A new middle Holocene varve diagram from the river Ångermanalven, northern Sweden: indications for a possible error in the Holocene varve chronology

A new varve diagram from the river Ångermanälven could be correlated to the postglacial varve chronology to between 4903 and 4415 varve years BP. An AMS ¹⁴C measurement on terrestrial macrofossils obtained between 4715 and 4706 varve years BP gave a calibrated age of between 5730 and 5040 calendar years BP. The discrepancy between varve and calender-year age indicates that an error or part of an error in the Swedish varve chronology may be situated between 2000 and 5000 varve years BP.     

The use of varved clay chronology for dating paleoseismic events: the Erstavik record in the Stockholm area, south Sweden

Geological structures suggest that the Fennoscandian Shield was subjected to a higher seismicity at the end of the last glaciation than today. This article demonstrates the use of varved clay chronology for dating paleoseismic events. It is argued that the deposited annually layered glacial varves were sensitive to past ground movements. In the Stockholm area, the Erstavik varved clay chronology suggests four paleoseismic events: a first (I) dating from varve year 10,473 to 10,468 BP; a second (II) 10,451 to 10,445 BP; a third (III) 10,429 to 10,425 BP; and a fourth (IV) 10,409 to 10,404 BP. In De Geer's ‘old' (1940) chronology the first (I) dating corresponds with −1117 to −1112, the second (II) with −1095 to −1089, the third (III) with −1073 to −1069, and the fourth (IV) with −1053 to −1048. The most pronounced event was the one at around varve year 10,429 BP (varve −1073 in De Geer's ‘old' chronology). The recurrence time of about 20 years suggests a totally different seismic regime at the time of deglaciation than what exists today. It coincided with the period of maximum isostatic uplift. The complexity of the varved clay response to seismic events is also discussed.     

AMS 14C measurements and macrofossil analyses of a varved sequence near Pudozh, eastern Karelia, NW Russia

The laminated sediments at Pudozh in eastern Karelia are generally assumed to have been deposited between 13 000 and 16 000 ¹⁴C yr BP and have been used to date the recession of the active ice margin. However, 17 AMS ¹⁴C measurements performed on terrestrial plant macrofossils contained in these sediments show that deposition began during the late Allerφd, when the ice margin had already receded to the northern part of Lake Onega. Based on an age model, we assume that the 1933-year-long varved sequence covers the time period between c. 12 900 and 11 000 calendar years BP. During this period, which comprises the later part of the Late Weichselian and the early Holocene, the local vegetation consisted of open, tree-less dwarf shrub heaths. Increased soil erosion may have occurred before 12 550 calendar years BP.     

Deglaciation and palaeoclimate of the Andfjord-Vågsfjord area, North Norway

This paper reviews the deglaciation history and palaeoclimate from 22 to 9.5 ¹⁴Cka BP in the Andfjord-Vagsfjord area. Eight main glacial events are recorded: The Egga-I (>22 ¹⁴Cka BP), the Bjerka, the Egga-II (>14.6 ¹⁴Cka BP), the Flesen (14.5 ¹⁴Cka BP), the D (13.8–13.2 ¹⁴Cka BP), the Skarpnes (12.2 ¹⁴Cka BP), the Tromsø–Lyngen (10.7–10.3 ¹⁴C ka BP) and the Stordal (10.0–9.5 ¹⁴Cka BP). Onset of the final deglaciation occurred about 14.6 ¹⁴Cka BP. Most of the western part of the Fennoscandian and Barents Sea Ice Sheets receded from the outer continental shelf 15–14 ¹⁴Cka BP. The delivery and melting of icebergs at this time to the Norwegian-Greenland Sea resulted in a low oxygen isotope event recorded in a number of cores in the region. Atlantic water intruded the area 13.2 ¹⁴Cka BP, and an atmospheric warming commenced 12.9/12.8 ¹⁴Cka BP. A marked glacial recession occurred before the Skarpnes event. During Allerød time, the glaciers retreated to the fjord heads or even farther inland. The Fennoscandian outlet glaciers readvanced (locally more than 40 km), reached their Younger Dryas outer limit after 10.7 ¹⁴Cka BP and retreated from this position before about 10.3 ¹⁴Cka BP.     

The deglaciation of southern Sweden 13,500-10,000 B.P.

