Lithological Influences on Contemporary and Long-Term Regolith Weathering at the Luquillo Critical Zone Observatory

1 Lithologic differences 2 variable silicate weathering fluxes, which provide an important negative feedback on climate over 3 geologic timescales. To isolate the influence of lithology on weathering rates and mechanisms, we 4 compare two nearby catchments in the Luquillo Critical Zone Observatory in Puerto Rico, which 5 have similar climate history, relief and vegetation, but differ in bedrock lithology. Regolith and pore 6 water samples with depth were collected from two ridgetops and at three sites along a slope transect 7 in the volcaniclastic Bisley catchment and compared to existing data from the granitic Río Icacos 8 catchment. The depth variations of solid-state and pore water chemistry and quantitative mineralogy 9 were used to calculate mass transfer (tau) and weathering solute profiles, which in turn were used to 10 determine weathering mechanisms and to estimate weathering rates. 11 Regolith formed on both lithologies is highly leached of most labile elements, although Mg 12 and K are less depleted in the granitic than in the volcaniclastic profiles, reflecting residual biotite 13 in the granitic regolith not present in the volcaniclastics. Profiles of both lithologies that terminate at 14 bedrock corestones are less weathered at depth, near the rock-regolith interfaces. Mg fluxes in the 15 volcaniclastics derive primarily from dissolution of chlorite near the rock-regolith interface and 16 from dissolution of illite and secondary phases in the upper regolith, whereas in the granitic profile, 17 Mg and K fluxes derive from biotite dissolution. Long-term mineral dissolution rates and 18 weathering fluxes were determined by integrating mass losses over the thickness of solid-state 19 weathering fronts, and are therefore averages over the timescale of regolith development. Resulting 20 long-term dissolution rates for minerals in the volcaniclastic regolith include chlorite: 8.9 x 10 -14 21 mol m -2 s -1 , illite: 2.1 x 10 -14 mol m -2 s -1 and kaolinite: 4.0 x 10 -14 mol m -2 s -1 . Long-term weathering 22 fluxes are several orders of magnitude lower in the granitic regolith than in the volcaniclastic, 23 despite higher abundances of several elements in the granitic regolith. Contemporary weathering 24 fluxes were determined from net (rain-corrected) solute profiles and thus represent rates over the 25 residence time of water in the regolith. Contemporary weathering fluxes within the granitic regolith 26 are similar to the long-term fluxes. In contrast, the long-term fluxes are faster than the 27 contemporary fluxes in the volcaniclastic regolith. Contemporary fluxes in the granitic regolith are 28 generally also slightly faster than in the volcaniclastic. The differences in weathering fluxes over 29 space and time between these two watersheds indicate significant lithologic control of chemical 30 weathering mechanisms and rates. 31