Dynamics of former ice lobes of the southernmost Patagonian Ice Sheet based on a glacial landsystems approach

Reconstructions of former ice masses from glacial geomorphology help to constrain the nature and timing of glaciation in relation to climatic forcing. This paper presents a new reconstruction of the glacial history of five ice lobes in southernmost South America: the Bahía Inútil − San Sebastián, Magellan, Otway, Skyring and Río Gallegos ice lobes. We use previous geomorphological mapping of glacial landforms to reconstruct former glacial limits and proglacial lakes, demarcate flow-sets from the distribution of glacial lineations, and evaluate glacial landsystem signatures and their palaeoglaciological implications. Evidence suggests that the ice lobes predominantly reflect active temperate glacial landsystems, which may have switched to polythermal systems when periods of cold-based ice developed ephemerally. This complex landsystem signature implies that the ice lobes were sensitive to regional climate variability, with active re-advances during overall retreat of the ice margins. There is also evidence for periods of fast ice flow and possible surge-like activity in the region, followed by the rapid retreat or even collapse of some of the ice lobes in association with proglacial lakes. Constraining our new reconstruction with published chronological information suggests that at least some of the ice lobes advanced before the global Last Glacial Maximum (gLGM: ca. 26.5–19 ka) during the last glacial cycle. Our new reconstruction demonstrates a more complex picture of ice dynamics than has previously been portrayed, and one in which the advance and retreat of the ice lobes was likely to have been primarily driven by changes in climate. As such, ice advances before the gLGM in the southernmost part of the Patagonian Ice Sheet are likely to indicate a wider climatic forcing at this time

The geomorphological record of an ice stream to ice shelf transition in Northeast Greenland

ACKNOWLEDGEMENTS This work was funded through NERC Standard Grant NE/N011228/1. We thank the Alfred Wegner Institute, and particularly Angelika Humbert and Hicham Rafiq, for their logistic support through the iGRIFF project. Further support was provided from Station Nord (Jorgen Skafte), Nordland Air, Air Greenland, and the Joint Arctic Command. Naalakkersuisut, Government of Greenland, provided Scientific Survey (VU-00121) and Export (046/2017) licences for this work. We thank Chris Orton for help with production of figures. Finally, we would like to thank our Field Ranger Isak (after which Isakdalen is informally named) and dog Ooni for keeping us safe in the field. We thank Rob Storrar and an anonymous reviewer for their comments which helped improve the manuscript.Peer reviewedPublisher PD

Holocene history of the 79°N ice shelf reconstructed from epishelf lake and uplifted glaciomarine sediments

Nioghalvfjerdsbrae, or 79∘ N Glacier, is the largest marine-terminating glacier draining the Northeast Greenland Ice Stream (NEGIS). In recent years, its ∼ 70 km long fringing ice shelf (hereafter referred to as the 79∘ N ice shelf) has thinned, and a number of small calving events highlight its sensitivity to climate warming. With the continued retreat of the 79∘ N ice shelf and the potential for accelerated discharge from NEGIS, which drains 16 % of the Greenland Ice Sheet (GrIS), it has become increasingly important to understand the long-term history of the ice shelf in order to put the recent changes into perspective and to judge their long-term significance. Here, we reconstruct the Holocene dynamics of the 79∘ N ice shelf by combining radiocarbon dating of marine molluscs from isostatically uplifted glaciomarine sediments with a multi-proxy investigation of two sediment cores recovered from Blåsø, a large epishelf lake 2–13 km from the current grounding line of 79∘ N Glacier. Our reconstructions suggest that the ice shelf retreated between 8.5 and 4.4 ka cal BP, which is consistent with previous work charting grounding line and ice shelf retreat to the coast as well as open marine conditions in Nioghalvfjerdsbrae. Ice shelf retreat followed a period of enhanced atmospheric and ocean warming in the Early Holocene. Based on our detailed sedimentological, microfaunal, and biomarker evidence, the ice shelf reformed at Blåsø after 4.4 ka cal BP, reaching a thickness similar to present by 4.0 ka cal BP. Reformation of the ice shelf coincides with decreasing atmospheric temperatures, the increased dominance of Polar Water, a reduction in Atlantic Water, and (near-)perennial sea-ice cover on the adjacent continental shelf. Along with available climate archives, our data indicate that the 79∘ N ice shelf is susceptible to collapse at mean atmospheric and ocean temperatures ∼ 2 ∘C warmer than present, which could be achieved by the middle of this century under some emission scenarios. Finally, the presence of “marine” markers in the uppermost part of the Blåsø sediment cores could record modern ice shelf thinning, although the significance and precise timing of these changes requires further work

