Paleo-environmental conditions and organic carbon accumulation during glacial events: New insights from saline lacustrine basins

Abstract

Organic carbon accumulation was common during glacial events, but previous studies have focused primarily on marine facies and few have investigated lacustrine facies. This study investigates the sedimentation and organic matter accumulation during the Late Paleozoic Ice Age (LPIA; Late Carboniferous–Early Permian) in the Junggar Basin of China, which was located in a mid-latitude region of Northern Pangea at the time. Organic-rich hydrocarbon source rocks in three sags were examined. Our findings indicate that the Fengcheng Formation (ca. 300 Ma) in the Mahu Sag was deposited in a saline (carbonate content = 40.8 ± 18.4 wt%), sub-oxic–anoxic (total S/Fe [TS/Fe] = 0.5 ± 0.3), and highly alkaline (δ15N = 19.6‰ ± 2.6‰) lacustrine environment. The Lucaogou Formation (ca. 290 Ma) in the Jimusaer Sag was deposited in a freshwater–saline (carbonate content = 28.9 ± 15.6 wt%), oxic–anoxic (TS/Fe = 0.4 ± 0.8), and circum-neutral–highly alkaline (δ15N = 10.7‰ ± 3.2‰) lacustrine setting. The Lucaogou Formation in the Fukang Sag was deposited in a freshwater–saline water (carbonate content = 11.5 ± 4.3 wt%), oxic–sub-oxic (TS/Fe = 0.1 ± 0.1), and relatively neutral pH (δ15N = 7.0‰ ± 0.9‰) lacustrine environment. The source rocks in these three sags represent a completely evolutionary sequence of a saline lacustrine basin, as the saline degrees follow the order of the Lucaogou Formation in the Fukang Sag < the Lucaogou Formation in the Jimusaer Sag < the Fengcheng Formation in the Mahu Sag. The co-evolution of depositional environment, particularly pH, and source rock quality is shown as the abundance of organic matter and hydrocarbon-generating potential first increasing and then decreasing with increasing pH, as the type of organic matter first became increasingly oil prone and then remained unchanged. This reflects the interplay between the bio-precursors, paleo-environment, and organic C accumulation. This may be a common feature of saline lacustrine basins throughout geological time worldwide, such as the Eocene Piceance Basin in USA (the Green River Formation), the Cretaceous Campos Basin in Brazil (the Itapema and Barra Velha formations), and the Eocene saline lacustrine basins in China (e.g., the Shahejie, Qianjiang, and Xiaganchaigou formations). During glacial periods, it is common for lacustrine basins to undergo salinization, which ultimately leads to such basins being ideal for fossil energy exploration. The accumulation of organic carbon drove and responded to the glacial period by sequestrating atmospheric CO2, and was thus an important part of the global carbon cycle that requires further research.