Chemical composition and stable sulfur isotope record of Black Sea sediments

The main terminal processes of organic matter mineralization in anoxic Black Sea sediments underlying the sulfidic water column are sulfate reduction in the upper 2-4 m and methanogenesis below the sulfate zone. The modern marine deposits comprise a ca. 1-m-deep layer of coccolith ooze and underlying sapropel, below which sea water ions penetrate deep down into the limnic Pleistocene deposits from >9000 years BP. Sulfate reduction rates have a subsurface maximum at the SO4[2-]-CH4 transition where H2S reaches maximum concentration. Because of an excess of reactive iron in the deep limnic deposits, most of the methane-derived H2S is drawn downward to a sulfidization front where it reacts with Fe(III) and with Fe2+ diffusing up from below. The H2S-Fe2+ transition is marked by a black band of amorphous iron sulfide above which distinct horizons of greigite and pyrite formation occur. The pore water gradients respond dynamically to environmental changes in the Black Sea with relatively short time constants of ca. 500 yr for SO4[2-] and 10 yr for H2S, whereas the FeS in the black band has taken ca. 3000 yr to accumulate. The dual diffusion interfaces of SO4[2-]-CH4 and H2S-Fe2+ cause the trapping of isotopically heavy iron sulfide with delta34S = +15 to +33 per mil at the sulfidization front. A diffusion model for sulfur isotopes shows that the SO4[2-] diffusing downward into the SO4[2-]-CH4 transition has an isotopic composition of +19 per mil, close to the +23 per mil of H2S diffusing upward. These isotopic compositions are, however, very different from the porewater SO4[2-] (+43 per mil) and H2S (-15 per mil) at the same depth. The model explains how methane-driven sulfate reduction combined with a deep H2S sink leads to isotopically heavy pyrite in a sediment open to diffusion. These results have general implications for the marine sulfur cycle and for the interpretation of sulfur isotopic data in modern sediments and in sedimentary rocks throughout earth's history.

在硫化水体柱之下的黑海缺氧沉积物中，有机质矿化的主要终端过程为：表层2~4米范围内的硫酸盐还原作用，以及硫酸盐还原带下方的甲烷生成作用。现代海相沉积包含一层约1米厚的颗石藻软泥，其下伏腐泥层；腐泥层之下，海水离子可渗透进入距今9000年以上的更新世淡水沉积中。硫酸盐还原速率在SO₄²⁻-CH₄过渡带达到地下峰值，该过渡带也是硫化氢浓度最高的位置。由于深层淡水沉积中活性铁含量过剩，大部分由甲烷生成的硫化氢会被向下搬运至硫化作用锋面，在此处与Fe(III)以及从下方向上扩散的Fe²+发生反应。H₂S-Fe²+过渡带以一层非晶态硫化铁黑带为标识，黑带上方发育有特征性的胶黄铁矿与黄铁矿赋存层位。孔隙水梯度会对黑海的环境变化做出动态响应，其响应时间常数相对较短：SO₄²⁻约为500年，H₂S约为10年；而黑带中的硫化铁则历经约3000年才得以堆积。SO₄²⁻-CH₄与H₂S-Fe²+这双重扩散界面，使得硫化作用锋面处富集的硫化铁具有偏重的同位素组成，δ³⁴S值介于+15‰至+33‰之间。硫同位素扩散模型显示，向下扩散进入SO₄²⁻-CH₄过渡带的SO₄²⁻同位素组成为+19‰，与向上扩散的H₂S的+23‰较为接近。但这两种同位素组成与同一深度孔隙水中的SO₄²⁻（+43‰）和H₂S（-15‰）差异显著。该模型阐释了甲烷驱动的硫酸盐还原作用与深层硫化氢汇共同作用，如何在开放扩散的沉积物中形成同位素组成偏重的黄铁矿。上述研究结果对全球海洋硫循环，以及现代沉积物和地球整个历史时期沉积岩中的硫同位素数据的解释，均具有普遍的参考价值。