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. firstname.lastname@example.org