Mid-Pleistocene Changes in the Composition of Periplatform Sands of the Great Bahama Bank Margins

Layers of sand-sized carbonate nodules have been observed in periplatform sediments of Holocene/Pleistocene age from the Great Bahama Bank (ODP Site 1006 and 633). These nodules are mainly composed of low-Mg calcite. Thin sections show a micritic matrix with interspersed planktonic foraminifera. Oxygen and carbon isotope analyses of the micritic matrix suggest an in situ formation in equilibrium with bottom water. On the western side of the Bahamas (Site 1006) nodules occur during glacial periods from marine isotope stage (MIS) 44 until 22 and in MIS 10. In contrast, the eastern margin (Site 633) is characterised by a consistant abundance of nodules during the Pleistocene until MIS 23, while afterwards they are mainly restricted to glacial periods. Their occurrence is generally linked to periods of low aragonite values. Thus the formation of the nodules may be caused by aragonite dissolution and cementation during early diagenesis. Dissolution could be triggered by the supply of organic material during periods of higher productivity and/or lower input of fine material from the platform top/margin during glacial stages. The latter effect together with a higher bottom current intensity may cause winnowing which leads to a higher pore space in the surface sediment enhancing the formation of cements. These environmental conditions could have been changed in relation to the 41 ky to 100 ky climate transition ("Mid-Pleistocene Revolution") at MIS 23 (approx. 0.9 Ma), causing the distinctive cut in the temporal distribution of nodular layers.