Because of its well-developed ice-marginal zones, SW Sweden is an important reference area for the study of deglaciation, chronology and palaeoclimate 13,500-10,000 B.P. The ice-marginal zones are described and defined. Earlier research and opinions concerning the deglaciation are summarized. Based on radiocarbon dates from shells, vertebrate bones and limnic sediments, a revised deglaciation chronology is presented. This chronology is supported by biostratigraphic transects of time-space diagrams. The radiocarbon and varve chronologies are compared. Some ice-marginal zones are supposed to be 400 to 900 years older than expected from the varve chronology. The deglaciation chronology is correlated within the southern margin of the Scandinavian inland ice. Various consequences for the interpretation of glacial dynamics, shoreline displacement, and the biological environment are mentioned.     

Younger Dryas deglaciation at Mt. Billingen, and clay varve dating of the Younger Dryas/Preboreal transition

A clay varve chronology has been established for the Late Weichselian ice recession east of Mt. Billingen in Västergötland, Sweden. In this area the Middle-Swedish end moraine zone was built up as a consequence of cold climate during the Younger Dryas stadial. A change-over from rapid to slow retreat as a result of climatic deterioration at the Alleröd/Younger Dryas transition cannot be traced with certainty in the varve sequences, but it seems to have taken place just before 11,600 varve years BP. The following deglaciation was very slow for about 700 years — within the Middle-Swedish end moraine zone the annual ice-front retreat was only c . 10 m on average. A considerable time-lag is to be expected between the Younger Dryas climatic event and this period of slow retreat. The 700 years of slow retreat were succeeded by 200 years of more rapid recession, about 50–75 m annually, and then by a mainly rapid and uncomplicated retreat of the ice-front by 100–200 m/year or more, characterizing the next 1500 years of deglaciation in south and central Sweden. The change from about 50–75 m to 100–200 m of annual ice-front retreat may reflect the Younger Dryas/Preboreal transition. Clay-stratigraph-ically defined, the transition is dated at c . 10,740 varve years BP, with an error of +100 to -250 years. In the countings of ice layers in Greenland ice cores (GRIP and GISP-2) the end of the Younger Dryas climatic event is 800–900 years older. However, a climatic amelioration after the cold part of the Younger Dryas and in early Preboreal should rapidly be reflected by for example chemical components and dust in Greenland ice cores, and by increasing δ¹³C content in tree rings. On the other hand, the start of a rapid retreat of the inland ice margin can be delayed by several centuries. This can explain at least a part of the discrepancy between the time-scales.     

Late Weichselian pollen stratigraphy, clay varve chronology and palaeomagnetic secular variations in Lake Bolmen, Småland, south Sweden

Pollen analysis, glacial varve chronology and palaeomagnetic measurements were carried out on Late Weichselian lake sediments from southwestern Smaland, south Sweden. The sequence is correlated to the GRIP event stratigraphy, expressed in calendar years BP, and covers the period from the deglaciation at c. 14 400 to 11 300 calendar years BP. The series encompasses c. 930 varves and has been connected to the local varve chronology. Varve thickness increases markedly after the Older Dryas stadial, which indicates an accelerated deglaciation and melting of dead ice. The pollen diagram displays the vegetation development from the deglaciation at c. 14 400 calendar years BP to the transition to the Holocene. The vegetation succession starts with an arctic pioneer vegetation at the deglaciation, changes to a more stable tundra environment and displays a development which concurs with the traditional lateglacial pollen stratigraphy for southern Sweden. A palaeo-magnetic secular variation curve is presented displaying two westerly declination swings at 14200-13800 and 12 800-11 600 calendar years BP, respectively. The upper one can be recognized from other palaeomagnetic stratigraphies from southern Sweden and Estonia.     

An 800-year long, radiocarbon-dated varve chronology from south-eastern Sweden

More than 50 varve-thickness diagrams, which were established from glacial varved clays in south-eastern Sweden were correlated with each other to form an 800-year long floating varve chronology. AMS |214|0C measurements on terrestrial macrofossils from the varved clays enabled synchronization of the record with other high-resolution archives. The synchronization indicates that the chronology spans between c. 13 150 and c. 12 350 calendar years BP and covers the later part of the Allerørd and the early part of the Younger Dryas. Calibrated radiocarbon dates, which were obtained on varved clays south of the floating chronology, indicate that the ice recession in south-eastern Sweden may have started during late Bølling. Our results indicate a longer time-span in varve years for the deglaciation than has been previously estimated     

