table1_Biomarker and Isotopic Composition of Seep Carbonates Record Environmental Conditions in Two Arctic Methane Seeps.docx

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.

现今海洋冷泉（cold seeps）的活动可通过海底甲烷逸出的直接观测、化能合成生物（chemosynthetic organisms）的存在，或沉积物孔隙水与水体中高气体浓度的定量检测得到明确证实。验证古冷泉活动及其生物地球化学特征则更具挑战性，但可通过自生冷泉碳酸盐岩（authigenic seep carbonates）的代用指标进行重建。本研究针对斯瓦尔巴群岛（Svalbard）西北侧（韦斯特内萨海岭（Vestnesa Ridge）；水深约1200米）与南侧（斯托夫约登纳海槽（Storfjordrenna）；水深约380米）两处活跃北极甲烷冷泉的伴生碳酸盐岩，开展了脂质生物标志物（lipid-biomarker）组成、碳酸盐矿物学（carbonate mineralogy）以及稳定碳氧同位素组成的分析。除一件碳酸盐样品外，其余全部样品均以文石主导的矿物学特征为代表，表明其形成于海底附近、硫酸盐依赖型甲烷厌氧氧化（AOM）作用强烈的环境中。与之相反，斯托夫约登纳海槽沉积物中采集的富镁方解石结核，似乎形成于沉积物更深层的硫酸盐甲烷转换带（sulfate-methane-transition zone），且甲烷厌氧氧化速率较低。碳酸盐岩的碳13贫化同位素特征（-20‰至-30‰，维也纳佩德贝列姆尼特标准（VPDB）），以及碳酸盐岩中高浓度的、指示厌氧甲烷氧化古菌（anaerobic methanotrophic archaea）的古菌醇（archaeol）与sn2-羟基古菌醇（sn2-hydroxyarchaeol）、硫酸盐还原菌（sulfate-reducing bacteria）的异、反异C15:0脂肪酸的碳13贫化脂质生物标志物，均指示碳酸盐岩形成时期存在甲烷厌氧氧化作用。本研究还检出了指示介导甲烷好氧氧化（MOx）的细菌的碳13贫化脂质生物标志物：双藿烯（diploptene）与4α-甲基甾醇（4α-methyl sterol）。这表明碳酸盐岩形成时期，甲烷厌氧氧化带与甲烷好氧氧化带的空间分隔相对狭窄，这在高甲烷通量体系（methane flux regimes）中是典型特征。该冷泉伴生碳酸盐岩还显示出较高的δ18O值（4.5‰至5‰ VPDB），指示其形成时期存在富18O的流体，这类流体可能源自失稳的甲烷水合物（methane gas hydrates）。综合同位素证据，本研究认为所有冷泉碳酸盐岩均形成于强甲烷渗漏时期的甲烷厌氧氧化作用。该渗漏事件可能与甲烷水合物失稳相关，进而导致甲烷从海底逸散进入水体。