Models of the Ocean Carbonate Cycle and the Glacial-Interglacial CO2 Variations

The mechanisms underlying the glacial-interglacial CO2 variations constitute one of the major unsolved problems in the Earth Sciences. Ice-core measurements have shown that during cold glacial periods, CO2 levels are around 190 p.p.m., but reach 280 p.p.m. during warm interglacials. Though many mechanisms have been hypothesized, it still remains unclear why these large CO2 variations occur, and where the excess CO2 is stored during glacial periods. While it has generally been assumed that the ocean carbonate cycle is (approximately) at steady state, both during glacial and interglacial periods we propose to explore the possibility, and potential impact, of autonomous dynamical behavior of the carbonate cycle.

A team of researchers at the Massachusetts Institute of Technology hypothesize the existence of a "biogeochemical oscillator" as a contributor to the glacial-interglacial CO2 variations, and in this project they will test that hypothesis. Their preliminary, highly-idealized model reveals the possibility of an internal oscillation of the ocean carbonate cycle which has several interesting features: (i) it produces oscillations in global ocean alkalinity (and thus atmospheric carbon dioxide) with the characteristic sawtooth shape observed in the ice-core record, (ii) the oscillations are due to ecological interactions between calcifying and noncalcifying primary producers, (iii) the period of the oscillations depends upon the chosen parameters but is in a range consistent with the observed glacial-interglacial variations, and (iv) the model predicts ?spikes? of enhanced calcifier productivity at the glacial-interglacial transitions, consistent with sedimentary records. 

Thus, the team will investigate the potential role of these mechanisms, employing a hierarchy of models of vastly different complexities, starting from a set of two ordinary differential equations and working up to a full general circulation model with explicit and detailed biogeochemical components including an explicit ecological model. The advantage of using such a hierarchical approach is that the idealized models can help to gain insight into the mechanisms behind and the robustness of the results from more realistic complex coupled simulations. They will test the model results against ice-core measurements of CO2, observations of glacial-interglacial variations in (carbonate) sediment accumulation, and observations of carbon isotope distributions in the oceans. 

Broader Impact : This work is obviously relevant for understanding historical climatic change, but should also be of significance in considering how the Earth system will respond to anthropogenically emitted CO2. The project is highly multidisciplinary, bringing together expertise in biogeochemical and ocean circulation models with advanced mathematical analysis and data from geological observations.