DETERMINATION OF PREDEVELOPMENT DENUDATION RATES OF AN AGRICULTURAL WATERSHED (CAYAGUAS RIVER, PUERTO-RICO) USING IN-SITU-PRODUCED BE-10 INRIVER-BORNE QUARTZ

Accurate estimates of watershed denudation absent anthropogenic effect s are required to develop strategies for mitigating accelerated physic al erosion resulting from human activities, to model global geochemica l cycles, and to examine interactions among climate, weathering, and u plift. We present a simple approach to estimate predevelopment denudat ion rates using in-situ-produced cosmogenic Be-10 in fluvial sediments . Denudation processes in an agricultural watershed (Cayaguas River Ba sin, Puerto Rico) and a matched undisturbed watershed (Icacos River Ba sin) were compared using Be-10 concentrations in quartz for various si ze fractions of bed material. The coarse fractions in both watersheds bear the imprint of long subsurface residence times. Fine material fro m old shallow soils contributes little, however, to the present-day se diment output of the Cayaguas. This confirms the recent and presumably anthropogenic origin of the modern high denudation rate in the Cayagu as Basin and suggests that pre-agricultural erosional conditions were comparable to those of the present-day Icacos. (C) 1998 Elsevier Scien ce B.V. All rights reserved.