Fossil benthic foraminifera are used to trace past methane release linked to climate change. However, it is still debated whether isotopic signatures of living foraminifera from methane-charged sediments reflect incorporation of methane-derived carbon. A deeper understanding of isotopic signatures of living benthic foraminifera from methane-rich environments will help to improve reconstructions of methane release in the past and better predict the impact of future climate warming on methane seepage. Here, we present isotopic signatures (δ13C and δ18O) of foraminiferal calcite together with biogeochemical data from Arctic seep environments from c. 1200 m water depth, Vestnesa Ridge, 79° N, Fram Strait. Lowest δ13C values were recorded in shells of Melonis barleeanus, − 5.2‰ in live specimens and − 6.5‰ in empty shells, from sediments dominated by aerobic (MOx) and anaerobic oxidation of methane (AOM), respectively. Our data indicate that foraminifera actively incorporate methane-derived carbon when living in sediments with moderate seepage activity, while in sediments with high seepage activity the poisonous sulfidic environment leads to death of the foraminifera and an overgrowth of their empty shells by methane-derived authigenic carbonates. We propose that the incorporation of methane-derived carbon in living foraminifera occurs via feeding on methanotrophic bacteria and/or incorporation of ambient dissolved inorganic carbon.

Highly negative δ13C values in fossil foraminifera from methane cold seeps have been proposed to reflect episodes of methane release from gas hydrate dissociation or free gas reservoirs triggered by climatic changes in the past. Because most studies on live foraminifera are based on the presence of Rose Bengal staining, that colors the cytoplasm of both live and recently dead individuals it remains unclear if, and to what extent live foraminifera incorporate methane-derived carbon during biomineralization, or whether the isotopic signature is mostly affected by authigenic overgrowth. In this paper, modern foraminiferal assemblages from a gas hydrate province Vestnesa Ridge (∼1,200 m water depth, northeastern Fram Strait) and from Storfjordrenna (∼400 m water depth in the western Barents Sea) is presented. By using the fluorescent viability assays CellTrackerTM Green (CTG) CMFDA and CellHunt Green (CHG) together with conventional Rose Bengal, it was possible to examine live and recently dead foraminifera separately. Metabolically active foraminifera were shown to inhabit methane-enriched sediments at both investigated locations. The benthic foraminiferal faunas were dominated by common Arctic species such as Melonis barleeanus, Cassidulina neoteretis, and Nonionellina labradorica. The combined usage of the fluorescence probe and Rose Bengal revealed only minor shifts in species compositions and differences in ratios between live and recently dead foraminifera from Storfjordrenna. There was no clear evidence that methane significantly affected the δ13C signature of the calcite of living specimens.

The main goal of CAGE 17-2 AMGG cruise was to study the gas-hydrate-bearing system and methane emission off south and east of Spitsbergen in Storfjordrenna and the northern flank of Olga Basin (named here Olga craters) respectively, and in the West Sentralbanken. We addressed this through a comprehensive scientific program comprising dives with the MISO-Tow Cam adapted to the multicorer frame from UiT-NPI (TowCam/Multicorer, TCM), methane measurements in sediments, water column, and in air, sediment coring (multicorer + gravity corer), water column and sediment biogeochemistry, microbiology, micropaleontology, and bathymetric mapping. Cruise CAGE 17-2 was also hosting this year’s AMGG research school cruise with masters, PhD and post-doc students participating. The areas investigated were: Storfjordrenna, Pingos site (ca 380 m water depth),Northern Flank of Olga Basin (ca 140 m water depth)West Sentralbanken (ca 200 m water depth) We planned the following activities during the CAGE 17-2 cruise: EM 302 Simrad swath bathymetry mapping to identify seabed morphology Mapping of flare distributionsCTD stations at different water depths and in different areas for measurements ofocean water masses characteristics, andwater sampling for water/gas chemistry and microbiology investigations across methane seeps.TCM surveys (video-camera) to image seabed fluid flow expressions, sites of bacteria mats, crusts and gas bubbles.Repeated deployments with TCM to sample surficial and shallow sediments with respect to microbiology, geochemistry, biogeochemistry, and micropaleontology.Gravity corer for studying sediment biogeochemistry, biomarkers, microbiology, and foraminifera.Scrape sampling to collect rocks and crusts.Gas Chromatographer (GC) to measure methane concentration in the water and sediment samples.Flasks Restek, Electro-Polished Miniature Canister (1000 cc) for air samples. Part of the cruise was supported by NPD, Oljedirektoratet. Special thanks to Rune Mattingsdal, NPD. The cruise may be known as: CAGE17_2