FIRST REPORT OF PALEOZOIC ALKALINE LAKE IN NORTHERN PANGEA: A NEW WINDOW FOR LATE PALEOZOIC ICE AGE (LPIA) AND TERRESTRIAL PETROLEUM GEOLOGY

If alkaline lakes can thrive in deep time is enigmatic. This work reports the first case of alkaline lake in the late Paleozoic era globally, which was found in northern Pangea, specifically the ca. 300 Ma Fengcheng Formation of the Junggar Basin, China. It is a rare mega-alkaline lake with an area of possibly up to 270000 km². Particularly, this is the first alkaline lake in the world currently found to host a huge amount of hydrocarbon (typically oil) resources of about 2 billion tons. In addition, the alkaline lake is developed across the late Paleozoic ice age (LPIA), and thus provides an invaluable new window for investigating the Earth science system during the LPIA and terrestrial petroleum geology.

The evidence for the Fengcheng alkaline lake is conclusive, including indicative alkaline minerals such as trona and shortite; complete sedimentary evolution sequence of alkaline lake; and heavy δ¹⁵N values (average ~18‰). The biological compositions of this alkaline lake are relatively simple, while the primary productivity is extremely high. Bio-precursors are distributed in a circular pattern indicative of biotic–environmental co-evolution; the paleo-lake center and margin is mainly composed of abundant haloalkaliphilic Dunaliella and cyanobacteria, respectively. Sporadic red algae cystocarps found in the margin area indicate that this alkaline lake is a "regression residual lake" closed in the Paleo-Asian Ocean. The unique biological and mineral compositions of this alkaline lake lead to the uniqueness of hydrocarbon generation, i.e., "more oil and less gas and two-stage oil generation peaks", which is different from the classic Tissot hydrocarbon generation model. The "saponification reaction" between sodium carbonate minerals and oils reflects the influence of organic–inorganic interactions on hydrocarbon generation. This is a new advance in the field of terrestrial petroleum geology, especially hydrocarbon generation. The Fengcheng alkaline lake sediments have a good response to the LPIA paleoclimate, particularly the microbial methanogenesis has important contribution to global warming. This is of significance for understanding the current global warming and future trends.

In a global perspective throughout geological history, further research is needed to investigate alkaline lake in terms of life, environment, and resource significance.