The Biogeochemical Aftermath of Neoproterozoic Ice Ages

【摘要】新元古代冰期的生物地球化学后果这项研究旨在严格检验著名的雪球地球假说，该假说预测，在新元古代（5.43 -10亿年前），整个地球从两极到赤道被大陆冰川和海冰覆盖了好几次。为了克服这种极端条件，有人提出，在数百万年的时间里，大气中的二氧化碳浓度必须上升到目前库存的100多倍，最终积累了足够的温室气体，使这个冰冻的星球变暖。一方面，神秘的碳酸盐（覆盖着全世界新元古代冰川沉积物）所需要的物质可能来自于这样一个充满二氧化碳的大气下大陆的强烈风化作用。或者，这些物质的来源可能来自深海，那里硫酸盐还原细菌主导着水柱——类似于今天缺氧的黑海——在广泛的，但不那么严重和较短的冰川时期。要验证这些端元假说的有效性，需要对盖层碳酸盐的元素、矿物学、结构和稳定同位素组成进行详细的研究。将分析来自纳米比亚奥塔维群（全球冰川作用的关键证据最初在那里收集）的样本，以及来自巴西班布伊和澳门群的明显等效地层的样本。对巴西碳酸盐为主的厚地层进行区域测绘和高分辨率地层学研究，将有助于评估那些试图解释这些前寒武纪气候剧烈振荡的模型。

ABSTRACTThe Biogeochemical Aftermath of Neoproterozoic Ice AgesThe proposed research aims to critically test the celebrated Snowball Earth hypothesis, which predicts that several times in the Neoproterozoic (543-1000 million years ago), the entire planet was blanketed from the poles to equator with continental glaciers and sea ice. To overcome such extreme conditions it has been suggested that over millions of years atmospheric carbon dioxide concentration must have risen to over 100 times its present inventory, eventually building up enough greenhouse gas to warm the frozen planet. On one hand, the materials required for the enigmatic carbonates (which cap these Neoproterozoic glacial sediments worldwide) may have come from the intense weathering of the continents under such a CO2-charged atmosphere. Alternatively, the source of these materials may have derived from the deep ocean where sulfate-reducing bacteria dominated the water column - similar to the present-day anoxic Black Sea - during widespread, yet less severe and shorter glacial epochs. Testing the validity of either of these end-member hypotheses will require detailed study of the elemental, mineralogical, textural, and stable isotopic compositions of the cap carbonates. Samples from the Otavi Group in Namibia (where the critical evidence for global glaciation was first amassed), and in demonstrably equivalent strata from the Bambui and Macaubas groups of Brazil will be analyzed. Regional mapping and high-resolution stratigraphic studies of the thick, carbonate-dominated Brazilian successions will aid in the evaluation of models that seek to explain these severe oscillations of Precambrian climate.