Quantifying the erosional impact of the Fennoscandian ice sheet in the Torneträsk–Narvik corridor, northern Sweden, based on cosmogenic radionuclide data

Despite spectacular landform evidence of a dominant role for glacial action in shaping landscapes under former northern hemisphere ice sheets, there is little quantitative evidence for rates and patterns of erosion associated with specific glaciations. Here we use cosmogenic nuclide data to assess rates of subglacial erosion underneath the Fennoscandian ice sheet. By testing whether there are remnant nuclide concentrations in samples taken from sites that include both relict areas and features and landscapes typically associated with vigorous glacial erosion, we can constrain the level and pattern of modification that resulted from the last glaciation. Cosmogenic ¹⁰Be and ³⁶Cl data from the Torneträsk region confirm the temporal and spatial variability of glacial erosion suggested by geomorphological mapping. At some sites, glacial erosion estimates in what appear to be heavily scoured areas indicate erosion of only c. 2 ± 0.4 m of bedrock, based on cosmogenic nuclide inheritance. This implies that the generation of severely scoured terrain in this study area required multiple glaciations. The overall modification produced by ice sheets along glacial corridors may be more restricted than previously thought, or may have occurred preferentially during earlier Quaternary glacial periods.     

NEW 10BE COSMOGENIC AGES FROM THE VIMMERBY MORAINE CONFIRM THE TIMING OF SCANDINAVIAN ICE SHEET DEGLACIATION IN SOUTHERN SWEDEN

The overall pattern of deglaciation of the southern part of the Scandinavian Ice Sheet has been considered established, although details of the chronology and ice sheet dynamics are less well known. Even less is known for the south Swedish Upland because the area was deglaciated mostly by stagnation. Within this area lies the conspicuous Vimmerby moraine, for which we have used the terrestrial cosmogenic nuclide (¹⁰Be) exposure dating technique to derive the exposure age of six glacially transported boulders. The six ¹⁰Be cosmogenic ages are internally consistent, ranging from 14.9 ± 1.5 to 12.4 ± 1.3 ka with a mean of 13.6 ±0.9 ka. Adjusting for the effects of surface erosion, snow burial and glacio-isostatic rebound causes the mean age to increase only by c. 6% to c. 14.4± 0.9 ka. The ¹⁰Be derived age for the Vimmerby moraine is in agreement with previous estimates forthe timing of deglaciation based on radiocarbon dating and varve chronology. This result shows promise for further terrestrial cosmogenic nuclide exposure studies in southern Sweden.     

Cosmogenic Be and Al exposure ages of tors and erratics, Cairngorm Mountains, Scotland: Timescales for the development of a classic landscape of selective linear glacial erosion

The occurrence of tors within glaciated regions has been widely cited as evidence for the preservation of relic pre-Quaternary landscapes beneath protective covers of non-erosive dry-based ice. Here, we test for the preservation of pre-Quaternary landscapes with cosmogenic surface exposure dating of tors. Numerous granite tors are present on summit plateaus in the Cairngorm Mountains of Scotland where they were covered by local ice caps many times during the Pleistocene. Cosmogenic ¹⁰Be and ²⁶Al data together with geomorphic relationships reveal that these landforms are more dynamic and younger than previously suspected. Many Cairngorm tors have been bulldozed and toppled along horizontal joints by ice motion, leaving event surfaces on tor remnants and erratics that can be dated with cosmogenic nuclides. As the surfaces have been subject to episodic burial by ice, an exposure model based upon ice and marine sediment core proxies for local glacial cover is necessary to interpret the cosmogenic nuclide data. Exposure ages and weathering characteristics of tors are closely correlated. Glacially modified tors and boulder erratics with slightly weathered surfaces have ¹⁰Be exposure ages of about 15 to 43 ka. Nuclide inheritance is present in many of these surfaces. Correction for inheritance indicates that the eastern Cairngorms were deglaciated at 15.6 ± 0.9 ka. Glacially modified tors with moderate to advanced weathering features have ¹⁰Be exposure ages of 19 to 92 ka. These surfaces were only slightly modified during the last glacial cycle and gained much of their exposure during the interstadial of marine Oxygen Isotope Stage 5 or earlier. Tors lacking evidence of glacial modification and exhibiting advanced weathering have ¹⁰Be exposure ages between 52 and 297 ka. Nuclide concentrations in these surfaces are probably controlled by bedrock erosion rates instead of discrete glacial events. Maximum erosion rates estimated from ¹⁰Be range from 2.8 to 12.0 mm/ka, with an error weighted mean of 4.1 ± 0.2 mm/ka. Three of these surfaces yield model exposure-plus-burial ages of 295_− 71^+ 84, 520_− 141^+ 178, and 626_− 85^+ 102 ka. A vertical cosmogenic nuclide profile across the oldest sampled tor indicates a long-term emergence rate of 31 ± 2 mm/ka. These findings show that dry-based ice caps are capable of substantially eroding tors by entraining blocks previously detached by weathering processes. Bedrock surfaces and erratic boulders in such settings are likely to have nuclide inheritance and may yield erroneous (too old) exposure ages. While many Cairngorm tors have survived multiple glacial cycles, rates of regolith stripping and bedrock erosion are too high to permit the widespread preservation of pre-Quaternary rock surfaces.     

