Abstract
Previous studies of marine sequences from the Faroe Islands region have identified a series of coarse-grained tephra horizons deposited during Marine Isotope Stage (MIS) 5. Here we reassess the MIS 5 tephrostratigraphy of the Faroe Islands region and focus on the cryptotephra deposits preserved within the fine-grained fraction of marine core LINK 16. We also extend the record to encompass the late MIS 6 and early MIS 4 periods. A density separation technique, commonly used for tephra investigations in lacustrine settings but rarely applied to marine sediments, is utilised to explore the fine-grained material and EPMA and LA-ICP-MS are employed to determine the major and trace element composition of individual tephra shards. In total, 3 basaltic and 3 rhyolitic Icelandic cryptotephra deposits with homogeneous geochemical compositions are identified — all of which have the potential to act as isochronous tie-lines. Geochemical results highlight that the Grímsvötn volcanic system of Iceland is the predominant source of the basaltic horizons and the Öraefajökull or Torfajökull systems are the likely sources of the rhyolitic deposits. Three of the horizons have been previously recognised in Faroe Islands region marine sequences, with two of these deposits traceable into a Norwegian Sea sequence. An early MIS 4 rhyolitic horizon is the most widespread deposit as it can be traced into the Norwegian Sea and to the south into a record from the Rockall Trough. Basaltic and rhyolitic horizons deposited during late MIS 6 have not been recognised in other sequences and represent new additions to the regional tephrostratigraphy.
Highlights
Atlantic Dansgaard–Oeschger (DO) events that relate to the warm Greenland Interstadial (GI) events recorded in the Greenland ice-core records (Fig. 2i; e.g. Rasmussen et al, 1996; North Greenland Ice Core Project Members, 2004)
The three cryptotephras added to the regional tephrostratigraphy (LINK 16 425–427 cm, 735–737 cm and 738–739 cm) resulted from extending investigations beyond Marine Isotope Stages (MIS) 5 into late MIS 6 and early MIS 4
We have shown that it was most likely deposited at all the sites via iceberg rafting, which precludes its use as a widespread isochronous horizon but it is useful as a regional marine– marine tie-line and may help in the reconstruction of past ocean surface circulation (e.g. Kuhs et al, 2014)
Summary
Tephrochronology and the synchronisation of palaeoclimatic sequences. Tephrochronology provides a powerful technique for the correlation and synchronisation of palaeoclimatic sequences through the tracing of common volcanic ash horizons that represent isochronous tie-lines. This is especially valuable for marine sequences that fall beyond the radiocarbon window as independent time-scales cannot be constructed (Austin and Hibbert, 2012). Tephras have the potential to resolve the relative timing of rapid atmospheric and oceanic climate changes preserved within ice and marine cores. During Marine Isotope Stages (MIS) 6–4 significant climatic re-organisations occurred in association with the transitions into and out of the last interglacial period (MIS 5e, Eemian).
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