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The effects of glacier ice block grounding on the morphology and sedimentology of proglacial fluvial outwash were examined daring a glacier nutburst flood or jökulhlaup, near Søndre Strømfjord, west Greenland. Observations made during and after the 1987 jökulhlaup both on the surface of an ice contact delta and within a confined valley sandur plain provided information about the formation of ice block obstacle marks and the significance of them bedforms for sandur morphology and sedimentology. Flow directions determined from obstacle mark morphology have been used successfully to chart flow direction changes on the falling limb of the jökulhlaup. Maximum flow depths for scour around stranded ice blocks may be given by 0.5-0.9 times the diameter of the ice block, as estimated from the depth of scour, the height of the obstacle shadow or the extent of ice block meltout sediments. Minimum flow depths can be represented by the height of the obstacle shadow above the mean bed level. The internal composition of the shadow indicates the ability of the flow to transport various sizes of material into the ice of obstacles. Ice block obstacle marks within the distal portion of the sandur initiated waning stage channel change. Proximal and lateral erosion around stranded ice blocks extended downstream from the ice block, forming chute channels which then captured waning stage flows, resulting in significant bar incision with associated deposition of lobate or deltaic deposits. It is suggested that ice block obstacle marks are important in terms of channel morphology, channel morphological change and their usefulness as palaeohydrological indicators. [ABSTRACT FROM AUTHOR]

The ultimate demise of the Laurentide Ice Sheet (LIS) and the preceding and succeeding oceanographic changes along the western Labrador Sea offer insights critically important to improve climate predictions of expected future climate warming and further melting of the Greenland ice cap. However, while the final disappearance of the LIS during the Holocene is rather well constrained, the response of sea ice during the resulting meltwater events is not fully understood. Here, we present reconstructions of paleoceanographic changes over the past 9.3 Kyr BP on the northwestern Labrador Shelf, with a special focus on the interaction between the final meltwater event around 8.2 Kyr BP and sea ice and phytoplankton productivity (e.g., IP25, HBI III (Z), brassicasterol, dinosterol, biogenic opal, total organic carbon). Our records indicate low sea‐ice cover and high phytoplankton productivity on the Labrador Shelf prior to 8.9 Kyr BP, sea‐ice formation was favored by decreased surface salinities due to the meltwater events from Lake Agassiz‐Ojibway and the Hudson Bay Ice Saddle from 8.55 Kyr BP onwards. For the past ca. 7.5 Kyr BP sea ice is mainly transported to the study area by local ocean currents such as the inner Labrador and Baffin Current. Our findings provide new insights into the response of sea ice to increased meltwater discharge as well as shifts in atmospheric and oceanic circulation. Key Points: Sea ice on the Labrador Shelf mainly follows the solar insolation and meltwater input from the decaying Laurentide Ice SheetSea ice increased following the Lake Agassiz outburst and Hudson Bay Ice Saddle Collapse between 8.5 and 8.2 Kyr BPLow sea ice conditions during the Holocene Thermal Maximum were replaced by an increase following the Neoglacial cooling trend [ABSTRACT FROM AUTHOR]