Glacial and palaeoenvironmental history of the Cape Chelyuskin area, Arctic Russia

Quaternary glacial stratigraphy and relative sea-level changes reveal at least two glacial expansions over the Chelyuskin Peninsula, bordering the Kara Sea at about 77°N in the Russian Arctic, as indicated from tills interbedded with marine sediments, exposed in stratigraphic superposition, and from raised-beach sequences mapped to altitudes of at least up to ca. 80 m a.s.l. Chronological control is provided by accelerator mass spectrometry ¹⁴C dating, electron-spin resonance and optically stimulated luminescence geochronology. Major glaciations, followed by deglaciation and marine inundation, occurred during marine oxygen isotope stages 6–5e (MIS 6–5e) and stages MIS 5d–5c. These glacial sediments overlie marine sediments of Pliocene age, which are draped by fluvial sediment of a pre-Saalian age, thereby forming palaeovalley/basin fills in the post-Cretaceous topography. Till fabrics and glacial tectonics record expansions of local ice caps exclusively, suggesting wet-based ice cap advance, followed by cold-based regional ice-sheet expansion. Local ice caps over highland sites along the perimeter of the shallow Kara Sea, including the Byrranga Mountains and the Severnaya Zemlya archipelago, appear to have repeatedly fostered initiation of a large Kara Sea ice sheet, with the exception of the Last Glacial Maximum (MIS 2), when Kara Sea ice neither impacted the Chelyuskin Peninsula nor Severnaya Zemlya, and barely touched the northern coastal areas of the Taymyr Peninsula.     

A GLACIAL STAGE SPANNING THE ANTARCTIC COLD REVERSAL IN TORRES DEL PAINE (51°S), CHILE, BASED ON PRELIMINARY COSMOGENIC EXPOSURE AGES

ABSTRACT. Initial cosmogenic ¹⁰Be results from a former ice limit in Torres del Paine indicate a shortlived stillstand or readvance of Patagonian ice culminating at 12–15 kyr BP with a mean age of 13.2 ± 0.8 kyr BP. The glacier extended some 40 km beyond the present ice margin and was within 15 km of the presumed Last Glacial Maximum limits. The timing of the glacier stage spans the cooling event recorded in Antarctic ice cores, termed the Antarctic Cold Reversal (14.5–12.9 kyr BP). This result implies that glaciers at these latitudes were out of phase with those in the northern hemisphere; instead they mirrored the climate structure of Antarctica during the last glacial to interglacial transition.     

Ice-free conditions in Novaya Zemlya 35 000–30 000 cal years B.P., as indicated by radiocarbon ages and amino acid racemization evidence from marine molluscs

Novaya Zemlya was covered by the eastern part of the Barents–Kara ice sheet during the glacial maximum of marine isotope stage 2 (MIS 2). We obtained ¹⁴C ages on 37 samples of mollusc shells from various sites on the islands. Most samples yielded ages in the range of 48–26 ¹⁴C Ky. Such old samples are sensitive to contamination by young ¹⁴C, and therefore their reliability was assessed using replicate analyses and amino acid geochronology. The extent of aspartic acid racemization (Asp D/L) indicates that many of the ¹⁴C ages are correct, whereas some are minimum ages only. The results indicate that a substantial part of Novaya Zemlya was ice-free about 35–27 ¹⁴C Kya, and probably even earlier. Corresponding shorelines up to >140 m a.s.l. indicate a large Barents–Kara ice sheet during early MIS 3. These results are consistent with findings from Svalbard and northern Russia: in both places a large MIS 4/3 Barents–Kara ice sheet is postulated to have retreated about 50 Kya, followed by an ice-free interstadial that lasted until up to ca. 25 Kya. The duration of the MIS 2 glaciation in Novaya Zemlya was calculated by applying the D/L values to a kinetic equation for Asp racemization. This indicates that the islands were ice covered for less than 3000 years if the basal temperature was 0^oC, and for less than 10 000 years if it was −5^oC.     

