Denudation rates based on accumulation of in situ produced ¹⁰Be compared with watershed mass balance results in the Luquillo Experimental Forest, Puerto Rico

¹(CNRS-IN2P3, CSNSM, Batiment 108, 91405 Campus Orsay, France; tel. (33 1) 69 41 5203;e-mail brown@csncls.in2p3.fr)

²(U.S. Geological Survey, 3215 Marine St., Boulder, CO 80303-1066, USA; tel. 303 541-3022)

Abstract

Nuclear reactions induced by cosmic rays produce rare nuclides within the mineral lattices of exposed surficial rocks and soils. After surface exposure of 10 ⁴ to 10 ⁵ yr in rapidly eroding environment s, these in situ produced cosmogenic nuclides attain steady-state concentrations which are inversely related to the mean denudation rate of the exposed surface. If river bedload sand is considered to represent an integrated average (weighted for flux) of the material exported from the various erosional environments of a basin, a few measurements of a cosmogenic nuclide in that sand may constrain the mean long-term erosion rate for the entire basin.

To examine the systematics of cosmogenic nuclides in soils and sands, we have initiated a preliminary study of in situ produced ¹⁰Be in quartz from the Icacos River basin in the Luquillo Experimental Forest (Puerto Rico). This relatively undisturbed lower montane wet forest has undergone extensive geological and biological surveys, permitting constraint of processes which must be considered in erosion rate calculations; soil mixing by bioturbation, and cosmic ray shielding by overlaying biomass and by local topography. Calculated erosion rates are more sensitive to soil mixing than to the other shielding parameters. Mass balance studies of the Icacos watershed (3.26 km²) indicate a present day mean erosion rate of ~75 meters/million yr(m/Myr).

Our samples included exposed horizontal rock faces, soil profiles, soils excavated by worms, soil and gravel exposed by mass wasting, and various size fractions of river bedload. Results from surface soils show that ¹⁰Be is constant throughout the bioturbated layer above the saprolite. Normalizing for 50 cm of bioturbation, the ¹⁰Be concentrations (~2 x 10⁵ atoms/gram) in stable ridgecrest soils and i n recently exposed flat bedrock surfaces correspond to denudation rates of ~20 m/ Myr. In contrast, much lower ¹⁰Be concentrations were found in gravel (<10⁴ atoms/gram) and 0.25-0.5 mm sand (3 x 10⁴) fr om a rapidly eroding landslide.

Riverine sands with a 0.25-0.5 mm grainsize had concentrations comparable to those of the ridgecrest soils, indicating the dominance of material from such stable environments. However, coarser sand had lower ¹⁰Be concentrations, suggesting that more material from areas undergoing more rapid denudation is present in the larger grain size fractions. Accurate constraint of the mean denudation of a basin will require samples in which the grain size distribution is representative of the long-term mean.

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