The effect of late holocene ice-mass changes on glacial isostatic adjustment in West Antarctica

PhD ThesisGlacial isostatic adjustment (GIA) describes the Earth’s response to changing ice and water loads as ice sheets grow and diminish. GIA is difficult to model in Antarctica due to limited knowledge of ice history and Earth properties. The signal confounds satellite gravity measurements of present-day ice-mass change and needs to be accurately removed, but remains the biggest uncertainty. One problem with current Antarctic GIA models is that they neglect ice-mass changes over the past few thousand years, which, in regions of low viscosity mantle, may dominate the present-day bedrock uplift. This study investigates deficiencies in millennial-scale GIA models arising from omission of Late Holocene and present-day ice-mass changes. In the Antarctic Peninsula increasing accumulation observed in ice cores since the 1850s has been shown to cause loading and present-day GIA-related subsidence, although results are dependent on the Earth model. This missing signal may help to reconcile the misfit between GIA model predictions and GPS-observed uplift. GPS records from the northern Peninsula provide an opportunity to place bounds on the regional Earth properties. Since 1995 several ice shelves have collapsed triggering ice-mass unloading that invokes a solid Earth response. However, non-linear GPS-observed uplift cannot be explained by elastic deformation alone. Using a viscoelastic model to predict uplift due to recent ice loss and testing the fit to GPS time series, an Earth model has been constrained with upper mantle viscosity much lower than previously suggested. Elsewhere, the stagnation of Kamb Ice Stream on the Siple Coast ~165 years ago has caused localised thickening of ice which may cause significant GIA-related subsidence if the regional mantle viscosity is low. Combining with an LGM deglacial history and comparing with an empirically-derived GIA model shows large misfits, indicating that the regional mantle viscosity is high and highlighting potential errors in the LGM deglacial model.NERC PhD studentshi

Meson exchange in the weak decay of Lambda hypernuclei and the Gamma_n/Gamma_p ratio

We take an approach to the Lambda non-mesonic weak decay in nuclei based on the exchange of mesons. The one pion and one kaon exchange are considered, together with the exchange of two pions, either correlated, leading to an important scalar-isoscalar exchange (sigma-like exchange), or uncorrelated (box diagrams). Extra effects of omega exchange in the scalar-isoscalar channel are also considered. Constraints of chiral dynamics are used to generate these exchanges. A drastic reduction of the OPE results for the Gamma_n/Gamma_p ratio is obtained and the new results are compatible with all present experiments within errors. The absolute rates obtained for different nuclei are also in good agreement with experiment.Comment: 30 pages, 16 figures, 8 tables, to be published in Nucl. Phys.

Postseismic Deformation in the Northern Antarctic Peninsula Following the 2003 and 2013 Scotia Sea Earthquakes

Large earthquakes in the vicinity of Antarctica have the potential to cause postseismic viscoelastic deformation affecting measurements of displacement that are used to constrain models of glacial isostatic adjustment (GIA). In November 2013, a Mw 7.7 strike‐slip earthquake occurred in the Scotia Sea, 650 km from the Antarctic Peninsula. GPS time series from the northern Peninsula show a change in rate after this event, indicating a far‐field postseismic deformation signal is present. In this study, we use a finite element model with a suite of 1D and 3D Earth structures to investigate the extent of postseismic deformation in the Antarctic Peninsula. Model output is compared with GPS time series to place constraints on the Earth structure in this region. The preferred Earth structure has a thin lithosphere combined with a Burgers rheology with steady‐state viscosity of 4 × 1018 Pa s and transient viscosity one order of magnitude lower. Our study shows that including 3D Earth structure does not improve the fit. Using the best fitting Earth structure, we run a forward model of the nearby 2003 Mw 7.6 strike‐slip earthquake and combine the predictions for both earthquakes. We show that postseismic deformation is widespread across the northern Peninsula with rates of horizontal deformation up to 1.65 mm/yr for the period 2015–2020, a signal that persists for decades. These results suggest that much of Antarctica may be deforming due to recent postseismic deformation and this signal needs to be accounted for when using GPS observations to constrain geophysical models

Glacial isostatic adjustment and post-seismic deformation in Antarctica

This chapter reviews glacial isostatic adjustment (GIA) and post-seismic deformation in Antarctica. It discusses numerical models and their inputs, and observations and inferences that have been made from them. Both processes are controlled by mantle viscosity but their forcings are different. Ongoing GIA induced by the loss of ice since the last glacial maximum (LGM) could have amounted to 5–15 m of global sea-level rise. However, mantle viscosity is so low in parts of West Antarctica (c. 1018 Pa s) that changes in ice thickness over the last centuries and decades have controlled the current uplift rates there. The uplift due to GIA has promoted ice-sheet stability since the LGM, and in West Antarctica GIA is a significant negative feedback on the current decline of the ice sheet. Post-seismic deformation following the 1998 earthquake near the Balleny Islands south of New Zealand has been detected in global navigation satellite system (GNSS) data and compared to model outputs. The best-fitting viscosity for this area is c. 1019 Pa s, similar to GIA-based estimates for the Antarctic Peninsula. Future work should focus on unifying descriptions of viscosity across geodynamic models, and integrating information from seismic, gravity, experimental and geological data

