东南极冰盖上新世大规模退缩事件

笔者近年对东南极内陆格罗夫山(Grove Mountains)开展了上新世以来冰盖表面波动的综合研究,运用冰川地质、地貌、土壤、沉积岩、孢粉组合及宇宙核素等各种方法手段,提出东南极大冰盖形成以后并非稳定演化至今,而在上新世早期时发生过大规模退缩,其前缘至少曾经退缩到格罗夫山地区,距现今冰盖边缘约400 km。之后,冰盖又迅速膨胀,到距今2.3 Ma时,冰面至少超过现今高度约400 m。以后冰面缓慢平稳下降,至1.6 Ma时,东南极冰盖进入第四纪振荡期,但重新上升的冰面再也没有超过现今高度的100 m以上。东南极冰盖大规模消融事件在全球尺度上也有所响应,例如北半球大冰盖形成,青藏高原整体剧烈隆升,塔里木盆地黄土出现等。这类行星尺度的气候变化可能与直布罗陀海峡关闭与地中海盐化事件,巴拿马地峡关闭等大地构造事件有关。     

东南极冰盖/冰架变化监测与预测技术研究

南北极是研究全球变化的关键区域。＂十一五＂期间我国在南极地区系统开展了东南极冰盖/冰架变化监测与预测技术研究,对认识全球气候变化具有重要作用。通过项目实施,建立和发展了一批现场观测体系,发展了冰盖观测新技术并集成应用于东南极冰盖的综合观测,拓展了对冰穹A地区的新认识和新发现;在冰穹A边缘地区钻取的一支浅冰芯恢复了过去约780年的气候记录,记录了东南极地区存在小冰期的明显证据;发展了东南极冰盖积累和等时年层流动模型,研究在冰盖浅层、中层和深部的变化特征,反演了冰穹A地区的古积累率分布。本文概要介绍该项目基本情况。     

Nature of the Antarctic Peninsula Ice Sheet during the Pliocene: Geological evidence and modelling results compared

In this paper, we examine the nature of the Pliocene Antarctic Peninsula Ice Sheet by comparing the terrestrial and marine geological records of the Antarctic Peninsula and surrounding sea floor with estimated net snow accumulation in the region derived from numerical palaeoclimate model experiments. Pliocene geological data and our new modelling results are consistent and mutually supportive in suggesting that an ice sheet was present even during the warmest episodes of the Pliocene. The combined results suggest that the ice sheet in the Antarctic Peninsula is more robust to globally warmer conditions than is generally assumed, at least up to the climatic limits examined in our study.     

A splendid book on the West Antarctic Ice Sheet

Book reviewed in this article:
Van der Veen, C. J. & Oerlemans, J. (eds.) 1986: Dynamics of the West Antarctic Ice Sheet.     

Ice-flow evolution of the Labrador Sector of the Laurentide Ice Sheet: a review,with new evidence from northern Quebec

Outcrops bearing stained, striated facets indicative of north–northeastward moving ice, truncated by unstained, striated facets indicative of various younger flows occur in the Caniapiscau area of north–central Quebec. This is the first report of differential staining of striated facets in the region. We propose that the staining occurred in an ice-free interval of probable interglacial age. This early ice flow probably occurred during ice retreat toward the Quebec highlands. Ice flow and glacial transport data from the southern Hudson Bay and James Bay basins indicate that the next major regional ice flow was toward the northwest and resulted from the expansion of an Early Wisconsinan glacier in the Quebec highlands. The northern part of this flow was diverted northwestward through Hudson Bay, and the southern part southwestward across James Bay, following a progressive counterclockwise rotation of flow. A zone of intersection (ZI) of two major glacier bedform systems, often referred to as the horseshoe-shaped Labrador Ice Divide, represents the head of a large northward convergent ice-flow system that extended to Ungava Bay and beyond. The Ungava flow propagated southward and captured the head of the opposing flow from an outflow centre located east of Caniaspiscau reservoir. We propose that this capture event correlates with the Gold Cove Advance in Ungava Bay and on Baffin Island at about 9900 14C yr BP. It is the largest advance of Quebec-Labrador ice yet proposed for the region. This correlation is based on the relative ice-flow chronology, accommodation of glacial lakes Naskaupi and McLean in the deglaciation sequence, the constraints placed on Last Glacial Maximum (LGM) ice configuration by the postglacial uplift pattern and events in the deep-sea record. Therefore, the Ungava ice-flow pattern is not a relict pre-Wisconsinan glacial landscape as recently proposed.     

