Abstract Past changes in the halogen composition of seawater are anticipated based on the differing behavior of chlorine and bromine that are strongly partitioned into seawater, relative to iodine, which is extremely depleted in modern seawater and enriched in marine sediments due to biological uptake. Here we assess the use of chert, a chemical sediment that precipitated throughout the Precambrian, as a proxy for halide ratios in ancient seawater. We determine a set of criteria that can be used to assess the primary nature of halogens and show that ancient seawater Br/Cl and I/Cl ratios can be resolved in chert samples from the 2.5 Ga Dales Gorge Member of the Brockman Banded Iron Formation, Hamersley Group, Western Australia. The values determined of Br/Cl ~2 × 10-3 M and I/Cl ~30 × 10-6 M are comparable to fluid inclusions in hydrothermal quartz from the 3.5 Ga North Pole area, Pilbara Craton, Western Australia, that were the subject of previous reconstructions of ancient ocean salinity and atmospheric isotopic composition. While the similar Br/Cl and I/Cl values indicate no substantial change in the ocean halide system over the interval 2.5–3.5Ga, compared to modern seawater, the ancient ocean was enriched in Br and I relative to Cl. The I/Cl value is intermediate between bulk Earth (assumed chondritic) and the modern seawater ratio, which can be explained by a smaller organic reservoir because this is the major control on marine iodine at the present day. Br/Cl ratios are about 30% higher than both modern seawater and contemporary seafloor hydrothermal systems, perhaps indicating a stronger mantle buffering of seawater halogens during the Archean.

中文翻译：

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由燧石和热液石英中的流体包裹体记录的太古代至古元古代海水卤素比

摘要 过去海水中卤素组成的变化是基于氯和溴强烈分配到海水中的不同行为，相对于碘的不同行为，碘在现代海水中极度枯竭，而由于生物吸收而在海洋沉积物中富集。在这里，我们评估了使用燧石（一种在整个前寒武纪沉淀的化学沉积物）作为古代海水中卤化物比率的代表。我们确定了一组可用于评估卤素的主要性质的标准，并表明可以在来自布罗克曼带状铁层的 2.5 Ga Dales 峡谷段的燧石样品中解析古代海水 Br/Cl 和 I/Cl 比率，西澳大利亚州哈默斯利集团。Br/Cl ~2 × 10-3 M 和 I/Cl ~30 × 10-6 M 的测定值与来自西澳大利亚皮尔巴拉克拉通 3.5 Ga 北极地区的热液石英中的流体包裹体相当，它们是古代海洋盐度和大气同位素组成的先前重建的主题。虽然相似的 Br/Cl 和 I/Cl 值表明在 2.5-3.5Ga 区间内海洋卤化物系统没有实质性变化，但与现代海水相比，古代海洋相对于 Cl 富含 Br 和 I。I/Cl 值介于大块地球（假设为球粒陨石）和现代海水比率之间，这可以用较小的有机储层来解释，因为这是当今海洋碘的主要控制因素。Br/Cl 比率比现代海水和现代海底热液系统高约 30%，