Ultramafic weathering in tropical volcanic arcs: Geochemistry and inverse modeling of rivers draining the Zambales Ophiolite Complex (ZOC), Philippines

Manuel Justin Custado , Daniel Enrique Ibarra , Hannah Tompkins, Jenna Serotta, Thomas Gotsch, Carlos Primo David, Justine Perry Domingo

Mafic and ultramafic rocks are highly susceptible to silicate weathering, a key negative feedback which modulates atmospheric CO2 over geological timescales. The tectonic exposure of these reactive lithologies, such as the Neogene uplift of the volcanic islands in maritime Southeast Asia, has been hypothesized to enhance this global feedback and contribute to long-term global cooling. However, the extent of its contribution is not clearly expressed in marine radiogenic strontium isotopic records. Evaluating this hypothesis motivates improving constraints on solute fluxes across the geologically complex region to refine interpretations of geochemical proxies for weathering. This work characterizes the silicate weathering process in the Zambales Ophiolite Complex (ZOC), Philippines using an updated geochemical dataset of major ions (Ca2+, Mg2+, Na+, Cl-) and 87Sr/86Sr, combining published data with new measurements of water, soil, and rocks collected from three sampling campaigns covering wet and dry seasons across 25 catchments. We employ inversion modeling to partition river geochemistry, explicitly accounting for lithological complexity and incorporating 87Sr/86Sr data and clay formation as a solute sink. Sensitivity analysis shows that the incorporation of these constraints substantially influences the calculated contributions of sources to the overall weathering flux. This approach yields a long-term silicate weathering-derived CO2 consumption of 2.15 x 106 mol CO2 km-2 yr-1 ± 25.77% (1σ) in the ZOC, significantly higher than global average. While adopting a more conservative framework returned an estimate lower than previous studies, it reinforces the disproportionately high area-normalized weathering fluxes from the ZOC. Principal component analysis additionally shows association between CO2 consumption and watershed features related to topography and runoff. Combining our calculations with published data, we estimate that SEA, despite occupying <2% of the Earth’s continental surface, accounts for approximately 7.6% of the global silicate weathering budget. This regional CO2 consumption per unit area corresponds to approximately four times the global continental average. Further, several altered ultramafic samples exhibit higher 87Sr/86Sr values than expected for mantle-derived material, a signature also expressed in a subset of river water samples. Compiling published data for mafic and ultramafic rocks across the region also show a considerable range of 87Sr/86Sr ratios (0.702-0.709). These findings highlight the importance of incorporating the geochemical variability of mafic and ultramafic bedrocks in inversion models to estimate weathering fluxes and motivates further work on investigating its influence on marine isotopic records to better understand the role of SEA in the evolution of the global carbon cycle.