UC Santa Barbara

Tectonic uplift has been hypothesized to accelerate silicate weathering rates and have driven atmospheric CO2 drawdown in the late Cenozoic, yet quantifying tectonic control on silicate weathering rates is complicated by other rate-controlling factors like climate and lithology. The Nicoya Peninsula, Costa Rica, provides a unique opportunity to assess how silicate weathering rates vary across a tenfold variation in uplift rates under near-constant climate and basaltic lithology. Here we report stream water chemistry and 87Sr/86 Sr ratios from 12 catchments draining mountainous terrain on Nicoya. From stream water chemistry, we partitioned weathering solutes from silicates and carbonates and then estimated catchment-scale stream discharge and sediment flux by calibrating global empirical models against hydrological gauging data in neighboring catchments. Combining these data, we determined the catchments’ total denudation and silicate weathering rates and find that weathering on Nicoya is ‘kinetically-limited’, or where silicate weathering rates no longer increase linearly with denudation rates. The deviation from linear behavior is related to the extent of low-relief geomorphic surfaces (known as the Cerro Azul surface) in catchments, which likely sets sediment residence time and weathering duration. Nicoya falls at the high end of a global trend between temperature and basaltic weathering rates, supporting the temperature dependence of basalt weathering. Sr isotopic data confirms low 87Sr/86Sr ratios from basalt weathering and suggest that the weathering of mafic lithologies in tropical margins would not increase Cenozoic seawater Sr ratios. This study enhances our understanding of silicate weathering in an uplifting tropical basaltic range, while also highlighting geomorphic controls on silicate weathering.