Chemical weathering and consumption of atmospheric carbon dioxide in the Alpine region

To determine the CO2 consumption due to chemical weathering in the Alps, water samples from the 32 main Alpine rivers were collected and analysed in two periods, spring 2011 and winter 2011/2012. Most of the river waters are characterized by a bicarbonate earth-alkaline composition with some samples showing a clear enrichment in sulphates and other samples showing a slight enrichment in alkaline metals. The amount of total dissolved solids (TDS) ranges between 96 and 551 mg/L. Considering the major ion composition and the Sr isotopic composition of water samples, coherently with the geological setting of the study area, three major reservoirs of dissolved load have been recognized: carbonates, evaporites and silicates. Based on a chemical mass balance, the flux of dissolved solids, and the flux of carbon dioxide consumed by chemical weathering have been computed for each basin and for the entire study area. Results show that the flux of dissolved solids, ranges from 8 × 103 to 411 × 103 kg km− 2 y− 1, with an average value of 127 × 103 kg km− 2 y− 1, while the flux of carbon dioxide consumed by chemical weathering in the short-term (< 1 Ma) is 5.03 × 105 mol km− 2 y− 1 on average. Since part of the CO2 is returned to the atmosphere through carbonate precipitation and reverse weathering once river water reaches the ocean, the CO2 removed from the atmosphere/soil system in the long-term (> 1 Ma) is much smaller than the CO2 consumed in the short-term and according to our calculations amounts to 2.01 × 104 mol km− 2 y− 1 on average. This value is almost certainly a minimum estimate of the total amount of CO2 fixed by weathering on the long-term because in our calculations we assumed that all the alkaline metals deriving from rock weathering in the continents are rapidly involved in the process of reverse weathering in the oceans, while there are still large uncertainties on the magnitude and significance of this process. The values of CO2 flux consumed by weathering are strongly correlated with runoff while other potential controlling factors show only weak correlations or no correlation. Our estimation of the CO2 consumed by weathering in the Alpine basins is in the same order of magnitude, but higher than the world average and is consistent with previous estimations made in river basins with similar climatic conditions and similar latitudes.