Abstract

The Cryogenian Period (717–635 Ma), or ‘Snowball Earth’, was an unusually cool period of Earth history when glaciers extended to low latitudes. Past ideas on causes of this widespread glaciation include increased consumption of atmospheric carbon dioxide by silicate weathering due to continental drift into tropical paleolatitudes, or by voluminous, easily weathered volcanic tuffs. Alternatively, carbon sequestration from the atmosphere may have been intensified by advances in biomass on land or at sea. These hypotheses are tested with a new study of red siltstones of the Johnnys Creek Formation (785–717 Ma) in central Australia, where paleosols have long been recognised. Although these dolomitic red siltstones look like shales, they lack lamination. Instead, they have the massive bedding and grainsize distribution of dolomitic and calcareous loess, which precede tillites of the Areyonga Formation. Paleomagnetic studies indicate little drift from a paleolatitude of 26.2° during accumulation of the Johnnys Creek Formation. Nor does the Johnnys Creek Formation contain easily weathered volcanic ash, only local basalt flows. Paleoproductivity of the paleosols increases up section, as estimated in ppm soil CO₂ from depth in paleosols to gypsic (By) and then calcic (Bk) horizons. Deepening and intensification of soil respiration reflects greater terrestrial carbon sequestration, and increased chemical weathering up section, and both would have drawn down atmospheric CO₂. Comparable transition from gypsic to calcic soils in modern deserts reflects change from cyanobacterial-gypsic to fungal–algal calcic ecosystems. Snowball Earth glaciation may have been induced by evolutionary advances to eukaryotic and multicellular life on land, in the same way as Ordovician glaciation was induced by land plants, Permo-Carboniferous glaciation by trees, and Pleistocene glaciation by grasslands.

1. KEY POINTS

2. Johnnys Creek Formation of central Australia has paleosols dated 785–717 Ma.

3. Little drift from paleolatitude 26.2°, and no volcanic ash, only local basalt flows.

4. Paleosol paleoproductivity and depth of weathering increased steadily up section.

5. Glaciation may have been induced by eukaryotic and multicellular life on land.

摘要

低温纪（717-635 Ma），或“雪球地球”，是地球历史上一个异常凉爽的时期，当时冰川延伸到低纬度地区。过去关于这种广泛冰川作用的原因的想法包括由于大陆漂移到热带古纬度而导致的硅酸盐风化或大量易风化的火山凝灰岩导致大气中二氧化碳的消耗增加。或者，陆地或海上生物量的进步可能已经加强了大气中的碳封存。对澳大利亚中部 Johnnys Creek 组（785-717 Ma）红色粉砂岩的一项新研究验证了这些假设，那里的古土壤早已被识别。虽然这些白云质红色粉砂岩看起来像页岩，但它们没有分层。相反，它们具有大量的白云质和钙质黄土的层理和粒度分布，它先于 Areyonga 组的分蘖。古地磁研究表明，在 Johnnys Creek 地层堆积期间，古纬度 26.2° 的漂移很小。Johnnys Creek Formation 也不包含容易风化的火山灰，只有当地的玄武岩流。古土壤的古生产力在剖面上增加，以 ppm 土壤 CO 估计₂从古土壤深度到吉普赛 (By) 和钙质 (Bk) 层。土壤呼吸的加深和强化反映了更大的陆地碳固存和化学风化上升部分的增加，两者都会吸收大气中的CO ₂。现代沙漠中从石膏土壤到钙质土壤的类似转变反映了从蓝藻-石膏质到真菌-藻类钙质生态系统的变化。雪球地球冰川作用可能是由陆地上真核生物和多细胞生命的进化进展引起的，就像奥陶纪冰川作用是由陆地植物引起的，二叠纪-石炭纪冰川作用是由树木引起的，而更新世冰川作用是由草原引起的。

1. 关键点

2. 澳大利亚中部的 Johnnys Creek 组具有年代为 785-717 Ma 的古土壤。

3. 古纬度26.2°的漂移很小，没有火山灰，只有局部玄武岩流。

4. 上段古土壤古生产力和风化深度稳步增加。

5. 冰川作用可能是由陆地上的真核生物和多细胞生命引起的。