Anaerobic oxidation of methane and sulphate reduction rates, hydrocarbons, sulphate and sulphide concentrations of sediment cores from the Larsen B embayment at the eastern site of the Antarctic Peninsula

First videographic indication of an Antarctic cold seep ecosystem was recently obtained from the collapsed Larsen B ice shelf, western Weddell Sea (Domack et al., 2005). Within the framework of the R/V Polarstern expedition ANTXXIII-8, we revisited this area for geochemical, microbiological and further videographical examinations. During two dives with ROV Cherokee (MARUM, Bremen), several bivalve shell agglomerations of the seep-associated, chemosynthetic clam Calyptogena sp. were found in the trough of the Crane and Evans glacier. The absence of living clam specimens indicates that the flux of sulphide and hence the seepage activity is diminished at present. This impression was further substantiated by our geochemical observations. Concentrations of thermogenic methane were moderately elevated with 2 µM in surface sediments of a clam patch, increasing up to 9 µM at a sediment depth of about 1 m in the bottom sections of the sediment cores. This correlated with a moderate decrease in sulphate from about 28 mM at the surface down to 23.4 mM, an increase in sulphide to up to 1.43 mM and elevated rates of the anaerobic oxidation of methane (AOM) of up to 600 pmol cm**-3 d**-1 at about 1 m below the seafloor. Molecular analyses indicate that methanotrophic archaea related to ANME-3 are the most likely candidates mediating AOM in sediments of the Larsen B seep.南极冷泉（cold seep）生态系统的首个影像记录，近期于威德尔海西部崩解的拉森B冰架区域获取（Domack等人，2005年）。在‘极地号’（R/V Polarstern）科考航次ANTXXIII-8的框架下，我们重返该区域开展地球化学、微生物学及补充影像勘查工作。依托不来梅MARUM的‘切诺基号’遥控无人潜水器（ROV Cherokee）开展的两次下潜作业中，我们在克兰冰川与埃文斯冰川的槽谷内，发现了与冷泉共生的化能合成蛤类Calyptogena属未定种（Calyptogena sp.）的多处双壳类贝壳堆积体。未采集到活蛤个体，这表明当前硫化物通量以及冷泉活动均已减弱。这一推断通过我们的地球化学观测得到了进一步验证。热成因甲烷（thermogenic methane）浓度在蛤群分布区的表层沉积物中为2微摩尔（µM），处于适度升高水平；在沉积物岩芯底部层段约1米深度处，浓度最高可达9微摩尔。该浓度变化与以下地球化学特征相关：表层沉积物硫酸盐浓度从约28毫摩尔（mM）适度降至23.4毫摩尔，硫化物浓度升高至最高1.43毫摩尔，同时海底以下约1米处的甲烷厌氧氧化（anaerobic oxidation of methane, AOM）速率最高可达600皮摩尔每立方厘米每天（pmol cm⁻³ d⁻¹）。分子生物学分析表明，与ANME-3类群相关的甲烷氧化古菌（methanotrophic archaea），是拉森B冰架冷泉沉积物中介导甲烷厌氧氧化的最可能功能类群。