Daniel Sigman and colleagues are working to explain the long-term changes in atmospheric CO2 content between ice ages and interglacial periods. Their work over the course of the CMI grant has resulted in a new technique for analyzing minute amounts of organic nitrogen in preserved in microfossils from deep sea sediments (Figure 13). The previously unrecoverable data have provided insight into two possible contributors to CO2 drawdown – iron fertilization and polar ocean stratification.
New down-core records from the Subantarctic Zone of the Southern Ocean provide evidence for more complete nutrient drawdown in the region during the last ice age. This finding represents some of the first evidence for naturally driven, long term iron fertilization of the ocean, in this case due to greater dust inputs to the region during the last ice age. Ice age enhancement of nitrate consumption in the Subantarctic Zone may also explain observations regarding the fertility of the tropical ocean during the last ice age, with implications for future climate-related changes in the global ocean.
Another possible contributor to glacial cooling is stratification of the polar regions of the oceans. Enhanced stratification would prevent CO2-rich deep waters from reaching the surface and expelling carbon to the atmosphere, leading to a lowering of atmospheric CO2 levels that would cool the planet (Note that this process would also affect ocean uptake of anthropogenic CO2, albeit in a somewhat different way). Previous work in the Antarctic Zone, the more polar portion of the Southern Ocean, suggested stratification in this region during the last ice age, but the quality of the records prevented this from being a clear conclusion. Subsequent studies on more conducive sediment records in the Subarctic North Pacific and Bering Sea have greatly strengthened the case for pervasive stratification of both these polar ocean regions during cold climates.
To explain the apparent link between climate and polar stratification, Sigman’s group has proposed that the reduced sensitivity of seawater density to temperature at low temperatures might contribute to high-latitude stratification in glacial periods. In the case of a globally colder ocean, temperature gradients, which now hinder stratification, would become less important in determining polar ocean density structure. Instead, atmospheric deposition of fresh water on polar ocean regions, which promotes stratification, would dominate. Postdoctoral researcher Agatha de Boer has used a general ocean circulation model to investigate this effect, and the results support a significant role for whole-ocean temperature change in determining polar ocean overturning. They also point to interactions among the polar regions of the different ocean basins that, along with the inclusion of other feedbacks, are leading us toward a more complete hypothesis for the role of polar ocean stratification in the Pleistocene cycles in CO2 and climate.