Towards interpretation of the radio-stratigraphy of Antarctic ice shelves from modeling and observations: A case study for the Roi Baudouin Ice Shelf, East Antarctica

Ice shelves surrounding the Antarctic perimeter buttress ice flow from the continent towards the ocean, and their disintegration leads to an increase in ice discharge and sea level rise. The evolution and integrity of ice shelves is governed by surface accumulation, basal melting, and ice dynamics. We find history of these processes imprinted in the ice-shelf stratigraphy, which is mapped using isochrones imaged with radar. As an observational archive, the radar obtained stratigraphy combined with ice flow modeling has high potential to assist model calibration and reduce uncertainties in projections for the ice-sheet evolution. In this study we use a simplistic and observationally driven ice-dynamic forward model to predict the ice-shelf stratigraphy. We validate this approach with the full Stokes ice-flow model Elmer/Ice, and present a test-case for the Roi Baudouin Ice Shelf (East Antarctica) - where our model predictions agree well with radar obtained observations. The presented method enables us to investigate whether ice shelves are in steady-state, as well as to map spatial variations of how much of the ice-shelf volume is determined by its local surface mass balance. In the case of Roi Baudouin, we find the ice-shelf volume in the western part to be dominated by ice inflowing from the ice sheet, while the eastern part of the ice shelf is dominated by ice locally accumulated on the shelf. Such analysis serves as a metric for the susceptibility of ice shelves to climate change. We further apply our approach to other ice shelves in Antarctica

Predicting the steady-state isochronal stratigraphy of ice shelves using observations and modeling

Ice shelves surrounding the Antarctic perimeter moderate ice discharge towards the ocean through buttressing. Ice-shelf evolution and integrity depend on the local surface accumulation, basal melting and on the spatially variable ice-shelf viscosity. These components of ice-shelf mass balance are often poorly constrained by observations and introduce uncertainties in ice-sheet projections. Isochronal radar stratigraphy is an observational archive for the atmospheric, oceanographic and ice-flow history of ice shelves. Here, we predict the stratigraphy of locally accumulated ice on ice shelves with a kinematic forward model for a given atmospheric and oceanographic scenario. This delineates the boundary between local meteoric ice (LMI) and continental meteoric ice (CMI). A large LMI to CMI ratio hereby marks ice shelves whose buttressing strength is more sensitive to changes in atmospheric precipitation patterns. A mismatch between the steady-state predictions of the kinematic forward model and observations from radar can highlight inconsistencies in the atmospheric and oceanographic input data or be an indicator for a transient ice-shelf history not accounted for in the model. We discuss pitfalls in numerical diffusion when calculating the age field and validate the kinematic model with the full Stokes ice-flow model Elmer/Ice. The Roi Baudouin Ice Shelf (East Antarctica) serves as a test case for this approach. There, we find a significant east–west gradient in the LMI / CMI ratio. The steady-state predictions concur with observations on larger spatial scales (>10 km), but deviations on smaller scales are significant, e.g., because local surface accumulation patterns near the grounding zone are underestimated in Antarctic-wide estimates. Future studies can use these mismatches to optimize the input data or to pinpoint transient signatures in the ice-shelf history using the ever growing archive of radar observations of internal ice stratigraphy

Towards assembling the internal ice stratigraphy in coastal regions of Dronning Maud Land, East Antarctica

The internal ice stratigraphy as imaged by radar is an integrated archive of the atmospheric- oceanographic, and ice-dynamic history that the ice sheet has experienced. It provides an observational constraint for ice flow modeling that has been used for instance to predict age-depth relationships at prospective ice-coring sites in Antarctica’s interior. The stratigraphy is typically more disturbed and more difficult to image in coastal regions due to faster ice flow. Yet, knowledge of ice stratigraphy across ice shelf grounding lines and further seawards is important to help constrain ocean-induced melting and associated stability. Here, we present preliminary results of synthesizing information from radar stratigraphic characteristics from airborne and ground-based radar surveys that have been collected for specific projects starting from the 1990s onwards focusing on ice marginal zones of Antarctica. The key data is based on airborne surveys from the German Alfred Wegener Institute’s polar aircrafts equipped with a 150 MHz radar. In the meantime this system has been replaced by an ultra-wide band 150-520 MHz radar. The older data will provide a baseline with extensive coverage that can be used for model calibration and change detection over time. We aim to provide metrics of the radio stratigraphy (e.g. shape and slope of internal reflection horizons) as well as classified prevalent stratigraphy types that can be used to calibrate machine learning approaches such as simulation based inference. The data obtained will be integrated in coordination efforts within the SCAR AntArchitecture Action Group

