Contrasting weathering behavior of mafic and ultramafic rocks in arc-collision zones

Consumption of CO₂ through weathering of volcanic arc and ophiolite rocks in arc-collision zones in the humid tropics has been proposed to regulate Earth’s climate over geological timescales. Evaluating this hypothesis requires quantifying the factors that control CO₂ consumption from weathering of mafic and ultramafic rocks. Temperature, runoff, and physical erosion may each influence weathering rates. To determine their relative importance for arc and ophiolite rocks, we examine co-variation of stream solutes, sediment geochemistry, and erosion rates inferred from cosmogenic ³⁶Cl in magnetite in an ancient, uplifted arc-collision zone in Puerto Rico. The data reveal contrasting weathering behavior between mafic and ultramafic rocks. Consumption of CO₂ from mafic rock weathering is primarily limited by the ability of erosion to strip regolith from the landscape. Conversely, ultramafic rock weathering is more strongly controlled by runoff than by erosion. This difference likely results from the low Al concentrations in ultramafic rocks, which inhibit the formation of aluminosilicate clays and thick regolith. Additionally, we find that CO₂ consumption is not limited by temperature for either mafic or ultramafic rocks in the tropics. These results have implications for the role of tropical arc-collision zones in the Earth’s negative silicate weathering feedback.