AMS 14C ages and isotopic compositions of samples from the Derugin Basin, Sea of Okhotsk@en

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 d13C values of these carbonates (>-43.5 per mill PDB) indicate methane as major carbon source; d18O values between 4.04 and 5.88 per mill 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 SO4**2- concentrations and a parallel increase of H2S with depth. Additionally, S and O isotope data of barite sulfate (d34S: 21.0-38.6 per mill CDT; d18O: 9.0-17.6 per mill 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. […]研究了鄂霍次克海德鲁金盆地（Derugin Basin）水深1500米处构造地垒（tectonic horst）区域的大规模重晶石（barite）沉积。海底观测与拖网采样（dredge samples）结果显示，沿3.5千米长的观测轨迹上散布着高度可达10米的不规则块状至柱状重晶石堆积体。水柱（water column）中检测到高浓度甲烷，表明甲烷泄溢（methane expulsion）及伴随的碳酸盐沉积是近期活跃的地质过程。海底分布的小片化能自养双壳类（chemoautotrophic clams）群落，包含索足蛤属未定种（Calyptogena sp.）与胡桃蛤属未定种（Acharax sp.），为活跃流体渗漏（active fluid venting）提供了额外佐证。该类重晶石呈白色至黄色，内部结构多孔隙且常具层理构造，表面通常覆盖深褐色富锰沉积物；对子样品开展的电子探针光谱（electron microprobe spectroscopy）测试显示，钡的摩尔锶替代量最高可达10.5 mol%。重晶石结构中少量自形黄铁矿晶体（idiomorphic pyrite crystals，占比约1%）指示了硫化氢（H₂S）的存在，这一结论在重晶石内部孔隙与通道中发现的活体化能自养管栖蠕虫（chemoautotrophic tube worms，直径约1毫米）集群中得到了验证。显微镜观测表明，微晶文石（micritic aragonite）与镁方解石（Mg-calcite）集合体或结壳是重晶石结构中常见的自生沉积（authigenic precipitations）产物；在重晶石堆积区附近获取的沉积物岩芯（sediment cores）中，也采集到了类似的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（> -43.5‰ PDB）表明甲烷为其主要碳源；氧同位素δ¹⁸O值介于4.04‰至5.88‰ PDB之间，对应形成温度显然低于5℃。其中一支岩芯在不同深度层位均发现了索足蛤属未定种（Calyptogena sp.）壳体，其¹⁴C测年结果介于20680年至>49080年之间。孔隙水（pore water）分析显示，流体中富含钡；同时随深度增加，硫酸根（SO₄²⁻）浓度逐渐降低，而硫化氢浓度同步升高。此外，重晶石硫酸盐的硫、氧同位素数据[δ³⁴S：21.0‰~38.6‰ CDT；δ¹⁸O：9.0‰~17.6‰ SMOW]强烈指向生物硫酸盐还原作用（biological sulfate reduction processes）；硫、氧同位素的分布范围仅能通过生物硫酸盐还原后的残留硫酸盐（residual sulfate）与“正常”海水硫酸盐发生同位素分馏（isotopic fractionation）后的混合过程来解释。[…]