THE MAJOR ROLE OF THE DIFFERENT CONTINENTAL ROCK OUTCROP ABUNDANCE ON THE GLOBAL CARBON CYCLE: IMPLICATION FOR ATMOSPHERIC/SOIL CO2 UPTAKE BY CONTINENTAL WEATHERING AND ALKALINITY RIVER TRANSPORT TO THE OCEANS

The silicate rock weathering followed by the formation of carbonate rocks in the ocean, transfers CO₂ from the atmosphere to the lithosphere. This CO₂ uptake plays a major role in the regulation of atmospheric CO₂ concentrations at the geological timescale and is mainly controlled by the chemical properties of rocks. A compilation of lithological, soil and geological maps available for regional and continental areas allowed us to construct for the first time a world lithological map with a grid resolution of 1x1 degree (Amiotte Suchet et al., 2003). The data are available on the IGCP 459 website. This study analyses the spatial distribution of the six main rock types by latitude, continents, ocean drainage basins and for 49 large river basins. As we already showed in previous works (Amiotte Suchet and Probst, 1993a and b, 1995), the flux of CO₂ consumed by rock weathering is greatly variable according to the rock types. Consequently, the relative outcrop abundance of the different rock types as well as their spatial distribution in relation to the latitudinal and altitudinal variations of the main hydroclimatic factors (precipitation, runoff and temperature) have a great influence on the global CO₂ consumed by rock weathering and on the riverine transports of dissolved inorganic carbon into the oceans. Coupling the GEM-CO₂ modeling (Amiotte Suchet and Probst, 1995) with the spatial distribution of the different rock types, it was possible in this study to estimate for each large river basin and for the global scale, the present-day CO₂ uptake by each rock type. The dissolution of carbonates and the chemical alteration of shales consume both 80% (40% each) of the total CO₂ uptake by continental weathering, even if their respective outcrop area represent only 13% and 25% of the total land area. On the contrary, sands plus sandstones and plutonic plus metamorphic rocks consume both only 12 % (respectively 5% and 7%) of the total CO₂ uptake while their outcrop areas occupy 54 % of the continents (26% and 28% respectively). Finally, the contribution of volcanic rock (acid and basalts) weathering (8%) to the total CO₂ flux is proportional to their outcrop abundance (7%). Nevertheless, the chemical alteration of basalts which cover only 5% of the total land areas represent 7% of the total CO₂ uptake.