Changes of gas hydrate stability zone and fluid overpressure over the past 25 ka at GMGS2-16 site in the Dongsha area of northern South China Sea
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摘要:
海平面下降和海底温度上升可以引起海底水合物分解，进而导致天然气水合物稳定带底界处沉积物孔隙形成超压，一旦超压积聚突破地层有效应力，就会在海底产生甲烷渗漏。本文通过建立与此相关的稳定带底界变化的数值模型，以分析南海北部东沙海域GMGS2-16水合物钻探站位25 ka BP以来稳定带底界的动态变化。结果显示，在海平面上升的大背景下，海底温度的波动是稳定带底界动态变化的主要因素，主导了水合物生成和分解的周期性变化。底水温度升高导致稳定带底界上移，水合物分解，造成大量甲烷气体的释放，然而这种响应呈现一定的滞后，大约滞后1～3 ka。此外，水合物钻探获取的相应层位沉积物中出现了Mo元素富集的现象，表明稳定带底界上升及水合物分解形成的气体超压可以形成海底冷泉活动。因此，天然气水合物分解可能是冷泉渗漏活动的驱动机制。
Abstract:The decrease in sea level and the increase in seafloor temperature may lead to an upward shift of the base of the hydrate stability zone. This shift could trigger the decomposition of marine gas hydrates, resulting in the generation of overpressure within the sediment pore spaces near the base of hydrate stability zone. We developed a numerical model to simulate the dynamic movement of the base of the gas hydrate stability zone over the period of 0-25 ka BP in the Dongsha area, located in the northern part of the South China Sea. The simulation results indicate that fluctuation in seafloor temperature is the primary factor on the movement of the base of the hydrate stability zone, especially in the context of rising sea levels. The temperature fluctuation dominates the cyclic processes of hydrate formation and decomposition. An increase in bottom water temperature would result in an upward shift of the lower boundary of the stability zone, leading to the decomposition of hydrates and the subsequent release of significant amounts of methane gas. Notably, this response exhibits a lag of approximately 1-3 ka. Furthermore, the corresponding sediment layers that were drilled show an enrichment of molybdenum (Mo) elements, suggesting that the overpressure generated by the bottom boundary rise of the stability zone and the decomposition of hydrates may contribute to the formation of cold seep activities at seafloor. Therefore, the decomposition of gas hydrates may serve as a driving mechanism for cold seepage activities.
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