Complex fabric development revealed by englacial seismic reflectivity: Jakobshavn Isbræ, Greenland

This is the published version. Copyright 2008 American Geophysical Union. All Rights Reserved.High-resolution reflection seismic data from Jakobshavn Isbræ, Greenland, reveal complex fabric development. Abundant englacial reflectivity occurs for approximately half the thickness of the ice (the lower half), and disruption of the englacial reflectors occurs in the lower 10–15% of the ice-thickness. These depths correspond to the higher impurity-content, and more easily deformed, ice from the Younger Dryas and Last Glacial Maximum to Stage-3. We conclude that the reflectivity results from contrasting seismic velocities due to changes in the crystal orientation fabric of the ice, and suggest that these fabric changes are caused by variations in impurity loading and subsequent deformation history. These findings emphasize the difference between ice-divide and ice-stream crystal orientation fabrics and have implications for predictive ice sheet modeling

Tracing of internal layers in radar echograms from a Greenland study region

The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file.Title from title screen of research .pdf file viewed on (June 25, 2007)Includes bibliographical references.Thesis (M.S.) University of Missouri-Columbia 2006.Dissertations, Academic -- University of Missouri--Columbia -- Electrical engineering.Signs of long-term glaciological processes and past ice sheet structure are preserved in the internal layer signatures of the Greenland ice sheet. Internal layer data have been collected over a considerable portion of the Greenland ice sheet using ice-sounding radar. We traced these layers along thousands of kilometers of flight lines from the ice divide toward Jakobshavn, which is the most active glacier in Greenland. We determined the traced-radar layers age at the GRIP site using the GRIP core age-depth relationship. Since the depth varies spatially for a layer of a specific dated age, an age-depth relationship for each position along the flight lines of this study can be found using the traced layers. We analyzed 31 points where flight lines crossover one another. From the flight line crossover analysis, we found a 9 m maximum difference, which is less than a 1% difference

Non-linear retreat of Jakobshavn Isbræ since the Little Ice Age controlled by geometry

Rapid acceleration and retreat of Greenland's marine-terminating glaciers during the last two decades have initiated questions on the trigger and processes governing observed changes. Destabilization of these glaciers coincides with atmosphere and ocean warming, which broadly has been used to explain the rapid changes. To assess the relative role of external forcing versus fjord geometry, we investigate the retreat of Jakobshavn Isbræ in West Greenland, where margin positions exist since the Little Ice Age maximum in 1850. We use a one-dimensional ice flow model and isolate geometric effects on the retreat using a linear increase in external forcing. We find that the observed retreat of 43 km from 1850 until 2014 can only be simulated when multiple forcing parameters – such as hydrofracturing, submarine melt and frontal buttressing by sea ice – are changed simultaneously. Surface mass balance, in contrast, has a negligible effect. While changing external forcing initiates retreat, fjord geometry controls the retreat pattern. Basal and lateral topography govern shifts from temporary stabilization to rapid retreat, resulting in a highly non-linear glacier response. For example, we simulate a disintegration of a 15 km long floating tongue within one model year, which dislodges the grounding line onto the next pinning point. The retreat pattern loses complexity and becomes linear when we artificially straighten the glacier walls and bed, confirming the topographic controls. For real complex fjord systems such as Jakobshavn Isbræ, geometric pinning points predetermine grounding line stabilization and may therefore be used as a proxy for moraine build-up. Also, we find that after decades of stability and with constant external forcing, grounding lines may retreat rapidly without any trigger. This means that past changes may precondition marine-terminating glaciers to reach tipping-points, and that retreat can occur without additional climate warming. Present-day changes and future projections can therefore not be viewed in isolation of historic retreat.submittedVersio

Reconstructing surface mass balance from the englacial stratigraphy of the Greenland Ice Sheet

