Isotopic and microtextural signatures of Holocene foraminifera at a methane seepage site in the western South Atlantic

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Methane seepage associated with gas hydrate systems represents a key fluid-driven process controlling carbonate geochemistry and diagenesis along continental margins. Recognition of seep signatures in foraminiferal shells, however, remains a research challenge. Here we investigate benthic and planktonic foraminifera from a degassing pockmark in the Rio Grande Cone, western South Atlantic, comparing the seep site with gas hydrates and bubble streams with a background site at the pockmark rim. We integrate δ 13 C and δ 18 O data of Uvigerina (benthic) and Globigerinoides ruber albus (planktonic) with synchrotron-based micro–X-ray fluorescence (μXRF) elemental mapping and micro-Raman spectroscopy. Seep foraminifera exhibit systematic 13 C depletion and 18 O enrichment relative to background specimens, and multivariate analyses confirm significant isotopic separation between the seep and background sites . No differences between living and dead Uvigerina indicate isotopic signals are acquired during life rather than post-mortem. Near-zero sulfate at the sediment surface and positive δ 13 C–dissolved inorganic carbon (DIC) values indicate the sulfate–methane transition zone (SMTZ) is suppressed to the sediment–water interface by intense gas flux, limiting in-sediment anaerobic oxidation of methane (AOM) and suggesting deep advective DIC dominates calcification. Elemental maps reveal heterogeneous distributions of redox-sensitive trace metals (Fe, Mn, Zn, Cr, V) associated with putative microbial coatings , whereas calcium remains homogeneous within the primary calcitic matrix. Micro-Raman analyses identify pyrite and organic carbon bands, but no evidence for methane-derived authigenic carbonate overgrowth, indicating limited diagenetic overprinting and preservation of primary geochemical signals in this Holocene high-flux seep system. This study provides the first multiproxy evidence from the western South Atlantic that seep signals in foraminifera are acquired primarily during life, with limited diagenetic overprinting in this Holocene high-flux system.