Understanding the origin of Cambrian marine oncoids: A petrologic, geochemical, and isotopic perspective

Exquisitely exposed Cambrian carbonates with exceptional thickness in North China offer a unique opportunity for studying the biota evolution and Earth's history. Here we characterize Cambrian oncoids by integrating petrological, geochemical, and isotopic analyses in order to unravel their genesis, assess the role of detrital fractions, and reconstruct the paleoenvironment of these ancient strata. The presence of nanosphere, microbial fossils, and relics of extracellular polymeric substances enables us to infer an important role of the microbial consortium in the origin of oncoids. These interpretations are also reinforced by fluorescence microscopy, ultraviolet excitation, and Raman spectral signature. Likewise, the depleted values of δ¹³C isotopes (−1.11 to −0.46 ‰) strongly support a significant input of microbial relics in the formation of oncoids. The geochemical modeling of trace and rare earth elements (REEs) advocates for oxygenated conditions in surface waters in North China during the Cambrian. Geochemical and isotopic data also reveal that oncoids are influenced by siliciclastic detrital contamination and stress the negligible role of diagenetic processes. These results provide evidence that oncoids can represent a reliable proxy of palaeoceanographic changes in the early history of Earth.