Chemical Weathering of Mafic and Ultramafic Volcanic Arc Bedrock in a Tropical Montane Setting

Abstract

Weathering of volcanic arc rocks in the tropics is an important carbon sink over geological timescales and exerts a strong influence on the evolution of Earth‘s climate. However, the relative importance of erosion and fresh mineral supply vs. kinetic or equilibrium limits in setting weathering rates of mafic and ultramafic rocks in the tropics remains poorly constrained. To investigate the sensitivity of weathering to erosion, chemical weathering rates were determined from stream-water chemistry in 24 montane watersheds developed on basalt and serpentinite bedrock in Puerto Rico. Each watershed is underlain almost entirely by the rock type of interest and experience similar climate conditions; however, they span a gradient in mean watershed slope to capture a range of erosion rates. Landslide volume was calculated for each watershed using a landslide inventory from the Hurricane María landslide events. Total dissolved solids (TDS) and discharge at each watershed were measured during the wet and dry season baseflow to explore seasonal variations in water chemistry. No seasonal variations in the dissolved load were observed, supporting chemo-static behavior. In serpentinite watersheds, silicate TDS decreases with increasing watershed slope, whereas in the basalt watersheds TDS is invariant with watershed slope. Chemical weathering rates determined from TDS and discharge are poorly correlated with watershed slope and Hurricane María landslide volume for both rock types. This suggests that kinetic or equilibrium limits are more important than erosion and fresh mineral supply in setting serpentinite and basalt weathering rates in the tropics. Ongoing work using in situ-produced ³⁶Cl, meteoric ¹⁰Be/⁹Be, and chemical depletion factors derived from stream-sediment geochemistry will further elucidate the weathering-erosion relationship by directly constraining erosion rates and providing independent estimates of chemical denudation rates.