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Drilling on Crary Ice Rise, Antarctica

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Drilling on Crary Ice Rise, Antarctica 1988), the 20- or 30-year mean accumulation rate can be cal- variations with time on ice stream B (McDonald and Whillans culated. This mean accumulation rate is integrated over the 1988). catchment area as obtained from the best available map (Shab- This research was supported by National Science Founda- taie, Whillans, and Bentley 1987). The output by flow is mea- tion grants DPP 83-17235, DPP 85-17590, and DPP 87-16447. sured by the repeat tracking of Transit (also called Doppler) satellites for ground control followed by repeat aerial photog- raphy and photogrammetry. On these controlled photographs References separate crevasses are traced from epoch to epoch to obtain velocity profiles across the ice stream. Together with data on MacAyeal, DR., R.A. Bindschadler, S. Shabtaie, S.N. Stephenson, ice thickness, the discharge is calculated. The result indicates and C.R. Bentley. 1987. Force, mass and energy budgets of the Crary that ice stream B and its catchment are slowly thinning. Ice Rise complex, Antarctica. Journal of Glaciology, 33(114), 218-230. More detailed studies suggest that the thinning of ice stream McDonald, 1 . and I. Whillans. 1988. Comparison of results from B is not uniform but is especially large and irregular near the TRANSIT satellite tracking. Annals of Glaciology. 11, 83-88 transition from inland ice flow to streaming flow (Shabtaie et McDonald J . , and I.M. Whillans. 1988. Search for short-term velocity al. 1988; Whillans, Boizan, and Shabtaie 1987). In contrast, variation on ice stream "B," West Antarctica. Eos, (Abstract,) 69(16), 365. farther downstream, ice stream B appears to be thickening Shabtaie, S., C.R. Bentley, R.A. Bindschadler, and D.R. MacAyeal. (MacAyeal and others 1987). 1988. Mass-balance studies of ice streams A, B, and C, West Ant- The main portion of ice stream C is nearly stagnant (Mc- arctica, and possible surging behavior of ice stream B. Annals of Donald and Whillans 1988), as had been suspected. The in- Glaciology. 11, 137-149 terstream ridges are, in contrast, relatively steady in flow Shabtaie, S., 1. M. Whillans, and C. R. Bentley. 1987. Surface elevations (Whillans et al. 1987). on ice streams A, B, and C, West Antarctica, and their environs. The next step in the study of ice streams is to deduce the Journal of Geophysical Research, 92(139), 8865-8883. mechanics controlling their flow. Once this is understood, it Van der Veen, C.J., and I.M. Whillans. In press a. Force budget: Part may be possible to address more fully the causes for the on- I, general theory and numerical methods. Journal of Glaciology. going changes in the ice streams and ice sheet as a whole. To Van der Veen, C.J., and I.M. Whillans. In press b. Force budget, Part II, application to the Byrd Station Strain Network. Journal of Gla- this end, very complete surveys of the velocity field of ice ciology. stream B are being obtained from repeat aerial photogram- Vornberger, P.L., and I.M. Whillans. 1986. Surface features of ice metry. The results are just becoming available, but the tech- stream B, Marie Byrd Land, West Antarctica. Annals of Glaciology, niques for interpreting these data have been more fully 8, 168-170. developed through theory (Van der Veen and Whillans in press Whillans, O.M., and R.A. Bindschadler. 1988. Mass balance of ice a) and application to the Byrd Station strain network (Van der stream B, West Antarctica. Annals of Glaciology. 11, 187-193 Veen and Whillans in press b) and to Byrd Glacier (Whillans Whillans, I. M., and J . Bolzan. 1988. A method for computing shallow et al. in press). ice-core depths. Journal of Glaciology. 34(118), 355-357 Other major efforts have been the interpretation of crevasse Whillans, I. M., J . Bolzan, and S. Shabtaie. 1987. Velocity of ice streams shapes on remote imagery to infer velocity patterns (Vorn B and C, Antarctica. Journal of Geophysical Research, 92(B9), 8895- - 8902. berger and Whillans 1986), a careful study of the reproduci- Whillans, I.M., Y.H. Chen, C.J. Van der Veen, and T.J. Hughes. In bility of positions calculated using transit- satellite tracking- press. Force budget, Part III: Application to three-dimensional flow data (McDonald and Whillans 1988), and a search for velocity on Byrd Glacier. Journal of Glaciology. Drilling on Crary Ice Rise, During the 1987-1988 field season, two holes were drilled through Crary Ice Rise (83°S 170°W) to install thermistor cables. Antarctica The hot-water drill, designed by the Polar Ice Coring Office melted a hole averaging 26 centimeters in diameter at an av- R.A. BINDSCHADLER erage drilling rate of 0.5 meters per minute. Instrumentation on the drill stem