Organic carbon released from carbonates and shales by chemical weathering: implications for the global organic carbon cycle understanding;

We propose in this study an initial estimation of the annual organic matter yield induced by chemical weathering of carbonates and shales considering their global surface at outcrop and their organic matter content. The calculation also uses data on river fluxes resulting from carbonate rocks and shales weathering in major world watersheds, published by numerous authors [1]. The results obtained from the studied watersheds have been extrapolated to a global scale. Despite rather large incertainty inherent to such an approach, the calculated value [2] of ca. 0.1 Gt implies that the annual organic carbon yield related to carbonates and shales chemical weathering might be a non-negligible component of the global carbon cycle. The calculated yield does not constitute a direct supply to soils and the rivers because a part of the produced organic matter can be mineralised, thus escaping recycling. Depending on the real fate of the organic carbon released from weathered ancient formations, i.e. mineralisation or recycling, this process might affect the efficiency of chemical weathering as a carbon sink, the global soil organic carbon amount estimation, or the evaluation of the total amount of the organic matter occurring in rivers. Whatever the hypothesis considered, this study suggests that the inherited organic yield is a key-component of the global organic carbon cycle understanding that has been to much omitted up to now. Because it exists and in addition because it might represent a non negligible carbon pool, fossil organic carbon deserves to be taken into account for a better evaluation of the organic stocks in soils and rivers presently only based on climatic data and current vegetal production. [1] Amiotte-Suchet 1994. Mémoire Sciences Géologiques, 97, 100p. [2] Di-Giovanni Ch., Disnar J. R., Macaire J. J. 2002. Global and Planetary Change, 32, 195-210.