Younger Dryas Readvance Research Articles

Configuration of the Scandinavian Ice Sheet in southwestern Norway during the Younger Dryas

The extent of the Scandinavian Ice Sheet in southwestern Norway is precisely located during the well-characterized Younger Dryas re-advance. However, the thickness of the ice sheet is less well constrained inland from the terminal position. Some exceptions include lateral moraines traced inland and up to 1000 m a.s.l. along Hardangerfjorden. Here, we apply 10Be dating in two areas: (1) bedrock and boulders in the high-relief landscapes near the Younger Dryas margin around the Bergen urban area, and (2) boulders from an upland 1600 m a.s.l. much farther (120 km) inland. We find that coastal summits ranging from ~400 to ~680 m a.s.l. and located only ~10–15 km up-flow from the ice margin, were covered by the Scandinavian Ice Sheet during the Younger Dryas. The scatter in the 10Be age population of 22 boulder samples is best explained by isotopic inheritance owing to inefficient subglacial erosion during the foregoing glaciation. Most of the 11 bedrock samples also exhibit inheritance, pointing to the source of inheritance in boulders and implying inefficient subglacial erosion during the last glaciation even in valley-bottoms near Bergen. Regional glacial striae compilations suggest that ice flow during maximum Younger Dryas ice-sheet configurations was for the most part cross-valley, with potentially low basal slip rates. Five new 10Be ages from the inland site help to constrain ice height far inland. We combine these new results with prior information to generate a cross profile of the Younger Dryas ice sheet in southern Norway.

Glacial stages in the Peña Negra valley, Iberian Range, northern Iberian Peninsula: Assessing the importance of the glacial record in small cirques in a marginal mountain area

The objective of this study was to assess whether it is possible to obtain a comprehensive record of the last deglaciation from the small Peña Negra glacial cirque in a marginal mountain range in the Iberian Range (northern Iberian Peninsula). The cirque is located in Sierra Cebollera, at an elevation slightly over 2000 m a.s.l. in the divide, and has relatively low elevational difference between its uppermost and lowest parts. The Peña Negra glacial cirque was selected because of the presence of several lateral moraines indicating the occurrence of a glacial tongue of 1.8 km length and a small rock glacier, enabling identification of the main glacial stages that affected the paleo-evolution of the glacier. Using cosmogenic exposure dating techniques and geomorphological mapping it was possible to reconstruct the historical Peña Negra glacier and to distinguish: (i) the maximum ice extent (MIE) at approximately 18–20 ka, coinciding with the Global Last Glacial Maximum (GLGM), although the occurrence of a previous stage was not ruled out; (ii) a stage of glacial re-advance at 16–17 ka, coinciding with the Oldest Dryas, followed by rapid ice melting; (iii) the development of a rock glacier in a minor structural cirque near the headwall at 14–15 ka, when paraglacial processes were probably dominant; and (iv) evidence of ice melting in the uppermost cirque at 13.5 ka. Study of the Peña Negra glacier revealed the history of a minor valley glacier affected by distinct stages, including the formation of a small rock glacier in the final phases of deglaciation. No evidence of a Younger Dryas re-advance was found.

A Younger Dryas re-advance of local glaciers in north Greenland

Abstract The Younger Dryas (YD) is a well-constrained cold event from 12,900 to 11,700 years ago but it remains unclear how the cooling and subsequent abrupt warming recorded in ice cores was translated into ice margin fluctuations in Greenland. Here we present 10Be surface exposure ages from three moraines in front of local glaciers on a 50 km stretch along the north coast of Greenland, facing the Arctic Ocean. Ten ages range from 11.6 ± 0.5 to 27.2 ± 0.9 ka with a mean age of 12.5 ± 0.7 ka after exclusion of two outliers. We consider this to be a minimum age for the abandonment of the moraines. The ages of the moraines are furthermore constrained using Optically Stimulated Luminescence (OSL) dating of epishelf sediments, which were deposited prior to the ice advance that formed the moraines, yielding a maximum age of 12.4 ± 0.6 ka, and bracketing the formation and subsequent abandonment of the moraines to within the interval 11.8–13.0 ka ago. This is the first time a synchronous YD glacier advance and subsequent retreat has been recorded for several independent glaciers in Greenland. In most other areas, there is no evidence for re-advance and glaciers were retreating during YD. We explain the different behaviour of the glaciers in northernmost Greenland as a function of their remoteness from the Atlantic Meridional Overturning Circulation (AMOC), which in other areas has been held responsible for modifying the YD drop in temperatures.

