Abstract:
Models of the ocean phosphorus cycle suggest that much of the phosphorus (P) entering the oceans is deposited on continental margins. However, during times of sea level low stand (i.e. glacial intervals), this P is actually deposited in deeper water off the continental margin. Furthermore, as rivers cut through the continental shelf, the shelf sediments may become a source of P to deeper waters. As a result of the change in the locus of deposition and the new source of P to deeper waters, higher levels of primary productivity in the open ocean may result. To test this hypothesis, we have used bulk sediment geochemistry and detailed P geochemistry from cores recovered during ODP Leg 177 and ODP Leg 189 from the Southern Ocean. We have sampled at 2-5 kyr resolution in order to investigate glacial/interglacial variability in P burial and export production. Detailed P geochemistry allows for the determination of P that was once associated with organic matter compared to detrital sources. Comparison of detailed P geochemistry with P/Ti ratios reveals that the ratios do reflect enhanced burial or preservation of organic P. All sites do exhibit glacial increases in export production as recorded by reactive P concentrations and P/Ti ratios. However, P accumulation rates lead P/Ti ratios suggesting that the two records may reflect the differences in local productivity versus more global influences of P delivery to the oceans. These findings support the idea that changes in sea level may influence open ocean primary productivity. While the residence time of P in the oceans is on the order of 10-20 kyr, which is of the same order as the sea level change, we see evidence for higher rates of P delivery to the open ocean during glacial times. Using P accumulation rates, we will evaluate the P mass balance during glacial and interglacial times in the Southern Ocean and attempt to link the variability to sea level high and low stands.