Biological Weathering : A Big-Picture View of Scaling and Variability.

Mass balance and the rock record’s evidence of sustained (Ga) environmental stability indicate that Earth’s global chemical weathering rate has been approximately constant since the advent of the atmosphere and oceans, capped by planetary volcanic CO₂emissions at roughly (total cations, constant continental area basis) 1 keq/ha/yr.  Before the emergence of rooted plants, terrestrial chemical weathering was a relatively diffuse process mimicking runoff in its spatiotemporal distribution. In the modern world photosynthesis and vascularity confer a portfolio of high-powered weathering tactics, a sort of parallel-hybrid chemical weathering system operating in rhizospheric and bulk soils and subsoils, suitable for rapid deployment and accumulation of ecosystem nutrient capital under a wide range of conditions.  However on the whole biology must live under the geochemical weathering cap and so its great chemical-weathering power is brought to bear only in spatiotemporal spurts, punctuated by spaces and times characterized by little to no weathering.

Modern biologically-driven chemical weathering dynamics presumably correspond to those of ecosystem succession.  Landscape renewal and disturbance are obvious deployment opportunities.  We studied chemical weathering and the fate of products of weathering in outdoor 7.5 m x 7.5 m x 1.5 m deep forest mesocosms constructed on glacial outwash at the Hubbard Brook Experimental Forest.  Weathering rates in an aggrading, then disturbed mesocosm varied from approximately zero to 10 keq/ha/yr over a timespan of only 20 yr.  Rates and mechanisms of cation storage and loss also varied dramatically over that interval. Variations within and among natural systems are likely greater.  These considerations may provide context for the very large range of rates of chemical weathering estimated by various methods.