A trace metal perspective on the evolution of Antarctic Circumpolar Deep Water chemistry

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Abstract
A new benthic foraminiferal Ba/Ca and Cd/Ca data set from core RC13‐229 in the deep Cape Basin indicates only small variations in bottom water nutrient concentrations in Circumpolar Deep Water (CPDW) over the last 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 Ocean during glacial periods. The small changes in the Ba and Cd records contrast with the large and systematic increase in CPDW nutrients during glacial periods implied by the benthic δ13C record. This discrepancy, essentially an extension of the well‐known Southern Ocean Cd‐δ13C conflict, is evaluated by transforming RC13‐229 paleochemical data into carbonate parameters using the modern oceanic relationships between δ13C, Cd, and ΣCO2 and between Ba and alkalinity. Calculations using Cd/Ca to estimate past variations in CPDW ΣCO2 and Ba/Ca to estimate past variations in CPDW alkalinity yield carbonate ion concentrations that exceed calcite saturation throughout the record length, with generally higher carbonate ion values associated with glacial intervals (opposite in sense to the RC13‐229 %CaCO3 record). Substituting δ13C to estimate ΣCO2 leads to extreme calcite undersaturation at this site during glacial periods, clearly inconsistent 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 ΣCO2 concentrations requires that both the Cd and Ba evidence for limited nutrient and alkalinity changes be disregarded.