MULTI-PROXY INDICATORS OF LAND-USE CHANGE RECORDED IN RESERVOIR SEDIMENTS FROM THE UPPER SUSQUEHANNA RIVER BASIN, NEW YORK

Can a multi-proxy record of organic, inorganic, and isotopic geochemical indicators from sediment cores be used to document recent land-use change? Multiple sediment cores obtained from two reservoirs (Whitney Point Reservoir (WPR), and East Sidney Reservoir (ESR)) within the Upper Susquehanna River Basin in New York provided an ideal location to test such an approach. Both reservoirs are used for flood control and were constructed in the 1942 and 1952, respectively, by the U.S. Army Corps of Engineers. Land use within the watersheds has transitioned post-reservoir construction from predominantly agricultural to forested conditions.

Sediment cores were analyzed for 137Cs and 210Pb, trace metals, major cations, stable isotopes, LOI, and grain size. C/N ratios range between 3-15 at WPR and 6-13 at ESR with the higher ratios corresponding to the most recent reservoir sediments. Trace metal trends at WPR exhibit decreasing concentrations toward the sediment-water interface whereas at ESR Cu, Zn, Pb and Fe concentrations increase toward the sediment-water interface. The metal concentrations correlate well with one another and with d15N values, suggesting the metals are from the same source contributing nutrients to the watershed. Stable carbon isotope data suggests that both watersheds contained predominantly C3 plants now and in the past, but the isotopic signature recorded reflects changes in the mixture of C3 and C4 plants since the reservoirs were constructed. d13C values range between –28 and –22 in WPR and between –28 and –25 in ESR. The -22 d13C values correspond to older sediments, whereas the -28 d13C values reflect sediments deposited more recently. The carbon isotope data suggest that WPR and EPR are now receiving less C4 (maize) plant materials compared to when the reservoirs were constructed. This carbon isotope fingerprint best reflects the land use change corresponding to the agricultural to forest transition within both watersheds.