Pore Fluid Origins, Circulation, and Links With Methane Hydrate on the South-Central Chilean Margin

Abstract

The chemical composition of marine sedimentary pore waters, notably freshening signals inferred from decreases in dissolved Cl⁻, have demonstrated a link between methane hydrate accumulation and the circulation of fluids and gases in convergent margin systems. However, the south-central Chilean Margin (32–46°S) lacks geochemical evidence for this relationship. In 2019, D/V JOIDES Resolution Expedition 379T drilled two sites (J1005 and J1006) near legacy site ODP 1233 (41°S) and recovered 120 m sediment cores from a seafloor venting structure. The sites are less than 10 km apart but exhibit differences in pore water chemistry and methane hydrate occurrence. The extent of Cl⁻ decrease is a function of distance from the venting structure, with the greatest freshening (and only recovery of methane hydrate) occurring at the closest site. Methane fluxes follow the same pattern, suggesting a common influence. Increasing oxygen and decreasing hydrogen isotopes point to mineral bound water originating ∼2.5 km below the seafloor as the primary source of pore water freshening. In contrast, marine silicate weathering coupled to methanogenesis, authigenic carbonate formation, and the alteration of oceanic crust regulate Sr systematics. These spatial heterogeneities indicate that fluid migration is attributable to regional overpressures in the accretionary complex and flows along narrow fault structures. We suggest that the focused migration of deep, gas-charged fluids serves as a model for regional methane hydrate accumulation, reconciling model estimates and field observations. Collectively, our results highlight an important link between regional hydrogeology, diagenetic processes, and methane hydrate formation on the south-central Chilean Margin.