Paleocean Circulation During the Last Deglaciation: A Bipolar Seesaw?

Wallace S. Broecker (Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, PO Box 1000, Palisades, NY 10964-8000; ph. 914-365-8413; fax 914-365-8155; Internet: broecker@ldeo.columbia.edu) (AGU Sponsor: Wallace S. Broecker)

Hughen et al. [1998] have documented that during the first 200 years of Younger Dryas time the 14C content of atmospheric CO2 increased by ~50) and that during the remainder of this 1200-year-duration cold event it steadily declined. The initial increase in 14C/C was likely the result of a reduction in the Atlantic's conveyor circulation. However, were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump, then it is difficult to understand why the climate conditions in the northern Atlantic basin remained cold throughout the Younger Dryas. Modeling exercises by Stocker and Wright [1996], Mikolajewicz [1998], and Schiller et al. [1998] show that if the conveyor is terminated, the transfer of radiocarbon into the deep sea shifts to the Southern Ocean, thereby stabilizing the atmospheric 14C/C ratio. Paleoclimatic evidence from the Antarctic continent suggests that this model-based scenario might have been played out in the real world. While the Younger Dryas cooling has been documented in many places around the world, including New Zealand [Denton and Hendy, 1994], Sowers and Bender [1995], using their 18O in O2-based correlation between the ice core 18O in ice records for Antarctica and Greenland, have demonstrated that in Antarctica the Younger Dryas was a time of maximum warming. The point of this paper is that the steep rise in 18O rise in Antarctic ice which commenced close to the onset of the Younger Dryas might have been caused by heat released to the atmosphere in response to an increase in deep-sea ventilation in the Southern Ocean.