Methane discharge in sediments of the Dvurechenskii mud volcano

During the MARGASCH cruise M52/1 in 2001 with RV Meteor we sampled surface sediments from three stations in the crater of the Dvurechenskii mud volcano (DMV, located in the Sorokin Trough of the Black Sea) and one reference station situated 15 km to the northeast of the DMV. We analysed the pore water for sulphide, methane, alkalinity, sulphate, and chloride concentrations and determined the concentrations of particulate organic carbon, carbonate and sulphur in surface sediments. Rates of anaerobic oxidation of methane (AOM) were determined using a radiotracer (14CH4) incubation method. Numerical transport-reaction models were applied to derive the velocity of upward fluid flow through the quiescently dewatering DMV, to calculate rates of AOM in surface sediments, and to determine methane fluxes into the overlying water column. According to the model, AOM consumes 79% of the average methane flux from depth (8.9 x 10**+ 6 mol a**-1), such that the resulting dissolved methane emission from the volcano into the overlying bottom water can be determined as 1.9 x 10**+ 6 mol a**-1. If it is assumed that all submarine mud volcanoes (SMVs) in the Black Sea are at an activity level like the DMV, the resulting seepage represents less than 0.1% of the total methane flux into this anoxic marginal sea. The new data from the DMV and previously published studies indicate that an average SMV emits about 2.0 x 10**+ 6 mol a**-1 into the ocean via quiescent dewatering. The global flux of dissolved methane from SMVs into the ocean is estimated to fall into the order of 10**+10 mol a**-1. Additional methane fluxes arise during periods of active mud expulsion and gas bubbling occurring episodically at the DMV and other SMVs.

2001年，我们搭乘RV Meteor科考船执行MARGASCH航次M52/1期间，在德武列琴斯基泥火山（Dvurechenskii Mud Volcano，简称DMV，位于黑海索罗金海槽）的火山口内的3个站位采集了表层沉积物样品，并在DMV东北15公里处布设了1个对照站位。我们对孔隙水的硫化物、甲烷、碱度、硫酸盐及氯化物浓度进行了分析，并测定了表层沉积物中颗粒有机碳、碳酸盐与硫的含量。本研究采用放射性示踪剂（¹⁴CH₄）培养法测定了甲烷厌氧氧化（Anaerobic Oxidation of Methane，AOM）速率。通过数值运移-反应模型，我们推导了静排水状态下DMV的向上流体流速，计算了表层沉积物中的AOM速率，并确定了进入上覆水柱的甲烷通量。模型结果显示，AOM消耗了来自深部的平均甲烷通量的79%（8.9×10⁶ mol·a⁻¹），因此该火山向上覆底层水排放的溶解甲烷通量经计算为1.9×10⁶ mol·a⁻¹。若假设黑海所有海底泥火山（Submarine Mud Volcanoes，SMVs）的活动水平均与DMV相当，则由此产生的渗漏甲烷通量占该缺氧边缘海总甲烷输入通量的比例不足0.1%。基于DMV的新数据与已发表的研究结果可知，单座海底泥火山通过静排水方式向海洋排放的甲烷通量约为2.0×10⁶ mol·a⁻¹。据估算，全球海底泥火山向海洋排放的溶解甲烷通量约为10¹⁰ mol·a⁻¹量级。在DMV及其他海底泥火山间歇性发生的活跃泥浆喷发与气体冒泡阶段，还会产生额外的甲烷通量。