Early Deglaciation Research Articles

Ichnological analysis of glacially-influenced sediments from the late Pleistocene to Holocene on the southeastern Canadian margin: Implications for palaeoclimate and palaeoceanography

The study of trace fossils in glacially-influenced margins provides valuable insights into the changes in palaeoclimate and palaeoceanography during glacial-interglacial periods, by providing biological and ecological data of the benthic environment. Here we present a comprehensive ichnofabric characterisation of a sediment core located on an inter-canyon ridge on the SW Grand Banks Slope off Newfoundland, covering the Late Pleistocene–Holocene period (0–∼23 ka). The core analysis reveals 12 ichnogenera and an escape structure, with Phymatoderma, Tasselia, and Zoophycos being prominent and relatively abundant at specific levels. By examining the distribution of bioturbation content and degree along the core, we establish a strong association between moderate to high bioturbation and cold periods. The Last Glacial Maximum exhibits the highest abundance and diversity of trace fossils, particularly the deposit-feeding Phymatoderma and the deposit-feeding/equilibrium trace Tasselia, indicating benthic adaptation to seasonal food input and relatively high sedimentation rates. Conversely, the warmer deglaciation period shows limited bioturbation. Salinity stress may account for the abrupt decline in bioturbation degree during the early deglaciation. Additionally, the occurrence of large-sized Zoophycos coincides with the Younger Dryas cold event during the last deglaciation. These findings highlight the complexity of palaeoceanographic conditions and benthic environments in proximal glacially-influenced sites compared to distal hemipelagic and pelagic sites. The interplay between down-slope processes (such as turbidity currents and hyperpycnal flows) and along-slope processes (like contour currents) plays a crucial role in shaping the unique sedimentological regime of depositional sites and subsequently affecting the ichnofauna in glacially-influenced margins.

Strong Climate Control on the Millennial‐Scale Dust Variability and Sediment Provenances in the Equatorial Indian Ocean Inferred From Sr‐Nd Isotopes

AbstractHigh‐resolution Sr and Nd isotope compositions along with major and trace element abundances have been analyzed in silicate fraction of sediments core, SSD004‐GC03, from the Equatorial Indian Ocean (7.2°N and 77.9°E) at 1,540 m water depth with a depositional history of ∼38 ka to determine source variabilities and their controlling factors. 87Sr/86Sr (0.71978–0.72491), ƐNd (−14.8 to −21.9), and a couple of source diagnostic elements display profound variability over the depositional time scale and point toward major sediment contribution from the Peninsular Gneissic Complex (PGC) and the Deccan Basalts along with aeolian dust flux, their relative proportions being determined by climate variability. The cold/arid periods are characterized by an enhanced proportion of aeolian dust and the Deccan Basalts, whereas the sediment contribution from the PGC is augmented during the warm/humid periods. The sediment provenance variations at the Equatorial Indian Ocean coincide very well with known cold/arid (Heinrich Stadial events: HS 1–4, LGM, Younger Dryas, 8.2 ka, 5.2 ka, and 1.1 ka) and warm/humid (Early Deglacial, Holocene Intensified Monsoon) climatic events reported in the tropical region and sea‐level change which are strongly captured by the Sr‐Nd isotope and elemental composition of sediments. The present investigation underscores the significant role of climate, mainly the aridity, in modulating the dust fluxes and erosion intensity and the strong coupling between Indian monsoon and North Atlantic climatic oscillations and further demonstrates minimal time delay between the production and transport of sediment from source to sink.

Responses of small mountain glaciers in the Maritime Alps (south-western European Alps) to climatic changes during the Last Glacial Maximum

