The Svalbard exemplar of Neoproterozoic glaciation

虽然生命成功地缓和了地球的表面条件，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20亿年中，最深刻和最持久的挑战是近全球规模的冰川作用（泛冰川作用），最有记录的事件是大约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，冰雪是如此广泛，以至于地球变得更加反射太阳辐射，并冷却到平均温度约-50摄氏度。冰川作用最终被火山释放的二氧化碳所终结，这些二氧化碳并没有被岩石的风化所消耗，因为岩石被埋在广阔的冰雪覆盖层之下。虽然极端寒冷和冰川终止的方式受到了挑战，但人们普遍认为冰川到达了海平面的热带纬度。新的数据将大大限制未来的建模工作。我们最近取得了突破性进展，在斯瓦尔巴特群岛的威尔逊布林组岩石中的盐水冰川湖中产生了一套保存异常完好的碳酸盐沉淀物的新化学数据，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”。首先，我们发现，就氧同位素而言，这些碳酸盐是迄今发现的蒸发性最强的，因此必须在极端干旱的环境中形成。第二，我们利用关于同位素17-O丰度的意义的新发现，与我们对硫同位素比率的测量相联系，以表明大气与年轻冰川时期存在的大气有着深刻的不同：最简单的解释是大气中二氧化碳含量非常高。这意味着风化作用确实受到大面积冰盖的抑制。这项研究和以前的各种研究已经证明了这些偏远地区的岩石暴露对于理解地球历史上这一非凡事件的重要性-事实上，它们是我们可以找到化学沉积记录的唯一地方，使我们能够了解地球表面和大气中的条件。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，并收集一套样本，使我们能够更清楚地了解保存的证据。我们将利用磁性来重建冰川沉积物的古纬度，并试图通过放射性方法直接确定年龄，看看它是否与“马里诺”一致。我们最喜欢的威尔逊布林形成盐水冰川湖的现代模拟发现在南极洲的非常寒冷的麦克默多干燥山谷。我们将使用沉积物的物理性质来测试这一想法，而化学性质将用于限制有多少水在大气中循环，大气的氧化程度以及二氧化碳是否已经在第一个冰川湖形成时在大气中积累。我们的工作还将扩展到斯瓦尔巴群岛两个冰川单元之上和之下发现的明显的温暖和寒冷气候海洋沉积物，以了解更广泛的背景。我们的工作包括许多新方法，以及在新领域应用久经考验的现代测年和磁性分析方法。我们希望在地球历史上最极端的寒冷事件之一期间，对陆地表面的性质有一个清晰和生动的了解。我们还将发现这个位置是否是世界上正式在Cryogenian地质时期的基础上放置“金钉”的最佳地点。这些信息将以新的方式传播和存档。

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.