Meltwater Discharge Research Articles

Heinrich summers

Millennial-scale climate oscillations of the last ice age registered in Greenland and Antarctic ice cores did not always vary in unison. A striking example is that the strongest Antarctic warming episodes occurred during Heinrich episodes in the North Atlantic region. Although the bipolar seesaw affords a possible explanation for such anti-phasing, it does not account for the equally striking observation that climate varied in unison between the hemispheres about half the time. Such phasing differences suggest the need for an alternative hypothesis in which the polar regions at times responded in unison to common forcing, and at other times left the impression of a bipolar seesaw. We posit that this impression arose from the effect of warmer-than-usual summers on continental ice sheets adjacent to the North Atlantic Ocean during each Heinrich episode. The relatively warm Heinrich summers produced discharges of meltwater and icebergs of sufficient volume to stimulate very cold winter conditions from widespread sea ice on a freshened ocean surface. The intervals between Heinrich episodes featured relaxation of sea-ice-induced winter severity from reduced summertime influx of meltwater and icebergs, indicating relatively cooler summer conditions. It is postulated that the causative variations in freshwater fluxes were driven by a climate signal most evident in Antarctic ice cores but also recognized in other paleoclimate records in both polar hemispheres. We suggest that this widespread signal arose from changes in the latitude and strength of the austral westerlies and the resulting effect on the western Pacific tropical warm pool, a mechanism dubbed the Zealandia Switch.

Fine‐Scale Geothermal Heat Flow in Antarctica Can Increase Simulated Subglacial Melt Estimates

AbstractAntarctic geothermal heat flow (GHF) affects the thermal regime of ice sheets and simulations of ice and subglacial meltwater discharge to the ocean, but remains poorly constrained. We use an ice sheet model to investigate the impact of GHF anomalies on subglacial meltwater production in the Aurora Subglacial Basin, East Antarctica. We find that spatially‐variable GHF fields produce more meltwater than a constant GHF with the same background mean, and meltwater production increases as the resolution of GHF anomalies increases. Our results suggest that model simulations of this region systematically underestimate meltwater production using current GHF models. We determine the minimum basal heating required to bring the basal ice temperature to the pressure melting point, which should be taken together with the scale‐length of likely local variability in targeting in‐situ GHF field campaigns.

Late Pleistocene- Holocene architecture and sedimentary processes on the glacially influenced SW Grand Banks Slope off Newfoundland

Complex inter-relationships between alongslope and downslope sediment dispersion exist on glaciated continental margins and vary widely along continental margins depending on sediment supply and bottom current strength. In eastern Canada, proglacial sedimentation rates are relatively high on the SW Grand Banks Slope compared to the sediment starved SE Grand Banks margin, but relatively low compared to the glacially dominated Scotian margin off eastern Canada. As on other parts of the Canadian margin, its late Quaternary sediment architecture has been constructed by interacting alongslope and downslope processes. These include sediment transported by downslope glacial meltwater discharge, alongslope bottom currents and ice-rafting. Based on the analysis of sediment cores going back to 24 ka (Heinrich event 2), this study investigates fine-grained sedimentary facies and the development of variable depositional patterns on the glacially influenced SW Grand Banks Slope off Newfoundland (eastern Canada). Both turbidites and contourites show stratification, but differ in internal structure, the presence of IRD, and the nature of their upper and lower boundaries. Sandy contourites are mostly massive, occurring either as lenses or as part of the ideal bi-gradational vertical sequence with mottled silt-mud. Glacial silty contourites have distinct rhythmic laminae with the long axis of IRD parallel to bedding. Regional scale thickness variations allow discrimination between hemipelagites and muddy contourites. Depositional architecture is built through temporal and spatial coupling of the diverse sedimentary processes. During the last glacial maximum and early deglaciation, turbidity currents fed either by meltwater or oceanographic processes flowed in canyons, and a contourite depositional system developed between the canyons. The two systems interacted on inter-canyon ridges, where contourite sedimentation was not completely overwhelmed by energetic turbidity currents. In the Holocene, alongslope processes became dominant, building a drift with clearly variable thickness, in part related to seabed morphology. A conceptual model is proposed to present the key elements of depositional processes in this depositional system, and a similar evolutionary history can be expected on other distal glacial margins.

