Combustion-related thermal overprinting of pristane/phytane ratios across the Triassic–Jurassic boundary in a continental basin

Pristane/phytane (Pr/Ph) ratios are widely used as proxies for depositional redox conditions, yet their reliability in continental successions subjected to rapid environmental change remains uncertain. Here, anomalously low Pr/Ph ratios across the Triassic–Jurassic boundary were investigated in the Haojiagou section, southern Junggar Basin (NW China), a fully continental, higher-plant–dominated succession lacking evidence for marine influence. High-resolution sampling was combined with total organic carbon (TOC), Soxhlet extraction, compound separation, and GC–MS analysis of saturated and aromatic hydrocarbons, alongside integrated biomarker evaluation of n -alkanes, isoprenoids, terpanes, and hopane-based parameters. A discrete interval associated with Central Atlantic Magmatic Province (CAMP)-related environmental perturbation is characterized by abrupt TOC depletion (<1.0 wt% in most samples), contrasting with organic-rich pre-CAMP strata that locally exceed 10 wt% TOC and reach 52.0 wt%, together with consistently low Pr/Ph ratios (∼0.5–0.8). Quantitative n -alkane parameters show increased Σ n -C 20 − /Σ n -C 21 + ratios and modified carbon preference index (CPI) values, documenting interval-wide redistribution toward lower-carbon-number homologues and attenuation of the higher-plant odd-over-even signature. In contrast, Pr/ n -C₁₇–Ph/ n -C₁₈ relationships and terrigenous/aquatic ratios show no systematic shift toward Type II organic matter, while low gammacerane/C₃₀ hopane and dibenzothiophene/phenanthrene ratios provide little evidence for sustained stratified, hypersaline, or strongly reducing conditions. Hopane-based maturity and rearrangement parameters display coordinated, stratigraphically restricted excursions during the CAMP interval; although these parameters are subject to multiple controls, their behavior together with the quantitative n -alkane changes and overall immature character of the succession is more consistent with transient thermal overprinting than with ordinary progressive burial maturation. Independent increases in inertinite, pyrolytic PAHs, and coronene indices further document enhanced wildfire and combustion activity within the same event interval. Collectively, these observations support combustion-related thermal overprinting associated with CAMP-related environmental perturbation and enhanced wildfire activity as the most plausible explanation for the observed Pr/Ph anomaly. The results demonstrate that transient thermal processes can decouple Pr/Ph from depositional redox, emphasizing the need for multiproxy evaluation of this widely applied biomarker in continental successions affected by rapid environmental change.