Sulfate-driven phosphorus recycling and the collapse of lacustrine ecological diversity in the mid-Triassic

The Middle Triassic Ordos Basin witnessed the earliest rehabilitation of complex lacustrine ecosystems after the end-Permian mass extinction (EPME). The specific challenges faced by freshwater ecosystems during this interval remain unclear, however, owing to the limited spatiotemporal coverage of integrated biogeochemical studies. Here, we combine high-resolution geochronology, mineralogical and multi-proxy geochemical data from the mid-Triassic Ordos Basin with temporal and spatial biogeochemical modelling to reconstruct lake redox structure and nutrient dynamics. Our results indicate that a transient increase in external sulfate input strengthened endogenous phosphorus recycling and eutrophication, promoting shoaling and intensification of a metastable sulfidic zone at mid-depths. This shoaling would have led to poisoning of benthic habitats, causing a collapse of the oldest known Mesozoic lacustrine ecosystem. We propose that sulfate loading prolonged anoxia and ecological stress by extending the residence time of phosphorus, a mechanism that may be relevant to deoxygenation events and resulting biocrises in both ancient and modern lacustrine ecosystems.