Deep internal layers

Current projects:

Unraveling the history of the Greenland Ice Sheet from its internal radiostratigraphy, with Knut Christianson and collaborator Joe MacGregor, funded by NSF

Internal structure of glaciers and ice sheets revealed by ice-penetrating radars has transformed our ability to understand ice evolution, and internal reflections imaged across large portions of the Greenland Ice Sheet (GrIS) are an excellent spatially resolved record of annual to multi-millennial change in Greenland. Access to this radiostratigraphy and age structure for the GrIS motivates and facilitates the unraveling of histories of surface accumulation, basal melt, and ice flow from these data. Given the recent proliferation of radar data, traced internal reflections, and increasingly sophisticated ice-flow and climate models, our understanding of the dynamics of the GrIS from these data is poised to grow significantly. This project will use the wealth of available airborne and ground-based radar data to constrain past climate and ice-sheet evolution, which is critical information for both paleoclimatic and predictive studies. Due to the inherent non-uniqueness of inferring multiple spatiotemporally variable parameters from a dated radiostratigraphy, reliable assumptions and constraints are required to construct tractable problems. Instead of attempting to solve a single inverse problem to infer many or all unknown parameters, this project will isolate the key unknowns while prescribing other values. The major objectives of this project include: 1) validating inferences from the recently available ice-sheet-wide dated radiostratigraphy of the GrIS against those from internal reflections mapped by targeted ground-based and airborne data collected along flowlines; 2) defining and applying strategies to use GrIS radiostratigraphy to constrain its history and to evaluate hypotheses regarding its evolution; 3) determining reconstructions of Greenland’s past climate and ice flow that match the radiostratigraphy; and 4) providing a foundation for future continental-scale ice-flow modeling that will seek to assimilate an extensive radiostratigraphy. In meeting these objectives we will evaluate specific hypothesis regarding accumulation-rate patterns across the GrIS, basal melt and ice-flow history near the Northeast Greenland Ice Stream (NEGIS) onset, divide-position history in central Greenland, and the ice-thickness history along the ice divide.


Past projects:

Using IceBridge data to relate the response of interior ice to past and future changes in flow of ice streams and fast outlet glaciers, with PI Howard Conway and Ed Waddington, funded by NASA

Changes in activity of fast-flowing outlet glaciers and ice streams in Antarctica and in Greenland exert strong control on the mass balance of the ice sheets through discharge of inland ice to the ocean. Furthermore, recent rapid changes in discharge of these fast-flowing outlets suggest that dynamical responses to warming play a much larger role in the future mass balance of ice sheets than previously considered. Observations show strong coupling between coastal regions and inland ice, which opens the possibility that continued ice discharge from the interior of the ice sheets to the ocean could cause ongoing and possibly rapid rise in sea level. We use data and models to study the response of the interior of ice sheets to past changes at their margins. This is a necessary step toward understanding how ice sheets will respond to future possible forcing; understanding the past is also needed to give context to present-day behavior.

Accumulation rate, ice flow, and divide-position history of Central West Antarctica with Ed Waddington, Tom Neumann, Howard Conway, and TJ Fudge, funded by NSF

In my research I also employ inverse methods. Ice-flow models are powerful tools, but they require estimates of initial conditions and boundary conditions that are often largely unknown. In an inverse problem the data have resulted from a known process that depends on some unknown parameter values or boundary conditions that we want to find. In particular, radar-observed internal layers in glaciers and ice sheets are well-suited data for inverse problems. We use radar layers, accumulation-rate estimates, ice-surface velocities, and modern ice thickness to infer histories of accumulation rate, ice thickness, and ice-divide position. Using our ice-flow model we assess how the signal we want to recover is represented in realizations of the available data. Using the available data we solve a suite of inverse problesm to bracket the most likely history of the West Antarctic Ice Sheet (WAIS) Divide. Understanding the history of the ice divide is necessary to interpret ice-core records and to estimate future behavior of the WAIS.