Earth surface erosion and weathering from the 10Be (meteoric)/9Be ratio

The isotope ratio of the meteoric cosmogenic nuclide Be-10 to the mineral-derived stable isotope Be-9 discloses both the Earth surface denudation rate and its weathering intensity. We develop a set of steady state mass balance equations that describes this system from a soil column over the hillslope scale to an entire river basin. The prerequisites making this new approach possible are: (1) the Be-9 concentration of parent rock (typically 2.5 +/- 0.5 ppm in granitic and clastic sedimentary lithologies) is known; (2) both Be isotopes equilibrate between the fluids decomposing rock and reactive solids formed during weathering; and (3) a critical spatial scale is exceeded at which the fluxes of both isotopes into and out of the weathering zone are at steady state over the time scale of weathering (typically similar to 10 kyr). For these cases the isotope ratios can be determined in bulk sediment or soil, on leachates from the reactive (adsorbed and pedogenic mineral-bound) phase in sediment or soil, and even on the dissolved phase in river water. The Be-10/Be-9 ratio offers substantial advantages over the single-isotope system of meteoric Be. The latter system allows to directly determine erosion rates only in the case that Be-10 is fully retentive in the weathering zone and that riverine sorting has not introduced grain size-dependent I Be concentration gradients in sediments. We show the feasibility of the Be-10/Be-9 tracer approach at the river scale for sediment and water samples in the Amazon basin, where independent estimates of denudation rates from in situ-produced I Be exist. We furthermore calculate meaningful denudation rates from a set of published Be-10/Be-9 ratios measured in the dissolved load of globally distributed rivers. We conclude that this isotope ratio can be used to reconstruct global paleo-denudation from sedimentary records. (C) 2012 Elsevier B.V. All rights reserved.