Assessment of ultramafic silicate weathering fluxes derived from high-resolution sampling: an example from the Zambales ophiolite, Luzon, Philippines

Abstract:

Ultramafic weathering in the wet and hot tropics has been proposed as an important driver in global weatherability changes influencing the long-term carbon cycle and atmospheric CO₂ over >10⁵ year timescales. However, weathering fluxes and the processes that drive high area-normalized fluxes in small tropical mafic and ultramafic catchments are relatively understudied. In this work we present new data from three adjacent ultramafic-dominated catchments (2.6 to 3.4 km²) from the Acoje Block of the Zambales ophiolite, Luzon, Philippines. Previous work based on one-time dry season samples scaled to annual fluxes from nearby catchments suggests that the Luzon arc produces some of the highest area-normalized weathering rates in the world. In 2018-19 weekly water samples were collected for 14 months and analyzed for major cations and anions, as well as δD and δ¹⁸O, by laser spectroscopy. Sampling during the wet season includes daily resolution over major storm events. Combined with discharge measurements, rainfall δD and δ¹⁸O data, and suspended sediment flux data from a complementary study, we analyze seasonal changes in weathering and assess the role of silicate versus carbonate weathering in the observed weathering fluxes.

Over the seasonal cycle we observe that weathering fluxes vary by ~2 orders of magnitude. We observe δD-δ¹⁸O relationships with shallower slopes and less total variability than rainfall, and intermittent streamflow in one catchment during the dry season, demonstrating that inter-catchment exchange by groundwater flow may influence differences in weathering fluxes from adjacent catchments. Concentration-discharge relationships demonstrate some significant dilution behavior in 2 of the 3 catchments for weathering products (SiO₂(aq), Mg²⁺ and Ca²⁺). Inverse model results, correcting for the rainwater and carbonate components, demonstrate that cation-equivalent (Ca*+Mg*+K*+Na*) fluxes are dominated (68%) by silicate weathering. Finally, our area-normalized annual silicate weathering fluxes are 1.6 to 2.5 times higher than previous work from catchments draining the Zambales Ophiolite, demonstrating the importance of paired concentration-discharge observations and high-resolution monitoring.