Relict proglacial lake of Spituk (Leh), northwest (NW) Himalaya: A repository of hydrological changes during Marine Isotopic Stage (MIS)-2

In the northwestern Himalaya (Ladakh and Karakoram), the spatial and temporal changes in the late Quaternary glaciation were modulated by a combination of two contrasting moisture sources (viz., the Indian Summer Monsoon and the mid-latitude westerlies). The inferences about their relative contribution to glacier expansion are based on the ages obtained on moraines. Since the moraines suffer from poor preservation and chronological uncertainty, the present study, therefore, resorted to a near-continuous relict proglacial lake succession in order to reconstruct the continuous pattern of glacier fluctuations during Marine Isotopic Stage-2 (MIS-2). Since the AMS radiocarbon ages (both organic and inorganic matter) suffered from various complexities such as dead carbon contribution and hard water effect, the paleohydrological inferences in this study were drawn primarily based on optical ages (along with sedimentological and geochemical analysis).The proglacial lake sedimentation was modulated by the temporal changes in meltwater discharge. This is attributed to the waning and waxing of proximal valley glaciers. Evidence accrued from the lake sediment supported by the published moraine chronology suggests that valley glaciers expanded during the onset of MIS-2 and persisted until around the Last Glacial Maximum (LGM). The beginning of MIS-2 is marked by the onset of cooling (~ 30 ka), which led to a marginal advance in the glaciers. Additionally, early MIS-2 was characterised by millennial-scale climatic fluctuations. For example, a short-lived increase in humidity was observed between 30.2 ka and 29.3 ka, followed by a reversal towards a drier climate between 29.3 ka and 28.1 ka. During the early part of MIS-2, glaciers were driven by the decrease in temperature, whereas the enhanced moisture contribution during mid MIS-2 was responsible for extensive glacier growth. We hypothesise that the enhanced moisture was contributed by the mid-latitude westerlies, with subordinate contribution from the Indian Summer Monsoon (ISM). This we attribute to the southward positioning of the Intertropical Convergence Zone (ITCZ) during MIS-2.

Mid-Late Holocene climate variabilities in the Bransfield Strait, Antarctic Peninsula driven by insolation and ENSO activities

The Antarctic Peninsula (AP) is sensitive to global climate changes. Diatom composition was investigated in a sediment core recovered from the Bransfield Strait, in order to reconstruct the changes in surface water conditions over the past ca. 5.8 kyr (mid-late Holocene) in the AP region. A relatively stable mid-Holocene (5.8–3.8 ka) with persistent presence of sea ice was punctuated by a cooling episode at 4.9–4.2 ka, followed by a cooling trend with stronger sea-ice variability during the late Holocene (since 3.8 ka), which enhanced the surface water stratification in the study area. Frequency analysis reveals centennial- to millennial-scale cyclicities resembling the characteristics of solar irradiance and El Niño-Southern Oscillation (ENSO). In particular, the increasing ENSO activities during the late Holocene resulted in intensified cold Weddell Sea outflow to the Bransfield Strait that increased sea ice presence and duration. This pattern is in contrast to a northerly warm air intrusion induced glacial meltwater discharge to the western AP region, leading to similar sea ice variability to that in the east AP region.

