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We developed a coupling scheme for the Community Earth System Model version 1.2 (CESM1.2) and the Model of Early Diagenesis in the Upper Sediment of Adjustable complexity (MEDUSA), and explored the effects of the coupling on solid components in the upper sediment and on bottom seawater chemistry by comparing the coupled model's behaviour with that of the uncoupled CESM having a simplified treatment of sediment processes. CESM is a fully-coupled atmosphere-ocean-sea ice-land model and its ocean component (the Parallel Ocean Program version 2, POP2) includes a biogeochemical component (BEC). MEDUSA was coupled to POP2 in an off-line manner so that each of the models ran separately and sequentially with regular exchanges of necessary boundary condition fields. This development was done with the ambitious aim of a future application for long-term (spanning a full glacial cycle; i.e., ~105 years) climate simulations with a state-of-the-art comprehensive climate model including the carbon cycle, and was motivated by the fact that until now such simulations have been done only with less-complex climate models. We found that the sediment-model coupling already had non-negligible immediate advantages for ocean biogeochemistry in millennial-time-scale simulations. First, the MEDUSA-coupled CESM outperformed the uncoupled CESM in reproducing an observation-based global distribution of sediment properties, especially for organic carbon and opal. Thus, the coupled model is expected to act as a better "bridge" between climate dynamics and sedimentary data, which will provide another measure of model performance. Second, in our experiments, the MEDUSA-coupled model and the uncoupled model had a difference of 0.2‰ or larger in terms of δ13C of bottom water over large areas, which implied potential significant model biases for bottom seawater chemical composition due to a different way of sediment treatment. Such a model bias would be a fundamental issue for paleo model-data comparison often relying on data derived from benthic foraminifera. [ABSTRACT FROM AUTHOR]

A range of estimates for the glacial–interglacial variations in CO2 consumption and HCO3− production rates by continental weathering processes were calculated with two models of continental weathering: the Gibbs and Kump Weathering Model (GKWM) [Paleoceanography 9(4) (1994) 529] and an adapted version of Amiotte Suchet and Probst''s Global Erosion Model for CO2 Consumption (GEM-CO2) [C. R. Acad. Sci. Paris, Ser. II 317 (1993) 615; Tellus 47B (1995) 273]. Both models link CO2 consumption and HCO3− production rates to the global distributions of lithology and runoff. A spectrum of 16 estimates for the runoff distribution at the Last Glacial Maximum (LGM) was constructed on the basis of two different data sets for present-day runoff and climate results from eight GCM climate simulation experiments carried out in the framework of the Paleo Modelling Intercomparison Project (PMIP). With these forcings, GKWM produced 3.55–9.0 Tmol/year higher and GEM-CO2 4.7–13.25 Tmol/year higher global HCO3− (1 Tmol=1012 mol) production rates at the LGM. Mean variations (plus/minus one standard error of the mean with 7 df) were 6.2±0.6 and 9.4±1.0 Tmol/year, respectively. The global CO2 consumption rates obtained with GKWM were 1.05–4.5 Tmol/year (mean: 2.8±0.4 Tmol/year) higher at the LGM than at present. With GEM-CO2, this increase was 1.95–7.15 Tmol/year (mean: 4.8±0.6 Tmol/year). The large variability in the changes obtained with each weathering model was primarily due to the variability in the GCM results.The increase in the CO2 consumption rate due to continental shelf exposure at the LGM was always more than 60% larger than its reduction due to ice cover. For HCO3− production rates, the increase related to shelf exposure was always more than twice as large as the decrease due to ice cover. Flux variations in the areas exposed both now and at the LGM were, in absolute value, always more than 3.5 times lower than those in the shelf environment.The calculated CO2 consumption rates by carbonate weathering were consistently higher at the LGM, by 2.45–4.5 Tmol/year (mean: 3.4±0.2 Tmol/year) according to GKWM and by 2.75–6.25 Tmol/year (mean: 4.6±0.4 Tmol/year) according to GEM-CO2. For silicate weathering, GKWM produced variations ranging between a 1.9 Tmol/year decrease and a 0.4 Tmol/year increase for the LGM (mean variation: −0.7±0.2 Tmol/year); GEM-CO2 produced variations ranging between a 0.8 Tmol/year decrease and a 1.05 Tmol/year increase (mean variation: +0.2±0.2 Tmol/year). In the mean, the calculated variations of CO2 and HCO3− fluxes would contribute to reduce atmospheric pCO2 by 5.7±1.3 ppmv (GKWM) or 12.1±1.7 ppmv (GEM-CO2), which might thus represent a non-negligible part of the observed glacial–interglacial variation of ∼75 ppmv. [Copyright &y& Elsevier]