Burrowing fauna mediate alternative stable states in the redox cycling of salt marsh sediments

The East Anglian salt marsh system (UK) has recently generated intriguing data with respect to sediment biogeochemistry. Neighbouring ponds in these salt marshes show two distinct regimes of redox cycling: the sediments are either iron-rich and bioturbated, or they are sulphide-rich and unbioturbated. No conclusive explanation has yet been given for this remarkable spatial co-occurrence.  Using pore-water analysis and solid-phase speciation, I will demonstrate that differences in solid-phase carbon and iron inputs are likely small between pond types, so these cannot act as the direct driver of the observed redox dichotomy. Instead, the results suggest that the presence of bioturbation is the driving force behind the transition from sulphur-dominated to iron-dominated sediments. The presence of burrowing fauna in marine sediments stimulates the mineralisation of organic matter, increases the iron cycling and limits the build-up of free sulphide. Subsequent early diagenetic modelling confirms that the observed regimes in pond geochemistry are caused by negligible differences in solid-phase inputs, which are amplified by positive feedbacks resulting from the impact of bioturbation on iron and sulphur cycling.