The Early Cretaceous Aptian stage was marked by a major carbon cycle disturbance(OAE1a) on a global scale^[1], which exerted profound impacts not only on the marine but also terrestrial environment and ecosystem. In this study, core samples were obtained from the Jiufotang Formation strata in the Kazuo Basin, western Liaoning Province (YSDP-4 borehole). The age of the strata is 121-112 Ma revealed by U-Pb dating^[2]. The Third Member of the Jiufotang Formation corresponds to the Aptian perturbation of the global carbon cycle C1-C7^[2]. By analyzing lipid biomarkers and their compound-specific carbon isotopes in Third Member mudstones and shales, we investigated the paleolake productivity and revealed the effects of micro-biological activities on organic carbon (OC) cycling and burial.
Abundant C_16-25 regular acyclic isoprenoid alkanes including pristane (Pr) and phytane (Ph) and an irregular acyclic isoprenoid alkane squalane were detected in the Third Member samples. The n-alkane distribution is dominated by nC₁₇, and the average value of Pr/Ph is 0.9. Notably, the compound-specific carbon isotopes of these lipid biomarkers exhibit significantly more positive values than those in the Second Member samples. The Short-chain n-alkanes (nC₁₃-₂₀) exhibit an average δ¹³C value of -28.6‰, while mid-chain homologues (nC₂₁-₂₆) are approximately 2‰ lower. Notably, acyclic isoprenoid alkanes demonstrate ¹³C-enrichment, with δ¹³C values reaching up tp -20‰.
Previous studies indicated that the Third Member of the Jiufotang Formation was deposited under a hot and dry climate^[3] which intensified the lake water evaporation, resulting in elevated salinity and favoring an anoxic and reducing deposition condition. The relatively high Pr/Ph ratios indicate that Pr and Ph in this interval are not solely sourced from photosynthetic organisms containing chlorophyll a. Highly positive δ¹³C values for these long-chain isoprenoid alkanes further suggest vigorous microbial activity, especially halophilic archaea and bacteria adapted to high salinity and elevated temperatures ^[4]. Although the Third Member exhibited favorable conditions for organic matter preservation, the overall TOC content was relatively low. This discrepancy suggests that while favorable conditions exist, other factors may limit OC accumulation. Microbial activity likely plays a critical role in regulating OC burial and perturbations of the OC cycle, particularly through processes such as anaerobic respiration that enhance carbon remineralization and influence net productivity.


Acknowledgments
This research was supported by Guangdong Natural Science Foundation of China (Grant No. 2025A1515010356) and the National Natural Science Foundation of China (Grant No. 42288201).

References
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[2] SUN M-D, LIN Q, et al. Terrestrial ecosystem response to Early Cretaceous global environmental change: A calibrated, high-resolution Aptian record from Northeast China [J]. Earth and Planetary Science Letters, 2025, 653: 119206.
[3] LU C, LIN M-Q, et al. A continental record of Early Cretaceous (Aptian) vegetation and climate change based on palynology and clay mineralogy from the North China Craton [J]. Palaeogeography, Palaeoclimatology, Palaeoecology, 2025, 662: 112750.
[4] SAITO R, KAIHO K, et al. Predominance of archaea-derived hydrocarbons in an Early Triassic microbialite [J]. Organic Geochemistry, 2015, 85: 66-75.
 

Microbial activity,organic carbon burial,terristrial OAE1a,Kazuo Basin in NE China