The Antarctic glaciation that occurred about 33.7 million years ago is a major turning point in Earth’s history. Pagani et al. (2011) look back to this glaciation to see what may have caused it, with a focus on CO2. Previous work done on CO2 levels during this time period have shown that CO2 was increasing, which contradicts greenhouse gas theory. However, Pagani. et al. determine that CO2 was overestimated, and that, in fact, CO2 was decreasing well before and during the Antarctic glaciation. This finding is pivotal in our understanding of how CO2 and the glaciation cycle are interlinked. –Mathew Harreld
Pagani, M., Huber, M., Liu, Z., Bohaty, S.M., Henderiks, J., Sijp, W., Krishnan, S., DeConto, R.M., 2011. The role of carbon dioxide during the onset of Antarctic glaciation. Science 334, 1261–1265
About 33.7 million years ago a major glaciation in the Antarctic shifted the climate towards what it is today. The onset of this Antarctic glaciation has been studied intensively because CO2 levels at the time of its occurrence seem counter intuitively high. Previous papers have suggested that CO2 was increasing along with the new glaciation, but this is counter to current theory and understanding of the relationship between temperature and CO2. Pagani et al. revaluate the alkenone (a substrate found in phytoplankton)-based record to determine why this counter intuitive finding may be occurring. A more recent paper did, in fact, model global CO2 levels during the period to be decreasing, but used boron records to develop CO2 levels. Pagani et al. explored this curious contradiction, and looked to use alkenone records to correctly model CO2 during the Antarctic glaciation 33.7 million years ago.
To evaluate the CO2 during the glaciation period, the authors used the same coring samples used by previous papers that used alkenone. The drilling sites were spread throughout the Atlantic, mostly near South America and the Southern Ocean, and one near New Zealand. Two of these drilling sites show evidence of increasing CO2 levels throughout the glaciation period, however the quality of the cores is very poor.
The authors used the carbon-13 values from the alkenones from six drilling sites, allowing for a wide range of environments and algal-growth patterns. The carbon-13 is derived from methyl ketone, which is found in the coring samples. The authors also used carbon isotopes levels to determine algal growth. Using various testing methods the authors compiled the algal growth and carbon-13 values. Their analysis provided evidence for lower carbon-13 and algal growth levels in the Antarctic region than in northern regions. However the results here do not show lower levels than reported previously.
The authors than ran tests on how cell size may affect the levels of CO2 in the oceans, but determined that the changes in cell size, which varied by location, had more to do with the available nutrient cycle than that of CO2. The authors also attempted to remodel possible ocean dynamics, which revealed that there were indeed lower levels of nutrients in the study region, as well as differing levels of other key materials used to study and calculate CO2. Many of the sites initially picked for this study are not of use because of this finding. The uncertainty of temperature and nutrient cycles in lower latitude regions makes it very difficult to determine CO2 levels, so the authors refocused their work on two sites at higher Southern latitudes, which are less susceptible to effects of the variables.
The results from these two sites revealed that CO2 declined about two million years before the rapid glaciation, and that the decrease continues into the event. There are possible places where CO2 increased, but full evidence is still lacking. This paper’s results determined that previous papers did not take full account of pre-glaciation values, and thus were left with poorer results. The results from this paper and from boron-based testing match with model estimates of this Antarctic glaciation. The authors concluded that there was a certain CO2 decline during the period, but cannot yet determine the absolute values.
This paper highlights the important role CO2 may have on affecting climate, and that CO2 decrease is critical for global cooling to occur and for the evolution of Earth’s cryosphere. The decrease in CO2 had a great effect on the earth’s climate 33.7 million years ago, but it didn’t act alone. The combination of long-term decreasing CO2 as well as many other important factors pushed the Earth’s climate into a long glaciation.