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The Last Glacial-Interglacial Transition

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The Last Glacial-Interglacial Transition 226 by Wim Z. Hoek The Last Glacial-Interglacial Transition Department of Physical Geography, Faculty of Geosciences, Utrecht University, Heidelberglaan 2, NL 3508 TC Utrecht, The Netherlands. E-mail: [email protected] The Last Glacial-Interglacial Transition is one of the tarandus), an organic layer with birch remains (Betula spp.) and fos- most intensively studied periods in Earth History. The sils of the European elk (Alces alces) was present, indicating a change from tundra to a more forested environment followed by a rapid climate and environmental changes that occurred return to tundra. Based on palynology, Jessen (1935) was able to during the transition can be used to test ideas about the define three successive pollen zones: Early Dryas (I), Allerød (II), functioning of our climate system. The stratigraphy of and Late Dryas (III). This subdivision was further refined by Iversen (1942), who added another interstadial phase that supposedly this period has been thoroughly investigated and, in occurred before the Allerød and was based on evidence from the site particular, the recently proposed event stratigraphy for at Bølling Sø (Denmark). the Last Glacial-Interglacial Transition based on the The palynological sub-division of the Weichselian Late Glacial established by Jessen (1935) and Iversen (1942), was introduced into Greenland ice core records serves as a tool for synchro- The Netherlands by van der Hammen (1949), who recognized the nisation of records from the ice, marine and terrestrial Bølling and Allerød oscillations, separated by the colder Dryas sta- environment. The causes behind the rapid climate dials in the lake deposits of Hijkermeer. The interstadial deposits were characterised by a higher content of organics in comparison changes are most likely to have been changes in ocean with the stadial deposits. Van der Hammen (1951) went on to circulation, partly triggered by ice-melting during demonstrate a similar vegetation development at different locations deglaciation. While the picture for the North Atlantic in The Netherlands and nearby areas. In Central Europe, the scheme described by Firbas (1954) has been more widely used. His defini- region is becoming more and more clear, complex pat- tion, particularly of the Oldest Dryas which is not so characteristic in terns of change over the globe remain to be studied. The NW Europe, has continued to be used in the Alpine region. Since functioning of the complex feed-back mechanisms then, hundreds of pollen diagrams from Late Glacial deposits have requires an interdisciplinary approach between geosci- been constructed using the subdivisions suggested by van der Ham- men and Firbas. entists from different disciplines. Introduction Chronostratigraphy of the Last Glacial Interglacial Transition The Last Glacial-Interglacial Transition (LGIT), often referred to as 14 Weichselian Lateglacial or Last Termination (ca 13,000–10,000 C With the introduction of the radiocarbon dating method, biostrati- yrs BP), marks the transition from the cold Weichselian Glacial to graphic correlation became less important and pollen diagrams were the warm Holocene. The LGIT is classically sub-divided into a considered more frequently in a chronological context. Moreover, series of cold stadials such as the Younger Dryas Stadial (named the terminology that was originally developed for biostratigraphical after the occurrence of the characteristic arctic-alpine plant species zones (Bølling, Allerød, etc.) has been used throughout northern and Dryas octopetala) separated by warm interstadials named after the western Europe in a chronostratigraphic sense (Mangerud et al., type localities Bølling and Allerød in Denmark. This climatostrati- 1974). Since then, the terms Bølling and Allerød have been con- graphic sub-division has been extensively used in NW-Europe, usu- nected worldwide to the time period between 13,000 and 11,000 14C ally in a chronostratigraphic sense, and is still widely employed yr BP as defined by Mangerud et al. (1974), while the Younger Dryas globally. chronozone falls between 11,000 and 10,000 14C yr BP (Table 1). The rapid climate changes that occurred during the LGIT, have This chronological subdivision was initially based on the radiocar- stimulated the palaeoclimate research community to study this bon dated biostratigraphic zone boundaries from southern Scandi- period intensively. This is not only because it provides the opportu- navia, and predated important later advances in radiocarbon dating nity to investigate the nature of the climate changes in high-resolu- such as AMS and radiocarbon calibration. tion, but also to test hypotheses on the mechanisms behind the abrupt changes using