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Beiträge aus geophysikalischen Messungen in Dronning Maud Land, Antarktis, zur Auffindung eines optimalen Bohrpunktes für eine Eiskerntiefbohrung = Contributions of geophysical measurements in Dronning Maud Land, Antarctica, locating an optimal drill site for a deep ice core drilling
Steinhage, D. (2001). Beiträge aus geophysikalischen Messungen in Dronning Maud Land, Antarktis, zur Auffindung eines optimalen Bohrpunktes für eine Eiskerntiefbohrung = Contributions of geophysical measurements in Dronning Maud Land, Antarctica, locating an optimal drill site for a deep ice core drilling. Ber. Polarforsch. Meeresforsch. 384: 1-91
In: Berichte zur Polar- und Meeresforschung = Reports on Polar and Marine Research. Alfred-Wegener-Institut für Polar- und Meeresforschung: Bremerhaven. ISSN 1618-3193, more
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    Marine

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  • Steinhage, D.

Abstract
    For the assessment of the future climate evolution of the earth a detailed knowledge of the climate system in the past is most important. Paleoclimatic information is incorporated into the large ice sheets of Greenland and Antarctica together with the annual deposition of snow. The air circulating in the snow cover is enclosed and preserved within small bubbles in the ice during the transformation of firn to ice. This paleoatmospheric archive is an unique feature of ice cores compared to any other paleoclimatic archive. Using physical and chemical analyses on the ice and the enclosed air various proxy parameters can be measured to reconstruct paleoclimatic information. To gain a reliable record, however, it is necessary that the original sequence of the ice layers is preserved and is not disturbed by any dynamic process within the ice sheet. An optimum site for a deep ice core drilling should be located in an area with no or nearly negligible lateral movement and absence of any basal melting as well as an undisturbed stratification and a sufficient large ice thickness. Another criterion is given by the source regions of air masses influencing the drill site and the annual accumulation rate, determining to large parts the temporal resolution to be achieved in an ice core record. The comparison of ice cores from Greenland and Antarctica revealed that rapid climatic changes during the last glacial period (Dansgaard-Oeschger events) are not reflected the same way in the northern and southern hemisphere. Also the temporal coupling between the two hemispheres during climatic changes is still unsufficiently known. These are two of the most important questions to be answered by two new ice cores to be drilled within the European Project for Ice Coring in Antarctica (EPICA). The first of these ice core drillings is currently carried out at Dome Concordia (123.1°East/75.15°South). The second will be in Dronning Maud Land (D ML), which due to the influence of air masses originating over the South Atlantic, is expected to be a direct Antarctic counterpart to the well known ice core records from central Greenland. In order to determine the actual drill location in DML an extensive airborne geophysical programme has been carried out. In the following the results of this reconaissance study will be presented. It has been shown in the past that radio echo sounding (RES) is capable of mapping ice thicknesses as well as the internal structure of ice sheets, ice shelves and glaciers with very high resolution. Using airborne RES it is possible to cover large areas in a short period. In chapter 2 the physical and technical principles of RES and data evaluation will be introduced. The data analysis includes all profiles with a total length of 91,500 km flown during the austral summer 1995-99 in DML. Maps of the ice thickness, internal structure of the ice sheet and the subglacial topography were derived and are presented and discussed in chapter 3. The area covered in DML contains 1,040,000 km², which is equal to 7.5 % of the continent. The average ice thickness is 1678 m. This is 288 m more than previously computed based on earlier sparse seismic and airborne RES measurements. The subglacial topography of large areas in the western part as well as north of the coastal mountains of the investigated region are below present sea level. Several internal horizons could be mapped in the vicinity of shallow ice core drilling B32 (±0°/75° South), showing all undisturbed stratification of the ice. Using simple model conceptions, known in the literature, as well as new data on snow accumulation and surface topography further glaciological parameters relevant for the selection of the drill site could be derived such as the balance velocity and isochrones. Surface velocities of the ice have been estimated from the balance velocities. The lowest velocities are observed along the ice divides. Usually they do not exceed 2 ma-1. Based on the good correlation of modelled isochrones and inter- nal horizons it can be concluded that basal melting does not occur along the profile chosen for the comparison. Based on all the information gained during this presite survey (ice thickness map derived from the airborne RES measurements, given surface topography and accumulation distribution, modelled balance velocities and isochrones) the best drill site in DML has been determined to be in the vicinity of B32 at ±0°and 75° South. The ice thickness in that area is 2750 m, which is sufficiently large to drill an ice core covering the last glacial cycle. The accumulation rate is 0.065 m ice equivalent a-1, which is three times higher than at Dome Concordia. The estimated surface velocities is lower than 1.5 ma-1. Based on the presented data and the age model by Nye the height of the 110,000 a isochrone is 475 m above the relatively smooth bedrock. Therefore it should be possible to achieve an undisturbed high-resolution ice core record over the full last glacial cycle in DML in the vicinity of B32.

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