地表岩石侵蚀速率对宇生核素暴露测年影响的研究

利用原地生宇生核素测定暴露年代时,通常会假设地貌体侵蚀速率为0。研究表明,该假设会低估地貌体的真实暴露年代。搜集2009~2012年全球不同区域56个岩石样品的宇生核素¹⁰Be测年数据,探讨侵蚀速率为0对于侵蚀速率为0.5、1以及2 mm/ka的样品,在不同暴露尺度上对暴露年代计算的影响幅度。结果表明,对于1×10⁴a尺度的样品暴露年代可能低估约0.5%,1%,2%;对于10×10⁴a尺度的样品可能低估约5%,7%,20%;对于50×10⁴a尺度的样品可能低估约40%,70%甚至100%以上。     

Unraveling complex exposure-burial histories of bedrock surfaces under ice sheets by integrating cosmogenic nuclide concentrations with climate proxy records

The production, accumulation, and decay of cosmogenic radionuclides in rock surfaces subjected to episodes of exposure and burial by ice results in nuclide concentrations in present day rock surfaces that can be used to address a variety of questions in glacial geomorphology and Quaternary geology. Of particular importance is the fact that these nuclide concentrations reflect both the timing of initial exposure of the rock surface and the chronology of subsequent exposure, burial, and erosion episodes. For landscapes where geomorphic evidence indicates that little/no erosion occurred, constraining the timing of initial exposure and the number of phases of exposure and burial that a rock surface has been subjected to is possible using multiple cosmogenic radionuclide concentrations combined with proxies for the timing and duration of periods of ice cover, such as ice core or marine isotope records. However, interpretations based on this approach require determination of an appropriate cutoff value to separate the proxy record into ice-free and ice-covered conditions and assessment of the sensitivity of the results to different cutoff values.We have developed a numerical model to evaluate variations in total exposure and burial durations as a function of different proxy records and cutoff values. This program is available at http://www.missouri.edu/~liyk/ClimateProxyCurve.zip. Initial results for sites in West Antarctica and northern Sweden show that the method provides a quick and robust way to derive best-fit cutoff values and chronologies of burial and exposure, and small changes in cutoff values can result in significant shifts in results. The method described here provides new insight into the interpretation and reliability of multiple nuclide samples. This approach also has the potential to provide improved constraints for ice sheet dynamics and landscape evolution, and a means to assess the sensitivity of calculated initial exposure dates to assumptions about ice sheet history.     

Using cosmogenic beryllium–7 as a tracer in sediment budget investigations

Recent advances in the use of the environmental radionuclides caesium–137 and unsupported lead–210 to quantify medium– and longer–term rates of erosion and sediment accumulation have proved of considerable value in catchment sediment budget investigations. However, there remains a need to explore the potential for using other shorter–lived radionuclides to provide evidence of sediment mobilisation, transport and storage over shorter timescales and particularly for individual events. This contribution reports the results of a study aimed at exploring the potential for using beryllium–7 (⁷Be, t _½= 53.3 days) to meet this requirement. The study investigated the use of ⁷Be as a sediment tracer in three key components of the sediment budget, namely, soil erosion and sediment mobilisation from slopes, the transport, storage and remobilisation of fine sediment in river channels and overbank deposition on river floodplains. The results presented clearly demonstrate the potential for using ⁷Be to obtain information on short–term and event–based sediment redistribution rates for use in catchment sediment budget investigations.     

Extent and Chronology of the Ross Sea Ice Sheet and the Wilson Piedmont Glacier along the Scott Coast at and Since the Last Glacial Maximum

During the last glacial maximum, a coalescent ice mass consisting of the grounded Ross Sea ice sheet and an expanded Wilson Piedmont Glacier covered the southern Scott Coast. This coalescent ice mass was part of a larger grounded ice sheet that occupied the Ross Sea Embayment during the last glacial maximum. Deglaciation of the western Ross Sea Embayment adjacent to the southern Scott Coast was delayed until shortly before 6500 ¹⁴C yr bp , aconclusion based on ages of marine shells from McMurdo Sound, a relative sea-level curve, and algae that lived in ice-dammed lakes. Therefore, most recession of grounded ice in the Ross Sea Embayment occurred in mid to late Holocene time, after deglacial sea-level rise due to melting of Northern Hemisphere ice sheets essentially was accomplished. Rising sea level alone could not have driven grounding-line retreat back to the present-day Siple Coast.     

