Description: Author(s): M. Beck, M. Klammer, I. Rousseau, M. Obergfell, P. Leiderer, M. Helm, V. V. Kabanov, I. Diamant, A. Rabinowicz, Y. Dagan, and J. Demsar In the conventional theory of superconductivity the critical temperature Tc is determined by the electron-phonon coupling constant and the phonon cut-off frequency. The hallmark experiments of McMillan and Rowell demonstrated that bosons (phonons) responsible for pairing can be observed through the frequency dependence of the gap parameter. Determination of the electron-boson coupling strength in high- T c cuprates is, however, not an easy task. One of the promising ways is to measure the energy relaxation rate of photoexcited carriers by using femtosecond real-time techniques. Here, considering the electron relaxation process within the conduction band, it is commonly assumed that the underlying Eliashberg electron-boson coupling function is independent of electron excess energy. Conversely, studies of light-induced suppression of superconductivity in Pr 1 . 85 Ce 0 . 15 CuO 4 reported here imply a strong variation of the electron-boson coupling function on electron energy. Considering the competing scenarios of superconductivity being mediated by either phonons or magnetic excitations, the results suggest that high-energy electrons strongly couple either to phonons or magnetic modes, while the situation is reversed when considering pairing of low-energy electrons. [Phys. Rev. B 95, 085106] Published Fri Feb 03, 2017

Description: We report the discovery of a large impact crater beneath Hiawatha Glacier in northwest Greenland. From airborne radar surveys, we identify a 31-kilometer-wide, circular bedrock depression beneath up to a kilometer of ice. This depression has an elevated rim that cross-cuts tributary subglacial channels and a subdued central uplift that appears to be actively eroding. From ground investigations of the deglaciated foreland, we identify overprinted structures within Precambrian bedrock along the ice margin that strike tangent to the subglacial rim. Glaciofluvial sediment from the largest river draining the crater contains shocked quartz and other impact-related grains. Geochemical analysis of this sediment indicates that the impactor was a fractionated iron asteroid, which must have been more than a kilometer wide to produce the identified crater. Radiostratigraphy of the ice in the crater shows that the Holocene ice is continuous and conformable, but all deeper and older ice appears to be debris rich or heavily disturbed. The age of this impact crater is presently unknown, but from our geological and geophysical evidence, we conclude that it is unlikely to predate the Pleistocene inception of the Greenland Ice Sheet.

Description: Accurate quantification of the millennial-scale mass balance of the Greenland ice sheet (GrIS) and its contribution to global sea-level rise remain challenging because of sparse in situ observations in key regions. Glacial isostatic adjustment (GIA) is the ongoing response of the solid Earth to ice and ocean load changes occurring since the Last Glacial Maximum (LGM; ~21 thousand years ago) and may be used to constrain the GrIS deglaciation history. We use data from the Greenland Global Positioning System network to directly measure GIA and estimate basin-wide mass changes since the LGM. Unpredicted, large GIA uplift rates of +12 mm/year are found in southeast Greenland. These rates are due to low upper mantle viscosity in the region, from when Greenland passed over the Iceland hot spot about 40 million years ago. This region of concentrated soft rheology has a profound influence on reconstructing the deglaciation history of Greenland. We reevaluate the evolution of the GrIS since LGM and obtain a loss of 1.5-m sea-level equivalent from the northwest and southeast. These same sectors are dominating modern mass loss. We suggest that the present destabilization of these marine-based sectors may increase sea level for centuries to come. Our new deglaciation history and GIA uplift estimates suggest that studies that use the Gravity Recovery and Climate Experiment satellite mission to infer present-day changes in the GrIS may have erroneously corrected for GIA and underestimated the mass loss by about 20 gigatons/year.

