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Abstract 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.

This paper presents the results of a study on the response of living benthic foraminifera to progressing environmental successions in a cold-seep ecosystem. Sediment samples were collected from Vestnesa Ridge (79°N, Fram Strait) at ~1200 m water depth. The distribution of live (Rose Bengal-stained) foraminifera were analyzed in the upper sediment layers in relation to pore water biogeochemical data together with the distribution of sulfur-bacterial mats and Siboglinidae tubeworms. At methane cold seeps, the process of environmental succession is strongly connected to the duration and strength of methane seepage and the intensity of methane-related biological processes, e.g, aerobic and anaerobic oxidation of methane (MOx and AOM, respectively). The results show that the distribution patterns of benthic foraminifera change according to the progressing environmental succession. The benthic foraminifera seemed to thrive in sediments with a moderate activity of seepage, dominated by MOx, i.e, at an early stage of seepage or when seepage decreases at a late stage of the succession. Species composition of the foraminiferal fauna under these conditions was similar to the control sites (outside of pockmarks with no seepage); the dominant species being Melonis barleeanus and Cassidulina neoteretis. In sediments with strong seepage and high AOM activity, the hostile environmental conditions due to the presence of toxic sulfide caused a reduction in the foraminiferal population, and samples were almost barren of foraminifera. In environments of moderate methane seepage, the presence of chemosynthetic Siboglinidae tube worms potentially support communities of the epibenthic species Cibicidoides wuellerstorfi. Despite the very different environmental conditions, the foraminiferal assemblages were very similar (or nearly absent). Therefore, the foraminiferal faunas cannot be used as exclusive indicators of past strength of methane seepage in palaeoceanographic interpretations.

Four cores from 1200 m water depth from Vestnesa Ridge on the western Svalbard margin in the eastern Fram Strait were studied for their content of fossil macrofaunas. Three of the cores were collected from a pockmark with active methane seepage, and one core (control core) was taken just outside the seepage area for comparison. Together the cores cover the last 45 000 years (mid-late Weichselian glacial, the deglaciation and the Holocene). The records show a range of influence of methane from no seepage (control core) and, although variable through time, from moderate seepage, to strong and very strong seepage. All cores have been analysed for the macrofossils >1 mm, trace fossils, planktic foraminifera, stable isotopes, geochemistry and sedimentology. The main purpose of the study is to improve our knowledge of the fossil macrofauna and past environmental changes related to the impact of methane emissions in the area. The core recovered outside the pockmark contained no fossil macrofaunas, while cores from inside the pockmark contained chemosymbiotic bivalves, and in some cases a rich macrofauna. The faunal relationships with the sedimentary environments confirm a close connection between the macrofauna and the variability in influence of cold seepage, particularly seen in the occurrence of chemosymbiotic bivalves Archivesica arctica, Isorropodon nyeggaensis, potentially chemosymbiotic bivalve Rhacothyas kolgae, polychaetes and an associated rich fauna of small epifaunal gastropods, showing that Arctic seeps were oases for macrofaunas in the past as they are today.