Timing of the last deglaciation in Lithuania

Boulders from the Grūda Moraine, which is associated with the maximum extent of the Scandinavian Ice Sheet (SIS) during the last glaciation, and the Baltija (also referred to as the South Lithuanian), the Middle and North Lithuanian moraines, which are associated with recessional stages of the SIS, were sampled for surface exposure dating using ¹⁰Be. By combining these data with existing radiocarbon ages, we developed a chronology for the retreat of the SIS margin in Lithuania. Our new ¹⁰Be ages suggest that the SIS margin began to retreat from its maximum extent at 18.3 ± 0.8 ¹⁰Be kyr. Based on a probable correlation of the Baltija Moraine with the Pomeranian Moraine in Poland, we infer that the Baltija Moraine was formed following a re-advance of the SIS margin. The ice margin retreated from the Baltija position at 14.0 ± 0.4 ¹⁰Be kyr. The SIS-margin retreat paused at least two more times to form the Middle Lithuanian Moraine at 13.5 ± 0.6 ¹⁰Be kyr and the North Lithuanian Moraine (tentatively correlated to the Pajūris Moraine) at 13.3 ± 0.7 ¹⁰Be kyr. Subsequent ice-margin retreat from the North Lithuanian Moraine represented the final deglaciation of Lithuania. Direct dating of these moraines better constrains the relation of ice-margin positions in Lithuania to those in adjacent countries as well as the SIS response to climate change.     

Chronology of deglaciation based on 10Be dates of glacial erosional features in the Grimsel Pass region, central Swiss Alps

Surface exposure dating, using in situ produced cosmogenic ¹⁰Be, is applied to determine the time since deglaciation of bedrock surfaces in the Grimsel Pass region. Nine ¹⁰Be dates from bedrock surfaces corrected for cover by snow are minimum ages for deglaciation of the pass. Four ¹⁰Be dates from surfaces below 2500 meters above sea level (m a.s.l.) on Nägelisgrätli, east of Grimsel Pass, yield ages that range from about 14 000 to 11 300 years. Three ¹⁰Be dates from locations above 2600 m a.s.l. on Nägelisgrätli are between about 11 700 and 10 400 years. Two ¹⁰Be dates from locations at 2560 m a.s.l. below Juchlistock are about 12 100 and 11 000 years. The geographical distribution of ¹⁰Be dates on Nägelisgrätli either may show the timing of progressive deglaciation of Grimsel Pass or may reflect differences in subglacial erosion of bedrock in the pass region. All dates are discussed in the context of deglaciation of the late Würmian Alpine ice cap and deglaciation from Last Glacial Maximum (LGM) ice extents in other regions.     

AMS dating Swedish varved clays of the last glacial/interglacial transition and the potential/difficulties of calibrating Late Weichselian 'absolute' chronologies

The increasing focus on the chronology of environmental and climatic changes of the last glacial-interglacial transition has led to several independent attempts to try to calibrate the ¹⁴C time-scale beyond the Holocene. The Late Weichselian Gotiglacial varved clays of the Swedish Time Scale could potentially be used for this purpose. The reliability of the Swedish Time Scale is discussed as well as different ways of using the Swedish varved clays for calibrating the ¹⁴C chronology. The strategy and initial results from an ongoing calibration project are presented. They show clearly that, if the right strategy is adopted, varved clay may be dated by accelerator mass spectrometry (AMS) ¹⁴C measurement of terrestrial macrofossils. A Late Weichselian 'event stratigraphy', including the Vedde Ash fall-out, is established for south Scandinavia using three dating categories: clay varve measurements, terrestrial macrofossil measurement, and lake sediment (including aquatic mosses) measurements. It suggests that a ¹⁴C chronology based on terrestrial organic remains is not consistent with the traditional Late Weichselian chronostratigraphy based on lake-sediment samples, and that 'clay varve years' exceed 'terrestrial ¹⁴C years' by c. 900 years at the end of, and by 1100–1200 years at the beginning of the Younger Dryas Chronozone. Further back in time, the time-scales appear to converge. These results are compared with other recently published calibration studies.     

