Written by Anne Morée, who completed her PhD at the Bjerknes Centre and the Geophysical Institute at the University of Bergen in 2020, and is now a postdoc at the University of Bern.
Approximately 21,000 years ago, the world experienced its Last Glacial Maximum. Large ice sheets covered the poles, while atmospheric carbon concentrations were just about 40 percent of today’s values. Anne Morée writes how she and her colleagues found that both biological cycling and circulation in the oceans must have been drastically different from today.
Being the ‘holy grail’ of paleoclimatology (the study of past climate), many researchers study the Last Glacial Maximum and its carbon cycle in order to find out how carbon was redistributed between the atmosphere, land biosphere and global oceans. Through studying these redistributions, we can improve our understanding of the climate system and how it responds to change.
Some aspects of the Last Glacial Maximum climate are relatively well understood. We for example know from geological records that temperatures decreased, ice sheets expanded and sea level lowered by about 120 meters. At the same time, we know that the carbon cycle changed:
- The land biosphere stored less carbon, in vegetation and soils.
- The atmosphere contained only about 180 ppm CO2, as compared to about 280ppm in pre-industrial times and about 415 ppm at present).
- The ocean must have stored extra carbon, as both the atmosphere and land biosphere stored less carbon during the Last Glacial Maximum.
So, what changes occurred in the oceans for them to be able to store all this extra carbon?
To answer this question, we have combined geological records with a model simulation of the Last Glacial Maximum ocean. Such model simulations can be used to study the driving mechanisms of the changes in carbon cycling. Besides the model data, we used geological records of carbon isotopes from deep ocean sediments. The isotopes of carbon in such sediment records provide information about past ocean circulation and biological cycling.
As for the model, we applied the ocean component of the Norwegian Earth System Model (NorESM) to simulate a Last Glacial Maximum ocean. We also implemented the isotopes of carbon (13C and 14C) in this model, such that we could compare the model data directly to the carbon isotopes from the geological records.
Our comparison between the geological records and the model simulation provided two main results. First, the model did not capture the changes in biological cycling required to satisfy the geological records. Nevertheless, we could perform analyses on the model simulation to look at the drivers of this mismatch between the model data and the geological records.
Secondly, we found that the geological records can be explained by the combination of large circulation changes and a profound strengthening of the biological pump, which is a measure of the extent to which biological processes manage to store carbon and nutrients at depth.
Our study shows how important the ocean is for the global carbon cycle on these timescales. As a next step, we would like to learn what exact processes could have driven these major changes in ocean circulation and biological cycling.
Morée, A. L., Schwinger, J., Ninnemann, U. S., Jeltsch-Thömmes, A., Bethke, I., and Heinze, C.: Evaluating the biological pump efficiency of the Last Glacial Maximum ocean using δ13C, Clim. Past, 17, 753–774, https://doi.org/10.5194/cp-17-753-2021, 2021.