We present new chronological data and Equilibrium Line Altitude (ELA) information for palaeoglaciers in the Maritime Alps during the Last Glacial Maximum (LGM) and the early deglaciation. Three relatively small catchments were investigated to test if the response of small (1–10 km2) glaciers to LGM climatic forcing was distinguishable from that of larger glacial systems. Palaeoglacier geometries and ELAs were reconstructed using geomorphological evidence and toolboxes in a geographic information system. Chronological control was provided through 10Be surface exposure dating of erratic boulders located on frontal and lateral moraine ridges. Our data indicate two phases of glacier advance or stabilisation in the Maritime Alps, the first occurring at ca. 25 to 24 ka and the second at ca. 20.5 to 19 ka. This is consistent with ages that have been reported from larger outlet lobes along the south-western Alpine fringe, where the second LGM advance is usually preserved in the form of a recessional moraine. Within the uncertainty of the dating technique, there are no distinguishable differences between the responses of small and large Alpine glaciers to the LGM climate. The reconstructed ELAs for palaeoglaciers in the Maritime Alps indicate a considerable variability, which appears to be linked to differences in received solar radiation, from ca. 1600 m a.s.l. for north-facing cirque glaciers to almost 2000 m a.s.l. for glaciers with a southerly aspect. We argue that such influence of solar radiation needs to be considered when using ELAs for palaeoclimatic inferences at regional scales. The calculated ELAs are up to 450 m higher than those in the northern Apennines or the Corsican mountains, indicating a relatively dry LGM climate on the Italian side of the Maritime Alps.

Large shift in the upper water column characteristics of the west-central Bay of Bengal during the last glacial-interglacial transition

The upper water column structure in the Bay of Bengal is strongly influenced by the monsoon and associated processes. However, the records of variation in water column stratification during different monsoon phases, are scarce from this region. Here, we have reconstructed the variation in monsoon induced upwelling, mixed layer and thermocline during the last glacial-interglacial transition, by using the relative abundance of mixed layer and thermocline dwelling planktic foraminifera, relative abundance of Globigerina bulloides and the stable oxygen isotopic ratio (δ18O) of surface dwelling Globigerinoides ruber. We report a weaker upwelling during the last glacial interval and late Greenlandian-Northgrippian Age, and strong upwelling during the deglaciation and Meghalayan Age. The intense upwelling shoaled the thermocline during the early glacial interval, last glacial maximum, late deglaciation and the Meghalayan Age. The mixed layer depth increased during the early deglaciation and Late Greenlandian-Northgrippian due to the reduced upwelling and stronger winter monsoon. The shift in the position of the core site with respect to the low salinity coastal currents, due to the rising sea level, also influenced the upper water column structure in the west-central Bay of Bengal. Therefore, the reorganization of the upper water column structure in the west-central Bay of Bengal, on the glacial-interglacial time scale, was mainly influenced by a combination of the upwelling, fresh water influx, sea level and winter monsoon.

The post-fire shift of temperate white pine-birch forest to boreal balsam fir forest in eastern Canada: climate-fire implications

Extensive 14C dating and botanical identification of charcoal fragments located in the organic surface soil layer and buried in the mineral podzolic solum were used to reconstruct the successional pathways of a balsam fir forest site. The studied forest site developed in a context of continuous fire disturbance over the last 9000 years with at least 26 fires occurring at a mean interval of 330 years. Tree vegetation of the site followed a four-step trajectory consisting of an early-Holocene spruce forest and a late-Holocene mixedwood balsam fir forest. Boreal-like spruce-birch and temperate-like white pine-birch forests dominated the site between 7900 and 5900 cal. B.P. and 5600 and 1275 cal. B.P., respectively. Because all forest types developed repeatedly after fire since early deglaciation, changes in forest composition, in particular the shift of white pine forest to balsam fir forest, and concurrent decline of birch (yellow birch and/or paper birch) and pine populations were most likely related to progressive cooler and wetter conditions from mid- to late Holocene. Fire disturbance on this part of the southern boreal biome has been a continuous, positive regenerative process over the Holocene, allowing the successional turnover of boreal and temperate trees under the influence of climatic change.

A global synthesis of high-resolution stable isotope data from benthic foraminifera of the last deglaciation

We present the first version of the Ocean Circulation and Carbon Cycling (OC3) working group database, of oxygen and carbon stable isotope ratios from benthic foraminifera in deep ocean sediment cores from the Last Glacial Maximum (LGM, 23-19 ky) to the Holocene (<10 ky) with a particular focus on the early last deglaciation (19-15 ky BP). It includes 287 globally distributed coring sites, with metadata, isotopic and chronostratigraphic information, and age models. A quality check was performed for all data and age models, and sites with at least millennial resolution were preferred. Deep water mass structure as well as differences between the early deglaciation and LGM are captured by the data, even though its coverage is still sparse in many regions. We find high correlations among time series calculated with different age models at sites that allow such analysis. The database provides a useful dynamical approach to map physical and biogeochemical changes of the ocean throughout the last deglaciation.

