WIDESPREAD MARINE SILICATE WEATHERING IN ANOXIC SEDIMENTS ON THE CHILE MARGIN

Marine silicate weathering (MSiW) in anoxic sediments has been shown to serve as a potentially significant sink of carbon and is associated with other diagenetic processes like clay dehydration through ion exchange. Compared with the Northern Hemisphere, however, there has been less work on MSiW in Southern Hemisphere continental margin sediments. One exception is the Chile Margin, where MSiW was assumed based on high alkalinity at ODP Site 1234 (36°S). Recent coring revisited the Chile Margin (JOIDES Resolution Exp. 379T) and recovered 130-meter-long sediment cores that document sulfate-methane transition zones ≤5 mbsf, widespread methanogenesis, reactive detrital silicates, authigenic carbonates, and downcore increases in alkalinity ≥60 mM. Here, we report high-resolution pore water profiles of major and minor ions (Cl^-, Na⁺, Ca²⁺, Mg²⁺, K⁺, Sr²⁺, B³⁺) and oxygen isotopes (δ¹⁸O) from Sites J1002 (46°S) and J1007 (36°S) to test the hypothesis that these observations are driven by MSiW.

Ion profiles at J1002 and J1007 undergo non-steady state changes below the sediment-seawater interface. In both cores, modern seawater δ¹⁸O and Cl^- values persist or slightly increase 0-40 mbsf before lowering >1.0 ‰ and 40-70 mM by ~130 mbsf, respectively. We normalize ion profiles to Cl^- to account for possible pore water freshening with depth, which allows us to compare changes between sites. K⁺/Cl^- and Na⁺/Cl^- increase with depth, whereas Mg²⁺/Cl^-, Ca²⁺/Cl^-, and Sr²⁺/Cl^- decrease with depth. With the exception of Cl^-, these profiles are in general agreement with ODP Site 1234. We suggest that these observations support the occurrence of MSiW across a large swath of the Chile Margin.

MSiW raises pore water Cl^-, alkalinity, and the concentration of major/minor ions, but lowers δ¹⁸O. We posit that authigenic carbonate precipitation acts as a sink for Mg²⁺, Ca²⁺, and Sr²⁺ and explains the downcore ion decreases. We also suggest that low-Cl^- fluids from illitization at depth have advected vertically owing to steep geothermal gradients in the region (40-100°C/km), which has been hypothesized in the Ulleung Basin and elsewhere. Higher B³⁺/Cl^- and reductions in K⁺ at the base of J1002 and J1007 support a link between MSiW and illitization through K⁺ exchange. Taken together, we present a dynamic system on the Chile Margin with widespread MSiW and concurrent clay dehydration at depth. Given the latitudinal extent of MSiW, further work is needed to assess the influence of this region on global K⁺ and CO₂ budgets.