Thickness evolution of the Scandinavian Ice Sheet during the Late Weichselian in Nordfjord, western Norway: evidence from ice-flow modeling

Winguth, C., Mickelson, D. M., Larsen, E., Darter, J. R., Moeller, C. A. & Stalsberg, K. 2005 (May): Thickness evolution of the Scandinavian Ice Sheet during the Late Weichselian in Nordfjord, western Norway: evidence from ice-flow modeling. Boreas , Vol. 34, pp. 176–185. Oslo. ISSN 0300–9483.
Results from experiments with a two-dimensional ice-flow model, applied along a west-east transect in western Norway, provide new constraints on the thickness evolution of the Scandinavian Ice Sheet throughout the Late Weichselian glaciation and deglaciation. Investigations took place along an E-W flowline of the former ice sheet at c. 62N, from the modern glacier Jostedalsbreen, through the Nordfjord, and across the continental shelf. A paleoclimate record from Kråkenes, which is located directly at the flowline, provides temperature and precipitation information for the time between 13 800 and 9200 cal. yr BP. LGM climate conditions for the study area are estimated from various GCM studies. The GISP2 δ¹⁸O record has been tuned to the local data in order to provide a continuous temperature record as input for time-transgressive model runs. The results of all experiments suggest that the ice did not cover the highest mountain peaks in this area, and that nunataks persisted throughout the Late Weichselian glaciation. These findings are in contrast to results from many previous model studies and other ice-sheet reconstructions, but agree well with minimum thickness estimates from cosmogenic dating and with vertical ice limits inferred from lower block field boundaries and trimlines.     

A model for Holocene retreat of the West Antarctic Ice Sheet

Marine ice sheets are grounded on land which was below sea level before it became depressed under the ice-sheet load. They are inherently unstable and, because of bedrock topography after depression, the collapse of a marine ice sheet may be very rapid. In this paper equations are derived that can be used to make a quantitative estimate of the maximum size of a marine ice sheet and of when and how rapidly retreat would take place under prescribed conditions. Ice-sheet growth is favored by falling sea level and uplift of the seabed. In most cases the buttressing effect of a partially grounded ice shelf is a prerequisite for maximum growth out to the edge of the continental shelf. Collapse is triggered most easily by eustatic rise in sea level, but it is possible that the ice sheet may self-destruct by depressing the edge of the continental shelf so that sea depth is increased at the equilibrium grounding line.Application of the equations to a hypothetical “Ross Ice Sheet” that 18,000 yr ago may have covered the present-day Ross Ice Shelf indicates that, if the ice sheet existed, it probably extended to a line of sills parallel to the edge of the Ross Sea continental shelf. By allowing world sea level to rise from its late-Wisconsin minimum it was possible to calculate retreat rates for individual ice streams that drained the “Ross Ice Sheet.” For all the models tested, retreat began soon after sea level began to rise (～15,000 yr B.P.). The first 100 km of retreat took between 1500 and 2500 yr but then retreat rates rapidly accelerated to between 0.5 and 25 km yr^?1, depending on whether an ice shelf was present or not, with corresponding ice velocities across the grounding line of 4 to 70 km yr^?1. All models indicate that most of the present-day Ross Ice Shelf was free of grounded ice by about 7000 yr B.P. As the ice streams retreated floating ice shelves may have formed between promontories of slowly collapsing stagnant ice left behind by the rapidly retreating ice streams. If ice shelves did not form during retreat then the analysis indicates that most of the West Antarctic Ice Sheet would have collapsed by 9000 yr B.P. Thus, the present-day Ross Ice Shelf (and probably the Ronne Ice Shelf) serves to stabilize the West Antarctic Ice Sheet, which would collapse very rapidly if the ice shelves were removed. This provides support for the suggestion that the 6-m sea-level high during the Sangamon Interglacial was caused by collapse of the West Antarctic Ice Sheet after climatic warming had sufficiently weakened the ice shelves. Since the West Antarctic Ice Sheet still exists it seems likely that ice shelves did form during Holocene retreat. Their effect was to slow and, finally, to halt retreat. The models that best fit available data require a rather low shear stress between the ice shelf and its sides, and this implies that rapid shear in this region encouraged the formation of a band of ice with a preferred crystal fabric, as appears to be happening today in the floating portions of fast bounded glaciers.Rebound of the seabed after the ice sheet had retreated to an equilibrium position would allow the ice sheet to advance once more. This may be taking place today since analysis of data from the Ross Ice Shelf indicates that the southeast corner is probably growing thicker with time, and if this persists then large areas of ice shelf must become grounded. This would restrict drainage from West Antarctic ice streams which would tend to thicken and advance their grounding lines into the ice shelf.     

