Global ice-sheet system interlocked by sea level

Denton and Hughes (1983, Quaternary Research 20, 125–144) postulated that sea level linked a global ice-sheet system with both terrestrial and grounded marine components during late Quaternary ice ages. Summer temperature changes near Northern Hemisphere melting margins initiated sea-level fluctuations that controlled marine components in both polar hemispheres. It was further proposed that variations of this ice-sheet system amplified and transmitted Milankovitch summer half-year insolation changes between 45 and 75°N into global climatic changes. New tests of this hypothesis implicate sea level as a major control of the areal extent of grounded portions of the Antarctic Ice Sheet, thus fitting the concept of a globally interlocked ice-sheet system. But recent atmospheric modeling results (Manabe and Broccoli, 1985, Journal of Geophysical Research 90, 2167–2190) suggest that factors other than areal changes of the grounded Antarctic Ice Sheet strongly influenced Southern Hemisphere climate and terminated the last ice age simultaneously in both polar hemispheres. Atmospheric carbon dioxide linked to high-latitude oceans is the most likely candidate (Shackleton and Pisias, 1985, Atmospheric carbon dioxide, orbital forcing, and climate. In “The Carbon Cycle and Atmospheric CO₂: Natural Variations Archean to Present” (E. T. Sundquest and W. S. Broecker, Eds.), pp. 303–318. Geophysical Monograph 32, American Geophysical Union, Washington, D.C.), but another potential influence was high-frequency climatic oscillations (2500 yr). It is postulated that variations in atmospheric carbon dioxide acted through an Antarctic ice shelf linked to the grounded ice sheet to produce and terminate Southern Hemisphere ice-age climate. It is further postulated that Milankovitch summer insolation combined with a warm high-frequency oscillation caused marked recession of Northern Hemisphere ice-sheet melting margins and the North Atlantic polar front about 14,000 ¹⁴C yr B.P. This permitted renewed formation of North Atlantic Deep Water, which could well have controlled atmospheric carbon dioxide (W. S. Broecker, D. M. Peteet, and D. Rind, 1985, Nature (London) 315, 21–26). Combined melting and consequent sea-level rise from the three warming factors initiated irreversible collapse of the interlocked global ice-sheet system, which was at its largest but most vulnerable configuration.     

A simple holistic hypothesis for the self-destruction of ice sheets

Ice sheets are the only components of Earth’s climate system that can self-destruct. This paper presents the quantitative force balance for bottom-up modeling of ice sheets, as first presented qualitatively in this journal as a way to quantify ice-bed uncoupling leading to self-destruction of ice sheets (Hughes, 2009a). Rapid changes in sea level and climate can result if a large ice-sheet self-destructs quickly, as did the former Laurentide Ice Sheet of North America between 8100 and 7900 BP, thereby terminating the last cycle of Quaternary glaciation. Ice streams discharge up to 90 percent of ice from past and present ice sheets. A hypothesis is presented in which self-destruction of an ice sheet begins when ubiquitous ice-bed decoupling, quantified as a floating fraction of ice, proceeds along ice streams. This causes ice streams to surge and reduce thickness by some 90 percent, and height above sea level by up to 99 percent for floating ice, so the ice sheet undergoes gravitational collapse. Ice collapsing over marine embayments becomes floating ice shelves that may then disintegrate rapidly. This floods the world ocean with icebergs that reduce the ocean-to-atmosphere heat exchange, thereby triggering climate change. Calving bays migrate up low stagnating ice streams and carve out the accumulation zone of the collapsed ice sheet, which prevents its recovery, decreases Earth’s albedo, and terminates the glaciation cycle. This sequence of events may coincide with a proposed life cycle of ice streams that drain the ice sheet. A first-order treatment of these life cycles is presented that depends on the longitudinal force balance along the flowbands of ice streams and gives a first approximation to ice-bed uncoupling at snapshots during gravitational collapse into ice shelves that disintegrate, thereby removing the ice sheet. The stability of the Antarctic Ice Sheet is assessed using this bottom-up approach.     