Accumulation Rate Measurements at Taylor Dome, East Antarctica: Techniques and Strategies for Mass Balance Measurements in Polar Environments

Accumulation rate measurements on the East Antarctic plateau are challenging due to both spatial and temporal variability. Annual stratigraphy is often not reliably or consistently preserved in the firn, and so accumulation cannot be determined from snow pit stratigraphy. We present a suite of accumulation rate measurements collected over several seasons at Taylor Dome, East Antarctica. We compare net accumulation results from direct burial rate measurements and β -activity firn cores along a 35 km traverse. The two methods are consistent and show that the net accumulation varies from greater than 10 cm a⁻¹ to about 1 cm a⁻¹ (ice equivalent) southwest to northeast across the dome. We map the depth of shallow radar layering to interpolate and extrapolate these point-location measurements and show that considerable variations occur over kilometer scales resulting from subtle surface topography. We also present accumulation rates estimated from concentration of the cosmogenic isotope ¹⁰Be and from activity profiles of ²¹⁰Pb. Finally, satellite passive microwave data are used to estimate spatially averaged accumulation rates on the regional to continental scale to provide a context for these local observations. We show that robust mass balance measurements in this environment must rely on spatial and/or temporal averaging.     

Can glacial erosion limit the extent of glaciation?

In southern South America, the maximum areal extent of ice during the Quaternary Period, the Greatest Patagonian Glaciation (GPG, [Mercer, J.H., 1983. Cenozoic glaciation in the southern hemisphere. Annual Reviews of Earth and Planetary Science 11, 99–132.]), occurred at 1.1 Ma and subsequent glaciations were overall less extensive. The GPG preceded global minimum temperatures and maximal volume of ice, which occurred in the last ~ 800 kyr, as recorded in the marine δ¹⁸O record. Significant modification of the drainage morphology of the southern Andes from a non-glaciated to glaciated landscape occurred throughout the Quaternary Period. We infer a non-climatic relationship between glacial modification of the mountains and the decreasing extent of ice and we discuss processes of landscape development that could have caused the trend. Specifically, these include modification of valleys, such as development from a V- to a U-shape, and lowering of mass-accumulation areas. Such changes would strongly affect glacial dynamics, the mass balance profile and mass-flux during succeeding glaciations, especially for low-gradient outlet glaciers occupying low areas. Other areas around Earth (at least where ice has been warm-based) also may exhibit a non-random trend of decreasing extent of ice with time, ultimately because of glacial erosion in the Quaternary Period.     

Late Holocene palaeoceanography in Van Mijenfjorden, Svalbard

Based on detailed stratigraphical analysis of sediment cores spanning the last ca. 4000 calendar years, we reconstruct the palaeoceanograhic changes in the fiord Van Mijenfjorden, western Svalbard. Benthic foraminiferal δ¹⁸O indicate a gradual reduction in bottom water salinities between 2200 BC and 500 BC. This reduction was probably mainly a function of reduced inflow of oceanic water to the fiord, due to isostatic shallowing of the outer fiord sill. Stable salinity conditions prevailed between 500 BC and. 1300 AD. After the onset of a major glacial surge of the tidewater Paulabreen (Paula Glacier) system (PGS) around 1300 AD, there was a foraminiferal faunal change towards glacier proximal conditions, associated with a slight bottom water salinity depletion. During a series of glacial surges occuring from 1300 AD up the present salinity in the fiord has further decreased, corresponding to a δ¹⁸O depletion of 0.5 %o. This salinity decrease corresponds to the period when the PGS lost an equivalent of 30 – 40 % of its present ice volume, mainly through calving in the fiord.     

