Dissolved organic matter and the glacial-interglacial pCO 2 problem

International audienceTwo box models, one analytical and one numerical, are used to investigate systematically a broad range of oceanic circulation changes on the atmospheric carbon dioxide (CO2) concentration. A number of oceanic carbon cycle models have failed to reproduce the 30% increase in CO2 partial pressure (pCO2) during the last deglaciation as reconstructed from polar ice cores. We apply therefore this approach of exploring the model's parameter space to examine the effect of long-lived dissolved organic matter on the system. The results from the two models complement each other, in terms of insight versus detail. Carbon is usually assumed to be transported from the surface into the deep ocean through the sedimentation of particulate matter. If there exists in the ocean a pool of dissolved organic matter (DOM) with a regeneration time comparable with the advection time, then the associated carbon can also be advected, resulting in a different clistribution. Such a DOM reservoir acts as a smoother on the spatial distribution of nutrients in the sea, particularly in the equatorial intermediate waters. We establish the role of intermediate waters as one of the key components of the oceanic carbon cycle and show that DOM reduces the sensitivity of the carbon cycle to oceanic circulation pattern changes, mainly because of its smoothing effect. Consequently, the possible existence of DOM species with a time constant of the order of a century tends to reduce, rather than enhance, the glacial-interglacial difference in pCO2 levels due to changes in the thermohaline circulation. 1Also at Centre des Faibles Radioactivitts, Domaine d