Similar glacial and Holocene Southern Ocean hydrography

We present new Holocene and glacial d 18 O data measured on planktonic foraminifera Neogloboquadrina pachyderma and benthic foraminifera Cibicidoides species from the Atlantic and Pacific sectors of the Southern Ocean in order to better understand its glacial hydrography. Combined with previously published data, the latitudinal d 18 O distributions of these foraminifera show no appreciable difference during the Holocene and the Last Glacial Maximum. This suggests that the basic Southern Ocean hydrography, including stratification and the mean position of the Antarctic Circumpolar Current frontal system, remained largely unchanged, although the small number of Pacific data render any conclusion in that sector preliminary. The Southern Ocean is a critical part of the global climate system. The continental shelves of the Weddell and Ross Seas are sites of Antarctic Bottom Water (AABW) formation. The highly barotropic Antarctic Circumpolar Current (ACC) reaches the ocean floor and is able to mix efficiently the North Atlantic Deep Water (NADW) and deep waters from the Indian and Pacific Oceans. The mixture of these deep waters, the Circumpolar Deep Water (CDW), then spreads back into the other ocean basins in one form or another. The formation of Antarctic Intermediate Water (AAIW) by Ekman pumping occurs in the convergence zone at the Polar Front and to the north in the subantarctic zone. The northward advection of temperatures and salinities of AABW, CDW, and AAIW from the Southern Ocean to other ocean basins has a profound effect on the global ocean circulation and thus on global climate. In addition, the surface waters of the Southern Ocean are key in controlling the atmospheric CO2 content because they link the atmosphere and the large carbon reservoir of the world deep ocean. The Southern Ocean surface waters have thus been a focus of attention in connection with the investigations of low atmospheric CO2 content during the Last Glacial Maximum (LGM) (Barnola et al., 1987) as well as with the forecasts of future atmospheric pCO2 levels (e.g., Sarmiento et al., 1998). Specifically, the relatively abundant surface macronutrients of the modern Southern Ocean provide an opportunity for enhanced biological pump (export of surface nutrients and CO2 to the deep ocean) to reduce the atmos- phericCO2content(MartinandFitzwater,1988). Also,reducingthe vertical mixing of Southern Ocean surface waters can lower the atmospheric pCO2 by limiting the transfer of CO2 from the deep ocean to the atmosphere (Ennever and McElroy, 1985; Toggweiler and Sarmiento, 1985; Wenk and Siegenthaler, 1985). In order to better understand the glacial Southern Ocean hydrog- raphy near the surface and at depth we examine the latitudinal distributions of d 18 O in the calcite test of planktonic foraminifera Neogloboquadrina pachyderma and benthic foraminifera Cibici- doides species. Their d 18 O reflects the seawater temperature and salinity in which they calcify their shells. The planktonic fora- miniferal carbon isotope ratios associated with this data set are discussed extensively by Kohfeld (1998). Also, benthic foramini- feral carbon isotope ratios in the Southern Ocean have been discussed recently at length (Matsumoto and Lynch-Stieglitz, 1999; Ninnemann and Charles, 1997; Oppo and Horowitz, 2000). Our new data in combination with those in the literature show that the spatial distribution ofd 18 O in planktonic and benthic foraminiferal tests remained nearly the same during the LGM as during the Holocene, a conclusion similar to that reached by Charles and Fairbanks (1990) on the basis of a subset of the planktonic foraminiferal d 18 O data shown here. The additional work in the Southern Ocean since that of Charles and Fairbanks (1990) more strongly constrains the LGM Southern Ocean hydrog- raphy, and we also infer that the circulation was similar during the Holocene and the LGM.