35. SECULAR VARIATIONS IN SEDIMENTARY ORGANIC DURING THE LAST 35 M.Y. IN THE TROPICAL ATLANTIC, SITE 925¹

Naohiko Ohkouchi²,³ and Eitaro Wada²

ABSTRACT

We present the sedimentary organic carbon isotope () record at Site 925 (Ceara Rise) for the last 35 m.y. The values increase from -24‰ in the Oligocene to -21‰ in the middle Quaternary. At the Oligocene/Miocene boundary, values shift about +1‰, and during the latest Miocene to late Pliocene, values again shift by nearly +2‰. Sedimentary aluminum and titanium contents suggest that the secular variations of the organic are mainly controlled by the marine value. The isotopic composition of total dissolved CO₂ in the Oligocene was probably slightly heavier than the present value. We suggest three mechanisms to explain the secular variations of _org during the last 35 m.y.: changes in (1) surface water CO₂(aq) concentration, (2) activity of carbon fixation process, and (3) relative abundance of organic compounds.

¹Shackleton, N.J., Curry, W.B., Richter, C., and Bralower, T.J. (Eds.), 1997. Proc. ODP, Sci. Results, 154: College Station, TX (Ocean Drilling Program).
²Center for Ecological Research, Kyoto University, Shimo-Sakamoto Otsu 520-01, Japan.
³Present address: Institute of Low Temperature Sciences, Hokkaido University, Sapporo 060, Japan. ohkouchi@soya.lowtem.hokudai.ac.jp