HR: 1340h
AN: PP13A-1481    [Abstracts]
TI: The Early Geological History of Long Island Sound
AU: * Varekamp, J C
EM: jvarekamp@wesleyan.edu
AF: Wesleyan University, Earth & Environmental Sciences, Middletown, CT 06459 United States
AU: Thomas, E
EM: ethomas@wesleyan.edu
AF: Yale University, Dept of Geology and Geophysics, New Haven, CT 06520 United States
AB: Long Island Sound (LIS), an urban estuary on the east coast of the USA, formed when terminal moraines created Long Island (NY) during the last glaciation. During early deglaciation, a periglacial lake developed behind this moraine (Glacial Lake Connecticut), which was rapidly filled with red, varved lake clays. Glacial Lake Connecticut drained when a barrier at its eastern end towards the Atlantic Ocean failed, at about 18 (calibrated) ka. The LIS basin then dried out, and a fluvial drainage pattern was carved into the lake beds (Lewis and Stone, 1991). With ongoing rise in global sea level, the sea entered LIS from the east and a marine-estuarine sediment sequence was deposited on the partially eroded varved clays. Lake Hitchcock formed behind the rapidly retreating Laurentide ice sheet in eastern New England, drained (possibly catastrophically) around 14 (calibrated) ka (Brigham-Grette, 2001), and created a coarse deltaic structure in LIS. Radiocarbon dates on bulk organic matter from the varved lake clays and marine silts indicate a time frame for Glacial Lake Connecticut between 18 and 16 ka, with the marine transgression possibly occurring around 15 ka (Lewis and DiGiacomo-Cohen, 2000). We dated organic carbon (hand picked terrestrial plant fragments) and mollusk fragments from core samples. Calibrated radiocarbon ages from organic material (ours and existing dates) do not show a clear relationship between age and depth-in-core. Radiocarbon dates show consistently older dates of organic matter than mollusk fragments from the same samples, with differences of up to 3000 years. We infer that the plant remains were stored on land for extensive periods of time before being deposited in LIS. The radiocarbon ages from mollusks show a curvi-linear relationship with depth, and suggest an age of about 10 (calibrated) ka for the marine transgression at the location of core LISAT 12 (in eastern LIS off the coast near Orient Point, Long Island). This new time frame suggests that the marine transgression occurred much later than thought, just after the Younger Dryas cold period and during Melt Water Pulse 1b of Fairbanks et al. (1989). Estimated rates of relative sea level rise from dated oyster beds (about 1.2 cm/yr) agree with the high rates indicated by these authors. This new timing of the flooding of the LIS basin poses the question whether Lake Hitchcock drained into Glacial Lake Connecticut rather than in marine waters. Preliminary stable isotope data on Elphidium excavatum from core LISAT 12 indicate that early LIS may have had slightly lower salinities than modern LIS, but foraminiferal faunas strongly resemble assemblages described from samples dated at 1000-1800 AD.
DE: 1105 Quaternary geochronology
DE: 1115 Radioisotope geochronology
DE: 1165 Sedimentary geochronology
DE: 1641 Sea level change (1222, 1225, 4556)
DE: 4217 Coastal processes
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005