Late Quaternary Evolution of a Submerged Karst Basin Influenced by Active Tectonics (Koločep Bay, Croatia)

Koločep bay is a 30 km elongated karst basin located between the Elafiti Islands and the mainland with a NW–SE general direction. The bay lies within the seismically active southern Dalmatia zone. Irregular grid sub-bottom profiles and two legacy reflection seismic profiles have been used to determine the overall morphology of the bay and to establish the seismic stratigraphy of its sedimentary infill. Three major seismic–stratigraphic units have been identified in the upper ~50 m of the ~120-meter-thick sedimentary sequence that lies above the karstified limestone bedrock. The karst polje basin was flooded due to sea-level rise before 12.1 kyr BP. The morphology of the bay implies complex influences of Late Glacial meltwater discharges, aeolian sand deposition, the existence of paleo–ponor/karst spring zones and buried Pleistocene river channels. The Pleistocene seismotectonic units are deformed in the NW and SE parts of the basin. The central part of the basin has no signatures of intensive tectonic activity during the Holocene. A major erosion event was identified that led to the formation of a basin within the older sedimentary infill. In the southern part of the basin, we have evidence of Holocene tectonic activity with the formation of erosional scarps on the seafloor of the bay.

The timing, duration and magnitude of the 8.2 ka event in global speleothem records

Abrupt events are a feature of many palaeoclimate records during the Holocene. The best example is the 8.2 ka event, which was triggered by a release of meltwater into the Labrador Sea and resulted in a weakening of poleward heat transport in the North Atlantic. We use an objective method to identify rapid climate events in globally distributed speleothem oxygen isotope records during the Holocene. We show that the 8.2 ka event can be identified in >70% of the speleothem records and is the most coherent signal of abrupt climate change during the last 12,000 years. The isotopic changes during the event are regionally homogenous: positive oxygen isotope anomalies are observed across Asia and negative anomalies are seen across Europe, the Mediterranean, South America and southern Africa. The magnitude of the isotopic excursions in Europe and Asia are statistically indistinguishable. There is no significant difference in the duration and timing of the 8.2 ka event between regions, or between the speleothem records and Greenland ice core records. Our study supports a rapid and global climate response to the 8.2 ka freshwater pulse into the North Atlantic, likely transmitted globally via atmospheric teleconnections.

Intermediate ocean circulation and cryosphere dynamics in the northeast Atlantic during Heinrich Stadials: benthic foraminiferal assemblage response

AbstractThe interaction between ocean circulation and ice‐sheet dynamics plays a key role in the Quaternary climate. Compared with the surface and deep regions of the Atlantic Meridional Overturning Circulation (AMOC), the study of intermediate depths during key time periods, such as Heinrich Stadials (HSs), remains poorly documented, especially for the northeast Atlantic. Here we use benthic foraminiferal assemblage data to trace palaeoenvironmental changes from ~32 to 14 ka cal bp at ~1000 m depth in the Bay of Biscay. Our results highlight the high sensitivity of foraminifera, with species‐specific responses, to continental (European Ice Sheet dynamics) and marine (AMOC) forcing factors during the last three HSs. In general, HSs were characterised by the concomitant presence of meso‐oligotrophic and anoxia indicator species and the low abundance of high‐energy indicator species. This confirms an overall sluggish intermediate circulation during the three HSs in the northeast Atlantic. HS1 is distinctive for its abundance of high‐organic flux indicator species during its early phase. This is consistent with the fact that HS1 was by far the most important period of ice‐sheet retreat and meltwater release into the ocean over the studied time interval. Finally, foraminifera depict the mid‐HS2 reventilation event due to regional glacier instabilities.

