DISEQUILIBRIUM EROSION IN EASTERN PUERTO RICO–IS THIS HOW MANY CLASTIC SEDIMENT DEPOSITS ARE MADE?

Eastern Puerto Rico is a setting that may be representative of the island-arc source of a large portion of the first-cycle clastic sediments in the geologic record. Four doubly paired catchments (two on volcanic rocks, two on granitic rocks, with one of each rock type forested and the other agricultural) were evaluated in this montane, humid-tropical landscape. The sampling of large storms was emphasized, and we captured some of the wettest storms during a 15-year period. The common assumption of equilibrium erosion in forested landscapes in the tropics is hard to reconcile with the depositional styles of clastic deposits associated with mountain building–one of rapid deposition and long intervening periods of no deposition.

To correctly estimate denudation, atmospherically derived mass must be removed from the solute and solid loads. Solid samples were heated to 550C to estimate suspended bedrock, and solute concentrations were recalculated as oxides (after subtracting carbonate-system ions and atmospherically derived salts) to estimate dissolved bedrock. Equilibrium erosion rates were determined two ways. In the granitic watersheds, Be-10 in quartz extracted from soils, rock outcrops, and sediments provides an estimate. In all watersheds, sediments derived from erosion have very low ratios of Na₂O to SiO₂ compared with bedrock. This allows a robust calculation of equilibrium rates using representative bedrock compositions and discharge-weighted average concentrations of bedrock-derived Na₂O, and SiO₂. Despite land cover, observed suspended-bedrock yields are fivefold to tenfold the equilibrium yields on the granitic rocks, but only slightly elevated on the volcanic rocks. Accordingly, this landscape, except perhaps for the forested-volcanic watershed, is far from equilibrium and probably is more representative of conditions during major erosion cycles in the Earth’s history than the more quiescent times in between. The models presented here, however, are considerable simplifications of these complex interacting processes. It is hoped that these results inspire others to develop physically based models and geochemical tests that better define the processes and how they respond to environmental and climate change.