Late Pleistocene History of Tongue of the Ocean, Bahamas

Abstract

The late Pleistocene history of Tongue of the Ocean, Bahamas, is delineated by study of six piston cores, 6 to 10 m in length, from the axial region of this deep marine embayment. Quantitative biostratigraphic analysis of four of the cores indicates the history of the last glacial cycle. One of these cores contains an unconformity between late Pleistocene and middle Eocene sediments. Paleoecologic analysis of planktonic foraminiferal thanato-coenoses indicates paleotemperature fluctuations of 3° to 4°C between interglacial and glacial stages, and a trend toward lower paleosalinity from the interglacial to postglacial stages. Both the microfauna and sediments in Tongue of the Ocean reflect the influence of late Pleistocene paleoclimates. Sediment in Tongue of the Ocean is derived primarily from the adjacent Bahamian banks; the sediment history, therefore, has been controlled mainly by eustatic variations in sea level. A low stand of sea level during the glacial stage effectively shut down the shallow-water carbonate production on the banks and resulted in a lower rate of sediment influx. A lower rate of production of planktonic foraminiferal tests during the glacial stage also contributed to this reduced sedimentation rate. The sedimentary record of Tongue of the Ocean is just the opposite of that found on most deep continental margins where glacial stages produced higher rates of sedimentation than interglacial stages. Most of the sediment in Tongue of the Ocean was winnowed from the banks, probably mainly during storms, and was deposited as a pelagic “snow.” Turbidites may not be as important in the axial region as previously reported. The carbonate mineralogy of Tongue of the Ocean sediment reflects the mineralogic complexity of organisms on the adjacent banks and possibly diagenetic alteration of carbonate sediment on the sea floor.