POST-GLACIAL ENVIRONMENTAL CHANGE INFERRED FROM THE SEDIMENTARY RECORD OF NULHEGAN POND, VERMONT, USA

A continuous sediment core retrieved from a pond in northeastern Vermont provides information about environmental changes during the post-glacial period. Nulhegan Pond is located at 352 m asl in the Nulhegan Basin, a prominent topographic lowland blanketed by sandy glacial sediments. Given the permeability of these deposits, the pond, which has a maximum depth of ~4 m and perennial inflowing and outflowing streams, is assumed to reflect the local water table elevation. The core, retrieved with a 5-cm diameter Livingstone corer operated from the ice surface, reached a depth of 14.71 m below the sediment-water interface. A depth-age model based on AMS radiocarbon dating of 6 terrestrial macrofossils indicates that the record extends back to 13.5 ka BP. Lab analyses included determination of organic content through loss-on-ignition (LOI), C:N ratio, biogenic silica content (bSi), and grain size (GS) distribution. Coarse sediment with negligible LOI and bSi was deposited before 12 ka BP. LOI values rose between 12 and 10.5 ka BP while GS and C:N decreased. Values of bSi began to rise at 10.5 ka BP, nearly 1500 years after the initial LOI rise. All four proxies shifted dramatically between 8.8 and 5.5 ka BP: GS coarsened, LOI increased, and C:N and bSi fell. Values recovered and were stable from 5.5 to ~2 ka BP, after which LOI, C:N, and GS increased again. Proxy shifts between 8.8 and 5.5 ka BP may represent a drop in water level that moved the shoreline closer to the coring site. Rising values over the last 2 ka may indicate a shift to wetter conditions that transported increased amounts of terrestrial material (with coarser GS and higher C:N) to the coring site. An interpretation of dry conditions in the early- to middle-Holocene is consistent with elevated summer insolation values and some water level reconstructions from northern New England and southern Quebec, but runs contrary to other records from southern New England and New York suggesting a regional discrepancy in effective moisture changes during the Holocene.