Description: Antarctic Peninsula (AP) ice shelves have been affected by ice front retreat and surface lowering over the past decades. 12 major ice shelves have disintegrated or significantly retreated and have been affected by volume loss. Longterm ice shelf thinning is twice as high at western AP ice shelves than at eastern AP ice shelves. Wilkins Ice Shelf (WIS), located at the western AP, has undergone considerable ice front retreat since the 1990s. It lost ca. 5000 km2 of its size since then. Surface lowering at WIS was found to be the largest at AP ice shelves between 1978 and 2008. Here, we analyze time-series of satellite data in order to assess dynamic changes of WIS following the ice front retreat between 1994 and 2010. We present multi-temporal changes in surface velocities and deduced products, such as strain rate and stress regimes. Surface flow was derived from SAR intensity offset tracking applied to ALOS PALSAR image pairs. In addition, we show variations in ice thickness between 2003 and 2012 derived from TanDEM-X satellite acquisitions and altimetry datasets (CryoSAT-2, ICESat). The bistatic TanDEM-X acquisitions are very suitable for interferometric processing due to highly coherent image pairs. The results showed surface velocity speed up during break-up of an ice bridge between two confining islands in 2006-2008, when an area of ca. 1800 km2 broke off. A sharp transition between compressive and extensive in-flow strain rates evolved at the narrowest part of the ice bridge, which contributed to the formation of a crack and hence, failure of the ice bridge in April 2009. First principal stresses were estimated to amount to ca. 250 kPa in the vicinity of the crack formation. The imaging TanDEM-X radar geometry allowed for a comprehensive ice thickness mapping of the ice shelf in 2012 and resolved many details due to the high spatial resolution. The ice thickness at WIS was found to be very heterogeneous. Thickness changes between 2003 and 2012 revealed increased thickness loss in recent years 2009-2012 at the western WIS as well as partially very thin ice there (〈30 m in 2012). Decoupling of the ice shelf from the stabilizing Latady Island might be inevitable in the near future. In summary, the comprehensive analysis of WIS based on remote sensing data and derived products revealed dynamic changes after ice front retreat. The future stability of WIS might be considered weak, given considerable ice thickness loss and partially very thin ice, ice flow acceleration and the identification of developing fractures during recent years.

Description: The surface reflection of solar radiation comprises an important boundary condition for solar radiative transfer simulations. In polar regions above snow surfaces, the surface reflection is particularly anisotropic due to low Sun elevations and the highly anisotropic scattering phase function of the snow crystals. The characterization of this surface reflection anisotropy is essential for satellite remote sensing over both the Arctic and Antarctica. To quantify the angular snow reflection properties, the hemispherical-directional reflectance factor (HDRF) of snow surfaces was derived from airborne measurements in Antarctica during austral summer in 2013/14. For this purpose, a digital 180∘ fish-eye camera (green channel, 490–585 nm wavelength band) was used. The HDRF was measured for different surface roughness conditions, optical-equivalent snow grain sizes, and solar zenith angles. The airborne observations covered an area of around 1000 km × 1000 km in the vicinity of Kohnen Station (75∘0′ S, 0∘4′ E) at the outer part of the East Antarctic Plateau. The observations include regions with higher (coastal areas) and lower (inner Antarctica) precipitation amounts and frequencies. The digital camera provided upward, angular-dependent radiance measurements from the lower hemisphere. The comparison of the measured HDRF derived for smooth and rough snow surfaces (sastrugi) showed significant differences, which are superimposed on the diurnal cycle. By inverting a semi-empirical kernel-driven bidirectional reflectance distribution function (BRDF) model, the measured HDRF of snow surfaces was parameterized as a function of solar zenith angle, surface roughness, and optical-equivalent snow grain size. This allows a direct comparison of the HDRF measurements with the BRDF derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite product MCD43. For the analyzed cases, MODIS observations (545–565 nm wavelength band) generally underestimated the anisotropy of the surface reflection. The largest deviations were found for the volumetric model weight fvol (average underestimation by a factor of 10). These deviations are likely linked to short-term changes in snow properties.

