Determination of predevelopment denudation rates of an agricultural watershed (Cayaguás River, Puerto Rico) using in situ-produced ¹⁰Be in river-borne quartz

¹Large Lakes Observatory, University of Minnesota, Duluth, MN 55812, USA (etbrown@d.umn.edu)

²U.S. Geological Survey, 3215 Marine St., Boulder, CO 80303-1066, USA

³2U.S. Geological Survey, GSA Center, 651 Federal Drive, Suite 400-15, Guaynabo, Puerto Rico, 00965-5703, USA

⁴4CNRS-IN2P3, CSNSM, Batiment 108, 91405 Campus Orsay, France; tel. (33 1) 69 41 5203

Abstract

Accurate estimates of watershed denudation absent anthropogenic effects are required to develop strategies for mitigating accelerated physical erosion resulting from human activities, to model global geochemical cycles, and to examine interactions among climate, weathering, and uplift. We present a simple approach to estimate predevelopment denudation rates using in-situ produced cosmogenic ¹⁰Be in fluvial sediments. Denudation processes in an agricultural watershed (Cayaguás River basin, Puerto Rico) and a matched undisturbed watershed (Icacos River basin) were compared using ¹⁰Be concentrations in quartz for various size fractions of bed material. The coarse fractions in both watersheds bear the imprint of long subsurface residence times. Fine material from old shallow soils contributes little, however, to the present-day sediment output of the Cayaguás. This confirms the recent and presumably anthropogenic origin of the modern high denudation rate in the Cayaguás basin and suggests that pre- agricultural erosional conditions were comparable to those of the present-day Icacos.

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