The high temperature creep deformation of ice: new laboratory measurements

Adam Treverrow1, Bridie Le’Gallais2

1Antarctic Climate And Ecosystems Cooperative Research Centre, Hobart, Australia, 2University of Tasmania, Hobart, Australia

The dynamic behaviour of the Antarctic ice sheet is controlled by both creep deformation and the occurrence of sliding at the base of the ice sheet. The activity of these processes is highly temperature dependent and as such they are influenced by the geothermal heat flux at the ice-solid Earth interface.

 

The constitutive relation describing the rate of ice deformation in numerical ice sheet models is typically a power-law relationship between the stresses driving the flow and the corresponding strain rates, with a separate term describing the temperature dependence. Many models use a simplified prescription of the temperature dependence which does not adequately describe the sensitivity of deformation rates to temperature within ~5°C of the melting point. This leads to an underestimation of strain rates in the warmest ice.

 

While the increased sensitivity of deformation rates to temperature near the melting point is clearly demonstrated by laboratory experiments, it is constrained by a relatively small number of observations due to the inherent difficulties in conducting experiments at high temperatures. Here we present preliminary results from an experimental program designed to improve the constraint on deformation rates at temperatures close to the melting point. Simple shear deformation experiments were conducted at temperatures between -2°C and -0.3°C at 0.1 MPa (octahedral shear stress). Unlike previous studies investigating temperatures close to the melting point, these experiments were continued through to high shear strains (>10%) to ensure that samples had developed the mechanical anisotropy and corresponding enhanced flow rates that are associated with the microstructural evolution that is typical of polar ice sheets.

 

These data contribute to the continued development of a constitutive relation for polycrystalline ice that will improve the accuracy of ice sheet models, and are relevant to model studies utilizing inverse methods to infer the spatial extent of basal sliding.

The importance of geothermal heat flux in modelling of the Antarctic Ice Sheet

Steven Phipps1, Jacqueline Halpin1

1Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Australia

Ice sheet models are the only physically-based tools that allow us to simulate the future evolution of the Antarctic Ice Sheet, including its contribution towards changes in global sea level. However, due to limitations in our understanding of ice sheet dynamics, modelling is an inherently uncertain exercise. A typical approach towards optimising ice sheet models is to “tune” key physical parameters by finding the values that give the most realistic simulations of the present-day ice sheet, based on criteria such as ice sheet geometry or ice velocity. However, this approach assumes that there are no errors in the boundary conditions being used to drive the models.

 

Here, we use the Parallel Ice Sheet Model to explore the sensitivity of the simulated Antarctic Ice Sheet to the available geothermal heat flux (GHF) datasets. We find that the choice of GHF is a significant source of uncertainty, leading to basin-wide differences in excess of 1000m in the simulated ice thickness. Using different GHF datasets to drive the model, we then “tune” it by determining the optimal values of key physical parameters. We show that the parameter combinations obtained are sensitive to the choice of GHF.

 

Our results highlight the importance of GHF in ice sheet modelling. Reliable GHF estimates are critical to optimising numerical models of the Antarctic Ice Sheet and, therefore, to reducing uncertainty in projections of future global sea level rise.

 

Heat production and heat flow variations in Australian continental terranes, lessons for geological based estimates of sub-glacial heat flow in Antarctica

Sandra Mclaren1, Chris Carson2, Roger Powell1

1University Of Melbourne, University Of Melbourne, Australia, 2Geoscience Australia, Canberra, AUSTRALIA

Prior to the breakup of Gondwana (beginning around the late Cretaceous) cratonic rocks of southern Australia, including the Gawler and Curnamona cratons, are thought to have been contiguous with similar aged rocks in East Antarctica. In Australia, excellent outcrop exposure means that these rocks are reasonably well understood. One of the key characteristics of these Australian Proterozoic-aged cratonic blocks is unusually high measured surface heat flow, averaging 2-3 times that of similarly-aged cratonic blocks elsewhere globally. This high heat flow arises from anomalously high concentrations of the heat producing elements, U, Th and K, which have been demonstrated to profoundly impact a range of temperature-dependent geological processes, such as metamorphism and magmatism. But the spatial distribution of these rocks is highly variable. Moreover, the vertical distribution of the heat producing elements is also important.

 

Geochemical analysis of rocks from the George V Land-Terre Adelie and eastern Prydz Bay regions suggest heat flow is highly heterogenous in East Antarctica, with the presence and variable distribution of U, Th and K enriched crustal rocks providing a first-order control on sub-glacial heat flow variations. Our data show that variations in abundance and distribution of heat producing elements within the Antarctic continental crust results in greater and much more variable regional sub-glacial heat flows than currently assumed in ice modelling studies. Such elevated heat flows may have significant effect on ice sheet behavior and highlights the importance of assessing the geological controls on heat flow for predictions of ice mass balance and sea-level change.

