Superhigh-organic‑sulfur (SHOS) coals (coals with organic sulfur content >4 wt%) are unique coal deposits found at a few notable locations in the world. Specific peat accumulation and preservation conditions must be met to form SHOS coals. Organic sulfur is a major constituent of such coals, and it may have various sources depending on the prevailing paleomire conditions. Understanding such paleomire conditions sheds light on the formation mechanisms of SHOS coals. This investigation decodes the paleomire conditions of the Paleogene SHOS coals from Meghalaya, India, using sulfur isotopic compositions (S) of organic sulfur (S) and pyritic sulfur (S) along with organic petrography, pyrite morphology and trace element ratios. Thirty coal samples were collected from the Jaintia Hills in the east, Khasi Hills in the middle, and Garo Hills in the west of Meghalaya. The organic sulfur content in the Garo, Khasi, and Jaintia coals varies from 1.0 to 3.3 wt%, 1.4 to 13.8 wt%, and 1.0 to 7.2 wt%, respectively. Further, after separation from pyritic sulfur and sulfate sulfur phases, the organic sulfur content ranges from 54.4 to 69.2%, 63.8 to 79.9%, and 59.3 to 73.8%, in the Garo, Khasi, and Jaintia Hills, respectively, suggesting the SHOS nature of these coal samples. The S varies from −29.3 ‰ to +5.7 ‰, −21.3 ‰ to +27.3 ‰, and −12.1 ‰ to −4.3 ‰, in the Jaintia, Khasi, and Garo Hills, respectively, while the S fluctuates from −4.6 ‰ to +3.7 ‰, −9.3 ‰ to +7.8 ‰, and − 9.0 ‰ to −5.0 ‰, respectively. The S values of pyrite and organic sulfur (OS) in Jaintia coals are S depleted compared to seawater sulfate (+22 ‰), leading to fractionations in the range of −51.3 ‰ to −16.3 ‰ (mean − 31.6 ‰) and − 26.6 ‰ to −18.3 ‰ (mean − 23.1 ‰) for pyritic and organic sulfur (OS), respectively. Pyrite in Khasi coals show a relatively heavier S composition averaging at −20.5 ‰, whereas organic sulfur (OS) isotope compositions range from −31.3 ‰ to −14.2 ‰ with a mean of −22.6 ‰. Pyrite and OS in the Garo coals are depleted compared to seawater sulfate. Isotope variations in the Jaintia, Khasi, and Garo coals indicate microbial sulfate reduction (MSR) of seawater sulfate. Large isotopic fractionations between Eocene seawater sulfate and pyritic sulfur (S = up to −51.3 ‰; mean − 31.6 ‰) in Jaintia coals indicate their possible formation in the water column/near the sediment-seawater interface (open system) and also hint toward dissimilatory sulfate reduction pathways that prevailed under anoxic redox conditions. However, mean values of S (−20.5 ‰) in the Khasi coals imply pyrite formation deeper in the sediments (more closed system) under dysoxic conditions. The dominance of OS over pyritic sulfur, framboidal pyrite, and its microcrystal size distributions in Jaintia coals may suggest syngenetic pyrite formation in open water reducing/anoxic conditions under paralic environments. Elevated Sr/Ba and U/Th values in these coals further confirm the anoxic conditions. Nevertheless, the presence of euhedral pyrite with the alleviated pyrite framboids in the Khasi coals and their complete absence in the Garo coals may suggest dysoxic-suboxic and suboxic-oxic depositional conditions, respectively. The isotopic signatures of the Garo coals suggest sulfur contribution from the parent paleobiota and MSR under a freshwater-oxic environment. Insignificant fractionations between S and S indicate limited iron and sulfate availability for additional sulfur cycling and disproportionation reactions, typical of oxic conditions. The absence of framboidal pyrite, elevated sulfate concentration, and mean Sr/Ba and U/Th values of 0.5 and 0.3, respectively, further suggest the freshwater peat deposition in the Garo Hills under limnotelmatic to telmatic freshwater conditions. Moreover, high inertinite content (I = 9.77–33.16 vol%), possibly induced by atmospheric peat exposure, supports the interpretation of suboxic-oxic paleomire conditions in Garo Hills. Gradually decreasing mineral matter content from Jaintia (mean 13.6 vol%) to Garo coals (mean 7.4 vol%) additionally projects a transition from mesotrophic brackish to freshwater limnotelmatic environment, complementing the shift in the paleomire condition from eastern (Jaintia) to western (Garo) Meghalayan Hills.