米兰科维奇冰期旋回理论:挑战与机遇

米兰科维奇理论认为,北半球高纬夏季太阳辐射变化是驱动第四纪冰期旋回的主因。这个理论的核心是单一敏感区的触发驱动机制,即北半球高纬气候变化信号被放大、传输进而影响全球。最近,由于大量高分辨率及精确定年的气候变化记录的获得,从以下4个方面构成了对米氏理论的挑战:1)一些低纬地区并没有明显的10万年冰量周期,而是以2万年岁差周期为主,表明北半球冰盖的扩张、收缩变化并没有完全控制低纬区的气候变化;2)在最近几次冰消期时,南半球和低纬区的温度增高,要早于北半球冰盖的融化,表明冰消期的触发机制并非是北半球高纬夏季太阳辐射;3)大气CO₂浓度在第2冰消期的增加同南极升温相一致,表明该时大气CO₂浓度增加亦有可能早于北半球冰盖消融;4)南半球的末次冰盛期有可能早于北半球。这就说明单一敏感区触发驱动机制已难以圆满解释所有观察事实,天文因素控制下轨道尺度气候变化机制研究正面临理论突破的新需求和新机遇。     

Astronomical theory of climatic change: Status and problem

Quaternary paleotemperatures and sea level records, from both deep sea cores and dated shorelines, provide the basis for testing the Milankovitch hypothesis of climatic change. The longest and most detailed records include (1) oxygen isotope analyses of Caribbean and Atlantic deep sea cores, (2) paleoecological analyses of the same cores, and (3) radiometrically dated raised coral reefs from New Guinea and elsewhere, representing times of relatively high Quaternary sea levels. Time-domain and frequency-domain analysis of these records, shows with a high degree of certainty that Quaternary climatic changes are strongly influenced by the obliquity perturbations and precession of the Earth's orbit. The same analyses also suggest that the time scale adopted by Emiliani for deep sea cores may be more nearly correct than alternative time scales of other workers.The question of whether insolation changes arising from orbital perturbations can generate ice ages, has been disputed by climatologists. It is shown here that orbital perturbations cannot affect climate indirectly through agencies originating within the Earth, such as vulcanism, and that the primary climatic control is therefore through variation of insolation distribution, as Milankovitch suggested. The conclusion is that climatologic theory must accommodate these facts.     

大洋环流型式转换在冰期旋回中的作用及经典第四纪冰期理论质疑(续)

大洋环流型式转换在冰期旋回中的作用及经典第四纪冰期理论质疑(续)汉景泰Ｗ．Ｓ．Ｆｙｆｅ(加拿大西安大略大学地质系)４问题及讨论大洋环流－气候学说认为从冰期到间冰期海洋－大气系统全球性巨型再组合导致了末次冰期的终止。     

No significant ice-sheet expansion beyond present ice margins during the past 4500 yr at Rauer Group, East Antarctica

The history of glacial advances and retreats of the East Antarctic ice sheet during the Holocene is not well-known, due to limited field evidence in both the marine and terrestrial realm. A 257-cm-long sediment core was recovered from a marine inlet in the Rauer Group, East Antarctica, 1.8 km in front of the present ice-sheet margin. Radiocarbon dating and lithological characteristics reveal that the core comprises a complete marine record since 4500 yr. A significant ice-sheet expansion beyond present ice margins therefore did not occur during this period.     

Pattern,style and timing of British–Irish Ice Sheet advance and retreat over the last 45 000 years: evidence from NW Scotland and the adjacent continental shelf

