Lateglacial glacier and climate variations in NW Europe

High-frequency climatic fluctuations during the last déglaciation (ca. 14,000-9000 14cyrbp) are well documented in terrestrial data, marine records and ice cores from the North Atlantic region (e.g. Lowe and Walker, 1997, and references therein). These climatic changes occurred at a time of maximum solar radiation receipt in the northern hemisphere and cannot therefore be explained by orbital forcing. The cause(s) of these climatic fluctuations must therefore be sought in the ocean/atmosphere/ climate system.

In northwestern Europe, several climatic oscillations occurred towards the end of the Weichselian/Devensian glaciation. These Late-glacial oscillations are dated to approximately 15,000-9000 radiocarbon years bp. Four periods are recognized based on biostrati-graphical evidence: two episodes of mild conditions (the B0lling (13,000-12,000 yr bp) and Allerod (11,800-11,000 yr bp) interstadials) separated by two cold periods (the Older Dryas (12,000-11,800 yr bp) and the Younger Dryas (11,000-10,000 yr bp) stadials). In Britain, the Late-glacial is divided into the Windermere

Interstadial (13,000-11,000 yr bp) and the Loch Lomond Stadial (11,000-10,000 yr bp) (Lowe and Gray, 1980). From the European mainland, a sequence of two interstadials (Belling and Altered) was separated by the Older Dryas short cold episode, and the Allerad was followed by the Younger Dryas Stadial (Fig. 5.9).

The stratigraphic sequence of Late-glacial chronozones has, however, been hampered by problems with accuracy of radiocarbon dating during this time interval. In northwest Europe, the chronozones are radiocarbon-dated biozones, but biozones are commonly time-transgressive as a response to climatic change, which are geographically and temporally diachronous. The Younger Dryas chronozone spans the time interval between 11,000 and 10,000 yr bp, but the Younger Dryas biozone is part of a stratigraphic sequence characterized by cold fossil assemblages, and these two rarely coincide.

Based on De Geer's (1912) varve investigations, the Swedish Time Scale has been divided into 'postglacial', 'finiglacial', and 'gotiglacial' parts. About 9266 'postglacial' varves were deposited during the Holocene along the river Angermanalven in central Sweden. The 'finiglacial' (early Holocene) varves have been connected to the oldest 'postglacial' varves, and this series consists of 1191 varves (Stromberg, 1989). These varves have then been connected to the youngest Late Weichselian ('gotiglacial') varves through overlapping varve diagrams (e.g. Brunnberg, 1995). To test whether the visual correlations were statistically correct, Holmquist and Wohlfarth (1998) used cross-correlation measures with overlapping varve diagrams from two local varve chronologies established in southeastern Sweden. Of a total of 363 analysed connections, only 78 fulfilled the statistical requirements for a perfect match. In 96 cases, the statistical approach suggested alternative links. In addition, they found that 179 correlations were not statistically valid and that 11 overlaps were too short to allow valid cross-correlation. The authors therefore suggested statistical analysis of the varve chronology links before the Swedish varve chronology can be regarded as a valid, high-precision time-scale.

TERRESTRIAL DATA

MARINE DATA

ICE-CORE DATA

'<C ka BP

central france (massif central)

england, wales + s. scotland

s. sweden + denmark

s. w. norway

n. norway

e. norwegian sea

s. e. norwegian sea

greenland

greenland

14.0

A 1 Poller and

B1 Coleoptera i

c| — Coleoptera macrofossils

i i

P| Pollen + plant macrofossils

E 1 Pollen + plant macrofossils

F 1 Diatoms

Planktonic foraminifera ç

Cold Warm

Hi Dust profile GISP2 core

Cold Warm

1 1 Snow accumulation GISP2 core

Cold Warm

8 10 12 14 16 1012141618 10 12 14 16 18 10 12 14 16 18 2 -4 6 8 1012

Mean July temperature °C

July SST °C

1 1 1 1 1 1 1 1 1 1 1 100 80 60 40 20 0 % Neogloboquadrina pachyderma

0 2040 6O8OIOC ECM Current (MA)

0.1 0.2 Accumulation (m ice per yr)

Figure 5.9 Climate development in Europe and adjacent areas of the North Atlantic based on terrestrial, marine and ice-core data. (Modified from Walker, 1995)

variations of local glaciers during the last glaciation 137

New and advanced dating possibilities, including accelerator mass spectrometry (AMS) measurements on terrestrial macrofos-sils, have demonstrated the problems inherent in the present subdivision of the Late-glacial period (e.g. Ammann and Lotter, 1989). Atmospheric 4C variations during the Late-glacial and early Holocene periods led to significant differences in radiocarbon and calendar years, and plateaux of constant radiocarbon age at ca. 12,700, 10,400 and 10,000 yr bp (Ammann and Lotter, 1989). The present model of radiocarbon-dated calendar-year time-scales is based on dendrochronology, varve chronology and U/Th dates on corals (Wohlfarth, 1996). It has therefore been suggested that the subdivision into chronozones should be abandoned and that isotope signals should be used instead (Broecker, 1992). This was suggested because plateaux of nearly constant radiocarbon age coincide with periods of rapid climate change (e.g. Becker et al, 1991), making it impossible to date the transitions precisely using the radiocarbon method. In addition, AMS radiocarbon dates obtained from terrestrial plant macrofos-sils are several hundred years younger than the corresponding bulk samples from the same stratigraphic levels, mainly due to hard-water effects (e.g. Wohlfarth et al, 1993). The terminological and chronological complexity arising from use of the radiocarbon method leads to further complications.

A series of 70 AMS radiocarbon dates has been used to date the Younger Dryas/Holocene (YD/H) transition in the lacustrine sediments of Krakenes Lake, western Norway to 11,530 +40/-60 cal. yr bp (Gulliksen et al, 1998). This age estimate for the YD/H transition is in close agreement with evidence presented from the Greenland GISP2 and GRIP ice cores, German pine series, European lake sediments and Baltic varves, indicating that the YD/H transition in the North Atlantic region occurred between 11,600 and 11,500 cal. yr bp.

During the last two decades, the Late-glacial climatic and glacial histories of the North Atlantic region have been interpreted in term of shifts in the North Atlantic oceanic Polar Front (Ruddiman and Mclntyre, 1981; Karpuz et al, 1993). Variations in the position of the Polar

Front have been assumed to reflect oceano-graphic changes, such as the thermohaline ocean conveyor belt (Broecker and Denton, 1990), a sudden influx of freshwater from melting ice sheets (Broecker et al, 1989), and energy transfer in connection with the formation of deepwater in the North Atlantic (Broecker et al, 1985). Several feedback mechanisms operate, like the effects of the Late Cenozoic ice sheets and sea-ice cover on atmospheric and surface ocean circulation (Kutzbach and Wright, 1985; Karpuz and Jansen, 1992). The Greenland ice cores show climatic changes roughly in phase with the fluctuations of the oceanic Polar Front (e.g. Dansgaard et al, 1989). Both the ice-core and marine records show that large-scale climatic changes occurred very rapidly in the North Atlantic region.

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