LABORATORY STUDY OF THE INFLUENCE OF FLUID RESIDENCE TIME ON GRANITE WEATHERING RATES

Silicate weathering functions as a long-term sink of atmospheric CO₂. Thus over million-year timescales it plays a crucial, though still controversial role in Earth’s long term climatic evolution. Recent mathematical modeling suggests that silicate weathering rates should be sensitive to fluid residence times, implying that topography may moderate climate through its influence on fluid flow rates in the subsurface. To quantify the influence of flow rate on mineral degradation, we assembled a system of six through-flow reactors for the analysis of effluent waters and their dissolved constituents. We loaded reactor columns with crushed, unaltered granite (approximately 50% feldspars, 35% quartz, < 10% hornblende and biotite and trace accessory minerals). Two replicates of three capillary-regulated flow rates (7 ml hr^-1, 1 ml hr^-1, and 0.2 ml hr^-1) were tested, corresponding to a factor of ~35 difference in fluid residence times. Thus we are able to quantify the influence of flow rate on chemical weathering without the potentially confounding factors (e.g. light, organic inputs, biotic influence) of natural settings. We sampled reactor effluent over a period of five months, and analyzed its chemical constituents. Thus we captured the initiation of chemical weathering in the rock. The highest rates of solute production occurred within the first 20-60 days. Across a range of elements, we generally found a decrease in solute production over time. Additionally, higher flow rates yielded higher rates of solute flux. Here we will discuss these results and the connection between flow rate and silicate weathering in the context of our ongoing experiments regarding differential weathering and erosion within granitoids of the Laramie Range, Wyoming.