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 cor als from the Southern Ocean. These data are used to reconstruct ventilation ages, both at present and at the end of the last glacial period approximat ely 16500 years ago. We apply an improved two-component mixing approach to correct uranium-series dates for contaminant thorium and protactinium prese nt in oxide coatings. Calculated seawater radiocarbon values for contempora ry samples decrease with depth in the water column and agree with direct se awater radiocarbon measurements for this area. This indicates that deep-sea corals can accurately record seawater radiocarbon distributions. Two of th ree glacial samples experienced open-system uranium-series systematics, how ever, a third sample from the Drake Passage yields concordant thorium and p rotactinium dates as well as seawater values for initial U-234/U-238. This coral yields a ventilation age that is approximately 20-40% greater than mo dern values for its location. This increase is consistent with published de ep-sea coral and calibrated planktonic-benthic foraminifera radiocarbon dat a. suggesting that the glacial oceans as a whole may have been substantiall y less ventilated, presumably due to decreased formation of North Atlantic Deep Water. An overall decrease in oceanic mixing rates could have contribu ted to lower dissolved carbon in surface ocean water and lower atmospheric pCO(2) during the past glacial period. (C) 2001 Elsevier Science B.V. All r ights reserved.