Predicting the future response of ice sheets to climate warming and rising global sea level is important but difficult. This is especially so when fast-flowing glaciers or ice streams, buffered by ice shelves, are grounded on beds below sea level. What happens when these ice shelves are removed? And how do the ice stream and the surrounding ice sheet respond to the abruptly altered boundary conditions? To address these questions and others we present new geological, geomorphological, geophysical and geochronological data from the ice-stream-dominated NW sector of the last British–Irish Ice Sheet (BIIS). The study area covers around 45 000 km² of NW Scotland and the surrounding continental shelf. Alongside seabed geomorphological mapping and Quaternary sediment analysis, we use a suite of over 100 new absolute ages (including cosmogenic-nuclide exposure ages, optically stimulated luminescence ages and radiocarbon dates) collected from onshore and offshore, to build a sector-wide ice-sheet reconstruction combining all available evidence with Bayesian chronosequence modelling. Using this information we present a detailed assessment of ice-sheet advance/retreat history, and the glaciological connections between different areas of the NW BIIS sector, at different times during the last glacial cycle. The results show a highly dynamic, partly marine, partly terrestrial, ice-sheet sector undergoing large size variations in response to sub-millennial-scale climatic (Dansgaard–Oeschger) cycles over the last 45 000 years. Superimposed on these trends we identify internally driven instabilities, operating at higher frequency, conditioned by local topographic factors, tidewater dynamics and glaciological feedbacks during deglaciation. Specifically, our new evidence indicates extensive marine-terminating ice-sheet glaciation of the NW BIIS sector during Greenland Stadials 12 to 9 – prior to the main ‘Late Weichselian’ ice-sheet glaciation. After a period of restricted glaciation, in Greenland Interstadials 8 to 6, we find good evidence for rapid renewed ice-sheet build-up in NW Scotland, with the Minch ice-stream terminus reaching the continental shelf edge in Greenland Stadial 5, perhaps only briefly. Deglaciation of the NW sector took place in numerous stages. Several grounding-zone wedges and moraines on the mid- and inner continental shelf attest to significant stabilizations of the ice-sheet grounding line, or ice margin, during overall retreat in Greenland Stadials 3 and 2, and to the development of ice shelves. NW Lewis was the first substantial present-day land area to deglaciate, in the first half of Greenland Stadial 3 at a time of globally reduced sea-level c. 26 kabp , followed by Cape Wrath at c. 24 kabp. The topographic confinement of the Minch straits probably promoted ice-shelf development in early Greenland Stadial 2, providing the ice stream with additional support and buffering it somewhat from external drivers. However, c. 20–19 kabp , as the grounding-line migrated into shoreward deepening water, coinciding with a marked change in marine geology and bed strength, the ice stream became unstable. We find that, once underway, grounding-line retreat proceeded in an uninterrupted fashion with the rapid loss of fronting ice shelves – first in the west, then the east troughs – before eventual glacier stabilization at fjord mouths in NW Scotland by ~17 kabp. Around the same time, ~19–17 kabp , ice-sheet lobes readvanced into the East Minch – possibly a glaciological response to the marine-instability-triggered loss of adjacent ice stream (and/or ice shelf) support in the Minch trough. An independent ice cap on Lewis also experienced margin oscillations during mid-Greenland Stadial 2, with an ice-accumulation centre in West Lewis existing into the latter part of Heinrich Stadial 1. Final ice-sheet deglaciation of NW mainland Scotland was punctuated by at least one other coherent readvance at c. 15.5 kabp , before significant ice-mass losses thereafter. At the glacial termination, c. 14.5 kabp , glaciers fed outwash sediment to now-abandoned coastal deltas in NW mainland Scotland around the time of global Meltwater Pulse 1A. Overall, this work on the BIIS NW sector reconstructs a highly dynamic ice-sheet oscillating in extent and volume for much of the last 45 000 years. Periods of expansive ice-sheet glaciation dominated by ice-streaming were interspersed with periods of much more restricted ice-cap or tidewater/fjordic glaciation. Finally, this work indicates that the role of ice streams in ice-sheet evolution is complex but mechanistically important throughout the lifetime of an ice sheet – with ice streams contributing to the regulation of ice-sheet health but also to the acceleration of ice-sheet demise via marine ice-sheet instabilities.     

