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Implications from glacial-interglacial rain ratio changes for atmospheric CO2 and ocean sediment interaction in a coupled Carbon Cycle model

Background and Developments
Ice-core records have documented a close relationship between atmospheric CO2 and climate for the past few hundreds of thousand years, with peak cold periods being characterised by about 80-100ppmv (parts per million by volume) lower concentrations than peak interglacials. Although there is agreement that the ocean must play a major role in driving the observed variations, the exact mechanisms remain unclear. A reduction of the carbonate-carbon to organic-carbon ratio in the biogenic export production (the "rain ratio") during glacial times has been advanced to explain the observed variations. This hypothesis is tested and implications for the dynamics of sedimentary carbonate preservation and dissolution are explored with MBM ("Multi-Box Model"), a ten-box model of the ocean carbon cycle, fully coupled to the sediment model MEDUSA ("Model of Early Diagenesis in the Upper Sediment (A)").

MEDUSA is a new transient one-dimensional advection-diffusion-reaction model describing the coupled early diagenesis processes of carbonates, opal and organic matter in the surface sediment. Sediment is represented as a two-phase porous medium (solids and pore-water). In the vertical, it is subdivided into two different zones. Solids raining down from the surface of the ocean are collected by the reactive mixed-layer at the top. Here, solids are transported by bioturbation and advection, solutes by molecular diffusion. Chemical reactions are restricted to this zone. Solids that get transported deeper than the bottom boundary of the reactive mixed-layer enter the second zone, the historical zone, where sediment accumulates. The historical zone is made up by a pile of layers representing a synthetic sediment core. The two zones may exchange material in a fully bi-directional way, i.e., the model can take chemical erosion into account when dissolution in the top reactive layer exceeds the supply of material from above.

Results Description
A transient scenario with 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 40ppm with the coupled MBM-MEDUSA. Changing shelf carbonate accumulation rates and continental weathering inputs produced a 55-60ppm reduction. The combination of the two mechanisms generates a pCO2 change of 90-95ppm, which compares well with the observed data.

However, the resulting model sedimentary record is in phase opposition with available data. The calcite transition zone (CTZ), i.e., the reconstructed separation between the Calcite Saturation Horizon (CSH) and the Carbonate Compensation Depth (CCD), gets about 1.5km thicker during glacial times than at present-day (interglacial). Sedimentary evidence from the Equatorial Pacific witnesses an extended transition zone during interglacial and a thin one during glacial times. Although the actual figures might be biased by the box-model approach adopted here, we do not expect that more realistic models using similar scenarios will provide a response that is opposite in phase. Realistic changes in the aragonite fraction of the carbonate rain were found to have only a minimal impact on atmospheric pCO2. Finally, chemical erosion of deep-sea sediment was shown to reduce the amplitude of variation of the sedimentary CCD by about 10-20%. It may provide a mechanism to improve the model-data agreement.

Related additional conclusions and issues that deserve further study, preferably with higher-resolution models, are given in the section on the "Potential Offered for Further Dissemination and Use". The sediment model MEDUSA has not only been coupled to the box-model MBM but also to a 3D ocean carbon cycle model available in our research group. Several 30000-year steady-state as well as 20000-year (Last Glacial Maximum to present-day) and 120000-year (a complete climatic cycle) transient simulation experiments have been carried out.

Key Innovative Features
The newly developed sediment model MEDUSA offers a flexible, fast and robust representation of time-dependent sedimentary exchange process, with excellent mass conservation properties; is fully bi-directional, i.e., can consistently take chemical erosion into account; can be coupled to carbon cycle models of any complexity; can be used locally at high vertical resolution.

Expected Benefits
MEDUSA allows realistic long-term simulation experiments with 3D models in open system set-ups, where riverine carbon and alkalinity inputs to the ocean drive carbonate burial fluxes in the deep sea. It helps to overcome a major shortcoming of the commonly used unrealistic closed-system model set-ups where riverine inputs are only used to balance the model-generated burial flux. MEDUSA also enables us to verify paleoceanographic hypotheses in a quantitative way.

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