Sensitivity of ice flow to local and regional variations in geothermal heat flux

Mark Pittard1, Ben Galton-Fenzi2,3, Jason Roberts2,3, Christopher Watson4

1Department of Geography, Durham University, Durham, United Kingdom, 2Australian Antarctic Division, Kingston, Australia, 3Antarctic Climate & Ecosystems Cooperative Research Centre, Hobart, Australia, 4School of Land and Food, University of Tasmania, Hobart, Australia

Geothermal heat flux is one of the key thermal boundary conditions for simulations of ice flow. We assess the sensitivity of the Lambert-Amery glacial system within the East Antarctic Ice Sheet to both local and regional variations in geothermal heat flux using the Parallel Ice Sheet Model. Geothermal heat flux can vary locally through elevated radiogenic heat production, with heat flow modelling within the Lambert-Amery glacial system estimating localised elevated geothermal heat flux of at least 120 mWm-2. To assess the influence of these local high heat flux regions on ice flow, we insert a geothermal heat flux anomaly into the dataset beneath five different types of ice flow. Geothermal heat flux can also vary more broadly, with different techniques of estimating geothermal heat flux producing different spatial patterns of heat flux. Using four different geothermal datasets scaled to the same median heat flux we assess the importance of the spatial variation on regional ice flow.

 

The simulations show that localised high heat flow regions can significantly enhance flow in slow-moving ice, with the influence extending both upstream and downstream of the anomaly. The scaled regional simulations demonstrate this further, with the ice divides being the most sensitive to regional variations in ice flow. Additionally, the position of the onset of basal sliding, in addition to the width of the region experiencing basal sliding was dependent on the underlying geothermal heat flux. Our results suggest that localized regions of elevated geothermal heat flux may play an important role in the organisation of ice sheet flow.

 

Promising Oldest Ice sites in East Antarctica based on thermodynamical modelling

Brice Van Liefferinge1, Frank Pattyn1, Marie Cavitte2, Nanna Karlsson3,4, Duncan Young2, Johannes Sutter4,5, Olaf Eisen4,6

1Université Libre De Bruxelles, Brussels, Belgium, 2University of Texas at Austin, Austin, USA, 3Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark, 4Alfred Wegener Institute, Bremerhaven, Germany, 5University of Bern, Bern, Switzerland, 6University of Bremen, , Germany

To resolve the mechanisms behind the major climate reorganisation which occurred between 0.9 and 1.2 Ma, the recovery of a suitable 1.5 million-year-old ice core is fundamental. The quest for such an Oldest Ice core requires a number of key boundary conditions, of which the poorly known basal geothermal heat flux (GHF) is lacking. We use a transient thermodynamical 1D vertical model that solves for the rate of change of temperature in the vertical, with surface temperature and modelled GHF as boundary conditions. For each point on the ice sheet, the model is forced with variations in atmospheric conditions over the last 2 Ma, and modelled ice-thickness variations. The process is repeated for a range of GHF values to determine the value of GHF that marks the limit between frozen and melting conditions over the whole ice sheet, taking into account 2 Ma of climate history. These threshold values of GHF are statistically compared to existing GHF data sets (Shapiro and Ritzwoller, 2004; Fox-Maule et al., 2005; Puruker, 2013; An et al., 2015). The new probabilistic GHF fields obtained for the ice sheet thus provide the missing boundary conditions in the search for Oldest Ice. High spatial resolution radar data are examined locally in the Dome Fuji and Dome C regions (Karlson et al., in prep; Young et al., 2017), as these represent the ice core community’s primary drilling sites. GHF, bedrock variability, ice thickness and other essential criteria combined highlight a dozen major potential Oldest Ice sites in the vicinity of Dome Fuji and Dome C.

 

Uncertainty reduction of geothermal heat flux from assimilating seismic tomography and depth to Curie temperature

Ben Mather1, Javier Fullea1, Louis Moresi2, Thomas Farrell1, Robert Delhaye1

1Dublin Institute For Advanced Studies, Dublin, Ireland, 2The University of Melbourne, Melbourne, Australia

Surface heat flux is highly sensitive to small temperature fluctuations at varying timescales. The magnitude of these variations depend on the duration of climatic fluctuations and add significant uncertainty to present-day surface heat flow estimates. We assimilate heat flow data with multiple geophysical observations that resolve the crust in its present-day state to improve the constraints on subsurface thermal models and quantify the uncertainty in surface heat flux. The Curie depth isotherm is computed from magnetic data to constrain temperatures in the middle-lower crust, while P and S-wave velocities that we extract from tomographic models are sensitive to temperature to varying degrees throughout the lithosphere. We integrate these within an adjoint inversion framework that we apply to Southeastern Australia to invert the structure of thermal conductivity and heat sources within the crust. Based on previous inversions solely constrained by surface heat flow points and seismic velocity, we found that relatively high rates of heat production in Proterozoic crust control the variation of heat flux at the surface. This Proterozoic crust shares tectonic provenance with Antarctica and may have significant implications for its heat flow regime. Here, we will quantify the uncertainty reduction of thermal structure from assimilating Curie depth in addition to seismic velocity and heat flow observations. Our inversion framework can be easily adapted to integrate additional data types available for Antarctica to improve the precision of geothermal heat flux estimates.

