Physical oceanography during POLARSTERN cruise ANT-XXIX/4

Recent studies have suggested that the marine contribution of methane from shallow regions and melting marine terminating glaciers may have been underestimated. Here we report on methane sources and potential sinks associated with methane seeps in Cumberland Bay, South Georgia's largest fjord system. The average organic carbon content in the upper 8 meters of the sediment is around 0.65 wt.%; this observation combined with Parasound data suggest that the methane gas accumulations probably originate from peat-bearing sediments currently located several tens of meters below the seafloor. Only one of our cores indicates upward advection; instead most of the methane is transported via diffusion. Sulfate and methane flux estimates indicate that a large fraction of methane is consumed by anaerobic oxidation of methane (AOM). Carbon cycling at the sulfate-methane transition (SMT) results in a marked fractionation of the d13C-CH4 from an estimated source value of -65 per mil to a value as low as -96 per mil just below the SMT. Methane concentrations in sediments are high, especially close to the seepage sites (~40 mM); however, concentrations in the water column are relatively low (max. 58 nM) and can be observed only close to the seafloor. Methane is trapped in the lowermost water mass, however, measured microbial oxidation rates reveal very low activity with an average turnover of 3.1 years. We therefore infer that methane must be transported out of the bay in the bottom water layer. A mean sea-air flux of only 0.005 nM/m²s confirms that almost no methane reaches the atmosphere.

近期研究表明，浅海区域及消融中的海洋终止冰川（marine terminating glaciers）所产生的甲烷的海洋贡献量可能被低估了。本文针对南乔治亚岛最大峡湾系统——坎伯兰湾（Cumberland Bay）内的甲烷渗漏（methane seeps）相关的甲烷源与潜在汇开展研究并进行报道。沉积物上层8米的平均有机碳含量（organic carbon content）约为0.65 wt.%；结合旁侧声呐（Parasound）数据可知，甲烷气藏大概率源自当前位于海底下数十米处的含泥炭沉积物。仅1个沉积物岩芯观测到向上平流（upward advection）现象，绝大多数甲烷通过扩散作用运移。硫酸盐与甲烷通量估算结果显示，大部分甲烷通过甲烷厌氧氧化（anaerobic oxidation of methane, AOM）被消耗。硫酸盐-甲烷转换带（sulfate-methane transition, SMT）处的碳循环使得甲烷的碳同位素比值δ¹³C-CH₄发生显著分馏，从估算的源值-65‰降至转换带下方低至-96‰的数值。沉积物中的甲烷浓度较高，尤其在渗漏位点附近（约40 mM）；然而水柱中的甲烷浓度相对较低（最高58 nM），且仅能在靠近海底的区域被观测到。甲烷被困在最下层水团中，但测得的微生物氧化速率极低，平均周转时间为3.1年。因此我们推断，甲烷必须通过底层水层运移出海湾。平均海-气通量仅为0.005 nM/(m²·s)，这证实几乎没有甲烷能够进入大气。