Autonomous rover enables radar profiling of ice-fabric properties in Antarctica

A ground-penetrating radar is an extensively used geophysical tool in cryosphere sciences (ice sheets and glaciers) with sounding depths of several kilometers due to the small radio-wave attenuation in ice sheets. The detection of the ice thickness and internal ice stratigraphy with commercial radars has become standard. However, there is still an observational gap in determining dielectric and mechanical ice-fabric anisotropy and basal properties using these systems. Recently, a ground-based phase-coherent radar showed its potential to fill this gap. However, this requires that the corresponding ground-based radars cover profiles several tens of kilometers in length. We address this challenge by modifying an autonomous rover to collect phase-coherent, quad-polarimetric radar data geolocated with real-time kinematic (RTK) positioning. In a proof-of-concept study in Antarctica, we demonstrate that this allows the collection of quad-polarimetric data along a 23-km profile, mapping anisotropic ice-fabric properties at <100-m intervals across the transition of grounded to floating ice. This study shows the possibility of collecting data that will refine ice-flow models by providing missing rheological parameters. This work also demonstrates the versatility of the autonomous ground vehicle with its ability to tow more than 200-kg payload, with a battery run time of over 6 h, and with a modular design that enables future integration of different radars or other geophysical sensors

A case study using 2019 pre-monsoon snow and stream chemistry in the Khumbu region, Nepal

This case study provides a framework for future monitoring and evidence for human source pollution in the Khumbu region, Nepal. We analyzed the chemical composition (major ions, major/trace elements, black carbon, and stable water isotopes) of pre-monsoon stream water (4300–5250 m) and snow (5200–6665 m) samples collected from Mt. Everest, Mt. Lobuche, and the Imja Valley during the 2019 pre-monsoon season, in addition to a shallow ice core recovered from the Khumbu Glacier (5300 m). In agreement with previous work, pre-monsoon aerosol deposition is dominated by dust originating from western sources and less frequently by transport from southerly air mass sources as demonstrated by evidence of one of the strongest recorded pre-monsoon events emanating from the Bay of Bengal, Cyclone Fani. Elevated concentrations of human-sourced metals (e.g., Pb, Bi, As) are found in surface snow and stream chemistry collected in the Khumbu region. As the most comprehensive case study of environmental chemistry in the Khumbu region, this research offers sufficient evidence for increased monitoring in this watershed and surrounding areas

Marine ice formation and deformation at the Southern McMurdo Ice Shelf, Antarctica

Marine ice accretes at the base of ice shelves often infilling open structural weaknesses and is thus thought to increase ice shelf stability. However, marine ice formation and deformation processes still remain poorly understood. Through measurements of marine ice properties, this study indirectly infers processes that occur during ice shelf flow and in the ice shelf cavity. Marine ice water isotope and solute chemistry are examined in ice cores from the Southern McMurdo Ice Shelf (SMIS) to derive marine ice source water composition and its origin. Marine ice microstructure (ice fabric, crystals size and shape) is also investigated in ice samples collected along an ice shelf flowline of increasing total strain to establish marine ice deformation in situ and compare it to deformation of ice formed from solid precipitation (meteoric ice). The measured marine ice water isotope composition together with the output of a boundary-layer freezing model indicate a spatio-temporally varying water source of sea water and relatively fresher water, such as melted meteoric or marine ice. This is in agreement with the occurrence of primarily banded and granular ice crystal facies typical for frazil ice crystals that nucleate in a supercooled mixture of water masses. It is proposed that marine ice exposed at the surface of SMIS, which experiences summer melt, is routed to the ice shelf base via the tide crack. Here frazil crystals nucleate in a double diffusion mechanism of heat and salt between two water masses at their salinity-dependent freezing point and accrete at the ice shelf where they consolidate to marine ice. Recycling of previously formed marine ice facilitates ice shelf selfsustenance with increasing air temperatures. Marine ice microstructure dynamically recrystallizes as a response to 20 - 25% total shear strain and vertical extension/horizontal compression. The marine ice extracted closer to shore develops a slightly less pointed anisotropic fabric, loses some of its horizontal shape preferred orientations (SPO) (with reference to vertical thin sections). Marine ice also adjusts its microstructure differentially downcore, indicating that it does not deform uniformly but shears in distinct planes. However, there is no evidence that SMIS marine ice deforms more easily than meteoric ice. Even though total strains at the meteoric and marine ice core sites are not equal, annual strain rates are in the order of x10-4 and the different ice types have similar minimum ages (of several thousand years). This makes their microstructural response to strain comparable. Meteoric ice shows stronger circle girdle fabrics, development of a vertical SPO and a decrease in its mean grain size with increasing total vertical extension and shear strain to 20% and 60% respectively downflow. The development of a circle girdle fabric and large increase in total strain downflow at the meteoric ice sites suggests that meteoric ice microstructure is preferentially oriented for horizontal compression as SMIS flows against shore. In contrast the marine ice microstructure is harder to deform in the ambient strain setting. The presence of marine ice thus could thus slow ice shelf dynamics and hence contribute to prolonging ice shelf life. This study relates ice shelf surface melting to basal marine ice accretion in a coldbased ice shelf cavity and the presence of marine ice to decelerated shelf ice deformation. Thus, knowledge gained in this study contributes to a better assessment of the behaviour of heterogenic ice shelves. In a changing climate, ocean circulation patterns and atmospheric conditions will change and it is important to understand current ice shelf behaviour in order to make sound predictions of their future buffering capability of land ice