The "Snowball Earths" were cataclysmic events during the late Neoproterozoic's Cryogenian period (720-635 Ma) in which most, if not all, of Earth’s surface was covered in ice. Paleoenvironmental reconstructions of these events utilize isotopic systems, such as Δ¹⁷O and barium isotopes of barites. Other isotopic systems, such as zinc (Zn), can reflect seawater composition or environmental conditions (e.g., temperature changes) and biological productivity. We report here a multi-isotopic C, O, and Zn data set for carbonates deposited immediately after the Marinoan glaciation (635 Ma) from the Otavi Group in northern Namibia. In this study, we chemically separated calcite and non-calcitic carbonate phases, finding isotopically distinct carbon and oxygen isotopes. These could reflect changes in the source seawater composition and conditions during carbonate formation. Our key finding is largescale Zn isotopic variations over the oldest parts of the distal foreslope cap carbonate sections. The magnitude of variation is larger than any found throughout post-snowball cap carbonates to date, and in a far shorter sequence. This shows a heretofore undiscovered difficulty for Zn isotopic interpretations. The primary Zn sources are likely to be aeolian or alluvial, associated with the massive deglaciation related run-off from the thawing continent and a greater exposed surface for atmospheric aerosol entrainment. The samples with the lightest Zn isotopic compositions (δ⁶⁶Zn < 0.3 ‰) potentially reflect hydrothermally sourced Zn dominating the carbonates’ Zn budget. This finding is likely unique to the oldest carbonates, when the meltwater lid was thinnest and surface waters most prone to upwelling of hydrothermally dominated Snowball Earth brine. On the other hand, local variations could be related to bioproductivity affecting the Zn isotopic composition of the seawater. Similarly, fluctuations in sea-level could bring the depositional site below and above a redoxcline, causing isotopic variations. These variations in Zn isotope ratios preclude the estimation of a global Zn isotopic signature, potentially indicating localized resumption of export production.

“雪球地球”是新元古代晚期成冰期（720-635 Ma）的灾难性事件，地球表面的大部分（如果不是全部）都被冰覆盖。这些事件的古环境重建利用同位素系统，例如 Δ ¹⁷重晶石的 O 和钡同位素。其他同位素系统，例如锌 (Zn)，可以反映海水成分或环境条件（例如温度变化）和生物生产力。我们在此报告纳米比亚北部 Otavi 群的 Marinoan 冰川作用 (635 Ma) 后立即沉积的碳酸盐的多同位素 C、O 和 Zn 数据集。在这项研究中，我们对方解石和非方解石碳酸盐相进行了化学分离，发现了同位素不同的碳和氧同位素。这些可以反映碳酸盐形成过程中源海水成分和条件的变化。我们的主要发现是远前斜坡盖碳酸盐岩剖面最古老部分的大规模锌同位素变化。变化的幅度大于迄今为止在整个后雪球帽碳酸盐岩中发现的任何变化，并且顺序要短得多。这显示了 Zn 同位素解释迄今未发现的困难。主要的锌来源可能是风成或冲积层，与融化大陆的大量冰川消融相关的径流和大气气溶胶夹带的更大暴露表面有关。Zn 同位素组成最轻的样品 (δ⁶⁶ Zn < 0.3 ‰) 可能反映出热液来源的 Zn 主导了碳酸盐岩的 Zn 收支。这一发现可能是最古老的碳酸盐岩所独有的，当时融水盖最薄，地表水最容易出现以热液为主的雪球地球盐水的上升流。另一方面，局部变化可能与影响海水锌同位素组成的生物生产力有关。同样，海平面的波动可能会使沉积地点低于和高于氧化还原斜线，从而导致同位素变化。Zn 同位素比率的这些变化排除了对全球 Zn 同位素特征的估计，这可能表明出口生产在局部恢复。