A glacial geomorphological map of the southernmost ice lobes of Patagonia: the Bahía Inútil - San Sebastián, Magellan, Otway, Skyring and Bella Vista lobes

This paper presents a glacial geomorphological map of the landforms created by five large ice lobes that extended eastwards from the southernmost reaches of the Patagonian Ice Sheet during the Quaternary period. The study is focussed on Tierra del Fuego, but also updates previous mapping of the Skyring and Otway lobes, and the resulting level of detail and extent is a significant advance on previous work in the region. The map has been created as the necessary precursor for an improved understanding of the glacial history of the region, and to underpin a programme of dating glacial limits in the region. It was produced using Landsat ETM+ and ASTER satellite imagery and vertical aerial photography, supplemented by Google Earth™ imagery and field-checking. Eleven landform types were mapped: moraine ridges, subdued moraine topography, kettle-kame topography, glacial lineations, irregular and regular hummocky terrain, irregular dissected ridges, eskers, meltwater channels, former shorelines and outwash plains. The map reveals three important characteristics of the glacial geomorphology. First, the geomorphic systems are largely dominated by landforms associated with meltwater (channels, outwash plains and kettle-kame topography). Second, there is a difference in the nature of landforms associated with the northern three ice lobes, where limits are generally marked by numerous clear moraine ridges, compared to those to the south, where hummocky terrain and drift limits prevail. Finally, cross-cutting landforms offer evidence of multiple advances, in places, which has implications for the timing of limit deposition, and thus for the design and interpretation of a dating programme

The timing and cause of glacial advances in the southern mid-latitudes during the last glacial cycle based on a synthesis of exposure ages from Patagonia and New Zealand

Glacier advances in the southern mid-latitudes during the last glacial cycle (ca. 110–10 ka) were controlled by changes in temperature and precipitation linked to several important ocean-climate systems. As such, the timing of glacial advance and retreat can yield important insights into the mechanisms of Southern Hemisphere climate change. This is particularly important given that several recent studies have demonstrated significant glacial advances prior to the global Last Glacial Maximum (gLGM) in Patagonia and New Zealand, the cause of which are uncertain. The recent increase in chronological studies in these regions offers the opportunity to compare regional trends in glacial activity. Here, we compile the first consistent 10Be exposure-dating chronologies for Patagonia and New Zealand to highlight the broad pattern of mid-latitude glacial activity over the last glacial cycle. Our results show that advances or still stands culminated at 26–27 ka, 18–19 ka and 13–14 ka in both Patagonia and New Zealand and were broadly synchronous, but with an offset between regions of up to 900 years that cannot be explained by age calculation or physically plausible erosion differences. Furthermore, there is evidence in both regions for glacial advances culminating from at least 45 ka, during the latter half of Marine Isotope Stage (MIS) 3. Glacial activity prior to the gLGM differed from the large Northern Hemisphere ice sheets, likely due to favourable Southern Hemisphere conditions during late MIS 3: summer insolation reached a minimum, seasonality was reduced, winter duration was increasing, and sea ice had expanded significantly, inducing stratification of the ocean and triggering northward migration of oceanic fronts and the Southern Westerly Winds. Glacial advances in Patagonia and New Zealand during the gLGM were probably primed by underlying orbital parameters. However, the precise timing is likely to have been intrinsically linked to migration of the coupled ocean-atmosphere system, which may account for the small offset between Patagonia and New Zealand due to differences in oceanic frontal migration. During deglaciation, advances or still stands occurred in both regions during the southern Antarctic Cold Reversal (ca. 14.5–12.9 ka) rather than the northern Younger Dryas (ca. 12.9–11.7 ka). Our findings suggest that major rearrangements of the Southern Hemisphere climate system occurred at various times during the last glacial cycle, with associated impacts on the position and intensity of the Southern Westerly Winds and oceanic fronts, as well as wind-driven upwelling and degassing of the deep Southern Ocean. Thus, reconstructing the timing of glacial advance/retreat using our compilation is a powerful way to understand the mechanisms of past interhemispheric climate change

