Controls on Gas Emission Distribution on the Continental Slope of the Western Black Sea
Controls on Gas Emission Distribution on the Continental Slope of the Western Black Sea
复制标题
黑海西部大陆坡瓦斯排放分布的控制
DOI：
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发表时间：
2021
影响因子：
2.9
通讯作者：
G. Bohrmann
中科院分区：
文献类型：
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作者：
M. Riedel;L. Hähnel;J. Bialas;A. Bachmann;Stefanie Gaide;P. Wintersteller;I. Klaucke;G. Bohrmann
黑海大陆坡在水深<720米处有大量的天然气渗漏现象，但仍不完全了解其背后的控制因素。在这里，我们调查气体渗漏沿着保加利亚和罗马尼亚黑海边缘使用声学多波束水柱，测深，后向散射，和次海底剖面数据，以确定次海底结构，海底气体渗漏，气体排放到水柱之间的联系。查明了10，000多个渗漏点，面积达13，000平方公里。天然气渗漏的最大水深受Ⅰ型构造天然气水合物稳定带的起始位置控制，最大水深约为720 m。然而，气体渗漏并不是随机分布在其他地方。将控制天然气渗漏部位的因素分为沉积、剥蚀和构造三大类。沉积因素与区域发生的沉积物波形成聚焦效应和有限的沉积物披覆质量传输存款（MTD）。与断层和气体渗漏有关的拉长的海底凹陷在相邻沉积物波之间的底部发育。拉长的凹陷逐渐变宽，向浅水深度更深，并在某些位置达到顶峰，成为麻点集群。MTD覆盖更大的区域，并使古地形变平。它们的表面形态导致断层状变形模式的沉积物披覆顶部的MTDs，是本地用于气体运移。沿沿着沟道和峡谷以及边坡破坏（气体排放沿沿着头坎和山脊发生）可看到侵蚀因素。斜坡崩塌带走的沉积物覆盖了斜坡下更大的区域。这些地区没有天然气渗漏，因为它们形成了不渗透的天然气迁移屏障，或移走了以前富含天然气的沉积物。研究区东部深层构造对天然气运移具有控制作用，广泛分布的正断层对天然气运移起促进作用。总体而言，气体渗漏是广泛的沿着边缘。天然气运移在水深160 m以下的浅沃茨表现得更为活跃，但火炬点的数量并不一定是天然气释放总量的指标。
The continental slopes of the Black Sea show abundant manifestations of gas seepage in water depth of <720 m, but underlying controls are still not fully understood. Here, we investigate gas seepage along the Bulgarian and Romanian Black Sea margin using acoustic multibeam water column, bathymetry, backscatter, and sub-bottom profiler data to determine linkages between sub-seafloor structures, seafloor gas seeps, and gas discharge into the water column. More than 10,000 seepage sites over an area of ∼3,000 km2 were identified. The maximum water depth of gas seepage is controlled by the onset of the structure I gas hydrate stability zone in ∼720 m depth. However, gas seepage is not randomly distributed elsewhere. We classify three factors controlling on gas seepage locations into depositional, erosional, and tectonic factors. Depositional factors are associated with regionally occurring sediment waves forming focusing effects and mass-transport deposits (MTDs) with limited sediment drape. Elongated seafloor depressions linked to faulting and gas seepage develop at the base between adjacent sediment waves. The elongated depressions become progressively wider and deeper toward shallow water depths and culminate in some locations into clusters of pockmarks. MTDs cover larger regions and level out paleo-topography. Their surface morphology results in fault-like deformation patterns of the sediment drape on top of the MTDs that is locally utilized for gas migration. Erosional factors are seen along channels and canyons as well as slope failures, where gas discharge occurs along head-scarps and ridges. Sediment that was removed by slope failures cover larger regions down-slope. Those regions are devoid of gas seepage either by forming impermeable barriers to gas migration or by removal of the formerly gas-rich sediments. Deep-rooted tectonic control on gas migration is seen in the eastern study region with wide-spread normal faulting promoting gas migration. Overall, gas seepage is widespread along the margin. Gas migration appears more vigorous in shallow waters below ∼160 m water depth, but the number of flare sites is not necessarily an indicator of the total volume of gas released.