Glacial survival of blockfields on the Varanger Peninsula, northern Norway

For more than hundred years it has been debated whether blockfields in mountain summit areas can be used to delimit the vertical extent of Pleistocene ice sheets. In this study the relationship between blockfields, developed in quartzites and sandstones on the Varanger Peninsula, northern Norway, and glacially derived features have been evaluated. Erratics and circular ablation moraines are superimposed on the blockfields and lateral meltwater channels are eroded into them. Glacial striations and other signs of glacial sculpturing are restricted to low-lying areas with channelled ice flow. Relative ages of the blockfields and the features in them are inferred, and the first measurements of in-situ produced cosmogenic nuclides from the Varanger Peninsula are reported. We conclude that the blockfields have survived underneath at least one thick, cold-based ice sheet. Thus, these blockfields cannot be used as indicators of ice-free conditions as previously suggested for southern Norway. Our results have implications for the potential for land surface preservation beneath ice sheets and for glacial reconstructions in northern Fennoscandia.     

Mountain-derived versus shelf-based glaciations on the western Taymyr Peninsula, Siberia

The early Russian researchers working in central Siberia seem to have preferred scenarios in which glaciations, in accordance with the classical glaciological concept, originated in the mountains. However, during the last 30 years or so the interest in the glacial history of the region has concentrated on ice sheets spreading from the Kara Sea shelf. There, they could have originated from ice caps formed on areas that, for eustatic reasons, became dry land during global glacial maximum periods, or from grounded ice shelves. Such ice sheets have been shown to repeatedly inundate much of the Taymyr Peninsula from the north-west. However, work on westernmost Taymyr has now also documented glaciations coming from inland. On at least two occasions, with the latest one dated to the Saale glaciation (marine isotope stage 6 [MIS 6]), warm-based, bedrock-sculpturing glaciers originating in the Byrranga Mountains, and in the hills west of the range, expanded westwards, and at least once did such glaciers, after moving 50–60 km or more over the present land areas, cross today's Kara Sea coastline. The last major glaciation affecting south-western Taymyr did, however, come from the Kara Sea shelf. According to optically stimulated luminescence dates, this was during the Early or Middle Weichselian (MIS 5 or 4), and was most probably not later than 70 Kya. South-western Taymyr was not extensively glaciated during the last global glacial maximum ca. 20 Kya, although local cold-based ice caps may have existed.     

Rapid and rhythmic ice sheet fluctuations in western Scandinavia 15-40 Kya–a review

The onshore record of Middle to Late Weichselian sediments and glacial history in Norway indicates a succession of four major ice advances alternating with rapid, considerable ice recessions and interstadial conditions. During all the glacial advances the ice sheet expanded from onshore/inland positions to the shelf areas. The basis for visualizing these variations in glaciation curves constructed along nine transects from inland to shelf, and for interpretation of the palaeoclimatic history, is the regional Quaternary stratigraphy, more than 300 datings, fossil content and some palaeomagnetic data. The methods applied in recent years for AMS radiocarbon dating of glacial sediments with low organic carbon content have given promising results with respect to accuracy and precision, and the results of such datings were an important tool for our reconstructions and for timing of the ice oscillations. The rapid and rhythmic ice fluctuations, as reconstructed in our new model, have been fairly synchronous in most parts of Norway. Ice advances commenced and culminated at 40, 30-28, 24-21 and 18-15 (¹⁴C) Kya. We describe three intervening interstadials from inland sites: Hattfjelldal I, Hattfjelldal II and Trofors. Our stratigraphical record also includes many indications of high, pre-Holocene, relative sea levels, suggesting a considerable glacioisostatic depression of western Scandinavia during the interstadials. In our glaciation model we suggest that, in addition to precipitation, the mountainous fjord and valley topography, glacial isostasy and relative sea level changes were probably more important for the size of the glacial fluctuations than were air temperature changes.     

Late glacial palaeoceanography of Hinlopen Strait, northern Svalbard

Timing and structure of the Late and post-glacial development of the northern Svalbard margin, together with the initial influx of Atlantic water into the Arctic Ocean are still very poorly constrained. We investigated a sediment core (NP94-51) from a high accumulation area on the continental shelf north of Hinlopen Strait with the purpose of resolving the timing and structure of the last deglaciation. Detailed analyses of ice-rafted detritus, benthic and planktonic foraminiferal fauna, diatom flora, grain size and radiocarbon dates are used to reconstruct the palaeoceanographic evolution of the area. Our results indicate that the disintegration of Hinlopen Strait ice and possibly the northern margin of the Svalbard Ice Sheet commenced between 13.7 and 13.9 ¹⁴C Ky BP. Influx of subsurface Atlantic waters into the area (12.6 ¹⁴C Ky BP) and the retreat of the sea ice cover, with the accompanying opening of the surface waters (10.8 ¹⁴C Ky BP), happened at different times and both much later than the disintegration of the ice sheets. The transition into the Holocene shows a two-step warming.     

