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Abstract :
[en] A reduction of the carbonate-carbon to organic-carbon export rain ratio during glacial times is commonly advanced to explain an important part of the observed glacial-interglacial atmospheric CO2 variation. This hypothesis was tested and side-effects on the evolution of carbonate preservation/dissolution in the surface sediment explored with a multi-box model (MBM) of the ocean carbon cycle, fully coupled to a new transient advection-diffusion-reaction model (called MEDUSA) representing early diagenesis processes of carbonate minerals in the surface sediment. MEDUSA explicitly considers the role of organic matter remineralisation in the sediment column to enhance calcite (and aragonite) dissolution. It is fully bi-directional and takes chemical erosion into account in times when carbonate dissolution makes the sediment mixed-layer collapse faster than the sediment supply to the surface is able to counterbalance. Coupled model experiments were run for 240,000 years, forced by variable sea-level, temperature and salinity histories, and variable continental weathering inputs. Various scenarios for the evolution of the rain ratio over glacial to interglacial periods were adopted.
A peak reduction of the rain ratio by 40% at the Last Glacial Maximum (LGM) was found to produce a net atmospheric pCO2 reduction of about 30ppm, on top of a 60ppm reduction produced by changing continental shelf carbonate accumulation and changing continental weathering inputs. The overall 90ppm oscillation compares well with the observed data. However, the effect on the model sedimentary record is clearly at odds with actual sediment records. The changes related to continental shelf processes and variable weathering flux depress the calcite saturation horizon by about 1km at the LGM; if rain ratio variations are also considered, that depression increases by another km. An assessment of the respective contributions from various model parameters will be presented.