The last deglaciation of the Franz Victoria Trough, northern Barents Sea

A study of two piston cores and a 3.5 kHz seismic profile from the Franz Victoria Trough provides new stratigraphic, stable isotopic and foraminiferal AMS ¹⁴C data that help constrain the timing of ice-sheet retreat in the northern Barents Sea and the nature of the deglacial marine environment. Silty diamicton at the base of each core, interpreted as till or ice-marginal debris flow, suggests that the Barents ice sheet was grounded at the core sites (470 m water depth). Eight AMS ¹⁴C dates on sediment overlying the diamicton indicate that the ice sheet retreated from both core sites by 12.9 ka and that postglacial sedimentation began 10 ka ago. These dates, combined with a recently published ¹⁴C date from a nearby core, suggest that the Franz Victoria Trough may not have been deglaciated until c . 13 ka, 2000 years later than modeled ice-sheet reconstructions indicate. In the trough, oxygen isotopic ratios in planktonic foraminifera ^N. pachyderma (sinistral) were 0.5–0.750, lower during deglaciation than after, probably as a result of ice-sheet and/or iceberg melting. Foraminiferal assemblages suggest that Atlantic-derived intermediate water may have begun to penetrate the trough c . 13 ka ago.     

Lake Mullsjön - a key site for understanding the final stage of the Baltic Ice Lake east of Mt. Billingen

The deglaciation pattern at Mt. Billingen, within the Middle Swedish end moraine zone, and its relationship with dramatic water level changes in the Baltic Ice Lake is a classic topic of Swedish Quaternary Geology. Based on data west of Mt. Billingen, the authors (in two earlier papers) presented a stratigraphic model associated with this subject. This study is an attempt to test the model east of Mt. Billingen, i.e. inside the Baltic Ice Lake itself. Lake Mullsjon is situated 30 km southeast of the drainage area of the Baltic Ice Lake and within the final drainage zone. About 8 m of Late Weichselian sediments (mostly varved clay) were recovered from the lake and analysed from different stratigraphic viewpoints, including lithology, grainsize, varve chronology, and pollen. These analyses show that the site was deglaciated in the later part of the Allerød Chronozone. Shortly thereafter the first drainage of the Baltice Ice Lake took place but without isolating Lake Mullsjon. After a short period of disturbed sedimentation varved clay continued to form as the glacier receded for another 120 varve years until the onset of the Younger Dryas cooling, as registered both in the pollen and in the varve stratigraphies. After c. another 120 varve years our analyses suggest that the Baltic Ice Lake was dammed once again. About 230 varve years of further ice readvance followed west of Mt. Billingen, while the ice margin in the east was more or less stationary. Rapid melting set in, at first producing coarse varves, but soon the clay was thin-varved and fine. This continued for 140 varve years until suddenly the lake became isolated. At this isolation thick beds of silty-sandy deposits were deposited on the lake floor. The isolation is dated to 10,400–10,500 ¹⁴C years B.P., which corresponds to the assumed age of the final drainage of the Baltic Ice Lake. It was also isolated at the same time as lakes (on the same isobase) situated 20 m lower, but west of Mt. Billingen, were raised above sea level. This strongly suggests that Lake Mullsjön was isolated as an effect of the drainage of the Baltic Ice Lake. Significant differences between the clay-varve and the ¹⁴C chronologies are also presented.     

Palaeomagnetic records from two varved clay sequences in the Middle Swedish ice marginal zone

Two varved clay sequences, at Rystad and Tottnäs, situated in the Middle Swedish ice marginal zone were analysed palaeomagnetically. Two parallel profiles were sampled and analysed at each site. The varved clay at Rystad was dated by floating varve chronology. The varves at Tottnäs can be linked to Swedish time scale, expressed in calendar years B.P. Due to the distance between the sites they cannot be correlated by means of varve diagrams. Palaemagnetic methods were used as an alternative. Based an AF demanetization of pilot samples, the palaeomanetic to be too low, in the order of15, compared to the site latitude. At Tottnäs the inclination records are very close to the expected inclination with respect to the site latitude. Because of a systematic inclination error in the Rystad profiles the correlation was based on the declination records. Statistical comparisons of these records between the two sites indicate that the sediment successions are partly synchronous. It is concluded that the deglaciation at Tottnäs started c. 130 years earlier than at Rystad. This mean that the Swedish ice marginal zone east of Rystad will have a more northeasterly extension than previously thought.     

