CARBONATE PRESERVATION HISTORY IN THE PERU BASIN - PALEOCEANOGRAPHIC IMPLICATIONS

We studied preservation/dissolution cycles and paleoproductivity in ei ght sediment cores from the Peru Basin south of the highly productive surface waters of the eastern equatorial Pacific. Stratigraphy is base d on stable oxygen isotopes and on combined magnetostratigraphy and bi ostratigraphy. Sediment cores which span the last 8 m.y., were retriev ed during cruise 79 with RV SONNE close to the carbonate compensation depth (CCD). In general, sediments show Pacific-type carbonate cycles. We interpret a pronounced carbonate peak between 6 and 7 Ma as the re sult of a western and northern extension of the highly productive Peru Current. Decreased carbonate contents from the late Miocene to the la te Pliocene might be associated with a slow contraction of the latitud inal extent of the high-productivity belt north of the study areas. Du ring the Pliocene, carbonate variations showed 400 kyr cycles indicati ng the growth and decay of ice sheets, which should have been associat ed with pulsations of the Antarctic ice cap. An abrupt collapse of the carbonate system occurred at 2.4 Ma. Higher frequency variations of t he carbonate record indicate the major increase of the northern hemisp here glaciation. During the Quaternary, carbonate fluxes are high duri ng glacials and low during interglacials. Large amplitude variations w ith long broad minima and maxima, associated with small migrations of the lysocline and the CCD (< 200 m), are indicative of the preservatio n/dissolution history in the Peru Basin. During the early Pleistocene, climatic forcing by the 41 kyr obliquity cycle is not observed in the carbonate record. During the last 800 kyr, variability in the carbona te record was dominated by the 100 kyr eccentricity cycle. Fluxes of b iogenic material (calcium carbonate, organic carbon, opal, and barium) were greatest during glacials, which imply higher productivity and ex port production of the Peru Current during cold climatic periods. Diss olution was greatest during interglacials as inferred from the relativ ely poor preservation of planktonic foraminifera and from the low accu mulation rate of carbonate. After the Mid-Brunhes Event (400 ka), we o bserve a plateaulike shift to enhanced dissolution and to intensified productivity.