Gas hydrate precipitation and microbial transformation affect the composition of gases migrating through sediments drilled off SW Taiwan

Molecular and stable isotopic compositions of light (C1–C3) hydrocarbons in sediments provide information on their formation pathways and postgenetic alterations. In 2018, gas hydrate-bearing sediment cores and borehole logging data were collected with R/V SONNE (cruise SO266) and the seafloor drill rig MARUM-MeBo200 at two sites off SW Taiwan (Bohrmann et al., 2023). The two sites are located on the passive continental margin and on the tectonically active convergent margin in the northern part of the South China Sea (SCS). Geophysical surveys have demonstrated the presence of hydrates at both sites as well as the base of the hydrate stability zone at ∼400 and 450 meters below seafloor (mbsf), respectively (Berndt et al., 2019; Bohrmann et al., 2023). At the passive margin, holes were drilled to a depth of ∼126 mbsf at the southern summit of Formosa Ridge (SSFR, ∼1,140 m water depth). A depth of ~144 mbsf was reached at Four-Way Closure Ridge (FWCR, ~1,320 water depth) on the active margin. Macroscopically, no hydrates were detected in recovered cores from either site, but hydrate-related proxies unequivocally demonstrated the in-situ presence of hydrates. For example, signals in sediment electrical resistivity detected during well logging correlated with anomalies in sediment temperature and pore water chloride concentrations detected in cores. Whereas two hydrate-bearing intervals were identified on SSFR (∼13–39 mbsf, ∼98–120 mbsf), a single interval was found on FWCR (∼65–120 mbsf).Considerable variations in relative hydrocarbon concentrations expressed as C1/(C2–C3) values were observed in gas accumulated in voids in the cores from each site. C1/(C2–C3) values