Mineralogical and stable isotope analyses of carbonates from the eastern Black Sea

Authigenic carbonates associated with cold seeps provide valuable archives of changes in the long-term seepage activity. To investigate the role of shallow-buried hydrates on the seepage strength and fluid composition we analysed methane-derived carbonate precipitates from a high-flux hydrocarbon seepage area ("Batumi seep area") located on the south-eastern Black Sea slope in ca. 850 m. In a novel approach, we combined computerized X-ray tomography (CT) with mineralogical and isotope geochemical methods to get additional insights into the three-dimensional internal structure of the carbonate build-ups. X-ray diffractometry revealed the presence of two different authigenic carbonate phases, i.e. pure aragonitic rims associated with vital microbial mats and high-Mg calcite cementing the hemipelagic sediment. As indicated by the CT images, the initial sediment has been strongly deformed, first plastic then brittle, leading to brecciation of the progressively cemented sediment. The aragonitic rims on the other hand, represent a presumably recent carbonate growth phase since they cover the already deformed sediment. The stable oxygen isotope signature indicates that the high-Mg calcite cement incorporated pore water mixed with substantial hydrate water amounts. This points at a dominant role of high gas/fluid flux from decomposing gas hydrates leading to the deformation and cementation of the overlying sediment. In contrast, the aragonitic rims do not show an influence of 18O-enriched hydrate water. The differences in d18O between the presumably recent aragonite precipitates and the older high-Mg cements suggest that periods of hydrate dissociation and vigorous fluid discharge alternated with times of hydrate stability and moderate fluid flow. These results indicate that shallow-buried gas hydrates are prone to episodic decomposition with associated vigorous fluid flow. This might have a profound impact on the seafloor morphology resulting e.g. in the formation of carbonate pavements and pockmark-like structures but might also affect the local carbon cycle.与冷泉（cold seeps）伴生的自生碳酸盐岩，是记录长期渗漏活动变化的珍贵档案。为探究浅埋藏水合物（shallow-buried hydrates）对渗漏强度与流体组分的影响，我们对位于黑海东南陆坡约850米水深处的高通量烃类渗漏区——"巴统渗漏区（Batumi seep area）"内的甲烷成因碳酸盐沉淀物开展了分析。本研究采用创新性方法，将计算机X射线断层扫描（CT）与矿物学、同位素地球化学方法相结合，以深入解析碳酸盐建造的三维内部结构。 X射线衍射法检测到两种不同的自生碳酸盐相：一是与活性微生物席伴生的纯文石质壳层，二是胶结半远洋沉积物的高镁方解石。CT图像显示，原始沉积物经历了强烈变形，先后呈现塑性变形与脆性变形特征，进而使逐步胶结的沉积物发生角砾岩化。而文石质壳层覆盖于已变形的沉积物之上，表明其可能代表了近期的碳酸盐生长阶段。 稳定氧同位素组成分析表明，高镁方解石胶结物中混入了大量与孔隙水混合的水合物来源水。这说明分解的天然气水合物所释放的高流量气/流体，对上覆沉积物的变形与胶结起到了主导作用。与之相反，文石质壳层并未受到富¹⁸O水合物水的影响。近期形成的文石质沉淀物与较古老的高镁方解石胶结物之间的δ¹⁸O差异表明，水合物分解与强流体排放的时期，与水合物稳定及流体活动平缓的时期交替出现。上述结果表明，浅埋藏天然气水合物易于发生幕式分解，并伴随强烈的流体活动。这一过程可能对海底形貌产生显著影响，例如形成碳酸盐岩坪与麻坑状构造，同时也会对局部碳循环造成扰动。