摘要:

ABSTRACTThere are two primary views concerning the stability of the East Antarctic Ice Sheet. One view, relying critically on the interpretation of Sirius Group glacial deposits in the Transantarctic Mountains, is that the ice sheet has been fluctuating dramatically throughout its existence and that it last disappeared during the Pliocene∼3 Ma ago. By analogy with the warmer Pliocene, it is argued that the current ice sheet is susceptible to global warming. The other view, originating from marine and terrestrial work in the 1970s and 1980s is that the ice sheet has been stable for∼14 Ma and that the continent has been subjected to unbroken, cold polar conditions subsequently. After summarising the status of the two hypotheses, we explain the rationale for this volume. Building on the Vega Syposium of April 1993, it presents the case for the stability of the East Antarctic Ice Sheet and includes new work on terrestrial geomorphology and geology, marine cores and ice-sheet modelling.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880392

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTA case is made for the stability of the East Antarctic Ice Sheet during Pliocene time from landscape development and surficial sediments in the Dry Valleys sector of the Transantarctic Mountains. The alternate hypothesis of Pliocene meltdown requires atmospheric temperatures 20°C above present values, late Pliocene ice-sheet overriding of the Transantarctic Mountains, and possible rapid late Pliocene mountain uplift of 1000–3000 m. The geomorphological results suggest that these conditions were not met in the Dry Valleys region. Rather, Pliocene mean annual atmospheric temperatures were at most only 3°to 8°C above present values; ice-sheet overriding occurred in Miocene time (>13.6 Ma); Pliocene glacier expansion was limited; and Pliocene surface uplift was only about 250 to 300 m. These conclusions are based on field studies in Taylor and Wright Valleys, in the western Asgard Range, and in the Quartermain Mountains. The chronology comes from numerous40Ar/39Ar dates onin-situvolcanic ashes that occur in stratigraphic association with unconsolidated diamictons in the western Dry Valleys, basaltic lava flows interbedded with widespread tills in Taylor Valley, and reworked basaltic clasts in alpine moraines in east-central Wright Valley. The combined evidence from the Dry Valleys region indicates that slope evolution was severely restricted throughout Pliocene time, and has been so since at least the middle Miocene. The implication is that most of the Dry Valleys landscape is relict and that it reflects ancient erosion, possibly under semi-arid climate conditions, prior to middle-Miocene time.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880393

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTBefore∼3 Ma, during the early Pliocene, polar ice sheets were restricted to Antarctica, and much evidence suggests global warmth greater than at any time during the last seven million years, including today. Did the East Antarctic Ice Sheet withstand this early Pliocene warmth or did it experience major instability and deglaciation? This problem is of central importance in considering the consequences of global warming that might result from anthropogenic increases in atmospheric CO2and other greenhouse gases.Marine stratigraphic data from the southern high latitudes is incompatible with earlier suggestions that invoke major Pliocene warming of the Antarctic continent, increase in temperature of the surrounding Antarctic water mass to 5–10°C, and associated major deglaciation (>50% ice volume reduction) of the East Antarctic Ice Sheet. An oxygen isotopic record of high stratigraphic resolution from the Subantarctic region indicates an increase in average sea surface temperatures of no more than∼3°C during the warmest interval of the Pliocene (4.8 to 3.2 Ma). These data suggest that the Antarctic ice sheets did not experience major deglaciation during this warmer interval. Instead the oxygen isotopic data indicate relative stability of the Antarctic climate/cryosphere system during the early Pliocene compared with the large-scale fluctuations that mark the latest Miocene prior to∼4.8 Ma. We suggest that the well known early Pliocene marine transgression was marked by sea levels no higher than 25 m above the present, and probably significantly less than this for most of this interval.Relative stability of the Antarctic cryosphere during the early Pliocene is supported by other evidence. The deep sea carbon isotopic record lacks large changes inδ13C values that would have resulted from major glacioeustatic changes. Significant ice rafting of sedimentary detritus to the Subantarctic far to the north of the Antarctic continent continued throughout the entire Pliocene, indicating a continued presence of major ice sheets on Antarctica even during the warmest Pliocene intervals. An almost complete lack of biocalcareous sediments in Antarctic waters during the Pliocene and continued dominance of biosiliceous sediments suggests oceanographic conditions broadly similar to those of the present day. This would not have been the case if the surface waters in the Antarctic had increased by more than 5°C, reflecting values of present day southern Subantarctic waters. Limited stratigraphic evidence suggests that marine sedimentary basins adjacent to West Antarctica have been relatively starved of terrigenous sediments throughout the Pliocene and Quaternary, probably as a result of the near-permanency of the East and perhaps the West Antarctic Ice Sheets since the Miocene, thereby reducing continental erosion.Early Pliocene planktonic microfossils suggest warmer average surface water temperatures in the Antarctic compared with the present day. However, conditions were not sufficiently warm to affect increasing endemism of Pliocene siliceous planktonic assemblages. Biosiliceous faunas and floras dominate throughout the Pliocene and there is no return of calcareous nannofossil deposition which had ceased during the late Miocene. During the Pliocene, conditions were never sufficiently warm to cause significant displacement of Antarctic by Subantarctic planktonic assemblages.The marine sedimentary record of oceanographic, climatic and cryospheric stability in the Antarctic during the early Pliocene agrees with predictions based on coupled ocean-atmospheric models of relatively stable Antarctic sea surface temperatures in response to global warming.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880394

