Northeastern Section - 37th Annual Meeting (March 25-27, 2002)

Paper No. 0
Presentation Time: 11:25 AM

PALEOPRODUCTIVITY VARIATIONS IN HEMLOCK LAKE, NEW YORK: MAGNETIC PROPERTIES MEASUREMENTS AND DIATOM COUNTS


CHAISSON, William P., ARNOLD, Mary R. and COTTRELL, Rory D., Earth and Environmental Sciences, Univ of Rochester, Hutchison Hall, Rochester, NY 14627, chaisson@earth.rochester.edu

A 32-cm gravity core was recovered from Hemlock Lake, New York (42°46'N, 77°36'; elevation 275.8 m). The bottom of the core, based on estimated sediment accumulation rates, consists of sediment that collected ~1870 AD. Diatom counts indicate that productivity in the lake increased after the turn of the 20th century (25 cm) and remained relatively high for ~40 years before declining (15 cm) toward late 19th century levels. The oligomesotrophic Aulacosiera subarctica (56.54%) shows the highest relative abundance in the Hemlock Lake 30 cm sample. Cyclotella comensis (53.1%) is indicative of low total phosphorus (Hall and Smol, 1992), and Aulacosiera subarctica (21.9%) are prevalent in the 1 cm sample from Hemlock Lake. In contrast, samples between 15 and 25 cm include significant proportions of Aulacosiera italica (14%) and Tabellaria fenestrata (18%), which are mesoeutrophic indicators. Magnetic properties of the sediments, including coercivity, coercivity of remanence, saturation remanence and saturation magnetization, were measured. Coercivity of remanence shows a significant excursion to lower values between 25 and 15 cm in the core. This can indicate an increase in the grain size of the magnetic minerals present. Hemlock Lake has been the water supply of the city of Rochester since 1876. Historical records reveal that the early 20th century was marked by repeated droughts in western New York state, leading to lowered lake level and water shortages. Exposure of the shoreline may have induced downslope movement of larger grain sizes and resuspension of sediments may have elevated productivity in the lake. The droughts of this period were associated with a prolonged positive phase of the North Atlantic Oscillation. Similar to conditions have prevailed since the late 1970s, but have not been accompanied by comparable changes in diatom abundance or magnetic properties.