论文标题
美元
$δ^{44/40}$Ca-$δ^{88/86}$Sr multi-proxy constrains primary origin of Marinoan cap carbonates
论文作者
论文摘要
新元古代地球至少经历了两个全球尺度的冰川，称为雪球事件。事件发生后，“帽碳酸盐”被广泛沉积，但争议围绕着它们的起源。 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.在外部平台部分中，多洛斯通$δ^{44/40} $ ca和$δ^{88/86} $ sr值定义了针对动态质量依赖的同位素分数预测的线。这种多洛酮主要是从熔体中沉淀出来的。此外，在地层上表现出最快的沉淀率的较高样品与升高的87SR/86SR比率相关，这与长期以来的期望是快速的冰冰风化脉冲强制碳酸盐形成。内平台的多洛酮对水质混合的影响更大，但仍表明降水速率增加了上段。上覆的石灰石显示了海水最大的CA和SR贡献。在全球冰片倒塌期间的当地沿海过程的扩增提供了一个简单但充分的命题，可以解释碳酸盐含量的Ca同位素异质性。岩石记录中动力学同位素效应的检测为开发$δ^{44/40} $ CA- $δ^{88/86} $ SR多螺纹作为饱和状态和$ P $ CO $ $ _2 $的指标。
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$_2$.
