A sedimentological characterisation of a 510 m lacustrine sequence recovered from the ICDP Nam Co Drilling Project (NamCore), Tibet

The Indian and East Asian Summer Monsoons are key atmospheric controls on hydrological variability across the Tibetan Plateau. This region, often referred to as the Asian Water Tower, provides freshwater resources for approximately two billion people via the major river systems that originate there. Consequently, reliable hydrological projections require improved constraints on the timing, duration, and magnitude of climate variability, and an understanding of the environmental responses to this change, in this climatically sensitive high-altitude region, particularly over long geological timescales.

Nam Co is one of the largest and deepest lakes on the Tibetan Plateau (4,718 m a.s.l.; 2,020 km² surface area; 98.9 m max. water depth; 10,680 km² catchment area) and is an exceptional archive for investigating long-term climatic and environmental variability. To reconstruct past long-term climate variability and to examine its impacts, the lake was chosen as a target for ICDP drilling (NamCore) to recover a long, continuous record, with the aim of examining paleoenvironmental evolution, geomicrobiology, tectonics, and paleomagnetism. During the field operations conducted between May and July 2024, a total of 1415.45 m was drilled and 1175.99 m cored with 950.77 m of sediment recovered (core recovery of 80.8 %). Comparisons between magnetic susceptibility from (i) borehole logging, (ii) whole-round sediment cores, and (iii) core catcher material show widely similar trends across the depth dimension, suggesting a highly accurate depth control of the drilling depths recorded.

Here, we present an overview of the project and sedimentological perspectives on the recovered sequence based on an integrated dataset comprising (i) core catcher material, (ii) sediment core samples and (iii) spectrophotometer and magnetic susceptibility measurements supported by (iv) detailed lithostratigraphic descriptions. The NamCore sedimentary succession is subdivided into five lithological units, defined by variations in (i) carbonate content and carbonate mineralogy, (ii) grain-size distributions, (iii) colour (likely related to iron speciation), and (iv) frequency-dependent magnetic susceptibility. The succession is characterised by recurring transitions among four lithofacies: (i) calcareous mud, (ii) ferric-stained calcareous mud, (iii) fine- to medium-grained sand, and (iv) non-calcareous mud. These alternations are interpreted to reflect major climatic and/or environmental changes, including variations in water-column, redox conditions, sediment accumulation rates, fluctuations in the extent of surrounding mountain glaciers, and broader-scale shifts in atmospheric circulation over the Tibetan Plateau associated with glacial–interglacial climate variability, changes in Pleistocene climate boundary conditions and related changes in catchment processes. In particular, colour shifts (green/red, a*) may represent shifts between warmer and wetter to colder and drier climatic conditions, whereas variations in frequency-dependent magnetic susceptibility likely reflect changes in pedogenesis, both of which might be linked to large-scale hydroclimatic forcing.