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We present the sedimentary organic carbon isotope (δ13C) record at Site 925 (Ceara Rise) for the last 35 m.y. The δ13C 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 nium contents suggest that the secular variations of the organic δ13C are mainly controlled by the marine δ13C value. The isotopic composition of total dissolved CO 2 in the Oligocene was probably slightly heavier than the present value. We suggest thre mechanisms to explain the secular variations of δCorg during the last 35 m.y.: changes in (1) surface water CO 2(aq) concentration, (2) activity of carbon fixation process, and (3) relative abundance of organic compounds.

Secular variations in sedimentary organic during the last 35 m.y. in the tropical Atlantic, Site 925