Youngest Toba Tuff (YTT) beds as a Late Pleistocene isochron in the Indian subcontinent? New geochronological and sedimentological insights

The ~75 ka Young Toba super-eruption was a catastrophic extreme event that spewed vast quantities of volcanic ash and aerosols and is argued to have caused a drastic volcanic winter, climatic upheaval, and a human genetic bottleneck. The global and regional impacts of the eruption on climate dynamics and past hominin populations are strongly contested due to conflicting global climatic records and archeological datasets. The Indian subcontinent is a key place for Toba research due to its proximity to Mt. Toba (Indonesia). Most of central and peninsular India is estimated to have been covered by a ~5 cm ash layer, with deposits found in different fluvial systems across the subcontinent. For a long time, many of these Youngest Toba Tuff (YTT) deposits, occurring in both archeological and non-archeological contexts, were believed to be primary tephra deposits dating to ~75 ka. However, the use of YTT as a Late Pleistocene chronological marker is surrounded by many uncertainties, primarily due to the complex and varying nature of the depositional and sedimentary contexts of YTT and chronological irregularities. Recent sedimentological and geochronological data indicate that most of the YTT deposits in India are secondary deposits, resulting from later erosion and redeposition occurring thousands of years after the eruption. This highlights the unreliability of YTT as an isochron and raises questions about the credibility of paleoclimate (terrestrial and marine) and archeological datasets that use YTT as a chronological marker. Our study contributes to this debate and reveals the complex redepositional history of YTT deposits in the Indian subcontinent through geochronological and geoarchaeological analyses of YTT-bearing sequences from Motravulapadu, Andhra Pradesh, India. Our findings suggest that the final burial of YTT occurred around 30 ka, highlighting a temporal gap of more than 40 000 years after the eruption. Notably, the redeposited tephra horizon appears visually uniform and compositionally pure, despite its secondary context, adding complexity to its stratigraphic interpretation. Our multiproxy sedimentological approach demonstrates the importance of integrating detailed stratigraphic, magnetic, granulometric, and chronological analyses to resolve such depositional ambiguities; it underscores the need for similar frameworks in future studies of YTT-bearing sequences. We suggest that YTT in India cannot be used as a reliable chronological marker without independent age estimations of its deposition. However, it has significant potential for identifying regional geological events.