 

Revealing the geothermal heat flux of the Antarctic continent

Yasmina M Martos1,2, Manuel Catalan3, Tom A Jordan4, Alexander Golynsky5, Dmitry Golynsky5, Graeme Eagles6

1NASA Goddard Space Flight Center, Greenbelt, United States, 2University of Maryland, College Park, United States, 3Royal Observatory of the Spanish Navy, San Fernando, Spain, 4British Antarctic Survey, Cambridge, United Kingdom, 5The All-Russia Scientific Research Institute for Geology and Mineral Resources of the Ocean, Saint-Petersburg, Russia, 6Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

Antarctica is the largest reservoir of ice on Earth and it contains around 70% of the world’s fresh water. Understanding its ice sheet dynamics is crucial to unraveling past global climate change and making robust climatic and sea level predictions. Of the basic parameters that shape and control ice flow, the most poorly known is geothermal heat flux. Direct observations of heat flux are difficult to obtain in Antarctica, and until now continent-wide heat flux maps have only been derived from low-resolution satellite magnetic and seismological data. We present the most advanced geothermal heat flux model and associated uncertainty derived from spectral analysis of the latest continental compilation of airborne magnetic data. Small-scale spatial variability and features consistent with known geology are better reproduced than in previous models, between 36% and 50%. Our results have the potential to contribute to more realistic and precise studies of subglacial hydrology distribution, improved ice‐core site selection, and enhance ice-sheet and sea-level modeling to better reconstruct past and predict future changes.

 

Changes in Greenland ice bed conditions inferred from seismology

Genti  Toyokuni2, Masaki Kanao1, Hiroshi Takenaka3, Ryota Takagi2, Seiji Tsuboi4, Yoko Tono5, Dean Childs6, Dapeng Zhao2

1National Institute of Polar Research, Tachikawa, Japan, 2Tohoku University, Sendai , Japan, 3Okayama University, Kita-ku, Japan, 4JAMSTEC, Kanazawa-ku, Japan, 5MEXT, Chiyoda-ku, Japan, 6PASSCAL/IRIS, Socorro, USA

Basal conditions of the Greenland Ice Sheet (GrIS) are a key research topic in climate change studies. The recent construction of a seismic network has provided a new opportunity for direct, real-time, and continuous monitoring of the GrIS. Here we use ambient noise surface wave data from seismic stations all over Greenland for a 4.5-year period to detect changes in Rayleigh-wave phase velocity between seismic station pairs. We observe clear seasonal and long-term velocity changes for many pairs, and propose a plausible mechanism for these changes. Dominant factors driving the velocity changes might be seasonal and long-term pressurization/depressurization of the GrIS and shallow bedrock by air and ice mass loading/unloading. However, heterogeneity of the GrIS basal conditions might impose strong regionalities on the results. An interesting feature is that, even at adjacent two station pairs in the inland GrIS, one pair shows velocity decrease while another shows velocity increase as a response to the high air and snow pressure. The former pair might be located on a thawed bed that decreases velocity by increased meltwater due to pressure melting, whereas the latter pair might be located on a frozen bed that increases velocity by compaction of ice and shallow bedrock. The results suggest that surface waves are very sensitive to the GrIS basal conditions, and further observations will contribute to a more direct and quantitative estimation of water balance in the Arctic region.

Constraints on Heat Flow in Antarctica based on Thermomechanical Models of the Tectonic Evolution

Audrey Huerta1

1Central Washington University, Ellensburg, United States

Results from thermomechanical models of tectonic systems can be used to constrain the magnitude and spatial variability of geothermal heat flux across continents. In the case of the Antarctic Continent, the unique geologic evolution provides stringent constraints on the past and current thermal structures of East and West Antarctica. For example, previous results of models simulating the Mesozoic to Cenozoic tectonic evolution of the West Antarctic Rift System show that the geometry and evolution of rifting is a direct consequence of the initial and evolving thermal structure of the lithosphere. Thus, this suite of successful simulations can be used to constraint the spatially and temporally varying contributions of mantle and crustal heat sources to the surface geothermal heat flux.

Thermal isostatic contributions to elevation: implications for the thermal state of Antarctica

Derrick Hasterok1, Matthew Gard1, Samuel Jennings1

1University Of Adelaide, Adelaide, Australia

Fifty years ago isostatic subsidence was proposed as a method to investigate the thermal state of the oceanic lithosphere. But it has always been more difficult to use isostatic methods on the continents due to significantly larger variations in composition, crustal thickness, and thermal properties (i.e., thermal conductivity and heat production).   The development of continent-wide seismic models of velocities and crustal thickness in recent years combined with laboratory-constrained models of physical properties as a function of composition allow for sufficiently accurate estimates of compositional buoyancy to reveal the thermal contribution to elevation. As a result isostatic methods applied to North America and Australia have shown great potential for improving estimates of the thermal state, particularly if heat production and/or sublithospheric heat flux can be constrained independently.   In this study, we will discuss the potential—and pitfalls—for applying thermal isostatic methods to predict the thermal state beneath the Antarctica ice sheet.  We will review previous constraints on the thermal state of the Antarctic lithosphere, and how these can be used with new estimates of crustal heat production and thermal conductivity to conduct thermal isostatic analysis.

