The clay fraction of sediments drilled at Sites 903 and 905 is composed of variable proportions of chlorite, illite, random mixed layers (including illite-smectite and illite-vermiculite), smectite, and kaolinite. The coincidence of major changes in clay mineralogy with increasing values of oxygen isotopes suggests a dominant climatic control of the clay sedimentation. At Site 903, we distinguish four intervals (Eocene, upper Oligocene–middle Miocene, middle Miocene–upper Miocene, and Pleistocene) characterized by distinct clay assemblages.
  The Eocene clay sedimentation is dominated largely by land-derived smectites, except close to a microtektites layer, where smectites probably formed by submarine alteration of glass. Across the major unconformity separating the Eocene chalks from Oligocene–Miocene siliciclastic sediments, the sharp increase in illite and kaolinite reflects the active erosion of the Appalachians Highlands. Sea-level lowering associated with Cenozoic global cooling seems to be responsible for the reworking of these minerals. Kaolinite was probably reworked from ancient rocks and from alterations. The step-by-step increase of both clay minerals lasted until the middle Miocene. The highest percentages of kaolinite from that time also indicate increasing humidity on land.
  During the late middle Miocene and late Miocene, illite still increased, but kaolinite decreased. This major change in clay mineralogy probably resulted from the combination of a glacioeustatic event previously documented by delta¹⁸O data and the erosion of uplifted crystalline terrains in the Appalachians. The Pleistocene clay sedimentation is characterized by abundant chlorite and illite resulting from the mechanical erosion of crystalline rocks outcropping in northern areas (e.g., Canadian Shield).
  A similar trend is recorded at Site 905, but in comparison with sediments from the upper slope (Site 903), sediments deposited on the upper rise are significantly enriched with illite-smectite mixed layers over either illite and kaolinite. This difference results from the combination of differential settling processes of clay minerals and from the north-south circulation of bottom water carrying suspended clays to the region from sources farther north.

Date of initial receipt: 6 March 1995
Date of acceptance: 17 October 1995

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