13. SULFUR-ISOTOPE STUDIES OF UPPER ALBIAN SEDIMENTS AT THE CÔTE D’IVOIRE-GHANA TRANSFORM MARGIN1Ken-ichiro Hisada,2 Yoshimichi Kajiwara,2 and Takako Yamaguchi2 |
The Côte d’Ivoire-Ghana Marginal Ridge in the Atlantic Ocean was drilled during Ocean Drilling Program Leg 159. Sulfur isotopes of Cretaceous sediments were examined, using 28 samples of mudstone and very fine-grained sandstone from Unit V (late Albian) at Site 959, Unit V (pre-Turonian) at Site 960, Unit III (Bajocian to Maastrichtian) at Site 961, and Unit III (late Albian) at Site 962. Among these, Site 962 is the most westwardly located. Each unit is the basal part of a cored interval. All units except for Unit III at Site 962 are barren of nannofossils or rarely yield poorly preserved nannofossils. The aim of this study was to discriminate the paleogeographical environment by means of sulfur isotopes. The ranges of 34Ssulfide are as follows: 13.8 to 2.0 at Site 959, 13.1 to -3.9 at Site 960, 5.9 to -27.5 at Site 961, and 12.2 to -15.4 at Site 962. The data might indicate a partially closed anoxic environment for the sulfide deposition with coeval seawater sulfate as a major source of sulfur, as suggested by a gypsum isotope signature (about +15). If the sedimentary features and occurrences of microfossils are taken into consideration, the paleoenvironment of the mid-Cretaceous Côte d’Ivoire-Ghana Marginal Ridge appears to have been a temporarily closed environment in an open sea related to the gateway connecting the proto-North to the South Atlantic Oceans. |
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