Chemical evolution of the Mid-Paleozoic Earth system and biotic response

The Mid-Paleozoic marks the transition from the low-diversity oceans of the Cambrian to a Devonian world with established terrestrial ecosystems and a substantially higher diversity of marine benthos, especially in the pelagic realm. Key events were the dramatic radiation termed the Great Ordovician Biodiversification Event (GOBE) and the nekton revolution of the Devonian. The interval also includes two mass extinction events (end-Ordovician and Late Devonian) with numerous minor biotic crises in the Silurian and Devonian but, curiously, not in the Ordovician. These remarkable changes are set against a proposed backdrop of increasing atmospheric oxygen levels and ocean ventilation. However, reviewing current ideas shows little consensus regarding either the timing or magnitude of these changes, with different approaches (geochemical proxies and modelling) often producing conflicting interpretations. Broadly, Mid-Paleozoic oceans are likely to have been less well oxygenated than the present day but notions that the marine biota was potentially more resilient to dysoxic conditions than modern taxa is not supported by the frequent association of crises and oceanic anoxic events. There is more of an agreement regarding temperature trends, with the Ordovician marked by a long-term cooling trend, partially coincident with the GOBE, culminating with an intense, end-Ordovician glaciation.