Orbital changes and climate

At the 41,000-period of orbital tilt, summer insolation forces a lagged response in northern ice sheets. This delayed ice signal is rapidly transferred to nearby northern oceans and landmasses by atmospheric dynamics. These ice-driven responses lead to late-phased changes in atmospheric CO₂ that provide positive feedback to the ice sheets and also project ‘late’ 41-K forcing across the tropics and the Southern Hemisphere. Responses in austral regions are also influenced by a fast response to summer insolation forcing at high southern latitudes.At the 22,000-year precession period, northern summer insolation again forces a lagged ice-sheet response, but with muted transfers to proximal regions and no subsequent effect on atmospheric CO₂. Most 22,000-year greenhouse-gas responses have the ‘early’ phase of July insolation. July forcing of monsoonal and boreal wetlands explains the early CH₄ response. The slightly later 22-K CO₂ response originates in the southern hemisphere. The early 22-K CH₄ and CO₂ responses add to insolation forcing of the ice sheets.The dominant 100,000-year response of ice sheets is not externally forced, nor does it result from internal resonance. Internal forcing appears to play at most a minor role. The origin of this signal lies mainly in internal feedbacks (CO₂ and ice albedo) that drive the gradual build-up of large ice sheets and then their rapid destruction. Ice melting during terminations is initiated by uniquely coincident forcing from insolation and greenhouse gases at the periods of tilt and precession.     

The Late Devensian (<22,000 BP) Irish Sea Basin: The sedimentary record of a collapsed ice sheet margin

The Late Devensian (<20 ka BP) glacial geology of the Irish Sea Basin (4000 km²) is an event stratigraphy recording the entry of marine waters into a glacio-isostatically-depressed basin, and the rapid retreat of the Irish Sea Glacier as a tidewater ice margin. Marine limits occur up to 140 m O.D. Across much of the central basin, the ice margin was uncoupled from its bed exposing a subglacially-scoured topography to glaciomarine processes. The Irish Sea Glacier was a major drainage conduit of the last British Ice Sheet; calving of the marine ice margin resulted in fast flow (surging) of ice streams recorded by drumlin fields around the northern basin margin and tunnel valleys. Rapid evacuation of the basin may have stranded large areas of dead ice in peripheral zones (e.g. Cheshire/Shropshire Lowlands) and initiated the collapse of the ice sheet.Thick wedges of ice-contact glaciomarine sediments were deposited during ice retreat as morainal bank complexes by successive tidewater ice margins stabilized at pinning points around the Irish Sea coast. Where morainal banks occur on the seaward side of drumlin swarms there is a clear sequential relationship between rapid ice loss from calving ice margins, the development of fast flowing ice streams, drumlinization and the pumping of subglacial sediment to tidewater. Raised delta complexes are locally associated with marine limits along the high relief coastal margins of Wales, east central Ireland, and the Lake District. Associated valley infill complexes record downslope resedimentation of heterogenous sediments into the marine environment during ice retreat. Co-eval offshore deposits are represented by well-stratified glaciomarine complexes that infill a subglacially-scoured topography that shows networks of tunnel valleys. Glaciomarine mud drapes occur well to the south of the maximum limit of grounded ice in the basin (e.g. North Devon, Scilly Islands, Southern Ireland). The age of these distal sediments, previously mapped as pre-Devensian tills, is constrained by amino acid ratios.Basin rebound following deglaciation was rapid, with over 100 m recovery in 3 ka, and was followed by a low marine still stand. Peat, accumulating in offshore areas now as much as 55 m below sea level has been drowned by the postglacial eustatic rise in sea level.The glacio-sedimentary model identified in this paper, involving rapid ice retreat and related sedimentation triggered by rising relative sea level, suggests that isotatic downwarping is an important mechanism for deglaciating continental shelves.     

Phase relationship between sea level and abrupt climate change

Direct traces of past sea levels are based on the elevation of old coral reefs at times of sea level highstands. However, these measurements are discontinuous and cannot be easily correlated with climate records from ice cores. In this study we show a new approach to recognizing the imprint of sea level changes in continuous sediment records taken from the continental slope at locations that were continuously submerged, even during periods of sea level lowstand. By using a sediment core precisely synchronized with Greenland ice cores, we were able to recognize major floods of the Mediterranean continental shelf over the past 270 kyr. During the last glacial period five flooding events were observed at the onset of the warmest Greenland interstadials. Consistent correspondence between warm climate episodes and eustatic sea level rises shows that these global flooding events were generated by pronounced melting of the Northern Hemisphere ice sheets, due to rapid intensification of Atlantic Meridional Overturning Circulation.The method described in this study opens a new perspective for inter-hemispheric synchronization of marine climate records if applied in other continental margins from the Southern Hemisphere or the equatorial regions.     