The accelerating flux of glacial meltwater to the oceans due to global warming is a potential trigger for future climate disturbance. Past disruption of Atlantic Ocean circulation, driven by melting of land‐based ice, is linked in models to reduced ocean‐atmosphere heat transfer and abrupt cooling during stadial events. The most recent stadial, the Younger Dryas (YD), is traditionally viewed as a severe cooling centered on the North Atlantic but with hemispheric influence. However, indications of summer warmth question whether YD cooling was truly year‐round or restricted to winter. Here, we present a beryllium‐10‐dated glacier record from the north‐east North Atlantic, coupled with 2‐D glacier‐climate modeling, to reconstruct Lateglacial summer air temperature patterns. Our record reveals that, contrary to the prevailing model, the last glacial advance in Scotland did not occur during the YD but predated the stadial, while the YD itself was characterized by warming‐driven deglaciation. We argue that these apparently paradoxical findings can be reconciled with regional and global climate events by invoking enhanced North Atlantic seasonality—with anomalously cold winters but warming summers—as an intrinsic response to globally increased poleward heat fluxes. Plain Language Summary: The accelerating melting of the Greenland Ice Sheet has key ramifications for North Atlantic Ocean circulation and heat transport. In the paleoclimate record, discrete periods of ocean current weakening are linked to increased meltwater input from collapsing ice sheets and are believed to have driven severe year‐round cooling of adjacent landmasses; these periods are known as stadials. Yet, research on the Younger Dryas (YD) stadial suggests that summers may have remained warm. We present a new glacier record from northwest Scotland to reconstruct patterns of stadial summer air temperature in the North Atlantic. Our data reveal that the last glacial advance did not occur during the YD, as widely assumed, but predated the stadial. The YD itself was a period of deglaciation indicative of summer climate warming. We propose that stadials are anomalous periods of high seasonality in the North Atlantic region, with cold winters but warming summers, and might be a persistent feature of AMOC disruption, both past and future. Key Points: Lateglacial fluctuations of Scottish glaciers are dated with 10Be. Equilibrium line altitudes are resolved via glacier‐climate modelingThe last pulse of glaciation culminated prior to the Younger Dryas (YD), while the stadial was characterized by deglaciation and summer warmingThe YD stadial was defined by anomalous thermal seasonality in the North Atlantic region, rather than by year‐round cooling [ABSTRACT FROM AUTHOR]

Lakes fed by Greenlandic mountain glaciers and ice caps (GICs) contain important archives of Arctic palaeoenvironmental change. GIC proglacial lake records have been increasingly used to reconstruct Holocene glacier behaviour, largely focusing on macrostratigraphy. However, despite the wide range of topographic settings and catchment characteristics, there has been little systematic analysis of the ways that catchment conditions are registered in the clastic sediments of GIC lakes. Such signals provide valuable insights into landscape processes and palaeoenvironmental conditions that are not routinely captured in other Quaternary glacial morphosedimentary archives. This review synthesises sedimentological and geochemical evidence from existing Holocene GIC proglacial lake records to establish: how catchment‐wide conditions have been recorded in the lacustrine sequences; and our ability to isolate these signals to enhance palaeoenvironmental reconstruction. Our review shows that with careful sedimentological and targeted (bio)geochemical analyses coupled with a clear process‐based understanding, catchment and in‐lake signals can be effectively identified in the microstratigraphic and mineral grain record. Such signals include wind patterns, mass wasting, precipitation events and seasonal lake ice cover, that can complement broader palaeoclimatic proxy evidence. The approaches collated here, if more widely applied, could considerably enhance environmental reconstructions not only in Greenland, but in glaciated catchments elsewhere. [ABSTRACT FROM AUTHOR]

