Using Bathymodiolus tissue stable isotope signatures to infer biogeochemical process at hydrocarbon seeps

Abstract:

Here we use stable isotopes of carbon, nitrogen and sulfur in the tissue of two bathymodiolin mussel species with different chemotrophic symbionts (methanotrophs in B. platifrons and sulfide-oxidizers in B. aduloides) to gain insights into the biogeochemical processes at an active site in 1120 m depth on the Formosa Ridge, called Site F. Because mussels with methanotrophic symbionts acquire the isotope signature of the used methane, the average δ¹³C values of B. platifrons (–70.3‰; n=36) indicates a biogenic methane source at Site F, consistent with the measured carbon isotope signature of methane (–61.1‰ to –58.7‰) sampled 1.5 m above the mussel beds. The only small offset between the δ¹³C signatures of the ascending methane and the authigenic carbonate at site F (as low as –55.3‰) suggests only minor mixing of the pore water with marine bicarbonate, which in turn may be used as an indicator for advective rather than diffusive seepage at this site. B. aduloides has much higher average δ¹³C values of –34.4‰ (n=9), indicating inorganic carbon (DIC) dissolved in epibenthic bottom water as its main carbon source. The DIC was apparently marine bicarbonate with a small contribution of ¹³C-depleted carbon from locally oxidized methane. The δ³⁴S values of the two mussel species indicate that they used two different sulfur sources. B. platifrons (average δ³⁴S = +6.4±2.6‰; n=36) used seawater sulfate mixed with isotopically light re-oxidized sulfide from the sulfate-dependent anaerobic oxidation of methane (AOM), while the sulfur source of B. aduloides (δ³⁴S = –8.0±3.1‰; n=9) was AOM-derived sulfide used by its symbionts. δ¹⁵N values differed between the mussels, with B. platifrons having a wider range of on average slightly lower values (mean = +0.5±0.7‰, n=36) than B. aduloides (mean = +1.1±0.0‰). These values are significantly lower than δ¹⁵N values of South China Sea deep-sea sediments (+5‰ to +6‰), indicating that the organic nitrogen is of local origin, possibly resulting from the activity of autotrophic bacteria and due to assimilation of isotopically light nitrate or ammonium by the symbionts.

Acknowledgments: Financial support was provided by the NSF of China (Grants: 41422602 and 41373085) and the “Hundred Talents Program” of CAS.