Supraglacial streamflow and meteorological drivers from southwest Greenland

Abstract. Greenland ice sheet surface runoff is drained through supraglacial stream networks. This evacuation influences surface mass balance as well as ice dynamics. However, in situ observations of meltwater discharge through these stream networks are rare. In this study, we present 46 discrete discharge measurements and continuous water level measurements for 62 d spanning the majority of of the melt season (13 June to 13 August) in 2016 for a 0.6 km2 supraglacial stream catchment in southwest Greenland. The result is an unprecedentedly long record of supraglacial discharge that captures both diurnal variability and changes over the melt season. A comparison of surface energy fluxes to stream discharge reveals shortwave radiation as the primary driver of melting. However, during high-melt episodes, the contribution of shortwave radiation to melt energy is reduced by ∼40 % (from 1.13 to 0.73 proportion). Instead, the relative contribution of longwave radiation, sensible heat fluxes, and latent heat fluxes to overall melt increases by ∼24 %, 6 %, and 10 % (proportion increased from −0.32 to −0.08, 0.28 to 0.34, and −0.04 to 0.06) respectively. Our data also identify that the timing of daily maximum discharge during clear-sky days shifts from 16:00 local time (i.e., 2 h 45 min after solar noon) in late June to 14:00 in late July and then rapidly returns to 16:00 in early August. The change in the timing of daily maximum discharge could be attributed to the expansion and contraction of the stream network, caused by skin temperatures that likely fell below freezing at night. The abrupt shift, in early August, in the timing of daily maximum discharge coincides with a drop in air temperature, a drop in the amount of water temporarily stored in weathering crust, and a decreasing covariance between stream velocity and discharge. Further work is needed to investigate if these results can be transferable to larger catchments and uncover if rapid shifts in the timing of peak discharge are widespread across Greenland supraglacial streams and thus have an impact on meltwater delivery to the subglacial system and ice dynamics.

Delta size variability at Lake General Carrera, Patagonia

Punctuated lake level drops at Lake General Carrera, Chile, over the last ~13 ka are recorded in multiple stacked deltas that differ in scale. Fifteen delta terrace suites are studied to examine the factors that influence deposit morphology. Surprisingly, the pronounced longitudinal precipitation gradient is inversely correlated with the observed variations in delta size. We find the drainage basins with long-term water storage in glaciers are associated with larger deltas. In these cases, the added contribution of rare, high-magnitude ice meltwater release events (> 600 m3/s) is suggested as the differential driver to enhance delta growth. The overall progradation rate is modest (0.2-1 m/yr), and appears to have slowed over time. For the youngest deltas, we estimate the range in the intermittency factor, a dimensionless measure of the frequency of floods, is 1x10-2 to 4x10-5. Our results illustrate the natural variability of delta growth in a small region (

Impacts of glacier and sea ice melt on methane pathways on the Northeast Greenland shelf

Global warming has led to a sharp decrease in Arctic summer sea ice extent and a dramatic ice mass loss of the Greenland Ice Sheet over the past three decades. The Northeast Greenland continental shelf is a site of intense water mass transformation involving both sea ice processes and glacier dynamics. The Arctic shelf waters are considered to be a net source of atmospheric methane (CH4); however, the effect of glacier and sea ice melt on oceanic CH4 concentrations still needs to be investigated. To better understand the effect of meltwater on the CH4 budget of the ocean, our study constrains the CH4 pathways by following changes in water mass properties and infers potential CH4 sources and sinks. Based on measurements of concentration and carbon isotope delta (δ13C) of CH4, the water mass tracer δ18O(H2O) and physical properties of the water masses, we detected CH4 excess in surface waters, which we attribute to brine release during sea ice formation. We show that this CH4 excess is sustained throughout the melt season, due to a freshwater lid formed at the ocean surface. The meltwater hardly alters the CH4 excess, but enhances water stratification, which, in turn, restricts the sea-to-air flux. The CH4 excess is subject to mixing with surrounding shelf waters influenced by basal glacial meltwater discharge. We suggest that the CH4 excess of Northeast Greenland continental shelf waters is redistributed in the marine environment, while CH4 emission to the atmosphere is limited to regions not covered by sea ice.

Detrital Carbonate Minerals in Earth's Element Cycles.