Lake Store Finnsjøen – a key for understanding Lateglacial/early Holocene vegetation and ice sheet dynamics in the central Scandes Mountains

The Lateglacial (LG) deglaciation and vegetation development in the Scandes Mountains has been debated for a century. Here we present new evidence from microfossils, radiocarbon dated plant macrofossils and sedimentary ancient DNA from laminated sediments in Lake Store Finnsjøen (1260 m a.s.l.) at Dovre, Central Norway. Combined with previous results from three other Dovre lakes, this allows for new interpretations of events during and immediately after the LG deglaciation. The Finnsjøen sediments present the first uninterrupted record of local vegetation development in the Scandes Mountains from the late Younger Dryas (YD), ca 12,000 cal years BP, to the early Holocene around 9700 cal years BP. The local vegetation in late YD/early Holocene was extremely sparse with pioneer herbs (e.g. Artemisia norvegica, Beckwithia, Campanula cf. uniflora, Koenigia, Oxyria, Papaver, Saxifraga spp.) and dwarf-shrubs (Betula nana, Salix including Salix polaris). From 11,300 cal years BP, local vegetation rapidly closed with dominant Dryas, Saxifraga spp., and Silene acaulis. From ca 10,700 cal years BP, open birch-forests with juniper, Empetrum nigrum and other dwarf-shrubs developed. Pine forests established within the area from 10,300 cal years BP. We identified the cold Preboreal Oscillation (PBO), not earlier described from pollen data in South Norway, around 11,400 cal years BP by a regional pollen signal. Distinct local vegetation changes were not detected until the post-PBO warming around 11,300 cal years BP. Apparently, the earlier warming at the YD/Holocene transition at 11,650 cal years BP was too weak and short-lived for vegetation closure at high altitudes at Dovre. For the first time, we demonstrate a regional glacier readvance and local ice cap formations during the YD in the Scandes Mountains. In two of the deep lakes with small catchments, YD glaciation blocked sedimentation without removing old sediments and caused a hiatus separating sediments of the ice-free LG interstadial (LGI) from those of the ice-free Holocene period. Both regional glaciers and local ice caps caused hiati. Ice-free pre-YD conditions at Dovre followed by a YD readvance point to a scenario that is intermediate between the maximum ice model postulating a thick glacier during the entire LG, and the minimum ice model postulating thin and multi-domed early LG ice.

Weathering rinds as mirror images of palaeosols: examples from the Western Alps with correlation to Antarctica and Mars

Weathering rinds have been used for decades as relative age indicators to differentiate glacial deposits in long Quaternary sequences, but only recently has it been shown that rinds contain long and extensive palaeoenvironmental records that often extend far beyond mere repositories of chemical weathering on both Earth and Mars. When compared with associated palaeosols in deposits of the same age, rinds often carry a zonal weathering record that can be correlated with palaeosol horizon characteristics, with respect to both abiotic and biotic parameters. As demonstrated with examples from the French and Italian Alps, rinds in coarse clastic sediment contain weathering zones that correlate closely with horizon development in associated palaeosols of presumed Late Glacial age. In addition to weathering histories in both rinds and palaeosols, considerable evidence exists to indicate that the black mat impact (12.8 ka) reached the European Alps, a connection with the Younger Dryas readvance supported by both mineral and chemical composition. Preliminary metagenomic microbial analysis using density gradient gel electrophoresis suggests that the eubacterial microbial population found in at least one Ah palaeosol horizon associated with a rind impact site is different from that in other Late Glacial and Younger Dryas surface palaeosol horizons.

New Evidence from a Black Mat Site in the Northern Andes Supporting a Cosmic Impact 12,800 Years Ago

AbstractPrevious work has ascribed a cosmic impact origin to black, high-temperature, carbon-encrusted beds (2–3 cm thick), associated with the Younger Dryas readvance of ice at 12.8 ka during the Late Glacial in the northern Andes of Venezuela. The evidence for this includes carbon spherules, aluminosilicate melt rocks, melted coatings of glass-like amorphous carbon, and Fe-Mn on sands and clasts derived from local felsic gneiss and granite. These sediments have been subjected to renewed investigation using high-resolution scanning electron microscopy and energy-dispersive spectrometry, and new data show that spherules at site MUM7B exhibit unique morphologies and compositions. Molar oxide weight percentages prove the spherules are not volcanic and show little overlap with cosmic materials. Spherule microstructures display quench melting and, thus, could not have formed from slow geological authigenic, diagenetic, or metamorphic processes. Instead, geochemical values for the Venezuelan samples plot withi...

