The Trezona carbon isotope excursion is recorded on five different continents in platform carbonates deposited prior to the end-Cryogenian Marinoan glaciation (>635 Ma) and represents a change in carbon isotope values of 16–18‰. Based on the spatial and temporal reproducibility, the excursion previously has been interpreted as tracking the carbon isotopic composition of dissolved inorganic carbon in the global ocean before the descent into a snowball Earth. However, in modern restricted shallow marine and freshwater settings, carbon isotope values have a similarly large range, which is mostly independent from open ocean chemistry and instead reflects local processes. In this study, we combine calcium, magnesium, and strontium isotope geochemistry with a numerical model of carbonate diagenesis to disentangle the degree to which the Trezona excursion reflects changes in global seawater chemistry versus local shallow-water platform environments. Our analysis demonstrates that the most extreme carbon isotope values (∼-10‰ versus +10‰) are preserved in former platform aragonite that was neomorphosed to calcite during sediment-buffered conditions and record the primary carbon isotope composition of platform-top surface waters. In contrast, the downturn and recovery of the Trezona excursion are recorded in carbonates that were altered during early fluid-buffered diagenesis and commonly are dolomitized. We also find that the nadir of the Trezona excursion is associated with a fractional increase in siliciclastic sediments, whereas the recovery from the excursion correlates with a relative increase in carbonate. This relationship suggests that the extreme negative isotopic shift in platform aragonite occurred in concert with periods of increased input of siliciclastic sediments, changes in water depth, and possibly nutrients to platform environments. Although the process for generating extremely negative carbon isotope values in Neoproterozoic platform carbonates remains enigmatic, we speculate that these excursions reflect kinetic isotope effects associated with CO₂ invasion in platform waters during periods of intense primary productivity.

特雷佐纳 (Trezona) 碳同位素漂移记录在五个不同大陆的平台碳酸盐岩中，该碳酸盐沉积在末次低温纪马里诺安冰期 (>635 Ma) 之前，代表碳同位素值的变化为 16–18‰。基于空间和时间的可重复性，此前的漂移被解释为在下降到雪球地球之前追踪全球海洋中溶解无机碳的碳同位素组成。然而，在现代受限制的浅海和淡水环境中，碳同位素值具有类似的大范围，这主要与开阔海洋化学无关，而是反映了局部过程。在这项研究中，我们将钙，镁，和锶同位素地球化学与碳酸盐成岩作用的数值模型，以解开 Trezona 偏移反映全球海水化学变化与当地浅水平台环境变化的程度。我们的分析表明，最极端的碳同位素值（~-10‰ 与 +10‰）保留在前平台文石中，该文石在沉积物缓冲条件下新变形为方解石，并记录了平台顶部地表水的主要碳同位素组成。相反，Trezona偏移的下降和恢复记录在碳酸盐中，碳酸盐在早期流体缓冲成岩作用中发生了改变，并且通常是白云石化的。我们还发现Trezona偏移的最低点与硅质碎屑沉积物的分数增加有关，而偏移的恢复与碳酸盐的相对增加相关。这种关系表明，平台文石的极端负同位素变化与硅质碎屑沉积物输入增加、水深变化以及可能对平台环境的养分的变化同时发生。尽管在新元古代平台碳酸盐中产生极负碳同位素值的过程仍然是个谜，但我们推测这些偏移反映了与 CO 相关的动力学同位素效应₂在初级生产力密集时期侵入台地水域。