白云凹陷东南部近海底溶解甲烷浓度变化及其对冷泉活动的指示意义

发布时间：2019-01-06 08:16

【摘要】：海底冷泉活动及其伴生的甲烷渗漏，是目前全球变化研究中的热点问题。作为天然气水合物的赋存标志及其分解释放甲烷的重要途径，冷泉活动在海水-沉积物界面引发了极为复杂的生物地球化学过程，并对全球碳循环和气候变化产生重要影响。南海是全球重要的甲烷储库之一，其冷泉区的甲烷渗漏通量与变化过程无疑急待评估和量化。深入开展这方面的研究，对于深入理解天然气水合物分解释放对全球变暖的响应及其反馈机制有重要的科学意义和应用价值。本文基于深海海底边界层原位环境观测系统（Benvir）‖，于2012年9月在南海北部陆坡中段神狐海域白云凹陷水合物远景区获得的一系列原位观测数据，针对研究区海底甲烷渗漏强度，疑似冷泉活动区位置，海底甲烷渗漏强度变化及影响，海底生物地球化学环境对海底甲烷渗漏的响应等科学问题进行了较深入的讨论。取得了以下认识。 1.参考世界典型冷泉区近底层海水溶解甲烷浓度，，将海底甲烷渗漏环境划分为背景地区、低甲烷渗漏区、中甲烷渗漏区和高甲烷渗漏区四种。 2.深入研究了白云凹陷地区近底层海水中溶解甲烷浓度及其时序变化特征，认为本文研究区属于中甲烷渗漏区。 3.揭示了温度、压力和盐度变化对溶解甲烷浓度的影响，提出：（1）周期性的压力变化（潮汐）是控制冷泉活动的因素之一，但由于白云凹陷水深较深，压力对海底界面的影响较小。（2）温度的变化会影响产甲烷菌和甲烷厌氧氧化菌的活性，进而影响水体中溶解甲烷浓度的变化。较压力而言，近底层海水的温度对沉积物中水合物稳定性的影响较小，但对海水溶解甲烷浓度的控制更为直接。（3）近底层物理化学环境的变化控制溶解甲烷的浓度的同时，影响了海水中离子浓度，引起了盐度的变化，因此盐度和甲烷的变化趋势较相似。且盐度可能会通过影响天然气水合物的稳定性来控制水体中溶解甲烷的浓度。（4）三者都不是影响近底层水体中溶解甲烷浓度变化的主要因素。 4.讨论了近底层海水溶解甲烷浓度变化对生物地球化学环境的响应，认为海流或外来水团引起的二氧化碳、溶解氧浓度和pH值的变化，可能是控制近底层海水中溶解甲烷浓度的主要因素。其中，外源二氧化碳和溶解氧会引起甲烷氧化还原平衡的变化，从而导致溶解甲烷浓度的变化。pH值的改变会影响产甲烷菌的活度，从而控制近底层海水中的溶解甲烷浓度。以上研究将有助于深入理解海底甲烷渗漏及其通量变化及其影响及制约因素，海底环境和生物地球化学过程对海底甲烷渗漏的通量变化的响应机制。
[Abstract]:Undersea cold spring activity and associated methane leakage are hot issues in global change research. As a sign of gas hydrate occurrence and an important way to decompose and release methane, cold spring activities have triggered a very complex biogeochemical process at the interface between seawater and sediment, and have an important impact on the global carbon cycle and climate change. The South China Sea is one of the most important methane reservoirs in the world. Further research in this field is of great scientific significance and practical value in understanding the response of gas hydrate decomposition and release to global warming and its feedback mechanism. Based on the deep-sea seabed boundary layer in situ environmental observation system (Benvir), a series of in-situ observation data were obtained in September 2012 in the Baiyun Sag Hydrate area of Shenhu Sea in the northern continental slope of the South China Sea. The depth of methane leakage in the study area, the location of the suspected cold spring active area, the variation and influence of methane leakage intensity, and the response of the marine biogeochemical environment to the methane leakage are discussed in depth. The following knowledge has been obtained. 1. Referring to the concentration of dissolved methane near the bottom of sea water in typical cold spring area of the world, the environment of methane leakage is divided into four types: background area, low methane leakage area, middle methane leakage area and high methane leakage area. 2. In this paper, the concentration of dissolved methane in the near bottom sea water and its temporal variation in Baiyun sag are studied, and it is considered that the study area belongs to the middle methane leakage area. 3. The effects of temperature, pressure and salinity on the concentration of dissolved methane are revealed. It is suggested that: (1) the periodic pressure variation (tide) is one of the factors controlling the cold spring activity, but because of the deep water depth in Baiyun depression, Pressure has little effect on the bottom interface. (2) the change of temperature will affect the activity of methanogenic bacteria and methane-anaerobic bacteria, and then affect the concentration of dissolved methane in water. Compared with pressure, the temperature of near-bottom seawater has little effect on the stability of hydrate in sediment, but the concentration of dissolved methane in seawater is controlled more directly. (3) the concentration of dissolved methane is controlled by the change of physical and chemical environment near bottom layer. The change of salinity is similar to that of methane. The salinity may control the concentration of dissolved methane by affecting the stability of natural gas hydrate. (4) none of them is the main factor affecting the concentration of dissolved methane in the near bottom water. 4. The response of dissolved methane concentration to biogeochemical environment in near-bottom seawater is discussed. The change of carbon dioxide, dissolved oxygen concentration and pH value caused by ocean current or foreign water mass is considered. It may be the main factor to control the concentration of dissolved methane in the near-bottom seawater. Exogenous carbon dioxide and dissolved oxygen can change the redox equilibrium of methane and lead to the change of dissolved methane concentration. The change of pH value will affect the activity of methanogenic bacteria and control the concentration of dissolved methane in the near bottom water. The above studies will be helpful to further understand the mechanism of benthic methane leakage and its flux change and its influence and restriction factors, and the response mechanism of seabed environment and biogeochemical process to the flux change of seabed methane leakage.
【学位授予单位】：中国海洋大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：P734;P744.4

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