Storage and release of fossil organic carbon related to weathering of sedimentary rocks

The biogeochemical carbon cycle, which plays an undeniable role in global climate change, is defined both by the size of carbon reservoirs (such as the atmosphere, biomass, soil and bedrock) and the exchange between them of various mineral and organic carbon forms. Among these carbon forms, fossil organic carbon (FOC) (i.e., the ancient organic matter stored in sedimentary rocks) is widely observed in modem environments but is not included in the supergene carbon budget. Using a digitized map of the world and an existing model of CO2 consumption associated with rock weathering, we establish the global distribution of FOC stored in the first meter of sedimentary rocks and a first estimation of annual FOC delivery to the modem environment resulting from chemical weathering of these rocks. Results are given for the world's 40 major river basins and extended to the entire continental surface. With a mean value of I 100 10(9) t, mainly controlled by shale distribution, the global FOC stock is significant and comparable to that of soil organic carbon (1500 10(9) t). The annual chemical delivery of FOC, estimated at 43 10(6) t yr(-1) and controlled by the areal distribution of shales and runoff is of the same order of magnitude as the FOC output flux to oceans. Chemical weathering of bedrock within the Amazon basin produces one-quarter of the total global flux of FOC derived from chemical weathering, and thus is expected to govern FOC release on a global scale. These results raise important questions concerning the role of FOC in the modem carbon cycle as well as the origin and the budget of carbon in soils and rivers. (C) 2007 Elsevier B.V. All rights reserved.