Towards a new Late-glacial to early Holocene varve chronology for the Swedish Timescale using microfacies analysis and tephrochronology

The Swedish Timescale (STS) is a composite varve chronology that spans the last ca 14,000 years and comprises over 1,000 individual varve thickness records. It has primarily been used to reconstruct the deglaciation of the Fennoscandian Ice Sheet but has also been used to derive age estimates for Baltic Ice Lake drainage events. Several researchers have suggested that there may be up to 900 varve years missing during the Lateglacial and Holocene periods. This research presents the results of microfacies analysis and tephrochronology from three sites in Östergötland to test the existing regional varve chronology and timing of deglaciation. Comparison between macro and microscale analyses of varves indicate that the analytical precision of µXRF, X-radiograph and macroscale sedimentology do not allow for accurate characterisation of sediments. Up to 40% more varves were identified under thin section than from macroscale counts which is the traditional means of varve analysis within the STS. Microfacies analysis enabled reconstruction of local sediment processes that are fundamental for the construction of accurate varve chronologies. These results suggest that there may be fundamental flaws within the STS that stem from a lack of detailed sediment descriptions and sediment process information. The identification of tephra at all three sites demonstrates the potential to independently correlate and date varve records within the STS. Bayesian age modelling integrates the varve count data and tephra ages and has: 1) enabled more precise age estimates of the tephras; 2) anchored the varve chronologies to a calendar-year timescale; and 3) provided calendar year estimates for deglaciation. The identification of the Askja-S tephra within glacial varves at Asplången indicates that the site became ice-free around 11,044±43 cal. yrs BP which is in disagreement with the published Younger Dryas age for site deglaciation. Critically, this suggests that the Baltic Ice Lake existed during the early Holocene. The presence of proximal varves in Östergötland during the early Holocene suggests that the ice margin was in the Middle Swedish End Moraine Zone at its pre-drainage position at Mt Billingen during this time. Therefore, it is proposed that the final drainage of the Baltic Ice Lake occurred sometime after 11,044±43 cal. yrs BP. These results do not support the current timing for the final Baltic Ice Lake drainage which at present is dated to the Younger Dryas-Preboreal transition ca 11,500 cal. yrs BP. If this inference is correct, the final Baltic Ice Lake drainage may have impacted early Holocene climatic cooling events.