Sr isotopes as a tracer of weathering processes and dust inputs in a tropical granitoid watershed, Luquillo Mountains, Puerto Rico

Sr isotope data from soils, water, and atmospheric inputs in a small tropical granitoid watershed in the Luquillo Mountains
of Puerto Rico constrain soil mineral development, weathering fluxes, and atmospheric deposition. This study provides
new information on pedogenic processes and geochemical fluxes that is not apparent in watershed mass balances based on
major elements alone. 87Sr/86Sr data reveal that Saharan mineral aerosol dust contributes significantly to atmospheric inputs.
Watershed-scale Sr isotope mass balance calculations indicate that the dust deposition flux for the watershed is
2100 ± 700 mg cm2 ka1. Nd isotope analyses of soil and saprolite samples provide independent evidence for the presence
of Saharan dust in the regolith. Watershed-scale Sr isotope mass balance calculations are used to calculate the overall
short-term chemical denudation velocity for the watershed, which agrees well with previous denudation rate estimates based
on major element chemistry and cosmogenic nuclides. The dissolved streamwater Sr flux is dominated by weathering of plagioclase
and hornblende and partial weathering of biotite in the saprock zone. A steep gradient in regolith porewater 87Sr/86Sr
ratio with depth, from 0.70635 to as high as 0.71395, reflects the transition from primary mineral-derived Sr to a combination
of residual biotite-derived Sr and atmospherically-derived Sr near the surface, and allows multiple origins of kaolinite to be