地球气候变化的米兰科维奇理论研究进展

米兰科维奇理论是从全球尺度上研究日射量与地球气候之间关系的天文理论（以下简称为“米氏理论”）。该理论认为,地球轨道偏心率、黄赤交角及岁差等三要素变化引起的到达北半球中高纬度夏季日射量变化是造成冰期—间冰期旋回的根本原因。详细回顾了米氏理论的发展历程,并以南极东方站过去42万年大气和气候变化的历史资料为例,讨论了经典米氏理论中有待研究的若干问题。     

更新世冰盖与大洋碳储库相互作用的箱式模型模拟

全球大洋深海有孔虫碳同位素(δ13C)记录中广泛发现40万年周期,这一周期可能与偏心率长周期的轨道驱动有关.1.6 Ma以来,δ13C的这一长周期拉长到50万年,且重值期不再与偏心率低值对应.目前对δ13C 40万年周期的成因及其周期拉长的机制还不明确.这里使用了包含9个箱体的箱式模型,用于研究热带过程与冰盖相互作用及其对大洋碳循环的影响.模拟结果显示当北半球高纬海区海冰迅速增大时冰盖迅速融化,进入冰消期,而当海冰快速消失后,冰盖则重新缓慢增长.冰盖变化具有冰期长,间冰期短的非对称形态.在季节性太阳辐射量的驱动下冰盖变化具有10万年冰期-间冰期旋回.当冰盖融化速率受北半球高纬夏季太阳辐射量控制时,冰盖变化的岁差周期明显加强,相位与地质记录一致,说明轨道驱动可以通过非线性相位锁定机制使冰盖变化与其在相位上保持一致.海冰的阻隔效应使大气中CO₂在冰消期时增多.冰期时大洋环流减弱使大气中CO₂逐渐减少.当模型只有ETP驱动的风化作用而不考虑冰盖变化时,模拟的δ13C记录显示极强的40万年周期,体现了大洋碳储库对热带风化过程的响应.当同时考虑冰盖变化和风化作用时,模拟的δ13C结果中40万年周期减弱而10万年周期加强,并且40万年周期上碳储库与偏心率的相位与不考虑冰盖变化时的相位也存在差异,反映了冰盖变化引起的洋流改组压制了大洋碳循环对热带过程的响应.      

Continental ice sheets and the planetary radiation budget

The interaction between continential ice sheets and the planetary radiation budget is potentially important in climate-sensitivity studies. A simple ice-sheet model incorporated in an energybalance climate model provides a tool for studying this interaction in a quantitative way. Experiments in which the ice-sheet model is coupled step by step to the climate model show that ice sheets hardly affect the zonal mean radiation balance because the albedo feedback due to sea ice and snow cover is dominating. The model requires a 5% drop in the solar constant to create ice sheets of ice-age size.If the feedback between surface elevation and ice-mass balance is included (in a very crude way), the ice-sheet size (L, measured southward from 70°N) becomes much more sensitive to in insolation. For a range of normalized solar constants, roughly from 0.98 to 1.02, two stable solutions exist: L 0 and L 2000 km. This result demonstrates that the response of ice sheets to insolation variations is far from linear. It also stresses the need for explicit modeling of the ice-mass balance of ice sheets, particularly its dependence on surface elevation.     

