Environmental controls on Early Cambrian macroevolution: Insights from the Tarim Basin, Northwest China

The terminal Ediacaran to lower Cambrian (538–521 Ma) documents the disappearance of Ediacaran soft-bodied biota and the diversification of early animals, including the emergence of small shelly fauna, archaeocyath sponges and trilobites. Despite extensive study, the role of oceanic oxygenation in macroevolutionary events across this interval remains unclear, with understanding hindered by limited constraints on temporal and spatial variability in geochemical conditions, both regionally and globally. Here, we report multi-proxy geochemical data (including organic carbon concentrations, carbonate and organic carbon isotopes, Fe speciation and redox sensitive trace elements) from three sections documenting different water-depths through the early Cambrian (Terreneuvian) Yurtus Formation of the Tarim Basin, North China. Our data reveal a highly dynamic oxygen minimum zone (OMZ), distinguished by a core of unstable ferruginous to dysoxic conditions, with peripheral dysoxic to oxic conditions that developed on the inner-outer shelf at different stages. The temporal and spatial extent of the OMZ appears to have been controlled by changes in productivity, driven by sea-level and climatic influences on upwelling. We expand on regional observations by considering published geochemical data from globally distributed successions across this interval, in addition to the lowest global occurrence of key fossil taxa. Our integration of regional and global geochemical datasets, alongside mechanistic insight from regional and global stratal stacking patterns, suggest that marine redox fluctuations responded dynamically to changes in upwelling driven by major sea level transgression and climate. These connected processes and palaeoenvironmental conditions formed the backdrop for the main phase of the Cambrian explosion.