Radiocarbon evidence for reduced deep water ventilation of the northern Indian Ocean during the last glacial maxima and early deglaciation

Enhanced outgassing of sequestered carbon to the atmosphere by upwelling of deep water in the Southern Ocean during the last deglaciation is assumed to be the main factor that modulated changes in atmospheric CO2 and atmospheric radiocarbon activity; however, the exact mechanism behind this deglacial CO2 release still remains unclear. In this study, we have investigated ventilation changes in the deep waters of the northern Indian Ocean over the past 25 kyr BP using paired benthic and planktic foraminiferal radiocarbon, along with stable carbon and oxygen isotopes of benthic foraminifera, sedimentary CaCO3 content, and coarse fraction from the gravity core SSD-044/GC-01 (3160 m water depth) collected from Central Equatorial Indian Ocean. The results indicate a greater proportion of southern sourced CO2-rich, poorly-ventilated Antarctic Bottom Water (AABW) during the Last Glacial Maximum (LGM) and Heinrich Stadial (HS1), in the northern Indian Ocean. The observed poor ventilation during the last glaciation shows that the northern Indian Ocean was a part of the glacial ocean aged carbon pool. Improved ventilation during the Bølling-Allerød (B-A) is consistent with the rapid resumption of Atlantic overturning circulation and inflow of North Atlantic Deep Water (NADW) into the northern Indian Ocean at the onset of the B-A, which caused flushing of the deep-carbon pool with evidence of degassing from the northern Indian Ocean. This study thus supports the role of Southern Ocean in modulating atmospheric CO2 variations during the last deglaciation.

Deglacial records of terrigenous organic matter accumulation off the Yukon and Amur rivers based on lignin phenols and long-chain n-alkanes

Abstract. Arctic warming and sea level change will lead to widespread permafrost thaw and subsequent mobilization. Sedimentary records of past warming events during the Last Glacial–interglacial transition can be used to study the conditions under which permafrost mobilization occurs and which changes in vegetation on land are associated with such warming. The Amur and Yukon rivers discharging into the Okhotsk and Bering seas, respectively, drain catchments that have been, or remain until today, covered by permafrost. Here we study two marine sediment cores recovered off the mouths of these rivers. We use lignin phenols as biomarkers, which are excellently suited for the reconstruction of terrestrial higher plant vegetation, and compare them with previously published lipid biomarker data. We find that in the Yukon basin, vegetation change and wetland expansion began already in the early deglaciation (ED; 14.6–19 ka). This timing is different from observed changes in the Okhotsk Sea reflecting input from the Amur basin, where wetland expansion and vegetation change occurred later in the Pre-Boreal (PB). In the two basins, angiosperm contribution and wetland extent all reached maxima during the PB, both decreasing and stabilizing after the PB. The permafrost of the Amur basin began to become remobilized in the PB. Retreat of sea ice coupled with increased sea surface temperatures in the Bering Sea during the ED might have promoted early permafrost mobilization. In modern Arctic river systems, lignin and n-alkanes are transported from land to the ocean via different pathways, i.e., surface runoff vs. erosion of deeper deposits, respectively. However, accumulation rates of lignin phenols and lipids are similar in our records, suggesting that under conditions of rapid sea level rise and shelf flooding, both types of terrestrial biomarkers are delivered by the same transport pathway. This finding suggests that the fate of terrigenous organic matter in the Arctic differs on both temporal and spatial scales.

