Ecological and biogeochemical consequences of benthic ecosystem engineer responses to the end-Permian mass extinction

Organisms whose activities impact the availability of resources in their environments, known as ecosystem engineers, are known to have profound controls on ecological and evolutionary dynamics throughout Earth history. Bioturbators – animals that mix seafloor sediments – are especially powerful ecosystem engineers due to their direct impacts on key benthic biogeochemical cycles. The emergence or loss of bioturbators throughout Earth history is associated with unique and profound shifts in benthic ecology and biogeochemistry. The end-Permian mass extinction (EPME), regarded as the most devastating climate-driven mass extinction in Earth history, saw devastating losses in marine benthic biodiversity and bioturbators, with the bioturbation-driven sedimentary mixed layer completely collapsing in some regions. The loss of bioturbating ecosystem engineers during the EPME has long been implicated in the rates of benthic recovery in the Early Triassic, although the precise impacts of bioturbator responses have remain unconstrained. Here, we test the hypothesis that loss of bioturbating ecosystem engineers during the EPME led to unique ecological and biogeochemical consequences in Early Triassic communities. Combining trace fossil data from literature and body fossil data from the Paleobiology Database for continuous stratigraphic sections across the EPME, we construct multiple comparative local time series of ecological responses of bioturbators and local benthic communities. We use the Earth system model cGENIE to reconstruct marine environmental conditions across the EPME, which also serve as boundary conditions for local biogeochemical models. For each region represented by continuous stratigraphic sections, we then use the fossil record to parameterise pre-EPME and post-EPME bioturbation in biogeochemical reactive-transport models and compare the impacts of the complete loss, reduction, or persistence of bioturbation on benthic biogeochemistry. Finally, we run local sensitivity analyses to constrain the impacts of bioturbation responses on biogeochemical change, and effect size analyses to quantify the relative roles of bioturbators and climate change on ecological responses across the EPME. These results address long-standing assumptions about the role of bioturbation in benthic ecosystem recovery through the Early Triassic and underscore the importance of local environments and community ecology for contextualising recovery in the aftermath of mass extinctions.