Successional stages of methane seep systems off central Chile: from active sediment-hosted seepage to fossil carbonate chemoherms

Methane seeps along active continental margins record the interaction between fluid flow, biogeochemical processes, benthic ecosystems, and tectonic forcing. Here we present an integrated analysis of methane seep environments from the Concepción Methane Seep Area (CMSA) on the central Chile margin (~36°S), combining seafloor imagery, multibeam bathymetry, sediment biogeochemistry, authigenic carbonate geochemistry, and U–Th chronology from the R/V Sonne 210 expedition in 2010. High-definition ROV surveys reveal a mosaic of seep habitats ranging from soft-sediment sites with active seepage and chemosynthetic fauna to extensive carbonate chemoherms representing older, largely inactive seep stages, dominated by background deep-sea fauna that utilize the carbonates as hard substrate and refuge.

Carbonates from multiple sites show strongly depleted δ¹³C values (down to ~–50‰ VPDB), confirming methane-derived carbon sources. U–Th ages span from very young (<5 ka) carbonates associated with active seepage to late Pleistocene and older structures (>100 ka), documenting long-lived and multi-phase seep activity. In contrast, some massive carbonate blocks exhibit complex internal architectures and anomalous U–Th systematics, indicating open-system behavior and requiring cautious age interpretation. Sediment biogeochemical data reveal high rates of benthic methane oxidation at active seep sites, characterized by shallow sulfate depletion and elevated sulfide concentrations. In contrast, carbonate-dominated sites lack comparable sedimentary biogeochemical signatures, primarily due to the limited presence of soft sediments, although methane oxidation may still partially occur within the carbonate framework. Water-column methane measurements indicate active methane release from the seafloor, with highest concentrations near the bottom and a pronounced decrease within the first 100–200 m above the seafloor.

By comparing multiple subregions within the CMSA, we identify distinct successional stages of seepage, progressing from sediment-hosted sites through mixed sediment–carbonate settings to predominantly fossil chemoherms. We discuss how these stages reflect temporal variability in methane flux, carbonate precipitation, and biological colonization, potentially modulated by episodic tectonic activity along the Chilean margin. Our results highlight the value of combining geomorphological, geochemical, and ecological data to reconstruct the life cycle of methane seep systems on active margins.