Pre-anthropogenic denudation rates of a perturbed watershed (Cayaguás River, Puerto Rico) estimated from in situ produced ¹⁰ Be in river borne quartz

¹CNRS-IN2P3, CSNSM, Batiment 108, 91405 Campus Orsay, France

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

Abstract

Earlier work has shown that in situ-produced ¹⁰Be in fluvial quartz eroded from small geologically-simple watersheds has an average concentration that is inversely related to the long-term mean denudation rate (Brown et al., EPSL, 1995). This study applies this approach to examination of anthropogenically perturbed catchments in Puerto Rico, where we determined concentrations of ¹⁰Be as a function of grain size in fluvial quartz. All ¹⁰Be measurements were made at the Tandtron AMS Facility, Gif-sur-Yvette, France.

Here we compare results from the nearly pristine Icacos River basin (3.26 km²) and from the Cayaguás River basin (26.4 km²), a geologically-similar watershed that was strongly affected by conversion to agriculture in the 19th and 20th centuries. The resultant rapid erosion stripped off the upper layer of unconsolidated soil. Subsequent denudation rates may have decreased with the exposure of more stable saprolite at the surface. Present-day mass balance for the Cayaguás indicates an annual suspended sediment yield corresponding to an erosion rate of ~750 mm/ka. In the Icacos catchment, ¹⁰Be distributions indicated an average denudation rate of 43 mm/ka, comparable to a rate of 75 mm/ka based on present-day mass balance. Lower ¹⁰Be concentrations observed in Cayaguás stream sediments correspond to a rate of ~85 mm/ka. Although this value is somewhat higher than that determined for the Icacos catchment, it is of the same order of magnitude, and demonstrates that the sediments retain the signal of pre-anthropogenic denudation. Assuming that the initial conditions in the Cayaguás were comparable to those observed in the Icacos, the lower concentrations in the Cayaguás suggest an average loss of ~50 cm of soil and saprolite since the initiation of human activity in the basin.

The distribution of ¹⁰Be concentrations as a function of grain size i n river sediment reflects the style of erosion within a basin. Coarse material is more abundant at greater depth in weathering profiles, so the ratio of the ¹⁰Be concentration in fine to coarse material at a g iven site is a function of the contribution of energetic processes to mass wasting. We determined ratios of ~5 to 6 for the Icacos basin as a whole, but only ~3.5 in the Guabá sub-basin where few active landslides have been observed. In the Cayaguás basin, where most of the surface soil layer has been lost by intense erosion related to human activity, the ratio is only ~2 to 3, consistent with the bulk of the sediment coming from a relatively thin near-surface horizon.

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