Dynamics of the Scharffenbergbotnen blue-ice area, Dronning Maud Land, Antarctica Anna SINISALO, 1,2 Aslak GRINSTED, 1,2 John MOORE 1 1 Arctic Centre, University of Lapland, P.O. Box 122, FIN-96101 Rovaniemi, Finland E-mail: anna.sinisalo@ulapland.fi 2 Department of Geophysics, University of Oulu, P.O. Box 3000, FIN-90014 Oulu, Finland ABSTRACT. Ground-penetrating radar (GPR) surveys in Scharffenbergbotnen valley, Dronning Maud Land, Antarctica, complement earlier, relatively sparse data on the ice-flow dynamics and mass-balance distribution of the area. The negative net surface mass balance in the valley appears to be balanced by the inflow. The flow regime in Scharffenbergbotnen defines four separate mass-balance areas, and about 60 times more ice enters the valley from the northwestern entrance than via the narrow western gate. We formalize and compare three methods of determining both the surface age gradient of the blue ice and the dip angles of isochrones in the firn/blue-ice transition zone: observed and dated radar internal reflections, a geometrical model of isochrones, and output from a flowline model. The geometrical analysis provides generally applicable relationships between ice surface velocity and surface age gradient or isochrone dip angle. INTRODUCTION Many Antarctic blue-ice areas (BIAs) are known to have very old ice at the surface (Whillans and Cassidy, 1983; Bintanja, 1999). However, the dating of the surface ice is still problematic. The easily recoverable ancient surface ice could be of great value for palaeoclimatic purposes if the dynamics and the internal structure of the BIAs were better known (Bintanja, 1999). Scharffenbergbotnen is the best-studied Antarctic BIA from the glaciological point of view. However, the flow regime and the surface age distribution of the area are still partially unknown. Flow models and 14 C analysis show that the age of most of the surface blue ice varies between 10 000 and 100 000 years (Van Roijen, 1996; Grinsted and others, 2003), but there are large differences in ages found by each method. No significant changes in surface mass balance have been observed over a 14 year measuring period in Scharffenbergbotnen (Sinisalo and others, 2003a). The temporal mass-balance record from the area therefore suggests that the BIA is relatively stable. However, there are large spatial variations in accumulation rates in the valley. Ground-penetrating radar (GPR) has been successfully used to study the spatial accumulation distribution else- where in Antarctica (e.g. Richardson and others, 1997; Sinisalo and others, 2003b). In this paper, we use GPR to study the spatial mass-balance distribution and the internal ice dynamics of Scharffenbergbotnen. We calculate the ages for continuous reflecting horizons taking them to be isochrones, which are then used to complement the accumulation data in the valley. There have been several attempts to model the ice flow in Antarctic BIAs (Naruse and Hashimoto, 1982; Whillans and Cassidy, 1983; Azuma and others, 1985). The ice flow has been modelled in Scharffenbergbotnen by Van Roijen (1996) and Grinsted and others (2003). The latter model is tested here by comparing the modelled isochrones with those observed by GPR near the equilibrium line between the firn and BIAs. FIELD SITE Scharffenbergbotnen, northwest Sivorgfjella, Heimefront- fjella, is a closed valley with an inflow from the surrounding ice sheet. There are two separate BIAs in the valley (Fig. 1). Although the BIAs are characterized by ablation, the surrounding glacier firn areas present positive accumulation, with equilibrium lines that approximately follow the perimeter of the BIAs. The large closed-type BIA dammed by the mountains covers the eastern part of the valley. A smaller, open-type BIA where the ice flow is not stopped by mountains forms a surface depression at the northwestern entrance. Ice apparently flows into the valley from the wide northwestern and the shallow, narrow western entrances, and from an icefall at the eastern end of the valley. The maximum ice thickness is about 1000 m (Herzfeld and Holmlund, 1990). The area is described in detail by Jonsson (1992) and Sinisalo and others (2003a). The age of the surface blue ice in Scharffenbergbotnen has been deter- mined by 14 C analysis of shallow ice cores. From this analysis the surface ice appeared to be 10  5 kyr old, though occasionally >24kyr (Van Roijen, 1996). However, the oldest surface ice is >100 kyr old according to the latest flow model (Grinsted and others, 2003). MEASUREMENTS AND METHODS The GPR survey (Fig. 1) with precise global positioning system (GPS) was made using a 50 MHz Mala ˚ Geoscience pulse radar and the same settings as described in Sinisalo and others (2003b). The objective of the survey was to track and date radar isochrones to map the mass balance in the valley, to determine the surface age gradient of the blue ice and to study the dip angles of the isochrones at the firn/blue- ice transition zone. It is essentially proven that the continuous reflecting horizons detected by GPR represent isochrones in the firn pack (Eisen and others, 2003a, b; Sinisalo and others, 2003b). The two-way travel times t tw of the radar wavelet Annals of Glaciology 39 2004 417 Downloaded from https://www.cambridge.org/core. 16 Dec 2021 at 15:22:29, subject to the Cambridge Core terms of use.