 

Devil in the detail: enhanced imaging of Antarctic crustal and lithospheric provinces to aid future geothermal heat flux estimation

Fausto Ferraccioli1, Jörg Ebbing2

1Nerc/British Antarctic Survey, Cambridge, United Kingdom, 2University of Kiel, Kiel, Germany

Geothermal heat flux is a key and yet poorly understood boundary condition that can affect past, present and future ice sheet dynamics via e.g. its influence on subglacial hydrology, and sediment and basal ice deformation, and it also important to determine in the quest for retrieving the oldest ice via new deep ice core drilling. In addition to its relevance for glaciology and paleo ice sheet studies GHF is important also as both a tracer and an influence on the tectonic and magmatic evolution of the Antarctic continent.

Studies to date of Antarctic geothermal heat flux comprise e.g. estimates derived from seismology, satellite and airborne magnetics, radar-derived estimates of basal reflectivity and basal water, direct measurements via drilling and rock samples, erratics and modelling approaches. The results of these studies vary significantly both in terms of spatial distribution, spatial resolution, magnitudes and uncertainties.

The recent availability of continental scale compilations of airborne gravity, new satellite gravity gradient data and notably a new magnetic anomaly compilation (ADMAP 2.0) that includes almost 3.5 Ml line km of data provide the means to start tackling the issues surrounding GHF estimation in  additional ways too. As part of a new European Space Agency initiative ADMAP 2.0+, an extension to the 3D Earth project of ESA, which will be launched in Feb. 2018 we plan to image and model the variability in crustal and lithospheric architecture of Antarctica in unprecedented detail and also assess its implications for the spatial variability of GHF. Here we will present the approaches that the project intends to develop further. A new basement province map for a large part of East Antarctica will be compared and contrasted with the currently available estimates of GHF and several key areas where new thermal models are required will be discussed.

 

Linking GHF to crustal structures and DBMS Estimates in the Amundsen Sea Sector

Ricarda Dziadek1, Fausto Ferraccioli2, Karsten Gohl1

1Alfred Wegener Institute – Helmholtz Centre For Polar And Marine Research, Bremerhaven, Germany, 2British Antarctic Survey, Cambridge, UK

The West Antarctic Rift System is one of the least understood rift systems on earth, but displays a unique coupled relationship between tectonic processes and ice sheet dynamics. Geothermal heat flux (GHF) is a poorly constrained parameter in Antarctica and suspected to affect basal conditions of ice sheets, i.e., basal melting and subglacial hydrology. Thermomechanical models demonstrate the influential boundary condition of geothermal heat flux for (paleo) ice sheet stability. Young, continental rift systems are regions with significantly elevated geothermal heat flux (GHF), because the transient thermal perturbation to the lithosphere caused by rifting requires ~100 Ma to reach long-term thermal equilibrium. We discuss airborne, high-resolution magnetic anomaly data from the Amundsen Sea Sector, to provide additional insight into deeper crustal structures related to the West Antarctic Rift System in the Amundsen/Bellingshausen sector. With the depth-to-the-bottom of the magnetic source (DBMS) estimates we reveal spatial changes at the bottom of the igneous crust and the thickness of the magnetic layer, which can be further incorporated into tectonic interpretations.

 

GHF inferred from in-situ temperature measurements in the Amundsen Sea, West Antarctica

Ricarda Dziadek1, Karsten Gohl1, Norbert Kaul2

1Alfred Wegener Institute – Helmholtz Centre For Polar And Marine Research, Bremerhaven, Germany, 2University of Bremen, Department of Geosciences, Bremen, Germany

Due to a complex tectonic and magmatic history of West Antarctica, the region is suspected to exhibit strong heterogeneous geothermal heat flux variations. Although the maximum ice extent has retreated from the shelf since the last glacial maximum, the trends of offshore GHF patterns and the overall order of magnitude are hypothetically related to those areas onshore where the West Antarctic Ice Sheet (WAIS) rests on geologically related structures. High-resolution GHF will aid the understanding of the paleo-retreat of the ice sheet in the Amundsen Sea Sector. This presentation builds on our previous studies in which we discussed geothermal heat flux based on 26 in-situ temperature measurements that were conducted in 2010 in the Amundsen Sea Embayment (ASE) in West Antarctica. We found, that the shallow (3 m) in-situ temperature measurements were likely influenced by inter-annual bottom-water temperature variability, leading to GHF estimates biased towards lower values (mean = 33 mWm-²). During RV Polarstern expedition PS104 in early 2017 we collected additional 28 in-situ temperature measurements in marine sediments (11 m) for deriving geothermal heat flux in the ASE, which will overall improve the spatial coverage of this region. Furthermore, we monitored the vertical temperature profile of the water column at these stations, which allows to map Circumpolar Deep Water (CDW) distributions across the inner Pine Island Shelf with greater detail.

 

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