Description: 3000 m of ice sheet thickness has ensured that central Greenland has kept it geothermal heat flow (GHF) distribution enigmatic. Some few direct ice temperature measurements from deep ice cores reveal a GHF of 50 to 60 mW/m2 in the Summit region and this is noticeably above what would be expected for the underlying Early Proterozoic lithosphere. In addition, indirect estimates from zones of rapid basal melting suggest extreme anomalies 15 to 30 times continental background. Subglacial topography indicates caldera like topographic features in the zones hinting at possible volcanic activity in the past [1], and all of these observations combined hint at an anomalous lithospheric structure. Further supporting this comes from new high-resolution P-wave tomography, which shows a strong thermal anomaly in the lithosphere crossing Greenland from east to west [2]. Rock outcrops at the eastern and western end of this zone indicate significant former magmatic activity, older in the east and younger in the west. Additionally, plate modelling studies suggest that the Greenland plate passed over the mantle plume that is currently under Iceland from late Cretaceous to Neogene times, consistent with the evidence from age of magmatism. Evidence of rapid basal melt revealed by ice penetrating radar along the hypocentre of the putative plume track indicates that it continues to affect the Greenland continental geotherm today. We analyse plume-induced thermal disturbance of the present-day lithosphere and their effects on the central Greenland ice sheet by using a novel evolutionary model of the climate-ice-lithosphere-upper mantle system. Our results indicate that mantle plume-induced erosion of the lithosphere has occurred, explaining caldera-type volcanic structures, the GHF anomaly, and requiring dyke intrusion into the crust during the early Cenozoic. The residual thermo-mechanical effect of the mantle plume has raised deep-sourced heat flow by over 25 mW/m2 since 60 Ma and explains the high basal melting rates of the Greenland ice sheet observed in the study area. [1] Fahnestock, M., Abdalati, W., Joughin, I., Brozena, J., Gogineni, P., 2001. High geothermal heat flow, Basal melt, and the origin of rapid ice flow in central Greenland. Science (New York, N.Y.). 294, 2338–2342. [2] Jakovlev, A.V., Bushenkova, N.A., Koulakov, I.Y., Dobretsov, N.L., 2012. Structure of the upper mantle in the Circum-Arctic region from regional seismic tomography. Russian Geology and Geophysics. 53, 963–971.

Description: Heat entering large ice sheets and glaciers from the underlying bedrock controls the thermal regime of the basal ice layers in terms of basal melting, the formation of basal temperate ice layers and its influence on sliding processes. In this study, we estimate the effect of the uncertainties in geothermal heat flux (GHF) on the topography, dynamics and basal thermal conditions of simulations of the present-day Greenland Ice Sheet (GIS), using the dynamic polythermal ice-sheet model SICOPOLIS. Today’s GHF at the base of the GIS has been measured at only the locations of the deepest ice cores. It should therefore be considered as highly uncertain forcing component for the dynamic modeling of the major part of the GIS. Three GHF models are applied as lithosphere boundary conditions driving a series of paleoclimatic simulations. The first is based on a 3-D global seismic model of the crust and upper mantle, constrained in regions where heat-flow measurements are available. The other two models are based on tectonic regionalization and a combination of tectonic regionalization and deep ice-core measurements. Although the tectonic regionalization model was originally taken as a starting point for both the seismic mantle/crust and tectonic/ice-core GHF models, the resulting heat flow distributions across the extent of the GIS differ significantly, not only by the mean values but also by their spatial patterns. As a consequence, the present-day GIS driven by the seismic GHF model is 450 meters thicker in the northern and central parts of the GIS, and 450 meters thinner in the southern part relative to the GIS forced by the tectonic/ice-core GHF model. In addition, the horizontal ice transport simulated using the seismic GHF model is 10-50% higher in the northern and western GIS, and 10-30% lower in the southern and eastern GIS than the transport resulting from the simulation driven by the tectonic/ice-core GHF model. These alterations in the GIS topography and horizontal transport induced by the two latter simulations with respect to the simulation driven by the tectonic regionalization model have the opposite tendencies. Finally, we discuss different scenarios of the evolution of the temperate base in the ice-sheet simulations driven by the seismic-based, tectonic regionalization and tectonic/ice-core GHF models. The modeled present-day temperate ice areas with non-zero basal melting cover 34%, 40.6% and 56.8% of the ice-covered area, respectively, for each of the GHF models examined. The minimum coverage of the temperate ice is reached shortly after the last glacial maximum and amounts to less than 20% of the ice-covered area, as determined using the seismic and tectonic GHF models, and about 40% when employing the tectonic/ice-core model. Although the response of the modeled ice-sheet to changing basal conditions may be influenced by the limitations in the classical polythermal approach used in SICOPOLIS (neglecting water diffusivity), we may conclude that the modeled present-day thermodynamical state of the GIS is largely sensitive to the GHF model employed

