Radar attenuation demonstrates advective cooling in the Siple Coast ice streams

Abstract

Ice streams are warmed by shear strain, both vertical shear near the bed and lateral shear at the margins. Warm ice deforms more easily, establishing a positive feedback loop in an ice stream where fast flow leads to warm ice and then to even faster flow. Here, we use radar attenuation measurements to show that the Siple Coast ice streams are colder than previously thought, which we hypothesize is due to along-flow advection of cold ice from upstream. We interpret the attenuation results within the context of previous ice-temperature measurements from nearby sites where hot-water boreholes were drilled. These in-situ temperatures are notably colder than model predictions, both in the ice streams and in an ice-stream shear margin. We then model ice temperature using a 1.5-dimensional numerical model which includes a parameterization for along-flow advection. Compared to analytical solutions, we find depth-averaged temperatures that are colder by 0.7°C in the Bindschadler Ice Stream, 2.7°C in the Kamb Ice Stream and 6.2–8.2°C in the Dragon Shear Margin of Whillans Ice Stream, closer to the borehole measurements at all locations. Modelled cooling corresponds to shear-margin thermal strengthening by 3–3.5 times compared to the warm-ice case, which must be compensated by some other weakening mechanism such as material damage or ice-crystal fabric anisotropy.

Evidence for and Against Temperate Ice in Antarctic Shear Margins From Radar‐Depth Sounding Data

“…Advective cooling has been observed to impact the temperature profile of ice streams and shear margins (Hills et al., 2022), and could be particularly important for regions of rapid ice flow like the Amundsen Sea Embayment. The difference between cold ice and temperate ice represents up to a 20× change in ice effective viscosity, which strongly impacts the location and stability of a shear margin, as well as the speed of the ice stream it bounds (Hunter et al., 2021; Perol & Rice, 2015).…”

“…In this manuscript we refer to this case as the Meyer & Minchew model ( T MM ), with both a “full shear heating” case (Λ = 0) and an “intermediate shear heating” case $$\left({\Lambda }=\frac{1}{2}\mathtt{B}\mathtt{r}\right)$$ after Hills et al. (2022), where $$\mathtt{B}\mathtt{r}$$ is the Brinkman number representing the ratio of heat generation to heat diffusion. We assume an n = 3 power law for ice rheology, including a constant prefactor chosen as the prefactor for temperate ice (Cuffey & Paterson, 2010) Section 3.4.6.…”

“…Englacial attenuation is primarily dependent on ice chemistry and temperature. Ice chemistry is unlikely to vary significantly over short length scales, and attenuation is very sensitive to ice temperature particularly for temperatures near melting (Hills et al., 2022; MacGregor et al., 2015; Schroeder, Seroussi, et al., 2016). Thus, we associate temperature directly with attenuation, while controlling for ice thickness and geometric spreading.…”

“…Lateral advection can significantly reduce modeled temperate ice in shear margins with lateral flow rates as low as 1-10 m/yr (Suckale et al, 2014), making it a difficult parameter to constrain. In this manuscript we refer to this case as the Meyer & Minchew model (T MM ), with both a "full shear heating" case (Λ = 0) and an "intermediate shear heating" case Λ = 1 2 Br) after Hills et al (2022), where Br is the Brinkman number representing the ratio of heat generation to heat diffusion. We assume an n = 3 power law for ice rheology, including a constant prefactor chosen as the prefactor for temperate ice (Cuffey & Paterson, 2010) Section 3.4.6.…”

Paths forward in radioglaciology

“…If frozen patches like this (or others closer to the coast) thaw, they have the potential to reshape catchment boundaries and mass loss projections (Dawson and others, 2022). Radar observations of the ice-sheet thermal state can also constrain the stability of subglacial lake systems as well as poorly constrained ice-sheet boundary conditions like accumulation rate, melt, advection and geothermal flux (Wolovick and others, 2021;Zeising and Humbert, 2021;Hills andothers 2022a, 2022b).…”