Organic compositions of the embryo-bearing lowermost Cambrian Kuanchuanpu formation on the northern Yangtze platform, China

We investigated the origin of organic matter and paleoceanographic conditions of the embryo-bearing lowermost Cambrian Kuanchuanpu Formation in Shaanxi Province, China, in terms of sedimentary facies, fossil morphometry, and organic geochemistry, including biomarker compositions. The results indicated the abundance of algae and eukaryotic phytoplankton that fueled a diverse ecosystem including cnidarians and small shelly fossils. The water column contained enough oxygen to provide habitat for benthic cnidarians such as Olivooides. On the other hand, low pristane/phytane ratios, detection of squalane, and hopane composition indicate reductive conditions within the sediments, and that the redox front was likely near the water-sediment interface. Common occurrences of pyrite and barite indicate that redox reactions of sulfur species were activated at the water-sediment interface. Such circumstances and the high phosphate concentration in ambient water may recall the presence of sulfur-oxidizing bacteria that promoted the phosphatization of animal embryos, as suggested for in the Neoproterozoic Doushantuo Formation. Nitrogen was the limiting nutrient for primary productivity, and the high phosphate concentration in the ocean promoted the phosphatization of animal embryos at the reductive water–sediment interface.