Retreat and stabilization of a marine-based ice margin along a high arctic fjord-cross-shelf trough system

Multibeam bathymetric and seismostratigraphic data collected in the Clyde fjord-cross-shelf trough system (eastern Baffin Island, Canadian Arctic Archipelago) display glacial landforms and depositional assemblages that enable the identification of the maximal extent of the Laurentide Ice Sheet (LIS) margin and delineating the patterns and controls on its subsequent retreat. Additionally, 10 new sediment cores – from which seven radiocarbon ages were acquired – allow the recognition of depositional processes. Results show that, during the Last Glacial Maximum, the LIS margin extended almost to the edge of the continental shelf. Early deglaciation of the trough was marked by an initial ice-shelf collapse and rapid retreat of the ice stream, as evidenced by the absence of ice marginal landforms and the presence of extensive iceberg ploughmarks across a large portion of the outer trough. It was followed by a slow retreat and successive stabilizations of the ice margin that led to the deposition of recessional moraines and grounding-zone wedges (GZWs). Deglaciation of the fjord in the early Holocene occurred in an episodic style, whereby rapid retreat was punctuated by relatively long standstills that enabled major moraine formation. Long-term stabilizations of the ice margin in the Clyde fjord-cross-shelf trough system are interpreted to coincide with major climatic cooling events, such as the Younger Dryas and early Holocene cold reversals. Ages derived from sediment cores and previous work suggest that higher retreat rates correspond with periods of significant global sea level rise, suggesting that oceanic forcing exerted a minor control on the deglaciation. GZWs and large moraine ridges are observed at pinning points in the trough and fjord, indicating that the location of ice margin stabilizations was influenced by topography. The reconstruction of the deglaciation of the Clyde fjord-cross-shelf trough system allows us to refine deglacial models for similar systems of northeastern Baffin Island, in particular beyond the coast and along the steeper section of the fjord where chronological gaps remained.

A paleo-perspective on West Antarctic Ice Sheet retreat

Geological records of ice sheet collapse can provide perspective on the ongoing retreat of grounded and floating ice. An abrupt retreat of the West Antarctic Ice Sheet (WAIS) that occurred during the early deglaciation is well recorded on the eastern Ross Sea continental shelf. There, an ice shelf breakup at 12.3 ± 0.6 cal. (calibrated) kyr BP caused accelerated ice-mass loss from the Bindschadler Ice Stream (BIS). The accelerated mass loss led to a significant negative mass balance that re-organized WAIS flow across the central and eastern Ross Sea. By ~ 11.5 ± 0.3 cal kyr BP, dynamic thinning of grounded ice triggered a retreat that opened a ~ 200-km grounding-line embayment on the Whales Deep Basin (WDB) middle continental shelf. Here, we reconstruct the pattern, duration and rate of retreat from a backstepping succession of small-scale grounding-zone ridges that formed on the embayment’s eastern flank. We used two end-member paleo-sediment fluxes, i.e., accumulation rates, to convert the cumulative sediment volumes of the ridge field to elapsed time for measured distances of grounding-line retreat. The end-members fluxes correspond to deposition rates for buttressed and unbuttressed ice stream flow. Both scenarios require sustained rapid retreat that exceeded several centuries. Grounding-line retreat is estimated to have averaged between ~ 100 ± 32 and ~ 700 ± 79 ma−1. The evidence favors the latter scenario because iceberg furrows that cross cut the ridges in deep water require weakly buttressed flow as the embayment opened. In comparison with the modern grounding-zone dynamics, this paleo-perspective provides confidence in model projections that a large-scale sustained contraction of grounded ice is underway in several Pacific-Ocean sectors of the WAIS.

Younger Dryas and early Holocene climate in south Greenland inferred from oxygen isotopes of chironomids, aquatic Moss, and Moss cellulose

