Intellectual merit: Evidence of episodes of extreme glaciation during the Neoproterozoic time (600-800 million years ago) lead to formulation of the Snowball Earth hypothesis in the early 1990's. A central but controversial idea of this hypothesis is that ice covered nearly the entire ocean for periods of up to several million years. Climate models differ greatly in their simulations of this time period suggesting everything from an ice free tropical ocean, to a "hard" snowball or fully frozen ocean to "slushy" scenario in between. It is known that climate models are highly sensitive to prescribed albedos although modelers have yet to settle on a preferred treatment in part because the full spectral albedos of the likely surfaces are uncertain. To provide clearer guidance in this realm, the Office of Polar Programs, Antarctic Integrated System Science Program has made this award to study the albedo of snow and ice surfaces that may have been pivotal to initiation and maintenance of an ice-covered ocean. Albedo may have also been key factor in determining sea ice thickness, thus light transmission to the water below and so governing possible refugia for the more delicate eukaryotic life forms that are known to have survived this time period. It is argued that today only Antarctica provides sufficiently analogous ice surfaces to the specialized ones that are thought to have occurred under a Snowball Earth scenario. A combination of field observations of cold (meaning less than -23C) snow-free sea ice, salt encrusted sea-ice surfaces and blue (subject to net ablation) glacial ice, laboratory experiments (various salt encrusted ice surfaces) and modeling (of radiative transfer and conditions required for potential refugia under thin, light transmitting ice in marginal seas such as the Mediterranean) will be carried out to test the viability of the Snowball Earth hypothesis. In collaboration with climate modelers, the field and laboratory results will be applied to better constraining global scale models of Snowball Earth.
Broader impacts: This basic research program is aimed at understanding past extreme climate conditions on Earth. Such conditions have implications for biological evolution; while prokaryotes could readily survive the challenge, it is not clear how the more susceptible eukaryotes did. The measurements should also provide information for sea-ice and glacial-ice in the modern environment that may be relevant to projecting future climate. They might also provide insights for the understanding of ice and salts surfaces on the moons of Jupiter and Saturn. The project will involve an undergraduate in laboratory observations and a graduate student whose PhD thesis will derive from this study. This research program integrates the expertise of an atmospheric scientist, sea-ice scientist and glaciologist. One of the coPIs is a member of an underrepresented group (female) in sea ice physics. The sea-ice component is collaborative with a New Zealand International Polar Year project focused on seasonal evolution of sea-ice properties.
The climatic changes of the Neoproterozoic 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. The goals of this project were to study processes that would have been important on the ice-covered ocean during such an event. The study focused on the albedos (reflectivity for sunlight) of snow and ice surfaces, which, because of their amplifying feedback, are crucial to the initiation, maintenance, and termination of a snowball event, as well as determining the ice thickness on the ocean. Some kinds of ice that are rare on the modern Earth may have been pivotal in allowing the tropical ocean to glaciate. The results of this research will aid in evaluation of the snowball-earth hypothesis, and help identify possible liquid-water refugia for photosynthetic surface life that apparently survived these events. The refugia may have been small and isolated, thereby causing extinctions, or promoting evolution of diversity, or both. Alternatively, the refugia may have been widespread and connected. To evaluate these alternatives, the oceanic surfaces that would have existed at various times and places during a snowball event were investigated, with fieldwork in Antarctica, laboratory work in a coldroom, and modeling of radiative transfer, heat transfer, and ice flow. In tropical regions where evaporation (sublimation) exceeded precipitation, ice surfaces may have included (a) bare sea ice, cold enough that salts precipitated, (b) sea ice with a salt crust left on the surface as the ice sublimated, and (c) cold glacier ice exposed by sublimation of "sea-glaciers" (self-sustaining ice shelves) flowing from polar seas into the dry tropics. Two of these three now exist in Antarctica: bare cold sea ice near the coast of Antarctica in early spring, and "blue ice" areas of the Transantarctic Mountains that have not experienced melting. Their albedos were investigated, as well as microphysical properties such as bubble content to explain the albedos. The salt crust does not exist in nature on the modern Earth, so it was investigated in the freezer laboratory. Laboratory work also investigated the migration of salt in sea ice and the migration of air bubbles in glacier ice. In both cases the migration rates were found to be too slow to remove light-scattering inclusions from the surface of the ice, so that the albedo would be expected to remain high, preserving the cold climate of Snowball Earth. One proposed refuge for photosynthesis 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 was used to determine the dimensions of the inlet necessary to allow the sublimation/evaporation rate to exceed the sea-glacier inflow rate. The conclusion is that realistic geometries can prevent sea-glacier invasion, so that a refuge is possible if the climate of the surrounding desert is warm enough. At the equator of Snowball Earth, climate models predict thick ice, or thin ice, or open water, depending largely on their albedo parameterizations; .The albedos of ice types in the Transantarctic Mountains representative of tropical sea glaciers appear to be within the range that favors ice hundreds of meters thick in the snowball tropics. Broader impacts: The results of this research will be used to constrain climate models of Snowball Earth, thus aiding in understanding the dramatic climate changes of the Neoproterozoic glaciations. It will have implications for biological evolution through this climatic bottleneck, and for interpretation of Neoproterozoic glacial deposits. The work has also illuminated processes in sea ice and glacier ice that occur on the modern Earth, and will aid in understanding surface-albedo feedbacks in the present climate. It may also aid in understanding the behavior of ice and salts on the surfaces of satellites of Jupiter and Saturn. The project has involved two undergraduates, two graduate students, and one postdoctoral fellow.
