Ice Sheet Retreat Research Articles

The development and impact of an ice‐contact proglacial lake during the Last Glacial Termination, Palaeolake Riada, central Ireland

ABSTRACTThe retreat of the last Irish Ice Sheet across the Irish midlands resulted in the formation of an ice‐contact lake, Palaeolake Riada, along its eastern margin during recession. However, the full extent and characteristics of the lake and its impact on ice dynamics are unknown, as glacigenic landforms and deposits relating to the lake have not been mapped in sufficient detail to allow reconstruction of ice‐marginal behaviour. This paper presents new mapping of subglacial, ice‐marginal and glaciolacustrine landforms and deposits in the Irish midlands. Glacial landforms and landform associations across this wide area have been identified using aerial photographs and high‐resolution digital elevation models derived from LiDAR and radar data. Reconstructions of successive ice‐margin retreat positions are combined with glacio‐isostatically adjusted digital elevation models to identify the probable position of lake outflows and reconstruct the development of the proglacial Palaeolake Riada through time. These reconstructions indicate that at its largest, Palaeolake Riada had an ice‐contact margin of 113 km, and covered an area of over 2300 km2. Initially, ice recession was concentrated around meltwater discharge outlets, but these became less important as the lake expanded and thermo‐mechanical erosion occurred along the entire subaqueous margin.

Layered seawater intrusion and melt under grounded ice

Abstract. Increasing melt of ice sheets at their floating or vertical interfaces with the ocean is a major driver of marine ice sheet retreat and sea level rise. However, the extent to which warm, salty seawater may drive melting under the grounded portions of ice sheets is still not well understood. Previous work has explored the possibility that dense seawater intrudes beneath relatively light subglacial freshwater discharge, similar to the “salt wedge” observed in many estuarine systems. In this study, we develop a generalized theory of layered seawater intrusion under grounded ice, including where subglacial hydrology occurs as a macroporous water sheet over impermeable beds or as microporous Darcy flow through permeable till. Using predictions from this theory, we show that seawater intrusion over flat or reverse-sloping impermeable beds may feasibly occur up to tens of kilometers upstream of a glacier terminus or grounding line. On the other hand, seawater is unlikely to intrude more than tens of meters through permeable till. Simulations using the Ice-sheet and Sea-level System Model (ISSM) show that even just a few hundred meters of basal melt caused by seawater intrusion upstream of marine ice sheet grounding lines can cause projections of marine ice sheet volume loss to be 10 %–50 % higher. Kilometers of intrusion-induced basal melt can cause projected ice sheet volume loss to more than double. These results suggest that further observational, experimental and numerical investigations are needed to determine the conditions under which seawater intrusion occurs and whether it will indeed drive rapid marine ice sheet retreat and sea level rise in the future.

Blue-ice moraines formation in the Heritage Range, West Antarctica: Implications for ice sheet history and climate reconstruction

Blue ice is found in areas of Antarctica where katabatic winds, focussed by steep surface slopes or by topography around nunataks, cause enhanced surface ablation. This process draws up deeper, older ice to the ice sheet surface, often bringing with it englacial sediment. Prevailing theories for dynamically stable moraine surfaces in East Antarctica suggest that: (i) it is this material, once concentrated, that forms blue-ice moraines (BIM), (ii) that the moraine formation can be dated using cosmogenic isotope approaches, and that, (iii) since we expect an increase in exposure age moving away from the ice margin towards bedrock, dating across the moraine can be used to constrain ice-sheet history. To test this lateral accretion model for BIM formation we visited Patriot, Marble and Independence Hills in the southern Heritage Range, West Antarctica. Detailed field surveys of surface form, sediment and moraine dynamics were combined with geophysical surveys of the englacial structure of the moraines and cosmogenic nuclide analysis of surface clasts. Results suggest sediment is supplied mainly by basal entrainment, supplemented by debris-covered valley glaciers transferring material onto the ice sheet surface, direct deposition from rock-fall and slope processes from nunataks. We find that once sediment coalesces in BIM, significant reworking occurs through differential ablation, slope and periglacial processes. We bring these processes together in a conceptual model, concluding that many BIM in West Antarctica are dynamic and, whilst they persist through glacial cycles, they do not always neatly record ice sheet retreat patterns since linear distance from the ice margin does not always relate to increased clast exposure age. Understanding the dynamic processes involved in moraine formation is critical to the effective interpretation of the typically large scatter of cosmogenic nuclide exposure ages, opening a deep window into the million-year history of the West Antarctic Ice Sheet.

