256-9 Reconstructing the Middle Paleozoic Sedimentary Mixed Layer: a Combined Stratigraphic and Ichnological Approach

Session: Life and Environments Through Time and Space: Multi-Record Approaches to Stratigraphic Paleobiology, Part I

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Burrowing and sediment-mixing animals, also known as bioturbators, are powerful ecosystem engineers, as their activities strongly shape surrounding ecosystems and environments.  Intense sediment mixing by bioturbators produces the sedimentary mixed layer, which in modern oceans consists of, on average, ten centimeters of fluid-rich, homogenized sediments. Previous work has revealed that the development of a deep mixed layer lagged significantly behind the appearance and expansion of other forms of bioturbation, and that mixed layer depths remained well below those of modern oceans through at least the Silurian. The advent of modern-style sediment mixing in ancient oceans would have substantially altered the biological and chemical character of the seafloor, leading to shifts in infaunal and epifaunal community structures, as well as in biogeochemical cycling of critical nutrients. Thus, constraining the evolutionary development of the sedimentary mixed layer allows us to decipher the ecological and environmental changes brought about by changes in bioturbation. Here, we present a field- and core-based stratigraphic exploration of bioturbation intensity and mixed layer development in middle Paleozoic successions from the Appalachian and Great Basins. Approximately 1200 meters of Lower Devonian to Pennsylvanian strata were logged at the centimeter to decimeter scale across twenty-two outcrops and sixteen cores, focusing on background (non-extinction) time intervals and facies recording normal shallow marine conditions. Middle Paleozoic units in our resulting dataset are generally characterized by intensive bedding plane-parallel bioturbation, with abundant and diverse assemblages of horizontal trace fossils reaching centimeter-scale diameters. High-fidelity preservation of many of these traces, especially in Appalachian Basin units, suggest that seafloor sediments were commonly firm at the time of burrow emplacement. Despite the presence of vertical burrows of centimeter-scale depths and high ichnofabric index (ii) values in some beds, especially those of Mississippian age, ii was low on average and vertical burrowing relatively sparse. Additionally, thin sandstone event beds are commonly preserved in siliciclastic successions and carbonates are often thinly bedded; the high preservational fidelity of these thin bedforms suggests limited intensive mixing in these settings. These data suggest that despite previously proposed Devonian radiations in bulldozing taxa, mixed layer depths in a range of shallow marine settings across Laurentia remained relatively shallow during this interval, with implications for our understanding of middle Paleozoic seafloor ecology and biogeochemistry.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10259

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