Neoproterozoic carbon isotope excursions are commonly attributed to changes in the global fraction of organic carbon burial associated with climate instability and/or oxygenation. Here we show that carbonate sediment deposited during the ca. 661 – <651 Ma Cryogenian non-glacial interlude between the Sturtian and Marinoan glaciations exhibit lateral offsets in carbonate-carbon isotope values from coeval units by as much as 10‰. Within the Thorndike submember of the Cryogenian succession in the Panamint Range, California, USA, carbonate-carbon isotope values can be linked to a laterally discontinuous dolomitization front: limestones exhibit ${\delta }^{13}$C_carb values of ∼+4 to +9‰, whereas values of stratigraphically equivalent dolostones are consistently lower, between ∼-4 and +4‰. Field observations and analyses of clasts from the overlying Marinoan glacial diamictite show that the offset in ${\delta }^{13}$C_carb values resulted from pre- to syn-Marinoan dolomitization. Further, ${\delta }^{44/40}$Ca and ${\delta }^{26}$Mg data indicate that this isotopic variability resulted from sediment-buffered diagenesis. We propose that extremely positive ${\delta }^{13}$C_carb values record local primary productivity within restricted platform surface waters and/or oxygenated pore fluids and negative values reflect anaerobic remineralization of organic carbon within sediment pore waters. In this scenario, neither the original calcite/aragonite nor subsequent dolomite precipitates of the Thorndike submember record ${\delta }^{13}$C_carb values that are representative of global Cryogenian seawater, and instead they archive the evolution of local dissolved inorganic carbon pools.

新元古代碳同位素漂移通常归因于与气候不稳定和/或充氧有关的全球有机碳埋藏比例的变化。在这里，我们显示了碳酸盐沉积物在ca期间沉积。661 – <651 Ma Sturtian和Marinoan冰河之间的冰河系非冰川夹层显示，碳酸盐-碳同位素值与同代单位的横向偏移量高达10‰。在美国加利福尼亚州Panamint山脉的深冷层演替的Thorndike子成员中，碳酸盐-碳同位素值可与侧向不连续的白云石化锋面相关：石灰岩表现出${\delta }^{13}$C _carb值约为+4至+ 9‰，而地层等效白云岩的值始终较低，约为-4至+ 4‰。实地观测和对上覆的Marinoan冰河白铁矿碎屑的分析表明，${\delta }^{13}$C_{碳水化合物的}价值是由Marinoan的白云石化前到合成的白云石化引起的。更多，${\delta }^{44/40}$钙和 ${\delta }^{26}$镁数据表明这种同位素变异性是由沉积物缓冲的成岩作用引起的。我们建议，非常积极${\delta }^{13}$C_{碳水化合物}值记录了受限平台地表水和/或充氧孔隙流体中的局部初级生产力，负值反映了沉积物孔隙水中有机碳的厌氧再矿化。在这种情况下，Thorndike子成员记录的原始方解石/文石或随后的白云石均不会析出${\delta }^{13}$代表全球低温冰冻海水的C_{碳水化合物}值，而是归档了局部溶解的无机碳库的演变。