The formation of authigenic pyrite in marine sediments involves multiple reactions between ferrous iron (Fe) and hydrogen sulfide (HS). Ferrous iron is commonly provided through the reductive dissolution of Fe-(oxyhydr)oxides by organic matter (i.e., dissimilatory Fe reduction), dissolved sulfide (i.e., abiotic Fe reduction) or methane (i.e., Fe-AOM), whereas sulfide is supplied by organoclastic sulfate reduction (OSR) or sulfate-driven anaerobic oxidation of methane (SD-AOM). Since Rayleigh-type distillation operates widely in sediments of gas-hydrate-bearing zones, sulfur and nickel isotope compositions (i.e., δS and δNi) cannot readily distinguish OSR- from SD-AOM-associated pyrite. However, these microbial pathways may yield different patterns of trace-element enrichment in pyrite. To better understand the linkage of trace-element patterns to specific microbial pathways (i.e., Fe reduction, Fe-AOM, OSR and SD-AOM), and to evaluate the use of S and Ni isotopic signatures as tracers for pyrite formation pathways in methane-rich sediments, we report pyrite-associated trace element and δS and δNi isotope analyses of sediments from a gas hydrate borehole (Site GMGS4-SC-03) from the Shenhu area, Pearl River Mouth Basin, South China Sea. Pyrite formed in conjunction with Fe- and/or SD-AOM exhibits abundant framboidal overgrowths and extremely high δS (up to +142.8‰) and δNi (up to +2.72‰), representing the highest stable S and Ni isotopic compositions of pyrite reported to date. These pyrite morphologies are enriched in Co and Ni, which may be a diagnostic signature of an SD-AOM pathway. By contrast, OSR-associated pyrite is enriched in Cu and Zn due to OSR-induced release of trace elements from decaying organic matter. In addition, the relationship of As to Cu and/or Zn can distinguish microbial Fe/Mn reduction from Fe/Mn-AOM, because microbial Fe/Mn reduction releases trace elements from both Fe/Mn-(oxyhydr)oxides (i.e., As) and organic matter (i.e., Cu and Zn), whereas Fe/Mn-AOM only releases trace elements from Fe/Mn-(oxyhydr)oxides. Furthermore, an observed covariation between As and either Co or Ni in most pyrite with high δS, indicates that this pyrite captured both As released during Fe/Mn-AOM and Co and Ni from SD-AOM. Thus, the high nickel isotope values measured in this study likely dominantly reflect release of isotopically heavy Ni from Fe- and Mn-(oxyhydr)oxides. Our results demonstrate that the trace-element composition of pyrite in gas-hydrate-bearing sediments can record the geochemical signature of the dominant microbial processes.

中文翻译：

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应用黄铁矿痕量金属以及硫和镍同位素特征来区分硫酸盐驱动的甲烷厌氧氧化与铁驱动的甲烷厌氧氧化


海洋沉积物中自生黄铁矿的形成涉及亚铁（Fe）和硫化氢（HS）之间的多重反应。二价铁通常通过有机物（即异化铁还原）、溶解的硫化物（即非生物铁还原）或甲烷（即 Fe-AOM）还原溶解铁（羟基）氧化物来提供，而硫化物则提供通过有机碎屑硫酸盐还原（OSR）或硫酸盐驱动的甲烷厌氧氧化（SD-AOM）。由于瑞利型蒸馏广泛应用于含天然气水合物区域的沉积物中，因此硫和镍同位素组成（即 δS 和 δNi）无法轻易区分 OSR- 与 SD-AOM 相关的黄铁矿。然而，这些微生物途径可能会在黄铁矿中产生不同的微量元素富集模式。更好地了解微量元素模式与特定微生物途径（即 Fe 还原、Fe-AOM、OSR 和 SD-AOM）的联系，并评估使用 S 和 Ni 同位素特征作为甲烷中黄铁矿形成途径的示踪剂-富含黄铁矿的沉积物，我们报告了南海珠江口盆地神狐地区天然气水合物钻孔（GMGS4-SC-03）沉积物的黄铁矿相关微量元素以及δS和δNi同位素分析。与 Fe- 和/或 SD-AOM 一起形成的黄铁矿表现出丰富的草莓状过度生长和极高的 δS（高达 +142.8‰）和 δNi（高达 +2.72‰），代表了已报道的黄铁矿最高稳定的 S 和 Ni 同位素组成迄今为止。这些黄铁矿形态富含 Co 和 Ni，这可能是 SD-AOM 途径的诊断特征。相比之下，由于 OSR 诱导从腐烂的有机物中释放微量元素，因此与 OSR 相关的黄铁矿富含 Cu 和 Zn。 此外，As 与 Cu 和/或 Zn 的关系可以区分微生物 Fe/Mn 还原与 Fe/Mn-AOM，因为微生物 Fe/Mn 还原从 Fe/Mn-（羟基）氧化物（即 As）中释放微量元素。 ）和有机物（即 Cu 和 Zn），而 Fe/Mn-AOM 仅从 Fe/Mn-（羟基）氧化物中释放微量元素。此外，在大多数具有高 δS 的黄铁矿中观察到的 As 与 Co 或 Ni 之间的共变表明，该黄铁矿捕获了 Fe/Mn-AOM 期间释放的 As 以及 SD-AOM 中释放的 Co 和 Ni。因此，本研究中测量的高镍同位素值可能主要反映了铁和锰（羟基）氧化物中重同位素镍的释放。我们的结果表明，含天然气水合物沉积物中黄铁矿的微量元素组成可以记录主要微生物过程的地球化学特征。