Scientific Drilling on the Third Pole: achievements of the highest ICDP lake drilling project on the Tibetan Plateau (Nam Co, 4718 m.a.s.l)

The Tibetan Plateau (TP), often referred to as the “Third Pole” and “Asian Water Tower”, serves about two billion people downstream with its water resources; thus, investigations of past climate changes on the TP have significant socio-economic implications for both the scientific community and governmental concerns. Numerous lakes on the plateau provide valuable archives to carry out paleoenvironmental change studies on different time scales by drilling sediment cores. With the support of the ICDP (NamCore project, Expedition 5073) and other funding, we accomplished a drilling campaign in a high-altitude, deep lake (Nam Co, 4718 m) on the Tibetan Plateau in the summer of 2024. In total, ~950 m of cores was recovered from seven holes at one site, with a deepest drilling depth of 510 m b.l.f., making NamCore a great success among ICDP lake drilling projects in the past several decades with respect to its altitude and maximum penetration depth. These achievements enable us to study past climate changes in this area potentially back to ~1 Ma and their linkages with other regions globally. Three core opening and sampling parties of the NamCore project have been organized in Beijing, where the cores were stored, to complete the splitting of all cores. Core descriptions, magnetic susceptibility scanning of the entire sequence combined with other analyses (e.g., grain size, organic/inorganic carbon content, biomarkers and pollen, etc.) on core catcher samples have revealed sediment variations, which can distinctly show the fluctuations between glacial and interglacial cycles, although the chronology using various approaches is still challenging. The results show four major lithologies throughout the drilled cores including calcareous mud, non-calcareous mud, sand and calcareous mud with ferric staining. Calcareous mud dominates the upper ~120 m and ferric-stained mud mainly appear in the sections deeper than ~320 m. Many sand layers with different thickness occur in the entire sequence but mostly in the middle part. Nothing has been retrieved in a section greater than 30 m in thickness in the lower part, which probably indicates a remarkable change in the sedimentary environment associated with a glacial period. Time series analysis on the magnetic susceptibility data shows two prominent cycles at 10.1 m and 21.4 m, which potentially correspond to the orbital precession and obliquity forcing of 21 ka and 41 ka, respectively. This cyclostratigraphic approach will be helpful to constrain the chronology and, by comparison with stalagmites in monsoonal areas and ice cores in polar regions, plays an important role in discovering the different drivers of climate change from low and high latitudes. However, more efforts are still needed to obtain absolute ages to establish a precise timeframe for these cores.