Carbon Geochemistry of Cold Seeps: Methane Fluxes and Transformation in Sediments from Kazan Mud Volcano, Eastern Mediterranean Sea

Despite growing concerns about potential enhancement of global warming and slope failure by methane produced by gas hydrate dissociation, much uncertainty surrounds estimates of gas hydrate reservoir sizes, as well as methane fluxes and oxidation rates at the sea floor. For cold seep sediments of the eastern Mediterranean Sea, depth-dependent methane concentrations and rates of anaerobic oxidation of methane (AOM) are constrained by modeling the measured pore-water sulfate profile. The calculated dissolved methane distribution and flux are sensitive to the advective flow velocity, which is estimated from the depth distributions of conservative pore-water constituents (Na, B). Near-complete anaerobic oxidation of the upward methane flux is supported by the depth distributions of indicative biomarkers, and the carbon isotopic compositions of organic matter and dissolved inorganic carbon. Pore-water and solid-phase data are consistent with a narrow depth interval of AOM, 14-18 cm below the sediment-water interface. Based on an isotopic mass balance, the biomass of the microbial population carrying out oxidation of methane coupled to sulfate reduction at the given methane flux represents about 20% of the total organic carbon, which is a significant pool of in situ formed organic matter. Model results indicate that the asymptotic methane concentration is reached a few meters below the sediment surface. The predicted asymptotic concentration is close to the in situ saturation value with respect to gas hydrate, suggesting that the rate of shallow gas hydrate formation is controlled by the ascending methane flux. The proposed model approach can be used to predict the formation of gas hydrate, and to quantify methane fluxes plus transformation rates in surface sediments where fluid advection is an important transport mechanism.尽管人们日益担忧天然气水合物分解产生的甲烷会加剧全球变暖并诱发斜坡失稳，但目前针对天然气水合物储层规模、海底甲烷通量及氧化速率的估算仍存在大量不确定性。针对地中海东部冷泉沉积物，研究人员通过对实测孔隙水硫酸盐剖面开展建模，限定了甲烷浓度随深度的变化特征以及甲烷厌氧氧化（anaerobic oxidation of methane, AOM）速率。计算得到的溶解甲烷分布与通量对平流流速极为敏感，而平流流速可通过保守性孔隙水组分（钠、硼）的深度分布进行估算。指示性生物标志物的深度分布，以及有机质与溶解无机碳的碳同位素组成，均证实向上运移的甲烷通量几乎完全被厌氧氧化。孔隙水与固相数据表明，甲烷厌氧氧化发生在沉积物-水界面下14~18 cm的狭窄深度区间内。基于同位素质量平衡分析，在给定甲烷通量下，介导甲烷厌氧氧化并耦合硫酸盐还原的微生物群落生物量约占总有机碳的20%，该类生物量是一类重要的原位形成有机质储库。模型结果显示，在沉积物表层以下数米处即可达到渐近甲烷浓度。预测得到的渐近浓度与天然气水合物的原位饱和值相近，表明浅层天然气水合物的形成速率受向上运移的甲烷通量控制。本研究提出的模型方法可用于预测天然气水合物的形成，并量化以流体平流作为重要输运机制的表层沉积物中的甲烷通量与转化速率。