Tephrochronology of the Last Interglacial in Northwest Europe

Understanding the timing of natural climate system feedbacks (i.e. sea-level rise) during the Last Interglacial (c. 130 to 115 ka BP) is crucial for refining model predictions of near future warming. However, Last Interglacial archives in Northern Europe are particularly challenging to date because they lie beyond the upper age limit of radiocarbon dating, and they lack anchored chronologies that extend to present day. A recently published reinvestigation of the annually varved Bispingen Palaeolake record (Lauterbach et al. 2024) suggested that the Last Interglacial actually lasted 4000 years longer than previously thought, as estimated using pollen zones. This study revisits the composite sediment succession in search of tephra, an independent dating and correlating method frequently used in Holocene-age sediments. This study first confirms the presence of cryptotephra shards in the Bispingen record, extracted from 70 5-cm thick low-resolution samples and from 29 1-cm high-resolution samples using a standard density separation technique (Blockley et al. 2005). This is followed by the discovery of five major tephra horizons, which were identified by their distinct shard concentration maxima. To understand the geochemistry of the tephra, the study also presents the results of Electron Microprobe Analysis from two tephra peaks. A rhyolitic population composed five tephra shards shows a distinct total high alkali signature (>10 wt%) that cannot be matched to any known Last Interglacial eruptions documented in North Atlantic marine cores. Total alkali and silica oxide comparison suggests that some trachytic shards may be associated with unknown Jan Mayen and Icelandic volcanism, although they differ in other Major element-oxide ratios. More Last Interglacial tephra studies in Northern Europe are needed to harness the chronostratigraphic potential of the tephra horizons found in this study, as linking sites together on the basis of a geochemically common tephra isochron will help constrain the regional timing of climate responses.