Coupled oceanic oxygenation and metazoan diversification during the early–middle Cambrian?

Abstract

The early-middle Cambrian (Fortunian to Age 4) is characterized by a significant increase in metazoan diversification. Furthermore, this interval is marked by a prominent environmental and ecological expansion of arthropod-and echinoderm-rich biotas. Recent redox work has suggested that this shift occurred during stable or decreasing marine oxygen levels, suggesting that these paleobiological and paleoecological transformations were decoupled from a redox control. We tested this idea by conducting new paleoredox analyses on Age 2-Age 4 Cambrian outer shelf (Jiuqunao-Wangjiaping), slope (Wuhe-Geyi), and basinal (Zhalagou) sections of the South China Craton. Multiple sections indicate that mid-depth waters transitioned from anoxic conditions during Cambrian Age 2 to stable oxic conditions during Cambrian Age 4. These findings suggest a stepwise expansion of oxic waters from shallow to deep settings during the early-middle Cambrian, consistent with a redox control of metazoan diversification and ecological expansion. More broadly, despite the surge in redox work over the past decade, this study highlights the need for continued coupled redox and paleontological studies to directly test models about the links between the evolution of animals, ecosystems, and marine redox conditions.

Oxygen minimum zones in the early Cambrian ocean

“…By contrast, the recognition of such structure would explain the large discrepancies reported through the Neoproterozoic-Cambrian transition (e.g., Sperling et al, 2015b). Our data augments emerging views of early Cambrian expanded marine oxygenated conditions (Chen et al, 2015;Li et al, 2017), supporting the establishment of a modern-like ocean architecture during the 'explosion' of metazoan diversity (Butterfield, 2018).…”

“…The dramatically increased diversity of metazoan life through the late Ediacaran to early Cambrian transition (~585-510 Ma) coincides with a protracted transition from global ocean anoxia to widespread ocean oxygenation (Canfield et al, 2007;Chen et al, 2015;Von Strandmann et al, 2015;Li et al, 2017). Throughout the Proterozoic Eon (2500 to 541 Ma), the ocean is thought to have been characterised by oxygenated surface waters overlying euxinic (anoxic and sulphidic) mid-depth waters along productive ocean margins (Poulton et al, 2010), and ferruginous (anoxic, Fe-containing) deeper waters (Planavsky et al, 2011;Poulton and Canfield, 2011).…”

Dynamic and synchronous changes in metazoan body size during the Cambrian Explosion

“…2A). We also note that increased extinction rates peak during the middle Meichucunian and Canglangpuan in South China when euxinic and/or anoxic waters were widespread in marine basins 33,34 .…”

Evidence for Highly Complex Redox Conditions and Strong Water‐Column Stratification in an Early Cambrian Continental‐Margin Sea

“…There are multiple lines of evidence in support of water‐column oxygenation of marine shelves/basins during the early Cambrian, particularly on the Yangtze Block of South China (e.g., Fe speciation and RSTEs: Cheng et al, , Jin et al, ; Li et al, ; and Mo‐U isotope: Chen et al, , Cheng et al, ; Dahl et al, ), although the deep ocean is likely to have remained anoxic at that time (Sperling et al, ; Stolper & Keller, ). Early Cambrian surface‐ocean oxygenation is likely to have been driven by rising atmospheric O 2 levels (Li et al, , ). Long‐term expansion of the oxygenated ocean‐surface layer would have caused a downward movement of the O 2 /H 2 S redoxcline and progressive water‐column oxygenation, as recorded by intermediate‐depth sections such as Silikou.…”