TY - JOUR

T1 - “Hypoxic” Silurian oceans suggest early animals thrived in a low-O2 world

AU - Haxen, Emma R.

AU - Schovsbo, Niels H.

AU - Nielsen, Arne T.

AU - Richoz, Sylvain

AU - Loydell, David K.

AU - Posth, Nicole R.

AU - Canfield, Donald E.

AU - Hammarlund, Emma U.

N1 - Funding Information: Funding: This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No 949538 ). NRP was supported by the Villum Fonden (grant 15397 ), DEC was supported by a research grant ( VIL16518 ) from Villum Fonden . Publisher Copyright: © 2023 The Author(s)

PY - 2023

Y1 - 2023

N2 - 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.

AB - 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.

KW - animal evolution

KW - Baltic basin

KW - caledonian foreland basin

KW - ocean oxygenation

KW - redox

KW - Silurian

U2 - 10.1016/j.epsl.2023.118416

DO - 10.1016/j.epsl.2023.118416

M3 - Journal article

AN - SCOPUS:85173480893

VL - 622

JO - Earth and Planetary Science Letters

JF - Earth and Planetary Science Letters

SN - 0012-821X

M1 - 118416

ER -