PROTRACTED DEVELOPMENT OF BIOTURBATION AND IMPLICATIONS FOR GLOBAL BIOGEOCHEMICAL CYCLING THROUGH THE EARLY PALEOZOIC (Invited Presentation)

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 not only sediment properties but also 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. Recent work, however, synthesizing ichnological, stratigraphic, sedimentological and taphonomic data from a range of lower Paleozoic siliciclastic successions spanning four paleocontinents, indicates that sediment mixing in marine shelfal environments remained limited until at least the late Silurian, 120 million years after the Precambrian–Cambrian transition. The protracted development of the sediment mixed layer is also consistent with sulfur data and supported by global sulfur model simulations, which indicate that bioturbation exercised a first-order control upon Paleozoic sulfur cycling and was responsible for low sulfate concentrations in the early Paleozoic global ocean. The delayed development of intensive sediment mixing may also be linked to secular variations in early Paleozoic atmospheric oxygen concentrations, as well as to the anomalous preponderance of exceptionally preserved soft-bodied Lagerstätten characteristic of the lower Paleozoic stratigraphic record. 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, and that evolutionary advances in sediment colonization significantly outpaced advances in sediment mixing. Ecosystem restructuring caused by the onset of significant infaunal mobile deposit feeding (‘bulldozing’), unlike other major paleobiological and paleoecological innovations, appears to have occurred well after both the Cambrian Explosion and the Great Ordovician Biodiversification Event.