Uncertainty Assessment of Ice Discharge Using GPR-Derived Ice Thickness from Gourdon Glacier, Antarctic Peninsula

Ice cliffs within a glacier represent a challenge for the continuity equations used in many glacier models by interrupting the validity of input parameters. In the case of Gourdon Glacier on James Ross Island, Antarctica, a ∼300–500 m high, almost vertical cliff, separates the outlet glacier from its main accumulation area on the plateau of the island. In 2017 and 2018 we conducted ice thickness measurements during two airborne ground penetrating radar campaigns in order to evaluate differences to older measurements from the 1990s. The observed differences are mostly smaller than the estimated error bars. In comparison to the in situ data, the published “consensus ice thickness estimate” strongly overestimates the ice thickness at the outlet. We analyse three different interpolation and ice thickness reconstruction methods. One approach additionally includes the mass input from the plateau. Differences between the interpolation methods have a minor impact on the ice discharge estimation if the used flux gates are in areas with a good coverage of in situ measurements. A much stronger influence was observed by uncertainties in the glacier velocities derived from remote sensing, especially in the direction of the velocity vector in proximity to the ice cliff. We conclude that the amount of in situ measurements should be increased for specific glacier types in order to detect biases in modeled ice thickness and ice discharge estimations

Dynamics of the ice cap on King George Island, Antarctica: field measurements and numerical simulations

ABSTRACT. King George Island is located at the northern tip of the Antarctic Peninsula, which is influenced by maritime climate conditions. The observed mean annual air temperature at sea level is -2.

Geophysikalische Prospektion

Mit dem Beginn der neuen Grabungsserie in Troia 1988 wurde als wichtiges Teilprojekt die genauere Untersuchung des ca. 0,75 km2 umfassenden Gebiets südlich und östlich des Burgbergs in Angriff genommen, das schon bei den früheren Grabungen durch etliche Sondagen als Siedlungsfläche der Unterstadt von Troia identifiziert worden war. Flankierend zu einigen gezielten Grabungsflächen wurden verschiedene geophysikalische Meßmethoden zur möglichst vollständigen Erfassung der weitgehend unter der Oberfläche verborgenen archäologischen Baureste erprobt. Dabei erwies sich die flächendeckende Messung von lokalen Veränderungen des Erdmagnetfeldes durch menschliche Einwirkung als die effektivste Methode. Zwischen 1989 und 2007 wurde der überwiegende Teil der Siedlungsfläche mit einer Dichte von im Mittel acht Messpunkten pro m2 durch Teams von vier verschiedenen Institutionen gemessen. Die Auswertung ergab klare Vorstellungen des orthogonalen hellenistisch/römischen Straßensystems bis hin zu einzelnen Gebäudefundamenten, des Be- und Entwässerungssystems und des Verlaufs der hellenistischen Stadtmauer. Außerdem konnte über ca. 700m der Verlauf eines in den Fels geschlagenen bis zu 3m breiten und etwa 1,5m tiefen Grabens aus der Spätbronzezeit (Troia VI/VII) geortet werden, der als Annäherungshindernis zum Schutz der Untersiedlung interpretiert wird. Die Befunde wurden durch gezielte Grabungssondagen verifiziert.Planning for the renewal of excavations at Troia in 1988 included as an important project the detailed investigation of the approx. 0.75 km2 large area to the south and southeast of the citadel mound, which in previous excavations had already been identified as the lower settlement area of Troia. In conjunction with some excavation soundings several geophysical methods were tried for an almost complete registration of the archaeological features mostly buried under the surface. Measurement of the local variations of the magnetic field of the earth as caused by human interaction proved to be the most effective method. Between 1989 and 2007 most of the settlement area was measured with a density of eight measurement points per m2 by teams from four different institutions. Evaluation of the measurements resulted in a clear view of the orthogonal Hellenistic/Roman street-system including details of single building structures, parts of the irrigation and sewage system and the main course of the Hellenistic citywall. In addition a rock-cut ditch of up to 3m width and 1.5 m depth originating in the late Bronze Age could be spotted for a length of approx. 700 m. The ditch is interpreted as a defense ditch around the lower settlement. The identified features were verified by selected excavation soundings

