Evolution of Microstructure in Ice Sheets
The microstructure of polycrystalline ice, such as that found in a glacier, is defined by its texture (size and shape of the crystals) and fabric (the overall orientation of the crystals). Ice crystals grow, rotate, and recrystallize with time and stress. The rate at which each of these processes occur depends on the temperature, the impurity content of the ice, and on the stress exerted by overburden and glacier flow.
Collaborative Research: VeLveT Ice - eVoLution of Fabric and Texture in Ice at WAIS Divide, West Antarctica
VeLveT Ice is a comprehensive study of the relationship between ice microstructure, impurities, and ice flow and their connection to climate history for the West Antarctic Ice Sheet (WAIS) Divide ice core site (79°28.058' S, 112°05.189' W). Many scientists have observed that the microstructure of ice evolves with depth and time in an ice sheet. This evolution of microstructure depends on the ice flow field, temperature, and impurity content. The flow field, in turn, depends on glacier microstructure, leading to feedbacks that produce layered variations in microstructure that are related to climate and flow history. Our objective is to understand how the evolution of microstructure with time and stress in the West Antarctic ice sheet is related to impurity content, temperature, and strain rate and how the spatial variability of microstructure and its effect on ice flow affects our interpretation of climate history in the WAIS Divide ice core. This project combines a detailed study of ice crystal orientation fabrics obtained through scanning electron microscope based Electron Backscatter Diffraction (EBSD) with measurements of borehole deformation made using logging instruments. We will incorporate and build on data collected by related WAIS Divide projects, including borehole sonic velocity measurements, borehole optical dust log measurements, borehole temperature, crystal size and shape, and ice core chemistry.
Erin Pettit (http://glaciers.gi.alaska.edu/people/pettit)
Dr. Erin Pettit is an Assistant Professor of Geophysics at the University of Alaska Fairbanks. Her research emphasis is in ice dynamics, and in this case the relation between ice crystal microstructure, climate history, and ice flow at the WAIS ice core site.
Rachel Obbard (http://www.polartrec.com/member/rachel-obbard)
Our goals for the December 2013-January 2014 field season were to measure the shape and tilt of the borehole at high resolution to form a basis for measuring future changes in borehole shape and tilt (in the 2015/2016 season). We used a logging instrument called an acoustic televiewer, which emits a 1.2MHzbeam towards the formation and records the amplitude and the travel time of the reflected signal. The amplitude represents impedance contrast between different layers of the ice sheet, and can be used to study stratigraphy. The travel time data can be used to derive borehole shape. Data processing produces 360° unwrapped maps and 3D images of the borehole wall and ice sheet layers, which we can compare to laboratory measurements of ice microstructure and the other ice core and borehole measurements from other WAIS projects. The team included Pettit, Obbard, UAF graduate student Tiffany Green, and PolarTREC teacher Yamini Bala.
We returned to WAIS Divide in December 2016-2017 to relog the borehole, so that we could analyze changes in borehole shape (both inclination and cross-sectional shape) caused by flow in the ice sheet. The project team included Dartmouth undergraduate Elena Bird ’18, graduate student Emilie Sinkler (UAF), and undergraduate Annie Sainvil (Smith).
Bird managed the outreach for the group, including our blog. We Skyped from Ob Hill in McMurdo with an AP Physics class at Monona Grove High School in Madison Wisconsin (teacher: Juan Botella) on 12/6/17.
At Dartmouth, we use electron backscatter diffraction (EBSD) to measure the orientation of ice crystals with respect to a reference coordinate system, in this case the borehole at WAIS. EBSD is done using a scanning electron microscope (SEM) and requires more time than optical methods for measuring orientation, but produces more complete orientation information, with both c-axis and a-axis directions, which are necessary to distinguish between certain types of recrystallization.
This research was supported by US National Science Foundation (NSF) grant NSF-1142035. The views and conclusions contained herein are those of the authors and should not be interpreted as representing official policies, either expressed or implied, of the NSF or the United States Government.