The Svalbard exemplar of Neoproterozoic glaciation

虽然生命成功地缓和了地球表面的环境，但地球历史上的一些事件已经威胁到大多数生命形式的生存能力。可以说，在过去20亿年里，最深刻和最持久的挑战是近全球范围的冰川作用（泛冰川作用），有记录的最佳事件发生在6.5亿至6.3亿年前（“马里诺”冰川作用）。一个覆盖模型（雪球地球假说）提出，冰雪分布如此广泛，以至于地球对太阳辐射的反射能力大大增强，平均温度降至零下50摄氏度左右。由于岩石被埋在广阔的冰雪覆盖之下，火山喷发的二氧化碳积聚并没有被岩石的风化作用所消耗，冰川作用最终终止了。尽管人们对寒冷的极端程度和冰川终止的方式提出了质疑，但人们普遍认为，在海平面上，冰川作用达到了热带纬度。新的数据将极大地限制未来的建模工作。我们最近取得了一项突破，生成了一套新的化学数据，这些数据来自斯瓦尔巴群岛威尔逊布林组（Wilsonbreen Formation）岩石中的含盐冰川湖中保存异常完好的碳酸盐沉淀物，这些沉淀物被认为与所有大陆上发现的冰川沉积物年龄相同，被称为“马里诺安”（Marinoan）。首先，我们发现，就氧同位素而言，这些碳酸盐是迄今为止发现的最具蒸发性的，因此一定是在极度干旱的环境中形成的。其次，我们利用同位素17-O丰度的新发现，将其与硫同位素比值的测量结果联系起来，以表明大气与年轻冰期时期存在的大气有很大的不同：对此最简单的解释是大气中二氧化碳含量很高。这意味着风化作用确实被广泛的冰层所抑制。这项研究和之前的各种研究已经证明了这些偏远地区的岩石暴露对理解地球历史上这一非凡事件的重要性——事实上，它们是我们唯一能找到化学沉积记录的地方，这使我们能够了解地球表面和大气的状况。我们建议在这个偏远的野外地区进行两个夏季的新研究，使我们能够充分描述和存档野外关系，并收集样品套件，使我们能够更清楚地了解保存的证据。我们将使用磁性来重建冰川沉积物的古纬度，并将尝试通过辐射测量法直接确定年龄，看看它是否与“马里诺安”相一致。我们最喜欢的威尔逊布林构造的现代类似物是在南极洲极度寒冷的麦克默多干谷中发现的含盐冰川湖。我们将利用沉积物的物理特性来测试这个想法，同时利用化学特性来限制大气中有多少水在循环，大气的氧化程度如何，以及在第一个冰川湖形成时大气中是否已经积累了二氧化碳。我们的工作还将扩展到在斯瓦尔巴群岛两个冰川单元上下发现的明显温暖和寒冷气候的海洋沉积物，以了解更广泛的背景。我们的工作包括许多新的方法，以及在一个新的领域应用久经考验的现代测年和磁分析方法。我们期望在地球历史上最极端的寒冷事件之一期间，清晰而生动地描绘出陆地表面的性质。我们还将了解这个地方是否可能是世界上最好的地方，在冰原地质时期的底部正式放置一个“金钉”。这些信息将以新颖的方式传播和存档。

Although life successfully moderates surface conditions on Earth, some events in Earth History have threatened the viability of most life forms. Arguably the most profound and long-lasting challenge in the last 2 billion years was glaciation on a near-global scale (pan-glaciation), with the best documented event being around 650 to 630 million years ago ('Marinoan' glaciation). One overaching model (Snowball Earth hypothesis) proposes that snow and ice was so widespread that the Earth become much more reflective of solar radiation and cooled to a mean temperature of around -50 degrees Celsius. Glaciation was eventually terminated by the build-up of carbon dioxide emitted from volcanoes, that was not used up by the weathering of rocks, since rocks were buried beneath the extensive snow and ice cover. Although the extremity of the cold and the way in which glaciation terminated have been challenged, there is widespread agreement that glaciation reached tropical latitudes at sea level. New data will significantly constrain future modelling efforts. We have recently made a breakthrough in generating a new suite of chemical data on exceptionally well-preserved carbonate precipitates in saline glacial lakes in the Wilsonbreen Formation rocks of Svalbard, thought to be the same age as glacial deposits found on all the continents and referred to as 'Marinoan'. Firstly we find that in terms of oxygen isotopes, these carbonates are the most evaporative yet discovered and so must have formed in a hyperarid environment. Secondly we use new discoveries about the meaning of the abundances of the isotope 17-O in relation to our measurements of sulphur isotope ratios in order to show that the atmosphere was profoundly different from that which existed during younger glaciations: the simplest explanation for it is that the atmosphere was very high in carbon dioxide. This implies that weathering was indeed inhibited by an extensive ice cover. This study and various previous studies have demonstrated the outstanding importance of the rock exposures in these remote locations to understanding this extraordinary event in Earth history - indeed they are the only place where we can find a chemical sedimentary record that allows us to understand conditions on the Earth surface and in the atmosphere. We propose to make new studies over two summer seasons in this remote field area to enable us to fully describe and archive the field relationships and collect suites of samples that will enable us to understand more clearly the preserved evidence. We will use magnetic properties to reconstruct the palaeolatitude of the glacial deposits and will try to determine the age directly by radiometric methods to see if it is consistent with the 'Marinoan'. Our favoured modern analogue for the Wilsonbreen formation saline glacial lakes are found in the intensely cold McMurdo Dry Valleys of Antarctica. We will test this idea using physical properties of the sediment whilst the chemical properties will be used to constrain how much water is cycled through the atmosphere, how oxidizing the atmosphere was and whether carbon dioxide had already built up in the atmosphere by the time the first glacial lakes formed. Our work will also extend to the apparently warm- and cold-climate marine deposits that are found above and below two glacial units in the Svalbard in order to understand the broader context. Our work includes a number of new approaches as well as applying tried-and-tested modern methods of dating and magnetic analysis in a new area. We expect to emerge with a clear and vivid picture of the nature of the land surface during one of the most extreme cold events in the history of the planet. We will also find out whether this location could be the best place in the world to formally place a 'golden spike' at the base of the Cryogenian geological period. The information will be disseminated and archived in novel ways.