This implies that as a minimum, a regional ice sheet centred on the Ellsworth-Whitmore uplands may have survived Pleistocene warm periods.
If so, it constrains the WAIS contribution to global sea level rise during interglacials to about 3. Recent studies have suggested that such a collapse may already be underway in the Pacific-facing sector of the ice sheet 4 , 5. Constraining the past ice behaviour would allow a more confident assessment of its potential contribution to past and future sea level change.
Marine biological evidence based on diatoms and the similarity of octopus and Bryozoa between the Pacific and Atlantic sectors suggests that much of the ice sheet disappeared during interglacials, creating an open seaway between these sectors 6 , 7 , 8. Such a conclusion is reinforced by estimates of higher-than-present global sea levels during interglacials 9 , Efforts to constrain the minimum configuration of the ice sheet in the past have relied on numerical ice-sheet models, each with its own set of assumptions on boundary conditions, internal dynamics and external forcing for example, climate and sea level 3 , 11 , The models suggest that most upland areas could have remained glaciated even during the warmest interglacials, but whether as individual mountain glaciers or as larger regional ice sheets remains uncertain, and there is no direct evidence from the continent to constrain this.
The rose diagram shows persistent katabatic winds from the south-southwest recorded at the Patriot Hills blue-ice aircraft runway over two months in the austral summer of ref.
Wind-drift glaciers can be seen along the summit ridge to the right of Mt. Fordell see Supplementary Fig. Two component massifs, Patriot Hills and Marble Hills are summits of a km-wide upland bounded by troughs excavated to below sea level At present, ice from the central WAIS flows around and between these mountains to the grounding line.
Katabatic winds flow down the ice slope from the divide towards Hercules Inlet crossing the mountains and creating blue-ice areas in their lee Fig. The winds cause ablation of surface ice that in turn causes a compensating upward flow of ice that brings basal debris to the ice-surface as blue-ice moraines 14 , This ice-marginal, basally derived material is deposited higher on the mountain flanks and records past changes in ice thickness.
The use of cosmogenic nuclide dating on bedrock and glacially transported material on nunataks in Antarctica has provided much quantitative data on the history of ice thickness changes over time 16 , 17 , 18 , 19 , 20 , 21 , 22 , These data led to the untested hypothesis that the spread in ages represented the continuous presence of an ice sheet that fluctuated in thickness in response to glacial—interglacial cycles The range of ages reflects preservation of some erratics and deposition of others during successive glaciations.
An alternative possibility is that the moraines represent composite features formed by multiple ice-sheet inundations interspersed with periods of local mountain glaciation or deglaciation. The combination of geomorphological analysis of landforms and measurement of multiple cosmogenic nuclides can provide rare insight into ice-sheet history.
The advantage of measuring multiple cosmogenic nuclides in single samples is that both the age and exposure history can be constrained For example, if a previously exposed clast is buried by ice long enough for the shorter lived of two nuclides to decay preferentially, the signal will be observed in the isotopic ratio By measuring multiple isotopes in three adjacent erratics at each specific sampling site, the degree of scatter and extent to which the erratics have shared the same history of exposure can be determined.
Thus, one can gain information on the age of deposition and possible subsequent overriding and disturbance by ice. Here we use geomorphological analysis of landforms and deposits supported by in situ cosmogenic 26 Al, 10 Be and 21 Ne from newly collected, quartz-bearing erratics to investigate elevated blue-ice moraines. Our evidence reveals several relict ice-marginal blue-ice moraine deposits as old as 1. The isotopic evidence indicates that the highest deposits have not been disturbed by ice since deposition, but lower deposits have experienced subsequent burial.
All geomorphic and cosmogenic nuclide data are consistent with an ice sheet that thickened and thinned in response to quaternary glacial—interglacial cycles. We find no evidence to suggest a change in glaciological conditions that would accompany the loss of the entire ice sheet and the build-up of individual mountain glaciers.