Our limited understanding of millennial‐scale variability in the context of the last glacial period can be explained by the lack of a reliable modeling framework to study abrupt climate changes under realistic glacial backgrounds. In this article, we describe a new set of long‐run Last Glacial Maximum experiments where such climate shifts were triggered by different snapshots of ice‐sheet meltwater derived from the early stages of the last deglaciation. Depending on the location and the magnitude of the forcing, we observe three distinct dynamical regimes and highlight a subtle window of opportunity where the climate can sustain oscillations between cold and warm modes. We identify the Eurasian Arctic and Nordic Seas regions as being most sensitive to meltwater discharge in the context of switching to a cold mode, compared to freshwater fluxes from the Laurentide ice sheets. These cold climates follow a consistent pattern in temperature, sea ice, and convection, and are largely independent from freshwater release as a result of effective AMOC collapse. Warm modes, on the other hand, show more complexity in their response to the regional pattern of the meltwater input, and within them, we observe significant differences linked to the reorganization of deep water formation sites and the subpolar gyre. Broadly, the main characteristics of the oscillations, obtained under full‐glacial conditions with ice‐sheet reconstruction derived meltwater patterns, share similar characteristics with δ18O records of the last glacial period, although our experiment design prevents detailed conclusions from being drawn on whether these represent actual Dansgaard‐Oeschger events. Plain Language Summary: During the last glacial period (115,000–12,000 years before present), the baseline cold climate was continuously disturbed by intense and abrupt climate changes. They completely modified the climate for a few thousand years or so, resulting, for instance, in massive temperature shifts and complete reorganizations of ocean circulation. These abrupt changes have been observed in climate records from the Northern Hemisphere and also can be traced in records from the Southern Hemisphere. Yet, we still do not know what triggers these changes, and often cannot simulate them at the right time under known environmental conditions. In the context of the Last Glacial Maximum, a cold period 21,000 years ago with extensive ice over the Northern Hemisphere, this article analyses a new set of climate model simulations that test the effects of freshwater melting from the ice sheets at different periods of the early deglaciation (∼21,000 to 18,000 years before present). Under some conditions, the resulting experiments displayed an Atlantic Ocean that oscillates between strong and collapsed basin‐wide circulation, causing approximately 10°C of temperature change over Greenland; a behavior that resembles observed abrupt climate changes. Key Points: New set of long‐run Last Glacial Maximum general circulation model experiments showing millennial‐scale variabilityDetailed description of the impact of ice sheet reconstruction‐derived meltwater distributions on abrupt climate changesIntroduction for future studies of the underlying physical processes of abrupt climate changes [ABSTRACT FROM AUTHOR]

Glacial outburst floods or ‘jökulhlaups’ are a key part of deglaciation of both alpine mountains and of ice sheet margins. They produce widespread and intense landscape change through erosion and deposition and are a hazard to populations and infrastructure. This article reviews these aspects with an emphasis on the current ‘state of play’ of research on the phenomena. Three case studies are presented: the 2010 Eyafjallajökull and 2007 Mt Ruapehu events, and the recent jökulhlaups in western Greenland. These illustrate that, where there is sufficient lead-in time for the development of projects, the application of modern technology and of multi-disciplinary research teams enables unprecedented acquisition quality, resolution and extent of data to be gained. These exciting data should enable development of new insights towards jökulhlaup impacts and flow phenomena. [ABSTRACT FROM AUTHOR]

The Atlantic Meridional Overturning Circulation (AMOC) is thought to be relatively vigorous and stable during Interglacial periods on multimillennial (equilibrium) timescales. However, recent proxy (δ13C benthic) reconstructions suggest that higher frequency variability in deep water circulation may have been common during some interglacial periods, including the Last Interglacial (LIG, 130–115 ka). The origin of these isotope variations and their implications for past AMOC remain poorly understood. Using iLOVECLIM, an Earth system model of intermediate complexity (EMIC) allowing the computation of δ13CDIC and direct comparison to proxy reconstructions, we perform a transient experiment of the LIG (125–115 ka) forced only by boundary conditions of greenhouse gases and orbital forcings. The model simulates large centennial‐scale variations in the interior δ13CDIC of the North Atlantic similar in timescale and amplitude to changes resolved by high‐resolution reconstructions from the LIG. In the model, these variations are caused by changes in the relative influence of North Atlantic Deep Water (NADW) and southern source water (SSW) and are closely linked to large (∼50%) changes in AMOC strength provoked by saline input and associated deep convection areas south of Greenland. We identify regions within the subpolar North Atlantic with different sensitivity and response to changes in preformed δ13CDIC of NADW and to changes in NADW versus SSW influence, which is useful for proxy record interpretation. This could explain the relatively large δ13C gradient (∼0.4%0) observed at ∼3 km water depth in the subpolar North Atlantic at the inception of the last glacial. Key Points: Similar variations of bottom water δ13C to that depicted by data reconstructions are simulated in the North Atlantic for the LIG periodChanges in AMOC strength may be a key process in distributing δ13C in the ocean interiorMagnitude of local bottom water δ13C response depends on position relative to water mass geometry [ABSTRACT FROM AUTHOR]