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTOn the basis of geological arguments, two widely different hypotheses have been proposed for the late-Tertiary glacial history of East Antarctica. These invoke ice-sheet reconstructions ranging from severe glaciation completely burying the Transantarctic Mountains to a situation, in which an unstable East Antarctic Ice Sheet repeatedly collapses to produce ice-free conditions over interior basins. Experiments were performed with a 3-D model of the Antarctic Ice Sheet to determine the ice sheet geometries to be expected under various kinds of climatic conditions and the physical mechanisms that may be involved. The results support the concept of a stable East Antarctic Ice Sheet with respect to a climatic warming, and point to the glaciological difficulties involved to explain an ice-free corridor over the Pensacola and Wilkes subglacial basins. The latter event is a crucial element in the‘waxing and waning ice sheet hypothesis’and would require a temperature rise of between 17 and 20K above present levels. For a temperature rise of less than 5K, the model actually predicts a larger Antarctic Ice Sheet than today as a result of increased snowfall, whereas the West Antarctic Ice Sheet was found not to survive temperatures 8–10K above present values. Furthermore, basal temperature conditions in these experiments point to the difficulties involved in raising the ice-sheet base to the pressure melting point over the large areas necessary to consider the possibility of sliding instability. A further sensitivity study in terms of topographical and climatic boundary conditions showed that even with a flat bedrock and mass-balance parameters stretched to their limits, the ice sheet would still need a temperature rise of the order of + 15K to melt down entirely. Based on these findings, it appears difficult to reconcile a highly variable East Antarctic Ice Sheet with the modest warmings recorded in, for instance, the deep sea records for the late Neogene.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880395

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTWright Valley, which extends from the edge of the East Antarctic Ice Sheet through the Transantarctic Mountains to the Ross Sea, contains widespread Neogene surficial drift deposits.40Ar39Ar ages of reworked basalt clasts in these drifts afford a limiting chronology for glacial events. Peleus till (>3.8 Ma old) is stratigraphically lowest and represents expansion of an East Antarctic outlet glacier through Wright Valley. Alpine I, II, III (3.7 Ma old) drifts, which flank alpine glaciers on the south valley wall, and Onyx, Wright, and Loop drifts, which record expansion of an ice lobe westward (inland) into Wright Valley, are all younger than Peleus till. Onyx, Wright, and Loop drift sheets (all Quaternary in age) postdate old alpine drifts (III and IV) and antedate young alpine drifts (I and II). Cold-desert conditions have persisted for at least 3.7 Ma and are reflected by the presence of coarse-grained tills, ventifacts, and abundant salts in the soils, and by the lack of temperate-ice landforms and glaciomarine deposits in east-central Wright Valley.The evidence of persistent cold paleoclimate contradicts the postulated Pliocene deglaciation of East Antarctica, which requires atmospheric temperatures 20°C warmer than present to produce melting ablation surfaces on the East Antarctic Ice Sheet. Furthermore, an East Antarctic outlet glacier has not expanded in to east-central Wright Valley in at least the last 3.8 Ma, thus precluding ice-sheet overriding of the Dry Valleys region during this interval. Overall, Wright Valley data imply that the adjacent East Antarctic Ice Sheet has been stable for at least the last 3.8 Ma.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880396