Occurrence of varved lake sediment sequences in Varmland, west central Sweden: lake characteristics, varve chronology and AMS radiocarbon dating

Varved lake sediments can be used to set multiple environmental proxies within a calendar year time scale. We undertook a systematic survey of lakes in the Province of Värmland, west central Sweden, with the aim of finding continuous varved lake sediment sequences covering the majority of the Holocene. In Fennoscandia, such sediments have previously only been recorded in northern Sweden and in southern and central Finland. By following a selective process and fieldwork we discovered three new varved sites (i.e. Furskogstjärnet, Mötterudstjärnet and Kälksjön). We found that lakes with varved sediments have several common lake morphometry properties and lake catchment characteristics such as maximum water depth, maximum water depth/lake surface area ratio, catchment soil types, altitude and number of inflows. Varve chronologies, supported by AMS-¹⁴C dating and tephrochronology were established for two of the sediment profiles. These varve chronologies are the longest geological records with an annual resolution known to exist in Sweden. In Furskogstjärnet, the AMS- ¹⁴C dates based on terrestrial plant macrofossils at several levels deviate significantly from the varve based time-depth curve. In Motterudstjarnet, a fully reasonable time-depth model based on the ¹⁴C dates gives older ages in the lower part of the sequence compared to the varve chronology. These results highlight that seemingly acceptable AMS radiocarbon dates may be erroneous. They also point to the fact that varved lake sediments are reliable geological archives with respect to chronological control and accuracy. Thus, these archives should be of prime interest for studies of climate and environmental change undertaken with the aim of providing sub-decadal resolution proxy data sets.     

Chronology and paleoenvironments during the late Weichselian deglaciation of the southwest Iceland shelf

Foraminifera, sedimentology, and tephra geochemistry in core 93030-006 LCF from the southwestern Iceland shelf were used to reconstruct paleoenvironments between 12.7 and 9.4 ¹⁴C ka BP. Seismic-reflection profiles place the core in glacial-marine and marine sediments within one meter of the underlying glacial till. Foraminifers in the earliest glacial-marine sediments provide a record of ice-distal conditions and immigration of slope species onto the shelf in association with warm Atlantic water. Meltwater increased during the Allerød under a weakened Atlantic water influence. Arctic conditions began by 11.14 ₁₄C ka BP with an abrupt increase in meltwater and near exclusion of boreal fauna from the shelf. Meltwater diminished in the early Younger Dryas, coinciding with sea-surface cooling between 11.14 and 10.5 ¹⁴C ka BP. A slight warming recorded in the uppermost glacial-marine sediments was interrupted by an inferred jökulhlaup event emanating from glacier ice on the Western Volcanic Zone. Retreat of the ice margin from the sea sometime between c. 10.3 and 9.94 ¹⁴C ka BP coincided with this event. The onset of postglacial marine sedimentation occurred along with increasing evidence of Atlantic water c. 9.94 ¹⁴C ka BP and was interrupted by a short-lived Pre-boreal cooling of the Irminger Current c. 9.91 ¹⁴C ka BP. Conditions similar to those today were established by 9.7 ¹⁴C ka BP.     

Postglacial emergence and Late Quaternary glaciation on northern Novaya Zemlya, Arctic Russia

Recent observations on postglacial emergence and past glacier extent for one of the least accessible areas in the Arctic, northern Novaya Zemlya are here united. The postglacial marine limit formed 5 to 6 ka is registered on the east and west coasts of the north island at 10 ± 1 and 18 ± 2 m aht, respectively. This modest and late isostatic response along with deglacial ages of >9.2 ka on adjacent marine cores from the northern Barents Sea indicate either early (>13 ka) deglaciation or modest ice sheet loading (<1500 m thick ice sheet) of Novaya Zemlya. Older and higher (up to 50 m aht) raised beaches were identified beneath a discontinuous glacial drift. Shells from the drift and underlying sublittoral sediments yield minimum limiting ¹⁴C ages of 26 to 30 ka on an earlier deglacial event(s). The only moraines identified are within 4 km of present glacier margins and reflect at least three neoglacial advances in the past 2.4 ka.     

