SIMULATING THE ROLE OF MARINE BIOTA IN GLACIAL-INTERGLACIAL CO2 CYLES

Several hypotheses invoke changes in marine productivity as a cause for the 80-100 ppm change in atmospheric carbon dioxide content observed on glacial-interglacial timescales. However the exact cause has not yet been identified. Recent developments in ocean biogeochemistry models have permitted a more detailed exploration of linkages between various physical (e.g. circulation, dust input, and sea ice changes) and biological (e.g. ecosystem changes) processes in the ocean, and thus quantification of the contribution of marine biota to changes in atmospheric CO2.

This study assesses the impact of high dust deposition rates on marine biota and atmospheric CO2 using a state-of-the-art ocean biogeochemistry model that includes two size classes of zooplankton and two size classes of phytoplankton (nanophytoplankton and diatoms) which are co-limited by light, phosphate, iron, and silicate. This representation enables us to explicitly simulate the impacts of iron on ecosystem structure and carbon export. Diatoms are favored in regions with high iron concentrations, require silicate to build their shells, contribute to a “large cells” grazing loop that more efficiently exports carbon to the sea floor, and therefore have a larger impact on atmospheric CO2. In contrast, most of the carbon produced by the “small cells” loop (nanophytoplankton) is regenerated.

High dust deposition rates from the Last Glacial Maximum (LGM) produce an increase in the relative abundance of diatoms in today's iron-limited regions, causing a 6% increase in export production and an atmospheric CO2 drawdown of 15 ppm. This result is consistent with the relative timing of changes in dust and CO2 in ice cores as well as the results from other models of varying complexities that suggest that iron fertilization effect of only 10-40 ppm. When the combined effects of glacial-interglacial changes in dust, temperature, ice cover and circulation are included, the model reproduces general regional changes in export production during the LGM observed in a reconstruction of marine export production proxies, but only reduces atmospheric CO2 by 30 ppm. This suggests additional physical or biological processes must be explored as potential contributors to the glacial-interglacial changes in atmospheric CO2.