Description: Since the International Geophysical Year (1957), a view has prevailed that East Antarctica has a relatively homogeneous lithospheric structure, consisting of a craton-like mosaic of Precambrian terranes, stable since the Pan-African orogeny 500 million years ago (e.g. Ferracioli et al. 2011). Recent recognition of a continental-scale rift system cutting the East Antarctic interior has crystallised an alternative view of much more recent geological activity with important implications. The newly defined East Antarctic Rift System (EARS) (Ferraccioli et al. 2011) appears to extend from at least the South Pole to the continental margin at the Lambert Rift, a distance of 2500 km. This is comparable in scale to the well-studied East African rift system. New analysis of RadarSat data by Golynsky & Golynsky (2009) indicates that further rift zones may form widely distributed extension zones within the continent. A pilot study (Vaughan et al. 2012), using a newly developed gravity inversion technique (Chappell & Kusznir 2008) with existing public domain satellite data, shows distinct crustal thickness provinces with overall high average thickness separated by thinner, possibly rifted, crust. Understanding the nature of crustal thickness in East Antarctica is critical because: 1) this is poorly known along the ocean–continent transition, but is necessary to improve the plate reconstruction fit between Antarctica, Australia and India in Gondwana, which will also better define how and when these continents separated; 2) lateral variation in crustal thickness can be used to test supercontinent reconstructions and assess the effects of crystalline basement architecture and mechanical properties on rifting; 3) rift zone trajectories through East Antarctica will define the geometry of zones of crustal and lithospheric thinning at plate-scale; 4) it is not clear why or when the crust of East Antarctica became so thick and elevated, but knowing this can be used to test models of Cenozoic ice sheet formation and stability. References Chappell, A.R. & Kusznir, N.J. 2008. Three-dimensional gravity inversion for Moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction. Geophysical Journal International, 174 (1), 1–13. Ferraccioli, F., Finn, C.A., Jordan, T.A., Bell, R.E., Anderson, L.M. & Damaske, D. 2011. East Antarctic rifting triggers uplift of the Gamburtsev Mountains Nature, 479, 388–392. Golynsky, A.V. & Golynsky, D.A. 2009. Rifts in the tectonic structure of East Antarctica (in Russian). Russian Earth Science Research in Antarctica, 2, 132–162. Vaughan, A.P.M., Kusznir, N.J., Ferraccioli, F. & Jordan, T.A.R.M. 2012. Regional heat-[U+FB02]ow prediction for Antarctica using gravity inversion mapping of crustal thickness and lithosphere thinning. Geophysical Research Abstracts, 14, EGU2012–8095.

Description: We present a new parameterization of surface mass balance (SMB) of the Greenland Ice Sheet (GIS) under interglacial climate conditions validated against recent satellite observations on a regional scale. Based on detailed analysis of the modeled surface melting and refreezing rates, we conclude that the existing SMB parameterizations fail to capture either spatial pattern or amplitude of the observed surface response of the GIS. This is due to multiple simplifying assumptions adopted by the majority of modeling studies within the frame of the positive degree day method.Modeled spatial distribution of surface melting is found to be highly sensitive to a choice of daily temperature standard deviation (SD) and degree-day factors, which are generally assumed to have uniform distribution across the entire Greenland region. However, the use of uniform SD distribution and the range of commonly used SD values are absolutely unsupported by the ERA-40 and ERA-Interim climate data. In this region, SD distribution is highly inhomogeneous and characterized by low amplitudes during the summer months in the areas where most surface ice melting occurs. In addition, the use of identical degree day factors on both the eastern and western slopes of the GIS results in overestimation of surface runoff along the western coast of Greenland and significant underestimation along its eastern coast. Our approach is to make use of (i) spatially and seasonally variable SDs derived from ERA-40 and ERA-Interim time series, and (ii) spatially variable degree-day factors, measured across Greenland, Arctic Canada, Norway, Spitsbergen and Iceland. We demonstrate that the new approach is extremely efficient for modeling the evolution of the GIS during the observational period and the entire Holocene interglacial.

Description: At the Earth’s surface, heat fluxes from the interior are generally insignificant when compared with fluxes from the sun and atmosphere; however, in areas permanently blanketed by ice these become very important. Modelling studies show that they are key to understanding the internal thermal structure of ice sheets and the distribution of melt water at their bases, information which is crucial for planning deep ice drilling campaigns and climate reconstructions. Unfortunately, the challenging conditions in ice-covered regions make measurement difficult in exactly the places where it is needed most. Until now, proxy methodologies have been considered best for determining geothermal heat flux (GHF) beneath ice sheets. Our method is to use a novel interdisciplinary approach, integrating a time-evolved climate-ice-lithosphere coupled model with a wide range of data such as direct ice-core measurements, past climate reconstructions and indirect estimates of the lithospheric thermal state. Here we show that the oldest (and thickest) part of the Greenland Ice Sheet (GIS) is strongly thermally influenced by both GHF increasing from west to east and glaciation-induced perturbations of the thermal structure of the upper crust. A pronounced lateral gradient in GHF across the Summit region of the GIS is due to anomalously thin lithosphere, which has only about 25 to 66% of the thickness typical for Archaean to early Proterozoic areas. Our findings suggest that the thermal basal conditions of the present-day central GIS are characterized by surprising rapid lateral variations in ice temperatures of up to 12ºC along relatively small distances of 100 to 150 km. We reveal two areas of rapid basal melt in central Greenland, only one of which was previously predicted by ice-penetrating radar measurements and age-depth relations from internal layering (Fahnestock et al. [2001]). The endothermic phase transition associated with rapid basal ice melt is found to increase subglacial heat flow in the uppermost layers of the crust by a factor of three to values well above 100 mW/m2. Fahnestock, M., Abdalati, W., Joughin, I., Brozena, J. & Gogineni, P. High geothermal heat flow, Basal melt, and the origin of rapid ice flow in central Greenland. Science 294, 2338–2342 (2001)