When was Summit, Greenland last ice-free: 81Kr dating of dirty ice at the bottom of the GISP2 ice core

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Ice sheets melt as Planet Earth becomes warmer. This melting causes sea level to rise, constituting one of the most serious problems resulting from global warming. Complete melting of the Greenland Ice Sheet would raise the average level of the oceans by 7 meters, with significant consequences for cities and towns along the coasts of the world. Understanding how extensively the Greenland Ice Sheet melted in the past will help us predict how fast it will melt in the future. The investigators will determine the age of ice at the very bottom of the ice sheet. The measurements will be made on ice from a site called “GISP2”, in central Greenland, where a hole has been drilled through the 3053-meter thick ice sheet. The ice at the bottom of the hole is likely to be the oldest ice on Greenland and the dating of this bottom ice will tell us the last time that Greenland was not covered by a big ice sheet. By comparing the time when Greenland was ice-free with global climate events that occurred in the past, we hope to learn why the Greenland Ice Sheet melted in the past, and what human activities might cause it to melt in the future. This work will have 2 major benefits to society. First, it will lead to a better understanding of how human activities are likely to affect sea level in the future. Second, it will lead to the training of a postdoctoral fellow, who will collect most of the data, understand the results, and develop research skills that will complete her training as an outstanding independent scientist.The great planetary ice sheets may melt due to global warming, causing sea level to rise. Complete melting of the East Antarctic ice sheet would raise sea level by 53 m, melting of the West Antarctic Ice Sheet would lead to a 5 m rise, and melting of Greenland would cause a 7 m rise. The Greenland ice sheet is generally regarded as most susceptible to melting and therefore of greatest concern. Melting of the Greenland Ice Sheet would lead to a catastrophic rise in sea level, causing flooding of coastal cities as well as lowlands near the ocean. A premise of this proposal is that, if we determine when Greenland was last ice-free, we will have new information which will help us predict the rate of melting in the future, and, consequent rate of sea level rise. The investigators will determine the age of ice at the very bottom of the ice sheet. We will do this using a method called “krypton 81 dating”. Krypton 81 is a form of krypton that is radioactive and gradually changes into another element. The lower the krypton-81 concentration in air trapped in ancient ice, the older the ice. The measurements will be made on ice from a site called “GISP2”, in central Greenland, where a hole has been drilled through the 3053-meter thick ice sheet. The dating measurements will be made at the University of Science and Technology of China, in Hefei. This University has the one instrument in the world capable of measuring the abundance of 81Kr with the required accuracy. Having dated the oldest ice, we will examine the age in the context of known global climate events that occurred around the same time. For example, climate was particularly warm at several times in the past, including 400,000 years ago and 1,070,000 years ago. A date near one of those 2 times would suggest that the planetary warming was sufficient to melt the Greenland Ice Sheet, after which the planet cooled and Greenland was continuously glaciated. This information would challenge global climate models to account for global warming at the appropriate time and would clarify what is required for the melting and regrowth of the ice sheet. This new insight would, in turn, improve our ability to estimate the melting trajectory of the Greenland Ice Sheet on human-relevant timescales (say, thousands to hundreds of thousands of years). It would also improve our ability to understand past climate events and the role of Greenland melting. A Postdoctoral Fellow will be directly responsible for most aspects of the project, including preparing ice core samples for analysis, extracting dissolved gases, interpreting the results, preparing a paper reporting the data, and submitting the results to the NSF Arctic Data Center. The post doctoral scientist will be mentored by the PI and provide support for becoming an independent scientist playing an important role in moving the field forward.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.