Eemian and Weichselian correlation problems in Finland

The Quaternary stratigraphy in Finland is discussed on the basis of an example from the Oulainen area of Ostrobothnia. Organogenic deposits found beneath till at this site are correlated with the Eemian Interglacial on biostratigraphical evidence. This is confirmed by TL dates of 97,000 ± 18,000 B.P. and 150,000 ± 30,000 B.P., whereas a finite radiocarbon date of 63,200 +⁵⁵⁰⁰ -₃₂₀₀ B.P. is probably too young. Correlation of the Weichselian stratigraphy is based on deep-sea oxygen isotope data, in which the variations in isotope ratios are assumed to reflect global changes in climate and fluctuations in the volume of the ice-caps. It is concluded on the latter grounds that Finland must have been free of ice at two periods during the Early Weichselian but at least for the most part covered by ice thereafter up to the final deglaciation. The sediments attributed to the only known Weichselian interstadial in Finland, the Perapohjola Interstadial, are taken to correspond most probably to the Brørup, although some may represent the Odderade, Information on the Weichselian till stratigraphy in the Oulainen area is largely confined to the deglaciation phase, the relatively complex nature of which suggests that complete reconstruction of the earlier phases of the Weichselian in an area such as Finland, located towards the centre of the ice sheet, is scarcely feasible by the methods currently available.     

Depositional environments and chronology of Late Weichselian glaciation and deglaciation in the central North Sea

Graham, A.G.C., Lonergan, L. & Stoker, M.S. 2010: Depositional environments and chronology of Late Weichselian glaciation and deglaciation in the central North Sea. Boreas, Vol. 39, pp. 471–491. 10.1111/j.1502‐3885.2010.00144.x. ISSN 0300‐9483. Geological constraints on ice‐sheet deglaciation are essential for improving the modelling of ice masses and understanding their potential for future change. Here, we present a detailed interpretation of depositional environments from a new 30‐m‐long borehole in the central North Sea, with the aim of improving constraints on the history of the marine Late Pleistocene British–Fennoscandian Ice Sheet. Seven units characterize a sequence of compacted and distorted glaciomarine diamictons, which are overlain by interbedded glaciomarine diamictons and soft, bedded to homogeneous marine muds. Through correlation of borehole and 2D/3D seismic observations, we identify three palaeoregimes. These are: a period of advance and ice‐sheet overriding; a phase of deglaciation; and a phase of postglacial glaciomarine‐to‐marine sedimentation. Deformed subglacial sediments correlate with a buried suite of streamlined subglacial bedforms, and indicate overriding by the SE–NW‐flowing Witch Ground ice stream. AMS ¹⁴C dating confirms ice‐stream activity and extensive glaciation of the North Sea during the Last Glacial Maximum, between c. 30 and 16.2 ¹⁴C ka BP. Sediments overlying the ice‐compacted deposits have been reworked, but can be used to constrain initial deglaciation to no later than 16.2 ¹⁴C ka BP. A re‐advance of British ice during the last deglaciation, dated at 13.9 ¹⁴C ka BP, delivered ice‐proximal deposits to the core site and deposited glaciomarine sediments rapidly during the subsequent retreat. A transition to more temperate marine conditions is clear in lithostratigraphic and seismic records, marked by a regionally pervasive iceberg‐ploughmarked erosion surface. The iceberg discharges that formed this horizon are dated to between 13.9 and 12 ¹⁴C ka BP, and may correspond to oscillating ice‐sheet margins during final, dynamic ice‐sheet decay.     

Late Weichselian deglacial history of Disko Bugt, West Greenland, and the dynamics of the Jakobshavns Isbrae ice stream

New relative sea-level (RSL) data from Disko Bugt, a large marine embayment in West Greenland, are used to examine the deglacial history of the Jakobshavns Isbrae ice stream. RSL data show rapid deglaciation after 10.3 ka cal. yr BP. Once deglaciation began, a bedrock high in the west of the bay exerted no discernible influence on the deglacial chronology. Following initial rapid retreat, ice stream recession slowed as it approached the eastern shores of the bay. Seabed elevations increase here and the ice stream terminus lingered for several thousand years before retreating into the narrow bedrock-confined Jakobshavns Isfjord. The seabed topography of Disko Bugt includes several deep channels which probably record the former course of the ice stream. Using a simple water depth/calving velocity relationship it is estimated that the maximum calving velocity on deglaciation was c. 4.8 km a^-1. This is less than the present rate (6–7 km a^-1), although ice discharge was two to four times that observed today. Initiation of rapid ice stream retreat was probably caused by ice stream thinning and increased surface melting. A critical point in time was the retreat of the ice stream from shallow continental shelf waters ( c. 400 m) into the deep bedrock trough (>800 m) which marks the entrance to Disko Bugt.