This project constructs POLENET a network of GPS and seismic stations in West Antarctica to understand how the mass of the West Antarctic ice sheet (WAIS) changes with time. The information is ultimately used to predict sea level rise accompanying global warming and interpret climate change records. The GPS (global positioning system) stations measure vertical and horizontal movements of bedrock, while the seismic stations characterize physical properties of the ice/rock interface, lithosphere, and mantle. Combined with satellite data, this project offers a more complete picture of the ice sheet's current state, its likely change in the near future, and its overall size during the last glacial maximum. This data will also be used to infer sub-ice sheet geology and the terrestrial heat flux, critical inputs to models of glacier movement. As well, this project improves tomographic models of the earth's deep interior and core through its location in the Earth's poorly instrumented southern hemisphere.
Broader impacts of this project are varied. The work is relevant to society for improving our understanding of the impacts of global warming on sea level rise. It also supports education at the postdoctoral, graduate, and undergraduate levels, and outreach to groups underrepresented in the sciences. As an International Polar Year contribution, this project establishes a legacy of infrastructure for polar measurements. It also involves an international collaboration of twenty four countries. For more information see IPY Project #185 at IPY.org. NSF is supporting a complementary Arctic POLENET array being constructed in Greenland under NSF Award #0632320.
POLENET-Antarctica is a pioneering scientific investigation of the large-scale structure and geological processes of West Antarctica. The project uses coupled networks of seismographs (which sense earthquakes and other sources of seismic waves) and GPS-based positioning systems (which show the slow vertical and horizontal motions of the Earth) to learn about how the western part of the Antarctic continent (by which we mean approximately the western hemisphere side of the continent-spanning Transantarctic Mountains) has evolved over geologic time and is evolving/deforming today. A key aspect of this study is also how these geologic processes have interacted in the past and are interacting today with the West Antarctic Ice Sheet (WAIS), which, although encompassing only about 10% of Antarctica’s ice contains nearly as much ice as Greenland, or about the equivalent of 5 m of global sea level rise). POLENET-Antarctica investigations were made technically possible by the development of a new generation of polar-capable instruments that can reliably record seismic and GPS data year-round, even during the extremely cold and dark Antarctic winter. NSF logistics made available through the U.S. Antarctic program make it feasible to install and recover data from these instruments during the brief southern summer field season. Seismic imaging of the Earth’s seismic velocity structure (which tells about its temperature and composition) to depths of many hundreds of km in this project has shown in detail just how different the geology of West Antarctica is compared to East Antarctica. West Antarctica (the lower-elevation parts of the continent has a thin crust and hot mantle that in some ways resembles the tectonically active western United States (where much of the landscape is slowly puling apart by lateral extension), while the deep Earth in East Antarctica is much colder, geologically stable, and rigid. A result of this is that we can now use seismic structure determined from seismic measurements to estimate the viscosity of the mantle with much greater certainty across West Antarctica. This is particularly important to understand the interplay between the WAIS and the underlying Earth today and in the past, and to interpreting the signals that we have recovered from the GPS receivers. Basically, low-viscosity mantle, as we now know exists throughout West Antarctica, means that the mantle (which flows like a thick fluid over many thousands of years) responds quickly to changes in the amount and weight of ice above it, probably with characteristic times of just a few thousand years. This means that the amount of ongoing rebound from ice before a few thousand years in the region is probably not a strong determiner of what is happening today. The GPS receiver data now shows that there is a very rapid uplift occurring in areas where major glaciers (such as the massive Pine Island Glacier) drain ancient ice from the WAIS into the Amundusen sea (and contribute to raising Earth’s global sea level). This means that ice mass is decreasing rapidly today in the region (an observation also corroborated by satellite mass estimates and other methods). These GPS data, combined with the viscosity estimates of the mantle inferred from seismology will strongly improve estimates of when and where the mass losses are occurring, and to the amount of ice loss and attendant sea level change. Because this is the first time that we have instrumented many of these regions, we are also making new discoveries on how large glaciers slip (we have made the discovery that seismic signals from large earthquakes thousands of kilometers away can trigger small ice quakes in the ice sheet), on the activity of volcanoes in the region (we discovered the first evidence of magma moving deep within the crust below the Marie Byrd Land volcanic province), and on the geological structure of the region (we can see in the crustal thickness maps determined with seismology just how uniformly the extension of the continent has distributed itself over the past 60 million years). We expect further discoveries from this unique data, which has now been archived so that future generations of researchers and their students can analyze it, no doubt in ways that have not been though of yet.
