Лёд и Снег • 2012 • № 4 (120)
УДК 551.324.54
Geodetic observation and interpretation of ice flow velocities in the southern part
of subglacial Lake Vostok
© 2012 г. A. Richter1, D.V. Fedorov2, S.V. Popov3, M. Fritsche1, V.Ya. Lipenkov4, A.A. Ekaykin4,
V.V. Lukin4, A.Yu. Matveev2, R. Dietrich1
1Technische Universitat Dresden, Institut ffir Planetare Geo^sie, Dresden, Germany; 2OAO «Aerogeodeziya», St. Petersburg; 3Polar Marine Geosurvey Expedition, St. Petersburg; 4Arctic and Antarctic Research Institute St. Petersburg andreas.richter@tu-dresden.de
Статья принята к печати 3 сентября 2012 г.
Accumulation, Antarctica, flow velocity, flux gate, GPS, mass balance.
Аккумуляция, Антарктида, баланс массы, скорость течения, створ ледяного потока, GPS.
Results of geodetic in-situ observations of ice-flow velocities in the southern part of subglacial Lake Vostok are combined with data sets of the ice surface topography, ice thickness, surface accumulation, basal accretion and firn/ice density for interpretations regarding the glaciological setting of the Lake Vostok system. Based on the ice-flow velocities and the ice thickness, mean surface accumulation rates are derived applying the flux gate method. These are representative for surface segments extending from the southern part of Lake Vostok to the Ridge B ice divide. They are consistent with the present-day accumulation rate at Vostok station and its variation upstream and thus suggest that the area has been close to steady state. In addition, ice-flow dynamics are investigated along a flow line segment extending from 26 km upstream to 12 km downstream from Vostok station. The analysis suggests deficiencies in current modelling approaches within the transition zone from floating to grounded ice.
Introduction
The results derived from the deep drilling at Vostok station, and the recent penetration to subglacial Lake Vostok, have impressively demonstrated the wealth and diversity of scientific questions for the answer of which glaciology can play a central role. In this context, geodetic observations and glaciological interpretation are closely linked in the particular setting of the Lake Vostok system. In 2001, an intensive cooperation between the geodesists of Technische Uni-versMt Dresden (Germany) and Aerogeodeziya (St. Petersburg), along with the glaciologists from the Arctic and Antarctic Research Institute (St. Petersburg) and the geophysicists of the Polar Marine Geosurvey Expedition (St. Petersburg) and with logistic support of the Russian Antarctic Expedition (RAE, St. Petersburg) was begun with the aim to contribute geodetic results to the exploration of Lake Vostok and its glaciological setting. This cooperation has been and will be continued, including joint field work in the Lake Vostok region, also in the upcoming years.
A brief overview of the results achieved so far by the joint efforts is given in the following section. Section 3 describes the observational data available to date for investigations in the southern part of Lake Vostok. Section 4 comprises two examples for the application of these data with regard to questions of glaciological relevance: First, mean accumulation rates are inferred from flux gates across the ice-flow direction which is representative for surface segments extending from the southern part of Lake Vostok to the Ridge B ice divide. Second, the flow dynamics in the
transition zone from floating to grounded ice is investigated along a flow line segment extending from 26 km upstream to 12 km downstream from Vostok station.
Previous works — a review
Ice surface geometry. One geodetic contribution to the investigation of subglacial Lake Vostok consists in the precise determination of the geometry of the ice surface in the region. A refined analysis of satellite radar altimetry data of the ERS-1 mission [9] yielded a high-resolution digital elevation model (DEM) of the ice surface in the Lake Vostok area [23]. More recently, a regional crossover analysis was performed on the satellite laser altimetry data of the ICESat mission [25] resulting in a new regional DEM of the ice surface [13]. Compared to the radar altimetry data, the ICESat data set is characterized by:
a higher accuracy of the altimeter measurements; a smaller footprint diameter (60-70 m vs. > 2.5 km); laser altimeter measurements free of the systematic effect of signal penetration (volume backscatter); high along-track resolution (172 m vs. 350 m); a wider across-track spacing (up to 20 km vs. 2 km). The last fact implies an additional increase in the uncertainty of the DEM within the meshes between the satellite tracks, where the elevation values are obtained by interpolation.
