Search

Given the high number and diversity of events in a typical cryoseismic dataset, in particular those recorded on ice sheet margins, it is desirable to use a semi-automated method of grouping similar events for reconnaissance and ongoing analysis. We present a workflow for employing semi-unsupervised cluster analysis to inform investigations of the processes occurring in glaciers and ice sheets. In this demonstration study, we make use of a seismic event catalogue previously compiled for the Whillans Ice Stream, for the 2010–2011 austral summer outlined in Part 1,. We address the challenges of seismic event analysis for a complex wave field by clustering similar seismic events into groups using characteristic temporal, spectral, and polarization attributes of seismic time series with the k -means++ algorithm. This provides the basis for a reconnaissance analysis of a seismic wave field that contains local events (from the ice stream) set in an ambient wave field that itself contains a diversity of signals (mostly from the Ross Ice Shelf). As one result, we find that two clusters include stick-slip events that diverge in terms of length and initiation locality (i.e., central sticky spot and/or the grounding line). We also identify a swarm of high-frequency signals on 16–17 January 2011 that are potentially associated with a surface melt event from the Ross Ice Shelf. Used together with the event detection presented in Part 1, the semi-automated workflow could readily be generalized to other locations and, as a possible benchmark procedure, could enable the monitoring of remote glaciers over time and comparisons between locations. [ABSTRACT FROM AUTHOR]

The largest regional drivers of current surface elevation increases in the Antarctic Ice Sheet are associated with ice flow reconfiguration in previously active ice streams, highlighting the important role of ice dynamics in mass balance calculations. Here, we investigate controls on the evolution of the flow configuration of the Vanderford and Totten glaciers – key outlet glaciers of the Aurora Subglacial Basin (ASB) – the most rapidly thinning region of the East Antarctic Ice Sheet (EAIS). We synthesise factors that influence the ice flow in this region and use an ice sheet model to investigate the sensitivity of the catchment divide location to changes in surface elevation due to thinning at the Vanderford Glacier (VG) associated with ongoing retreat and thickening at the Totten Glacier (TG) associated with an intensification of the east–west snowfall gradient. The present-day catchment divide between the Totten and Vanderford glaciers is not constrained by the geology or topography but is determined by the large-scale ice sheet geometry and its long-term evolution in response to climate forcing. Furthermore, the catchment divide migrates under relatively small changes in surface elevation, leading to ice flow and basal water piracy from the Totten to the Vanderford Glacier. Our findings show that ice flow reconfigurations occur not only in regions of West Antarctica like the Siple Coast but also in the east, motivating further investigations of past, and the potential for future, ice flow reconfigurations around the whole Antarctic coastline. Modelling of ice flow and basal water piracy may require coupled ice sheet thermomechanical and subglacial hydrology models constrained by field observations of subglacial conditions. Our results have implications for ice sheet mass budget studies that integrate over catchments and the validity of the zero flow assumption when selecting sites for ice core records of past climate. [ABSTRACT FROM AUTHOR]

Seismic Rayleigh wave ellipticity measurements are the horizontal-to-vertical ratio of the Rayleigh wave particle motion, and are sensitive to the subsurface structure beneath a seismic station. H/V ratios measured from the ambient vibrations of the Earth are being increasingly used in glaciological applications to determine glacier and ice sheet thickness, seismic velocities and firn properties. Using the newly developed degree-of-polarisation (DOP-E) method which exploits the polarisation properties of seismic noise, we identify and extract Rayleigh waves from seismic stations in Greenland, and relate them to sea ice processes and the geology of the upper crust. Finally, we provide some suggestions for future applications of DOP-E method to gain greater insight into seasonal and long-term variability of sea ice formation and breakup as well as the monitoring of ice sheet thickness, subglacial environment and firn layers in the poles. [ABSTRACT FROM AUTHOR]

Given the high number and diversity of events in a typical cryoseismic dataset, in particular those recorded on ice sheet margins, it is desirable to use a semi-automated method of grouping similar events for reconnaissance and ongoing analysis. We present a workflow for employing semi-unsupervised cluster analysis to inform investigations of the processes occurring in glaciers and ice sheets. In this demonstration study, we make use of a seismic event catalogue previously compiled for the Whillans Ice Stream, for the 2010–2011 austral summer (outlined in companion paper, Latto et al., 2023). We address the challenges of seismic event analysis for a complex wavefield by clustering similar seismic events into groups using characteristic temporal, spectral, and polarization attributes of seismic time series with the k-means++ algorithm. This provides the basis for a reconnaissance analysis of a seismic wavefield that contains local events (from the ice stream) set in an ambient wavefield that itself contains a diversity of signals (mostly from the Ross Ice Shelf). As one result, we find that two clusters include stick-slip events that diverge in terms of length and initiation locality (i.e. Central Sticky Spot and/or the grounding line). We also identify a swarm of high frequency signals on January 16–17, 2011 that are potentially associated with a surface melt event from the Ross Ice Shelf. Used together with the event detection presented in the companion paper, the semi-automated workflow could readily generalize to other locations, and as a possible benchmark procedure, could enable the monitoring of remote glaciers over time and comparisons between locations. [ABSTRACT FROM AUTHOR]

The largest regional drivers of current surface elevation increases in the Antarctic Ice Sheet are associated with ice flow reconfiguration in previously active ice streams, highlighting the important role of ice dynamics in responding to climate change. Here, we investigate controls on the evolution of the flow configuration of the Vanderford and Totten Glaciers – key outlet glaciers of the Aurora Subglacial Basin, the most rapidly thinning region of the East Antarctic Ice Sheet. We review factors that influence the ice flow in this region, and use an ice sheet model to investigate the sensitivity of the catchment divide location to thinning at Vanderford Glacier associated with ongoing retreat, and thickening at Totten Glacier associated with an intensification of the east-west snowfall gradient. The present-day catchment divide between the Totten and Vanderford Glaciers is not constrained by the geology or topography, but is determined by the large-scale ice sheet geometry and its long-term evolution in response to climate forcing. Furthermore, the catchment divide is subject to migration under relatively small changes in surface elevation, leading to ice flow and basal water piracy from Totten to Vanderford Glacier. Our findings show that ice flow reconfigurations do not only occur in regions of West Antarctica like the Siple Coast, but also in the east, motivating further investigations of past, and potential for future, ice flow reconfigurations around the whole Antarctic coastline. Such modelling of ice flow and basal water piracy may require coupled ice sheet thermomechanical and subglacial hydrology models, constrained by field observations of subglacial conditions. Our results also have implications for ice sheet mass budget studies that integrate over catchments, and the validity of the zero flow assumption when selecting sites for ice core records of past climate. [ABSTRACT FROM AUTHOR]

Supraglacial lakes play a central role in storing melt water, enhancing surface melt and ultimately in driving ice flow and ice shelf melt through injecting water into the subglacial environment and through facilitating fracturing. Here, we develop a model for the drainage of supraglacial lakes through the dissipation-driven incision of a surface channel. The model consists of the St Venant equations for flow in the channel, fed by an upstream lake reservoir, coupled with an equation for the evolution of channel elevation due to advection, uplift and downward melting. After reduction to a 'stream power'-type hyperbolic model, we show that lake drainage occurs above a critical rate of water supply to the lake due to the backward migration of a shock that incises the lake seal. The critical water supply rate depends on advection velocity and uplift (or more precisely, drawdown downstream of the lake) as well as model parameters such as channel wall roughness and the parameters defining the relationship between channel cross-section and wetted perimeter. Once lake drainage does occur, it can either continue until the lake is empty, or terminate early, leading to oscillatory cycles of lake filling and draining, with the latter favoured by large lake volumes and relatively small water supply rates. [ABSTRACT FROM AUTHOR]

Seismic Rayleigh wave ellipticity measurements are the horizontal-to-vertical ratio of the Rayleigh wave particle motion, and are sensitive to the subsurface structure beneath a seismic station. H/V ratios measured from the ambient vibrations of the Earth are being increasingly used in glaciological applications to determine glacier and ice sheet thickness, seismic velocities and firn properties. Using the newly developed degree-of-polarisation (DOP-E) method which exploits the polarisation properties of seismic noise, we identify and extract Rayleigh waves from seismic stations in Greenland, and relate them to sea ice processes and the geology of the upper crust. Finally, we provide some suggestions for future applications of DOP-E method to gain greater insight into seasonal and long-term variability of sea ice formation and breakup as well as the monitoring of ice sheet thickness, subglacial environment and firn layers in the poles. [ABSTRACT FROM AUTHOR]

Ground-penetrating radar data acquired in the 2016/17 austral summer on Sørsdal Glacier, East Antarctica, provide evidence for meltwater lenses within porous surface ice that are conceptually similar to firn aquifers observed on the Greenland Ice Sheet and the Arctic and Alpine glaciers. These englacial water bodies are associated with a dry relict surface basin and consistent with perennial drainage into an interconnected englacial drainage system, which may explain a large englacial outburst flood observed in satellite imagery in the early 2016/17 melt season. Our observations indicate the rarely-documented presence of an englacial hydrological system in Antarctica, with implications for the storage and routing of surface meltwater. Future work should ascertain the spatial prevalence of such systems around the Antarctic coastline, and identify the degree of surface runoff redistribution and storage in the near surface, to quantify their impact on surface mass balance. [ABSTRACT FROM AUTHOR]

High-magnitude jökulhlaups, glacier margin position and ice-thickness have been identified as key controls on sandur evolution. Existing models however have focused primarily on observations made during short windows of time and often do not account for the subsequent modification of proglacial landsystems by repeated jökulhlaups or post-depositional modification due to melt out over decadal time-scales. Digital Elevation Models (DEMs) were used to reconstruct the development of large depressions on Skeiðarársandur, an outwash plain in southeast Iceland. These depressions measure up to 1 km in width and up to 13 m in depth and are associated with ice bodies up to 1 km in length and up to 150 m in height emplaced during a high-magnitude jökulhlaup in 1903 and subsequently buried by jökulhlaups in 1913 and 1922. The continued melting of the Harðaskriða ice bodies over a century following their emplacement, together with subsequent repeated burial, by high-magnitude jökulhlaups demonstrates that jökulhlaups may continue to serve as important controls on sandur evolution on a decadal to centennial timescale (101–102 years). The Harðaskriða depressions developed only following the retreat of the glacier margin after 1945, which highlights the controls of margin position on the evolution of the sandur. Margin position and thickness of the glacier profile was seen to affect not only the distribution and thickness of sediment emplaced during jökulhaups but also the rate and pattern of melt in the decades following the decoupling of the margin from the sandur. The jökulhlaup landsystem model signatures identified at this site may provide a useful analogue for interpreting landforms and strata emplaced by glacier margin fluctuations, jökulhlaups and melt out generated by retreating continental Pleistocene ice sheets. Unlabelled Image • New model for the origin of large (<1 km wide, <13 m deep) depressions on Skeiðarársandur, southeast Iceland. • 1 km long, 150 m high piece of glacier margin detached and transported during the 1903 high-magnitude jökulhlaup. • Large jökulhlaup-transported ice blocks subsequently buried by jökulhlaups in 1913 and 1922. • Digital Elevation Models (DEMs) and Ground Penetrating Radar (GPR) used to reconstruct the development of depressions. • Jökulhlaup-transported ice bodies continue to melt over a century following their emplacement at an average rate of 19.4 ± 2.6 cm per year. [ABSTRACT FROM AUTHOR]