The relationship of transient oxygen and carbon cycle changes in simulations of repeated glacial cycles

The glacial cycles of the late Pleistocene directly influenced the  oxygenation pattern of the global ocean, due to physical and biological changes responding to orbital and climatic change. These changes in past oxygenation had profound implications for carbon storage in seawater and marine sediments. Here we show how different Earth system processes affect benthic oxygen concentrations over repeated glacial cycles in the fully coupled Earth system model Bern3D. We further investigate how these oxygen changes relate to changes in marine carbon storage. Pioneering work on glacial-interglacial marine carbon cycle changes predicted linear relationships between Apparent Oxygen Utilization (AOU) and regenerated Dissolved Organic Carbon (DIC) changes, which are commonly used to estimate marine, and in some cases even atmospheric, carbon reservoir changes from reconstructions of these quantities. The new simulations show that the often-postulated linear correlations break in simulations of climatic change, often even in a closed atmosphere-ocean system that does not take into account weathering-burial imbalances. The underlying conceptual and box models do not capture essential dynamics of the real world systems, most importantly saturation disequilibria in the surface ocean. Hence, the linear relationships between these variables break in 3-d dynamic circulation models. AOU is therefore not a reliable measure for regenerated carbon in the ocean interior^1,2 over glacial cycles and thus not a direct tracer of remineralisation. However, the perturbations of the oxygen cycle are, albeit in an intricate manner, related to those of the carbon cycle, and thus oxygen proxy reconstructions provide a constraint on carbon flux changes. For example, spatial and temporal patterns of oxygen concentration changes provide strong constraints for temporal changes of several Earth system processes (e.g. sea ice expansion, circulation and remineralisation) and oxygen proxy records are thus indispensable to test alternative scenarios of past ocean carbon cycle and atmospheric CO₂ in Earth system models.

 

 

References

1 Cliff, E., Khatiwala, S. and Schmittner, A., 2021. Glacial deep ocean deoxygenation driven by biologically mediated air–sea disequilibrium. Nature Geoscience, 14(1), pp.43-50.

2 Schmittner, A. and Fillman, N.J., 2024. Carbon and carbon-13 in the preindustrial and glacial ocean. PLOS Climate, 3(7), p.e0000434.