Description: We derive recent surface mass balance (SMB) estimates from airborne radar observations along the iSTAR traverse (2013, 2014) at Pine Island Glacier (PIG), West Antarctica. Ground-based neutron probe measurements provide information of snow and firn density with depth at 22 locations and were used to date internal annual reflection layers. The 2005 layer was traced for a total distance of 2367 km to determine annual mean SMB for the period 2005–2014. Using complementary SMB estimates from two regional climate models, RACMO2.3p2 and MAR, and a geostatistical kriging scheme, we determine a regional-scale SMB distribution with similar main characteristics to that determined for the period 1985–2009 in previous studies. Local departures exist for the northern PIG slopes, where the orographic precipitation shadow effect appears to be more pronounced in our observations, and the southward interior, where the SMB gradient is more pronounced in previous studies. We derive total mass inputs of 79.9 +/- 19.2 and 82.1 +/- 19.2 Gt yr-1 to the PIG basin based on complementary ASIRAS–RACMO and ASIRAS–MAR SMB estimates, respectively. These are not significantly different to the value of 78.3 +/- 6.8 Gt yr-1 for the period 1985–2009. Thus, there is no evidence of a secular trend at decadal scales in total mass input to the PIG basin. We note, however, that our estimated uncertainty is more than twice the uncertainty for the 1985–2009 estimate on total mass input. Our error analysis indicates that uncertainty estimates on total mass input are highly sensitive to the selected krige methodology and assumptions made on the interpolation error, which we identify as the main cause for the increased uncertainty range compared to the 1985–2009 estimates.

Description: We report annual snow accumulation rates from 1959 to 2004 along a 250 km segment of the Expéditions Glaciologiques Internationales au Groenland (EGIG) line across central Greenland using Airborne SAR/Interferometric Radar Altimeter System (ASIRAS) radar layers and high resolution neutron-probe (NP) density profiles. ASIRAS-NP-derived accumulation rates are not statistically different (95 % confidence interval) from in situ EGIG accumulation measurements from 1985 to 2004. ASIRAS-NP-derived accumulation increases by 20 % below 3000 m elevation, and increases by 13 % above 3000 m elevation for the period 1995 to 2004 compared to 1985 to 1994. Three Regional Climate Models (PolarMM5, RACMO2.3, MAR) underestimate snow accumulation below 3000 m by 16–20 % compared to ASIRAS-NP from 1985 to 2004. We test radar-derived accumulation rates sensitivity to density using modeled density profiles in place of NP densities. ASIRAS radar layers combined with Herron and Langway (1980) model density profiles (ASIRAS-HL) produce accumulation rates within 3.5 % of ASIRAS-NP estimates in the dry snow region. We suggest using Herron and Langway (1980) density profiles to calibrate radar layers detected in dry snow regions of ice sheets lacking detailed in situ density measurements, such as those observed by the Operation IceBridge campaign.

Description: Surface melt and subsequent firn air depletion can ultimately lead to disintegration of Antarctic ice shelves1,2 causing grounded glaciers to accelerate3 and sea level to rise. In the Antarctic Peninsula, foehn winds enhance melting near the grounding line4, which in the recent past has led to the disintegration of the most northerly ice shelves5,6. Here, we provide observational and model evidence that this process also occurs over an East Antarctic ice shelf, where meltwaterinduced firn air depletion is found in the grounding zone. Unlike the Antarctic Peninsula, where foehn events originate from episodic interaction of the circumpolar westerlies with the topography, in coastal East Antarctica high temperatures are caused by persistent katabatic winds originating from the ice sheet’s interior. Katabatic winds warm and mix the air as it flows downward and cause widespread snow erosion, explaining 〉3 K higher near-surface temperatures in summer and surface melt doubling in the grounding zone compared with its surroundings. Additionally, these winds expose blue ice and firn with lower surface albedo, further enhancing melt. The in situ observation of supraglacial flow and englacial storage of meltwater suggests that ice-shelf grounding zones in East Antarctica, like their Antarctic Peninsula counterparts, are vulnerable to hydrofracturing7.