EARLY PHANEROZOIC SHALLOW MARINE SUBSTRATE EVOLUTION: THE SOLE MARK RECORD

Bioturbation, the physical and chemical mixing of sediments by burrowing animals, is a critical engineering process in modern seafloor environments and exerts an important control on benthic ecology, sediment properties and ocean-wide biogeochemical cycling. Well-mixed sediments have long been assumed to appear at the Precambrian-Cambrian boundary with the first occurrence of the index fossil and three-dimensional burrow Treptichnus pedum. However, field-based stratigraphic analyses, synthesizing ichnological, sedimentological and taphonomic data, indicate that sediment mixing in shallow marine environments remained limited until at least the late Silurian, 120 million years after the Precambrian-Cambrian transition. These field-based data are corroborated by a global compilation of Phanerozoic shallow marine erosive sole structures (e.g., tool and flute marks). The formation and preservation of sole structures require a hydroplastic and cohesive substrate. Therefore, the temporal distribution of these structures can be employed to track secular changes in seafloor rheology, which can significantly alter early diagenetic processes and thus shape global biogeochemical cycling. The frequency of sole mark preservation in shallow marine environments has gradually but significantly declined through the Phanerozoic, indicating that seafloor rheology experienced major secular transformations over the past 541 million years, in conjunction with and likely directly due to the development of well-bioturbated sediments.

However, the precise biogeochemical impact of early Paleozoic bioturbation remains debated. To further address this question, I use a new multi-component diagenetic model to explore the relationship between bioturbation, porosity and biogeochemical (e.g., C, P, O and S) cycling. Not only intensity but style of bioturbation (e.g., biodiffusion, bioirrigation and changes in porosity) influence the magnitude of both P recycling and S oxidation. On the basis of these model results, the gradual increase in shallow sediment porosity predicted by the lower Paleozoic sole mark record may have mediated secular decreases in P burial and thus enhanced P recycling and marine productivity.