Sulfur and oxygen isotope insights into sulfur cycling in shallow-sea hydrothermal vents, Milos, Greece
Sulfur and oxygen isotope insights into sulfur cycling in shallow-sea hydrothermal vents, Milos, Greece
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DOI：
10.1186/s12932-014-0012-y
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发表时间：
2014-08-12
影响因子：
2.3
通讯作者：
Amend, Jan P.
中科院分区：
文献类型：
--
作者：
Gilhooly, William P., III;Fike, David A.;Amend, Jan P.
米洛斯岛附近的浅海（5米深）热液喷发为微生物硫酸盐还原过程中火成岩非生物成因硫化物输入和生物成因硫化物生产之间的过渡提供了一个理想的机会。海底喷口特征包括含有热液矿物（元素硫和砷硫化物的橙子/黄色斑块）的大面积（大于1平方米）白色斑块以及硫氧化和还原微生物细胞。硫化物敏感膜部署在喷口和非喷口沉积物捕获强烈的地球化学空间格局，从平流到扩散硫化物从地下运输。尽管喷口生物和热液作用密切相关的明确的视觉证据，孔隙流体的硫和氧同位素组成不允许描绘一个生物信号从非生物信号分开。游离气体中的硫化氢（H2S）具有均匀的δ S-34值（2.5 +/-0.28%，n = 4），与孔隙水H2S（2.7 +/-0.36%，n = 21）几乎相同。在孔隙水硫酸盐中，如微生物硫酸盐还原所预期的，δ S-34（SO 4）和δ O-18（SO 4）没有成对增加。相反，孔隙水三角洲S-34（SO 4）值下降（从约千分之21至千分之17），随着温度的增加（高达97.4摄氏度）在每个热液功能。我们解释温度和三角洲S-34（SO 4）之间的反比关系，作为一个混合过程之间的氧化海水和S-34贫化热液输入海水夹带过程中被氧化。同位素质量平衡模型表明，硫化物氧化产生的次生硫酸盐至少占散装硫酸盐池的15%。与三角洲S-34（SO 4）中的这一趋势一致，硫酸盐的氧同位素组成在低pH值（75 ℃）孔隙沃茨中倾向于O-18富集。向高δ O-18（SO 4）的转变与酸性和高温条件下的平衡同位素交换一致。目前的数据集无法确定热液流体中所含H2S的来源;然而，排放到海底的H2S的端员三角洲S-34值与在类似于300摄氏度的温度下与地下硬石膏脉进行的平衡同位素交换一致。这些热液系统中的任何生物硫循环都被低硫酸盐、富含H2S的热液流体和富氧、硫酸盐的海水混合驱动的非生物化学反应所掩盖。
Shallow-sea (5 m depth) hydrothermal venting off Milos Island provides an ideal opportunity to target transitions between igneous abiogenic sulfide inputs and biogenic sulfide production during microbial sulfate reduction. Seafloor vent features include large (>1 m(2)) white patches containing hydrothermal minerals (elemental sulfur and orange/yellow patches of arsenic-sulfides) and cells of sulfur oxidizing and reducing microorganisms. Sulfide-sensitive film deployed in the vent and non-vent sediments captured strong geochemical spatial patterns that varied from advective to diffusive sulfide transport from the subsurface. Despite clear visual evidence for the close association of vent organisms and hydrothermalism, the sulfur and oxygen isotope composition of pore fluids did not permit delineation of a biotic signal separate from an abiotic signal. Hydrogen sulfide (H2S) in the free gas had uniform delta S-34 values (2.5 +/- 0.28%, n = 4) that were nearly identical to pore water H2S (2.7 +/- 0.36%, n = 21). In pore water sulfate, there were no paired increases in delta S-34(SO4) and delta O-18(SO4) as expected of microbial sulfate reduction. Instead, pore water delta S-34(SO4) values decreased (from approximately 21 parts per thousand to 17 parts per thousand) as temperature increased (up to 97.4 degrees C) across each hydrothermal feature. We interpret the inverse relationship between temperature and delta S-34(SO4) as a mixing process between oxic seawater and S-34-depleted hydrothermal inputs that are oxidized during seawater entrainment. An isotope mass balance model suggests secondary sulfate from sulfide oxidation provides at least 15% of the bulk sulfate pool. Coincident with this trend in delta S-34(SO4), the oxygen isotope composition of sulfate tended to be O-18-enriched in low pH (75 degrees C) pore waters. The shift toward high delta O-18(SO4) is consistent with equilibrium isotope exchange under acidic and high temperature conditions. The source of H2S contained in hydrothermal fluids could not be determined with the present dataset; however, the end-member delta S-34 value of H2S discharged to the seafloor is consistent with equilibrium isotope exchange with subsurface anhydrite veins at a temperature of similar to 300 degrees C. Any biological sulfur cycling within these hydrothermal systems is masked by abiotic chemical reactions driven by mixing between low-sulfate, H2S-rich hydrothermal fluids and oxic, sulfate-rich seawater.