Extensive MIS 3 glaciation in southernmost Patagonia revealed by cosmogenic nuclide dating of outwash sediments

The timing and extent of former glacial advances can demonstrate leads and lags during 2 periods of climatic change and their forcing, but this requires robust glacial chronologies. In 3 parts of southernmost Patagonia, dating pre-global Last Glacial Maximum (gLGM) ice limits 4 has proven difficult due to post-deposition processes affecting the build-up of cosmogenic 5 nuclides in moraine boulders. Here we provide ages for the Río Cullen and San Sebastián 6 glacial limits of the former Bahía Inútil-San Sebastián (BI-SSb) ice lobe on Tierra del Fuego 7 (53-54°S), previously hypothesised to represent advances during Marine Isotope Stages 8 (MIS) 12 and 10, respectively. Our approach uses cosmogenic 10Be and 26Al exposure 9 dating, but targets glacial outwash associated with these limits and uses depth-profiles and 10 surface cobble samples, thereby accounting for surface deflation and inheritance. The data 11 reveal that the limits formed more recently than previously thought, giving ages of 45.6 ka 12 (+139.9/-14.3) for the Río Cullen, and 30.1 ka (+45.6/-23.1) for the San Sebastián limits. These dates 13 indicate extensive glaciation in southern Patagonia during MIS 3, prior to the well-14 constrained, but much less extensive MIS 2 (gLGM) limit. This suggests the pattern of ice 15 advances in the region was different to northern Patagonia, with the terrestrial limits relating 16 to the last glacial cycle, rather than progressively less extensive glaciations over hundreds of 17 thousands of years. However, the dates are consistent with MIS 3 glaciation elsewhere in 18 the southern mid-latitudes, and the combination of cooler summers and warmer winters with 19 increased precipitation, may have caused extensive glaciation prior to the gLGM

Geomorphological record of a former ice stream to ice shelf lateral transition zone in Northeast Greenland

Understanding ice stream dynamics over decadal to millennial timescales is crucial for improving numerical model projections of ice sheet behaviour and future ice loss. In marine-terminating settings, ice shelves play a critical role in controlling ice-stream grounding line stability and ice flux to the ocean, but few studies have investigated the terrestrial lateral geomorphological imprint of ice shelves during deglaciation. Here, we document the terrestrial deglacial landsystem of Nioghalvfjerdsfjorden Glacier (79N) in Northeast Greenland, following the Last Glacial Maximum, and the margin’s lateral transition to a floating ice shelf. High-elevation areas are influenced by local ice caps and display autochthonous to allochthonous blockfields that mark the interaction of local ice caps with the ice stream below. A thermal transition from cold- to warm-based ice is denoted by the emplacement of erratics onto allochthonous blockfields. Below ~600 m a.s.l. glacially abraded bedrock surfaces and assemblages of lateral moraines, ‘hummocky’ moraine, fluted terrain, and ice-contact deltas record the former presence of warm-based ice and thinning of the grounded ice stream margin through time. In the outer fjord a range of landforms such as ice shelf moraines, dead-ice topography, and weakly developed ice marginal glaciofluvial outwash was produced by an ice shelf during deglaciation. Along the mid- and inner-fjord areas this ice shelf signal is absent, suggesting ice shelf disintegration prior to grounding line retreat under tidewater conditions. However, below the marine limit, the geomorphological record along the fjord indicates the expansion of the 79N ice shelf during the Neoglacial, which culminated in the Little Ice Age. This was followed by 20th Century recession, with the development of a suite of compressional ice shelf moraines, ice-marginal fluvioglacial corridors, kame terraces, dead-ice terrain, and crevasse infill ridges. These mark rapid ice shelf thinning and typify the present-day ice shelf landsystem in a warming climate