We investigate if the commonly neglected riverine detrital carbonate fluxes might reconciliate several chemical mass balances of the global ocean. Particulate inorganic carbon (PIC) concentrations in riverine suspended sediments, that is, carbon contained by these detrital carbonate minerals, were quantified at the basin and global scale. Our approach is based on globally representative data sets of riverine suspended sediment composition, catchment properties, and a two‐step regression procedure. The present‐day global riverine PIC flux is estimated at 3.1 ± 0.3 Tmol C/y (13% of total inorganic carbon export and 4% of total carbon export) with a flux‐weighted mean concentration of 0.26 ± 0.03 wt%. The flux prior to damming was 4.1 ± 0.5 Tmol C/y. PIC fluxes are concentrated in limestone‐rich, rather dry and mountainous catchments of large rivers near Arabia, South East Asia, and Europe with 2.2 Tmol C/y (67.6%) discharged between 15°N and 45°N. Greenlandic and Antarctic meltwater discharge and ice‐rafting additionally contribute 0.8 ± 0.3 Tmol C/y. This amount of detrital carbonate minerals annually discharged into the ocean implies a significant contribution of calcium (∼4.75 Tmol Ca/y) and alkalinity fluxes (∼10 Tmol (eq)/y) to marine mass balances and moderate inputs of strontium (∼5 Gmol Sr/y) based on undisturbed riverine and cryospheric inputs and a dolomite/calcite ratio of 0.1. Magnesium fluxes (∼0.25 Tmol Mg/y), mostly hosted by less‐soluble dolomite, are rather negligible. These unaccounted fluxes help in elucidating respective marine mass balances and potentially alter conclusions based on these budgets.

Beryllium isotope variations recorded in the Adélie Basin, East Antarctica reflect Holocene changes in ice dynamics, productivity, and scavenging efficiency

The Adélie Basin is a relatively small (∼1600 km2), semi-enclosed continental shelf bathymetric depression located adjacent to the Wilkes Subglacial Basin, a basin underlying a sector of the East Antarctic Ice Sheet that contains ∼3–4 m sea level equivalent of ice. Located within the Adélie Basin is a ∼184 m thick laminated sediment deposit, the Adélie Drift, ideal for examining regional changes in ice sheet and ocean dynamics. Here, we examine the ratio of reactive beryllium-10 to reactive beryllium-9 ((10Be/9Be)reac) in a marine sediment core obtained from the Adélie Drift to assess these changes during the Holocene epoch (11.7 ka BP to present). The (10Be/9Be)reac record provides insight into changes in freshwater input, primary productivity, and scavenging efficiency, while removing the influence of particle size on 10Be concentration. During the early Holocene, (10Be/9Be)reac ratios indicate increased meltwater discharge from ca. 11.7 to 10 ka BP, as grounded ice retreated from the Adélie Basin and adjacent bathymetric highs. After ∼10 ka BP, beryllium isotopes are influenced by scavenging efficiency and dilution controlled by ocean currents and accumulation rate, operating alongside meltwater input, suggesting there are additional factors to consider when using (10Be/9Be)reac as a proxy for ice shelf cover and glacial dynamics.

Morphodynamic evolution of paraglacial spit complexes on a tide-influenced Arctic fjord delta (Dicksonfjorden, Svalbard)

Recent global warming triggered pronounced geomorphic changes such as coastal retreat and delta progradation along the coastlines of the Arctic regions. Coastal morphodynamics and associated sediment transport at the Arctic fjord head remain relatively unexplored due to the logistically limited accessibility to the field area, especially at short-term temporal scales. A repeat survey using an unmanned aerial vehicle (UAV)-assisted photogrammetry was conducted to quantify the annual morphodynamics of gravel spit complexes developed on the tidal delta plain of the deglaciated Dicksonfjorden, Svalbard of Arctic. Results show that the spit morphodynamics varies in time and space with an overall downfjord increase in the growth and migration rate of the spits. The youngest spits elongated 22 m yr−1 and migrated landward 4.3 m yr−1 between 2015 and 2019, marking the most pronounced spit morphodynamics documented to date in the Svalbard fjord systems. The spit morphodynamics is driven primarily by longshore drift and to a lesser degree by overwash processes. Gravels constituting the spits originate from the unconsolidated debris-flow deposits of old alluvial fans, which locally retreat 0.5 m yr−1. The growth of the spit complexes is also fed by snow meltwater discharge on the alluvial fans, accounting for a downfjord imbrication of angular gravel layers that are intercalated with interlaminated sands and muds on the landward sides of the spits. The breached spits at the most upfjord location have remained stationary during the study period and presumably since the 1930s. Rapid delta progradation combined with an isostatic rebound after the Little Ice Age (LIA) has decreased spit morphodynamics on the tidal delta plain upfjord in Dicksonfjorden with infrequent and insignificant wave influence. Sparse distribution of the isolated spits signifies the intermittent spit development, which is constrained by the proximity to the protruded alluvial fans. The spit complexes in Dicksonfjorden highlight that climate change accelerates coastal geomorphic changes at the fjord head by enhancing wave intensity and regulating episodic sediment delivery that led to the downfjord shift in the locus of wave shoaling.

