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THE MECHANICAL-CHEMICAL WEATHERING LINKAGE: GLACIER EROSION AND SOLUTE FLUXES
ANDERSON, SUZANNE P. INSTAAR.
Field studies suggest a linkage between high physical erosion rates and rates of chemical denudation. Temperate glaciers are responsible for exceptionally high erosion rates, in which this linkage should be clear. Silicate-weathering fluxes from glaciers are consistent with the low temperature, dilute water chemistry, and high mineral surface area production in these environments. Low temperatures reduce silicate-weathering rates sufficiently to explain the difference between silica fluxes from glaciers and from non-glacierized basins. By analogy to laboratory flow-through reactors, glacial solute flux should depend on surface area production and mineral weathering rate constants. The surface area production is significant: a typical glacial erosion rate and grain-size distribution produces on the order of 104 km2 of mineral surface area per square kilometer per year. Mineral weathering rates decline with surface age; hence this new surface area is highly reactive. Application of the “reactor” model yields results consistent with measured solute fluxes for the example of Bench Glacier, Alaska. In part this reflects the far-from-equilibrium conditions in glacial runoff, so that mineral weathering rate constants are not limited by saturation state. In glacial catchments, both annual silica fluxes and mean concentrations increase with water discharge. This suggests that mineral surface area increases with water discharge from glaciers, an effect plausibly linked to erosion rates. A small set of glaciers for which both erosion rate and silica flux data are available support the idea that production of new reactive mineral surface area by glacial erosion drives silicate-weathering fluxes.
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