Strangeness nuclear physics: a critical review on selected topics

Selected topics in strangeness nuclear physics are critically reviewed. This includes production, structure and weak decay of $\Lambda$--Hypernuclei, the $\bar K$ nuclear interaction and the possible existence of $\bar K$ bound states in nuclei. Perspectives for future studies on these issues are also outlined.Comment: 63 pages, 51 figures, accepted for publication on European Physical Journal

Geodetic Observations Reveal Near‐Zero Uplift Rates in the Transantarctic Mountains: Implications of Surface Mass Loading Deformation

Plain Language Summary: Ground‐based Global Positioning System (GPS) receivers allow us to track subtle changes in the Earth's shape. In this study, we use GPS data on vertical land motion to measure the response of Antarctica's bedrock to the loading caused by present‐day changes in the ice sheets. We demonstrate that variations in snowfall over decades and subsequent Earth deformations, which appear as varying trends in the GPS‐recorded vertical position time series, influence these measurements. We focus on observations from the Transantarctic Mountains and show that these factors can lead to biased estimates of the Earth's response to long‐term changes in the ice sheets, dating back to the last glacial maximum around 20,000 years ago, unless decadal GPS data records are available

Surface Mass Balance Variability Causes Viscoelastic Solid Earth Deformation in the Antarctic Peninsula

Present‐day ice‐mass changes in Antarctica typically deform the solid Earth elastically, and this signal needs removing from Global Positioning System (GPS) observations of displacement before they can be used to constrain models of glacial isostatic adjustment. However, much of West Antarctica is underlain by weak upper mantle, meaning these short‐term fluctuations may also cause viscous (transient or steady‐state) deformation of the Earth. We model the viscoelastic response of the Earth to surface mass balance (SMB) variability in the Antarctic Peninsula and find an improved fit to GPS data at most sites compared to elastic only. Viscoelastic modeling constrains upper mantle steady‐state viscosity in the northern Peninsula to 5 × 1017 to 2 × 1018 Pa s, and >1 × 1018 Pa s for the mid to southern Peninsula. In the northern Peninsula, removing viscoelastic displacement caused by SMB variability from GPS time series increases estimated uplift rates by up to 3 mm/yr compared with using an elastic‐only correction

Resolving the paradox of conflicting glacial chronologies: Reconstructing the pattern of deglaciation of the Magellan cordilleran ice dome (53–54°S) during the last glacial – interglacial transition

Raised shorelines and associated lacustrine sediments in the central Estrecho de Magallanes (Strait of Magellan) have been interpreted as products of cordilleran glaciers impounding a large proglacial lake and preventing drainage to the South Pacific and Southern Ocean during the Late glacial between c. 15.0 and 12.0 cal ka BP. However, a growing body of glacial geological evidence points towards an earlier retreat of the Magellan cordilleran ice dome, insufficient to dam lakes at that time. We critically re-evaluate the extant evidence for the c. 15.0–12.0 cal ka BP lake, here named ‘Lago Kawésqar’, and provide further sedimentological and chronological evidence for its existence. We also provide new cosmogenic surface nuclide dating of erratic and bedrock samples collected from extensive field campaigns that confirm the rapid and widespread retreat of the Magellan ice fields to the inner fjords of the Fuegian archipelago by c. 16.0 ka. To resolve the apparent paradox between these two lines of evidence we propose that glacial isostatic adjustment led to a topographic barrier to lake drainage rather than an ice dam. We use Glacial Isostatic Adjustment modelling to demonstrate that rapid isostatic recovery following the early deglaciation after c. 17.0 cal ka BP likely led to elevation of the present shallow south-western coastal margin of the Fuegian archipelago. Final drainage of Lago Kawésqar was probably caused by neotectonic subsidence of the same margin along the boundary of the South American – Scotia tectonic plates at c. 12.0 cal ka BP

Mass balance of the Greenland Ice Sheet from 1992 to 2018

In recent decades, the Greenland Ice Sheet has been a major contributor to global sea-level rise1,2, and it is expected to be so in the future3. Although increases in glacier flow4–6 and surface melting7–9 have been driven by oceanic10–12 and atmospheric13,14 warming, the degree and trajectory of today’s imbalance remain uncertain. Here we compare and combine 26 individual satellite measurements of changes in the ice sheet’s volume, flow and gravitational potential to produce a reconciled estimate of its mass balance. Although the ice sheet was close to a state of balance in the 1990s, annual losses have risen since then, peaking at 335 ± 62 billion tonnes per year in 2011. In all, Greenland lost 3,800 ± 339 billion tonnes of ice between 1992 and 2018, causing the mean sea level to rise by 10.6 ± 0.9 millimetres. Using three regional climate models, we show that reduced surface mass balance has driven 1,971 ± 555 billion tonnes (52%) of the ice loss owing to increased meltwater runoff. The remaining 1,827 ± 538 billion tonnes (48%) of ice loss was due to increased glacier discharge, which rose from 41 ± 37 billion tonnes per year in the 1990s to 87 ± 25 billion tonnes per year since then. Between 2013 and 2017, the total rate of ice loss slowed to 217 ± 32 billion tonnes per year, on average, as atmospheric circulation favoured cooler conditions15 and as ocean temperatures fell at the terminus of Jakobshavn Isbræ16. Cumulative ice losses from Greenland as a whole have been close to the IPCC’s predicted rates for their high-end climate warming scenario17, which forecast an additional 50 to 120 millimetres of global sea-level rise by 2100 when compared to their central estimate