The Formation and Evolution of Long Island Sound

The sequence of events during the deglaciation of New England has been studied extensively. The Laurentide Ice Sheet reached its maximum extent on Long Island at 21.3-20.4 ka (calibrated radiocarbon ages) to 19.0-18.4 ka (radionuclide ages). During retreat of the icesheet, periglacial Lake Connecticut formed behind the moraines. After the lake drained through the moraine at its eastern end, fluvial valleys were cut into the lake beds. The sea then flooded the basin, with the flooding inferred to have occurred at about 15.5 ka through correlation with terrestrial events. Vibracores from eastern Long Island Sound contain varved lake-beds overlain by intertidal to shallow marine sediments with oysters in life position, foraminifera and littoral diatoms, somewhat laminated sandy silts, and coarser-grained sediments, the latter possibly deposited in a sand-wave environment. We provide direct age control of the post-glacial transgression with 30 radiocarbon dates on oysters, and compare these ages with those obtained on macrophytes and bulk organic carbon in the same samples (calibrated with CALIB 5.1). Most carbonate ages are considerably younger than those of bulk carbon and plant fragments: the organic matter must have resided on land for up to several millennia prior to deposition in Long Island Sound. The carbonate ages indicate that the main marine transgression occurred at 11-10 ka, although estuarine clays in the deeply incised valleys may reflect earlier inundation. The main flooding occurred at the end of the Younger Dryas (Melt Water Pulse 1B), when the rate of sea level rise accelerated, and possibly the crust subsided locally due to collapse of the forebulge (rate of relative sea level rise 20m/500yrs). Long Island Sound thus became an estuary at the beginning of the Holocene, and the earliest native Americans may have witnessed the marine inundation of a large section of the Long Island Sound basin.