42. PLIOCENE–PLEISTOCENE PALEOCLIMATIC AND PALEOCEANOGRAPHIC HISTORY OF SITE 959, EASTERN EQUATORIAL ATLANTIC OCEAN1Im Chul Shin2 |
ABSTRACTPliocene–Pleistocene paleoclimatic and paleoceanographic variables were examined based on the number (in 20 view fields) and relative abundances of calcareous nannofossil F. profunda, number of discoasters in 20 view fields, relative abundances of warm- and cool-water indicator calcareous nannofossils, and the species diversity indices S, H(S), and E. The early Pliocene is characterized by relatively shallow nutricline (by proxy, thermocline) compared to the late Pliocene as evidenced by lower relative abundances of F. profunda. The early Pliocene contains a higher abundance of discoasters, warm-water indicator calcareous nannofossils, and the species diversity indices S, H(S), and E all suggest warm and stable surface-water conditions. The late Pliocene contains the highest relative abundance of F. profunda, indicating the deepest nutricline and, by proxy, thermocline. Climatic shift and changes in nutricline/thermocline occurred between 2.82 and 3.03 Ma. The surface-water temperature became cool, the thermocline/nutricline depth became shallower, and the primary productivity began to get higher starting from the early late Pliocene (2.82 Ma) to the latest Pleistocene. These paleoceanographic events are shown by significant changes in both number and relative abundances of F. profunda, relative abundances of warm-water indicator calcareous nannofossils, and by the species diversity indices from the early late Pliocene (upper part of Subzone CN 12a; 2.82 Ma) through the latest Pleistocene. The early Pleistocene shows greater variations of the relative abundances of F. profunda and warm-water nannofossils, suggesting unstable and great fluctuation of surface-water temperature and nutricline/thermocline depth. Another climatic cooling starting approximately 2.4 Ma, also shown by the decreasing relative abundances of F. profunda, by warm-water indicator nannofossils, and by the species diversity indices for the younger sediment. The middle and late Pleistocene contain the lowest relative abundances of F. profunda, suggesting the shallowest thermocline/nutricline. |
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