Interiøren av isflakene består av mange lag akkumulert snø, hvis tykkelse avhenger av overflatemassebalanse (OMB) fra fortiden og effekten av dynamisk tynning etter lagenes avsetning. Denne oppgaven undersøker påvirkningen disse to faktorene har på den endelige stratigrafien av isflaket ved å bruk av en isokron numerisk isflakmodell. Modellen, som har SMB som en øvre grensebetingelse, simulerer utviklingen av lagene over tid. Målet med oppgaven er å invertere modellen og å rekonstuere mengden av fortidens OMB ved å bruke nåtidens stratigrafi. Den første delen av oppgaven bruker den isokrone numeriske isflakmodellen for å undersøke påvirkningen av OMB på lagtykkelse av et todimensjonalt, idealisert isflak. OMB fra denne idealiserte simuleringen blir deretter perturbert i hvert horisontale punkt og lag for å kvantifisere følsomhet av lagtykkelse til små forandringer i akkumulering, og resultatene blir formalisert i en følsomhetsmatrise. Etterpå blir et sett av simuleringer med forlenget forandring i OMB som spenner flere tusen år og lange avstander fremført. I alle tilfeller er innvirkningen av OMB sentral til stratigrafien av isflaket og påvirker det direkte på grunn av forandringer i selve OMBen og indirekte på grunn av forandringer i dynamisk tynning. Oppgaven fokuserer deretter på å gjenskape stratigrafien av simuleringen med forlenget forandring i OMB ved å ettablere et lineært forhold mellom OMB og lagtykkelse og ved å ekstrapolere følsomhetsmatrisen. Resultatene viser at virkelige forandringer i lagtykkelse på grunn av forandringer i OMB kan bli tilnærmet med et lineært forhold. Den andre delen av oppgaven fokuserer i å rekonstruere OMB fra en gitt lagtykkelse, altså inversjonen. Med det etablerte lineære sistemet av ligninger, blir dens løsning funnet med tre regulariseringsmetoder, Riley's, Truncated Singular Value Decomposition, og Conjugate Gradient. Rekonstrueringen av OMB ble utført for tilfellet av et idealisert isflak med flat berggrunn og et 2D meridionalt tverrsnitt av berggrunnen i Grønlands IsFlak (GrIF) over isskillet. Resultatene i alle tilfeller viser en akkurat rekostruering av OMB for alle lag i punkt i nærheten av isskillet, men jo lengre fra isskillet, jo færre lag nær overflaten har en akkurat OMB rekostruering. Dette resulterer i en V-form mønster hvor akkurate rekostrueringer av OMB kun er mulig innenfor denne formen. Oppgaven konkluderer med en applikasjon av metoden for stratigrafien av GrIF som er tatt fra radiostratigrafidata av NASA's Operation IceBridge.The interior of an ice sheet consists of several layers of accumulated snow, whose present thickness depends on the surface mass balance (SMB) of the past and the effect of dynamic thinning after the layer's deposition. This thesis examines the influence that these two factors have on the final stratigraphy of the ice sheet, by using an isochronal numerical ice sheet model. The model, for which SMB is the upper boundary condition, simulates the evolution of the layers through time. The aim of this thesis is to invert the forward model and for a given present stratigraphy to reconstruct the amount of past SMB. The first part of the thesis uses the isochronal numerical model to examine the influence of SMB on the layer thickness of a two dimensional, idealized ice sheet. The SMB of this idealized simulation is then perturbed at each horizontal location and layer in order to quantify the sensitivity of the layers' thickness to small changes in accumulation, and the results are formalized in a sensitivity matrix. Subsequently, a set of simulations with sustained change in SMB that spans several thousands of years and long distances is performed. In all cases, the impact of SMB is crucial for the stratigraphy of the ice sheet and affects it directly due to changes in SMB itself and indirectly due to alterations of dynamic thinning. The thesis then focuses on recreating the stratigraphy of the simulation with sustained changes in SMB by establishing a linear relation between SMB and layer thickness and extrapolating the sensitivity matrix. The results show that indeed changes in layer thickness due to alterations in SMB can be approximated with a linear relation. The second part of the thesis focuses on reconstructing SMB from a given layer thickness, the inversion. With the linear system of equations established, its solution is found with three regularization methods, Riley's, Truncated Singular Value Decomposition, and Conjugate Gradient. The SMB reconstruction was performed for the case of an idealized ice sheet with at bedrock and a 2D meridional cross section of the bedrock of the Greenland Ice Sheet (GrIS) across the ice divide. The results for all cases show an accurate reconstruction of the SMB for all layers at locations close to the ice divide, but the further away from the ice divide, the less layers close to the surface have their SMB accurately reconstructed. This results in a V-shape pattern, where accurate reconstruction of the SMB is only possible within this shape. The thesis concludes with the application of the method on the stratigraphy of GrIS as taken from radiostratigraphy data of NASA's Operation IceBridge.Doktorgradsavhandlin

