Damkohler Number Input to Transport-Limited Chemical Weathering Calculations

作者：Yu Fang; Hunt Allen Gerhard^*

来源：ACS Earth and Space Chemistry, 2017, 1(1): 30-38.

DOI：10.1021/acsearthspacechem.6b00007

• Damkohler number
• transport limited
• chemical weathering
• percolation theory
• solute transport

Chemical weathering of the silicate minerals in the Earth's crust is the dominant influence on variability of atmospheric carbon on time scales of about 1 Ma and up, lending significance to the ability to predict such weathering rates. Field weathering rates at such large time scales tend to be much slower than laboratory values, however. It has been proposed that the discrepancy occurs because field weathering rates are solute-transport-limited rather than reaction kinetic-limited. To assess the relative importance of reaction kinetics and solute transport, the Damkohler number, Da(1), a ratio of a transport time to a reaction time can be used. Unfortunately, even in those experiments that suggest importance of transport limitations, the traditional calculation of Da(1) yields values close to 1 and an extreme sensitivity of experimental reaction rates on Da(1). We develop a new method of calculating Da(1) based on the theory of non-Gaussian solute transport, appropriate for heterogeneous porous media. Our results are that weathering rates are less sensitive to the calculated Da(1) and that Da(1) is now equally sensitive to input parameters, generating a much more straightforward assessment of the relevance of transport limitations to chemical weathering. Our most important single inference appears to be that, for field conditions at larger time scales (roughly decadal and up), chemical weathering is practically always transport-limited.

• 出版日期2017-3