EVIDENCE FOR LATE-GLACIAL ICE DAMMED LAKES IN THE CENTRAL STRAIT OF MAGELLAN AND BAHÍA INÚTIL, SOUTHERNMOST SOUTH AMERICA

ABSTRACT. This paper critically appraises the evidence for a succession of ice-dammed lakes in the central Strait of Magellan ( c. 53°S) c. 17 000–12 250 cal. yr BP. The topographic configuration of islands and channels in the southern Strait of Magellan means that the presence of lakes provides compelling constraints on the position of former ice margins. Lake shorelines and glacio-lacustrine sediments have been dated by their association with a key tephra layer from Volcan Reclús (c. 15 510–14 350 cal. years bp ) and by ¹⁴C-dated peats. The timing of glacial lake formation and associated glacier readvances is at odds with the rapid and widespread glacier retreat of the Patagonian ice fields further north after c. 17 000 cal. yr bp , suggesting rather that the lakes were coeval with the Antarctic Cold Reversal and persisted to the Late-glacial/Holocene transition. This apparent asymmetrical latitudinal response in glacier behaviour may reflect overlapping spheres of northern hemisphere and Antarctic climatic influence in the Magellan region.     

Glacial rebound of the British Isles—II. A high-resolution, high-precision model

Observations of ice movements across the British Isles and of sea-level changes around the shorelines during Late Devensian time (after about 25 000 yr BP) have been used to establish a high spatial and temporal resolution model for the rebound of Great Britain and associated sea-level change. The sea-level observations include sites within the margins of the former ice sheet as well as observations outside the glaciated regions such that it has been possible to separate unknown earth model parameters from some ice-sheet model parameters in the inversion of the glacio-hydro-isostatic equations. The mantle viscosity profile is approximated by a number of radially symmetric layers representing the lithosphere, the upper mantle as two layers from the base of the lithosphere to the phase transition boundary at 400 km, the transition zone down to 670 km depth, and the lower mantle. No evidence is found to support a strong layering in viscosity above 670 km other than the high-viscosity lithospheric layer. Models with a low-viscosity zone in the upper mantle or models with a marked higher viscosity in the transition zone are less satisfactory than models in which the viscosity is constant from the base of the lithosphere to the 670 km boundary. In contrast, a marked increase in viscosity is required across this latter boundary. The optimum effective parameters for the mantle beneath Great Britain are: a lithospheric thickness of about 65 km, a mantle viscosity above 670 km of about (4-5) 10²⁰ Pa s, and a viscosity below 670 km greater than 4 × 10²¹ Pa s.     

Radiocarbon Chronology of Ross Sea Drift, Eastern Taylor Valley, Antarctica: Evidence for a Grounded Ice Sheet in the Ross Sea at the Last Glacial Maximum

More than 250 radiocarbon dates of lacustrine algae and marine shells afford a chronology for Ross Sea drift in eastern Taylor Valley. Dates of algae that lived in ice-dammed Glacial Lake Washburn show that grounded Ross Sea ice blocked the mouth of Taylor Valley between 8340 and 23,800 ¹⁴C yr bp . Ross Sea ice was at its maximum position at the Hjorth Hill moraine between 12,700 and 14,600 ¹⁴C yr bp and was within 500m distance of this position as late as 10,794 ¹⁴C yr bp . The implication is that the flow line of the Ross Sea ice sheet which extended around northern Ross Island and across McMurdo Sound to Taylor Valley must have remained intact, and hence that a grounded ice sheet must have existed east of Ross Island as late as 8340 ¹⁴C yr bp . Evidence from ice-dammed lakes in Taylor Valley and from shells from McMurdo Sound suggests grounding-line retreat from the vicinity of Ross Island between 6500 and 8340 ¹⁴C yr bp . If this is correct, then most recession to the present-day grounding line on the Siple Coast took place subsequently in the absence of significant deglacial sea-level rise. Rising sea level may have triggered internal mechanisms within the ice sheet that led to retreat, but did not in itself drive continued ice-sheet recession. Ice retreat, once set in motion, continued in the absence of sea-level forcing. If correct, this hypothesis implies that the grounding line could continue to recede into the interior reservoir of the West Antarctic Ice Sheet.