GLACIAL ENRICHMENTS OF AUTHIGENIC CD AND U IN SUB-ANTARCTIC SEDIMENTS- A CLIMATIC CONTROL ON THE ELEMENTS OCEANIC BUDGET

We examine the possibility that glacial increase in the areal extent o f reducing sediments might have changed the oceanic Cd inventory, ther eby decoupling Cd from PO4. We suggest that the precipitation of Cd-su lfide in suboxic sediments is the single largest sink in the oceanic C d budget and that the accumulation of authigenic Cd and U is tightly c oupled to the organic carbon flux into the seafloor. Sediments from th e Subantarctic Ocean and the Cape Basin (South Atlantic), where oxic c onditions currently prevail, show high accumulation rates of authigeni c Cd and U during glacial intervals associated with increased accumula tion of organic carbon. These elemental enrichments attest to more red ucing conditions in glacial sediments in response to an increased flux of organic carbon. A third core, overlain by Circumpolar Deep Water ( CPDW) as are the other two cores but located south of the Antarctic Po lar Front, shows an approximately inverse pattern to the Subantarctic record. The contrasting patterns to the north and south of the Antarct ic Polar Front suggest that higher accumulation rates of Cd and U in S ubantarctic sediments were driven primarily by increased productivity. This proposal is consistent with the hypothesis of glacial stage nort hward migration of the Antarctic Polar Front and its associated belt o f high siliceous productivity. However, the increase in authigenic Cd and U glacial accumulation rates is higher than expected simply from a northward shift of the polar fronts, suggesting greater partitioning of organic carbon into the sediments during glacial intervals. Lower o xygen content of CPDW and higher organic carbon to biogenic silica rai n rate ratio during glacial stages are possible causes. Higher glacial productivity in the Cape Basin record very likely reflects enhanced c oastal upwelling in response to increased wind speeds. We suggest that higher productivity might have doubled the areal extent of suboxic se diments during the last glacial maximum. However, our calculations sug gest low sensitivity of seawater Cd concentrations to glacial doubling of the extent of reducing sediments. The model suggests that during t he last 250 kyr seawater Cd concentrations fluctuated only slightly, b etween high levels (about 0.66 nmol kg(-1)) on glacial initiations and reaching lowest values (about 0.57 nmol kg(-1)) during glacial maxima . The estimated 5% lower Cd content at the last glacial maximum relati ve to modern levels (0.60 nmol kg(-1)) cannot explain the discordance between Cd and delta(13)C, such as observed in the Southern Ocean. Thi s low sensitivity is consistent with foraminiferal data, suggesting mi nimal change in the glacial Cd mean oceanic content.