Elemental fluxes to the ocean are expected to increase with the surface area of continental exposure to weathering and atmospheric P_CO2. The record of phosphorus in sediments, which has no notable source within the ocean, and the radiogenic strontium isotopes in Archean carbonates indicate that, prior to the Great Oxidation Event (GOE), subaerial expanses represented only about 20% of the modern continental surface area, i.e. 7% of the surface of the Earth. Because these simple first-order observations, in contrast to the low oxygen content of the pre-GOE atmosphere, have so far received only little attention in the appraisal of the marine chemistry of the early Earth, a reassessment of the chemistry of the pre-GOE ocean is warranted. Here we discuss some of the geochemical cycles of the Archean world, including protons, alkalinity, electrons, and other electrolytes, and attempt to build a first conceptual framework for Chemical Archeoceanography. The smaller subaerial exposures characterizing the Archean and the low abundance of Archean carbonates and mudstones imply that the flux of alkalinity to the ocean was much weaker than today and therefore that the capacity of the runoff to neutralize the high-temperature hydrothermal fluids was less important. Such a reduced flux in turn entails lower seawater pH and higher chlorinity. The lack of atmospheric O₂ allowed iron to be in its soluble Fe(II) form and to reduce ${\mathrm{HCO}}_3^{-}$ to CH₄ while liberating large amounts of protons. The low pH of the pre-GOE ocean and the reduced alkalinity input, reinforced by scant P supply, account for reduced carbonate precipitation. Ocean chemistry evolved under the control of two changing factors: (i) the balance between chemical weathering and hydrothermal fluxes and (ii) the oxygen pressure in the ocean and the atmosphere. Seawater iron concentration was controlled by high [Fe²⁺]/[H⁺]² ratios. With the temperature of hydrothermal fluids at mid-ocean ridges and other active volcanic edifices being determined by the expansion properties of seawater, the chemistry of hydrothermal fluids is controlled by the chlorinity of the ocean. Of all the major element cycles in seawater, those of Na, Mg, and P seem to be unbalanced when runoff falls below the modern value. The very low P content of banded iron formations is inconsistent with a major role of biological activity in the oxidation of Fe²⁺ dissolved in the Archean ocean. Prior to the GOE, banded iron formations were the major sedimentary sink of seawater cations, a role now played by carbonates. The controls of seawater alkalinity, which today are exerted by the Ca²⁺–CaCO₃ couple, was exerted by dissolved Fe²⁺-magnetite or Fe²⁺-ferrihydrite.

Plain language summary

We here evaluate the constraints on seawater chemistry before free oxygen was introduced into the atmosphere some 2.4 billion years ago. We suggest a simple conceptual framework which we validated on modern seawater and which is based on two so-far largely overlooked observations, namely that the phosphorus concentration and strontium isotopic records in ancient sediments require that emerged continental land masses were covering only ~7% of Earth's surface at the time of the Great Oxidation Event instead of 30% today. Atmospheric and seawater chemistry critically depend on this proportion as well as on the chlorinity of the ocean. The oxygen-free atmosphere allowed iron to remain in solution and to reduce atmospheric carbon dioxide to methane. The pH of the ancient oceans must have been lower because less river water was available to neutralize the acidic submarine high-temperature hydrothermal fluids emitted at mid-ocean ridges. Limited continental surface starved the sedimentary column for clays. The low pH and scant phosphorus drainage led to rare carbonate deposition. We conclude by proposing a new scheme for Archean marine chemistry and geochemical cycles that can explain the chemical and geological observations from the Archean and early Proterozoic rock record.

中文翻译：

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化学考古学

预计海洋的元素通量将随着大陆暴露于风化和大气中的P _CO2的表面积而增加。在海洋中没有明显来源的沉积物中磷的记录，以及太古代碳酸盐中的放射性锶同位素表明，在大氧化事件（GOE）之前，地下扩张仅占现代大陆表面积的20％ ，即地球表面的7％。由于这些简单的一阶观测与GOE之前大气中的低氧含量相比，到目前为止在评估早期地球的海洋化学，重新评估前地球化学方面仅受到很少的关注。 GOE海洋是有保证的。在这里，我们讨论了太古代世界的一些地球化学循环，包括质子，碱度，电子和其他电解质，并试图建立化学考古学的第一个概念框架。太古代的特征是较小的大气暴露以及太古代的碳酸盐和泥岩含量低，这意味着向海洋的碱度通量要比今天弱得多，因此径流中和高温热液的能力不那么重要。这样减少的通量又导致较低的海水pH和较高的氯含量。缺乏大气中的氧气₂允许铁以其可溶性Fe（II）形式还原${\mathrm{HCO}}_3^{-}$释放大量质子的过程中产生CH ₄。GOE前海洋的低pH值和减少的碱度输入（由于缺乏磷供应而增强）导致碳酸盐沉淀减少。海洋化学是在两个变化因素的控制下发展起来的：（i）化学风化和热液通量之间的平衡，以及（ii）海洋和大气中的氧气压力。海水铁浓度通过高[Fe ²⁺ ] / [H ⁺ ] ^2来控制比率。由于海洋的膨胀特性决定了洋中脊热液的温度和其他活跃的火山构造，因此热液的化学性质受海洋中氯含量的控制。在海水的所有主要元素循环中，当径流低于现代值时，Na，Mg和P的元素循环似乎不平衡。带状铁形成物中极低的P含量与在太古代海洋中溶解的Fe ²⁺的氧化中的生物活性的主要作用不一致。在GOE之前，带状铁层是海水阳离子的主要沉积汇，现在由碳酸盐发挥作用。Ca ²⁺ –CaCO _3可以控制海水的碱度耦合，被溶解的Fe施加²⁺ -magnetite或Fe ²⁺ -ferrihydrite。

普通语言摘要

我们在这里评估大约24亿年前将游离氧引入大气之前对海水化学的限制。我们提出了一个简单的概念框架，该框架在现代海水中得到了验证，并基于两个迄今为止被广泛忽视的观测结果，即古代沉积物中的磷浓度和锶同位素记录要求新兴大陆土地质量仅覆盖约7％的水。发生大氧化事件时的地球表面，而不是今天的30％。大气和海水的化学反应主要取决于这一比例以及海洋的氯含量。无氧气氛使铁保留在溶液中，并将大气中的二氧化碳还原为甲烷。古代海洋的pH必须较低，因为可用于中和洋中脊的酸性海底高温热液的河水较少。有限的大陆表面使沉积柱缺乏粘土。低pH值和少量磷排泄导致罕见的碳酸盐沉积。最后，我们提出了一种关于太古代海洋化学和地球化学循环的新方案，该方案可以解释太古代和原始元古代岩石记录中的化学和地质观测结果。