Ancient permafrost of the Batagay megaslump (East Siberia) – first insights into chronostratigraphy

Age information from ancient permafrost is key for understanding permafrost formation, stability and decay, and allows for interpreting past climate and environmental conditions over Pleistocene timescales. However, reliable permafrost geochronology is challenging, especially for deposits beyond the radiocarbon dating limit at about 50,000 years before present. The headwall of the world’s largest retrogressive thaw slump at Batagay in the Yana Upland, East Siberia (67.58 °N, 134.77 °E), exposes four generations of ice and sand–ice (composite) wedges that formed synchronously with permafrost aggradation (Opel et al., 2019). The exposed Batagay stratigraphy separates into a lower ice complex that is covered by a lower sand unit, an upper ice complex and an upper sand unit. Two woody beds below and above the lower sand are remarkable (Murton et al., 2017). We apply four dating methods to disentangle the chronology of the Batagay permafrost archive: optically-stimulated luminescence (OSL) dating of quartz and post-infrared-stimulated luminescence (pIR-IRSL) dating of feldspar as well as accelerator mass spectrometry-based Cl-36/Cl dating of wedge ice and radiocarbon dating of organic material (Murton et al., under review). All four chronometers produce stratigraphically consistent and comparable ages. However, OSL appears to date Marine Isotope Stage (MIS) 3 to MIS 2 deposits more reliably than pIR-IRSL, whereas the latter is more consistent with Cl-36/Cl ages for older deposits. The age information obtained so far indicates that the Batagay permafrost sequence is discontinuous. The lower ice complex developed at least 650,000 years ago, potentially during MIS 16 and represents the oldest dated permafrost in western Beringia and the second oldest known ice in the Northern Hemisphere. The age of the overlying lower sand is poorly constrained, indicating formation some time during MIS 16–4. The upper ice complex formed during MIS 4–2 and the upper sand during MIS 3–2, respectively. Thus, the ancient permafrost at Batagay potentially provides one of the longest terrestrial records of Pleistocene environments in western Beringia. Additional sampling for all dating approaches presented here took place in spring 2019, and is part of ongoing research to enhance the geochronology of the exceptional palaeoenvironmental archive of the Batagay megaslump. References Murton, J.B., Edwards, M.E., Lozhkin, A.V., Anderson, P.M., Savvinov, G.N., Bakulina, N., Bondarenko, O.V., Cherepanova, M., Danilov, P.P., Boeskorov, V., Goslar, T., Grigoriev, S., Gubin, S.V., Korzun, J., Lupachev, A.V., Tikhonov, A., Tsygankova, V.I., Vasilieva, G.V., & Zanina, O.G. (2017) - Preliminary palaeoenvironmental analysis of permafrost deposits at Batagaika megaslump, Yana Uplands, northern Siberia. Quat. Res., 87, 314–330 Murton, J.B., Toms, P., Blinov, A., Opel, T., Fuchs, M., Wood, J., Gärtner, A., Merchel, S., Rugel, G., Savvinov, G., & Wetterich, S. (under review) - A multi-method pilot dating study of ancient permafrost, Batagay megaslump, East Siberia. Quat. Res. Opel, T., Murton, J.B., Wetterich, S., Meyer, H., Ashastina, K., Günther, F., Grotheer, H., Mollenhauer, G., Danilov, P., Boeskorov, V., Savvinov, G.N., & Schirrmeister, L. (2019) - Past climate and continentality inferred from ice wedges at Batagay megaslump in the Northern Hemisphere’s most continental region, Yana Highlands, interior Yakutia. Clim. Past, 15, 1443–1461.