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 productivity and organic carbon burial. We present P accumulation rates from the eastern (Neogene) and western (Cenozoic) equatorial Pacific, a region chosen to reflect oceanic P input trends because of its importance in global models of new production and biogenic sedimentation. P accumulation rates range from 5 to 60 mu mol P cm(-2) kyr(-1) and exhibit a positive correlation with mass sediment accumulation rates, calcium carbonate being the major sediment component. The influences of surface productivity patterns, site migration through time relative to equatorial upwelling, and water depth are observed in the P accumulation rate records. These site-specific effects are relatively minor, however, compared to synchronous, significant trends in P accumulation rates in these equatorial Pacific sites. The most notable event occurred in 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 cm(-2) kyr(-1)), after which values increased slightly to the present. These changes in P accumulation rates in the equatorial Pacific translate 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, and 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 bulk of the change in P accumulation and burial in the equatorial Pacific. The P accumulation rate record is markedly different from the unidirectional increases in continental weathering rates over the last 40 m.y. interpreted from the Sr isotope record, suggesting a decoupling of nutrient input fluxes from input fluxes of other dissolved constituents.