(Table 3) Carbon and oxygen isotopic data of carbonates from Core SLR 37-1, HYC 25-1 and an aragonite crust from station DR 35-1

An area of massive barite precipitations was studied at a tectonic horst in 1500 m water depth in the Derugin Basin, Sea of Okhotsk. Seafloor observations and dredge samples showed irregular, block- to column-shaped barite build-ups up to 10 m high which were scattered over the seafloor along an observation track 3.5 km long. High methane concentrations in the water column show that methane expulsion and probably carbonate precipitation is a recently active process. Small fields of chemoautotrophic clams (Calyptogena sp., Acharax sp.) at the seafloor provide additional evidence for active fluid venting. The white to yellow barites show a very porous and often layered internal fabric, and are typically covered by dark-brown Mn-rich sediment; electron microprobe spectroscopy measurements of barite sub-samples show a Ba substitution of up to 10.5 mol% of Sr. Rare idiomorphic pyrite crystals (1%) in the barite fabric imply the presence of H2S. This was confirmed by clusters of living chemoautotrophic tube worms (1 mm in diameter) found in pores and channels within the barite. Microscopic examination showed that micritic aragonite and Mg-calcite aggregates or crusts are common authigenic precipitations within the barite fabric. Equivalent micritic carbonates and barite carbonate cemented worm tubes were recovered from sediment cores taken in the vicinity of the barite build-up area. Negative ?13C values of these carbonates (>?43.5‰ PDB) indicate methane as major carbon source; ?18O values between 4.04 and 5.88‰ PDB correspond to formation temperatures, which are certainly below 5°C. One core also contained shells of Calyptogena sp. at different core depths with 14C-ages ranging from 20 680 to >49 080 yr. Pore water analyses revealed that fluids also contain high amounts of Ba; they also show decreasing SO42- concentrations and a parallel increase of H2S with depth. Additionally, S and O isotope data of barite sulfate (?34S: 21.0–38.6‰ CDT; ?18O: 9.0–17.6‰ SMOW) strongly point to biological sulfate reduction processes. The isotope ranges of both S and O can be exclusively explained as the result of a mixture of residual sulfate after a biological sulfate reduction and isotopic fractionation with 'normal' seawater sulfate. […]

本研究针对鄂霍次克海德鲁金盆地（Derugin Basin）水深1500米处的构造地垒（tectonic horst）区域的大规模重晶石（barite）沉淀区开展了调查。海底现场观测与海底拖网采样结果显示，不规则状、从块状到柱状的重晶石堆积体最高可达10米，沿3.5千米长的观测轨迹散布于海底。水体中高浓度的甲烷表明，甲烷逸散及可能伴随的碳酸盐沉淀是近期仍在活动的过程。海底分布的小片化能自养（chemoautotrophic）双壳类群落（索足蛤属Calyptogena sp.、尖齿蛤属Acharax sp.）为活跃的流体喷溢（fluid venting）活动提供了额外佐证。呈白至黄色的重晶石具有极强的孔隙性，且多具层状内部组构（internal fabric），通常被深棕色富锰沉积物覆盖；重晶石亚样品的电子探针显微光谱法（electron microprobe spectroscopy）测量结果显示，钡（Ba）的锶（Sr）替代率最高可达10.5 mol%。重晶石组构中罕见的自形黄铁矿（idiomorphic pyrite）晶体（占比1%）指示体系中存在硫化氢（H₂S）。在重晶石内部的孔隙与通道中发现的活体化能自养管栖蠕虫（tube worms，直径1毫米）集群进一步验证了这一结论。显微镜观测显示，微晶文石（micritic aragonite）与镁方解石（Mg-calcite）集合体或结壳是重晶石组构内常见的自生沉淀（authigenic precipitations）产物。在重晶石堆积区附近获取的沉积岩芯（sediment cores）中，也发现了同类微晶碳酸盐以及被重晶石碳酸盐胶结的蠕虫管。这类碳酸盐的碳同位素δ¹³C值为负值（>−43.5‰，相对于佩迪箭石标准Peedee Belemnite，简称PDB），表明甲烷为其主要碳源；其氧同位素δ¹⁸O值介于4.04‰至5.88‰ PDB之间，对应形成温度必然低于5℃。其中一支岩芯在不同深度层位均发现了Calyptogena属双壳类壳体，其碳十四测年（¹⁴C-age）结果介于20680年至大于49080年之间。孔隙水（pore water）分析结果显示，流体中同样含有高浓度的钡；同时随着深度增加，硫酸根（SO₄²⁻）浓度逐渐降低，而硫化氢（H₂S）浓度则同步上升。此外，重晶石硫酸盐的硫同位素δ³⁴S（21.0‰~38.6‰，相对于迪亚布洛峡谷陨硫铁标准Canyon Diablo Troilite，简称CDT）与氧同位素δ¹⁸O（9.0‰~17.6‰，相对于标准平均海洋水Standard Mean Ocean Water，简称SMOW）数据，均强烈指示存在生物硫酸盐还原（biological sulfate reduction）过程。硫、氧同位素的分布范围仅能通过生物硫酸盐还原后的残余硫酸盐与‘正常’海水硫酸盐发生同位素分馏（isotopic fractionation）混合的过程来解释。[…]