Methane concentration in sediment cores during METEOR cruise M92

We studied the concurrence of methanogenesis and sulfate reduction in surface sediments (0-25 cm below sea floor, cmbsf) at six stations (70, 145, 253, 407, 770 and 1024 m) along the Peruvian margin (12° S). This oceanographic region is characterized by high carbon export to the seafloor, creating an extensive oxygen minimum zone (OMZ) on the shelf, both factors that could favor surface methanogenesis. Sediments sampled along the depth transect traversed areas of anoxic and oxic conditions in the bottom-near water. Net methane production (batch incubations) and sulfate reduction (35S-sulfate radiotracer incubation) were determined in the upper 0-25 cmbsf of multicorer cores from all stations, while deep hydrogenotrophic methanogenesis (> 30 cmbsf, 14C-bicarbonate radiotracer incubation) was determined in two gravity cores at selected sites (78 and 407 m). Furthermore, stimulation (methanol addition) and inhibition (molybdate addition) experiments were carried out to investigate the relationship between sulfate reduction and methanogenesis.
Highest rates of methanogenesis and sulfate reduction in the surface sediments, integrated over 0-25 cmbsf, were observed on the shelf (70-253 m, 0.06-0.1 and 0.5-4.7 mmol m-2 d-1, respectively), while lowest rates were discovered at the deepest site (1024 m, 0.03 and 0.2 mmol m-2 d-1, respectively). The addition of methanol resulted in significantly higher surface methanogenesis activity, suggesting that the process was mostly based on non-competitive substrates, i.e., substrates not used by sulfate reducers. In the deeper sediment horizons, where competition was probably relieved due to the decline of sulfate, the usage of competitive substrates was confirmed by the detection of hydrogenotrophic activity in the sulfate-depleted zone at the shallow shelf station (70 m).
Surface methanogenesis appeared to be correlated to the availability of labile organic matter (C / N ratio) and organic carbon degradation (DIC production), both of which support the supply of methanogenic substrates. A negative correlation of methanogenesis rates with dissolved oxygen in the bottom-near water was not obvious, however, anoxic conditions within the OMZ might be advantageous for methanogenic organisms at the sediment-water interface.
Our results revealed a high relevance of surface methanogenesis on the shelf, where the ratio between surface to deep (below sulfate penetration) methanogenic activity ranged between 0.13 and 105. In addition, methane concentration profiles indicate a partial release of surface methane into the water column as well as a partial consumption of methane by anaerobic methane oxidation (AOM) in the surface sediment. The present study suggests that surface methanogenesis might play a greater role in benthic methane budgeting than previously thought, especially for fueling AOM above the sulfate-methane transition zone.

本研究针对秘鲁大陆边缘（Peruvian margin）南纬12°区域的6个站位（水深70、145、253、407、770和1024米）的表层沉积物（海底以下0~25厘米，cmbsf）中甲烷生成作用（methanogenesis）与硫酸盐还原作用（sulfate reduction）的共存情况展开研究。该海洋学区域以向海底的高碳输出通量为特征，在大陆架上形成了大范围的氧最低带（oxygen minimum zone, OMZ），这两个因素均有利于表层甲烷生成作用的发生。本次沿深度断面采集的沉积物覆盖了近底层水（bottom-near water）中的缺氧与有氧环境区域。本研究在所有站位的多管采样岩芯（multicorer cores）上层0~25 cmbsf中，测定了净甲烷生成量（batch incubations，批量培养法）与硫酸盐还原作用的活性；同时在选定站位（78和407米）的两根重力岩芯（gravity cores）中，测定了深层氢营养型甲烷生成作用（hydrogenotrophic methanogenesis，>30 cmbsf，14C-碳酸氢盐放射性示踪剂培养法（14C-bicarbonate radiotracer incubation））。此外，还开展了刺激实验（methanol addition，添加甲醇）与抑制实验（molybdate addition，添加钼酸盐），以探究硫酸盐还原作用与甲烷生成作用之间的关联。

最高的表层沉积物0~25 cmbsf累计甲烷生成速率与硫酸盐还原速率出现在大陆架站位（70~253米），分别为0.06~0.1 mmol·m⁻²·d⁻¹和0.5~4.7 mmol·m⁻²·d⁻¹；而最低速率则出现在最深站位（1024米），分别为0.03和0.2 mmol·m⁻²·d⁻¹。添加甲醇可显著提升表层甲烷生成活性，表明该过程主要依赖非竞争性底物——即硫酸盐还原菌（sulfate reducers）无法利用的底物。在更深层沉积物层位中，由于硫酸盐含量下降，竞争压力得到缓解，通过在浅大陆架站位（70米）的硫酸盐耗尽区中检测到氢营养型甲烷生成活性，证实了竞争性底物的利用。

表层甲烷生成作用似乎与易降解有机质（labile organic matter，碳氮比）及有机碳降解的溶解无机碳（dissolved inorganic carbon, DIC）产量相关，这两个过程均为甲烷生成菌（methanogens）的底物供给提供支撑。甲烷生成速率与近底层水中溶解氧呈负相关的趋势并不显著，但氧最低带（OMZ）内的缺氧环境可能有利于沉积物-水界面处的甲烷生成菌。

本研究结果表明，大陆架区域的表层甲烷生成作用具有极高的重要性，该区域表层与深层（硫酸盐渗透层以下）甲烷生成活性的比值介于0.13~105。此外，甲烷浓度剖面显示，表层甲烷有一部分释放至水柱中，同时表层沉积物中还有一部分甲烷通过厌氧甲烷氧化作用（anaerobic methane oxidation, AOM）被消耗。本研究表明，表层甲烷生成作用在底栖甲烷收支（benthic methane budgeting）中的作用可能比此前认知的更为重要，尤其是为硫酸盐-甲烷转换带（sulfate-methane transition zone）上方的厌氧甲烷氧化作用（AOM）提供底物。