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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >The sulfur isotopic composition of Neoproterozoic seawater sulfate: implications for a snowball Earth
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The sulfur isotopic composition of Neoproterozoic seawater sulfate: implications for a snowball Earth

机译：新元古代海水硫酸盐的硫同位素组成：对雪球地球的影响

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The present study employs a method for analysis of the sulfur isotopic composition of trace sulfate extracted from carbonates collected in Namibia in order to document secular variations in the sulfur isotopic composition of Neoproterozoic oceanic sulfate and to assess variations in the sulfur cycle that may have accompanied profound climatic events that have been described as the snowball Earth hypothesis. The carbonates in the Otavi Group of Northwest Namibia contain 3-295 ppm sulfate. Positive excursions, to a high of 40‰ (CDT), occur above the lower (Chuos Formation) and upper (Ghaub Formation) glacial intervals in the Rasthof and Maieberg cap carbonates, respectively. Positive excursions at the top of the Rasthof Formation (reaching 51‰) and within the overlying Gruis Formation (34‰) do not appear to correspond to glaciation. The δ~(34)S_(sulfate) values within the Ombaatjie Formation exhibit shifts over relatively short stratigraphic intervals (tens of meters), varying between ～15 and 25‰. Cap carbonates from Australia exhibit positive δ~(34)S_(sulfate) trends with amplitudes similar to those of Namibian δ~(34)S_(sulfate), although, more data are necessary to firmly establish these δ~(34)S trends as global in nature. δ~(34)S_(sulfate) excursions found in Namibian cap carbonates are consistent with the snowball Earth hypothesis in that they appear to reflect nearly complete reduction of sulfate in an isolated, anoxic global ocean, although, there are other mechanisms that may have facilitated these large shifts in δ~(34)S_(sulfate). Regardless, the low sulfate concentrations in Otavi carbonates, the high amplitude variability of the δ~(34)S_(sulfate) curve, and the apparently full reduction of sulfate (as implied from δ~(34)S_(pyrite) data), even in strata low in C_(org), suggest that Neoproterozoic oceanic sulfate concentrations were much lower than modern values. Additionally, the buildup of ferrous iron and banded-iron formations during the Sturtian glacial event would indicate that Fe supply exceeded sulfide availability during the glacials and/or that all sulfide was fixed and buried. This could be construed as further evidence in support of low oceanic sulfate (and sulfide) at this time.
机译：本研究采用一种分析从纳米比亚收集的碳酸盐中提取的痕量硫酸盐的硫同位素组成的方法，以记录新元古代大洋硫酸盐的硫同位素组成的长期变化，并评估可能伴随着深刻的硫循环的变化。被描述为雪球地球假说的气候事件。西北纳米比亚的奥塔维组的碳酸盐含有3-295 ppm的硫酸盐。在Rasthof和Maieberg盖碳酸盐岩的下部（Chuos地层）和上部（Ghaub地层）冰川区间以上，分别发生正向偏移，高达40‰（CDT）。 Rasthof组顶部（达到51‰）和上覆的Gruis组（34‰）内的正偏移似乎并不对应于冰川。 Ombaatjie组内的δ〜（34）S_（硫酸盐）值在相对较短的地层间隔（几十米）内表现出位移，变化范围为〜15至25‰。来自澳大利亚的碳酸盐碳酸盐显示出正的δ〜（34）S_（硫酸盐）趋势，幅度类似于纳米比亚的δ〜（34）S_（硫酸盐），尽管要确定这些δ〜（34）S趋势还需要更多数据作为全球性的在纳米比亚碳酸盐岩中发现的δ〜（34）S_（硫酸盐）偏移与地球雪球假说相符，因为它们似乎反映了一个孤立的缺氧全球海洋中硫酸盐的几乎完全还原，尽管可能还有其他机理促进了δ〜（34）S_（硫酸盐）的这些大变化。无论如何，奥塔维碳酸盐中的硫酸盐浓度低，δ〜（34）S_（硫酸盐）曲线的幅度变化大，硫酸盐的表观完全还原（从δ〜（34）S_（黄铁矿）数据暗示），即使在C_（org）较低的地层中，也表明新元古代海洋硫酸盐浓度远低于现代值。另外，在斯图尔特冰川事件期间亚铁和带状铁层的堆积将表明，在冰川期间，铁的供应量超过了硫化物的供应量，并且/或者所有硫化物都被固定并埋藏了。这可以解释为此时支持低海洋硫酸盐（和硫化物）的进一步证据。

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