Exploring the potential links between ocean oxygenation and the evolution of early Paleozoic marine ecosystems

The beginning of the Phanerozoic saw two major diversification events in the history of life on our planet: the Cambrian Explosion (~530 to 510 Ma) and the Great Ordovician Biodiversification Event (~471 to 458 Ma), or GOBE. These two events are separated by a plateau diversity that spanned from the later Cambrian and Early Ordovician, roughly ~40 million years. On a finer temporal scale, the fossil record shows that this diversity plateau contains at least six rapid turnover events in the marine fauna: extinction events followed by rapid diversifications. The causes of this repetitive turnover in marine ecosystems remain an open question and was likely were driven by several environmental and biological factors.

Numerous lines of geochemical and mineralogical evidence indicate that the later Cambrian and Early Ordovician oceans were persistently less oxygenated than todays. Additionally, marine redox chemistry also varied considerably in conjunction with the biological turnover events. One of these events, the end-Marjuman extinction (~494 Ma), is by far the most well-studied in this regard. This interval was marked by perturbations in the global carbon and sulfur cycles, reflected by positive excursions in the marine carbon and sulfur isotope records referred to as the Steptoean Positive Carbon Isotope Excursion or SPICE. Extensive work, relying on multiple lines of evidence has linked these carbon and sulfur cycle perturbations to an interval of marine deoxygenation. Our work on three of the other extinction events has revealed both similar and dissimilar geochemical records to those of the SPICE, despite the similarity in the changes in marine communities across the events. However, all these events appear to coincide with evidence for declines in marine oxygenation. Therefore, the pause in building marine diversity in the early Paleozoic was in part both directly and in directly tied sustained low and variable marine oxygen levels.

Speaker: Benjamin Gill, Virginia Tech