Abstract The Cryogenian Period spans two major glaciations, the Sturtian Ice Age (~720–660 Ma) and the Marinoan Ice Age (~650–635 Ma), the termination of each of which was associated with a unique type of cap carbonate deposit. Cap carbonates are significant in providing a record of as-yet not fully understood ocean-chemical changes during the deglaciations following Snowball Earth events and in serving as readily recognizable event beds useful in global stratigraphic correlation of Neoproterozoic successions. Debate regarding the formation of cap carbonates has focused on three key issues: (1) alkalinity sources, (2) abiotic (chemical) versus biotic (microbial) precipitation, and (3) formation rates. Multiple hypotheses regarding cap carbonate formation have been advanced, including the weathering alkalinity model, the oceanic overturn model, the gas hydrate destabilization model, the plumeworld model, the starvation sediment model, the enhanced microbial activity model, and the calcareous loess model. We evaluated these models by considering their proposed solutions to the key issues above in the context of a global compilation of location, thickness, carbonate C-isotope, and paleomagnetic data for Cryogenian cap carbonates. Cap carbonates were probably produced through a combination of chemical and microbial processes in a strongly stratified deglacial ocean with a low-salinity lid. Correlation of δ13Ccarb profiles shows that cap carbonate precipitation began synchronously but terminated diachronously at a global scale. Cap carbonates are markedly thicker in low-paleolatitude regions, suggesting greater alkalinity production and/or more rapid carbonate precipitation in those regions, which favors alkalinity production through continental weathering rather than through oceanic upwelling or methane oxidation. Calculation of alkalinity production rates based on a range of cap carbonate masses and formation intervals shows that minimum masses (~2.2 × 1021 g, i.e., comprising only known continental deposits) could have been produced through intense continental weathering at short timescales (103–104 yr), validating rapid deposition models. Longer timescales of cap carbonate formation (105–106 yr) are not precluded by these calculations but are hard to reconcile with physical evidence of rapid accumulation and near-complete lack of terrigenous clastic impurities. Alkalinity production rate calculations also show that maximum cap carbonate masses (to ~14.4 × 1021 g; i.e., assuming unproven deep-ocean cap carbonate deposits) are probably unrealistic, requiring more alkalinity than could be generated even through a combination of mechanisms including continental weathering, deep-ocean microbial sulfate reduction, and methane release.

中文翻译：

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低温盖碳酸盐模型：回顾和批判性评估

摘要 低温纪跨越了两个主要的冰川期，Sturtian 冰河时代（~720-660 Ma）和 Marinoan 冰河时代（~650-635 Ma），每个冰期的终止都与独特类型的碳酸盐盖层沉积有关。在提供雪球地球事件后冰消期期间尚未完全了解的海洋化学变化的记录方面，以及在新元古代序列的全球地层相关性中用作易于识别的事件床方面，帽状碳酸盐具有重要意义。关于帽状碳酸盐形成的争论集中在三个关键问题上：（1）碱度来源，（2）非生物（化学）与生物（微生物）沉淀，以及（3）形成速率。已经提出了关于帽状碳酸盐形成的多种假设，包括风化碱度模型、海洋翻转模型、天然气水合物失稳模型、羽流世界模型、饥饿沉积物模型、微生物活动增强模型和钙质黄土模型。我们通过在全球汇编低温盖碳酸盐的位置、厚度、碳酸盐 C 同位素和古地磁数据的背景下考虑他们对上述关键问题提出的解决方案来评估这些模型。帽状碳酸盐可能是通过化学和微生物过程在具有低盐度盖子的强烈分层的冰消期海洋中的组合产生的。δ13​​Ccarb 剖面的相关性表明，在全球范围内，盖层碳酸盐沉淀同步开始但历时终止。低古纬度地区的盖层碳酸盐显着更厚，表明这些地区的碱度更高和/或碳酸盐沉淀更快，这有利于通过大陆风化而不是通过海洋上升流或甲烷氧化产生碱度。基于一系列盖层碳酸盐质量和形成间隔的碱度生产率计算表明，最小质量（~2.2 × 1021 g，即仅包含已知的大陆沉积物）可以通过短时间尺度的强烈大陆风化产生（103-104 yr)，验证快速沉积模型。这些计算并未排除更长的碳酸盐盖层形成时间尺度（105-106 年），但很难与快速积累和几乎完全缺乏陆源碎屑杂质的物理证据相协调。碱度生产率计算还表明，最大顶盖碳酸盐质量（至 ~14.4 × 1021 g；即，