OXYGEN MINIMUM ZONE-LIKE CONDITIONS AT CHENGJIANG

The chemical conditions that prevailed in early Cambrian marine environments and their relationships to early metazoan biotas have not been well documented. Here, we present multi-proxy geochemical data from two drill cores through the early Cambrian Yu’anshan Formation of Yunnan, China, which contains the Chengjiang biota, the most diverse Cambrian fossil assemblage known. In both cores, the interval bearing exceptionally preserved fossils is associated with geochemical data suggesting a stratified water column and extensive denitrification under oxygen-depleted conditions, similar to that of modern oxygen minimum zones. The stratigraphic succession suggests that after an interval of euxinic conditions, marine regression combined with decreasing carbon flux drove the nitrogenous conditions. Given the likely lower atmospheric oxygen concentrations during the Cambrian period, the water column would have been more susceptible to oxygen depletion than today. Thus, areas where nutrient supply was sufficient to stimulate high export of primary production would also have been higher-risk areas for early Phanerozoic ecosystems dominated by benthic fauna. Therefore, we argue that a more stable environment for these ecosystems would be in oligotrophic domains of the ocean and that the radiation of animals occurred in the well-mixed and relatively shallow oxic waters. As observed at Chengjiang, this habitable zone was separated from deeper anoxic waters as an ocean prone to stratification progressively flooded continental interiors throughout the Cambrian period.