THE OCEANIC PHOSPHORUS CYCLE AND CONTINENTAL WEATHERING DURING THE NEOGENE

Little is known about the history of dissolved phosphorus (P) input to the ocean during the Cenozoic, an important factor in reconstructing global change because of the role of P in controlling net oceanic prod uctivity and organic carbon burial. We present P accumulation rates fr om the eastern (Neogene) and western (Cenozoic) equatorial Pacific, a region chosen to reflect oceanic P input trends because of its importa nce in global models of new production and biogenic sedimentation. P a ccumulation rates range from 5 to 60 mu mol P cm(-2) kyr(-1) and exhib it a positive correlation with mass sediment accumulation rates, calci um carbonate being the major sediment component. The influences of sur face productivity patterns, site migration through time relative to eq uatorial upwelling, and water depth are observed in the P accumulation rate records. These site-specific effects are relatively minor, howev er, compared to synchronous, significant trends in P accumulation rate s in these equatorial Pacific sites. The most notable event occurred i n the late Miocene, when mean P accumulation rates exhibited a strong peak at 5-6 Ma, to the highest mean value of the Neogene (37 mu mol P cm(-2) kyr(-1)), followed by a sharp decrease to 1-2 Ma (14 mu mol P c m(-2) kyr(-1)), after which values increased slightly to the present. These changes in P accumulation rates in the equatorial Pacific transl ate into substantial changes in P burial; the decrease from 5-6 to 1-2 Ma signified a 62% decrease in P burial in the equatorial Pacific, an d the difference is equivalent to 14% of the estimated modern P input rate to the oceans. Some of the late Neogene change in the equatorial Pacific P accumulation rate record may have been due to redistribution of P burial to high-latitude regions. However, on the basis of P mass balance considerations, redistribution alone cannot account for the b ulk of the change in P accumulation and burial in the equatorial Pacif ic. The P accumulation rate record is markedly different from the unid irectional increases in continental weathering rates over the last 40 m.y. interpreted from the Sr isotope record, suggesting a decoupling o f nutrient input fluxes from input fluxes of other dissolved constitue nts.