δ⁴⁴/⁴⁰Ca-δ⁸⁸/⁸⁶Sr多指标约束Marinoan盖帽碳酸盐岩的原生成因
$δ^{44/40}$Ca-$δ^{88/86}$Sr multi-proxy constrains primary origin of Marinoan cap carbonates
AI总结：
本研究利用δ⁴⁴/⁴⁰Ca-δ⁸⁸/⁸⁶Sr多指标分析纳米比亚Marinoan盖帽碳酸盐岩，揭示其原生信号源于融水-海水混合、动力学分馏及冰消风化。
摘要：
新元古代地球至少经历了两次被称为“雪球地球”事件的全球尺度冰川作用。“盖帽碳酸盐岩”在这些事件之后广泛沉积，但其成因存在争议。在此，我们将新颖的δ⁴⁴/⁴⁰Ca-δ⁸⁸/⁸⁶Sr多指标应用于纳米比亚的两条Marinoan期（约635 Ma）盖帽碳酸盐岩序列，并表明这些岩石记录了来自海水-冰川融水混合与动力学同位素效应共同作用的原生环境信号。在一个外台地剖面中，白云岩的δ⁴⁴/⁴⁰Ca和δ⁸⁸/⁸⁶Sr值构成一条符合动力学质量依赖同位素分馏预测的直线。该白云岩主要从融水中沉淀。此外，层位上更高、表现出最快沉淀速率的样品与升高的⁸⁷Sr/⁸⁶Sr比值相关，这与长期以来的预期一致，即快速冰消风化脉冲驱动了盖帽碳酸盐岩的形成。内台地白云岩显示出更强的水团混合效应，但仍揭示沉淀速率沿剖面向上升高。上覆灰岩显示出最大的海水Ca和Sr贡献。在全球冰盖崩塌期间局地海岸过程的增强，为解释盖帽碳酸盐岩的Ca同位素非均质性提供了一个简单而充分的命题。在岩石记录中检测到动力学同位素效应，为将δ⁴⁴/⁴⁰Ca-δ⁸⁸/⁸⁶Sr多指标发展成为饱和状态和pCO₂指标提供了基础。
英文摘要：
The Neoproterozoic Earth experienced at least two global-scale glaciations termed Snowball Earth events. 'Cap carbonates' were widely deposited after the events, but controversy surrounds their origin. Here, we apply the novel $δ^{44/40}$Ca-$δ^{88/86}$Sr multi-proxy to two Marinoan (ca. 635 Ma) cap carbonate sequences from Namibia and show that the rocks archive primary environmental signals deriving from a combination of seawater-glacial meltwater mixing and kinetic isotope effects. In an outer platform section, dolostone $δ^{44/40}$Ca and $δ^{88/86}$Sr values define a line predicted for kinetic mass-dependent isotope fractionation. This dolostone mostly precipitated from meltwater. Moreover, stratigraphically higher samples exhibiting the fastest precipitation rates correlate with elevated 87Sr/86Sr ratios, consistent with long-held expectations that a rapid deglacial weathering pulse forced cap carbonate formation. An inner-platform dolostone shows greater effects from water-mass mixing but still reveals that precipitation rates increased up-section. Overlying limestones show the greatest Ca and Sr contributions from seawater. Amplification of local coastal processes during global ice sheet collapse offers a simple but sufficient proposition to explain the Ca isotope heterogeneity of cap carbonates. Detection of kinetic isotope effects in the rock record provides a basis for developing the $δ^{44/40}$Ca-$δ^{88/86}$Sr multi-proxy as an indicator of saturation state and $p$CO$_2$.
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