Influence of methane seepage on isotopic signatures in living deep-sea benthic foraminifera, 79° N

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.

Sulfide toxicity as key control on anaerobic oxidation of methane in eutrophic coastal sediments

“…(2) the concentration gradient of methane into the SMTZ from below and (3) the concentration gradient between the top layer of the sediment and the bottom water. We note that the methane fluxes may be underestimated in sediment sections with high methane concentrations because of loss of methane from the porewater due to degassing during sampling, as observed in previous studies (Egger et al, 2016;Melaniuk, Sztybor, Treude, Sommer, & Rasmussen, 2022).…”

“…where ‫ܬ‬ represents the diffusive flux (mmol m -2 d -1 ), ߶ represents the sediment porosity, ‫ܦ‬ ௦ represents the sediment diffusion coefficient for the ambient tortuosity, pressure, temperature and salinity at each site was calculated using the R package marelac (Soetaert, Petzoldt, & Meysman, 2010), which implements the constitutive relations previously listed (Boudreau, 1997) and dC/dz is: (1) the concentration gradient of sulfate from above the SMTZ into the SMTZ; (2) the concentration gradient of methane into the SMTZ from below and (3) the concentration gradient between the top layer of the sediment and the bottom water. We note that the methane fluxes may be underestimated in sediment sections with high methane concentrations because of loss of methane from the porewater due to degassing during sampling, as observed in previous studies (Egger et al, 2016;Melaniuk, Sztybor, Treude, Sommer, & Rasmussen, 2022).…”

“…The calculated downward flux of sulfate into the SMTZ (Table 3) is highest at Sites 5 and 7 (1.5 and 1.3 mmol m 2 d -1 ), but still substantial (almost 1 mmol m 2 d -1 ) at Site 3. The calculated upward flux of methane into the SMTZ at Site 3 is nearly absent, and around 0.5 mmol m 2 d -1 at Sites 5 and 7, which should be regarded as an absolute minimum estimate due to degassing of methane during sampling (Egger et al, 2016; Melaniuk et al, 2022). There is no benthic methane efflux at Site 3.…”

Response of benthic foraminifera to environmental successions of cold seeps from Vestnesa Ridge, Svalbard: Implications for interpretations of paleo-seepage environments

“…The sampling on POS419 and CAGE 15-2 cruises were done using a TV-guided multicorer to visually localize active methane seep sites within the pockmarks to select sampling spots. Porewater data for multicores MUC 10, MUC 11, and MUC 12 has been previously published (Melaniuk et al, 2022), and foraminiferal fauna data for multicores MC 893A and MC 886 in Melaniuk (2021). For this study we have added multicore MUC 8 (POS419) for additional analyses of porewater and foraminiferal faunas (Figure 1).…”

“…Rose Bengal was used in this study to make our results comparable with most other studies. All samples were kept onboard in a dark, cool (+4°C) room until further processing (for details see Melaniuk, 2021;Melaniuk et al, 2022).…”