palaeoclimate modelling. This makes the LGIT one of the most thoroughly investigated of all geological episodes. As a Bio- and chronostratigraphy result, the stratigraphy of this time period is relatively well devel- oped. Boundaries of similar biostratigraphical zones defined in different regions are in most cases diachronous due to differences in biologi- cal response to changes in climate. This means that the biostrati- History graphic zones as defined in Scandinavia are difficult to apply outside this geographical region. Terms such as Lateglacial Interstadial or In 1901, Hartz and Milthers used deposits in a clay-pit near Allerød Windermere Interstadial have been used for the Bølling-Allerød (Denmark) to demonstrate that a climatic warming occurred during interstadial complex in Britain, where it is often difficult to recog- the Lateglacial. Between two clay layers containing leaves of moun- nize stadial conditions between the two separate interstadial phases. tain avens (Dryas octopetala) and fossils of reindeer (Rangifer Furthermore, there are problems relating to biostratigraphic June 2008 227 Table 1 Chronostratigraphy of the Late Weichselian based on Table 2 Event stratigraphy for the Last Glacial-Interglacial the classical Stratigraphy of Norden after Mangerud et al. Transition based on the NGRIP oxygen isotope stratigraphy presented on the GICC05 timescale (Rasmussen et al., 2006): age b2k is before 2000 AD with the maximum counting error (MCE) and conversion to cal BP (before 1950). definition. In Germany, an additional interstadial called Meiendorf has often been recognized, which correlates to the Bølling intersta- dial in the surrounding countries. But in this particular case, the term Bölling is used for what is effectively the first part of the Allerød tinuous and annually resolved palaeoclimate archive, which pro- interstadial in the surrounding countries, leading to considerable vides an unprecedented level of precision and accuracy for deter- confusion. This apparent discrepancy between the definition of mining the timing and duration of climate events during the LGIT Bølling and Allerød is mainly the result of a difference of opinion (see further Lowe et al., 2008). over correlation by Iversen (1942) with the section at Bølling Sø (de The revised INTIMATE event stratigraphy for the LGIT in the North Atlantic region is presented in Table 2. The ages for the event Klerk, 2004). These examples highlight some of the problems of boundaries are derived from the GICC05 timescale (Rasmussen et transferring a biostratigraphic zonation scheme from one region to al., 2006) in years b2k (= before 2000 AD). Note that this ice-core- another. based time-scale is different from the calibrated 14C-timescale given The chronostratigraphic approach, originally developed for in cal BP (=before 1950 AD). In effect, there is an offset of 50 years Scandinavia, can be compared to that in other areas of northwest between the two timescales which must be taken into account when Europe, and again differences emerge. In The Netherlands, for 14 14 comparing ice-core years to C-dated events. example, where there are a number of C -dated Lateglacial pollen An additional basis for correlation in the North Atlantic region diagrams, the onset of the Lateglacial Interstadial falls between 14 during the LGIT is provided by tephrochronology. This time period 12,500 and 12,450 C yrs BP (Hoek, 2001). This is considerably in geological history is marked by the frequent occurrence of vol- later than the start of classically defined Bølling in Scandinavia canic eruptions, possibly related to the glacio-isostatic compensation (Table 1). The start and end of the Younger Dryas Stadial in the 14 since the last deglaciation, and both visible and microtephras form Netherlands, however, have been dated at 10,950 and 10,150 C yrs valuable time-parallel marked horizons in marine and terrestrial sed- BP, respectively, and are more in accordance with the chronostrati- iments, and also in the ice-core record (Turney et al., 2004; Alloway graphic boundaries defined by Mangerud et al. (1974). et al., 2007; Lowe et al., 2008). Event stratigraphy Discussion: climate changes during the As mentioned above, the application of biostratigraphic terminology Last Glacial-Interglacial Transition to records from different regions and different spheres often cause correlation problems. Although lithological and botanical evidence shows that marked In an attempt to resolve some of these difficulties, the INTI- environmental changes occurred during the Lateglacial, it is often MATE group of the INQUA Palaeoclimate Commission developed difficult to relate these signals directly to climate. The AP (Arboreal an Event Stratigraphy for the North Atlantic region, based on the Pollen) percentage was long considered as a direct temperature Greenland
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