A detailed understanding of calving processes at the lacustrine margins of the Greenland ice sheet is necessary for accurately forecasting its dynamic response to ongoing climate change. However, existing data sets of lacustrine calving are limited to summer seasons and to alpine glaciers. Here, we use an integrated time‐lapse and structure‐from‐motion approach to generate the first continuous year‐round volumetric record of calving processes at a lacustrine ice sheet margin. We identify two distinct calving regimes that are associated with melt‐undercutting and lake ice cover. We also find that calving rates respond rapidly to sudden lake drainage. Given that lake temperature, lake ice cover, and sudden lake drainages are controlled by air temperature and ice‐margin thinning, we suggest that climate change, manifested in lengthening summer seasons, will accelerate rates of mass loss and terminus recession at lacustrine ice‐margins in Greenland. Plain Language Summary: Lakes situated at the margins of glaciers and ice sheets are important because they promote the detachment of large blocks of ice to form icebergs, a process known as calving. Climate change is increasing the number of lakes at the margins of the Greenland ice sheet, and many of those already present are increasing in size. Consequently, a detailed understanding of lake calving processes is needed to accurately predict the future response of the ice sheet to climate change. This study improves our knowledge of calving processes by using time‐lapse cameras to record images of calving activity at a lake located in western Greenland over a continuous 14‐month period. We use the images to create 3D models that illustrate how calving processes change through time and find that rates and styles of calving vary seasonally in response to lake‐driven undercutting of the ice sheet margin and the presence or absence of lake ice cover. Our results suggest that ice loss via calving activity at the lake edges of the Greenland ice sheet will increase in future as summer seasons lengthen in response to climate change. Key Points: We present the first year round record of calving activity at a lacustrine ice‐margin and classify 216 calving events by mechanism and volumeMelt‐undercutting and the presence and absence of lake ice cover are associated with seasonality in calving rates and processesLengthening summer seasons and thinning ice‐margins are likely to increase calving rates and thus rates of mass loss at lacustrine margins [ABSTRACT FROM AUTHOR]

ABSTRACT After a period of early Palaeocene faulting and uplift of the Nuussuaq Basin, West Greenland, two valley systems were incised into the underlying sediments. Incision of the older Tupaasat valley took place during a single drainage event of large water masses, which resulted in catastrophic deposition. The valley was cut along early Palaeocene NW- to SE-trending normal faults, clearly showing that the trend and the relief of the valley were structurally controlled. The valley fill is up to 120 m thick and consists of a lower part of sandstones and conglomerates deposited from catastrophic flows characterized by very high concentrations of suspended coarse-grained sediment load. Catastrophic deposition was followed by rapid decrease in flow discharge and the establishment of a lacustrine environment within the valley characterized by the deposition of heterolithic sediments. The younger Paatuutkløften valley system was mainly cut into the Tupaasat valley fill, which was completely or nearly completely eroded away in many places. The younger valley is 1–2 km wide and up to 190 m deep. Incision of the Paatuutkløften valley probably reflected renewed tectonic activity and uplift of the basin. This phase was shortly followed by rapid major subsidence. The valley-fill deposits comprise a uniform succession of fluvial and estuarine sandstones. The valley fill is topped by shoreface sandstones, which are succeeded abruptly by offshore mudstones deposited shortly before and during the initial extrusion of a thick hyaloclastite succession. The Paatuutkløften valley fill is attributed to a very rapid rise in relative sea level contemporary with extensive volcanism. It is suggested that this sequence of events coincided with the arrival of the North Atlantic mantle plume. In several respects, the early Palaeocene valley-fill deposits of the Nuussuaq Basin are different from idealized facies models for incised valley systems and represent very... [ABSTRACT FROM AUTHOR]