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TOWARDS A TEPHROCHRONOLOGY FRAMEWORK FOR THE LAST GLACIAL/INTERGLACIAL TRANSITION IN SCANDINAVIA AND THE FAROE ISLANDS
WASTEGåRD, STEFAN Department of Physical Geography and Quaternary Geology, Stockholm University, Sweden.
Davies, Siwan M. Department of Geography, University of Wales Swansea, UK.
Turney, Chris S.M. School of Archaeology and Palaeoecology, Queen's University, Belfast, UK.
Wohlfarth, Barbara Department of Physical Geography and Quaternary Geology, Stockholm University, Sweden.
The Last Glacial/Interglacial transition (LGIT; ca 15-8 cal. ka BP) was a period of rapid climatic transitions around the North Atlantic. Although close similarities are evident in the palaeoclimatic reconstructions obtained from terrestrial, marine and ice-core records for the LGIT, uncertainties exist as to the degree of synchroneity (or asynchroneity) between them, largely due to the limitations of the radiocarbon dating method (radiocarbon plateaux, reservoir effects) and the lack of suitable dating methods for the time period before ca 40 ka BP. Therefore, new approaches are required for geochronology models and correlation of sequences and events. One method that holds much promise of effecting more precise regional correlations is tephrochronology.
Ten years ago, only three tephra horizons were described from this time period in Scandinavia and the Faroes: the Saksunarvatn Tephra (ca 10.2 cal. ka BP), the Vedde Ash (ca 12.0 cal. ka BP) and the Laacher See Tephra (LST, ca 12.9 cal. ka BP). The first two of these are of Icelandic origin while the LST has its origin in the Eifel volcanic field in SW Germany.
A technique for extracting cryptotephra (a tephra horizon invisible to the naked eye) has revolutionised the application of tephrochronology in minerogenic deposits from the LGIT (Turney, 1998). This technique relies upon the difference between the specific gravity of the tephra shards and the host sediment matrix and has led to the first discovery of the Vedde Ash on the British mainland as well as the previously unrecorded Borrobol Tephra, dated to ca. 14.4 cal. ka BP (Fig. 1; e.g. Turney et al., 1997). In Sweden, this technique led to the first discovery of the Vedde Ash, as well as a previously unrecorded rhyolitic tephra dated to ca 10.2 cal. ka BP (the Högstorpsmossen Tephra; Björck et al., 2002). The rhyolitic component of the Vedde Ash was also found in two sites on the Karelian Isthmus in NW Russia, greatly extending the distribution of this important marker horizon (Fig. 1; Wastegård et al., 2000). Recently, the Borrobol Tephra and two new tephra horizons, the Hässeldalen Tephra (ca 11.5 cal. ka BP) and the Askja 10-ka Tephra (ca 11.2 cal. ka BP) were discovered in LGIT deposits from Blekinge, SE Sweden, (Fig. 1; Davies et al., 2003). An effort to date the Borrobol Tephra in Sweden using wiggle-matching of AMS-dates to the Cariaco basin chronology (Hughen et al., 2004) yielded an age of ca 13.9 cal. ka BP, indicating that the Borrobol Tephra in Sweden and Scotland either represents two separate eruptions from the same volcanic system, or that the British age estimate is slightly too old (Davies et al., 2004). Lacustrine records from Andøya, north Norway (Fig. 1) extending back to ca 20 cal. ka BP are also under investigation as well as the classic Vallengaard mose site on Bornholm, Denmark (Fig. 1) that contains a visible occurrence of the Laacher See Tephra (ca 12.9 cal. ka BP).
Sediments from the Lateglacial seem to be missing on the Faroe Islands in the North Atlantic, probably due to an extensive ice cover during the Younger Dryas which may have removed older deposits. The Saksunarvatn Tephra is visible in several sections and in lake sediments and is an important marker horizon for the Early Holocene. Silicic tephra horizons below the Saksunarvatn Tephra have been found at two sites, the L3574 Tephra (Dugmore and Newton, 1998) from Lake Saksunarvatn (the type site for the Saksunarvatn Tephra) and the Hovsdalur Tephra dated to ca 10.5 cal. ka BP (Wastegård, 2002). The highly silicic Hovsdalur Tephra has an identical geochemistry to the Hässeldalen Tephra (Fig. 2), but is ca 1000 years younger. A rhyolitic tephra called the Suduroy Tephra, dated to 8 cal. ka BP was also found in the Hovsdalur site on the southern island of Suduroy. This tephra has a geochemistry similar to the rhyolitic component of the Vedde Ash and the IA2 tephra from the Rockall Trough, west of Ireland (ca 13.5-13.0 cal. ka BP; Bond et al., 2001). This indicates that “Vedde-like” rhyolitic eruptions of the Katla volcano may have persisted during the Lateglacial into the Early Holocene. After the Holmsá event (ca 7.6 cal. ka BP; Larsen, 2000) the composition of the silicic magma below the Katla volcano seems to have changed to a more dacitic composition. This is indicated by the fairly homogeneous composition of the so called SILK tephras erupted between the Holmsá and Eldgjá (AD 930s) events (Larsen et al., 2001).
Björck, J., Andrén, T., Wastegård, S., Possnert, G., and Schoning, K., 2002, An event stratigraphy for the Last Glacial-Holocene transition in eastern middle Sweden: results from investigations of varved clay and terrestrial sequences. Quaternary Science Reviews v. 21, p. 1489-1501.
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Figure 1. Map showing investigated sites in Sweden, Denmark (black diamonds) and the Faroe Islands. The type sites for the Vedde Ash, the Borrobol Tephra and the Saksunarvatn Tephras are also marked as well as volcanic centres on Iceland.
Figure 2. Biplot of SiO2 and K2O concentrations in tephras from the LGIT in Scandinavia and the Faroe Islands. The envelopes show the composition in tephra from some of the main European volcanic provinces (modified after Mangerud et al., 1984 and Davies et al., 2002).
Figure 3. Table 1. Tephra horizons from the LGIT (ca 15-8 cal. ka BP) found in terrestrial deposits in Scandinavia and the Faroe Islands. Ages are given as cal yr. BP
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