Does temperature or runoff control the feedback between chemical denudation and climate? Insights from NE Iceland

The relative role of temperature and runoff on the chemical denudation rates in seven NE Iceland river catchments were determined through the analysis of river water chemistry collected over a five-year period. Denudation rates are quantified from the instantaneous riverine Na fluxes. As sodium is the major element least incorporated into secondary phases, its denudation rate is directly related to the dissolution rate of the catchments' primary rocks. Chemical transport in rivers is taken to be analogous to laboratory mixed-flow reactors; dissolution rates in mixed flow-reactors at fixed temperature and flow rate are a function of fluid composition, degree of fluid undersaturation, the Arrhenius activation energy, and reactive surface area. Data analysis suggests that the Na chemical denudation rates of the NE Icelandic catchment rocks increase by 13% for each degrees C increase in temperature. Of this 13%, 5% stems from the Arrhenius activation energy and 8% stems from the temperature dependence of the fluid pH and aqueous Al activity. Increased runoff raises the height of the water table thus enhancing chemical denudation by increasing the reactive surface area of weathering minerals. Regression of measured Na fluxes indicate that the average active water-rock surface area of the studied catchments increase linearly with runoff such that a 1% rise in runoff produces a 1% increase in reactive surface area and chemical denudation rates. The relative role of temperature versus runoff on chemical denudation depends, therefore, on the relative changes in these two rate controlling parameters. The maximum temperature variation of the studied rivers was 15.4 degrees C, which would increase Na chemical denudation rates by a factor of 6. In contrast, instantaneous runoff varies by as much as a factor of 250. These observations suggest that runoff dominates the instantaneous variation of the Na chemical denudation rates of the studied catchments. (C) 2013 Elsevier Ltd. All rights reserved.