Structure and timing of the last deglaciation: Oxygen-isotope evidence

Foraminiferal oxygen-isotope data from 24 tropical Atlantic sediment cores, constrained by 77 ¹⁴C dates, are stacked to form a composite record of isotopic Termination 1.. This record indicates that most of the isotopic transition at the end of the last ice age occurred between 14 ka BP and 6 ka BP. Minor isotopic expression of deglaciation is permitted as early as 16 ka BP, but the most rapid rate of change occurred between 14 ka BP and 12 ka BP. Three ‘steps’ of maximum change are present. Although they are close to the statistical limits of detection in the composite record, the clear presence of the steps in individual records suggests that they are real. We estimate their timing at 14-12 ka BP (Termination 1-a), 10-9 ka BP (Termination 1-b), and 8-6 ka BP (Termination 1-c).Centering of the termination near 11 ka BP is consistent with the ‘Milankovitch’ hypothesis that high summer insolation caused deglaciation. In detail, however, maximum rates of change prior to the 11 ka BP insolation extreme, and the inferred steps require additional mechanisms controlling the tempo of glacial-interglacial climate change. Steps equivalent to those in δ¹⁸O have not been detected in ice-margin retreat data. Steps in the isotopic transition, if real, may record thinning of the ice sheets not accompanied by loss of area. Alternation between near-equilibrium and near-stagnant ice-sheet profiles during deglaciation is hypothesized, perhaps due to calving and unstable ‘down-draw’ of the ice sheets followed by partial re-equilibration. Significant problems remain. The effects of temperature on the isotope record are only partially constrained. Presently available data allow only semi-quantitative intercalibration of ice volume, sea level, and isotopic estimates of glaciation.     

Ice-age simulations with a calving ice-sheet model

Variations of ice-sheet volume during the Quaternary ice ages are simulated using a simple ice-sheet model for the Northern Hemisphere. The basic model predicts ice thickness and bedrock deformation in a north-south cross section, with a prescribed snow-budget distribution shifted uniformly in space to represent the orbital perturbations. An ice calving parameterization crudely representing proglacial lakes or marine incursions can attack the ice whenever the tip drops below sea level. The model produces a large ～ 100,000-yr response in fair agreement (correlation coefficient up to 0.8) with the δ¹⁸O deep-sea core records. To increase confidence in the results, several of the more uncertain model components are extended or replaced, using an alternative treatment of bedrock deformation, a more realistic ice-shelf model of ice calving, and a generalized parameterization for such features as the North Atlantic deglacial meltwater layer. Much the same ice-age simulations and agreement with the δ¹⁸O records, as with the original model, are still obtained. The model is run with different types of forcing to identify which aspect of the orbital forcing controls the phase of the 100,000-yr cycles. First, the model is shown to give a ～ 100,000-yr response to nearly any kind of higher-frequency forcing. Although over the last 2-million yrs the model phase is mainly controlled by the precessional modulation due to eccentricity, over just the last 500,000 yr the observed phase can also be simulated with eccentricity held constant. A definite conclusion on the phase control of the real 100,000-yr cycles is prevented by uncertainty in the deep-sea core time scales before ～600,000 yr B.P. The model is adapted to represent West Antarctica, and yields unforced internal oscillations with periods of about 50,000 yr.     

Record of a Mid-Pleistocene depositional anomaly in West Antarctic continental margin sediments: an indicator for ice-sheet collapse?