Formation of murtoos by repeated flooding of ribbed bedforms along subglacial meltwater corridors

Fluctuations in meltwater discharge below modern glaciers and ice sheets due to diurnal, seasonal and long-term temperature variations are modulated by complex interactions between subglacial drainage, basal processes and bedform development. The bed of palaeo-ice sheets contains a variety of bedforms recording these modulations and provides an open window into the subglacial environment. Through the morphometric analysis of natural and experimental bedforms, respectively mapped along Scandinavian meltwater corridors and produced in a physical model simulating transitory subglacial water flow, we observe a morphological and genetic bedform continuum corresponding to the progressive transformation of ribbed bedforms into murtoos. Two alternating drainage configurations, related to repeated subglacial flooding events, are involved in this transformation: (i) significant meltwater discharge, high hydraulic connectivity and ice-bed decoupling during flooding events lead to hydraulic alteration of ribbed bedforms by erosion, sediment deposition and channel incision, while (ii) limited meltwater flow, low hydraulic connectivity and ice-bed recoupling that follow flooding events lead to their deformational reshaping into murtoos. The degree of transformation of ribbed bedforms into murtoos can be quantified by combining two dimensionless morphometric parameters (circularity and sinuosity) and provides a convenient proxy to constrain magnitudes, durations and/or frequencies of subglacial floods in palaeo-meltwater corridors.

Submarine Geomorphology and Glacimarine Sedimentary Processes Associated to Deglaciation in Europa Fjord, Chilean Patagonia

In a period of high rates of glacier retreat, increasing meltwater discharge from tidewater glaciers can influence marine fjord ecosystems due to increase sediment delivery and accumulation rates in the proglacial environment. Glacier variations and associated changes in glacimarine processes are recorded in the sedimentary record and submarine geomorphology of fjords. In October-November 2017, CIMAR23 Cruise surveyed several fjords adjacent to the Southern Patagonian Icefield (SPI). In this study, we show detailed results of the multibeam bathymetry, backscatter and sub-bottom seismic profiles, and sediment core samples from Europa Fjord, located west of the central SPI area, to improve the analysis of the fjord glacial and sedimentary evolution and to connect its behavior with the other fjords in the region. For the cores 210Pb chronology, magnetic susceptibility, organic carbon, total nitrogen, and stable isotopes (δ13Corg, δ15N) were performed. The seafloor geomorphology showed that the most prominent submarine landforms are transverse morainal ridges, which indicate past stillstand glacier positions; these features formed during the deglaciation of the fjord (latest Pleistocene), and some, perhaps more recently during short episodic advance/retreat events. In the central and mouth fjord areas, erosional features such as lineations are found on the rocky bottom and interpreted as formed by past grounded glaciers. Several submarine moraines are generally well preserved, but some parts are crumbled by slope failures and erosion. Most of the seismic sections are interpreted as the expected subglacial-ice proximal-ice distal succession of facies, characteristic of single retreat environments. The sedimentary record includes bioturbed muds with muddy laminations and variable amounts of coarse sediments interpreted as ice rafted debris, suggesting a proglacial environment, punctuated by calving. From the beginning of the twentieth century there is a tendency to an increase in the terrigenous organic material content, including an abrupt increase during the 80–90s, and a decline in the last decade. This behavior is indicating possibly a general increase of surface terrestrial runoff and, for the last decade, an increase of glacial meltwater input which would be caused by an accelerated deglaciation.

