Our understanding of past climate change is a critical test of our ability to predict the future. In particular, ice age cycles have dominated Earth’s climate variation over the last two million years, but the mechanism of this variation remains unknown. Perhaps most critical for considerations of future climate, these cimate variations were associated with large changes in the atmospheric content of CO2. Danny Sigman, Jorge Sarmiento and colleagues are attempting to understand the interaction between CO2 and climate over ice age cycles by chemically analyzing the microfossils diatoms from ice age sediments to reconstruct past conditions in the ocean.


Iron fertilization

One long-posed hypothesis for lower CO2 in glacial times is that dry and windy conditions led to greater delivery of iron-rich dust to the Southern Ocean, the ocean region surrounding Antarctica, enhancing algal growth and thus drawing CO2 into the ocean. The Subantarctic Zone, the more equatorward region of the Southern Ocean, is most vulnerable to iron inputs from the winds. Over the last two years, Sigman and colleagues have measured nitrogen isotopes in Subantarctic diatoms from the last ice age and found evidence of such iron fertilization.

Most recently, after creating a record of a nearly full glacial cycle, however, the team has found that the fertilization effect came relatively late in the glacial cycle, suggesting that it only amplified latter stages of glacial cooling, rather than actually prompting the ice age. For this and other reasons, Subantarctic iron fertilization may represent an important amplifier of glacial CO2 decline and cooling, but it cannot be the main driver. The researchers are now looking for samples from other Subantarctic locations to verify this finding.


Polar stratification

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. The most important region to analyze for this effect is the Antarctic Zone, the more polar portion of the Southern Ocean. The team’s first efforts with the same diatom methods do indeed suggest Antarctic stratification during the last ice age, but the quality of the records prevent this from being a clear conclusion.

The Subarctic North Pacific and Bering Sea have proven to be a better testing ground for the hypothesis of polar ocean stratification during ice ages because of more conducive sediment records. The team’s recent activities in this region have provided startlingly strong support for the hypothesis. Taking into account the different forms of evidence for polar ocean stratification under cold climates, Sigman, in a 2004 Letter to Nature, provided a first skeleton argument for the physical processes that may be responsible. Sigman is currently investigating this subject using a numerical ocean model, in collaboration with scientists at the NOAA Geophysical Fluid Dynamics Laboratory on campus.


Impact of nutrient depletion on atmospheric CO2

Jorge Sarmiento’s group is using models to assess the ocean’s response to surface nutrient depletion in the southern ocean that might occur in response to iron fertilization. The Southern Ocean is thought to be the greatest sink of anthropogenic carbon at the present time and the most likely candidate for iron fertilization. However, the group’s work shows that changes in nutrients in different parts of the Southern Ocean would have dramatically different effects on atmospheric CO2.

The team’s modeling results support the existence of two separate overturning circulations in the ocean (Figure 8). One “loop” is similar to the well-known “ocean conveyor belt” – cold water sinks in the north Atlantic, travels south as deep water toward Antarctica, and is upwelled north of the polar front in the subantarctic zone of the Southern ocean before returning northward to complete the loop. This “productive” circulation brings nutrient-rich water to low-latitudes, fueling low-latitude productivity. In the second “unproductive” loop, deep water formed south of the polar front in the Antarctic zone sinks to the deepest part of the ocean but is later upwelled again in the Antarctic zone.

Figure 8. Two separate overturning circulations in the ocean (after Toggweiler). The northern loop approximates the “conveyor belt” circulation, and the southern loop upwells only in the Antarctic zone of the Southern Ocean.

Irina Marinov and colleagues find that nutrient depletion in the Subantarctic zone would have a relatively small impact on atmospheric CO2 levels, and lead to a dramatic reduction of low latitude biological productivity. In contrast, nutrient depletion in the deep water formation regions of the Antarctic zone would substantially draw down atmospheric CO2, and have a much smaller impact on low latitude biological productivity.

The findings suggest that surface nutrient concentrations in the deepwater formation regions of the Antarctic zone of the Southern Ocean control the ocean-atmosphere of CO2, and that past large changes in atmospheric CO2 were likely linked to Antarctic processes.