Weathering rinds: Unlikely host clasts for evidence of an impact-induced event

Interhemispheric evidence of a cosmic impact 12.9ka is known now from North to South America, Europe and Eurasia, all data supporting a cosmic event derived from cores and from geological sections. Most databases supporting the impact hypothesis at the Younger Dryas Boundary (YDB) rely on high-temperature microspherules, melted minerals, cracked clasts, presence of nanodiamond, high-temperature scoria, high Fe/Ni ratios, pdf's, shock melted quartz, high 10Be/9Be ratios and occasional presence of platinum metals. Controversy over the impact, the so-called Black Mat enigma, and its relation to the Younger Dryas readvance at the end of the last ice age, is fueled by arguments over whether a single extraterrestrial impact might sustain a 1kyr-long downturn in insolation engendering a substantial increase in worldwide ice volume. New evidence in the form of impact microfeatures – extreme breccia, high crack propagation, thick carbon encrustations and partial to full shock-melted/contorted grains – in weathering rinds from the Western Alps, France, as documented here, adds to the growing body of evidence that the event was truly widespread, if not worldwide in effect. Whereas evidence of cosmic impacts may be erased by glacial and fluvial erosion in high alpine areas, such events as demonstrated herein are recorded as punctuated time-stratigraphic events in microcosm, preserved in weathered clasts.

Scottish west coast fjords since the last glaciation: a review

Abstract Evidence from seismic profiles, sidescan and multibeam sonar surveys and sediment cores reveal important information about the depositional history of Scotland's fjords during the last deglacial transition and the subsequent Holocene. Devensian glaciation removed pre-existing sediments both inside and outside the fjord basins, and deposited a diamict over much of the subsequent erosional surface. Glaciomarine sequences deposited during the initial retreat of this ice margin are mainly preserved in the sea area outside the fjords. Younger glaciomarine units present within the fjord basins are often attributed to the later Younger Dryas re-advance (12.8–11.5 ka BP). Younger Dryas ice reworked Devensian sediments, depositing terminal moraines at the mouths of a number of fjords and glaciomarine units within the basins during its retreat. Ice margins oscillated throughout deglaciation, although the latter stages of post-Younger Dryas retreat may have occurred rapidly with topographic pinning an important factor in determining the style of retreat. Holocene records from the fjords are limited, but tentatively support the onshore evidence for a tundra-style landscape in the earliest Holocene, followed by a warming of the climate and, latterly ( c . 2000 ka BP), increasing humidity. In the more recent past, human activities such as deforestation, fishing, aquaculture and industry have been recorded in the fjord sediments.

Calibrating a glaciological model of the Greenland ice sheet from the Last Glacial Maximum to present-day using field observations of relative sea level and ice extent

We constrain a three-dimensional thermomechanical model of Greenland ice sheet (GrIS) evolution from the Last Glacial Maximum (LGM, 21 ka BP) to the present-day using, primarily, observations of relative sea level (RSL) as well as field data on past ice extent. Our new model (Huy2) fits a majority of the observations and is characterised by a number of key features: (i) the ice sheet had an excess volume (relative to present) of 4.1 m ice-equivalent sea level at the LGM, which increased to reach a maximum value of 4.6 m at 16.5 ka BP; (ii) retreat from the continental shelf was not continuous around the entire margin, as there was a Younger Dryas readvance in some areas. The final episode of marine retreat was rapid and relatively late (c. 12 ka BP), leaving the ice sheet land based by 10 ka BP; (iii) in response to the Holocene Thermal Maximum (HTM) the ice margin retreated behind its present-day position by up to 80 km in the southwest, 20 km in the south and 80 km in a small area of the northeast. As a result of this retreat the modelled ice sheet reaches a minimum extent between 5 and 4 ka BP, which corresponds to a deficit volume (relative to present) of 0.17 m ice-equivalent sea level. Our results suggest that remaining discrepancies between the model and the observations are likely associated with non-Greenland ice load, differences between modelled and observed present-day ice elevation around the margin, lateral variations in Earth structure and/or the pattern of ice margin retreat.

Sedimentary processes and depositional environments in glaciated fjord systems — Evidence from Nordfjord, Norway

