Sulphur cycling in a Neoarchaean microbial mat.
Sulphur cycling in a Neoarchaean microbial mat.
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DOI：
10.1111/gbi.12227
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发表时间：
2017-05
期刊：
影响因子：
3.7
通讯作者：
Fike DA
中科院分区：
文献类型：
--
作者：
Meyer NR;Zerkle AL;Fike DA
多种硫同位素比值是理解深部硫地球化学循环复杂性的有力指标。在<~2.4 Ga的岩石中，硫的质量无关分馏（S-MIF）信号的消失已被用于确定大气氧水平急剧上升的日期。然而，在大氧化事件之前，S循环的复杂性仍然知之甚少。例如，同时代大气中硫的同位素组成仍有争议。此外，黄铁矿δ 34 S值的变化已被广泛归因于微生物硫酸盐还原（MSR）。虽然侏罗纪早期成岩黄铁矿形成的岩相学证据很常见，但MSR存在和分布的结构证据仍然是谜。我们使用二次离子质谱（西姆斯）将详细的岩相学和原位高分辨率多S同位素研究（δ 34 S和Δ 33 S）相结合，以记录南非Ghaap群~2.65 Ga Lokammona地层页岩中保存异常完好的黄铁矿化微生物岩的S同位素特征。新太古代微生物席中MSR的存在得到了典型的生物成因结构的支持，包括波状褶皱纹层和早期成岩黄铁矿，δ 34 S和Δ 33 S的变化<26‰ μm，Δ 33 S = −0.21 ± 0.65‰（±1σ）。δ 34 S值的这些大的变化表明在高速率的MSR期间有限的硫酸盐池的瑞利蒸馏。此外，我们还发现了第二个形态上明显不同的黄铁矿相，它是在石化后沉淀的，δ 34 S = 8.36 ± 1.16‰，Δ 33 S = 5.54 ± 1.53‰（±1σ）。我们认为，这种次生黄铁矿的S-MIF特征并不反映沉积时的同期大气过程;相反，它是由含有继承的大气S-MIF信号的后期含硫流体的流入和/或热化学硫酸盐还原过程中的磁性同位素效应形成的。这些见解突出了岩相学和西姆斯研究的互补性，以解决地质记录中多代黄铁矿形成途径。
Multiple sulphur (S) isotope ratios are powerful proxies to understand the complexity of S biogeochemical cycling through Deep Time. The disappearance of a sulphur mass‐independent fractionation (S‐MIF) signal in rocks <~2.4 Ga has been used to date a dramatic rise in atmospheric oxygen levels. However, intricacies of the S‐cycle before the Great Oxidation Event remain poorly understood. For example, the isotope composition of coeval atmospherically derived sulphur species is still debated. Furthermore, variation in Archaean pyrite δ34S values has been widely attributed to microbial sulphate reduction (MSR). While petrographic evidence for Archaean early‐diagenetic pyrite formation is common, textural evidence for the presence and distribution of MSR remains enigmatic. We combined detailed petrographic and in situ, high‐resolution multiple S‐isotope studies (δ34S and Δ33S) using secondary ion mass spectrometry (SIMS) to document the S‐isotope signatures of exceptionally well‐preserved, pyritised microbialites in shales from the ~2.65‐Ga Lokammona Formation, Ghaap Group, South Africa. The presence of MSR in this Neoarchaean microbial mat is supported by typical biogenic textures including wavy crinkled laminae, and early‐diagenetic pyrite containing <26‰ μm‐scale variations in δ34S and Δ33S = −0.21 ± 0.65‰ (±1σ). These large variations in δ34S values suggest Rayleigh distillation of a limited sulphate pool during high rates of MSR. Furthermore, we identified a second, morphologically distinct pyrite phase that precipitated after lithification, with δ34S = 8.36 ± 1.16‰ and Δ33S = 5.54 ± 1.53‰ (±1σ). We propose that the S‐MIF signature of this secondary pyrite does not reflect contemporaneous atmospheric processes at the time of deposition; instead, it formed by the influx of later‐stage sulphur‐bearing fluids containing an inherited atmospheric S‐MIF signal and/or from magnetic isotope effects during thermochemical sulphate reduction. These insights highlight the complementary nature of petrography and SIMS studies to resolve multigenerational pyrite formation pathways in the geological record.