Antarctic stratification and glacial CO2

One way of accounting for lowered atmospheric carbon dioxide concentrations during Pleistocene glacial periods is by invoking the Antarctic stratification hypothesis, which links the reduction in CO2 to greater stratification of ocean surface waters around Antarctica1,2. As discussed by Sigman and Boyle3, this hypothesis assumes that increased stratification in the Antarctic zone (Fig. 1) was associated with reduced upwelling of deep waters around Antarctica, thereby allowing CO2 outgassing to be suppressed by biological production while also allowing biological production to decline, which is consistent with Antarctic sediment records4. We point out here, however, that the response of ocean eddies to increased Antarctic stratification can be expected to increase, rather than reduce, the upwelling rate of deep waters around Antarctica. The stratification hypothesis may have difficulty in accommodating eddy feedbacks on upwelling within the constraints imposed by reconstructions of winds and Antarctic-zone productivity in glacial periods. Figure 1 Northward Ekman transport of surface waters and contours of modern potential density averaged for each ocean zone. The approximate mean latitude of the Polar Front (northern boundary of Antarctic zone) and direction of Ekman and eddy transports that control the rate of deepwater upwelling are shown (lower panel). The relevant eddy transport involves a net southward flow, as opposed to north–south mixing, and ultimately results from the tendency of lighter water to spread over denser water.