Ice core records have long indicated that the Younger Dryas began and ended with large, abrupt climate shifts over Greenland. Key climatic features remain unknown, including the magnitude of warming during the Younger Dryas-Holocene transition along with the seasonality and spatial variability of Younger Dryas climate changes across Greenland. Here, we use geochemical and paleoecological proxies from lake sediments at Lake N14 in south Greenland to address these outstanding questions. Radiocarbon dating and diatom assemblages confirm early deglaciation and isolation of Lake N14 before ∼13,600 cal yr BP, consistent with previous work. Oxygen isotope ratios (δ18O) of chironomid head capsules, bulk aquatic moss, and aquatic moss-derived cellulose are used to reconstruct oxygen isotopes of past lake water and annual precipitation. Oxygen isotope proxies indicate annual precipitation δ18O values increased by 5.9–7.7‰ at the end of the Younger Dryas. Following the Younger Dryas, moss and cellulose δ18O values show a clear decline in precipitation δ18O values of 2–3‰ from ∼11,540–11,340 cal yr BP that may correspond with the Preboreal Oscillation. Reconstructed precipitation δ18O values then gradually increased from 11,300–10,100 cal yr BP. All three aquatic organic materials register similar shifts in precipitation δ18O values over time, and they closely parallel the δ18O shifts observed in ice cores. This evidence strongly supports the utility of these methods for reconstructing lake water δ18O, and furthermore precipitation δ18O values where lake water reflects precipitation. The relatively large shift in isotopic composition of precipitation at Lake N14 suggests that shifts in temperature, precipitation seasonality, and/or moisture sources at the end of the Younger Dryas were even larger in south Greenland than they were in central Greenland, most likely because of the proximity to major changes in North Atlantic Ocean circulation. The annual air temperature change estimated at Lake N14 at the end of the Younger Dryas is also very large (∼18 ± 7 °C) compared to the summer warming previously inferred from chironomid species assemblages there (∼6 °C). This indicates that the strongest warming at the end of the Younger Dryas occurred in the winter season, consistent with past observations of intensified Younger Dryas seasonality at Lake N14 and elsewhere in Greenland.

Last Glacial Maximum and early deglaciation in the Stura Valley, southwestern European Alps

We combined data from geomorphologic surveys, glacial modelling, and 10Be exposure ages of boulders on moraines, to investigate the Last Glacial Maximum (LGM) and the early retreat glacial phases in the Stura Valley of the Maritime Alps. We used the exposure ages to reconstruct the timing of standstills or readvances which interrupted the post-LGM withdrawal, initiated ∼24 ka. We mapped and dated the frontal moraines of a first glacial standstill/readvance at a short distance (∼7 km) from the maximum external limit of the LGM, which occurred at ∼22 ka, and a second one at ∼19 ka (Bühl stadial). This morpho-chronologic succession is congruent with that obtained in the adjacent Gesso Valley and, combined with the similarity of Equilibrium Line Altitude values, demonstrates a consistent glacial response in the Maritime Alps to climatic forcing.Our data are chronologically consistent with those of the southern flank of the European Alps, stressing not only a general synchroneity of the LGM across the various sectors, but also that of a LGM recessional standstill or readvance at ∼22 ka. The short distance between the LGM moraines and the recessionary phase moraines, and minimal difference in ELA indicate a modest variation in the mass balance of the Maritime Alps glaciers during this time interval. A similar modest variation between LGM and the first recessional phase glacier mass balance is also found throughout the western sector of the Southern Alps but is considerably more pronounced for the glaciers of the central-eastern sectors. This behaviour can be explained by the interplay between the moisture supplied by southern currents sourced in the Western Mediterranean and that advected by the westerlies sourced in the North Atlantic, which affected the various sectors of the Southern Alps differently.

Northeast Greenland: ice‐free shelf edge at 79.4°N around the Last Glacial Maximum 25.5–17.5 ka

The size of the last glacial ice sheet on the Northeast (NE) Greenland shelf and its interaction with ocean circulation have been the subject of debate. Here we provide insights into the extent of the ice sheet around the Last Glacial Maximum (LGM) and investigate timing and strength of changes in the flow of Atlantic Water. The study is based on a multiproxy investigation of a marine sediment core, gravity core DA17‐NG‐ST01‐019G, from 323‐m water depth at the NE Greenland shelf edge at 79.4°N. We present benthic and planktic foraminiferal distribution data, AMS‐14C dates, sedimentological (ice‐rafted debris (IRD) and grain sizes), and geochemical (XRF) data in combination with geophysical (sub‐bottom profiler) data. The oldest sediments at the study site are dated to 25.5–17.5 ka, encompassing the time frame from the beginning of the LGM to the early deglaciation. This part is overlain by sediments from the late deglaciation and Holocene. The deposits dating from the LGM are very rich in both planktic and benthic foraminifera and macrofossils of excellent preservation. The faunas show that the site generally was affected by a strong flow of relatively warm subsurface Atlantic Water during the LGM and Early Holocene. Conditions turned more polar with cold bottom water flow in the Middle–Late Holocene (c. 7.5 ka to Recent) with presence of mainly agglutinated benthic foraminiferal species. Our data from the LGM also indicate that the deposits were mixed by iceberg scouring, confirmed by the geophysical data showing extensive ploughing of the sediments on the outer shelf area. The results further indicate that the Greenland Ice Sheet did not reach to the edge of the NE Greenland shelf at 79.4°N during the LGM 24–18 ka.

