Insights on the formation of longitudinal surface structures on ice sheets from analysis of their spacing, spatial distribution and relationship to ice thickness and flow

Longitudinal surface structures (LSSs) are prevalent upon the ice streams, ice shelves and outlet glaciers of ice sheets. These features inform our understanding of past and present ice sheet behavior. However, consensus regarding their genesis has not been reached. Here we analyze 42,311 LSS segments mapped across Antarctica together with geophysical data to determine their morphological and glaciological properties. Most LSSs are spaced 450 to 1500 m apart, a distance positively correlated with the width of the ice flow unit on which they occur. The start points (upstream end locations) of LSSs have diverse ice thicknesses and velocities. The majority of LSSs occur where ice flow is converging or broadly parallel, and they are prominent at ice confluences. Some occur at slow-flowing ice stream onsets. Occasionally, LSSs relate to sudden variations in basal shear stress due to basal perturbations. From these observations, we argue that LSSs are the consequence of increased strain which occurs during the lateral compression and longitudinal extension of ice: (i) converging/flowing into a channel (this scenario characterizes most LSSs), (ii) at the onset of ice streaming, (iii) at flow unit confluence, and (iv) as ice flows over and around a basal perturbation

Recent Changes to Langjökull Icecap, Iceland: An investigation integrating airborne LiDAR and satellite imagery

Langjökull, Iceland’s second largest icecap (~950 km$^{2}$), was the subject of an incomplete airborne LiDAR survey in August 2007. This study investigates and evaluates the application of photoclinometry, which employs visible light imagery (here, Landsat ETM+ band 4) to interpolate unmeasured sections of this fragmented data set. A complete digital elevation model (DEM) of Langjökull was produced, and photoclinometry was determined to be a satisfactory and robust technique for topographic interpolation (RMS error = 3.4 m over a 3 km section). Future applications of photoclinometry can ensure optimal results by focusing on the consistent ability of their imager to accurately represent low contrast surfaces; also, consideration of setting characteristic such as solar azimuth, solar elevation, and moderate surface slope will make photoclinometric interpolation more effective. Photoclinometry it is proven to be a current and valuable technique, it is confirmed as a secondary rather than primary tool, and other possible applications of photoclinometry are considered. Using the completed DEM of Langjökull for summer 2007 and a previously prepared corresponding 1997 data set, Langjökull was found to have a specific annual mass balance of -0.99$\pm$0.1 meters per year of water equivalence (m yr$^{-1}$ w.e.), a number which confirms published predictions that Langjökull will likely disappear in the next 200 years. Comparison of remotely-sensed mass balance values and traditional $\textit{in situ}$ measurements revealed a possible systematic disparity; it is hypothesized that field measurements may not be sufficiently constraining behavior of interior areas and that the signal from strongly receding outlet glaciers may be skewing the $\textit{in situ}$ mass balance value calculated for the entire icecap. An additional DEM of outlet Hagafellsjökull Vestari allowed for calculation of specific mass balances of -2.28 m yr$^{-1}$ w.e. for 1997-2001, -3.86 m yr$^{-1}$ w.e. for 2001-2007, and -3.23 m yr$^{-1}$ w.e. for 1997-2007. Similarly, visual inspection and tracing of Landsat images showed a recession of -3.42.5 km$^{2}$ yr$^{-1}$ from 1994 to 2007. The new 2007 DEM allowed for clear visualization of strong recession on several Langjökull outlets as well as interior mass loss and terminus advance witnessing to the 1998 surge event of outlet Hagafellsjökull Eystri. In addition, slight interior elevation increase and anti-correlated mass loss and terminal retreat potentially indicate a future surge of outlet Hagafellsjökull Vestari. In sum, the technological and glaciological information put forward in this study provides a method for innovative cryospheric research, presents a much needed benchmark and update on the state of Langjökull, and ultimately facilitates and encourages continued monitoring of highly important smaller glaciers and icecaps

Flow Signatures on the Bed and the Surface of Ice Sheets

Ice flow produces morphological features at the bed and on the surface of ice sheets. These ‘flow signatures’ provide us with an insight into the mechanisms, history and characteristics of ice sheet flow. In this thesis I examine the characteristics of basal and surficial ice sheet flow signatures, as well as possible links between them. The first chapter introduces ice sheet flow signatures. At the bed, a suite of landforms known as subglacial bedforms are found. The surface of an ice sheet is home to longitudinal surface structures (LSSs) and transverse surface structures (TSSs). Whilst the two environments are mostly considered in isolation, links between the sets of flow signatures found at each have been suggested. Section A deals with basal flow signatures. Chapter 2 asks whether subglacial bedforms are patterned. Drumlins are found to be regularly placed within the landscape, and likely grow or shrink over time. Chapter 3 examines whether subglacial bedforms conform to a size and shape continuum. By collating and analysing a dataset of 96,900 measurements of size and shape it is found that 3 continua of subglacial bedforms exist: flutes, lineations and ribs. The latter two are joined by an understudied class of quasi-circular bedforms. Section B deals with surficial flow signatures. In Chapter 4 I present and analyse a map of the LSSs of the Antarctic Ice Sheet. The morphology, spatial distribution and glaciological context of LSSs leads to the proposal of a model for their formation. Chapter 5 presents the first systematic study of TSSs. Mapping and analysis reveals that TSSs are regularly spaced, differ little in their morphology between ice streams and are most likely stationary. Section C compares basal and surficial ice sheet flow signatures. In Chapter 6, morphological comparisons, a case study of the Rutford Ice Stream, analysis of ice penetrating radar and examination of ice flow modelling lead to the conclusion that the majority of basal and surficial flow signatures are separate entities. Chapter 7 concludes the thesis and provides suggestions for future research