Sulfur isotope composition of ODP Site 801 in the western Pacific Ocean

The subsurface biosphere in the basaltic ocean crust is potentially of major importance in affecting chemical exchange between the ocean and lithosphere. Alteration of the oceanic crust commonly yields secondary pyrite that is depleted in 34S relative to igneous sulfides. Although these 34S depleted sulfur isotope ratios may point to signatures of biological fractionation, previous interpretations of sulfur isotope fractionation in altered volcanic rocks have relied on abiotic fractionation processes between intermediate sulfur species formed during basalt alteration. Here, we report results for multiple S-isotope (32S, 33S, 34S) compositions of altered basalts at ODP Site 801 in the western Pacific and provide evidence for microbial sulfate reduction within the volcanic oceanic crust. In-situ ion-microprobe analyses of secondary pyrite in basement rocks show a large range of delta34S values, between -45 per mil and 1 per mil , whereas bulk rock delta34S analyses yield a more restricted range of -15.8 to 0.9 per mil . These low and variable delta34S values, together with bulk rock S concentrations ranging from 0.02% up to 1.28% are consistent with loss of magmatic primary mono-sulfide and addition of secondary sulfide via microbial sulfate reduction. High-precision multiple sulfur-isotope (32S/33S/34S) analyses suggest that secondary sulfides exhibit mass-dependent equilibrium fractionation relative to seawater sulfate in both delta33S and delta34S values. These relationships are explained by bacterial sulfate reduction proceeding at very low metabolic rates. The determination of the S-isotope composition of bulk altered oceanic crust demonstrates that S-based metabolic activity of subsurface life in oceanic basalt is widespread, and can affect the global S budget at the crust-seawater interface.

洋壳玄武岩中的地下生物圈，可能对海洋与岩石圈之间的化学交换过程具有重要影响。洋壳蚀变通常会形成次生黄铁矿，相较于火成硫化物，这类黄铁矿的³⁴S同位素丰度偏低。尽管这种³⁴S亏损的硫同位素比值可能暗示存在生物分馏作用，但此前针对蚀变火山岩中硫同位素分馏的解释，均基于玄武岩蚀变过程中形成的中间硫物种之间的非生物分馏过程。本研究针对西太平洋大洋钻探计划（Ocean Drilling Program, ODP）801站位的蚀变玄武岩，报道了其多硫同位素（³²S、³³S、³⁴S）组成的分析结果，并为火山洋壳内存在微生物硫酸盐还原作用提供了证据。对基底岩中次生黄铁矿的原位离子探针分析显示，其δ³⁴S值的分布范围极广，介于-45‰至1‰之间；而全岩δ³⁴S分析的结果范围则相对狭窄，为-15.8‰至0.9‰。这些偏低且波动范围大的δ³⁴S值，配合全岩硫浓度介于0.02%至1.28%之间的特征，与岩浆原生单硫化物的流失、以及通过微生物硫酸盐还原作用添加的次生硫化物的过程相吻合。高精度多硫同位素（³²S/³³S/³⁴S）分析结果表明，相较于海水硫酸盐，次生硫化物在δ³³S和δ³⁴S值上均表现出与质量相关的平衡分馏特征。上述同位素分馏关系可通过代谢速率极低的细菌硫酸盐还原作用得到解释。对蚀变洋壳全岩硫同位素组成的测定结果表明，洋壳玄武岩中地下生命基于硫的代谢活动广泛存在，且能够影响壳-海水界面的全球硫循环收支。