Organic molecular evidence in the ∼1.40 Ga Xiamaling Formation black shales in North China Craton for biological diversity and paleoenvironment of mid-Proterozoic ocean

Abstract

Indigenous hydrocarbon biomarkers in the black shales of the 1.40 billion-year-old Xiamaling Formation (XML) from the North China Craton were systematically analyzed to reveal the redox condition and biological community. These black shales are not only rich in organic matter, but also undergone an unexpectedly low thermal evolution process, both of which contribute to well-preserved indigenous hydrocarbons and promote the convincing discovery of the Precambrian biology. The combination of the absence or low concentrations of gammacerane, dibenzothiophene series and aryl isoprenoids, abundant rearranged hopanes, the occurrence of micro-ripples, and low level of redox-sensitive trace elements suggested shallow, low salinity and suboxic to anoxic conditions during the deposition period of XML. The absence of aryl isoprenoids as diagnostic biomarkers for green/purple sulfur bacteria strongly argued against the pervasively euxinic (sulphidic) bottom water in mid-Proterozoic ocean. The scenario of large hump unresolved complex mixture and abundant monomethyl alkanes as well as absent steranes may be derived from benthic microbial reworking processes for biolipids. Here the absence of saturated and aromatic steroids in the XML sediments suggested that eukaryotic algae have little ecological contribution to primitive biomass and/or are the result of heterotrophic reworking from benthic microbial mats. Abundant presence of hopanoid, tricyclic terpane and n-alkane in these samples supported the predominant contribution of (cyano) bacteria.