New geological insights fingerprint high heat producing crust in the remote interior of Wilkes Land, East Antarctica

Alessandro Maritati1, Jacqueline Halpin1, Joanne Whittaker1,

1Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Australia

Heat produced by radioactive decay within East Antarctic crust provides a significant contribution to the total heat flow balance that is supplied at the base of the East Antarctic Ice Sheet. However, the distribution of heat-producing crustal rocks in ice-covered regions remains poorly constrained. Wilkes Land, East Antarctica hosts the Totten Glacier catchment area, one of the largest ice drainage basins in East Antarctica, and is one region where an improved knowledge of sub-glacial geology is of fundamental importance to understanding solid Earth-cryosphere interactions.

 

We examined the provenance of three low-grade quartzite erratic samples from the Windmill Islands region to provide evidence of the age and composition of the ice-covered bedrock. We suggest these quartzite erratic samples were sourced from a previously-unknown intra-continental sedimentary basin in the interior of Wilkes Land. U–Pb ages of detrital zircons include dominant age peaks at c. 1200 Ma and c. 1490 Ma, which fingerprint the provenance of this sedimentary basin.

 

Based on the detrital zircon record in these quartzite erratic samples, and using recent Australia-Antarctica plate reconstructions, we suggest there are heat-producing granitic source rocks in Wilkes Land equivalent in age and composition to those present in the Coompana Province of southwestern Australia. We further suggest that the significant heat production variability that characterises the Coompana granitic basement (2.52–6.39 μW/m3) can therefore also be expected in the interior of Wilkes Land. These new geological insights indicate that a higher and more variable regional sub-glacial heat flow might be expected in this region than currently resolved in available models.

Heat and groundwater transport between the Antarctic Ice Sheet and subglacial sedimentary basins from electromagnetic geophysical measurements

Bernd Kulessa1, Kerry Key2, Sarah Thompson1, Martin Siegert3

1Glaciology Group, College of Science, Swansea University, UK, 2Lamont-Doherty Earth Observatory, Columbia University, USA, 3Grantham Institute, and Department of Earth Science and Engineering, Imperial College London, UK

Numerical models of contemporary as well as palaeo ice sheets suggest that groundwater and heat exchanges between subglacial sedimentary basins and the ice sheet above can be substantial and influence the ice flow. A strategy for the measurement and assessment of such fluxes beneath contemporary ice sheet has not so far been available, however. Here we summarise, first, existing evidence for groundwater and heat exchanges between contemporary and palaeo ice sheets and the substrate below. Second we explain the utility of electromagnetic (EM) geophysical measurements in elucidating such exchanges, and present magnetotelluric (MT) forward models of the deep sedimentary basin beneath the Institute Ice Stream in West Antarctica by way of illustration. Third we propose a simple empirical model by which heat exchanges between subglacial sedimentary basins and the overlying ice sheet can be estimated to first-order from electromagnetic data. We then apply this model to existing Antarctic magnetotelluric data and discuss upcoming field electromagnetic projects on the Whillans and Institute Ice Streams in West Antarctica.

Temporal–spatial variations in infrasound sources related to cryosphere dynamics in Lützow–Holm Bay Region, Antarctica

Masaki Kanao1

1National Institute of Polar Research, Tachikawa, Japan

Characteristic features of infrasound waves observed in the Antarctic reflect the physical interaction between the surface environment along the continental margin and the surrounding Southern Ocean. The temporal–spatial variability of the source locations for infrasound excitation during 2015 was investigated using recordings made by two infrasound arrays deployed along a section of the coast of Lützow–Holm Bay (LHB), Antarctica. The infrasound arrays clearly detected temporal variations in frequency content and propagation direction during this period. A number of infrasound sources were identified, many located north of the arrays. Many of the events had a predominant frequency content of a few Hz, higher than microbaroms from the ocean. A comparison of the results with MODIS satellite images revealed that these infrasound sources were ice-quakes associated with the calving of glaciers, the breaking off of sea ice, and collisions between this sea ice and icebergs around the LHB. Continuous measurements of infrasound in the Antarctic may serve as a proxy for monitoring the regional surface environment in terms of climate change at high southern latitudes.

 

Heat production estimates from a global geochemical dataset: A priori constraints on Antarctic heat production

Derrick Hasterok1, Matthew Gard1, Grant Cox1, Martin Hand1

1University Of Adelaide, Adelaide, Australia

Radiogenic heat production is one of the greatest uncertainties in thermal models of the lithosphere.  The difficulty in sensing radiogenic heat production using remote sensing techniques has been an impediment to developing accurate thermal models of the continental lithosphere.  Most studies simply assume heat production values based on an average continental lithospheric composition, but heat production varies considerably from region to region.  In this study, we present heat production estimates from >200,000 whole-rock analyses distributed globally.  We find that patterns of heat production fundamentally differ between igneous and sedimentary rocks as a function of major element chemistry and physical properties.  For igneous samples, heat production can be correlated to seismic velocity and density, which can be used as a proxy for estimating heat production with depth. Systematic variations in heat production also exist in space and with crystallization age that must also be accounted for when developing crustal models.  We demonstrate how this method can be used to estimate the heat production of the Australian lithosphere, which produces results similar to independent estimates of heat production derived from thermal isostatic methods.  From this dataset, it is possible to develop a set of predictors for heat production of Antarctic terranes both laterally and vertically.