中文翻译：

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破译古近纪超高有机硫煤的古煤岩条件


超高有机硫 (SHOS) 煤（有机硫含量 >4 wt% 的煤）是在世界上一些著名地点发现的独特煤矿。必须满足特定的泥炭积累和保存条件才能形成 SHOS 煤。有机硫是此类煤的主要成分，根据当时的古煤条件，有机硫可能有多种来源。了解这种古煤的条件有助于了解 SHOS 煤的形成机制。这项研究利用有机硫 (S) 和黄铁矿硫 (S) 的硫同位素组成 (S) 以及有机岩相学、黄铁矿形态和微量元素比率，解码了印度梅加拉亚邦古近纪 SHOS 煤的古矿条件。从梅加拉亚邦东部的 Jaintia 山、中部的 Khasi 山和西部的 Garo 山采集了 30 个煤炭样品。 Garo、Khasi 和 Jaintia 煤中的有机硫含量分别为 1.0 至 3.3 wt%、1.4 至 13.8 wt% 和 1.0 至 7.2 wt%。此外，在从黄铁矿硫和硫酸盐硫相分离后，加罗山、卡西山和贾因蒂亚山的有机硫含量范围分别为 54.4% 至 69.2%、63.8% 至 79.9% 和 59.3% 至 73.8%，这表明 SHOS 性质这些煤样。 Jaintia、Khasi 和 Garo 山的 S 变化范围分别为 -29.3 ‰ 至 +5.7 ‰、-21.3 ‰ 至 +27.3 ‰ 和 -12.1 ‰ 至 -4.3 ‰，而 S 波动范围为 -4.6 ‰ 至 -4.6 ‰分别为+3.7%、-9.3%至+7.8%、-9.0%至-5.0%。 Jaintia 煤中黄铁矿和有机硫 (OS) 的 S 值与海水硫酸盐 (+22 ‰) 相比是贫硫的，导致分馏范围为 -51.3 ‰ 至 -16.3 ‰（平均值 - 31.6 ‰）和 - 26.6黄铁矿和有机硫 (OS) 分别为 ‰ 至 -18.3 ‰（平均值 - 23.1 ‰）。 卡西煤中的黄铁矿显示出相对较重的硫成分，平均为-20.5 ‰，而有机硫（OS）同位素成分范围为-31.3 ‰至-14.2 ‰，平均值为-22.6 ‰。与海水硫酸盐相比，加罗煤中的黄铁矿和有机磷含量较低。 Jaintia、Khasi 和 Garo 煤中的同位素变化表明海水硫酸盐存在微生物硫酸盐还原 (MSR)。 Jaintia 煤中始新世海水硫酸盐和黄铁矿硫之间的大量同位素分馏（S = 高达 -51.3 ‰；平均值 - 31.6 ‰）表明它们可能在水柱中/靠近沉积物-海水界面（开放系统）形成，并且也暗示缺氧氧化还原条件下盛行的异化硫酸盐还原途径。然而，卡西煤中的 S 平均值（−20.5 ‰）意味着在缺氧条件下，黄铁矿在沉积物更深处（更封闭的系统）形成。 Jaintia煤中OS相对于黄铁矿硫、草莓状黄铁矿的优势及其微晶尺寸分布可能表明在湿地环境下开放水减少/缺氧条件下同生黄铁矿的形成。这些煤中 Sr/Ba 和 U/Th 值升高进一步证实了缺氧条件。然而，卡西煤中自形黄铁矿的存在以及黄铁矿片状黄铁矿的减少以及它们在加罗煤中完全不存在可能分别表明缺氧-缺氧和缺氧-缺氧沉积条件。加罗煤的同位素特征表明，在淡水含氧环境下，硫的贡献来自于母体古生物群和 MSR。 S 和 S 之间的微不足道的分馏表明，对于额外的硫循环和歧化反应（典型的含氧条件）而言，铁和硫酸盐的可用性有限。 缺乏草莓状黄铁矿、硫酸盐浓度升高以及平均 Sr/Ba 和 U/Th 值分别为 0.5 和 0.3，进一步表明加罗山的淡水泥炭沉积处于湖流到湖流淡水条件下。此外，惰性石含量高（I = 9.77–33.16 vol%），可能是由大气泥炭暴露引起的，支持对加罗山低氧-氧古矿条件的解释。从 Jaintia（平均 13.6 vol%）到 Garo 煤（平均 7.4 vol%）逐渐降低的矿物质含量还预示着中营养咸水环境向淡水湖环境的转变，补充了古煤层条件从东部（Jaintia）到西部（Garo）的转变）梅加拉亚山。