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• Ashes to ashes: Applications of tephrochronology in Scandinavia

  2022. Simon A. Larsson.

  Avhandling (Dok)

  The project presented in this thesis provides two examples of applications of tephrochronology in Scandinavia. The usage of tephra deposits preserved in sediments—i.e. horizons of volcanic ashes, detected either as visible layers or as low-concentration “cryptotephras”—is demonstrated to be a strong and versatile tool for chronological control of individual sediment sequences and for correlation between study sites on both local and wider-regional scales.

  Fieldwork was done at a number of study sites in southernmost Sweden and mid-Norway to sample sediments formed in lakes and peatlands during the Last Glacial–Interglacial Transition, i.e. c. 16,000–8,000 years ago. Labwork was then performed to analyse the sediments by methods to estimate organic matter, organic and carbonate carbon, and elemental contents as well as to find datable macrofossils and to detect tephra occurrences. Among the detected tephras between the different study sites, identifications were made of the Fosen Tephra, the Hässeldalen Tephra, the Vedde Ash, and the Laacher See Tephra. These were used for the purposes of either application in the project; (1) a palaeoclimate reconstruction at Körslättamossen, with new proxy analyses and correlations to a previous study of the same site as well as to other studies in Europe via linkages established by the tephras, and (2) a palaeoglaciological reconstruction on the Fosen peninsula, based on correlations between study sites using the detected tephras to assess the timing of glacial retreat in the area.

  The palaeoclimate reconstruction at Körslättamossen and correlation to previous studies provides new information about the climatic development in Europe after the end of the last ice age, specifically regarding the timing of the cold stage known as the Younger Dryas. Detailed knowledge of such events, spatially and temporally, is necessary to understand the behaviour of the climate system in the past and, thereby, its behaviour in the present and the future. This is one of several studies demonstrating the usefulness of tephrochronology for such research questions.

  The palaeoglaciological reconstruction on the Fosen peninsula, also relating to the Younger Dryas, is one of a few recent studies which demonstrate the strengths of tephra investigations in providing evidence for the timing of events on more local scales than the wider spatial perspective more often utilised in tephrochronological applications. Other than resulting in a significantly revised chronology for the retreat of the Scandinavian Ice Sheet in the study area, this study should also encourage new, similar tephra studies in future Quaternary research.

  In addition to the direct results of either study (mainly presented in the related scientific papers) and the addition of new reference data for later tephra studies, the thesis also discusses further implications of the results and observations made with emphasis on considerations and study design issues in Quaternary research at large and for the tephrochronologist in particular.

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• Synchronous or Not? The Timing of the Younger Dryas and Greenland Stadial-1 Reviewed Using Tephrochronology

  2022. Simon A. Larsson (et al.). Quaternary 5 (2)

  Artikel

  The exact spatial and temporal behaviour of rapid climate shifts during the Last Glacial–Interglacial Transition are still not entirely understood. In order to investigate these events, it is necessary to have detailed palaeoenvironmental reconstructions at geographically spread study sites combined with reliable correlations between them. Tephrochronology, i.e., using volcanic ash deposits in geological archives as a dating and correlation tool, offers opportunities to examine the timing of events across wider regional scales. This study aims to review the posited asynchrony of the Younger Dryas stadial in comparison with Greenland Stadial-1 by correlating new proxy data from southernmost Sweden to previous palaeoclimate reconstructions in Europe based on the presence of the Hässeldalen Tephra, the Vedde Ash, and the Laacher See Tephra. μ-XRF core-scanning data were projected using a recently published age–depth model based on these tephras and several radiocarbon dates, and compared to previous findings, including by adapting previous chronologies to the recently proposed earlier date of the Laacher See Tephra (13,006 ± 9 cal. a BP). Although the results to some extent support the idea of a more synchronous Younger Dryas event than previously assumed, this issue requires further high-resolution proxy studies to overcome limitations of temporal precision.

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• Palaeolake sediment records reveal a mid- to late Younger Dryas ice-sheet maximum in Mid-Norway

  2022. Fredrik Høgaas (et al.). Boreas 51 (2), 41-60

  Artikel

  We present a revised chronology of the Scandinavian Ice Sheet's glacial maximum during the Younger Dryas (Tautra event) in Mid-Norway. Sediment records from palaeolakes near Leksvik show the occurrence of thick, laminated silt units with numerous dropstones between organic-rich units and indicate that a proglacial lake was dammed between the Tautra ice margin and a local spillway. Ash beds and several radiocarbon-dated plant macrofossil samples from corresponding stratigraphical sequences in different basins provide robust chronological constraints for the timing and duration of the proglacial lake and, consequently, the Tautra event. The existing chronological constraints on the Tautra event suggest that the glacial episode occurred at 12.9-12.6 ka. Our new chronology indicates that the Younger Dryas ice-sheet re-advance culminated close to 12.1 cal. ka BP, maintained this position for a maximum of 700 years and started retreating inland at c. 11.4 cal. ka BP. Our revised age for retreat from the Younger Dryas glacial maximum thus differs from the existing deglaciation chronology by approximately a thousand years, and hints at a similar late Younger Dryas glacial maximum throughout most of southern Norway.

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• A high-resolution Lateglacial–Early Holocene tephrostratigraphy from southernmost Sweden with comments on the Borrobol–Penifiler tephra complex

  2022. Simon A. Larsson, Stefan Wastegård. Quaternary Geochronology 67

  Artikel

  We present results from a cryptotephra investigation performed at a high resolution (0.5 cm) on sediments from Körslättamossen in southernmost Sweden. Six peak concentration levels were detected and extracted for geochemical analysis by electron probe microanalyser. Five of these levels were successfully analysed and we propose correlations to the Hässeldalen Tephra, the Vedde Ash, and the Laacher See Tephra (adding new analysis results for the first geochemically confirmed finding of the latter in Sweden), as well as an undetermined Borrobol-type tephra. The tephra identifications were combined with radiocarbon dated macrofossils in order to create an age model for the sampled sediments based on Bayesian methods. Stratigraphical and chronological results were found to concur with a previous study of the site and our results form the basis for discussion concerning the issues surrounding Lateglacial Borrobol-type tephras, of which we suggest further review in order to unlock these tephras’ full potential for Quaternary studies.

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• The Laacher See Tephra discovered in southernmost Sweden

  2018. Simon A. Larsson, Stefan Wastegård. Journal of Quaternary Science 33 (5), 477-481

  Artikel

  We present the first geochemically confirmed finding of the Laacher See Tephra (LST) on the Swedish mainland, now the northernmost extension of the LST. Sediments were sampled at the Korslattamossen fen, southernmost Sweden, and a high-concentration cryptotephra occurrence (>65000shardscm(-3)) of the LST was found in a sequence of calcareous gyttja. Tephra identification was confirmed by geochemical analysis using field-emission electron probe microanalysis and through comparison of the results with published LST data from proximal sites and distal sites north-east of Laacher See. The LST has previously been divided into eruption phases suggested to have spread in several dispersal fans, but it was not possible to confidently determine the phase of the tephra here closer than to the MLST or ULST. The finding of the LST presented here further strengthens the potential of tephrochronological studies in the south Scandinavian region.

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