Iron cycling in Arctic methane seeps

Abstract

Anoxic marine sediments contribute a significant amount of dissolved iron (Fe 2+) to the ocean which is crucial for the global carbon cycle. Here, we investigate iron cycling in four Arctic cold seeps where sediments are anoxic and sulfidic due to the high rates of methane-fueled sulfate reduction. We estimated Fe 2+ diffusive fluxes towards the oxic sediment layer to be in the range of 0.8 to 138.7 μmole/m 2 /day and Fe 2+ fluxes across the sediment-water interface to be in the range of 0.3 to 102.2 μmole/m 2 /day. Such variable fluxes cannot be explained by Fe 2+ production from organic matter-coupled dissimilatory reduction alone. We propose that the reduction of dissolved and complexed Fe 3+ as well as the rapid formation of iron sulfide minerals are the most important reactions regulating the fluxes of Fe 2+ in these cold seeps. By comparing seafloor visual observations with subsurface pore fluid composition, we demonstrate how the joint cycling of iron and sulfur determines the distribution of chemosynthesisbased biota.

Methane transport and sources in an Arctic deep-water cold seep offshore NW Svalbard (Vestnesa Ridge, 79°N)

“…All cores from the Lunde and Lomvi pockmark showed low concentrations of ammonium and phosphate, which suggests low organic matter degradation rates (Froelich et al, 1979). Similar concentrations were reported from cold seeps south of Svalbard and the northern Norwegian margin (Hong et al, 2020). Peaks in Fe 2+ and Mn 2+ (as in PC020 and PC021) are commonly also associated with organic matter degradation utilizing Mn and Fe (oxyhydr)oxides as electron acceptors and releasing dissolved Mn 2+ and Fe 2+ to the pore water (Froelich et al, 1979).…”

The Impact of Methane on Microbial Communities at Marine Arctic Gas Hydrate Bearing Sediment

“…SO 4 2– was measured onshore using ion chromatography ( Hong et al, 2017 ), while ΣHS was measured using a spectrophotometer at a wavelength of 670 nm ( Cline, 1969 ). A detailed protocol on the measurement of ΣHS can be found in the Supplementary Material of Hong et al (2020) . Due to equipment availability on the two field cruises, ΣHS and DIC concentrations were not measured for all sediment cores while alkalinity and Fe 2+ concentrations were only measured for a selection of sediment layers ( Supplementary Tables 1 – 5 ).…”

“…Porewater geochemistry was measured for all cores, and data for BC_1029 and MC_1063 were collected from Hong et al (2020). CH 4 concentrations were measured with a head space technique and gas chromatography (Thermoscientific Trace 1310) equipped with a flame ionization detector (Hoehler et al, 2000;Panieri et al, 2017).…”

“…SO 4 2− was measured onshore using ion chromatography (Hong et al, 2017), while HS was measured using a spectrophotometer at a wavelength of 670 nm (Cline, 1969). A detailed protocol on the measurement of HS can be found in the Supplementary Material of Hong et al (2020). Due to equipment availability on the two field cruises, HS and DIC concentrations were not measured for all sediment cores while alkalinity and Fe 2+ concentrations were only measured for a selection of sediment layers ( Supplementary Tables 1-5). DNA Extraction, Sequencing, and Sequences Analyses Sediment cores were extruded and 2 cm thick layers were transferred in Whirl-Pak R sterile sampling bags (Nasco, United States) and stored at -80 • C. Following the measurements of the different environmental parameters in the laboratory, 55 of these samples were selected for amplicon libraries sequencing.…”

Distinct methane-dependent biogeochemical states in Arctic seafloor gas hydrate mounds

“…7-10 measurements were performed for every sample. TA was calculated from the recorded pH and amount of acid added using the Gran function 68 . Increases in porewater alkalinity determined by onboard titrations were used to roughly constrain the SMT depths (within 30 cm) for sampling purposes.…”