PROTRACTED DEVELOPMENT OF BIOTURBATION AND IMPLICATIONS FOR GLOBAL SULFUR CYCLING IN EARLY PALEOZOIC MARINE SHELFAL ENVIRONMENTS

Bioturbation or sediment mixing by burrowing animals is a critical engineering process in modern seafloor environments. The advent of bioturbation would, therefore, have profoundly impacted the development of Phanerozoic biogeochemical cycling, including nutrient fluxes, organic carbon burial and seafloor oxygenation, as well as the development of complex benthic ecosystems. However, the timing of the development of bioturbation has, historically, not been well constrained. 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. The onset of extensive bioturbation has accordingly been invoked as a trigger for a myriad of late Neoproterozoic and earliest Cambrian geochemical, ecological and taphonomic phenomena. In contrast to this paradigm, we present trace fossil, stratigraphic and taphonomic data, from a range of lower to middle Paleozoic siliciclastic successions spanning four paleocontinents, indicating that shelfal sediments in the early Cambrian were essentially unmixed. Moreover, even as late as the Middle–Late Silurian, nearly 120 million years after the Precambrian–Cambrian transition, infaunal mixing of shelfal sediments remained suppressed. These data indicate that in spite of concurrent advances in infaunalization, mixed layer development was a protracted process and did not occur with the first appearance of three-dimensional burrows. Further, we use these data to explore, in a mass balance framework, the impact of delayed mixed layer development upon global sulfur cycling. Our model suggests that bioturbation exercised a first-order control upon Paleozoic sulfur cycling and was responsible for suppressed sulfate concentrations in the early and middle Paleozoic global ocean. Protracted sediment mixing may also be linked to secular variations in atmospheric oxygen concentrations, as well as to the anomalous preponderance of exceptionally preserved soft-bodied biotas and shallow-tier trace fossil assemblages characteristic of the lower Paleozoic stratigraphic record.