Absence of β-carotane as proxies of hydrothermal activity in brackish lacustrine sediments

β-Carotene, the precursor of β-carotane, is commonly rich in brackish sediments and would become unstable theoretically under the exposure to high temperatures, bright light conditions, and acidic or oxdizing environments. However, the specific process and mechanism of the absence of such compounds are still not clear sufficiently. To fill the knowledge gap, in this study we explore the origin of the absence of β-carotane in a 50-m-thick section of the Middle Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin, northwest China, based on a combined mineralogical, petrographic, organic and inorganic geochemical study. Biomarker analyses indicate that the β-carotane-free section was deposited in a brackish, reducing, and semi-deep lacustrine environment. Hydrothermal fluid effects during sediment deposition are inferred to have caused the breakdown of β-carotene. The hydrothermal (<100 °C) effects are evidenced by mineralogy and geochemistry, e.g., the hydrothermal-indicative minerals (reedmergnerit, tuff and pyrite), high Fe/Ti and low Al/(Al + Fe) ratios, rare earth elements (REEs) distribution. Associated biomarker variations were found, e.g., low i-alkanes/n-alkanes, high C24Tetracyclic/C26Tricyclic terpanes, high (C19 + C20)/(C21 + C23) tricyclic terpanes, high C29/C30 hopanes, and very low gammacerane/C30 hopane ratios. Our data suggest that hydrothermal fluids can prevent the preservation of β-carotene, and the absence of β-carotane indicates a period of hydrothermal activity during deposition of the Lucaogou Formation in a 50-meter-thick sequence.