Tephrochronological constraints indicate a MIS 7 age for the middle Pleistocene glacial maximum in far northeastern Asia

Large-scale glacial deposits are widespread across far northeastern Asia; however, their ages remain poorly constrained, limiting understanding of the glacial history of Western Beringia and its relationship to other Northern Hemisphere glacial records. A key challenge is correlating glacial deposits from sites separated by hundreds of kilometers. Geochemically characterized tephras provide an opportunity to link glacial deposits from different localities and, in addition, to correlate onshore glacial records with marine paleoenvironmental archives and the LR04 oxygen-isotope curve. We present new tephrochronological constraints on glacial deposits from Western Beringia, including the Kamchatka Peninsula and the northeastern Asian mainland, and correlate these records with marine sediment cores using geochemically fingerprinted ash layers.
The Kamchatka Peninsula is located on the northeastern edge of Eurasia and is bordered by the Okhotsk Sea to the west and the Pacific Ocean and Bering Sea to the east. Kamchatka hosts two volcanic belts that were active throughout the Quaternary and produced large explosive eruptions, dispersing tephra over hundreds of kilometers. A series of tephra-bearing sediments, including glacial till, is exposed along the right bank of the Kamchatka River, which marks the maximum known advance of the Sredinny Range glaciers, approximately 100 km from the source area. The lowermost exposed unit of the sequence was dated to 350-250 ka, providing a minimum age constraint for the overlying till. Five tephra layers bracketing the till were correlated with tephras found in four marine cores from the Okhotsk Sea and the Pacific Ocean. Based on the core age models, the tephra sequence spans 230 to 177 ka BP, corresponding to MIS 7d – 6d. The till at its most distal position is sandwiched between ~230 ka and 205 ka old tephras indicating that the glacier reached its maximum extent during MIS 7d. The youngest ~177 ka old Rauchua tephra overlies the till closer to the source area, suggesting a slow glacier retreat toward MIS 6d.
The same ~230 ka tephra is preserved between regionally extensive glacial till and overlying glaciofluvial deposits on the northeastern Asian mainland, about 750 km northwest of the Kamchatka sites. This indicates a synchronous glacial advance and presence of large ice masses across vast areas of far northeastern Asia during MIS 7d. Based on published data, these deposits represent the most extensive glaciation in the region, whereas younger MIS 6 glaciations were confined to mountainous areas. Our findings challenge the conventional view of MIS 7 as a warm interglacial and underscores the significance of MIS 7d as a major cold stadial within the Penultimate Interglacial.