40Ar/39Ar dating of the Thorsmork ignimbrite and Icelandic sub-glacial rhyolites

Abstract

Robust correlation of paleoclimatic archives (e.g. marine sediments, ice, speleothems) necessitates an increase of the number of independent and accurately dated stratigraphic markers using numerical methods. Icelandic rhyolitic volcanism has a strong chronological interest because these products are found as tephra in the North Atlantic marine archives as well as in Greenland ice cores. In this study, we have dated, using the 40Ar/39Ar method, subglacial rhyolites of the Tindfjallajökull, Kerlingarfjöll and Torfajökull volcanoes which are the main sources of rhyolitic tephra identified in marine and glaciological records.The Thorsmork ignimbrite (Tindfjallajökull) which correlates with the rhyolitic component (II-RHY-1) of the North Atlantic Ash Zone II (NAAZ II) layer is dated at 55.6 ± 2.4 ka, an age that agrees with the ones given by the ice core chronologies (GICC05: 55.4 ± 1.2 ka). The eruptions of Rauðfossafjöll and Fannborg are respectively dated at 77.0 ± 3.0 ka and 149.3 ± 3.4 ka.These new ages along with the published ones confirm, as already concluded by Flude et al. (2010) that there is no clear evidence, at least at the latitude of Iceland, that climate warming is the main mechanism that triggered rhyolitic eruptions.Combining major and trace element compositions with our new age determinations, it appears that the Rauðfossafjöll and the Snaefell rhyolitic eruptions have not yet been discovered in marine sediment cores. Nevertheless, given the accuracy and precision of our dating, these rhyolitic eruptions have the potential to become chronostratigraphic markers, once tephra shards corresponding to these eruptions are discovered in ice and or marine sediment records. The Fannborg eruption might be recorded in the marine core LINK 16 during MIS 6 prior to the transition to MIS5e.