Diagenetic and detrital origin of moretane anomalies through the Permian–Triassic boundary

Abstract

Many biogeochemical anomalies coincide with the Late 1 Permian Extinction (LPE; 252.28 Ma). Several mechanisms have been proposed to explain the moretane/hopane anomaly that has been identified in samples from Meishan GSSP section in southeastern China. Here, we report homohopane, 2α- and 3β- methylhomohopane and lithological data for a drill core from the Meishan section in southeastern China. Three intervals of elevated C30 moretane/hopane ratios are recorded in the Lungtan, Yinkeng and Helongshan Formations. Moretane/hopane ratios of C31-34 homohopanes and the 2α- and 3β-methylhomohopanes display the same stratigraphic patterns as the C30 moretane/hopane record. In light of the multiple and parallel moretane anomalies for the homohopane and 2α- and 3β-methylhomohopane series, enhanced input from higher plant organic matter, such as coal and peat, does not adequately explain the observed isomer patterns. Correlation of high moretane/hopane ratios with low C35 HHI and high hopane/sterane values suggest increased input of hopanoids from oxic soils. Additionally, moretane/hopane ratios show excellent correlations with total clay percentages and specific clay types, particularly chlorite, illite, and mixed layer illite/smectite. We conclude that a combination of episodic hopanoid input from soil bacteria and diagenetic effects related to redox and detrital clays generated the unique moretane/hopane patterns at Meishan. Similar relationships of Ts/(Ts+Tm) with redox and source indicators and lithology indicate that Ts/(Ts+Tm) is affected by the same factors controlling the moretane/hopane ratios. Berthierine, a clay that requires reducing conditions for formation, was detected in samples from the Lungtan Formation. We are unable to determine from our results whether the berthierine is authigenic or detrital, but future determination of the origin of berthierine at Meishan may offer additional environmental insight. No link between diasteranes and lithology was observed in this study suggesting that diasteranes are relatively unaffected by the detrital clay component of the Meishan sediments. In total, the results point toward the complex role of source input, lithology, and depositional redox conditions in the transformation of organic matter during maturation. Future work is required to elucidate the lithological effects on diagenetic processes, including biomarker genesis isomerization, and thermal degradation.