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.

A spatial model of Marion Island's palaeo-ice extent

AbstractSub-Antarctic Marion Island's glacial history has acted as a control on abiotic terrestrial processes and the colonization and distribution of biotic species found on the island today. Recent chronological studies have shown an early deglaciation of the island and identified new geomorphological features associated with past ice dynamics. These permit a reassessment of ice extent during and after the island's last local glacial maximum. In this paper, we provide a revised reconstruction of the island's palaeo-ice extent by using a geomorphology-based approach to delineate palaeo-ice margins and demarcate possible glacial basins. The model presented here provides the needed spatial context for future studies on the variations in the distribution of species (e.g. microorganisms and plant species) and abiotic processes and forms (e.g. soil development and periglacial landforms). In addition, it highlights areas that require improved geophysical assessment in order to produce a more complete island-scale reconstruction of former ice extents (e.g. the west coast).

Palaeoclimate dynamics within the Summer Rainfall Zone of South Africa

South Africa has a particularly spatially heterogeneous climate driven by a complex set of ocean-atmospheric factors. This results in three distinct regions of rainfall seasonality: the summer- (SRZ), winter- (WRZ) and year-round (YRZ) rainfall zones. Even within these zones there is considerable spatio-temporal variability in precipitation patterns, especially in the SRZ which covers the largest terrestrial area. Over the past several thousand years the position of the boundaries of the southern extent of the SRZ have shifted due to the influences of major climate systems, as well as changes in the characteristics of the adjacent oceanic currents. Here we use the Modern Analogue Technique on a new synthesis of pollen records from across the SRZ to quantitatively reconstruct four climate variables (Mean Temperature of the Coldest month, Mean Temperature of the Warmest Month, Mean Summer Precipitation and Winter Climatic Moisture Index), over the past 36,000 years. The climate variables and number of analogues used were selected after thorough examination of the modern dataset. The results provide evidence for three key features: first, an early deglaciation following a cold, dry and prolonged Last Glacial Maximum. Second, a clear manifestation of the African Humid Period in the north-eastern part of the zone, supported by reconstructed sea surface temperatures. And third, a short-lived cold period at around 10 cal kyr BP, which may have been obscured by larger statistical errors in past studies. As it is still mostly within the margin of error for our study, this period requires further study. Our results show that the technique can be used successfully on large and diverse compilations of South African records, improving comparability with other regions.

Glacial landforms reveal dynamic ice-sheet behaviour along the mid-Norwegian margin during the last glacial-deglacial cycle

The analysis of glacial landforms preserved on mid- and high-latitude continental margins provides insights into the patterns and processes of sedimentation beneath contemporary ice sheets and aids predictions of the future resilience of ice sheets to ocean and atmospheric drivers of change. However, most previous high-resolution investigations of submarine glacial landforms have utilised data that are focused only on relatively small areas of the seafloor. Here we use an extensive database of high-resolution marine-geophysical data to map and interpret the distribution of glacial landforms over an 80,000 km2 area along a ∼600 km-long section of the mid-Norwegian margin. Our glacial-geomorphological mapping shows that the Scandinavian Ice Sheet displayed highly dynamic behaviour, including readvances and changes in ice-flow direction, during the last glacial-interglacial cycle. The shallow banks briefly became dynamic centres of ice flow during deglaciation, with ice readvances from these banks linked to the loss of ice-sheet buttressing through the early deglaciation of grounded ice in the deeper troughs. The geometry of the continental shelf, especially its troughs and banks, exerted an important control on the pattern of ice-sheet retreat. The distribution of small grounding-zone wedges shows that frequent, small-magnitude still-stands or readvances within overall ice-stream retreat were prevalent on prograding slopes that limited the flux of ice across the grounding zone. Although the pattern of ice-sheet retreat along the mid-Norwegian margin is now relatively well-understood, future marine sediment coring efforts are needed to better constrain the timing of these deglacial events.

