Carbon and oxygen isotopic paleoceanography of the Indian and South Atlantic Oceans: Paleoclimate and paleo-ocean circulation

SETO Koji

Journal of science of the Hiroshima University. Series C, Earth and planetary sciences

Ocean circulation is intimately associated with continental arrangement and global climate. The purposes of this study are to reconstruct the water mass structure and the deep ocean circulation in the Indian and South Atlantic Oceans during the Cenozoic.
Oxygen and carbon isotopes were studied in Cenozoic sediments at six sites (Sites 752, 754, 756, 757, 758, and 762) in ODP (Ocean Drilling Program) Legs 121 and 122 in the northeastern Indian Ocean. These isotopic records are related to global events occurring in middle Miocene, the Eocene / Oligocene boundary, middle to late Eocene, and the Paleocene / Eocene boundary. To compile those records along with a number of published isotopic data from the Indian and South Atlantic Oceans, adjustments to isotopic ratios have been calculated for different foraminiferal species, and benthic and planktonic foraminiferal isotopic data converted into δ values of dissolved inorganic carbon (DIC) of marine water. The general trends of oxygen and carbon isotopic values show an increase to the south.
Averaged values in one million year intervals of oxygen and carbon isotopes were calculated for each ODP and DSDP (Deep Sea Drilling Project) site, and the time and spatial distributions of the oxygen and carbon isotopic values were examined from the estimated paleodepth. In the Paleocene ocean, the vertical distribution of isotopic ratios is uniform. However, notable negative shift in oxygen isotopic the remarkable in the Miocene are recognized at about 1500m paleodepth in the northeastern Indian Ocean. The source of the water masses are assumed to be as follows: AABW (Antarctic Bottom Water) or proto-AABW formed in the Southern Ocean (Atlantic sector) throughout the Cenozoic. In the Paleocene, another water mass may have formed at low latitudes including the Tethyan Sea, and this water may could have been warm and highly saline, judging from oxygen isotopic ratios. This water mass corresponds to WSDW (Warm Saline Deep Water), which have encounterd Proto-AABW at mid latitudes during the early Paleogene. This water mass rapidly reduced in size with the closing of the Tethyan Sea at the Paleocene / Eocene boundary, but still continued to 50 Ma in the Indian Ocean and to 40 Ma in the South Atlantic Ocean. AAIW (Antarctic Intermediate Water) developed from the Oligocene (30 Ma) in the Indian Ocean. Proto-NADW (Proto-North Atlantic Deep Water) distinctly developed from the late Pliocene (3 Ma).

eng

Hiroshima University

[ISSN] 0075-4374

[NCID] AA11095875