Palaeoceanography: Antarctic stratification and glacial CO2
Palaeoceanography: Antarctic stratification and glacial CO2
复制标题
古海洋学：南极层结和冰川二氧化碳
作者：
D. Sigman;E. Boyle
Sigman and Boyle reply—Palaeoceanographic evidence indicates that there was more complete nutrient consumption in Antarctic surface waters during the last ice age1, 2, but lower biological production1. These results suggest that the Antarctic was stratified during glacial times, reducing the transport of sequestered nutrients and CO2 into the Antarctic surface. By sequestering CO2 in the ocean interior, this change could explain the observation of lower levels of atmospheric CO2 during the ice age3. Geological data offer two possible causes for this stratification. First, the Southern Hemisphere westerly winds apparently shifted northwards during glacial times4, which would have reduced Ekman-driven upwelling in the Antarctic5 (a ‘wind-shift’mechanism). Second, the Antarctic sea-ice cycle intensified during glacial times6, which may have allowed a low-salinity lid to accumulate in the open Antarctic, thus reducing vertical mixing and open-ocean overturning (a ‘sea-ice’mechanism).Keeling and Visbeck criticize these mechanisms for Antarctic stratification on theoretical grounds and highlight an alternative hypothesis for lowering glacial CO2—prevention of CO2 release from the Antarctic by covering the ocean with sea ice, thereby blocking ocean–atmosphere CO2 exchange7. Although we cannot be completely confident about the specific mechanisms for stratification outlined above, we believe that Antarctic stratification is a more plausible hypothesis for lower glacial CO2 than gas-exchange limitation, and it is also more directly supported by palaeoceanographic data3. With regard to the wind-shift mechanism, Keeling and Visbeck argue that a reduction in winds over the Antarctic was unlikely because of an increase in the Equator-to-Pole temperature gradient during glacial times. However, the modern meridianal variation in wind strength across the Southern Ocean is large enough for the observed northward migration in westerly winds during the ice age to have overcome the effects of a global average increase in winds, yielding less wind-driven upwelling in the glacial Antarctic. Winds depend on regional (not global) temperature gradients, and the temperature gradient across the Antarctic may well have been smaller during glacial times, potentially explaining the greater northward persistence of sea ice. But it must be admitted that the wind-shift mechanism is