In-situ ice thickness measurements derived with ground penetrating radar from Gourdon Glacier, Antarctic Peninsula

These data report in-situ ground penetrating radar (GPR) ice thickness measurements at the outlet and the catchment area of Gourdon Glacier on James Ross Island, northern Antarctic Peninsula. Data were conducted in 2017-02-15 and 2018-02-10 with a 25 MHz GPR antenna flown as a sling load by a Bell 212 helicopter. The processing of the GPR data was done in the software REFLEXW v.8.5 (Sandmeier geophysical research) and includes interpolation to equidistant traces, bandpass filtering (10 MHz lower cutoff, 50 MHz upper cutoff), correction to real start time, application of a gain function, and a 2D-finite-difference migration. The travel-time in ice was converted to ice-thickness in meter ["IceThickness(m)"] by applying a constant velocity of 168 m/µs. In case of data from the plateau, a standard correction value for firn and snow of +10 m is applied. The associated error resulting from the GPR measurements ["IceThickness_GPR_error(m)"] and the error additional considering the horizontal positioning accuracy ["IceThickness_Data_error(m)"] are calculated after Lapazaran et al. (2016). Further details about the processing and the error calculation are described in Lippl et al. (in submission). In addition, the surface elevation in reference to the WGS84 ellipsoid [SurfaceElevation(m)] derived from the GNSS signal and the GPR backscatter is included. However, surface elevations can be prone to errors of several meters due to the large foodprint of the radar and the GNSS accuracy

Mass balance measurements on King George Island ice cap in 2007 and 2008

The Antarctic Peninsula has been identified as a region of rapid on-going climate change with impacts on the cryosphere. The knowledge of glacial changes and freshwater budgets resulting from intensified glacier melt is an important boundary condition for many biological and integrated earth system science approaches. We provide a case study on glacier and mass balance changes for the ice cap of King George Island. The area loss between 2000 and 2008 amounted to about 20 km**2 (about 1.6% of the island area) and compares to glacier retreat rates observed in previous years. Measured net accumulation rates for two years (2007 and 2008) show a strong interannual variability with maximum net accumulation rates of 4950 mm w.e./a and 3184 mm w.e./a, respectively. These net accumulation rates are at least 4 times higher than reported mean values (1926-95) from an ice core. An elevation dependent precipitation rate of 343 mm w.e./a (2007) and 432 mm w.e./a (2008) per 100 m elevation increase was observed. Despite these rather high net accumulation rates on the main ice cap, consistent surface lowering was observed at elevations below 270 m above ellipsoid over an 11-year period. These DGPS records reveal a linear dependence of surface lowering with altitude with a maximum annual surface lowering rate of 1.44 m/a at 40 m and -0.20 m/a at 270 m above ellipsoid. These results fit well to observations by other authors and surface lowering rates derived from the ICESat laser altimeter. Assuming that climate conditions of the past 11 years continue, the small ice cap of Bellingshausen Dome will disappear in about 285 years

Measured ice surface velocities on the King George Island ice cap with DGPS

King George Island is located at the northern tip of the Antarctic Peninsula, which is influenced by maritime climate conditions. The observed mean annual air temperature at sea level is -2.4°C. Thus, the ice cap is regarded as sensitive to changing climatic conditions. Ground-penetrating radar surveys indicate a partly temperate ice cap with an extended water layer at the firn/ice transition of the up to 700 m high ice cap. Measured firn temperatures are close to 0°C at the higher elevations, and they differ considerably from the measured mean annual air temperature. The aim of this paper is to present ice-flow dynamics by means of observations and simulations of the flow velocities. During several field campaigns from 1997/98 to 2008/09, ice surface velocities were derived with repeated differential GPS measurements. Ice velocities vary from 0.7 m/a at the dome to 112.1 m/a along steep slopes. For the western part of the ice cap a three-dimensional diagnostic full-Stokes model was applied to calculate ice flow. Parameters of the numerical model were identified with respect to measured ice surface velocities. The simulations indicate cold ice at higher elevations, while temperate ice at lower elevations is consistent with the observations