Uranium-series and radiocarbon geochronology of deep-sea corals: implications for Southern Ocean ventilation rates and the oceanic carbon cycle

We present new uranium-series and radiocarbon measurements for deep-sea corals from the Southern Ocean. These data are used to reconstruct ventilation ages, both at present and at the end of the last glacial period approximately 16 500 years ago. We apply an improved two-component mixing approach to correct uranium-series dates for contaminant thorium and protactinium present in oxide coatings. Calculated seawater radiocarbon values for contemporary samples decrease with depth in the water column and agree with direct seawater radiocarbon measurements for this area. This indicates that deep-sea corals can accurately record seawater radiocarbon distributions. Two of three glacial samples experienced open-system uranium-series systematics, however, a third sample from the Drake Passage yields concordant thorium and protactinium dates as well as seawater values for initial 234U/238U. This coral yields a ventilation age that is approximately 20–40% greater than modern values for its location. This increase is consistent with published deep-sea coral and calibrated planktonic–benthic foraminifera radiocarbon data, suggesting that the glacial oceans as a whole may have been substantially less ventilated, presumably due to decreased formation of North Atlantic Deep Water. An overall decrease in oceanic mixing rates could have contributed to lower dissolved carbon in surface ocean water and lower atmospheric pCO2 during the past glacial period.