were taken by Köster et al. some 15 years earlier. Preliminary analysis of a piston core of 114 cm length, for which a basal AMS age of ca. 1500 cal yr BP was obtained, shows that planktonic:benthic diatom ratios were exceptionally low ca. 1000 years ago. Similar changes were found in a second piston core as well as in a diatom record from Wolf Lake in the Adirondacks of upstate New York. This suggests that the fluctuations at Walden represent regional-scale climatic variability, most likely due to severe droughts that also affected much of SW North America at that time. Preliminary analysis of two gravity cores which captured the flocculent mud-water interface suggests that percentages of diatoms that are commonly associated with cultural eutrophication (Asterionella formosa and Fragilaria cf. nanana) have remained high but relatively stable since the mid-20^th century, consistent with the results of Köster et al. Relative abundances of chrysophyte remains increased in the uppermost 5 cm of sediment which were likely deposited since the earlier study. A similar rise of chrysophyte abundances in other lakes worldwide has been taken to reflect recent warming trends which are also recorded in lake ice records from the northeastern United States. Additional radiometric and microfossil analyses of the Walden cores are forthcoming.