Controls on major solutes within the drainage network of a rapidly weathering tropical watershed

Abstract

Surface water chemistry in the main stem and source points of the Rio Icacos basin (Luquillo Experimental Forest, Puerto Rico) was studied to investigate the factors regulating spatial variability in major solutes in a rapidly weathering landscape. We sampled along the main stem as well as at small source points at high elevation where fresh bedrock is frequently exposed, and at low elevation in the floodplain/colluvial plain of the main stem. Concentrations of silicon, alkalinity, and the sum of base cations were lower at the source points than in the main stem, and were lowest in low-elevation source points. Calcium and sodium were the dominant cations at all sampling points after sea-salt correction, reflecting the weathering of plagioclase feldspar throughout the basin. The partial pressure of carbon dioxide (pCO 2) tended to be higher, and HCO3- concentrations were lower, in the low-elevation source points than at other positions in the landscape. When coupled with the relatively low concentrations of Si and base cations, this suggests that the availability of primary reactive minerals, rather than carbonic acid concentrations, limits weathering in these low-elevation sources. Mechanical denudation appears to enhance chemical weathering rates not only by refreshing reactive mineral surfaces but also by contributing carbon dioxide from the decomposition of organic-rich material in landslides, which occur frequently. The spatial variability of major solutes appears to depend primarily on the availability of fresh primary reactive minerals, carbon dioxide concentrations, and hydrolysis conditions. Copyright 2007 by the American Geophysical Union.