Both models were combined to a hybrid DEM [13]. At each ICESat measurement point, the height difference of the interpolated elevation from the radar-derived DEM minus the laser altimeter measurement was determined. Over the lake
area, this difference is very homogeneous (standard deviation 40 cm). The radar-derived elevations are, on average, 1.19 m lower than the laser altimetry data, which is interpreted as the effect of signal penetration of the radar altimeter measurements. The difference (radar minus laser) was interpolated between the ICESat tracks and applied as a correction to the elevations of the radar-derived DEM. In this way, the hybrid DEM was obtained which combines the high spatial resolution of the radar altimetry data with the accuracy of the laser measurements.
This DEM reveals interesting details on the surface relief in the area of Lake Vostok. Of particular interest are the striking troughs along the western (upstream), and bumps along the eastern (downstream) shores of the subglacial lake (see also [19]). The DEM, along with an ice-thickness model and a regional geoid model, was furthermore applied for a quantitative evaluation of the hydrostatic equilibrium of the ice sheet above Lake Vostok [13]. This study showed that most of the ice surface above the subglacial lake fulfils the hydrostatic equilibrium within a few meters. A large positive violation (+10 m) of the hydrostatic equilibrium is found within a fringe along the shore line, which would allow to map a generalized lake contour. Further, local violations of the hydrostatic equilibrium correspond to locations where the ice sheet is grounded on small bedrock islands. Finally, the HE examination revealed an area of anomalous, anthropogenic surface snow densifica-tion along the convoy track from Vostok station towards Mirny.
The ice surface above Lake Vostok was also used as a calibration area for the determination of ICESat laser operational period biases [13]. The relative systematic biases can now be applied as corrections to the ICESat data from other ice covered regions to separate this systematic effect from real temporal ice-surface elevation changes.
Ice flow velocities. Another geodetic result of high glacio-logical relevance consists in the accurate determination of ice flow velocities. Based on repeated GNSS (Global Navigation Satellite Systems, comprising to date GPS and GLO-NASS) observations on surface markers, horizontal velocity components were derived [21]. These velocity components are given relative to bedrock as the local velocity contribution of the rotation of the Antarctic tectonic plate was subtracted. The first of these in-situ observations, commenced in 2001 (47. RAE), were restricted to the southern part of Lake Vostok [31]. The participation of geodesists in the continental traverses conducted by RAE allowed extending these observations to the central and northern parts of the lake [22]. To date (including observations up to 2011, 56 RAE), surface flow velocities are determined for a total of 50 markers in the Lake Vostok region (Fig. 1). The accuracy of the velocity components depends essentially on the time span between the first and the last observation on the marker. For all of the 50 markers this time span amounts to one year or more; and accuracies of typically 10 mm/a (flow velocity) and 0.5° (flow direction) were achieved [22].
The observed ice-flow velocities were used to validate ice-flow models in the Lake Vostok region. At present, ice-dynamic models [18, 27] poorly predict the flow velocity field over the subglacial lake. On the other hand, especially in the southern
part of the lake, the flow velocity directions derived from the GNSS observations agree well with the flow trajectories inferred by structure tracking from radio-echo sounding data [28]. The surface velocities observed on GNSS markers along the flow line through Vostok station (VFL; e.g. [12]) were also used to derive constraints for the transit time of an ice particle across the lake and the rate of basal ice accretion [22, 31].
Furthermore, the observed velocity components form the basis for the determination of horizontal deformations at the surface of the ice sheet by means of a strain analysis [20]. Around Vostok station, a polygon of GNSS markers was established and four times (2001-2011) observed for a detailed monitoring of the surface deformations in the surroundings of the 5r drilling site [31]. Our results indicate that the surface deformation in the area of Vostok station is characterized by extension both along and across the ice flow direction. This agrees with the large scale surface deformations observed in the southern part of the lake, and there are no indications for an additional local effect around Vostok station, e.g. related to the 5r borehole. The flow velocities and deformation rates derived from the GNSS observations were combined with ice thickness data inferred from radio-echo sounding [6] and gla-ciological data to determine the local ice mass balance at Vostok station [20]. For the area of the deformation polygon, a change of the ice mass over time close to zero was revealed.
Surface height changes. The observation of changes in the surface height over Lake
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