Termination Ii Climate Benchmarks from Zhangjia Cave Speleothems as a Potential Upper Pleistocene GSSP

The ice-age Termination II (T-II, ~130 kyr) is a critical interval for understanding glacial–interglacial transitions and, at the same time, an important geochronological benchmark for potentially defining the beginning of a new epoch, the Upper Pleistocene. Following the establishment of the working group for the Upper Pleistocene GSSP (Global Boundary Stratotype Section and Point) in May, 2025, formal evaluation of candidate boundary section is now underway. A number of potential GSSPs have been proposed, including sections from marine sediments, ice cores, Chinese loess, and speleothems, each offering distinct advantages and limitations. Notably, among these archives, speleothems stand out because they can combine high temporal-resolution (up to seasonal) with precise absolute chronology based on U-Th dating technique, providing a level of chronologic control essentially unparalleled compared to other paleoclimate archives. To date, the most precisely dated, high-resolution δ¹⁸O records across T-II come from Zhangjia Cave in Guangyuan, southwestern China. Fast growth and high uranium concentration of Zhangjia speleothems yield ~2–3-year temporal-resolution and age uncertainties of ±350 years around T-II, which can be potentially improved by a few folds. The most significant hydroclimate transition in Zhangjia records that defines T-II started at 129.8 ± 0.5 kyr BP and persisted for approximately 300 years. This rapid transition is accompanied by a visible change in the color of the speleothems from dark brown to light yellow, and followed by a more gradual adjustment lasting over ~3 kyr. Three additional centennial-scale benchmarks surrounding T-II are also clearly expressed, with peaks at approximately 133.4, 134.3, and 136.3 kyr BP. These four benchmarks provide a robust framework for correlating Zhangjia speleothem records with North Atlantic marine sediments/speleothems and Antarctic ice cores, resolving the global expression of T-II with unprecedent clarity and providing a strong basis for defining the Upper Pleistocene GSSP. A suite of coeval speleothems from the same cave is also available, including specimens that exhibit annual-fluorescent-layers offering the prospect of refining the existing relative chronology to even near-annual precisions. In contrast, although Chinese loess deposits offer abundant environmental proxies, their low-resolution (typically hundreds of years), large chronological uncertainty (~5 kyr), and overprinting by pedogenesis/diagenesis processes preclude a precise and decisive correlation with Zhangjia speleothem records.