Abstract:
Lower Eocene to Holocene sediments recovered from Ocean Drilling Program Sites 918 and 919 were studied to determine the grain-size distribution and mineralogy of the <2 mu m size fraction. The clay minerals consist of chlorite, smectite, illite, kaolinite, and a mixed layered illite/smectite. Several non-clay minerals were identified including quartz, plagioclase, alkali-feldspar, amphibole, pyroxene, zeolite, and calcite. Relative abundances of the clay minerals were determined semi-quantitatively using an oriented internal standard method. Smectite abundances were found to increase with depth, while illite and chlorite abundances decrease with depth. The Eocene sediments of Site 918 are composed predominantly of smectite with some kaolinite and small amounts of chlorite and illite, indicative of warm climatic conditions at the time of deposition. Oligocene sediments show an increase in chlorite and illite reflecting a lowering of base level and/or climatic cooling. A warming trend in the early to middle Miocene is suggested by increased amounts of kaolinite. Variations in the relative amounts of chlorite and illite at this time may be the result of short-term eustatic sea level changes. Near the end of the Miocene, a cooling trend is indicated by increasing amounts of chlorite and illite. This follows deposition of glauconitic hardgrounds at the top of a chalk unit with coarse sediment deposited above them. This interval is believed to coincide with the spillover of Arctic seawater into the Irminger Basin. The hardgrounds indicate hiatuses of short duration followed by high density turbidity currents or Sparse chalk deposition. Non-clay minerals in the <2 mu m size fraction become common in upper Miocene to Holocene sediments of Sites 918 and 919, as glacial rock-flour that signals the onset of Greenland glaciation. Using R-mode factor analysis, the clay assemblages of Pliocene-Pleistocene age sediments were found to be significantly different from older sediments, with a dramatic transition at 457 mbsf, recording the beginning of significant Greenland glaciation.