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 <10.000 in the hydrate-bearing sections, which contrast with values ranging between 10.000 and 25.000 in sections lacking hydrates, indicate that ethane (C₂) and to a lesser extent propane (C₃) are enriched during hydrate precipitation. Molecular fractionation is also observed for CO₂, which is strongly depleted in the hydrate-bearing sections. The δ¹³C- (–79 to –69‰) and δ²H- (–197 to –187‰) values of methane (C₁) indicate that microbial carbonate reduction is the major source of light hydrocarbons (Milkov & Etiope, 2018) at both sites. Based on pore water sulfate and methane concentrations, the zone of the microbially-mediated sulfate-dependent anaerobic oxidation of methane was identified at a depth of ~10–12 mbsf at both sites. Preferential consumption of C₁ in this zone is indicated by low C₁/(C₂–C₃) values. The process also resulted in depletions of C₁ in ¹³C and ¹H (δ¹³C-C₁ as low as -100‰, δ²H-C₁ as high as -179‰ at 18 mbsf at FWCR) as reported from other regions (e.g., Nankai Trough off Japan; Riedinger et al., 2015).
Our results show that physical fractionation and bio(chemical) transformation of individual light hydrocarbons can significantly change the molecular and isotopic composition of upward migrating gases. Therefore, the composition of shallow gas does not necessarily reflect that of the gas in the deeper subsurface, for example as bound in capacious hydrate reservoirs. The cores from the SCS are excellent for studying how hydrate occurrences and microbial transformation lead to alteration of gas composition.