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTAn40Ar/39Ar chronology ofin-situto nearin-situvolcanic ashfall deposits indicates that the surficial stratigraphy of Arena Valley extends back at least to middle-Miocene time. Wet-based glacial ice occupied part of Arena Valley more than 11.3 Ma ago. Thick, northeast-flowing ice subsequently engulfed Arena Valley, again more than 11.3 Ma ago. Only minor glacier expansion occurred during Pliocene and Pleistocene time. The maximum Pliocene thickening of Taylor Dome, 35 km inland of Arena Valley, was certainly less than 475 m and probably less than 250 m. Maximum thickening of Taylor Dome was less than 160 m during the Pleistocene.The preservation of Miocene-and Pliocene-age ashes on steep valley slopes indicates that the major bedrock land-forms of Arena Valley are relict and that little slope evolution/colluviation has occurred during the last 11.3 Ma. The geologic record of Arena Valley glaciation and landscape evolution shows persistent cold-desert conditions and hence implies stability of the adjacent East Antarctic Ice Sheet for at least the last 11.3 Ma.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880397

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACT40Ar/39Ar datedin-situvolcanic ashfall deposits indicate that the surficial stratigraphy of the western Asgard Range in the Dry Valleys region extends back at least to 15.0 Ma. The preservation of Miocene and Pliocene colluvium and drift on valley slopes shows that major bedrock landforms are relict and that little slope evolution has occurred during the last 15.0 Ma. Major bedrock landforms can be compared directly with buttes, mesas, box canyons, and escarpments of platform deserts. Although glaciers played a role in shaping the present topography of the western Asgard Range, the main landforms were probably cut by progressive scarp retreat, propagation of embayments with theater-shaped heads, and isolation of buttes and mesas prior to 15.0 Ma.We also recognize a phase of wet-based alpine glacier expansion in theater-headed embayments prior to 15.0 Ma ago, along with a late-Miocene phase of northeast-flowing ice-sheet overriding between 14.8/ 15.2 Ma and 13.6 Ma. However, there is no evidence for notable glacier expansion during the last 13.6 Ma. Instead,in-situashfall deposits indicate persistent hyper-arid, cold-desert conditions with only minor advance (less than 2.5 km) of rock glaciers and glacierets at valley heads. Our results are consistent with Pliocene stability rather than meltdown of the adjacent East Antarctic Ice Sheet.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880398

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTLimited surface uplift, glacial extent, and climatic change since mid-Pliocene time are inferred from surficial deposits in middle Taylor Valley, Antarctica (77°30'-77°50'S., 160°0'-163°40'E.).40Ar/39Ar isotopie dates of whole-rock basalts constrain 14 subaerial volcanic eruptions between 3.89 and 1.50 Ma. The isotopic ages, subaerial nature, and present positions of these volcanic outcrops together are used to limit surface uplift to less than≈300 m since 2.57 Ma. On the north wall of Taylor Valley, glacial drifts at Rhone Platform are intercalated with isotopically dated volcanic rocks and record at least four Pliocene-Pleistocene expansions of Taylor Glacier (>3.47 Ma; 2.97 Ma-2.71 Ma;<2.71 Ma;<1.50 Ma). The most extensive expansion reached 1082 m elevation, and occurred between 2.97 Ma and 2.71 Ma, whereas the other three were only slightly less extensive. Specific dates of reduced alpine glacier and Taylor Glacier extent (1.50, 2.20, 2.55, 2.71, 2.97, 3.52, and 3.74 Ma) come from the ages of subaerially erupted volcanic deposits that are now in contact with alpine glaciers. The implication of these data is that there has not been a significant change in the magnitude or character of glacial activity since mid-Pliocene time. All glacial deposits in middle Taylor Valley contain clasts with ventifaction and desert varnish, suggesting that desert conditions existed prior to 3.47 Ma. Anin-situlayer of volcanic lapilli and bombs dated to 2.97 Ma rests on an intact desert pavement at 1075 m elevation on Rhone platform on the north wall of Taylor Valley.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880399

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880400

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor