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.

通过直接观察海底甲烷排放，化学合成生物的存在或沉积物孔隙水和水柱中高浓度气体的定量，可以看出当今海洋中冷渗漏的活动。验证过去的冷渗流活动和生物地球化学特征更具挑战性，但可以根据自生渗碳碳酸盐的代理记录进行重建。在这里，我们调查了位于西北部（Vestnesa Ridge；水深约1200 m）和南部（Storfjordrenna）的两个活跃的北极甲烷渗流的渗流相关碳酸盐的脂质生物标志物清单，碳酸盐矿物学以及稳定的碳和氧同位素组成。 ;约380 m水深）。除一个碳酸盐样品外，所有文石占主导地位的矿物学表明，在以高硫酸盐依赖性甲烷厌氧氧化（AOM）速率为特征的环境中，靠近海底的降水。相比之下，在Storfjordrenna沉积物中采样的富含Mg方解石的结核似乎在较低的AOM速率下在沉积物中更深的硫酸盐甲烷转换区形成。碳酸盐沉淀时的AOM活性由碳酸盐的¹³ C贫同位素特征[-20至-30‰Vienna Pee Dee Belemnite（VPDB）]，以及高浓度的¹³ C贫脂质生物标志物，可诊断厌氧性甲烷氧化营养古细菌（古生物和sn2-羟基古生物）和碳酸盐中的硫酸盐还原细菌（异和前异C15：0脂肪酸）。我们还发现了¹³种贫C的脂质生物标记（双倍戊烯和4α-甲基固醇），可诊断介导甲烷好氧氧化（MOx）的细菌。这表明，在碳酸盐形成时，AOM和MOx区域之间的空间间隔相对狭窄，这是高甲烷通量方案的典型特征。渗水相关碳酸盐也显示相对较高的δ ¹⁸O值（VPDB的4.5-5‰）表明在降水过程中存在¹⁸种富O流体，可能来自不稳定的甲烷水合物。基于联合的同位素证据，我们建议所有的渗透碳酸盐都是在强烈的甲烷渗透过程中甲烷的厌氧氧化产生的。渗漏可能与天然气水合物的失稳有关，这导致甲烷从海底蒸发到水柱中。