The dependence of chemical weathering rates on fluid residence time

In order to evaluate the importance of hydrologic processes in controlling chemical weathering rates, a reactive transport analysis is used to interpret chemical weathering rate data for a range of systems. An analysis of weathering rates for granitic material shows that weathering rates depend most strongly on fluid residence times and fluid flow rates, and depend very weakly on material age. Over moderate fluid residence times from 5 days to 10 yr, characteristic of soils and some aquifers, transport-controlled weathering explains the orders of magnitude variation in weathering rates to a better extent than material age. For fluid residence times greater than 10 yr, characteristic of some aquifers, saprolites, and most marine sediments, a purely thermodynamic-control on chemical weathering rates sustains chemical weathering—this control may be due to clay precipitation, which can drive weathering of primary minerals, or microbial processes which alter the fluid chemistry via the oxidation of organic matter. In addition, this analysis suggests that the apparent time dependence of chemical weathering rates commonly used to model the evolution of Earth's landforms may be attributable to transport-controlled weathering and the evolution of hydrologic properties over time. If hydrologic processes are the primary control on chemical weathering rates, the nature of the temperature dependence of chemical weathering rates is also altered.