Molecular and carbon isotopic evidence of pigments indicating a dynamic oceanic chemocline 1.4 billion years ago in north China

Abstract The redox state of the marine photic zone is crucial to the evolution of life, which is further linked to the degree of oxygenation of the atmosphere and ocean systems. Hydrocarbon molecular fossils (biomarkers) and isotopic fingerprints of anoxygenic photoautotrophs can provide direct evidence for seawater redox conditions within the photic zone. In this study, we identified a series of 2,3,6-trimethyl aryl isoprenoids (2,3,6-TMAI) from low-maturity sedimentary organic matter in the Xiamaling Formation (XML), deposited 1.4 billion years ago in Xiahuayuan area of north China. The δ13C values of C18 and C19 TMAI ranged from –19.8‰ to –21.3‰. These 13C-enriched values indicate their origin in anaerobic photosynthetic green sulfur bacteria (GSB). The episodic enrichments of 2,3,6-TMAI in the XML Formation suggest dynamic marine geochemistry. Specifically, low concentrations of 2,3,6-TMAI at the bottom of the studied profile reflect a shoal large scale oxygenic deposition environment (unit 4), where the organic matter was almost all decomposed in the oxic water column during deposition, while the gradually increasing 2,3,6-TMAI concentrations from the middle to the upper XML reflects alteration of oceanic redox condition from an oxygen minimum zone (OMZ) in unit 3 through periods of euxinia during the deposition of units 2 and 1. The absence of co-variation between total organic carbon (TOC) and enriched 13C values indicates that anoxygenic photoautotroph activities might have contributed little to sedimentary organic matter burial in the Mesoproterozoic XML sediments.