Variable C/P Composition of Organic Production and its Effect on Ocean Carbon Storage in Glacial Model Simulations

During the four most recent glacial maxima the atmospheric CO₂ concentration decreased by about 90–100 ppm compared to interglacial concentration. Most of this CO₂ was likely stored in the ocean. A strengthening of the biological pump (i.e. the biological carbon uptake in the surface ocean and export to the deep ocean) has been proposed as a key mechanism for the increased glacial oceanic CO₂ storage. The biological pump is strongly constrained by the amount of available surface nutrients. In most models it is assumed that the ratio between the elements in organic material, in particular the C/P ratio of carbon to phosphorus, is fixed at the classical Redfield ratio. However, while the C/P ratio appears to be approximately constant when averaged over basin scales, it is highly variable on regional scales and between species. Observations suggest that the C/P ratio increases when phosphorus is scarce.

We perform a sensitivity study to test how a phosphate–concentration dependent C/P ratio influences the oceanic CO₂ storage in cGENIE, an Earth system model of intermediate complexity. We carry out simulations of glacial–like changes in albedo, radiative forcing, wind–forced circulation, remineralization depth of organic matter, and mineral dust deposition. Simulations with a classical constant Redfield ratio are compared to simulations with an observationally motivated variable C/P ratio that increases with decreasing phosphate concentration. Our results show that a flexible C/P ratio in production of organic matter amplifies the increase of the CO₂ storage that is caused by circulation changes or by increased mineralization depth or dust deposition. Thus, the effect of a variable C/P ratio can be crucial for quantifying changes in oceanic carbon storage in glacial–to–interglacial transitions, as well as in the present context of increasing atmospheric CO₂ concentrations.