Fjords situated in past, or present, glacial regimes represent important repositories between land masses (source systems) and the open oceans (sink systems). Newly collected TOPAS high-resolution seismic profiles from the previously glaciated Nordfjord system, on the west coast of Norway, reveal sedimentary processes and depositional environments within fjords in more detail than before. The seismic data show that the fjord basins are characterised by a well-laminated lower unit that is overlain by acoustic transparent lensoidal bodies. We infer the lower unit, which is up to 350 m thick, to be composed of glacimarine sediments. The transparent bodies represent up to 0.25 km 3 large slide debrites that comprise < 5% of the total sediment volume in the fjord. The Nordfjord basin well-laminated sediments were deposited from the fjord mouth to the fjord head as the Fennoscandian Ice Sheet started to retreat from the outer coast at c. 12 700 14C yr BP (c. 15 000 cal. yr BP), during the last deglaciation. The entire Nordfjord system was ice free for a short time period before the ∼ 1000 year long Younger Dryas readvance halted the main Fennoscandian deglaciation and over-ran the eastern part of Nordfjord. In contrast to the Last Glacial Maximum, we suggest that the sediments in the ice-covered fjord basins were preserved during Younger Dryas. The rapid isostatic adjustments accompanying the withdrawal of the Younger Dryas ice sheet promoted conditions for failures. We relate the identified Nordfjord slide bodies to this paraglacial environment. This implies that the acoustic well-laminated glacimarine fjord unit has been deposited in about 2000 yr, and that sediment rates as high as 20 cm/yr have existed during deposition.

A Late Pleistocene-Holocene natural seismograph along the Boconó Fault (Mérida Andes, Venezuela): the moraine-dammed Los Zerpa paleo-lake

Abstract The Boconó Fault system is a major active tectonic feature accommodating an important part of the dextral relative motion between the Caribbean Plate and northern South-America. The main trace follows an axial valley running SW-NE within the Mérida Andes (northwestern Venezuela), and crosscuts a series of moraines related to late Pleistocene glaciers developments and retreats, at an altitude between 2600 and 5000 m. Several lakes were generated after the last retreat (between the Late Glacial Maximum –LGM– and the Younger Dryas re-advance), dammed by lateral and frontal moraines. Among them, the Los Zerpa moraine system yielded rich outcrops ranging from an upstream very coarse torrential to deltaic fill, to a downstream clayey-silty horizontal laminated lacustrine accumulation; a fore-set-type heterogeneous “prograding” body links the two sets. The whole system, as well as the surrounding moraines, underwent successive major earthquakes during the Late Glacial/lower Holocene period as evidenced by co-seismic scarps in the moraines, migrations of the outlet, and associated sagponds. Besides active faulting affecting both the moraines and the sedimentary fill, the latter –main purpose of our detail study– exhibits various evidence of strong disturbances which we relate to seismic shaking, such as: i) successive unconformities with co-seismic slips along fractures in the coarse proximal sediments; ii) successive dip changes, discontinuities, and slumps in the foreset-like set; iii) slumps with basal liquefaction, syn-sedimentary fractures, and instantaneous re-sedimentation in the fine-grained laminated accumulation. Lateral (temporal) correlations are established between the successive disturbances detected in the three situations; in turn, these sedimentary events are correlated with seismic activity of the Boconó Fault main trace. Thus, the whole paleo-lake may be considered as a natural seismograph which worked during several thousands years, after the end of the LGM and during early Holocene.

Deglaciation chronology and glaciomarine successions in the Malangen‐Målselv area, northern Norway

The deglaciation history of the Malangen‐Målselv fjord and valley area proximally to the Tromsø‐Lyngen (Younger Dryas) moraine at Bakkejord, Malangen, northern Norway, is reconstructed based on morphostratigraphic, lithostratigraphic and geophysical evidence, and 25 radiocarbon dates from marine shells and foraminifera. The results show that following the Skarpnes event c. 12 200 14Cyr BP, and prior to the Younger Dryas readvance, the area was deglaciated at least as far as Sandmo situated 22 km proximally to the Tromsø‐Lyngen moraine. Two moraine ridges crossing the fjord at Sandmo and buried beneath thick glaciomarine sediments are correlated with this period. The area was subsequently deglaciated between 10 300 and 9200 14Cyr BP, following the Tromsø‐Lyngen (Younger Dryas) readvance. Five ice‐front accumulations post‐dating the Tromsø‐Lyngen moraine and situated 19, 27, 42, 55 and 77 km behind it are identified and dated based on radiocarbon dates and correlation of marine limits: Målsnes (c. 10 050 14Cyr BP), Kjerresnes (c. 10 000 14Cyr BP), Solli (c. 9750 14Cyr BP), Bardufoss‐Brentmoen‐Storskogmoen (c. 9600–9700 14Cyr BP) and Alapmoen (c. 9200 Cyr BP). The largest of these, at Bardufoss‐Storskogmoen, possibly accumulated as a response to an ice advance. Fourteen dates of apparent late Allerød/Younger Dryas age (11 100–10 000 14Cyr BP), obtained from fossils in glaciomarine sediments in the Målselv valley up to 77 km proximally to the Tromsø‐Lyngen moraine, are interpreted as postdating rather than predating this moraine. Several of these are considered to be too old because of uncertain reservoir age, carbon‐dating plateaus and/or contamination. This highlights uncertainties associated with radiocarbon‐dating and the profound effect such uncertainties may have on interpreting geological events.