Deriving Antarctic crustal heat production using gamma ray spectrometry

Martin Hand1, Jacqueline Halpin2, Derrick Hasterok1, Sandra McLaren3, Tom Raimondo4

1University of Adelaide, Adelaide, Australia, 2Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Australia, 3University of Melbourne, Melbourne, Australia, 4University of South Australia, Adelaide, Australia

Current Antarctic geothermal heat flux (GHF) models employ simple representations of lithospheric composition that do not yet capture the significant heterogeneity known from geological studies. Radiogenic heating within the crust from decay of naturally occurring radioactive elements, which varies as a function of the geological evolution of a terrane, is particularly poorly resolved, and yet this component can dominate the total surface GHF.

 

Heat production estimates are derived from the main heat producing element (HPE) content (i.e. U-Th-K) of a rock. Generally these HPE concentrations are determined using geochemical methods, and while fairly routine, these techniques are both expensive (~AUD $200/sample) and destructive.

 

We aim to augment the current Antarctic heat production inventory based on compiled geochemical analyses, with a new dataset of heat production estimates using a novel non-destructive method. The U-Th-K content of samples will be determined using a calibrated gamma ray spectrometer containing a high-density bismuth germanate detector in a lead-lined analytical cavity. This system is already in operation at the universities of Adelaide and South Australia and we are establishing a further facility at the University of Tasmania. Using this technique, we will analyse the heat production of legacy rock samples that reside in collections across Australia, encompassing a very high proportion of the entire basement evolution of East Antarctica, at a fraction of the cost of conventional geochemical methods.

 

Using this technique we expect to generate an unprecedented volume of heat production data, which will represent a major advance in characterising the natural variability of radiogenic heat production in Antarctic crust for use in future GHF models.

A new heat flux model for the Antarctic Peninsula incorporating variable crustal radiogenic heat production

Alex Burton-Johnson1, Jacqueline Halpin2, Joanne Whittaker2, Felicity Graham2, Sally Watson2

1British Antarctic Survey, Cambridge, United Kingdom, 2Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Australia

We present findings recently published in GRL (Burton-Johnson et al., 2017) on the variability of Antarctic sub-glacial heat flux and the impact from upper crustal geology.

A new method reveals that the upper crust contributes up to 70% of the Antarctic Peninsula’s subglacial heat flux, and that heat flux values are more variable at smaller spatial resolutions than geophysical methods can resolve. Results indicate a higher heat flux on the east and south of the Peninsula (mean 81 mWm-2) where silicic rocks predominate, than on the west and north (mean 67 mWm-2) where volcanic arc and quartzose sediments are dominant. Whilst the data supports the contribution of heat producing element-enriched granitic rocks to high heat flux values, sedimentary rocks can be comparable dependent on their provenance and petrography. Models of subglacial heat flux must utilize a heterogeneous upper crust with variable radioactive heat production if they are to accurately predict basal conditions of the ice sheet. Our new methodology and dataset facilitate improved numerical model simulations of ice sheet dynamics.

The most significant challenge faced remains accurate determination of crustal structure, particularly the depths of the heat producing element-enriched sedimentary basins and the sub-glacial geology away from exposed outcrops. Continuing research (particularly detailed geophysical interpretation) will better constrain these unknowns and the effect of upper crustal geology on the Antarctic ice sheet.

 

The GeoMAP dataset of Antarctic rock exposures

Simon Cox1, Matilda Ballinger2, Jacqueline Halpin2, SCAR GeoMAP Action Group3

1GNS Science, Dunedin, New Zealand, 2Institute for Marine and Antarctic Studies, University Of Tasmania, Hobart, Australia, 3www.scar.org/ssg/geosciences/geomap,

The SCAR GeoMap action group has been building a detailed digital geological dataset of Antarctica. We have been capturing existing geological map data, refining its spatial reliability, improving representation of glacial sequences and geomorphology.  The initiative is aimed towards continent-wide perspectives and for cross-discipline use, our international team is collaboratively classifying and describing around 72,000 distinct areas that cover 51,000 km2. The dataset will describe ‘known geology’ of rock exposures rather than ‘interpreted’ sub-ice features. Glacial deposits are an important focus for their potential to contain records of ice fluctuations of relevance to climate change.  Here we present background on: (1) Degree of completion toward the first version of a continent-wide dataset. All rock outcrops will have geological attributes assigned to them in GeoSciML suitable for use at 1:250,000 (or more-regional) scale. (2) The large number of hard-copy geological maps and data sources, which range in scale and quality. (3) Development of local legends, which highlight geological variation across the region. (4) Progress towards a unified classification scheme. (5) Bibliographic links referencing authors of key original work. (6) Potential for the dataset to provide fresh perspectives, for example, through combined geological legends and interrogation of continent-wide time-space plots. It is our expectation that the dataset will be ideal to constrain and develop models of heat flow in the Antarctic continent.

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