Intraskeletal isotopic and elemental chemistry of chemosymbiont-bearing calyptogenid bivalves from an active methane seep: Implications for seep dynamics and reconstruction

Methane seeps are common continental margin features where elevated fluxes of methane- and/or sulfide-enriched pore-fluids fuel organic-carbon production by free-living and metazoan-hosted chemoautotrophic bacteria, which in turn support the development of island-like ecosystems of enhanced biomass and biodiversity. Although methane seeps may play an important role in biogeochemical cycling, their physicochemical dynamics remain poorly constrained; published data largely consist of "instantaneous" geochemical profiles of pore-waters and bulk geochemical variations in authigenic and skeletal carbonate. Spatiotemporal variation in pore-fluid flux and solute concentration is likely a key component for understanding methane seep environments and their ecological structure and stability. Such variations would alter ambient fluid chemistry at the sediment-water interface, which in turn should be recorded in the intraskeletal growth chemistry of in situ benthic macrofauna. We explored this first-order prediction through high-resolution stable-isotopic (δ18O,_ δ13C) and minor-element-ratio (Mg/Ca, Sr/Ca) analyses of intraskeletal growth in fifteen chemosymbiont-bearing Calyptogena kilmeri vesicomyid bivalves sampled from the outer-, middle-, and inner-portions of an active methane seep in 960 m depth within Monterey Canyon, off the central California coast. Intraskeletal transects reveal no clear geochemical evidence among specimens for cohesive methane-seep-wide environmental changes through time. In contrast, spatial analysis of chemical populations from the outer-, middle-, and inner-seep locations reveal significant, if subtle, higher Sr/Ca in the outer-seep location, and a significant δ13C decreases from the outer- to middle- to inner-seep locations. Maximum δ13C values in the outer-seep location are consistent with predicted molluscan δ13C enrichment above ambient seawater DIC δ13C -- a pattern in conflict with the Rio et al. (1992) model of chemosymbiont shell δ13C -enrichment through preferential 12C uptake by bacterial productivity. The decreasing δ13C values toward the cold-seep center likely reflect increasing 12C-enriched contributions of C. kilmeri-respired CO2 and/or pore-water DIC to the calcification fluid. Physiological principles would suggest an adaptive peak for C. kilmeri where (1) dissolved sulfide supply from pore-waters is optimal for bacterial productivity, but non-toxic for the bivalve, and (2) dissolved oxygen supply at the sediment-water interface is sufficient for bivalve respiration. Specimens from this adaptive peak would likely have the highest productivity:respiration ratios and exhibit the fastest tissue and shell growth-rates. While tempting to interpret this adaptive peak as proximal to the outer-seep location based on its higher Sr/Ca (i.e., faster shell growth driven by higher productivity) and higher δ13C (i.e., least affected by sulfide-rich pore-waters depleted in δ13C), the available geochemical and environmental constraints provide insufficient confidence for a unique interpretation. In contrast, δ18O values show no significant differences among seep locations, and are consistent with isotopic equilibrium precipitation from seawater DIC.