At least two global “Snowball Earth” glaciations occurred during the Neoproterozoic Era (1000-538.8 million years ago). Post-glacial surface environments during this time are recorded in cap carbonates: layers of limestone or dolostone that directly overlie glacial deposits. Postulated environmental conditions that created the cap carbonates lack consensus largely because single hypotheses fail to explain the cap carbonates’ global mass, depositional timescales, and geochemistry of parent waters. Here, we present a global geologic carbon cycle model before, during, and after the second glaciation (i.e. the Marinoan) that explains cap carbonate characteristics. We find a three-stage process for cap carbonate formation: (1) low-temperature seafloor weathering during glaciation generates deep-sea alkalinity; (2) vigorous post-glacial continental weathering supplies alkalinity to a carbonate-saturated freshwater layer, rapidly precipitating cap carbonates; (3) mixing of post-glacial meltwater with deep-sea alkalinity prolongs cap carbonate deposition. We suggest how future geochemical data and modeling refinements could further assess our hypothesis.

新元古代（1000-5.388亿年前）至少发生过两次全球性的“雪球地球”冰川作用。这段时间的冰期后地表环境被记录在盖碳酸盐中：直接覆盖在冰川沉积物上的石灰岩或白云岩层。形成盖帽碳酸盐的假设环境条件缺乏共识，主要是因为单一假设无法解释盖帽碳酸盐的整体质量、沉积时间尺度和母体水体的地球化学。在这里，我们提出了第二次冰川作用（即马里诺冰川）之前、期间和之后的全球地质碳循环模型，该模型解释了盖碳酸盐的特征。我们发现盖碳酸盐形成的三个阶段过程：（1）冰川作用期间的低温海底风化产生深海碱度； (2) 剧烈的冰期后大陆风化作用为碳酸盐饱和的淡水层提供碱度，迅速沉淀盖帽碳酸盐； (3)冰后融水与深海碱度的混合延长了盖帽碳酸盐的沉积。我们建议未来的地球化学数据和模型改进如何进一步评估我们的假设。