Links between spatially heterogeneous pore water geochemistry, fluid migration, and methane hydrate near a seafloor mound venting structure on the south Chilean Margin (41°S)

Abstract

Pore water freshening (i.e., decreases in dissolved Cl) has been documented in marine sediments along most active margins, with the migration of deep fluids or methane hydrate dissociation often invoked as sources of freshening in the sediment column. During D/V JOIDES Resolution Expedition 379T in 2019, two new sites (J1005 and J1006) were cored near ODP Site 1233 (41°S), adjacent to a seafloor mound venting structure. The three sites are less than 10 km apart but show marked differences in pore water chemistry and methane hydrate occurrence. The extent of Cl decrease is a function of distance from the mound, with the strongest freshening occurring at the closest site (J1006), which is the only site where methane hydrate was observed.Methane fluxes follow the same pattern, suggesting a common control. Increasing oxygen and decreasing hydrogen isotopes point to deep mineral bound water as the primary source of freshening near the mound, with fluids originating ˜2.5 km below seafloor near the décollement. Secondary influences from methane hydrate dissociation and ash diagenesis also appear to influence regional pore water chemistry. The variability in pore water freshening suggests that fluid migration and eventual expulsion at the venting structure follows narrow pathways, likely along faults within the forearc complex. The migration of deep, gascharged fluids may also support methane hydrate saturations greater than in situ organic carbon diagenesis would allow, but nonetheless consistent with geophysical estimates. Together, the data highlight an important link between fluid migration and methane hydrate formation on the Chilean Margin.