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Title: Mass balance investigation of Antarctica from budget methods
Author: Depoorter, Mathieu A.
ISNI:       0000 0004 6056 713X
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2016
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During the last 20 years, West Antarctica has experienced enhanced ice discharge to the ocean due to loss of buttressing from melting and collapsing ice shelves. On the other hand, increases in precipitation have been reported in East Antarctica in line with an expected wetter atmosphere in a warming climate. The big questions that still lie ahead are therefore: (i) Will the enhanced precipitations in East Antarctica compensate the dynamic mass losses observed in West Antarctica in the future? (ii) And what will be the resulting contribution to sea level rise (SLR)? To answer those questions we need to have a firm grip on the present day mass balance (MB) of Antarctica and on the mechanisms that govern both the surface mass balance (SMB) and the ice discharge (D) into the ocean. This thesis investigates the MB of Antarctica using the input-output method (10M) allowing for a direct diagnoses of local, regional and global MB in Antarctica. It does this for both the Antarctic Ice Sheet (AIS) and the ice shelves. Because the mass imbalance of AIS is of the order of 5-10% of both accumulation and attrition terms of the mass budget (~2000 Gt yr-1), all glaciers around Antarctica as well as each assumption made require precise attention. This thesis starts with a chapter exploring the grounding zone (Chapter 2), and then goes on to the actual mass balance calculations of the AIS in Chapter 3 and of the Antarctica ice shelves in Chapter 4. The Grounding lines (GL) of Antarctica have been widely studied using various techniques at a local and regional scale. In recent years GL datasets aiming for circumpolar coverage have been published using different approaches. However these datasets still bear unexplained discrepancies of up to tens of kilometres in numerous places around Antarctica. In Chapter 2 four recent datasets are compared which track either the surface break of slope (h) or the inward limit of tidal flexure (F) as proxies for the grounding point (G). From visual examination and from a particle tracking scheme (PTS), it is found that all GL datasets agree within 1-2 km on slow moving ice and on the sides of fast flowing features (FFFs). However it is confirmed that h, obtained from photogrametry or photo clinometry, is not a reliable proxy in central parts of FFFs because of multiple breaks-in-slope and artefacts. It is further confirmed that the most reliable methods to map G in such places are those tracking F. In addition, a gravitational driving stress (td) is computed from a 1 km Antarctic digital model elevation (DEM) and leads to the finding that driving stress mapping (DSM) supports dynamic approaches in grounding line location. This reconciles static and dynamic grounding line methods by showing that they map the same features providing that altimetry is used rather than imagery for static methods. Guided by these analyses a new, up-to-date, and complete grounding line of Antarctica is compiled. The potential of DSM is successfully tested on a grounding line migration case study in West Antarctica. To investigate the grounding zone around Antarctica and its ice dynamics, DSM is further used. DSM allows to map sharp transitions across G for slow moving ice, as well as complicated transitions on fast flowing features (FFFs). Complicated transitions on FFFs contradict the idea of there being an ideal transition occurring at G, whereby the ice flow regime switches from basal drag-dominated to lateral drag-dominated. Rather, it is found that acceleration occurs upstream of G and that deceleration occurs downstream of G. This changes the understanding of the grounding zone ice dynamics, where ice was believed to accelerate at G due to loss of basal drag. Using DSM in combination with ice penetrating radar (lPR), reported and new ice plains (i .e. lightly grounded areas) are detected and mapped. They extents cover ~55,000 km2 around the Ross, the Filchner-Ronne, and the Larsen C ice shelves. These findings have implications for our understanding of ice sheet stability since ice plains are particularly prone to grounding line migration and can stretch up to ~300 km inland of G. In Chapter 3 the MB of the AIS is assessed using the input-output method (lOM). The grounding line fluxes (GLF) and 5MB are estimated for 110 drainage basins covering the whole AIS. The GLF is computed using up to date grounding lines and additional radar ice thicknesses data compared to previous 10M studies. 5MB values are re-evaluated in light of new drainage basins defined from an ice velocity field rather than from topography. 5MB is taken as the 30 years mean of three regional climate models. Due to a number of improvements in the GLF methodology, an unprecedented 94% of the ice sheet area is surveyed, i.e. an increase of + 13% from the latest 10M study. Un-surveyed areas are accounted for using mass trends (MT) from a Bayesian hierarchical modelling solution from the RATES (Resolving Antarctic ice mass TrEndS) project. The integrated AIS mass balance is -63 ± 83 Gt yr- I and divides into -22 ± 28, -62 ± 45, and 22 ± 64 Gt yr- I for the Antarctic Peninsula (AP), the West Antarctic Ice Sheet (WAIS), and the East Antarctic Ice Sheet (EAIS), respectively. The integrated MB is therefore a lower 10M estimate compared to previous 10M studies and reconciles the 10M with the other MB methods of satellite gravimetry and altimetry. Because the stability of the AIS is intimately linked to the stability of ice shelves, Chapter 4 finally focuses on the mass balance of ice shelves around Antarctica, giving the partition between calving fluxes (CF) and basal mass balance (BMB), the main processes by which Antarctic ice is lost. Before this study, iceberg calving had been assumed the dominant cause of mass loss for the Antarctic ice sheet, with previous estimates of the calving flux exceeding 2,000 Gt yr- I . More recently, the importance of melting by the ocean had been demonstrated close to the grounding line and near the calving front. So far, however, no study had reliably quantified the calving flux and the BMB (the balance between accretion and ablation at the ice-shelf base) for the whole of Antarctica. The distribution of fresh water in the Southern Ocean and its partitioning between the liquid and solid phases was therefore poorly constrained. Here, a first estimate of the mass balance components for all ice shelves in Antarctica is produced using calving flux and grounding-line flux from satellite and airborne observations, modelled ice-shelf snow accumulation rates, and a regional scaling that accounts for un-surveyed areas. The total CF is 1321 ± 144 Gt yr- I and the total BMB is -1454 ± 174 Gt yr-1 . These numbers mean that about half of the ice-sheet surface mass gain is lost through oceanic erosion before reaching the ice front, and that the calving flux is about 34% less than previous estimates derived from iceberg tracking. In addition, the fraction of mass loss due to basal processes varies from about 10 to 90 % between ice shelves. A significant positive correlation between BMB and surface elevation change is found for ice shelves experiencing surface lowering and enhanced discharge. It is therefore suggested that basal mass loss is a valuable metric for predicting future ice-shelf vulnerability to oceanic forcing.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available