Linkages between ocean circulation and the Northeast Greenland Ice Stream in the Early Holocene

The melting of marine terminating glaciers in Northeast Greenland is a visible sign that our climate is changing. This melt has been partly attributed to changes in oceanic heat fluxes, particularly warming of Atlantic Water (AW). Yet our understanding of the interaction between glaciers and the ocean is limited by the length of instrumental records. Here, we present a multi-proxy study (benthic foraminifera assemblages, CT scans, grain size, XRF, and stable isotope data) on core DA17-NG-ST08-092G, located 90 km east of the Northeast Greenland Ice Stream (NEGIS). Whilst the exact timing of deglaciation is uncertain, it is certain to have occurred at least as early as 12.5 ka cal BP, and likely before 13.4 ka cal BP. The inflow of AW may have played a role in the seemingly early deglaciation on the Northeast Greenland continental shelf. Following deglaciation, the site was overlain by an ice shelf, with AW and Polar Water (PW) flowing beneath until 11.2 ka cal BP. The NEGIS briefly retreated westwards between 11.2 and 10.8 ka cal BP before our site returned to glacier-proximal conditions dominated by colder subsurface water and persistent AW flowing beneath (10.8–9.6 ka cal BP). Between 9.6 and 7.9 ka cal BP the NEGIS retreated westwards; there was a continued presence of AW and PW at the site. A drastic shift in ocean circulation occurred at 7.9 ka cal BP, with a decline in AW flow and dominance of PW flowing beneath perennial sea ice. During the Late Holocene, there was return of AW and likely breakup of perennial sea ice.

Multi-proxy reconstructions of productivity on the continental slope off the Mekong River in the southern South China Sea over the past 30,000 years

Marine productivity is a key process in controlling the global carbon cycle. To understand the paleo-productivity behavior in the southern South China Sea, multiple geochemical analyses on Core SO18383–3 recovered from the continental slope off the Mekong River were integrated to reveal the response of paleo-productivity to environmental change. The elemental ratios of Bio/Al, bBa/Al, Zn/Al, and Sr/Al allowed the reconstruction of paleo-productivity evolution. The results show that the paleo-productivity was low around the Last Glacial Maximum (LGM) and early deglaciation (29.9–14.8 cal ka BP), then increased during the main deglaciation period (14.8–8.0 cal ka BP), before gradually declining during the Holocene (8.0–1.1 cal ka BP). This variation in paleo-productivity intensity was significantly controlled by the change in phosphorus input driven by the East Asian summer monsoon. Moreover, the opal/CaCO3 proxy, which reflects the relative abundance of siliceous versus calcareous phytoplankton, showed a higher siliceous phytoplankton dominance in the latest Pleistocene (29.9–10.8 cal ka BP) than in the Holocene. This was possibly governed by the upwelling process with cold waters forced by a cold-core eddy during the strengthening of the East Asian winter monsoon. This study greatly increased our understanding of the principal forcing factors on paleo-productivity in the southern South China Sea over the last 30 ka.

Deep-water coral records of glacial and recent ocean-atmosphere dynamics from the Perth Canyon in the southeast Indian Ocean

We present radiocarbon and neodymium isotope records from accurately dated (U-series) live and fossil (<2 ka, 17.5 to 29.6 ka BP) deep-water corals, collected between 675 and 1788 m from the Perth Canyon, offshore southwest Australia. These records provide the first insights into recent and last glacial intermediate-deep water circulation and ventilation histories from the poorly represented southeast Indian Ocean. Ocean ventilation fluctuated significantly during the last glacial period, being highly ventilated during early Marine Isotope Stage 2 and the early deglaciation, yet less ventilated and more stratified during the Last Glacial Maximum. Furthermore, the Perth Canyon corals recorded a major and rapid (∼300 years) perturbation at 25 ka, indicating that mid-depth waters were exposed to poorly ventilated, unradiogenic Nd deeper waters. We link this to similar, age equivalent changes at geographically disparate sites from the southeast Atlantic to the Southern Ocean south of Tasmania, together with fluctuations in Antarctic sea-ice, oceanic fronts, and Atlantic Meridional Overturning Circulation (AMOC). This widespread Southern Ocean event indicates a transient breach in upper and lower ocean cell boundaries of the MOC, providing further evidence for the non-steady state, zonal dynamics of the glacial Southern Ocean.