Loss of the sedimentary mixed layer as a result of the end-Permian extinction
Authors:
Hofman et al
Abstract:
The end-Permian mass extinction resulted in the most dramatic degradation of marine bottom communities during the Phanerozoic. One result of this extinction was the long-recognized, extreme reduction in bioturbation of the Early Triassic seafloor. Several lines of evidence (i.e., preferential preservation of epifaunal and very shallow-tier infaunal trace fossils; paucity of mid- and deep-tier trace fossils; absence of mottled bioturbation textures; dominance of cohesive substrates; widespread occurrence of microbially induced sedimentary structures in open-marine environments; ecological composition of Early Triassic communities) show that the reduction in bioturbation was so extreme that the sedimentary mixed layer was eradicated at an interregional scale for the only time since it was established in the early Palaeozoic. The consequences of this for ecosystem function and geochemical cycling must have been profound and yet they have received little consideration. Biogenic mixing of sediments is fundamental to geochemical cycling in extant marine ecosystems, and it also governs ecologically limiting factors such as nutrient fluxes, benthic primary production, and availability of ecospace. The collapse of biogenic sediment mixing during the Early Triassic must have affected geochemical properties of sediments and the seawater, as reflected in the geologic record of the sulphur cycle. Additionally, many of the proxies traditionally used to infer Early Triassic seawater anoxia may rather reflect poor sediment oxygenation arising from the extinction of bioturbators. Because of its impact on seawater and sediment chemistry, the loss of the mixed layer may have been an important, but hitherto little-considered constraint on the recovery from the end-Permian mass extinction.
Widespread delayed mixing in early to middle Cambrian marine shelfal settings
Authors:
Tarhan et al
Abstract:
The radiation of burrowing metazoans in the early Phanerozoic dramatically altered the properties of marine sediment, an event commonly referred to as the “Cambrian substrate revolution” or “agronomic revolution.” The advent of infaunalization, and especially 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 mixed seafloor sediment has, historically, not been well constrained. Mixing has been assumed, in the absence of data, to occur at the Precambrian–Cambrian boundary with the appearance of the index fossil and three-dimensional burrow Treptichnus pedum. Herein we present new ichnological, stratigraphic and taphonomic data suggesting that, although significant developments in infaunalization— paleobiologically complex animal–substrate interactions, particularly burrow construction—occurred during the early and middle Cambrian, mixing remained suppressed throughout this interval. We demonstrate, using a novel multi-proxy approach, that shelfal sediment in the earliest Cambrian was essentially unmixed. Moreover, our findings indicate that even as late as the middle Cambrian—30 million years after the Precambrian–Cambrian transition and well after the appearance of supposedly deposit-feeding trilobites—seafloor heterolithic shelfal sediment remained largely unmixed on a global scale. These findings challenge the current assumption that mixing occurred with the first appearance of three-dimensional burrows and hold important implications for the advent and development of modern-style biogeochemical cycling.
