Firn densification
What is firn densification?
Snow that accumulates and persists for more than one year becomes firn, which can eventually densifies into glacial ice. Think about a snowball you might make - fresh snow will fall apart before it hits your target, so you have to compress the snow with your hands to compact it into something that sticks together.
Over time, a glacier does this to itself. As more snow accumulates, the weight of this snow presses down onto the old firn beneath it. The rate at which snow turns into glacial ice is called the densification rate, and we care about it for a few reasons.
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︎︎︎ Paleoclimate records
︎︎︎ Satellite-measured alitmetry
How do we measure firn densification?
Firn densification can be measured with a variety of tools: strain gauges embedded into boreholes, gamma ray transmission, repeat measurements of optical stratigraphy, and phase-sensitive radars2, among others.
My early Ph.D. work looked at the use of autonomous phse-sensitive radio echo sounders (ApRES) to measure firn densification. Using data collected over the Korff Ice Rise, Fletcher Promontory, and Skytrain Ice Rise, we removed the strain rate contribution of horizontal ice flow to isolate the vertical strain rate due to firn compaction.
![Core observations, modeled velocity and densities, and Method 1 compaction velocities. Left panels are from FP, the right panels are from SIR, top panels are compaction velocities and bottom panels are densities. In the top panels (a, b), blue squares are compaction velocities derived from pRES measurements using Method 1. Uncertainties are derived by taking into account the signal-to-noise ratio of each englacial reflector used to computed vertical velocities (Section 2.2) and are represented by the width of the bar symbols. Gray circles in the bottom panels are ice core-measured densities. The dashed and dotted curves show the model output (Section 2.5), where the dashed blue lines are generated from a model tuned to pRES vertical velocities, and the dotted black lines are generated from a model tuned to core densities. The gray horizontal dashed and dot-dashed lines show where core-derived densities are 550 and 830 kg m−3, respectively, corresponding to the compaction transition density identified by Herron and Langway (Reference Herron and Langway1980) and the lock-in density.]()
![Spatial variability of total velocities and compaction velocities at SIR and KIR obtained using Method 2. Method 2 can generate compaction velocity profiles at locations where we lack coincident density profiles across two transects, a northeastern–southwestern transect on KIR (black-to-blue gradient, Fig. 1b) and a north-to-south transect on SIR (black-to-blue gradient, Fig. 1c). Panels a and b show the total vertical velocity measured by each of the pRES points along KIR and SIR, respectively. Panels c and d show the firn compaction velocity. Note in panel a at KIR, the eastern flank (light blue) generally flows faster than the western flank, but that this trend is not evident in the compaction velocity estimates (c). The triangle to the right of the colorbar indicates the orientation of the transects in Figures 1b, c.]()
Snow that accumulates and persists for more than one year becomes firn, which can eventually densifies into glacial ice. Think about a snowball you might make - fresh snow will fall apart before it hits your target, so you have to compress the snow with your hands to compact it into something that sticks together.
Over time, a glacier does this to itself. As more snow accumulates, the weight of this snow presses down onto the old firn beneath it. The rate at which snow turns into glacial ice is called the densification rate, and we care about it for a few reasons.
Firn is a porous material, meaning air can travel through connected pores between ice grains. Eventually, at depth, these pores close and the air becomes trapped in bubbles. Ice core scientists use the air in these bubbles to measure what the climate was like in the past. We need to know how long it took firn to densify to know how long that air mixed with the atmosphere.
︎︎︎ Satellite-measured alitmetry
Satellites measure the change in height of the ice sheet over time. This is one way we know that some glaciers in Antarctica are thinning and losing mass. However, this change in height of the ice surface depends on many different processes. We have to account for all these in order to isolate the change due to ice thinning, and firn densification is one of them. If densification were in steady state and the same rate everywhere, we would know how to account for it. However, its dependence on accumulation rate, temperature, and grain size1, as well as wind, impurities, and water content, mean that densification rates vary in space and time.
How do we measure firn densification?
Firn densification can be measured with a variety of tools: strain gauges embedded into boreholes, gamma ray transmission, repeat measurements of optical stratigraphy, and phase-sensitive radars2, among others.
My early Ph.D. work looked at the use of autonomous phse-sensitive radio echo sounders (ApRES) to measure firn densification. Using data collected over the Korff Ice Rise, Fletcher Promontory, and Skytrain Ice Rise, we removed the strain rate contribution of horizontal ice flow to isolate the vertical strain rate due to firn compaction.
1 Kingslake J, Skarbek R, Case E, and McCarthy C (2022) “Grain-Size Evolution Controls the Accumulation Dependence of Modelled Firn Thickness.” The Cryosphere 16, no. 9, 3413–30. https://doi.org/10.5194/tc-16-3413-2022.
2 Case E and Kingslake J (2022) “Phase-Sensitive Radar as a Tool for Measuring Firn Compaction.” Journal of Glaciology 68, no. 267, 139–52. https://doi.org/10.1017/jog.2021.83.