Quaternary changes in level of the upper Taylor Glacier, Antarctica: implications for paleoclimate and East Antarctic Ice Sheet dynamics

Glacial drifts perched alongside outlet glaciers that drain through the Transantarctic Mountains constrain inland polar plateau ice elevations. The Taylor Glacier, which heads in the Taylor Dome (a peripheral dome of the East Antarctic Ice Sheet), drains East Antarctic ice into the Dry Valleys sector of Transantarctic Mountains and terminates in central Taylor Valley, about 24 km west of the Ross Sea. Five gravel-rich drifts (including 39 distinct moraine ridges) fringe a lateral lobe of the Taylor Glacier in the lower Arena Valley, Quartermain Mountains, southern Victoria Land. ³He and ¹⁰Be exposure age dating (from Brook et al . 1992), together with Arena Valley stratigraphy and soil morphologic data, provide chronologic control for these drifts and constrain maximum Quaternary thickening of the inland Taylor ice dome to less than 160 m. These minor Quaternary expansions of Taylor Glacier were out-of-phase with outlet glaciers that pass through the Transantarctic Mountains and terminate in the Ross Sea north and south of the Dry Valleys region. Textural analyses suggest that drift deposition occurred from cold-based ice, even though Taylor Glacier advances most likely occurred during global interglaciations. The thermal regime of former Taylor Glacier ice lobes, the character of geomorphic features superimposed on individual drifts, the chemical composition of soils developed on Taylor drifts, and the stability of in situ moraine ridges on steep valley walls suggest that the present cold-desert climate in Arena Valley has persisted for at least the last 2.2 Ma.     

Changes in the mass balance of the Antarctic Ice Sheet over 50 years

The mass balance of the Antarctic Ice Sheet has been calculated based on instrumental estimates of the grounded ice discharge and snow accumulation data. The boundaries and sectional areas of the main ice catchment basins in West and East Antarctica have been determined, and the data on the grounded ice discharge and snow accumulation in these basins have been systematized. The intensity of accumulation and ablation processes in Antarctica has noticeably increased over the last 50 years. The mass balance of the ice sheet in East Antarctica has been and remains positive, while in West Antarctica it was positive in the middle of the last century and has become negative by now. The mass balance of the entire Antarctic Ice Sheet has been and remains positive, while the mass growth has noticeably decreased over the last 50 years.     

The East Antarctic sea ice zone: Ice characteristics and drift

Results from studies of the surface energy balance and the ocean structure in the presence of fast ice near Mawson on the Antarctic coast are used to illustrate the important ways in which sea ice interacts with the ocean and atmosphere. Away from the coast, ship and drifting buoy observations are used to characterize the E Antarctic sea ice zone in a study area between 60° E and 120° E and S of 61° S. Divergent drift over most of the region plays a dominant role in expanding the ice extent in autumn and in determining the characteristics of the pack. Much of the sea ice in the region is young thin ice which forms in leads and polynyas, and in late spring in the study area, the ice thickness averaged over the total ocean surface within the ice edge less than 0.4 m. Even in winter the majority of ice floes off E Antarctica are probably less than 1 m thick.     

