The effects of bioturbation on porewater chemistry and early marine diagenesis: Evidence from a modern intertidal zone in Abu Dhabi, U.A.E.

Early marine carbonate cements generally form in CaCO 3 ‐supersaturated seawater at the seabed or shallow burial depths, resulting in syn‐sedimentary cemented firmgrounds and hardgrounds. The processes controlling early marine diagenesis are complex, particularly in coastal environments where geochemistry is influenced by different water sources and local biogenic activity. Investigating the modern firmgrounds and hardgrounds in the intertidal zones of the U.A.E. reveals how physical, chemical and biological factors influence early marine precipitation. In these environments, early marine diagenesis is governed by interacting hydrogeochemical, microbial and physical processes, whilst bioturbation modifies solute transport dynamics. In situ porewater and sediment analyses show that the intensely bioturbated lower intertidal zone experiences enhanced sediment permeability, persistent seawater flushing and sediment reworking, producing geochemically uniform porewaters and fewer firmground features. In the middle intertidal zone, reduced seawater exchange, shallower bioturbation and longer porewater residence times promote geochemical conditions favourable for early marine cement precipitation. Additionally, mixing between upwelling continental brines and intertidal seawaters affects porewater redox, salinity and ion concentrations, leading to localised diagenesis. Spatial variations in porewater and sediment chemistry can develop over short distances (<1 km) because of complex hydrogeochemical interactions, in which bioturbation acts as an important modifier. This study highlights the importance of integrating ichnological, geochemical and hydrogeological approaches.