POST-GLACIAL RELATIVE LAKE LEVEL FLUCTUATIONS IN THE SENECA LAKE BASIN, NY

The post-glacial history of the Finger Lakes, NY involved several changes in lake level throughout the late Pleistocene and Holocene, resulting from the changing position of the retreating Laurentide ice sheet, river outlet position, post-glacial rebound, and hydrologic budget. We propose that isostatic rebound played a larger role in changing lake level than previously recognized. To investigate this hypothesis, we collected a series of cores along a north-south transect in Seneca Lake and the marsh of the Catherine Creek Wetland area south of the lake. This transect spans ~54 km, and is oriented perpendicular to the isobase. Open lake cores (2-5 m long) were collected using a piston corer and wetland cores were collected using a Russian peat borer (2.5 m long), vibra-corer (~4 m long), and truck-mounted hollow stem auger (~25 m long).

Cores were described, photographed, analyzed for magnetic susceptibility, and sampled for loss-on-ignition analyses, grain size analyses, and microfossil identification and quantification. Cores collected from the lake generally contain laminated silt or sand whereas those from the wetland preserve massive silt and sand overlain by peat beds. Cores were correlated using distinctive changes in the profiles of loss-on-ignition, magnetic susceptibility, and grain size. Microfossils preserved in the wetland cores provide a means of estimating water depth. The presence of oogonia (reproductive structures of aquatic macroalgae Chara) and an abundance of Candona spp. ostracodes in the wetland cores indicate open fresh water with a depth of less than 5 m. Northern Seneca Lake cores document a regression (lowstand) during the Early to mid- Holocene and transgression during the mid- to Late Holocene. A ~40 m drop in lake level compared to modern likely occurred during the Early to mid-Holocene in the northern part of the basin. The opposite trend is preserved in wetland cores to the south. The sediment record in the southern wetland documents a transgression during the mid-Holocene followed by eutrophication during the mid- to Late Holocene. Only isostatic rebound can explain the opposing lake level trends in the north and south ends of the basin. A similar record is preserved in a long core collected from the southern end of neighboring Cayuga Lake, further implying a regional control on lake level.