PROTRACTED DEVELOPMENT OF THE MIXED LAYER: IMPLICATIONS FOR SUBSTRATE EVOLUTION AND EXCEPTIONAL PRESERVATION IN EARLY PALEOZOIC MARINE SHELFAL ENVIRONMENTS

The radiation of burrowing metazoans in the early Phanerozoic dramatically altered the properties of marine sediments, an event commonly referred to as the “Cambrian substrate revolution” or “agronomic revolution.” The advent of infaunalization and biogenically-mediated sediment mixing profoundly impacted the development of Phanerozoic biogeochemical cycling, including nutrient fluxes, organic carbon burial, seafloor oxygenation and sediment ecology. However, the timing of the development of mixing has, historically, not been well constrained. Mixing has long been assumed to occur at the Precambrian–Cambrian boundary with the appearance of the index fossil and three-dimensional burrow Treptichnus pedum. We present new ichnological, stratigraphic and taphonomic data, from a range of lower to middle Paleozoic siliciclastic successions spanning four paleocontinents, suggesting that shelfal sediments in the earliest Cambrian were essentially unmixed. Moreover, we demonstrate that even as late as the Middle–Late Silurian, nearly 120 million years after the Precambrian–Cambrian transition, infaunal mixing of shelfal sediments had still not attained modern reworking intensities.

Data were collected from units characterized by heterolithic facies considered representative of marine shelfal settings and binned into three temporal intervals: 1) lower–middle Cambrian, 2) upper Cambrian–middle Ordovician and 3) upper Ordovician–middle to upper Silurian. A variety of metrics, including ichnofabric index, bed thickness, depth of bioturbation and trace fossil taphonomy and paleoecology were employed to track the global pace of mixed layer development. Bedding planes from all intervals are characterized by dense, exceptionally preserved trace fossil assemblages. Mean ichnofabric index (ii 2; ii 2; ii 3) and bed thickness (1.16 cm; 3.30 cm; 4.31 cm) values for each interval indicate that although infaunalization was well-advanced in marine shelfal settings by the early Cambrian, mixed layer development was a protracted process. Ichnological data documenting global-scale suppression of mixing challenge current assumptions that mixing occurred with the first appearance of three-dimensional burrows and hold important implications for the advent of modern-style biogeochemical cycling.