Neoproterozoic loess and limits to snowball Earth

Neoproterozoic tillites overlain by limestones and dolostones (cap carbonates) have been interpreted as evidence of abrupt climate change from glaciers to tropical seas on the assumption that cap carbonates were marine. This assumption is here challenged by evidence for loess deposition of the Nuccaleena Formation of South Australia from granulometry (angular, randomly oriented, silt, modal diameter 7ϕ) and sedimentary structures (climbing-translatent cross-stratification, linear dunes). In addition, a variety of palaeosols in the Nuccaleena Formation have red clayey horizons, replacive micritization, expansion cracks, and thufur mounds, as well as low carbon and oxygen isotope values, high strontium isotope ratios, and geochemical evidence of leaching and clay formation. This cap carbonate has no definitively marine features, and is here interpreted as periglacial loess overlying the fluvioglacial–intertidal Elatina Formation, so it is not a record of abrupt global warming from snowball Earth. Other cap carbonates around the world differ in various ways, and may have formed in different depositional environments, but could profitably be reconsidered from the new perspective of the loess depositional model proposed here.新元古代冰碛岩（Neoproterozoic tillites）被灰岩与白云岩（盖帽碳酸盐岩，cap carbonates）覆盖，此前学界基于“盖帽碳酸盐岩形成于海洋环境”的假设，将其解读为冰川环境向热带海洋环境快速气候转变的证据。本次研究通过南澳大利亚努卡里纳组（Nuccaleena Formation）的粒度分析结果（具棱角状、随机定向的粉砂，众数粒径为7φ）与沉积构造特征（爬升迁移交错层理、线性沙丘），为其风成黄土沉积提供了实证，对上述假设提出了挑战。此外，努卡里纳组中发育的多套古土壤具有红色黏土层位、交代微晶化作用、膨胀裂缝与冰缘土丘，同时伴随偏低的碳、氧同位素值，偏高的锶同位素比值，以及反映淋滤作用与黏土形成的地球化学证据。该盖帽碳酸盐岩并无明确的海洋沉积特征，本次研究将其解读为覆于冰水-潮间带埃拉蒂纳组（Elatina Formation）之上的冰缘黄土，因此其并非雪球地球（snowball Earth）事件后全球快速变暖的沉积记录。全球其他地区的盖帽碳酸盐岩虽存在诸多差异，且可能形成于不同沉积环境，但均可通过本次提出的风成黄土沉积模型这一新视角进行重新审视与研究。