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Few ice cores from the Amundsen Sea and Bellingshausen Sea sectors of the West Antarctic Ice Sheet (WAIS) extend back in time further than a few hundred years. The WAIS is believed to be susceptible to collapse as a result of anthropogenic climate change and may have at least partially collapsed during the Last Interglacial (LIG) period. Understanding the stability of the WAIS during warm periods such as the LIG and Holocene is important. As part of the WACSWAIN (WArm Climate Stability of the West Antarctic ice sheet in the last INterglacial) project, the British Antarctic Survey's (BAS) Rapid Access Isotope Drill (RAID) was deployed in 2020 on Sherman Island in the Abbot Ice Shelf, West Antarctica. We drilled a 323 m deep borehole, with discrete samples of ice chippings collected that cover the entire depth range of the drilled ice. The samples were analysed for stable water isotope composition and major ion content at BAS from 2020–2022. Using annual layer counting of chemical records, volcanic horizon identification and ice modelling, an age scale for the record of 1724 discrete samples is presented. The Sherman Island ice record extends back to greater than 1240 years, providing the oldest, continuous ice-derived palaeoclimate records in the coastal Amundsen and Bellingshausen Sea sectors to date. We demonstrate the potential for recovery of a complete Holocene climate record from Sherman Island in the future and confidence in the ability of RAID samples to contain sufficiently resolved records for meaningful climatic interpretation. [ABSTRACT FROM AUTHOR]

During the concluding phase of the NASA Operation IceBridge (OIB), we successfully completed two airborne measurement campaigns (in 2018 and 2021, respectively) using a compact S and C band radar installed on a Single Otter aircraft and collected data over Alaskan mountains, ice fields, and glaciers. This paper reports seasonal snow depths derived from radar data. We found large variations in seasonal radar-inferred depths with multi-modal distributions assuming a constant relative permittivity for snow equal to 1.89. About 34 % of the snow depths observed in 2018 were between 3.2 and 4.2 m, and close to 30 % of the snow depths observed in 2021 were between 2.5 and 3.5 m. We observed snow strata in ice facies, combined percolation and wet-snow facies, and dry-snow facies from radar data and identified the transition areas from wet-snow facies to ice facies for multiple glaciers based on the snow strata and radar backscattering characteristics. Our analysis focuses on the measured strata of multiple years at the caldera of Mount Wrangell (K'elt'aeni) to estimate the local snow accumulation rate. We developed a method for using our radar readings of multi-year strata to constrain the uncertain parameters of interpretation models with the assumption that most of the snow layers detected by the radar at the caldera are annual accumulation layers. At a 2004 ice core and 2005 temperature sensor tower site, the locally estimated average snow accumulation rate is ∼2.89 m w.e. a -1 between the years 2003 and 2021. Our estimate of the snow accumulation rate between 2005 and 2006 is 2.82 m w.e. a -1 , which matches closely to the 2.75 m w.e. a -1 inferred from independent ground-truth measurements made the same year. The snow accumulation rate between the years 2003 and 2021 also showed a linear increasing trend of 0.011 m w.e. a -2. This trend is corroborated by comparisons with the surface mass balance (SMB) derived for the same period from the regional atmospheric climate model MAR (Modèle Atmosphérique Régional). According to MAR data, which show an increase of 0.86 ∘ C in this area for the period of 2003–2021, the linear upward trend is associated with the increase in snowfall and rainfall events, which may be attributed to elevated global temperatures. The findings of this study confirmed the viability of our methodology, as well as its underlying assumptions and interpretation models. [ABSTRACT FROM AUTHOR]

Ice crystal orientation fabric (COF) records information about past ice-sheet deformation and influences the present-day flow of ice. Polarimetric radar sounding provides a means to infer anisotropic COF patterns due to the associated birefringence of polar ice. Here, we develop a polarimetric coherence (phase-based) method to determine horizontal properties of the COF. The method utilizes the azimuth and depth dependence of the vertical gradient of the hhvv coherence phase to infer the dielectric principal axes and birefringence, which are then related to the second-order fabric orientation tensor. Specifically, under the assumption that one of the orientational eigenvectors is vertical, we can determine the horizontal eigenvectors and the difference between the horizontal eigenvalues (a measure of horizontal fabric asymmetry). The method exploits single-polarized data acquired with varying antenna orientation. It applies to ground-based “multi-polarization” surveys and is demonstrated using data acquired by Center for Remote Sensing of Ice Sheets (CReSIS) using Multi-Channel Coherent Radar Depth Sounder (MCRDS) from the North Greenland Eemian Ice Drilling (NEEM) ice core region in Greenland. The analysis is validated using a combination of polarimetric matrix backscatter simulations and comparison with COF data from the NEEM ice core. The results are consistent with a conventional model of ice deformation at an ice divide where a lateral tension component is present, with minor horizontal COF asymmetry and the greatest horizontal concentration of crystallographic axes orientated near parallel to the ice divide. [ABSTRACT FROM AUTHOR]

The orientation distribution of ice crystals – the ice fabric – is an important component of ice rheology. The anisotropy of present-day ice fabric provides a record of past deformation, and, in turn, influences the viscosity of ice during future deformation. Our understanding of ice fabric within the polar ice sheets is primarily informed by measurements from ice cores which tend to be located at ice divides. Therefore, remote sensing is necessary to place observational constraints upon spatial variation in ice fabric and to assess its expression and influence across different flow regimes. Polarimetric radar sounding provides a means to measure ice fabric anisotropy due to the associated dielectric anisotropy (birefringence) of polar ice. Past radar sounding analysis methods have focused upon polarimetric power to infer fabric properties. However, this approach is complicated by the combined effect of birefringent propagation and anisotropic scattering upon polarimetric power and can suffer from angular ambiguity. Here we describe a new phase-based 'polarimetric-coherence' method to determine ice fabric from a 'simple' ground-based radar sounding survey (single-polarized data as a function of azimuth). The coherence method provides a direct way to measure the polarimetric phase shift associated with birefringent propagation and from this we can determine horizontal fabric properties: specifically, the prevailing crystallographic axis and the asymmetry/strength of the fabric. We describe the application of the coherence method to two different radar systems in two different ice flow regimes. First, we consider data acquired using MCRDs (Multi Channel Coherent Radar Depth Sounder) from the NEEM ice core region in northwest Greenland. Second, we consider data acquired using ApRES (Autonomous phase-sensitive Radio-Echo Sounder) from the Whillans Ice Stream, West Antarctica. In both case studies, we demonstrate that the radar fabric measurements provide new insights about past ice deformation and the stability/instability of ice flow patterns. Additionally, we formulate the analysis method in terms of a second order orientation tensor which enables us to directly compare and cross-validate with ice core fabric data at NEEM. [ABSTRACT FROM AUTHOR]