included inclinometers to measure the tilt of National Aeronautics and Space Administration the hole, thermistors to measure the water temperature and Goddard Space Flight Center heat loss, and calipers to measure the size of the hole. Greenbelt, Maryland 20771 After the holes were drilled, cables with thermistors were installed in the holes and allowed to freeze in. Freezing took B. Koci only a few days after which each thermistor continued cooling to a final equilibrium temperature. This cooling required many Polar Ice Coring Office weeks, so final temperatures will not be obtained until re- University of Nebraska measurement next field season. Lincoln, Nebraska 68588-0200 The temperature data is used to date the time since the ice A. IKEN rise grounded. The premise of this technique, first applied by Lyons, Ragle, and Tamburi (1972), is that the bases of ice rises VAW/ETH-Zentrum are colder than floating ice shelves. Thus, as an ice shelf grounds, 8092 Zurich, Switzerland the basal ice must cool, a process requiring thousands of years 60 ANTARCTIC JOURNAL 168W 83.4S 170W 83.2S 172W 83S Ow 11 2ZOU- 100 .0-100 83.2S 83S 168W 82.8S 170W Figure 1. Surface elevation of Crary Ice Rise. Contours are in meters above mean sea level. The isolated dome and single ridge are evident. Data are from airborne radar sounding (Shabtaie personal communication) and optical leveling. Filled circles indicate drill sites. and affecting, eventually, the entire ice rise (MacAyeal and The ice thickness above buoyancy at this point is 70 meters— Thomas 1980). By numerically modeling this transient cooling, enough, we expect, to prevent warm sea water from seeping the time elapsed since grounding can be determined. under the ice rise. Thus, the warm basal ice implies an ice rise The first hole was 370 meters deep and located at highest which is very young and has only just begun to cool. We bedrock (using radio echo-sounding data collected by the Uni- estimate the time since grounding is only a century or two. versity of Wisconsin). This location corresponded to a local ice This estimate will be refined after remeasurement of the ther- dome on the ice rise (figure 1). We speculate that ice here has mistors in both holes next field season. been grounded longest and thus will provide a maximum age An ancillary, but no less significant, achievement of drilling for the ice rise. The second hole was located on the prominant the second hole (450 meters deep) was the unexpected recovery ridge southwest of the dome. This ridge is the highest feature of subglacial material. A rock 5 centimeters long and a mud of the ice rise, but radar sounding data indicate that it occurs clast 4 centimeters long were lodged in the caliper arms when on the side of a bedrock slope rather than a bedrock ridge. the drill stem was winched to the surface (figure 3). In addition, Temperature measurements in the first hole lasted for 11.5 approximately 1,000 cubic centimeters of sediment material days. Although this was not long enough for full recovery from the drilling to occur, there was a very clear indication that the basal ice is very close to pressure melting (figure 2). 4 8 q 0 LU cc I- <-10 cc uJ LU I- -15 _20o 100 200 300 TIME (hours) Figure 2. Cooling curves in first hole at 100, 200, 300, and 370 meters below the surface. Data at the bottom of hole (370 meters) Figure 3. Photograph of rock recovered from bottom of second is from two independent thermistors. (m denotes meters.) hole (450 meters). 1988 REVIEW 61 was spread over the drill stem filling most of the ledges and This research was supported under National Science Foun- holes. The rock appears to be basaltic in composition and strongly dation grant DPP 86-14407. faceted indicating subglacial transport over a considerable dis- tance (Ridky personal communication). Thin sections of the rock will be prepared, and it will be examined by electron microscopy for evidence of micro-striations. References The clast and mud appear to be equivalent in composition: a mixture of fine and coarse gravels. The mud may well have Lyons, J.B., R.H. Ragle, and A.J. Tamburi. 1972. Growth and ground- been formed when the high-pressure hot water broke and ing of the Ellesmere Island ice rises. Journal of Glaciology, 11(61), 43- 52. dissolved numerous subglacial clasts. Analysis of the biogenic MacAyeal, D.R., and R.H. Thomas. 1980. Ice-shelf grounding: Ice and composition of the mud and clast is underway at the Byrd bedrock temperature changes. Journal of Glaciology, 25(93), 397-400. Polar Research Center at Ohio State University. No younger Ridky, R. 1988. Personal communication. than upper Miocene have been identified in the samples (Scherer Scherer, R. 1988. Personal communication. personal communication). Shabtaie, S.
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