Ocean Surfaces on Snowball Earth

根据雪球地球假说，前寒武纪晚期（600-8亿年前）的气候变化包括极端冰川作用，在此期间冰可能几乎覆盖了整个海洋几百万年。 这些事件将在地球进化史上占有重要地位。它们本可以通过将生命形式困在狭小的孤立的无冰区域来促进生物多样性，或者它们可能导致大规模灭绝，为新的生命形式填补空缺扫清道路。 是什么导致地球结冰，以及后来又导致冰融化？ 对这些问题的科学调查将有助于更好地了解地球可能经历的气候变化，并能够更好地预测未来的气候。 该项目涉及南极实地观测以及实验室研究和计算机建模。该项目的目的不是证明或反驳雪球地球假说，而是量化对于在气候模型中模拟雪球事件非常重要的过程。 主要目标是确定冰冻海洋上可能存在的冰的类型，并确定它们会反射回太空的阳光量。 明亮的雪和冰表面反射阳光是维持低纬度寒冷气候的原因。 海洋的融化需要温室气体的积累，但沙漠尘埃和火山灰的沉积可能有助于使冰雪变暗。 地球表面有如此多的冰，即使冰吸收或反射的光量存在微小差异，也可能导致气候发生重大变化。 冰的特性也将决定光合作用生命在何处以及在什么情况下能够生存。现代地球上罕见的某些冰可能是导致热带海洋结冰的关键。 海洋表面可能包括一些现在仅存在于南极洲的冰类型：带有沉淀盐的裸露冷海冰，以及因升华而暴露且未经历融化的横贯南极山脉的“蓝冰”区域。 已开展实地考察来检查这些冰类型，数据分析正在进行中。 第三种冰，即带有盐壳的海冰，正在冷冻实验室进行研究。 模型将展示阳光如何与含有吸光灰尘和火山灰的冰相互作用。除了反射阳光之外，雪球海洋上的冰层厚度足以在自身重量作用下流动，侵入海洋的所有部分。 然而，光合作用生命生存的证据表明，海洋表面仍保留着一些液态水区域。 光合生物可能的避难所之一是几乎封闭的热带海洋远端的海湾，该海湾由大陆裂谷形成并被沙漠包围，例如现代的红海。 正在开发冰川流模型，以确定连接海洋的通道的尺寸，这对于防止流冰入侵并维持水供应以补充蒸发是必要的。

The climatic changes of late Precambrian time, 600-800 million years ago, included episodes of extreme glaciation, during which ice may have covered nearly the entire ocean for several million years, according to the Snowball Earth hypothesis. These episodes would hold an important place in Earth?s evolutionary history; they could have encouraged biodiversity by trapping life forms in small isolated ice-free areas, or they could have caused massive extinctions that cleared the path for new life forms to fill empty niches. What caused the Earth to become iced over, and what later caused the ice to melt? Scientific investigation of these questions will result in greater understanding of the climatic changes that the Earth can experience, and will enable better predictions of future climate. This project involves Antarctic field observations as well as laboratory studies and computer modeling.The aim of this project is not to prove or disprove the Snowball Earth hypothesis but rather to quantify processes that are important for simulating snowball events in climate models. The principal goal is to identify the types of ice that would have been present on the frozen ocean, and to determine how much sunlight they would reflect back to space. Reflection of sunlight by bright surfaces of snow and ice is what would maintain the cold climate at low latitudes. The melting of the ocean required buildup of greenhouse gases, but it was probably aided by deposition of desert dust and volcanic ash darkening the snow and ice. With so much ice on the Earth?s surface, even small differences in the amount of light that the ice absorbed or reflected could cause significant changes in climate. The properties of the ice would also determine where, and in what circumstances, photosynthetic life could have survived. Some kinds of ice that are rare on the modern Earth may have been pivotal in allowing the tropical ocean to freeze. The ocean surfaces would have included some ice types that now exist only in Antarctica: bare cold sea ice with precipitated salts, and "blue ice" areas of the Transantarctic Mountains that were exposed by sublimation and have not experienced melting. Field expeditions were mounted to examine these ice types, and the data analysis is underway. A third ice type, sea ice with a salt crust, is being studied in a freezer laboratory. Modeling will show how sunlight would interact with ice containing light-absorbing dust and volcanic ash. Aside from its reflection of sunlight, ice on the Snowball ocean would have been thick enough to flow under its own weight, invading all parts of the ocean. Yet evidence for the survival of photosynthetic life indicates that some regions of liquid water were maintained at the ocean surface. One possible refuge for photosynthetic organisms is a bay at the far end of a nearly enclosed tropical sea, formed by continental rifting and surrounded by desert, such as the modern Red Sea. A model of glacier flow is being developed to determine the dimensions of the channel, connecting the sea to the ocean, necessary to prevent invasion by the flowing ice yet maintain a water supply to replenish evaporation.