TROPICAL PACIFIC BIOGEOCHEMICAL ROLE IN GLOBAL CLIMATE CHANGE CONSTRAINED BY THE SOUTHERN OCEAN

The tropical Pacific plays an important role in the transfer of heat and water from low to higher latitudes; and the eastern equatorial Pacific (EEP) has a key role in the release of CO2 from the deeper ocean to the atmosphere. This CO2 release is significant in controlling atmospheric concentrations and thus, global temperature. The release, through thermal equilibration of colder upwelling water, is modulated by plankton production of both organic carbon (CO2 uptake) and calcite (CO2 release). Overall modulation depends on the quantity of the different types of carbon that settle out of the upper ocean as sinking particles. CO2 release depends on the quantity of carbon removed from the upper ocean by sinking matter, and the ratio of organic carbon to calcite in that matter. Deep ocean CO2 is supplied to the equatorial Pacific via the equatorial undercurrent (EUC), flowing several hundred meters below the surface and upwelling primarily off the coast of Peru. The EUC draws its water from the subantarctic in the SW Pacific. This water is enriched in CO2 and nutrients from the deep ocean by Southern Ocean meridional overturning under the influence of the westerlies.

We have examined the dynamics of the EEP system on the glacial-interglacial timescale in relation to the well known glacial reductions in atmospheric CO2 content. We have reconstructed regional and temporal patterns in overall biological productivity, nutrient concentrations, and calcite, opal and organic carbon particles fluxes to the deep sea. Particles fluxes depend on new proxies for preservation and ex230Th-normalization of sediment accumulation rates. We find that changes in the supply and chemistry of EUC source waters in the subantarctic led to reduced productivity during glacials and likely to lower efflux of CO2 to the atmosphere. Equally important, those chemical changes also increased the organic carbon to calcite flux ratio for matter settling out of the EEP. This would have further reduced the CO2 released to the atmosphere, adding a biological positive feedback on glacial age mechanisms reducing atmospheric CO2 content.