TRANSPORT OF CARBONATE SEDIMENT IN THE WESTERN NORTH-ATLANTIC - EVIDENCE FROM OXYGEN AND CARBON ISOTOPES

In most deep-ocean environments, the carbonate component is usually considered to be of pelagic origin, deposited on the sea bed without extensive transport. Even when evidence exists that carbonate fractions have been transported, the transport is usually viewed as being intrabasinal. Stable isotope analyses (O-18 and C-13) of the carbonate contained in the fine fraction (< 63 mum) of eight cores from the North American continental margin and the west flank of the Mid-Atlantic Ridge (MAR) suggest that carbonate sedimentation patterns may be influenced by transport to a greater extent than previously recognized. In Holocene sections of all the cores, the fine-fraction and foraminiferal isotopic signals are comparable. In cores from the flanks of the MAR, the fine-fraction and foraminiferal isotopic signals remain comparable during the entire glacial-interglacial cycle. On the North American margin, however, the pattern is strikingly different. Starting about 10 ka and extending back into glacial times, the fine-fraction isotopic signal in margin cores becomes increasingly depleted, reaching a value of -8 parts per thousand. (PDB). The foraminiferal signal from these same cores follows the typical glacial-interglacial pattern and gets heavier during glacial time. The Holocene correspondence of foraminiferal and fine-fraction isotope curves throughout the North American Basin suggests that both have the same intrabasinal, pelagic source. However, in late glacial age sediments from the North American margin, the divergence of foraminiferal and fine-fraction isotope signals indicates different sources for each component. Based on the isotopic signals and the timing of changes in the signal, a substantial proportion of the fine-fraction carbonate on the North American margin appears to be derived from diagenetically altered carbonate rock. This fine carbonate sediment probably resulted from physical erosion of carbonate rock by glacial processes and deposition in the North American Basin by the retreating Wisconsin glacier.