Younger Dryas readvance in Squamish river valley, southern Coast mountains, British Columbia

Stratigraphy and landforms in the Brohm Creek basin and lowermost Mamquam River valley provide evidence for a readvance of a valley glacier in the southern Coast Mountains of British Columbia during the Younger Dryas interval. At Brohm Creek, till containing wood 10,500 and 10,700 14C yr old (ca 12,500–12,900 cal yr old), overlies an ice-contact terrace that is correlative with similar terraces at the mouth of Mamquam River, 11 km to the south and 900 m lower in elevation. A line linking the two sites has a slope of 5°, which is similar to slopes of termini of present-day valley glaciers in the southern Coast Mountains. We used these observations to reconstruct the Younger Dryas glacier in Squamish valley at the head of Howe Sound. The glacier deflected landslide debris, derived from steep slopes at the head of Cheekye River basin on the west flank of Mt. Garibaldi, southward to an ice-marginal lake at the mouth of Mamquam River. Foreset-bedded sediments in the core of a large fan at the mouth of Cheekye River indicate the glacier terminus had retreated up Squamish valley beyond Cheekye and Mamquam rivers by 10,200 14C yr BP (11,900 cal yr old). Radiocarbon ages on the oldest postglacial sediments in a kettle on the landslide debris show that stagnant ice in the study area had melted before 10,000 14C yr BP (11,500 cal yr BP). The readvance of the Squamish valley glacier during the Younger Dryas Chronozone was probably driven by a regional climate reversal.

Sedimentary Architecture and Dynamic Stratigraphy of a Marine Ice-Contact System

ABSTRACT The Mona moraine in southern Norway is a sedimentary ridge, 3.5 km long, formed at the mouth of a shelf trough during a late Younger Dryas readvance of the Scandinavian ice sheet. The moraine architecture, sedimentation processes, and record of ice-front behavior have been reconstructed in a high-resolution allostratigraphic framework on the basis of ground-penetrating radar profiles and sedimentologic analysis of outcrop sections. The shelf trough was draining ice from adjacent platforms, which resulted in ice-front protrusion with a moraine offset relative to the coeval swarm of small moraines formed in the shallow-water zone. The grounding-line system at Mona commenced its development as an ice-contact submarine fan, while the glacier front was thinning and grinding to a halt on a bedrock threshold. The ice flux declined, but fluctuated, and the subglacial sediment supply by meltwater prevailed. The ice-front stillstand allowed the submarine fan to aggrade to the sea level and turn into a Gilbert-type delta with a short (160 m) distributary plain. The delta foreset, 50-60 m thick, consists of gravelly massflow deposits, and its sandy turbiditic toe has been extended on the threshold's steep lee slope to a distance of at least 2 km. The delta was abandoned by rapid calving of the ice front, and the ice-contact slope was covered with marine mud and resedimentation products. A radiocarbon date from the base of this thin cover indicates ice retreat at around 10,100 yr BP. The moraine was surrounded by the sea and subject to reworking by waves, tides, and longshore currents. A thick package of regressive foreshore deposits with downstepping wave-cut terraces on the moraine slopes reflects gradual emergence due to regional glacioisostatic uplift. The study demonstrates that the facies architecture of marine moraines bears an important record of glacier dynamics and ice-front behavior.

Glacioisostacy controls chemical and isotopic characteristics of tholeiites from the Reykjanes Peninsula, SW Iceland

We present chemical and isotopic data from Reykjanes Peninsula rift zone tholeiites which show dramatic changes during the past 20 kyr, effectively defining the 9–13 kyr BP period of fluctuations in glacioisostacy associated with the end of the Weichselian glaciation and the Younger Dryas readvance. During this period primitive lavas were erupted with `depleted' characteristics; low Nb/Zr, high 143Nd/ 144Nd, low 87Sr/ 86Sr, and low 206Pb/ 204Pb. We propose that the clustered eruption of these chemically and isotopically anomalous lavas is primarily a reflection of reduced crustal residence times for the parental magmas. These `depleted' magmas largely escaped crustal processes such as mixing, assimilation and fractional crystallisation which would overprint their distinctive chemical and isotopic characteristics. There is some evidence that the northern rift zone may also have been similarly affected during this period of rapid glacioisostatic readjustment. The parental melts to these depleted glacioisostatic lavas may only be produced during fluctuations in glaciation, or could be a normal component in all Iceland magmas.