Assessing the Potential of GTGT for Tracing Post-Impact Hydrothermal Activity: A Case Study from the Hapcheon Impact Crater, South Korea

Impact-generated hydrothermal systems are regarded as prime environments for habitability and biosignature preservation on early Earth and other planetary bodies. However, the reliability of lipid biomarkers for reconstructing such environments in dynamically evolving crater lakes remains poorly constrained. Here we investigate the distributions of glycerol dialkyl glycerol tetraethers (GDGTs) and glycerol trialkyl glycerol tetraethers (GTGTs) in lacustrine sediments from the Hapcheon impact crater, South Korea, to evaluate their utility as tracers of post-impact hydrothermal evolution. Radiocarbon ages indicate deposition during the late Pleistocene (~35–50 ka) within a highly unstable crater-lake system characterized by frequent slumping, soft-sediment deformation, and sediment remobilization. GTGT abundances closely track independent hydrothermal indicators, including calcite enrichment, and record the most intense hydrothermal conditions in the lowermost sediments, followed by a progressive long-term decline and sporadic persistence in younger intervals. In contrast, crenarchaeol shows repeated anti-covariation with GTGT, indicating transient shifts between hotter thermophilic habitats and cooler mesophilic conditions. These opposing trends suggest that the Hapcheon biomarker record captures both gradual post-impact cooling and short-lived environmental perturbations linked to basin-margin instability and water-column mixing. By contrast, conventional GDGT-based indices such as RI, RI-OH′, MBT and CBT display weak or inconsistent relationships with hydrothermal signals, particularly in disturbed intervals. We attribute this decoupling to hydrothermal overprinting, mixed microbial sources, and recurrent sedimentary reworking, which violate the ecological and depositional assumptions underlying standard GDGT calibrations. GTGT therefore emerges as a more sensitive and robust tracer of hydrothermal activity in extreme post-impact lake settings. These findings provide new insights into biomarker behaviour in impact-generated environments and demonstrate that proxy performance depends strongly on environmental stability. More broadly, GTGT may offer a valuable molecular tool for detecting hydrothermal niches and assessing habitability in ancient terrestrial and extraterrestrial crater systems.