A previously published hydrothermal brine-river water mixing model driven by ocean crust production suggests that the molar Mg/Ca ratio of seawater (mMg/Ca(sw)) has varied significantly (approximately 1.0-5.2) over Precambrian time, resulting in six intervals of aragonite-favouring seas (mMg/Ca(sw) > 2) and five intervals of calcite-favouring seas (mMg/Ca(sw) < 2) since the Late Archaean. To evaluate the viability of microbial carbonates as mineralogical proxy for Precambrian calcite-aragonite seas, calcifying microbial marine biofilms were cultured in experimental seawaters formulated over the range of Mg/Ca ratios believed to have characterized Precambrian seawater. Biofilms cultured in experimental aragonite seawater (mMg/Ca(sw) = 5.2) precipitated primarily aragonite with lesser amounts of high-Mg calcite (mMg/Ca(calcite) = 0.16), while biofilms cultured in experimental calcite seawater (mMg/Ca(sw) = 1.5) precipitated exclusively lower magnesian calcite (mMg/Ca(calcite) = 0.06). Furthermore, Mg/Ca(calcite )varied proportionally with Mg/Ca(sw). This nearly abiotic mineralogical response of the biofilm CaCO3 to altered Mg/Ca(sw) is consistent with the assertion that biofilm calcification proceeds more through the elevation of , via metabolic removal of CO2 and/or H+, than through the elevation of Ca2+, which would alter the Mg/Ca ratio of the biofilm's calcifying fluid causing its pattern of CaCO3 polymorph precipitation (aragonite vs. calcite; Mg-incorporation in calcite) to deviate from that of abiotic calcification. If previous assertions are correct that the physicochemical properties of Precambrian seawater were such that Mg/Ca(sw) was the primary variable influencing CaCO3 polymorph mineralogy, then the observed response of the biofilms' CaCO3 polymorph mineralogy to variations in Mg/Ca(sw), combined with the ubiquity of such microbial carbonates in Precambrian strata, suggests that the original polymorph mineralogy and Mg/Ca(calcite )of well-preserved microbial carbonates may be an archive of calcite-aragonite seas throughout Precambrian time. These results invite a systematic evaluation of microbial carbonate primary mineralogy to empirically constrain Precambrian seawater Mg/Ca.

中文翻译：

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海水中的Mg / Ca控制着微生物CaCO3的多晶型矿物学：这是前寒武纪时期方解石-sea石海的潜在替代物。

以前发布的由洋壳生产驱动的热液盐水河水混合模型表明，在前寒武纪时间内，海水的摩尔Mg / Ca比（mMg / Ca（sw））发生了显着变化（大约1.0-5.2），自古生代晚期以来，ar石有利海（mMg / Ca（sw）> 2）和五个方解石有利海（mMg / Ca（sw）<2）间隔。为了评估微生物碳酸盐作为前寒武纪方解石-ar石海的矿物学替代物的可行性，将钙化微生物海洋生物膜培养在实验海水中，该海水的配制范围被认为是特征在于前寒武纪海水的Mg / Ca比率。在实验文石海水中培养的生物膜（mMg / Ca（sw）= 5.2）沉淀的文石主要是少量的高镁方解石（mMg / Ca（方解石）= 0.16），而在实验方解石海水（mMg / Ca（sw）= 1.5）中培养的生物膜仅沉淀了较低的镁方解石（mMg / Ca（方解石）= 0.06）。此外，Mg / Ca（方解石）与Mg / Ca（sw）成比例变化。生物膜CaCO3对改变的Mg / Ca（sw）的这种近乎非生物的矿物学反应与以下观点一致：生物膜钙化更多地通过升高的代谢，通过代谢去除CO2和/或H +而不是通过提高Ca2 +来进行。会改变生物膜钙化液的Mg / Ca比值，从而导致其CaCO3多晶型物沉淀的模式（文石与方解石； Mg掺入方解石）偏离非生物钙化的模式。如果先前的断言是正确的，即前寒武纪海水的理化特性使得Mg / Ca（sw）是影响CaCO3多晶型矿物学的主要变量，则观察到的生物膜CaCO3多晶型矿物学对Mg / Ca（sw）变化的响应结合前寒武纪地层中此类微生物碳酸盐的普遍存在，表明保存良好的微生物碳酸盐的原始多晶型矿物学和Mg / Ca（方解石）可能是整个前寒武纪时期方解石-文石海的档案。这些结果要求对碳酸盐微生物的主要矿物学进行系统评估，以经验约束前寒武纪海水Mg / Ca。结合前寒武纪地层中这类微生物碳酸盐的普遍存在，表明保存良好的微生物碳酸盐的原始多晶型矿物学和Mg / Ca（方解石）可能是整个前寒武纪时期方解石-文石海的档案。这些结果要求对碳酸盐微生物的主要矿物学进行系统评估，以经验约束前寒武纪海水Mg / Ca。结合前寒武纪地层中这类微生物碳酸盐的普遍存在，表明保存良好的微生物碳酸盐的原始多晶型矿物学和Mg / Ca（方解石）可能是整个前寒武纪时期方解石-文石海的档案。这些结果要求对碳酸盐微生物的主要矿物学进行系统评估，以经验约束前寒武纪海水Mg / Ca。