A TRACE-METAL PERSPECTIVE ON THE EVOLUTION OF ANTARCTIC CIRCUMPOLAR DEEP-WATER CHEMISTRY

A new benthic foraminiferal Ba/Ca and Cd/Ca data set from core RC13-22 9 in the deep Cape Basin indicates only small variations in bottom wat er nutrient concentrations in Circumpolar Deep Water (CPDW) over the l ast 450 kyr. Variability in the Ba record is characterized by somewhat higher values during glacial periods, consistent with a reduction in the flux of Ba-depleted North Atlantic Deep Water to the Southern Ocea n during glacial periods. The small changes in the Ba and Cd records c ontrast with the large and systematic increase in CPDW nutrients durin g glacial periods implied by the benthic delta(13)C record. This discr epancy, essentially an extension of the well-known Southern Ocean Cd-d elta(13)C conflict, is evaluated by transforming RC13-229 paleochemica l data into carbonate parameters using the modern oceanic relationship s between delta(13)C, Cd, and Sigma CO2 and between Ba and alkalinity. Calculations using Cd/Ca to estimate past variations in CPDW Sigma CO 2 and Ba/Ca to estimate past variations in CPDW alkalinity yield carbo nate ion concentrations that exceed calcite saturation throughout the record length, with generally higher carbonate ion values associated w ith glacial intervals (opposite in sense to the RC13-229 %CaCO3 record ). Substituting delta(13)C to estimate Sigma CO2 leads to extreme calc ite undersaturation at this site during glacial periods, clearly incon sistent with the preservation of calcite throughout the length of RC13 -229. Accepting the carbon isotope record as a direct measure of past variations in CPDW Sigma CO2 concentrations requires that both the Cd and Ba evidence for limited nutrient and alkalinity changes be disrega rded.