Chemical weathering, river geochemistry and atmospheric carbon fluxes from volcanic and ultramafic regions on Luzon Island, the Philippines

Abstract

We investigated rates of chemical weathering of volcanic and ophiolitic rocks on Luzon Island, the Philippines. Luzon has a tropical climate and is volcanically and tectonically very active, all factors that should enhance chemical weathering. Seventy-five rivers and streams (10 draining ophiolites, 65 draining volcanic bedrock) and two volcanic hot springs were sampled and analyzed for major elements, alkalinity and 87Sr/ 86Sr. Cationic fluxes from the volcanic basins are dominated by Ca 2+ and Mg 2+ and dissolved silica concentrations are high (500–1900 μM). Silica concentrations in streams draining ophiolites are lower (400–900 μM), and the cationic charge is mostly Mg 2+. The areally weighted average CO 2 export flux from our study area is 3.89 ± 0.21 × 10 6 mol/km 2/yr, or 5.99 ± 0.64 × 10 6 mol/km 2/yr from ophiolites and 3.58 ± 0.23 × 10 6 mol/km 2/yr from volcanic areas (uncertainty given as ±1 standard error, s.e.). This is ∼6–10 times higher than the current best estimate of areally averaged global CO 2 export by basalt chemical weathering and ∼2–3 times higher than the current best estimate of CO 2 export by basalt chemical weathering in the tropics. Extrapolating our findings to all tropical arcs, we estimate that around one tenth of all atmospheric carbon exported via silicate weathering to the oceans annually is processed in these environments, which amount to ∼1% of the global exorheic drainage area. Chemical weathering of volcanic terranes in the tropics appears to make a disproportionately large impact on the long-term carbon cycle.