Deep subsurface shifts in microbial processes in Nam Co (Tibet) revealed by multidisciplinary investigations of an ICDP drill core

In the summer of 2024, Nam Co, one of the oldest lakes on the Tibetan Plateau, was the focus of the ICDP NamCore scientific drilling campaign aimed at reconstructing the Quaternary climate history of the region. Within this framework, the SNSF-funded DIGESTED project investigates biosphere-geosphere interactions across the entire lake system, encompassing water column conditions and deep sedimentary records. By integrating sedimentology, lake physics, biogeochemistry, and microbiology, the project seeks to assess the extent to which biological processes influence the sedimentary archive used to reconstruct paleoclimates and understand the ecological trajectory of the lake over the past million years.

We present the biogeochemical results from modern waters, recent and ancient sediments from the drill site. The water column is fully oxidized, with oxic conditions extending  8 cm below the sediment-water interface. Below this zone, microbially produced methane (supported by C and H isotopic ratios) shows a successive increase (0 to 7.8 mmol/L) to a depth of ~80 mblf. Methane is abundant in measurable quantities down to a depth of ~250 mblf, which marks a change in lithology from sand to non-calcareous mud. Biomarker ratios associated with methane cycling indicate a pronounced shift in microbial activity at this depth. Both the GDGT-0/Crenarchaeol ratio and the methane index (Zhang et al., 2011) increase sharply at and below 200 m, consistent with limited methanogenesis and methanotrophy above this boundary and substantially more active microbial processes below, despite the absence of detectable methane. This transition also coincides with changes in the composition of preserved and extractable subsurface microbial DNA. Our 16S rRNA gene sequence analyses reveal communities associated with fermentation and C1-based metabolisms below 200 m, whereas sediments above this depth are dominated by archived or transported taxa that are rarely active in such anoxic sedimentary environments.

With this study, we begin to piece together how microbial processes and their suppression, fluid migration, and paleoenvironmental conditions collectively shape the integrity of this climatic archive. A pronounced lithological and biogeochemical boundary at ~200 m separates a likely once-active methane cycling system from an overlying, energy-limited deep biosphere that permits methane accumulation and slow diffusive transport toward geological boundaries. Our ultimate goal is to disentangle the paleoenvironmental conditions leading to such strong shifts by coupling an age model with sedimentological, chemo-physical, and biological characterization of those archives.

 

Zhang, Y. G., Zhang, C. L., Liu, X.-L., Li, L., Hinrichs, K.-U., & Noakes, J. E. (2011). Methane Index: A tetraether archaeal lipid biomarker indicator for detecting the instability of marine gas hydrates. Earth and Planetary Science Letters, 307(3), 525–534. https://doi.org/10.1016/j.epsl.2011.05.031