(Table 1) Stable isotope records of clathrite and water

Aragonitic clathrites are methane-derived precipitates that are found at sites of massive near-seafloor gas hydrate (clathrate) accumulations at the summit of southern Hydrate Ridge, Cascadia margin. These platy carbonate precipitates form inside or in proximity to gas hydrate, which in our study site currently coexists with a fluid that is highly enriched in dissolved ions as salts are excluded during gas hydrate formation. The clathrites record the preferential incorporation of 18O into the hydrate structure and hence the enrichment of 16O in the surrounding brine. We measured d18O values as high as 2.27 per mil relative to Peedee belemnite that correspond to a fluid composition of -1.18 per mil relative to standard mean ocean water. The same trend can be observed in Ca isotopes. Ongoing clathrite precipitation causes enrichment of the 44Ca in the fluid and hence in the carbonates. Carbon isotopes confirm a methane source for the carbonates. Our triple stable isotope approach that uses the three main components of carbonates (Ca, C, O) provides insight into multiple parameters influencing the isotopic composition of the pore water and hence the isotopic composition of the clathrites. This approach provides a tool to monitor the geochemical processes during clathrate and clathrite formation, thus recording the evolution of the geochemical environment of gas hydrate systems.

文石质笼状碳酸盐沉淀物（Aragonitic clathrites）是甲烷成因的沉淀物，发现于卡斯卡迪亚大陆边缘南部水合物脊顶部的大型近海底天然气水合物（笼形水合物，clathrate）赋存区域。这类板状碳酸盐沉淀物形成于天然气水合物内部或其邻近区域；在本研究区域，天然气水合物形成过程中盐分被排出，因此其共存流体富含溶解离子。该笼状碳酸盐沉淀记录了¹⁸O优先掺入水合物晶格的过程，进而使周围卤水的¹⁶O发生富集。我们测得相对于佩迪箭石（Peedee belemnite）标准的d18O值最高可达2.27‰，对应的流体同位素组成相对于标准平均海水（standard mean ocean water）为-1.18‰。钙同位素也呈现出相同的变化趋势：持续的笼状碳酸盐沉淀会导致流体中⁴⁴Ca富集，进而使碳酸盐矿物中的⁴⁴Ca同样富集。碳同位素分析证实该碳酸盐的碳源为甲烷。本研究采用覆盖碳酸盐三大主要组分（钙、碳、氧）的三重稳定同位素分析方法，可解析影响孔隙水同位素组成、进而影响笼状碳酸盐沉淀物同位素组成的多项控制参数。该方法为监测水合物及笼状碳酸盐形成过程中的地球化学过程提供了有效工具，从而可完整记录天然气水合物系统的地球化学环境演化历程。