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Grounded Ice (grounded + ice)
Selected AbstractsGeological constraints on Antarctic palaeo-ice-stream retreatEARTH SURFACE PROCESSES AND LANDFORMS, Issue 4 2008Colm Ó Cofaigh Abstract Submarine landforms preserved in bathymetric troughs on the Antarctic continental shelf show that the style of ice stream retreat across the shelf following the last glacial maximum varied between different troughs. Three styles of retreat are inferred from the geological evidence: rapid, episodic and slow. Rapid retreat by ice stream floatation and calving is recorded by the preservation of a landform assemblage of unmodified streamlined subglacial bedforms including mega-scale glacial lineations (MSGLs) that record streaming flow along these troughs. These elongate bedforms are not overprinted by recessional glacial landforms formed transverse to ice flow such as moraines or grounding-zone wedges, and overlying deglacial sediments are thin. A second type of landform assemblage consists of MSGLs overprinted or interrupted by transverse grounding-zone wedges. This assemblage implies episodic retreat between successive grounding-zone positions. The third type of landform assemblage is that of numerous, closely spaced, recessional moraines and intermittent grounding-zone wedges that overlie and interrupt MSGLs. This assemblage records the slow retreat of grounded ice across the shelf. Variation in the style of ice stream retreat between the different bathymetric troughs indicates that Antarctic palaeo-ice-streams did not respond uniformly to external forcing at the end of the last glacial cycle. Rather, their diachronous retreat reflects the dominance of local controls in the form of bathymetry and drainage basin size. More broadly, these data show that retreat of marine-based ice sheets in areas of reverse bed slope is not necessarily catastrophic, and they provide important constraints for numerical models that attempt to predict the dynamics of large polar ice sheets. Copyright © 2008 John Wiley & Sons, Ltd. [source] Reconstruction of the Ross Ice Drainage System, Antarctica, at the Last Glacial MaximumGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 2-3 2000George H. Denton We present here a revised reconstruction of the Ross ice drainage system of Antarctica at the last glacial maximum (LGM) based on a recent convergence of terrestrial and marine data. The Ross drainage system includes all ice flowlines that enter the marine Ross Embayment. Today, it encompasses one-fourth of the ice-sheet surface, extending far inland into both East and West Antarctica. Grounding lines now situated in the inner Ross Embayment advanced seaward at the LGM (radiocarbon chronology in Denton and Marchant 2000 and in Hall and Denton 2000a, b), resulting in a thick grounded ice sheet across the Ross continental shelf. In response to this grounding in the Ross (and Weddell) Embayment, ice-surface elevations of the marine-based West Antarctic Ice Sheet were somewhat higher at the LGM than at present (Steig and White 1997; Borns et al. 1998; Ackert et al. 1999). At the same time, surface elevations of the East Antarctic Ice Sheet inland of the Transantarctic Mountains were slightly lower than now, except near outlet glaciers that were dammed by grounded ice in the Ross Embayment. The probable reason for this contrasting behavior is that lowered global sea level at the LGM, from growth of Northern Hemisphere ice sheets, caused widespread grounding of the marine portion of the Antarctic Ice Sheet, whereas decreased LGM accumulation led to slight surface lowering of the interior terrestrial ice sheet in East Antarctica. Rising sea level after the LGM tripped grounding-line recession in the Ross Embayment, which has probably continued to the present day (Conway et al. 1999). Hence, gravitational collapse of the grounded ice sheet from the Ross Embayment, accompanied by lowering of the interior West Antarctic ice surface and of outlet glaciers in the Transantarctic Mountains, occurred largely during the Holocene. At the same time, increased Holocene accumulation caused a slight rise of the inland East Antarctic ice surface. [source] The Geologic Basis for a Reconstruction of a Grounded Ice Sheet in McMurdo Sound, Antarctica, at the Last Glacial MaximumGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 2-3 2000George H. Denton A grounded ice sheet fed from the Ross Embayment filled McMurdo Sound at the last glacial maximum (LGM). This sheet deposited the little-weathered Ross Sea drift sheet, with far-traveled Transantarctic Mountains (TAM) erratics, on lower slopes of volcanic islands and peninsulas in the Sound, as well as on coastal forelands along the TAM front. The mapped upper limit of this drift, commonly marked by a distinctive moraine ridge, shows that the ice-sheet surface sloped landward across McMurdo Sound from 710 m elevation at Cape Crozier to 250 m in the eastern foothills of the Royal Society Range. Ice from the Ross Embayment flowed westward into the sound from both north and south of Ross Island. The northern flowlines were dominant, deflecting the southern flowlines toward the foothills of the southern Royal Society Range. Ice of the northern flowlines distributed distinctive kenyte erratics, derived from western Ross Island, in Ross Sea drift along the TAM front between Taylor and Miers Valleys. Lobes from grounded ice in McMurdo Sound blocked the mouths of TAM ice-free valleys, damming extensive proglacial lakes. A floating ice cover on each lake formed a conveyor that transported glacial debris from the grounded ice lobes deep into the valleys to deposit a unique glaciolacustrine facies of Ross Sea drift. The ice sheet in McMurdo Sound became grounded after 26,860 14C yr bp. It remained near its LGM position between 23,800 14C yr bp and 12,700 14C yr bp. Recession was then slow until sometime after 10,794 14C yr bp. Grounded ice lingered in New Harbor in the mouth of Taylor Valley until 8340 14C yr bp. The southward-retreating ice-sheet grounding line had penetrated deep into McMurdo Sound by 6500 14C yr bp. The existence of a thick ice sheet in McMurdo Sound is strong evidence for widespread grounding across the Ross Embayment at the LGM. Otherwise, the ice-sheet surface would not have sloped landward, nor could TAM erratics have been glacially transported westward into McMurdo Sound from farther offshore in the Ross Embayment. [source] Geodetic observations of ice flow velocities over the southern part of subglacial Lake Vostok, Antarctica, and their glaciological implicationsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2006Jens Wendt SUMMARY In the austral summer seasons 2001/02 and 2002/03, Global Positioning System (GPS) data were collected in the vicinity of Vostok Station to determine ice flow velocities over Lake Vostok. Ten GPS sites are located within a radius of 30 km around Vostok Station on floating ice as well as on grounded ice to the east and to the west of the lake. Additionally, a local deformation network around the ice core drilling site 5G-1 was installed. The derived ice flow velocity for Vostok Station is 2.00 m a,1± 0.01 m a,1. Along the flowline of Vostok Station an extension rate of about 10,5 a,1 (equivalent to 1 cm km,1 a,1) was determined. This significant velocity gradient results in a new estimate of 28 700 years for the transit time of an ice particle along the Vostok flowline from the bedrock ridge in the southwest of the lake to the eastern shoreline. With these lower velocities compared to earlier studies and, hence, larger transit times the basal accretion rate is estimated to be 4 mm a,1 along a portion of the Vostok flowline. An assessment of the local accretion rate at Vostok Station using the observed geodetic quantities yields an accretion rate in the same order of magnitude. Furthermore, the comparison of our geodetic observations with results inferred from ice-penetrating radar data indicates that the ice flow may not have changed significantly for several thousand years. [source] | ||||||||||