Hypoxic Silurian oceans suggest early animals thrived in a low-O2 world

Atmospheric oxygen (O2) concentrations likely remained below modern levels until the Silurian- Devonian, as indicated by several recent studies. Yet, the background redox state of early Paleozoic oceans remains poorly constrained, hampering our understanding of the relationship between early animal evolution and O2. Here, we present a multi-proxy analysis of redox conditions in the Caledonian foreland basin to Baltica from the early to the mid-Silurian. Our results indicate that anoxic to severely hypoxic bottom waters dominated during deposition of the Silurian sediments cored in the Sommerodde -1 well (Bornholm, Denmark), and regional comparison suggests that these conditions persisted across the Baltoscandian foreland basin. Indeed, even during times of relative oxygenation, ichnological observations indicate that conditions were, at most, very weakly oxic. The results suggest that dissolved O2 was generally scarce in the bottom waters of the extensive Silurian seaway between Baltica and Avalonia, even between Paleozoic Anoxic Events. In light of delayed oxygenation of the atmosphere-hydrosphere system, it may be time to consider that early animals were adapted to hypoxia and thrived through -100 million years of low-O2 conditions after the Cambrian.(c) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons .org /licenses /by /4 .0/).

Automated summary

Recent studies indicate that oxygen levels in the early Paleozoic oceans likely remained below modern levels. This suggests a prolonged period of hypoxia, lasting around 100 million years after the Cambrian, during which early animals thrived in low oxygen conditions. The results support the idea that early animal evolution and the oxygenation of the atmosphere-hydrosphere system were closely related.