Abstract The past Martian atmosphere is often compared to the Archean Earth’s as both were dominated by CO2-rich and O2-poor chemistries. Archean Earth rocks preserve mass-independently fractionated sulfur isotopes (S-MIF; non-zero Δ33S and Δ36S), originating from photochemistry in an anoxic atmosphere. Thus, Martian crustal rocks might also be expected to preserve a S-MIF signature, providing insights into past atmospheric chemistry. We have used secondary ion mass spectrometry (SIMS) to investigate in situ, the sulfur isotope systematics of NWA 8171 (paired to NWA 7034), a Martian polymict breccia containing pyrite that formed through hydrothermal sulfur addition in a near-surface regolith setting. In this meteorite, pyrite grains have a weighted mean of Δ33S of -0.14 ± 0.08 ‰ and Δ36S = -0.70 ± 0.40 ‰ (2 s.e.m.), so the S-MIF signature is subtle. Sulfur isotope data for four additional shergottites yield Δ33S values that are not resolvable from zero, as in previous studies of shergottites. At first glance the result for the polymict breccia might seem surprising, but no Martian meteorite yet has yielded a S-MIF signature akin to the large deviations seen on Earth. We suggest that S-MIF-bearing aerosols (H2SO4 and S8) were produced when volcanic activity pushed a typically oxidising Martian atmosphere into a reduced state. After rain-out of these aerosols, S8 would tend to be oxidised by chlorate, dampening the S-MIF signal, which might be somewhat retained in the more abundant photolytic sulfate. Then in the regolith, mixing of aqueous surface-derived sulfate with igneous sulfide (the latter with zero MIF), to form the abundant pyrite seen in NWA 8171, would further dampen the S-MIF signal. Nonetheless, the small negative Δ33S anomalies seen in Martian meteorites imply that volcanic activity was sufficient to produce a reducing atmosphere at times. This volcanically-driven atmospheric evolution would tend to produce high levels of carbonyl sulfide (OCS). Given that OCS is a relatively long-lived strong greenhouse gas, the S-MIF signal implies that volcanism periodically generated warmer conditions, perhaps offering an evidence-based solution to the young wet Mars paradox.

中文翻译：

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火星风化层黄铁矿中的一个小 S-MIF 信号：对大气的影响

摘要 过去的火星大气经常被比作太古宙地球，因为两者都以富含 CO2 和缺乏 O2 的化学物质为主。太古宙地球岩石保留了与质量无关的分馏硫同位素（S-MIF；非零 Δ33S 和 Δ36S），源自缺氧大气中的光化学。因此，火星地壳岩石也可能会保留 S-MIF 特征，从而提供对过去大气化学的见解。我们使用二次离子质谱法 (SIMS) 原位研究了 NWA 8171（与 NWA 7034 配对）的硫同位素系统学，这是一种含有黄铁矿的火星复合角砾岩，通过在近地表风化层环境中添加热液硫而形成。在这块陨石中，黄铁矿颗粒的加权平均 Δ33S 为 -0.14 ± 0.08 ‰ 和 Δ36S = -0.70 ± 0.40 ‰ (2 sem)，所以 S-MIF 签名很微妙。四个额外的 Shergottites 的硫同位素数据产生的 Δ33S 值无法从零开始解析，就像之前对 Shergottites 的研究一样。乍一看，polymict 角砾岩的结果似乎令人惊讶，但还没有火星陨石产生类似于地球上看到的大偏差的 S-MIF 特征。我们认为，当火山活动将典型的氧化火星大气推向还原状态时，会产生含 S-MIF 的气溶胶（H2SO4 和 S8）。在这些气溶胶下雨后，S8 往往会被氯酸盐氧化，从而抑制 S-MIF 信号，这可能在一定程度上保留在更丰富的光解硫酸盐中。然后在风化层中，将含水表面衍生的硫酸盐与火成岩硫化物（后者的 MIF 为零）混合，形成在 NWA 8171 中看到的丰富的黄铁矿，将进一步抑制 S-MIF 信号。尽管如此，在火星陨石中看到的小的负 Δ33S 异常意味着火山活动有时足以产生还原性大气。这种火山驱动的大气演化往往会产生高水平的硫化羰（OCS）。鉴于 OCS 是一种相对长期存在的强温室气体，S-MIF 信号暗示火山活动会周期性地产生更温暖的条件，这可能为年轻的潮湿火星悖论提供了一个基于证据的解决方案。