Biomarker and Isotopic Composition of Seep Carbonates Record Environmental Conditions in Two Arctic Methane Seeps

Abstract

Present-day activity of cold seeps in the ocean is evident from direct observations of methane emanating from the seafloor, the presence of chemosynthetic organisms, or the quantification of high gas concentrations in sediment pore waters and the water column. Verifying past cold seep activity and biogeochemical characteristics is more challenging but may be reconstructed from proxy records of authigenic seep carbonates. Here, we investigated the lipid-biomarker inventory, carbonate mineralogy, and stable carbon and oxygen isotope compositions of seep-associated carbonates from two active Arctic methane seeps, located to the northwest (Vestnesa Ridge; ∼1,200 m water depth) and south (Storfjordrenna; ∼380 m water depth) offshore Svalbard. The aragonite-dominated mineralogy of all but one carbonate sample indicate precipitation close to the seafloor in an environment characterized by high rates of sulfate-dependent anaerobic oxidation of methane (AOM). In contrast, Mg-calcite rich nodules sampled in sediments of Storfjordrenna appear to have formed at the sulfate-methane-transition zone deeper within the sediment at lower rates of AOM. AOM activity at the time of carbonate precipitation is indicated by the 13C-depleted isotope signature of the carbonates [−20 to −30‰ Vienna Pee Dee Belemnite (VPDB)], as well as high concentrations of 13C-depleted lipid biomarkers diagnostic for anaerobic methanotrophic archaea (archaeol and sn2-hydroxyarchaeol) and sulfate-reducing bacteria (iso and anteiso-C15:0 fatty acids) in the carbonates. We also found 13C-depleted lipid biomarkers (diploptene and a 4α-methyl sterol) that are diagnostic for bacteria mediating aerobic oxidation of methane (MOx). This suggests that the spatial separation between AOM and MOx zones was relatively narrow at the time of carbonate formation, as is typical for high methane-flux regimes. The seep-associated carbonates also displayed relatively high δ18O values (4.5–5‰ VPDB), indicating the presence of 18O-enriched fluids during precipitation, possibly derived from destabilized methane gas hydrates. Based on the combined isotopic evidence, we suggest that all the seep carbonates resulted from the anaerobic oxidation of methane during intense methane seepage. The seepage likely was associated to gas hydrates destabilization, which led to the methane ebullition from the seafloor into the water column.

Saturation sensitivity and influencing factors of marine DC resistivity inversion to submarine gas hydrate

“…Gas hydrates are mainly found in continental margins as well as in permafrost regions, which are formed in marine sediment under relatively low temperatures and high pressures. Having been viewed as a potential resource, the hydrates are not merely the targets of offshore drilling, infrastructure, and slope stability assessment, but also play a vital role in the study of global climate change and the carbon cycling (Kvenvolden, 2000;Zhang et al, 2020;Kars et al, 2021;Yao et al, 2021;Yu et al, 2022). Therefore, submarine hydrate identification has been of great interest to academic and related industries in recent years (Li et al, 2016;Becker et al, 2020;Merle et al, 2021;Wang et al, 2021).…”

Methane-Derived Authigenic Carbonates on the Seafloor of the Laptev Sea Shelf

“…The OM extracted from the studied carbonate crusts (δ 13 C-C org from -40.2 to -31.1 VPDB) is depleted in isotopically heavy C org , compared to the "host" sediments because AOM strongly discriminate against methane containing the heavy isotope 13 C (Peckmann and Thiel, 2004;Peckmann et al, 2009). It can be a result of capture of methanotrophic bacterial biomass at the time of carbonate precipitation and their incorporation into microcrystalline carbonate matrix (Marlow et al, 2014;Yao et al, 2021), which is typical for seep carbonates and known Arctic MDACs as well (Pauly, 1963;Schubert et al, 1997;Lein et al, 1999;Kravchishina et al, 2017). Methane oxidation resulted in formation of bicarbonate and OM (both as microbial biomass and as soluble extracellular secretions).…”

“…Methane oxidation resulted in formation of bicarbonate and OM (both as microbial biomass and as soluble extracellular secretions). Therefore 13 C-depleted C org of seep carbonates is diagnostic for bacteria mediating oxidation of methane (Yao et al, 2021).…”

Microbial Communities Involved in Methane, Sulfur, and Nitrogen Cycling in the Sediments of the Barents Sea

“…Station 6841 was located at the Sturfjord trough, one of the known methane seep areas [43,44], in the zone of the most pronounced torch reaching a height of more than 100 m above the bottom level (80th AMK cruise report, 2020). In the 0-2 cm horizon a silty-pelitic silt of dark olive-brown color was exposed, then along the core it turned into aleurite-pelitic silt of dark olive-gray and deep black-gray colors with a large amount of hydrotroilite smears, spots, and micro-layers.…”