Millennial-scale migration of the frozen/melted basal boundary, western Greenland ice sheet

AbstractThe geometry and thermal structure of western Greenland ice sheet are known to have undergone relatively substantial change over the Holocene. Evolution of the frozen and melted fractions of the bed associated with the ice-sheet retreat over this time frame remains unclear. We address this question using a thermo-mechanically coupled flowline model to simulate a 11 ka period of ice-sheet retreat in west central Greenland. Results indicate an episode of ~100 km of terminus retreat corresponded to ~16 km of upstream frozen/melted basal boundary migration. The majority of migration of the frozen area is associated with the enhancement of the frictional and strain heating fields, which are accentuated toward the retreating ice margin. The thermally active bedrock layer acts as a heat sink, tending to slow contraction of frozen-bed conditions. Since the bedrock heat flux in our region is relatively low compared to other regions of the ice sheet, the frozen region is relatively greater and therefore more susceptible to marginward changes in the frictional and strain heating fields. Migration of melted regions thus depends on both geometric changes and the antecedent thermal state of the bedrock and ice, both of which vary considerably around the ice sheet.

Investigating the roles of relative sea-level change and glacio-isostatic adjustment on the retreat of a marine based ice stream in NW Scotland

The record of ice-sheet demise since the last glacial maximum (LGM) provides an opportunity to test the relative importance of instability mechanisms, including relative sea-level (RSL) change, controlling ice-sheet retreat. Here we examine the record of RSL changes accompanying the retreat of the Minch Ice Stream (MnIS) of northwest Scotland during the deglaciation following the LGM as well as use the record to provide additional age constraints on a local late-glacial readvance known as the Wester Ross Readvance. We use new and existing records of RSL change obtained from isolation basins in Wester Ross along the flanks of the former MnIS to test available glacial-isostatic adjustment (GIA) predictions of the deglacial RSL history for the region. Using these GIA model predictions we examine the nature of RSL change across the retreating front of the MnIS through the early deglaciation. Our new radiocarbon ages from these basins confirm the timing of deglaciation within the inner trough of the former MnIS as well as refines the age of the Wester Ross Readvance, both established by earlier cosmogenic-based studies. We find that the Wester Ross Readvance culminated around 15.8 ± 0.1 ka, slightly earlier than recent suggestions. Near Gairloch, Wester Ross, RSL fell from a marine limit ∼20 m above present at ∼16.1–16.5 ka. Three isolation basins record RSL fall over the following ∼0.8 ka allowing a comparison between GIA predictions and RSL observations. Our new analyses suggest that the rate of RSL rise increased at the ice front, in concert with the MnIS encountering a landward sloping bed potentially aiding the rapid retreat of the MnIS from 17.6 to 16.4 ka BP. This observation suggests that GIA during deglaciation does not necessarily induce a stabilizing RSL change to marine-based ice streams as some models have suggested. Along indented ice margins, the RSL field at the front of individual ice streams may be governed by the regional GIA signal driven by the ice sheet as a whole, rather than the local ice front. In addition, the stabilizing impact of post-glacial rebound is dependent on an Earth rheology weak enough to respond quickly to the ice-sheet retreat. In the case of the MnIS, the RSL experienced at the front of the ice stream was likely governed by the earlier ice mass extent, the larger ice masses lying to the east and south of the highly indented ice front, and the relatively strong Earth rheology beneath the British Isles. Thus, the geometry of the ice sheet margins, such as those in Greenland and Antarctica today, and the Earth rheology beneath them need to be taken into account when considering the stabilizing impact of post-glacial rebound on marine ice sheet retreat.

Revised chronology of northwest Laurentide ice-sheet deglaciation from 10Be exposure ages on boulder erratics

We present new 10Be surface exposure ages from boulders on bedrock to directly date northwest Laurentide ice-sheet deglaciation through a wide swath of the western Canadian Shield that had no previous reliable temporal constraints on ice-margin retreat. Uplift-corrected boulder 10Be surface exposure ages are 13.9 ± 0.2 ka (n = 6) at a site on the western edge of the Slave Craton and 12.4 ± 0.2 ka (n = 5, 1 outlier) at a second site ∼110 km up-ice to the east. These direct 10Be ages for ice-margin retreat are ∼2.4 kyr and ∼1.6 kyr older, respectively, than the canonical deglacial chronology for the northwest Laurentide ice sheet that is based on minimum-limiting 14C dates. We infer an ice-margin retreat rate of 60–100 m yr−1 between the two sites over an interval spanning the transition from the Allerød warm period into the Younger Dryas cold period. This is significantly slower than the rapid >800 m yr−1 retreat rate for the northwest Laurentide ice sheet inferred from earlier deglacial chronologies, which has been hypothesized as a potential source of meltwater forcing for the Younger Dryas cold period. These first direct ages on spatio-temporal patterns of deglaciation in this data-poor region suggest that additional refinement of the deglacial chronology is needed to test hypotheses on the relation between ice-sheet retreat, associated meltwater discharge, abrupt climate change, and rapid sea-level rise.

Rapid retreat of the southwestern Laurentide Ice Sheet during the Bølling-Allerød interval

Abstract The timing of Laurentide Ice Sheet deglaciation along its southwestern margin controlled the evolution of large glacial lakes and has implications for human migration into the Americas. Accurate reconstruction of the ice sheet's retreat also constrains glacial isostatic adjustment models and is important for understanding ice-sheet sensitivity to climate forcing. Despite its significance, retreat of the southwestern Laurentide Ice Sheet (SWLIS) is poorly constrained by minimum-limiting 14C data. We present 26 new cosmogenic 10Be exposure ages spanning the western Interior Plains, Canada. Using a Bayesian framework, we combine these data with geomorphic mapping, 10Be, and high-quality minimum-limiting 14C ages to provide an updated chronology. This dataset presents an internally consistent retreat record and indicates that the initial detachment of the SWLIS from its convergence with the Cordilleran Ice Sheet began by ca. 15.0 ka, concurrent with or slightly prior to the onset of the Bølling-Allerød interval (14.7–12.9 ka) and retreated >1200 km to its Younger Dryas (YD) position in ~2500 yr. Ice-sheet stabilization at the Cree Lake Moraine facilitated a meltwater drainage route to the Arctic from glacial Lake Agassiz within the YD, but not necessarily at the beginning. Our record of deglaciation and new YD constraints demonstrate deglaciation of the Interior Plains was ~60% faster than suggested by minimum 14C constraints alone. Numerical modeling of this rapid retreat estimates a loss of ~3.7 m of sea-level equivalent from the SWLIS during the Bølling-Allerød interval.

Decadal-scale onset and termination of Antarctic ice-mass loss during the last deglaciation

Emerging ice-sheet modeling suggests once initiated, retreat of the Antarctic Ice Sheet (AIS) can continue for centuries. Unfortunately, the short observational record cannot resolve the tipping points, rate of change, and timescale of responses. Iceberg-rafted debris data from Iceberg Alley identify eight retreat phases after the Last Glacial Maximum that each destabilized the AIS within a decade, contributing to global sea-level rise for centuries to a millennium, which subsequently re-stabilized equally rapidly. This dynamic response of the AIS is supported by (i) a West Antarctic blue ice record of ice-elevation drawdown >600 m during three such retreat events related to globally recognized deglacial meltwater pulses, (ii) step-wise retreat up to 400 km across the Ross Sea shelf, (iii) independent ice sheet modeling, and (iv) tipping point analysis. Our findings are consistent with a growing body of evidence suggesting the recent acceleration of AIS mass loss may mark the beginning of a prolonged period of ice sheet retreat and substantial global sea level rise.

Climate and ice sheet evolutions from the last glacial maximum to the pre-industrial period with an ice-sheet–climate coupled model

Abstract. The last deglaciation offers an unique opportunity to understand the climate–ice-sheet interactions in a global warming context. In this paper, to tackle this question, we use an Earth system model of intermediate complexity coupled to an ice sheet model covering the Northern Hemisphere to simulate the last deglaciation and the Holocene (26–0 ka). We use a synchronous coupling every year between the ice sheet and the rest of the climate system and we ensure a closed water cycle considering the release of freshwater flux to the ocean due to ice sheet melting. Our reference experiment displays a gradual warming in response to the forcings, with no abrupt changes. In this case, while the amplitude of the freshwater flux to the ocean induced by ice sheet retreat is realistic, it is sufficient to shut down the Atlantic meridional overturning circulation from which the model does not recover within the time period simulated. However, with reduced freshwater flux we are nonetheless able to obtain different oceanic circulation evolutions, including some abrupt transitions between shut-down and active circulation states in the course of the deglaciation. The inclusion of a parameterisation for the sinking of brines around Antarctica also produces an abrupt recovery of the Atlantic meridional overturning circulation, absent in the reference experiment. The fast oceanic circulation recoveries lead to abrupt warming phases in Greenland. Our simulated ice sheet geometry evolution is in overall good agreement with available global reconstructions, even though the abrupt sea level rise at 14.6 ka is underestimated, possibly because the climate model underestimates the millennial-scale temperature variability. In the course of the deglaciation, large-scale grounding line instabilities are simulated both for the Eurasian and North American ice sheets. The first instability occurs in the Barents–Kara seas for the Eurasian ice sheet at 14.5 ka. A second grounding line instability occurs ca. 12 ka in the proglacial lake that formed at the southern margin of the North American ice sheet. With additional asynchronously coupled experiments, we assess the sensitivity of our results to different ice sheet model choices related to surface and sub-shelf mass balance, ice deformation and grounding line representation. While the ice sheet evolutions differ within this ensemble, the global climate trajectory is only weakly affected by these choices. In our experiments, only the abrupt shifts in the oceanic circulation due to freshwater fluxes are able to produce some millennial-scale variability since no self-generating abrupt transitions are simulated without these fluxes.

Deglaciation controls on sediment yield: Towards capturing spatio-temporal variability

Accelerated glacier and ice sheet retreat and thinning in recent decades has profound consequences for catchment sediment supply with attendant repercussions for nutrient cycling, carbon fluxes and natural resource management. This paper evaluates the impacts of deglaciation on sediment yields from glaciated, deglaciating and recently-deglaciated catchments. It summarises the key characteristics of sediment yields from glaciated catchments to be that they span five orders of magnitude, vary with latitude and are greatest in high-relief and tectonically-active regions. We review the available quantitative data on sediment yields from glaciated catchments and we comment extensively on spatio-temporal variability to understand global to local and inter- and intra-catchment controls. Significant gaps in the available sediment yield data and also in our knowledge of sediment sources, pathways and sinks are identified. We constrain a set of novel approaches by which these gaps could be addressed. In particular, we suggest that the opportunities presented by emerging datasets and analytical methods enabling landcover changes, Digital Elevation Model (DEM) change detection, analyses of connectivity and analyses of sediment plumes are exciting and these approaches should become practical tools for understanding intra- and inter-catchment sediment yields from deglaciating landscapes. We showcase preliminary studies utilising these datasets and they are used to formulate hypotheses designed to stimulate further research.

Retreat of the Antarctic Ice Sheet During the Last Interglaciation and Implications for Future Change

AbstractThe Antarctic Ice Sheet (AIS) response to past warming consistent with the 1.5–2°C “safe limit” of the United Nations Paris Agreement is currently not well known. Empirical evidence from the most recent comparable period, the Last Interglaciation, is sparse, and transient ice‐sheet experiments are few and inconsistent. Here, we present new, transient, GCM‐forced ice‐sheet simulations validated against proxy reconstructions. This is the first time such an evaluation has been attempted. Our empirically constrained simulations indicate that the AIS contributed 4 m to global mean sea level by 126 ka BP, with ice lost primarily from the Amundsen, but not Ross or Weddell Sea, sectors. We resolve the conflict between previous work and show that the AIS thinned in the Wilkes Subglacial Basin but did not retreat. We also find that the West AIS may be predisposed to future collapse even in the absence of further environmental change, consistent with previous studies.

Distribution, characteristics and formation of esker enlargements

Eskers are primarily ridges of glaciofluvial sediment deposited in subglacial, englacial and supraglacial conduits. They are typically straight to sinuous features, however, their planform morphology can be highly diverse. Esker enlargements are spatially confined ridge sections that are significantly wider than the trunk ridge (typically 250–400 m) and that reconverge downflow. The enlargements include complex ridge networks or coherent sediment bodies. We mapped >1400 esker enlargements across Fennoscandia and Keewatin, Canada, to investigate their distribution and morphological characteristics. Esker enlargements are less abundant below the marine limit, and tend to become more abundant in areas of faster ice retreat. They form local clusters along particular ridges, and can occasionally be traced laterally between adjacent esker systems. Based on morphological observations, we link their formation to roof collapses in subglacial conduits. The distribution of esker enlargements indicates that subglacial conduit collapse became an increasingly significant process during the final stages of deglaciation of both the Scandinavian and Laurentide ice sheets, and may have exerted a positive feedback on ice sheet retreat at land-terminating ice margins.

Southwest Greenland ice-sheet retreat during the 8.2 ka cold event

The century-long 8.2 ka cold event interrupted early Holocene boreal warmth and may have halted retreat of Greenland ice-sheet margins during the last deglaciation. Here, we synthesize new and existing glacial geological data to assess the behavior of the southwest Greenland ice sheet during the 8.2 ka cold event. In southwest Greenland near the town of Kangerlussuaq, existing and new 10Be surface exposure ages demonstrate that deposition of the Keglen moraines ended at 8.0 ± 0.1 ka (n = 10, 1 outlier), with prior studies arguing that these moraines represented an ice-margin stillstand in response to the 8.2 ka cold event. However, new 10Be ages show that the southwest Greenland ice-sheet margin retreated from the Umîvît moraines, which lie 5–10 km outboard of the Keglen moraines, at 8.2 ± 0.2 ka (n = 11). Accordingly, we suggest that the southwest Greenland ice-sheet margin in the Kangerlussuaq region retreated 5–10 km during the 8.2 ka cold event. Farther south and inland from Nuuk, new 10Be boulder-on-bedrock ages adjacent to the modern ice-sheet margin demonstrate that the ice-margin was retreating with no moraine deposition at 8.2 ± 0.1 ka (n = 4, 1 outlier), with this retreat continuing up to at least ∼7.5 ka according to an existing threshold lake record. Therefore, we propose that the southwest Greenland ice-sheet margin underwent continued retreat during the 8.2 ka cold event in response to elevated early Holocene boreal summer insolation that overwhelmed the impacts from century-scale cooling. Caution should be used in assuming climatic causation for moraine deposition based on temporal correlation.

Direct evidence for thinning and retreat of the southernmost Greenland ice sheet during the Younger Dryas

During the last deglaciation, North Atlantic climate abruptly warmed at the Bølling (∼14.7 ka), cooled into the Younger Dryas (∼12.9 ka) and abruptly warmed again into the Holocene (∼11.7 ka). While these events are defined by Greenland ice cores, there is still considerable uncertainty on Greenland ice-sheet margin responses to abrupt climate change. To refine the ice sheet's deglacial history, we present new cosmogenic nuclide surface exposure ages from boulders on bedrock at five sites in southernmost Greenland fjords located midway between the coast and inland ice margin. We find ice-sheet thinning below three local topographic highs at 12.7 ± 0.3 ka (n = 3), 13.1 ± 0.4 ka (n = 1, 2 outliers), and 12.3 ± 0.2 ka (n = 3), with up-fjord retreat at 12.5 ± 0.3 ka (n = 3) and 12.7 ± 0.2 ka (n = 4) based on two sites just above the mid-fjord marine limit. These mid-fjord 10Be ages therefore show southernmost Greenland ice-sheet thinning and retreat during the Younger Dryas. We hypothesize that this thinning and retreat was a response to ocean warming prior to the Holocene and/or summer shortwave radiative forcing during the Younger Dryas due to peak boreal summer insolation. Our results also support a previously hypothesized winter bias in proxy records of Younger Dryas atmospheric cooling, since a large summer cooling during the Younger Dryas could have counteracted the effects of ocean warming and direct radiative forcing, inhibiting ice-sheet retreat.

Climate driven declines in terrestrial input over the middle and late Holocene of perched boreal lakes in northeast Ontario (Canada) and teleconnections to the North Atlantic

The landscape of northeast Ontario, Canada, is dominated by glacial and postglacial landforms including numerous boreal lakes that formed following the retreat of the Laurentide Ice Sheet (LIS) and Glacial Lake Ojibway. This study uses micro-X-Ray Fluorescence elemental analysis to examine changes in sediment cores from two boreal lakes from northeast Ontario with the goal of inferring changes in terrestrial inputs and redox status of these lakes over the middle to late Holocene. These headwater lakes were selected as they have small watersheds that minimizes the complexity of hydrological responses, and as they are relatively proximal (∼130 km distance between lakes) to allow the detection of regional signals related to changes in climate over the Holocene. Strong correlations between Si, K, Ca, and Ti in Hogback Lake and Green Lake cores suggest that these elements represent proxies of terrestrial input. Standardized composite trends of these elements were very similar between the two lakes and were combined to represent a regional signal, with the highest terrestrial inputs between ∼6000 and ∼4000 cal yr BP, that subsequently declined through the Holocene. These results suggest that regional patterns in terrestrial input are linked to changes in hydroclimate and likely increased terrestrial stability over the middle to late Holocene. These composite trends are compared to similar geochemical composite trends from Iceland suggesting climate teleconnections between northeast North America and the North Atlantic. Cool conditions in the North Atlantic result in increased soil erosion in Iceland and correspond to cool and dry conditions in boreal northeast Ontario with corresponding low inputs of terrestrial material, and vice versa, over multidecadal time scales over the middle to late Holocene.

Late Miocene to Plio-Pleistocene volcanic and paleoenvironmental history of the Cerro Domo and Ventisqueros Mesa area, northwest James Ross Island, northern Antarctic Peninsula

We present the description and interpretation of a James Ross Island Volcanic Group (JRIVG) succession around the Cerro Domo and Ventisqueros Mesa area. Our aim is to reconstruct the volcanic evolution and provide new palaeoenvironmental information for the northwest sector of James Ross Island, northern Antarctic Peninsula. We present representative stratigraphic logs that are combined with new chronological constraints provided by 40Ar/39Ar ages and 87Sr/86Sr isotope analyses that demonstrate a record of the late Miocene to earliest Pleistocene. The first volcanic activity registered is associated with tuff cone deposits that resulted from marine hydromagmatic eruptions, close to 6.18 ± 0.14 Ma. Subsequently, lodgement till deposits record a grounded glacial advance. Overlying the glacigenic deposits, fossil-bearing sediments record a marine or glacimarine environment following the ice sheet retreat, and an 87Sr/86Sr age of 6.02 (+0.09, −0.09) Ma dates ice-poor conditions. A lava-fed delta system, which yields an age of 5.37 ± 0.12 Ma, prograded over the previous deposits. Stacked pillow lavas, curvy columnar-jointed basalts and subaerial tabular lavas, with a 4.9 ± 0.10 Ma age, were emplaced after the lava-fed delta deposition. Another lava-fed delta located to the north of the previously volcanic units yielded an age of 3.57 ± 0.19 Ma. A glacial pavement carved into the top of these deposits records a second ice advance in the area. The final volcanic activity emplaced Cerro Domo, a large volcanic edifice which was the centre of explosive hydromagmatic eruptions. The presence of pristine, articulated bivalves in life position within the lower volcanic deposits of Cerro Domo, together with an 87Sr/86Sr age of 3.16 (+0.59, −0.43) Ma, are strong indicators of a marine environment and suggest ice-poor “interglacial” conditions at this time. Dykes cutting the volcanic deposits of Cerro Domo and considered coeval with the eruptions afford ages of 2.62 ± 0.08 and 2.72 ± 0.09 Ma. Our results provide new insights on the volcanic and glaciation history of James Ross Island, as well as supporting marine, ice-poor conditions around ~6 Ma, and near or right before the Plio-Pleistocene boundary, between ~3.2 and ~2.6 Ma.

Comparing Glacial‐Geological Evidence and Model Simulations of Ice Sheet Change since the Last Glacial Period in the Amundsen Sea Sector of Antarctica

AbstractSince the Last Glacial Maximum ∼20,000 years ago, the Antarctic Ice Sheet has undergone extensive changes, resulting in a much smaller present‐day configuration. Improving our understanding of basic physical processes that played important roles during that retreat is critical to providing more robust model projections of future retreat and sea‐level rise. Here, a limited‐area nested ice sheet model was applied to the last deglacial retreat of the West Antarctic Ice Sheet in the Amundsen Sea Embayment (ASE), at 5 km resolution. The ice sheet response to climate and sea‐level forcing was examined at two sites along the flowlines of Pine Island Glacier and Pope Glacier, close to the Hudson Mountains and Mount Murphy respectively, and the simulated responses compared with ice sheet thinning histories derived from glacial‐geological data. The sensitivity of results to selected model parameters was also assessed. The model simulations predict a broadly similar response to ocean forcing in both the central and eastern ASE, with an initial rapid phase of thinning followed by a slower phase to the modern configuration. Although there is a mismatch of up to 5,000 years between the timing of simulated and observed thinning, the modeling suggests that the upstream geological records of ice surface elevation change reflect a response to retreat near the grounding line. The model‐data mismatch could potentially be improved by accounting for regional variations in mantle viscosity, sea‐surface heights and basal sliding properties across the continental shelf.

Timing, pace and controls on ice sheet retreat: an introduction to the BRITICE‐CHRONO transect reconstructions of the British–Irish Ice Sheet

ABSTRACTMotivated to help improve the robustness of predictions of sea level rise, the BRITICE‐CHRONO project advanced knowledge of the former British–Irish Ice Sheet, from 31 to 15 ka, so that it can be used as a data‐rich environment to improve ice sheet modelling. The project comprised over 40 palaeoglaciologists, covering expertise in terrestrial and marine geology and geomorphology, geochronometric dating and the modelling of ice sheets and oceans. A systematic and directed campaign, organised across eight transects from the continental shelf edge to a short distance (10s of kilometres) onshore, was used to collect 914 samples which yielded 639 new ages, tripling the number of dated sites constraining the timing and rates of change of the collapsing ice sheet. This special issue synthesises these findings of ice advancing to the maximum extent and its subsequent retreat for each of the eight transects to produce definitive palaeogeographic reconstructions of ice margin positions across the marine to terrestrial transition. These results are used to understand the controls that drove or modulated ice sheet retreat. A further paper reports on how ice sheet modelling experiments and empirical data can be used in combination, and another probes the glaciological meaning of ice‐rafted debris.

