Burrow with Synsedimentary Halo: Defining the Edge of Bioturbation Effects on Substrate During the Cambrian Explosion

The Cambrian explosion witnessed the innovation of body plans and animal activities, accompanied by complex environmental changes, including the seafloor substrate revolution. Early Cambrian burrowing organisms are well-known for disturbing seafloor sediments, promoting nutrient and oxygen cycling, and mixing the substrate. Seafloor bioturbations or burrows are recorded and preserved; however, animal-altered areas are not restricted to the bioturbations themselves. Open burrow systems dramatically improve sediment porosity and permeability, acting as conduits for fluids to interact with surrounding sediments below the sediment-water interface. During this process, fluids from the conduits diffuse into the surrounding matrix; nevertheless, records of such fluid alteration are rarely preserved. Thus, the impacts of bioturbation on substrate modification, especially chemically, remain far from resolved, except for studies based on numerical simulations rather than fossil records. Burrows are also referred to as “biogeochemical hotspots” due to local chemical changes within and around them induced by burrowing organisms. Oxidized zones have been found in modern polychaete burrows, representing oxygen introduced into the surrounding matrix. Dense, morphologically complex burrows with thick halos occur at several horizons in the Houjiashan Formation (Cambrian Stage 4), North China. Fragmented burrows, their irregular diagenetic halos, and synsedimentary fracturing within these halos indicate early (synsedimentary) and preferential diagenesis of sediments immediately adjacent to burrow margins. We propose that these synsedimentarily precipitated halos record burrow systems’ effects on substrate modification rather than sediment mixing. Semi-quantitative measurements suggest the entire substrate could be completely modified under a moderate rather than high BI. Using μ-XRF and in-situ analysis of major and rare earth elements, we propose burrow systems enhance Ca, Sr and other elements diffusion from the burrows into surrounding sediments. This research indicates the entire substrate can be modified without complete sediment mixing, providing an important reference for future research of organism-driven substrate modification.