Marine geophysical evidence for former expansion and flow of the Greenland Ice Sheet across the north‐east Greenland continental shelf

High‐resolution swath bathymetry and TOPAS sub‐bottom profiler acoustic data from the inner and middle continental shelf of north‐east Greenland record the presence of streamlined mega‐scale glacial lineations and other subglacial landforms that are formed in the surface of a continuous soft sediment layer. The best‐developed lineations are found in Westwind Trough, a bathymetric trough connecting Nioghalvfjerdsfjorden Gletscher and Zachariae Isstrøm to the continental shelf edge. The geomorphological and stratigraphical data indicate that the Greenland Ice Sheet covered the inner‐middle shelf in north‐east Greenland during the most recent ice advance of the Late Weichselian glaciation. Earlier sedimentological and chronological studies indicated that the last major delivery of glacigenic sediment to the shelf and Fram Strait was prior to the Holocene during Marine Isotope Stage 2, supporting our assertion that the subglacial landforms and ice sheet expansion in north‐east Greenland occurred during the Late Weichselian. Glacimarine sediment gravity flow deposits found on the north‐east Greenland continental slope imply that the ice sheet extended beyond the middle continental shelf, and supplied subglacial sediment direct to the shelf edge with subsequent remobilisation downslope. These marine geophysical data indicate that the flow of the Late Weichselian Greenland Ice Sheet through Westwind Trough was in the form of a fast‐flowing palaeo‐ice stream, and that it provides the first direct geomorphological evidence for the former presence of ice streams on the Greenland continental shelf. The presence of streamlined subglacially derived landforms and till layers on the shallow AWI Bank and Northwind Shoal indicates that ice sheet flow was not only channelled through the cross‐shelf bathymetric troughs but also occurred across the shallow intra‐trough regions of north‐east Greenland. Collectively these data record for the first time that ice streams were an important glacio‐dynamic feature that drained interior basins of the Late Weichselian Greenland Ice Sheet across the adjacent continental margin, and that the ice sheet was far more extensive in north‐east Greenland during the Last Glacial Maximum than the previous terrestrial–glacial reconstructions showed. Copyright © 2008 John Wiley & Sons, Ltd.     

Palaeo-ice streaming in the central sector of the British—Irish Ice Sheet during the Last Glacial Maximum: evidence from the northern Irish Sea Basin

Late Devensian glacial sediments and landforms of the Isle of Man record the advance and deglacial signature of the central sector of the British-Irish Ice Sheet. Evidence from the area, gathered from striae, erratic trains and drift limits, show ice was routed over and around the island in two flow phases post-36 kyr BP. In the south of the island, streamlined depositional bedforms with low elongation ratios suggest low ice-flow velocities resulting from one or more of (i) the up-ice location of the island within a regional onset zone, (ii) flow retardation of ice interacting with the margins of the island and (iii) localized drainage of the deforming bed. The deglacial landform assemblage of lateral marginal sandurs and drainage diversions, coupled with a lack of dead-ice features, suggests ice did not downwaste in situ but retreated intact along the coastal margins as Manx Upland ice thinned. In the north of the island, however, the Bride Moraine complex indicates a change in deglacial ice-sheet dynamics, with temporary re-advance and marginal oscillation causing proglacial tectonism and thrusting of the glacial sediment pile, possibly during the Killard Point Stadial event (18.8-16.4 cal. kyr BP). From a basin-wide perspective, the Irish Sea Basin sector of the British-Irish Ice Sheet had many of the characteristics of an ice stream, such as a zone of flow convergence up-ice, a grounding line in the southern Celtic Sea and recessional limits characterized by proglacially tectonized and thrust dead-ice landscapes indicative of a rapidly oscillating ice margin.     