The historical emergence of a geometric and sinuous ridge network at the Hørbyebreen polythermal glacier snout, Svalbard and its use in the interpretation of ancient glacial landforms

The post Little Ice Age recession and downwasting of Hørbyebreen, a Svalbard polythermal glacier, has revealed a sub-polar glacial landsystem within which a geometric and sinuous ridge network (GSRN) has evolved by ice melt-out. Spatio-temporal evolution of these features is evaluated using time series of remote sensing data, aerial photography, DEMs and field observations. The GSRN is interpreted as an assemblage of crevasse and hydrofracture infills branching out from parent eskers, a signature of the rapid release of pressurised subglacial and englacial meltwater, likely related to surge-induced jökulhlaups. This gives rise to a range of component materials including till, gravel and interdigitated till and gravel. Ancient examples of such jökulhlaup-diagnostic GSRN in ice sheet settings are recognised as elongate zones of conjugate, reticulate and honeycomb patterned ridges in close association with eskers that often also display reticulate ridge patterns. We present a conceptual model of a process-form continuum for the production of different styles of GSRN that identifies: a) surge crevassing, indicated by glacier wide GRN and zig-zag eskers; b) ice stream surging plug flow, indicated by linear assemblages of GRN; c) sub-marginal till emplacement in radially crevassed active temperate snouts, indicated by narrow concentric arcs of GRN linked to push moraines; and d) jökulhlaup-induced hydrofracture, indicated by linear assemblages of GSRN.

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.

Deep-water sedimentation processes on a glaciated margin: The Foula Wedge trough mouth fan, West of Shetland

Trough Mouth Fans (TMF) are sedimentary depocenters located at glaciated continental margins and consist predominantly of glacigenic debris flow deposits. The Foula wedge is a Pleistocene TMF accumulated offshore West of Shetland over the Northeast Atlantic margin. This study presents an analysis of a 3D seismic reflection dataset imaging the distal Foula wedge basin fan deposits between 1010 and 1100 m water depth, directly downslope from a gully system which was active untill the end of the last deglaciation. Results reveal, in unprecedented detail, the basal surface of this fan system and its internal complex architecture. Features typical of both debris flow deposits and turbidites are identified, including a basin channel network with linear and diverging erosional features forming distinctive terminal lobes, stacked and backstepping. The study links the seafloor morphology of the basin fan with its subsurface geomorphology, showing connection with the downslope gully system to the east. It presents evidence for a complex distal depositional system on glaciated margins, characterised by heterogeneous sediment delivery processes and deposits. A conceptual evolution model is proposed, with a glacigenic debris flow-dominated TMF at the LGM, subsequently influenced by meltwater discharges, with deposition occurring as a function of the shelf margin and slope paleo-morphology, slope substrate composition, interaction of downslope and along slope processes and ice-margin dynamics.

The Roles of Orbital and Meltwater Climate Forcings on the Southern Ocean Dynamics during the Last Deglaciation

The last deglacial climate evolution, from 19 to 9 thousand years before the present, represents the vital role of feedback in the Earth’s climate system. The Southern Ocean played a fundamental role by exchanging nutrients and carbon-rich deep ocean water with the surface during the last deglaciation. This study employs a fully coupled Earth system model to investigate the evolution of Southern Ocean dynamics and the roles of changes in orbital and meltwater forcings during the last deglaciation. The simulation supports that the Southern Ocean upwelling was primarily driven by windstress. The results show that the melting and formation of Antarctic sea ice feedback influenced Southern Ocean surface buoyancy flux. The increase in Antarctic sea ice melt-induced freshwater flux resulted in a steepened north-south surface salinity gradient in the Southern Ocean, which enhanced the upwelling. The single-forcing experiments indicate that the deglacial changes in orbital insolation influenced the Southern Ocean upwelling. The experiments also highlight the dominant role of Northern Hemisphere meltwater discharge in the upper and lower branch of the Meridional Overturning Circulation. Furthermore, orbital forcing shows lesser deglacial Antarctic sea ice retreat than the Northern Hemisphere meltwater forcing, which follows the bipolar seesaw mechanism.