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首页> 外文期刊>Continental Shelf Research: A Companion Journal to Deep-Sea Research and Progress in Oceanography >Geological setting as background for methane distribution in Holocene mud deposits, Arhus Bay, Denmark
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Geological setting as background for methane distribution in Holocene mud deposits, Arhus Bay, Denmark

机译：丹麦Arhus湾全新世泥沉积中甲烷分布的地质背景

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摘要

Detailed acoustic mapping have been carried out in the Arhus Bay in order to establish the general Late Weichselian and Holocene stratigraphy, and to map the gas related acoustic blanking. The mapping results show that the oldest seismic unit is glacial till, probably related to the latest glacial advance in the region. The glacial till is covered by late-glacial ice-lake clay and silt reaching a thickness of up to 10 m. In the deeper part of the bay, early Holocene organic material and peat has been recorded in a few cores. A thin seismic unit is observed, which probably represents an early Holocene lowstand period, when most of the Arhus Bay was dry land. The three upper seismic units are related to the Holocene transgression of the region representing different hydrographical conditions. The lowermost unit (Marine unit 1) partly drapes the basin area with clay sediments and partly shows prograding sandy coastal deposits around glacial ridges. Marine shells from this unit date back to 8700 cal. years BP which are the oldest marine shells found south of the threshold in the northern Great Belt. The next unit (Marine unit 2) consists in general of mud to sandy mud, which cover most of the western central part of the Arhus Bay and in some places reach the present seabed in areas of erosion or non-deposition. The distribution of the youngest seismic unit (mud, Marine unit 3) is confined to the sub-recent to recent sedimentation basins in the eastern central part of the area. Acoustical blanking shows that the methane production takes place in the Holocene marine sediments. A map of the distribution and depth to free methane in the muddy sediments has been produced. Combined information from the different seismic equipment used allowed a mapping of the distribution and depth to free gas in the intervals 0.5-2, 2-4 and >4m. The map shows that acoustic blanking is found in the central part of Arhus Bay about 4 m below the seabed. In areas with high sedimentation rate, the acoustic blanking is found closer to the sediment surface and in selected key stations, pore-water chemistry have documented that depth to acoustical blanking is comparable to the methane saturation depth. Barotropic induced inflow dominates the present current system in the semi-enclosed Arhus Bay. The inflow events create turbulence in the outer eastern parts of the bay, followed by high sedimentation rates. The recent situation is reflected in the seabed sediments in the eastern part of the bay, which are characterised as soft sandy mud deposits, with gas bubbles close to the seabed. Seismic investigations have not previously been used to identify recent sedimentation areas and the most vulnerable areas in respect to possible escape of methane and toxic hydrogen-sulphide. The results of the present survey show that future monitoring must be focused in shallow gas areas in the easternmost part of Arhus Bay.
机译：已在Arhus湾进行了详细的声波测绘，以建立一般的后期Weichselian和全新世地层，并绘制与气体有关的声波消隐图。测绘结果表明，最古老的地震单元是冰期，可能与该地区最新的冰期有关。冰盖被晚冰期的冰湖粘土和粉砂覆盖，厚度达10 m。在海湾的较深部分，已在几个岩心中记录了全新世早期的有机物质和泥炭。观察到一个薄弱的地震单元，这可能代表了全新世低潮时期​​的早期，那时大多数Arhus海湾是旱地。三个上部地震单元与代表不同水文条件的区域的全新世海侵有关。最下部的单元（海洋单元1）部分用粘土沉积物覆盖盆地区域，部分显示冰川山脊附近的积沙海岸沉积物。该装置的船壳可追溯到8700 cal。 BP是位于北大带北部门槛以南的最古老的海贝壳，为BP年。下一个单元（海洋单元2）通常由泥浆到沙质泥浆组成，覆盖了Arhus湾西部中部的大部分地区，在某些地方到达了侵蚀或未沉积的海床。最年轻的地震单元（泥浆，海洋单元3）的分布仅限于该地区东部中部最近的沉积盆地。声学消隐表明甲烷的产生发生在全新世海洋沉积物中。绘制了泥泞沉积物中游离甲烷的分布和深度图。来自所使用的不同地震设备的组合信息允许以0.5-2、2-4和> 4m的间隔绘制自由气体的分布和深度。该地图显示，在海床以下约4 m的Arhus湾中部发现了消音。在高沉积速率的区域中，发现的声波消隐较接近沉积物表面，在选定的关键站点，孔隙水化学已证明，声波消隐的深度可与甲烷饱和深度相媲美。在半封闭的Arhus湾中，正压引起的流入控制了当前的系统。入流事件在海湾的东部造成湍流，随后沉积速率很高。最近的情况反映在海湾东部的海底沉积物上，这些沉积物的特征是软的沙质泥沙沉积物，气泡靠近海底。以前没有进行地震调查来确定最近的沉积区域和最易发生的甲烷和有毒硫化氢逸出区域。本次调查的结果表明，未来的监测必须集中在Arhus湾最东端的浅层天然气地区。

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