Calbuco, a monotonous andesitic high-flux volcano in the Southern Andes, Chile

Calbuco (41.3°S) is a highly active composite frontal arc volcano in the Andean Central Southern Volcanic Zone (CSVZ). It is currently ranked by Chile's Geological Survey (SERNAGEOMIN) as among the most dangerous volcanic centers in the Southern Andes, with sub-Plinian explosive activity occurring as recently as 2015. Compositional and geochronologic data from 65 samples constrain the Pleistocene-Holocene eruptive evolution of Calbuco. New 40Ar/39Ar ages from lava flows, revised mapping, and existing 14C dates for Holocene tephras reveal that much of the current edifice is <20 ka in age. Thus, Calbuco has produced a greater Holocene eruptive flux, 3.4 km3/ka averaged over the last 10 ka and 4.8 km3/ka over the last 4 ka, than other arc volcanoes both in the CSVZ and globally. Morphologic and stratigraphic observations along with new whole-rock geochemical data suggest that this remarkable magma flux is expressed as repeated eruptions of lavas and tephra with a narrow SiO2 range (54–58 wt%). This compositional monotony, conspicuous resorbed amphibole, and elevated fluid-mobile trace element abundances differ from the post-glacial compositions and mineralogy observed at other CSVZ arc front volcanoes. Mid-crustal processing at or just below the depth where amphibole is stable plays a key role in the genesis of andesite at Calbuco. The precision of 40Ar/39Ar dates from Latest Pleistocene to Holocene low K2O mafic lavas remains a challenge to constructing eruptive chronologies for comparison to records of glacier expansion and retreat. Notwithstanding, our findings from Calbuco imply that petrologic factors, including mantle melt flux, depth of magma storage, and melt H2O content, may promote distinctive eruptive responses at adjacent CSVZ volcanoes following rapid retreat of the Patagonian Ice Sheet at 18 ka.

Early and middle Miocene ice sheet dynamics in the Ross Sea: Results from integrated core-log-seismic interpretation

Abstract Oscillations in ice sheet extent during early and middle Miocene are intermittently preserved in the sedimentary record from the Antarctic continental shelf, with widespread erosion occurring during major ice sheet advances, and open marine deposition during times of ice sheet retreat. Data from seismic reflection surveys and drill sites from Deep Sea Drilling Project Leg 28 and International Ocean Discovery Program Expedition 374, located across the present-day middle continental shelf of the central Ross Sea (Antarctica), indicate the presence of expanded early to middle Miocene sedimentary sections. These include the Miocene climate optimum (MCO ca. 17–14.6 Ma) and the middle Miocene climate transition (MMCT ca. 14.6–13.9 Ma). Here, we correlate drill core records, wireline logs and reflection seismic data to elucidate the depositional architecture of the continental shelf and reconstruct the evolution and variability of dynamic ice sheets in the Ross Sea during the Miocene. Drill-site data are used to constrain seismic isopach maps that document the evolution of different ice sheets and ice caps which influenced sedimentary processes in the Ross Sea through the early to middle Miocene. In the early Miocene, periods of localized advance of the ice margin are revealed by the formation of thick sediment wedges prograding into the basins. At this time, morainal bank complexes are distinguished along the basin margins suggesting sediment supply derived from marine-terminating glaciers. During the MCO, biosiliceous-bearing sediments are regionally mapped within the depocenters of the major sedimentary basin across the Ross Sea, indicative of widespread open marine deposition with reduced glacimarine influence. At the MMCT, a distinct erosive surface is interpreted as representing large-scale marine-based ice sheet advance over most of the Ross Sea paleo-continental shelf. The regional mapping of the seismic stratigraphic architecture and its correlation to drilling data indicate a regional transition through the Miocene from growth of ice caps and inland ice sheets with marine-terminating margins, to widespread marine-based ice sheets extending across the outer continental shelf in the Ross Sea.