We interpret this consistency as evidence for continuous ice-sheet conditions in this part of the Weddell Sea sector. The minimum configuration that maintains strong katabatic winds is a regional ice sheet centred on the Ellsworth-Whitmore block. This interpretation, where the WAIS shows dynamic equilibrium about a continuous ice divide, supports numerical models that indicate a maximum WAIS contribution to sea level of about 3.
Such an interpretation is also consistent with marine biological evidence indicating an open seaway in West Antarctica during some interglacials 6 , 7 , 8. The geomorphological analysis of landforms and deposits reveals currently active blue-ice moraines at the edge of glaciers at the eastern foot of the mountains Supplementary Figs 1—4.
Striated, basally derived clasts occur in the moraines and in folded debris bands in the adjacent glacier surface. Above the ice margin are two formerly glaciated zones marked by an upper erosional trimline Fig. Lower down in this zone, the weathered erratics and till have been disturbed by eastward flowing ice. This is demonstrated by erratics preferentially trapped in irregularities in the bedrock and the preservation of till patches in basins and on the eastern side of bedrock bumps, leaving the western slopes and summits relatively free of debris.
The weathered deposits represent former ice-marginal blue-ice moraines. This conclusion is borne out by the location and concentration of till patches at the foot of a mountain escarpment athwart katabatic winds, their proximity to a former ice margin and the lack of erratics above the trimline Supplementary Fig.
Moreover, the shape and lithology of the erratics is the same as the quartz-rich lithologies in the moraines at the current ice edge. The lower unweathered zone is characterized by fresh erratics, perched boulders and ice-cored tills; it is thought to reflect deposition by ice during the Last Glacial Maximum 24 and is not considered here.
The origin of the debris dotted line , much of it locally derived, is from deep within the glacier trough, indeed close to present sea level. In addition to the lateral flow there is a limited longitudinal component of flow to the east into the page. We measured in situ cosmogenic 26 Al, 10 Be and 21 Ne on newly collected quartz-bearing erratics see Methods and Supplementary Tables 1—3.
The exposure ages of the weathered erratics decline with decreasing elevation towards the glacier surface Fig. Three adjacent erratics from each of two sites in the upper weathered zone in the Marble Hills have 10 Be exposure ages of 1. The samples at both sites yield tightly clustered ages for each isotope with ratios that do not indicate prolonged burial see Methods and Supplementary Fig. Lower down, erratics have younger 10 Be exposure ages of 0. Comparable erratics from a patch of weathered till in the Patriot Hills have similarly clustered 10 Be exposure ages of 0.
The striking feature of the data from the high elevation samples is that they reflect a shared origin and exposure history. Rather than the scatter of ages one might expect with repeated episodes of burial by ice, there is a consistent pattern of decreasing age of exposure and increasing degree of burial towards the present glacier margin.
This suggests that any subsequent burial at the different sites was by cold-based ice that did not move existing material or deposit new material in the process. The apparent exposure ages of weathered rock samples from the Marble Hills blue and Patriot Hills red plotted against elevation above the ice margin.
There is a clear decline in exposure ages with decreasing elevation at the Marble Hills. A similar exposure history involving burial is inferred for samples above the Last Glacial Maximum limit 24 in the Patriot Hills Supplementary Figs The simplest explanation of the pattern of cosmogenic nuclide data is that an ice margin fluctuated in elevation on the mountain flank Fig. The highest erratics are exposed for the longest time, while progressively lower erratics are exposed for increasingly shorter periods of time.
This explains both the younging trend and evidence of increased burial with decreased altitude. The implication of exposure ages of up to 1. Given the mountains are situated near the grounding line of today, increases in ice thickness near the mountains would accompany any seaward migration of the grounding line as ocean temperature cooled and global sea level fell Over millions of years one would expect glacial erosion to lower the ice-sheet surface relative to the mountains 29 , 30 and thus the cyclic changes in ice thickness would be superimposed on a trajectory of lowering relative to the mountains.