The deglaciation of the western sector of the Irish Ice Sheet from the inner continental shelf to its terrestrial margin

This paper provides a new deglacial chronology for retreat of the Irish Ice Sheet from the continental shelf of western Ireland to the adjoining coastline, a region where the timing and drivers of ice recession have never been fully constrained. Previous work suggests maximum ice-sheet extent on the outer western continental shelf occurred at ~26–24 cal. ka BP with the initial retreat of the ice marked by the production of grounding-zone wedges between 23–21.1 cal. ka BP. However, the timing and rate of ice-sheet retreat from the inner continental shelf to the present coast are largely unknown. This paper reports 31 new terrestrial cosmogenic nuclide (TCN) ages from erratics and ice-moulded bedrock and three new optically stimulated luminescence (OSL) ages on deglacial outwash. The TCN data constrain deglaciation of the near coast (Aran Islands) to ~19.5–18.5 ka. This infers ice retreated rapidly from the mid-shelf after 21 ka, but the combined effects of bathymetric shallowing and pinning acted to stabilize the ice at the Aran Islands. However, marginal stability was short-lived, with multiple coastal sites along the Connemara/Galway coasts demonstrating ice recession under terrestrial conditions by 18.2–17. ka. This pattern of retreat continued as ice retreated eastward through inner Galway Bay by 16.5 ka. South of Galway, the Kilkee–Kilrush Moraine Complex and Scattery Island moraines point to late stage re-advances of the ice sheet into southern County Clare ~14.1–13.3 ka, but the large errors associated with the OSL ages make correlation with other regional re-advances difficult. It seems more likely that these moraines are the product of regional ice lobes adjusting to internal ice-sheet dynamics during deglaciation in the time window 17–16 ka.     

南极冰盖GLIMMER模式移动边界试验研究

利用欧洲冰盖比较计划(EISMINT)的移动边界条件EISMINT-1测试集,通过三维有限差分南极冰盖GLIMMER模式,采用浅冰近似计算了一个定义在方形冰盖积分区域的流场特征量,研究了冰盖在大尺度长时间序列条件下对气候变化的反馈,考察了稳定态下冰盖演化的周期性行为.试验表明:GLIMMER模式在EISMINT-1移动边界条件下可以模拟出气候强迫下的冰盖演化趋势,在长时间尺度上可以刻画出气候变化的反馈机制.同时,对南极冰盖数值模拟面临的问题、GLIMMER模式以及EISMINT冰盖比较计划的内容和框架做了说明,对EISMINT比较计划目前的发展状况和遇到的问题做了分析,并且对GLIMMER模式下一步的研究方向做了一些探讨.     

南极大陆冰缘环境变迁与滨海盐湖生态特征(英)

Ecological investigation has been done in some saline lakes in the Vestfold Hills (60°38'S, 78°06' E), East Antarctica. The results show that changes of natural environment and physicalchemical factors, as well as the structure of biotic species and succession do occur in some lakes, due to seasonal and local climate changes and geographic variances in regions. These should be strong ecological proofs to explain that the processes of globle climate change and ice sheet retreating, and isostatic uplift raising, could be continueing in present time, and should be significant to the study on Antarctic and global changes.     

Hummocky moraine: sedimentary record of stagnant Laurentide Ice Sheet lobes resting on soft beds

Over large areas of the western interior plains of North America, hummocky moraine (HM) formed at the margins of Laurentide Ice Sheet (LIS) lobes that flowed upslope against topographic highs. Current depositional models argue that HM was deposited supraglacially from stagnant debris-rich ice (`disintegration moraine'). Across southern Alberta, Canada, map and outcrop data show that HM is composed of fine-grained till as much as 25 m thick containing rafts of soft, glaciotectonized bedrock and sediment. Chaotic, non-oriented HM commonly passes downslope into weakly-oriented hummocks (`washboard moraine') that are transitional to drumlins in topographic lows; the same subsurface stratigraphy and till facies is present throughout. These landforms, and others such as doughnut-like `rim ridges', flat-topped `moraine plateaux' and linear disintegration ridges, are identified as belonging to subglacially-deposited soft-bed terrain. This terrain is the record of ice lobes moving over deformation till derived from weakly-lithified, bentonite-rich shale. Drumlins record continued active ice flow in topographic lows during deglaciation whereas HM was produced below the outer stagnant margins of ice lobes by gravitational loading (`pressing') of remnant dead ice blocks into wet, plastic till. Intervening zones of washboard moraine mark the former boundary of active and stagnant ice and show `hybrid' drumlins whose streamlined form has been altered by subglacial pressing (`humdrums') below dead ice. The presence of hummocky moraine over a very large area of interior North America provides additional support for glaciological models of a soft-bedded Laurentide Ice Sheet.