Modelling the retreat of the Uummannaq ice stream system, central west Greenland.

I aim to understand what controlled the retreat pattern of the Uummannaq ice stream (UISS) during the last deglaciation. Evidence for the pattern of retreat is recorded in marine bathymetric and sedimentological data in the central trough (Ó Cofaigh et al., 2013). On land, on the islands that sit within the fjord (Ubekendt and Karrat) and on the fjord margins, geomorphological evidence records the thinning of the ice surface through time (Roberts et al., 2013; Lane et al., 2014). These records are set within a chronological framework of radiocarbon and cosmogenic dates, which suggest that the ice stream was grounded close to the continental shelf edge at the Last Glacial Maximum, and had begun rapid retreat by 17ka BP. However, it is unclear what controlled the retreat pattern identified in the Uummannaq system. Modelling the UISS using a 1-D numerical model provides the opportunity to combine the chronology and geometries inferred from the landforms and to test the influence of various controls upon the retreat of the ice stream. The model has the capability to dynamically and robustly simulate grounding line retreat behaviour over millennial timescales (Jamieson et al., 2014). Marine geophysical data and dates from islands are used to constrain the numerical model and sensitivity tests are conducted to explore its response to a range of forcing patterns. The model retreat is simulated from a steady-state LGM configuration and was subjected to a series of retreat perturbations forced independently or simultaneously by either rising sea level, sub marine melting, ice temperature and surface melt. Comparing the simulated behaviour of the UISS against the geomorphological and cosmogenic exposure evidence for ice surface thinning onshore confirms that the UISS responds non-linearly to the applied forcings. This is likely to be because of the influence of topographic controls in the system, which appears to be the key modulator of retreat. Ice temperature and climate also have an impact on retreat and thinning of the UISS, but ultimately, a combination of sea level rise, submarine melt, increasing atmospheric temperatures are needed to reconstruct the retreat of the UISS that fits with the geomorphic evidence presented

Glacier dynamics: influence of precursor crevasses and supraglacial lakes on calving at Sermeq Kujalleq in Kangia (Jakobshavn Isbræ), Greenland

Tidewater outlet glaciers, crucial in draining the Greenland Ice Sheet (GrIS), significantly contribute to global sea-level rise. However, the primary mechanism of their mass loss, calving, remains insufficiently understood. This study delves into the influence of crevasses and supraglacial lakes on the calving dynamics of the Sermeq Kujalleq in Kangia (Jakobshavn Isbræ), one of the fastest-flowing and most studied glaciers in the world. Utilizing optical remote sensing data from 2016 to 2022, 29 calving events are analyzed, of which 23 feature sections that coincide with pre-existing crevasses. These precursor crevasses predominantly appear in the southern and central sections of the glacier terminus, where there is a rise in glacier bed elevation, which is thought to contribute to the formation of deep crevasses. Conversely, supraglacial lakes are distributed across the glacier and their fill-and-drain cycles are irregular. Using a semi-automatic lake area detection algorithm to analyze 12 supraglacial lakes, 63 drainage events are identified, with 22 completing within four days or less. In 13 of these events, the drained lake area is 80% or more. Drainage events often occur in clusters and are believed to accelerate ice flow via basal lubrication, potentially influencing or triggering calving events. These lakes typically reach their maximum size before the end of summer, suggesting that the subglacial drainage system becomes efficient enough to handle surface meltwater, which reaches the bedrock through englacial routes like moulins. Additionally, this research identifies three instances where increases in glacier velocity coincide with calving events, and seven instances where the drainage of supraglacial lakes is followed by large calving events. This study accentuates the connection between precursor crevasses and calving events, and underscores the relationship between supraglacial lake drainage and the ensuing acceleration of glacier movement, which is likely to promote further calving. These findings offer valuable insights for improving our knowledge about calving dynamics and informing glacier calving models, enhancing our ability to predict future sea-level rise in a changing climate. Incorporating higher-resolution imagery in future studies could enable more accurate tracking of crevasses and calculations of lake areas, including assessments of depth. This enhanced precision may reveal new correlations between crevasse formation, lake drainage patterns, and calving events, further improving our understanding of tidewater glacier dynamics

Surface Expression of Basal and Englacial Features, Properties, and Processes of the Greenland Ice Sheet

Radar-sounding surveys measuring ice thickness in Greenland have enabled an increasingly “complete” knowledge of basal topography and glaciological processes. Where such observations are spatially limited, bed elevation has been interpolated through mass conservation or kriging. Ordinary kriging fails to resolve anisotropy in bed geometry, however, leaving complex topography misrepresented in elevation models of the ice sheet bed. Here, we demonstrate the potential of new high-resolution (≤5 m) surface topography data (ArcticDEM) to provide enhanced insight into basal and englacial geometry and processes. Notable surface features, quantified via residual surface elevation, are observed coincident with documented subglacial channels, and new, smaller-scale tributaries (<2,000 m in width) and valley-like structures are clearly identified. Residual surface elevation also allows the extent of basal ice units to be mapped, which in conjunction with radar data indicate that they act as “false bottoms,” likely due to a rheological contrast in the ice column

Five decades of radioglaciology

Radar sounding is a powerful geophysical approach for characterizing the subsurface conditions of terrestrial and planetary ice masses at local to global scales. As a result, a wide array of orbital, airborne, ground-based, and in situ instruments, platforms and data analysis approaches for radioglaciology have been developed, applied or proposed. Terrestrially, airborne radar sounding has been used in glaciology to observe ice thickness, basal topography and englacial layers for five decades. More recently, radar sounding data have also been exploited to estimate the extent and configuration of subglacial water, the geometry of subglacial bedforms and the subglacial and englacial thermal states of ice sheets. Planetary radar sounders have observed, or are planned to observe, the subsurfaces and near-surfaces of Mars, Earth's Moon, comets and the icy moons of Jupiter. In this review paper, and the thematic issue of the Annals of Glaciology on ‘Five decades of radioglaciology’ to which it belongs, we present recent advances in the fields of radar systems, missions, signal processing, data analysis, modeling and scientific interpretation. Our review presents progress in these fields since the last radio-glaciological Annals of Glaciology issue of 2014, the context of their history and future prospects

Understanding Greenland ice sheet hydrology using an integrated multi-scale approach

Improved understanding of Greenland ice sheet hydrology is critically important for assessing its impact on current and future ice sheet dynamics and global sea level rise. This has motivated the collection and integration of in situ observations, model development, and remote sensing efforts to quantify meltwater production, as well as its phase changes, transport, and export. Particularly urgent is a better understanding of albedo feedbacks leading to enhanced surface melt, potential positive feedbacks between ice sheet hydrology and dynamics, and meltwater retention in firn. These processes are not isolated, but must be understood as part of a continuum of processes within an integrated system. This letter describes a systems approach to the study of Greenland ice sheet hydrology, emphasizing component interconnections and feedbacks, and highlighting research and observational needs