(Table 1) Summary of sampling points described and main petrographic, morphological and geochemical characteristics

Targeted sampling on the Dolgovskoy Mound (northern Shatsky Ridge) revealed the presence of spectacular laterally extensive and differently shaped authigenic carbonates. The sampling stations were selected based on sidescan sonar and profiler images that show patchy backscatter and irregular and discontinuous reflections in the near subsurface. The interpretation of acoustic data from the top part of the mound supports the seafloor observations and the sampling that revealed the presence of a complex subsurface plumbing system characterized by carbonates and gas. The crusts sampled consist of carbonate cemented layered hemipelagic sedimentary Unit 1 associated with several centimetres thick microbial mats. Three different carbonate morphologies were observed: (a) tabular slabs, (b) subsurface cavernous carbonates consisting of void chambers up to 20 cm**3 in size and (c) chimney and tubular conduits vertically oriented or forming a subhorizontal network in the subsurface. The methanogenic origin of the carbonates is established based on visual observations of fluids seepage structures, 13C depletion of the carbonates (d13C varying between -36.7 per mil and -27.4 per mil), and by thin carbonate layers present within the thick microbial mats. Laboratory experiments with a Hele–Shaw cell were conducted in order to simulate the gas seepage through contrasting grain size media present on the seafloor. Combined petrography, visual observations and sandbox simulations allowed a characterization of the dynamics and the structures of the plumbing system in the near subsurface. Based on sample observations and the experiments, three observed morphologies of authigenic carbonates are interpreted, respectively, as (a) Darcian porous flow through the finely laminated clayey/coccolith-rich layers, (b) gas accumulation chambers at sites where significant fluid escape was impeded by thicker clayey layers forming the laminated Unit1 and (c) focussed vertical fluid venting and subhorizontal migration of overpressured fluids released from (b). The Hele–Shaw cell experiments represent a promising tool for investigating shallow fluid flow pathways in marine systems.

针对沙茨基海岭北部的多尔戈夫斯科伊海丘（Dolgovskoy Mound）开展的靶向采样发现，发育规模可观、横向延展且形态各异的自生碳酸盐岩（authigenic carbonates）。采样站位的选取依据侧扫声呐（sidescan sonar）与剖面仪（profiler）图像，这类图像显示近地表区域存在斑块状反向散射信号，以及不规则且不连续的地下反射特征。对该海丘顶部的声学数据进行解译，佐证了海底观测结果，同时也证实了采样所发现的、以碳酸盐岩与天然气为典型特征的复杂近地表流体输导系统的存在。采集的结壳样品由碳酸盐胶结的层状半远洋沉积单元1（hemipelagic sedimentary Unit 1）构成，该单元与数厘米厚的微生物席（microbial mats）伴生。共观测到三种不同形态的碳酸盐岩：(a) 板状块体；(b) 内部空腔尺寸可达20 cm³的地下洞穴状自生碳酸盐岩；(c) 垂直定向或在地下形成近水平网络的烟囱状与管状通道。该碳酸盐岩的产甲烷成因可通过以下证据得以确定：流体渗漏构造的肉眼观测结果、碳酸盐岩的碳13亏损特征（δ¹³C值介于-36.7‰与-27.4‰之间），以及厚层微生物席内部发育的薄层碳酸盐岩层。为模拟天然气通过海底不同粒度介质的渗漏过程，研究团队开展了海勒-肖盒（Hele–Shaw cell）实验室实验。结合岩石学（petrography）分析、肉眼观测与砂箱模拟（sandbox simulations），实现了对近地表流体输导系统的动力学特征与内部结构的系统表征。基于采样观测与实验结果，三种自生碳酸盐岩形态可分别被解译为：(a) 穿过富黏土质/颗石藻细层状地层的达西多孔流（Darcian porous flow）；(b) 在受较厚黏土质层（构成层状单元1）阻碍的流体大量逸出区域形成的气体聚集腔；(c) 源自(b)的超压流体的集中垂直喷溢与近水平运移。海勒-肖盒实验为研究海洋系统中的浅层流体运移通道提供了颇具前景的研究工具。