Soil erosion and atmospheric CO2 during the last glacial maximum: the rôle of riverine organic matter fluxes

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Atmospheric CO2 is consumed both by organic matter formation and chemical rock weathering, and subsequently discharged as dissolved organic carbon, particulate organic carbon, and dissolved inorganic carbon to the oceans by rivers. In the long term, varying the ratio of the amount of atmospheric CO2 consumed by continental erosion and the amount of CO2 released during carbonate precipitation and organic matter respiration in the oceans can change the CO2 content in the atmosphere. The purpose of this paper is to determine whether riverine organic carbon fluxes during the last glacial maximum (LGM) may have been different from today in order to assess the potential impact on atmospheric CO2. Previous studies mainly focused on the role of the river fluxes of inorganic carbon in this respect, but none of them examined possible variations in the fluxes of organic carbon, although the erosion of organic carbon actually represents the bulk of the atmospheric CO2 consumption by continental erosion. We therefore applied a global carbon erosion model to a LGM scenario in order to determine the riverine fluxes of organic matter during that time. The climatic conditions during the LGM were reconstructed using a computer simulation with a general circulation model. It is found that during the LGM the riverine organic carbon input into the oceans was at least ˜10% lower than today. Most of the reduction of the total organic matter fluxes is due to the reduction of the fluxes of dissolved organic carbon. The fluxes of particulate organic carbon remained almost unchanged. The oceanic response to the lower carbon input was estimated on the basis of a present-day steady state budget for organic river carbon in the oceans, and implies that the reduction of the river fluxes were more than counterbalanced by lower burial rates due to the smaller shelf area during the LGM. This suggests that both the lower river carbon input and the relatively greater share of this carbon being subjected to oceanic respiration, acted as a negative feedback to the low atmospheric CO2 content during the LGM.