Modern global warming is likely to cause future melting of Earth's polar ice sheets that may result in dramatic sea-level rise. A possible collapse of the West Antarctic Ice Sheet (WAIS) alone, which is considered highly vulnerable as it is mainly based below sea level, may raise global sea level by up to 5–6 m. Despite the importance of the WAIS for changes in global sea level, its response to the glacial–interglacial cycles of the Quaternary is poorly constrained. Moreover, the geological evidence for the disintegration of the WAIS at some time within the last ca. 750 kyr, possibly during Marine Isotope Stage (MIS) 11 (424–374 ka), is ambiguous. Here we present physical properties, palaeomagnetic, geochemical and clay mineralogical data from a glaciomarine sedimentary sequence that was recovered from the West Antarctic continental margin in the Amundsen Sea and spans more than the last 1 Myr. Within the sedimentary sequence, proxies for biological productivity (such as biogenic opal and the barium/aluminum ratio) and the supply of lithogenic detritus from the West Antarctic hinterland (such as ice-rafted debris and clay minerals) exhibit cyclic fluctuations in accordance with the glacial–interglacial cycles of the Quaternary. A prominent depositional anomaly spans MIS 15–MIS 13 (621–478 ka). The proxies for biological productivity and lithogenic sediment supply indicate that this interval has the characteristics of a single, prolonged interglacial period. Even though no proxy suggests environmental conditions much different from today, we conclude that, if the WAIS collapsed during the last 800 kyr, then MIS 15–MIS 13 was the most likely time period. Apparently, the duration rather than the strength of interglacial conditions was the crucial factor for the WAIS drawdown. A comparison with various marine and terrestrial climate archives from around the world corroborates that unusual environmental conditions prevailed throughout MIS 15–MIS 13. Some of these anomalies are observed in the pelagic Southern Ocean and the South Atlantic and might originate in major ice-sheet drawdown in Antarctica, but further research is required to test this hypothesis.     

Évolution géomorphologique de l'avant-pays du Sud-Ouest de la Nouvelle-Calédonie durant les derniers cycles glaciaires

We present the morphological analysis of a combined digital elevation model (DEM) obtained by merging data from terrestrial and coastal regions of southern New-Caledonia. Our aim is to describe the Quaternary morphotectonic evolution of the southwestern lagoon of New Caledonia, a region with a shallow average bathymetry (20 m), recently submerged in response to both tectonic and eustatic fluctuations. The marine DEM was obtained by interpolating 131 000 depth soundings from MOP–SHOM (‘Mission océanographique du Pacifique’–‘Service hydrographique et océanographique de la Marine’, France). We confirm that morphologies such as meandering channels and deeply incised canyons are the result of a sub-aerial fluviatile evolution during periods of low sea levels. The marine sedimentation in this region is very recent, probably corresponding to no more than two glacial–interglacial cycles. Geometrical relationships between submerged paleo-rivers and streams, including captures and incisions, strongly suggest that Quaternary active tectonics affected river development. We identify two hydrographic palaeo-networks suggesting outflow changes, from parallel to the present-day island axis to perpendicular. To cite this article: V. Chevillotte et al., C. R. Geoscience 337 (2005).     

Oceanic evidence for the mechanism of rapid northern hemisphere glaciation

The oxygen isotopic stage 5/4 boundary in deep-sea sediments marks a prominent interval of northern hemisphere ice-sheet growth that lasted about 10,000 yr. During much of this rapid ice growth, the North Atlantic Ocean from at least 40°N to 60°N maintained warm sea-surface temperatures, within 1° to 2°C of today's subpolar ocean. This oceanic warmth provided a local source of moisture for ice-sheet accretion on the adjacent continents. The unusually strong thermal gradient off the east coast of North America (an “interglacial” ocean alongside a “glacial” land mass) also should have directed low-pressure storms from warm southern latitudes north-ward toward the Laurentide Ice Sheet. In addition, minimal calving of ice into the North Atlantic occurred during most of the stage 5/4 transition, indicative of ice retention within the continents. Diminished summer and autumn insolation, a warm subpolar ocean, and minimal calving of ice are conducive to rapid and extensive episodes of northern hemisphere ice-sheet growth.     

中更新世气候转型与100ka周期研究

中更新世气候转型是第四纪气候变化中最重要的特征之一，它是指全球气候的主导周期在中更新世时从41ka转变为100ka，且气候波动的幅度也加大。经典的Milankovitch假说不能完全解释中更新世气候转型的原因以及100ka周期在气候记录中的强烈表现，因为太阳辐射与气候记录之间存在着相当的差异，尤其是二者在变化幅度上不匹配。近年来围绕这一转型过程的时代和原因获得了一些新的进展，主要是针对中更新世气候转型的时间、对气候记录中100ka周期的重新检讨以及非太阳辐射因素在这一转型过程中所起的作用。其它可能的转型原因包括大冰盖、温室气体、地球轨道面倾角、冰盖基底、构造隆升等。     

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.