This scenario is consistent with the great ages and minimal burial of the highest erratics. The reconstruction looking west shows a the present minimum, b past maximum blue-ice relationships and c features that would accompany local glaciation. The ages of weathered quartz-rich erratics on the ice-scoured bedrock upland decline and their burial history becomes more complex at lower elevations, the latter reflecting longer periods of burial by ice.
The Last Glacial Maximum limit is often the lowest on the upland plateau. The evidence is consistent with oscillations in ice thickness related to Pleistocene sea level fluctuations. There is no evidence of local glaciation radiating from the massifs.
It could be argued that the scenario above should produce a scatter of exposure ages of up to 1. The clustering may reflect episodes in the past when conditions were particularly favourable for the formation of blue-ice moraines at those particular locations, just as the modern concentration of blue-ice material varies with local topography and ice-surface elevation.
In addition, clustering would be an expected artefact of the sampling, which was concentrated on sites at specific altitudes. The implication of the evidence above is that the WAIS divide and associated katabatic winds have also been present for at least 1. The blue-ice moraines form because of strong katabatic winds and these in turn are strongest and most consistent when they flow downslope for hundreds of km.
The loss of the ice divide would diminish both katabatic winds and blue-ice moraine formation. Could such an assemblage of deposits survive loss of the WAIS?
The recorded ages leave adequate intervals of time for the ice divide to disappear. If the ice sheet disappeared, ice caps and glaciers would likely build up on mountain massifs in a fjord landscape. Each massif would have a different and locally radial pattern of flow depending on the type and scale of the topography Fig. We found none of the features characteristic of such a scenario. Rather than radial flow from the mountain axis, Marble and the Patriot Hills bear geomorphologic evidence of eastward ice flow Supplementary Fig.
In other fjord areas of the world, glacial deposits typically include marine traces, such as diatoms, shells and glaciomarine muds Scherer, personal communication, Local glaciation typically produces deposits associated with local corrie glaciers, as in the Asgard and Olympus ranges in the Transantarctic Mountains Rather than corrie glacier deposits, concentrations of material with a local origin in the study area are restricted to wind-drift glaciers that merged local rocks with exotic material in blue-ice moraines.
Indeed, the very existence of former wind-drift glaciers supports the existence of the ice sheet and associated katabatic winds. The lack of evidence of marine and local glaciation cannot on its own rule out short periods of complete deglaciation.
The cosmogenic nuclide data alone are not a direct test of this hypothesis. It is possible that some evidence may remain preserved beneath the ice sheet or that the characteristic geomorphology is missing or poorly developed. Corrie and wind-drift glaciers could produce geomorphology that may be indistinguishable, while cold-based glaciers may leave no mark at all. The extents in km 2 for the current and for the years of minimum and maximum extents are provided below the image.
The different shades of gray over land indicate the land elevation with the lightest gray being the highest elevation. Figure 3. Seasonal cycle of Northern Hemisphere sea ice extents a and areas b , given as daily averages, for the years through The vertical line represents the last data point plotted. Figure 4 : Color-coded animation displaying the last 2 weeks of the daily sea ice concentrations in the Northern Hemisphere.
Why live in Antarctica? How many people? Why icebergs can have colours and stripes Icebergs are formed from the glacial ice that has built up from snow falling on the Antarctic continent over millennia. Dr Steve Nicol. Photo gallery See all. Without the scattering effect of air bubbles, light can penetrate ice more deeply. To the human eye, ancient glacial ice acts like a filter, absorbing red and yellow light and reflecting blue light, creating the beautiful blue hues of a glacier.
In contrast, snow is white because it is chock full of air bubbles. Snow reflects back the full spectrum of white light, just like a freshly poured soda has bubbly, light-colored foam on top. Blue ice sometimes emerges at the edge of Antarctica, where glaciers tumble into the sea. Summertime melting can also create smooth patches of blue glacial ice. But by definition, true blue-ice areas most often appear near Antarctica's mountain ranges.
0コメント