Dynamic Observations of the Microstructural Evolution of Firn
There is broad interest in understanding firn compaction for a number of reasons, viz., in order to understand near-surface behavior of ice sheets for satellite observations to determine ice sheet mass balance; how the firn microstructure relates to ice sheet fabric; and, most importantly for better interpretation of paleoclimate from air that becomes trapped within the firn. Firn densification number of different mechanisms including pressure sintering, plastic deformation, grain rearrangement, and, near the surface, metamorphism due to the temperature gradient, which leads to vapor movement. There have been a number of attempts to model firn densification based on experimental observations of firn microstructure or density measurements. However, such post-mortem models assume the densification mechanisms. In this project, we will determine the mechanisms of firn densification and microstructural evolution as a function of depth using dynamic observations of the evolution of the firn using X-ray computed microtomography (μCT).
For the proposed work, we will drill an 80 m firn core (approx. pore-close-off depth), at Summit,
Greenland in June, 2017 and transport it to Dartmouth. We will perform two types of experiments. First, for selected firn depths close to the surface, where the firn is in a temperature gradient and is subject to diurnal temperature changes, we will perform in situ observations in the μCT of how the firn changes when subject to a temperature gradient. Second, for firn further down the firn column, which does not see a significant temperature gradient, but does see stresses from the overburden, we will perform in situ μCT observations of the microstructural evolution using a compression stage. In addition to observing the microstructure as a whole and determining a wide variety of microstructural parameters (% Object Volume; Specific Surface Area; object Surface area-to-Volume Ratio, Structure Model Index, Fragmentation Index, Mean Structure Thickness) that can be used to characterize the firn quantitatively, we can follow the evolution of single or small numbers of ice crystals to observe bond formation and bond-breaking under load in detail, as undertaken in some prior studies on snow (Chen and Baker, 2010). Both sets of observations will be performed over periods of days to weeks in the μCT, which is located in a cold room. After the μCT experiments, the firn will be sectioned and examined in a cold-stage-equipped scanning electron microscope (SEM). The latter will enable: 1) observations at much higher resolution than the μCT; 2) thresholding of the μCT reconstructions using the SEM images, as demonstrated by Lomonaco, Baker and Chen (2008); and the determination of both ice crystal orientations, using electron backscattered patterns, and the local microchemistry, using energy dispersive X-ray spectroscopy, as demonstrated by Baker et al. (2007) for firn. The ultimate aim is to elucidate the microstructural evolution of firn at the microscopic level using advanced imaging techniques so that ultimately this behavior can be accurately modeled and related to macroscopic phenomena.