Reorganisation of Earth’s biogeochemical cycles briefly oxygenated the oceans 520 Myr ago

Abstract

The Phanerozoic radiation of bilaterian animals has been linked to oxygenation of Earth's oceans, due to the oxygen demand of the evolving animal ecosystems. However, how early animals may have regulated Earth's surface oxygen budget via self-stabilising feedbacks is poorly understood. Here, we report parallel positive uranium, carbon, and sulphur isotope excursions from carbonate successions in Siberia that document a brief global oxygenation episode 521-520 Myr ago, at the onset of diversification of larger arthropods known from the fossil record. Our data and model indicate that an abrupt increase in the sinking rate of marine organic matter expanded the oxygenated zone in the oceans and that reducing conditions returned 1.3 ± 0.8 Myr after the onset of this transient oxygenation episode, necessitating a strong negative feedback to the increasing levels of oxygen. We speculate that larger zooplankton could have sourced both oxygen and food to the seafloor, fueling bioturbation over wider areas and, thereby, stabilising O 2 -rich habitats in the oceans. Thus, this reorganisation exemplifies how animal ecosystems might have influenced oxygen availability in Earth's surface environment soon after their establishment. LetterIn contrast to many geochemical proxies that evaluate local ancient marine redox including iron speciation and trace metal (Mo, U, V) enrichments, the uranium isotope composition (δ 238 U, the per mille deviation of the 238 U/ 235 U ratio relative to CRM 145 standard) of seawater can be used to evaluate ocean oxygenation at 1. Natural History Museum of Denmark, University of Copenhagen, Denmark * Corresponding author (email: tais.dahl@snm.ku.dk) 2. Centre for Star and Planet Formation, University of Copenhagen, Denmark 3. Swedish Museum of Natural History, Stockholm, Sweden 4. Virginia Polytechnic Institute and State University, Blacksburg, USA a globally integrated scale. This is possible due to the long residence time and uniform δ 238 U of uranium in the modern ocean and predicted for the Cambrian ocean (Weyer et al., 2008;Dahl et al., 2014;Tissot and Dauphas, 2015). The δ 238 U proxy has been utilised to track past global ocean redox during three known oceanic anoxic events (Montoya-Pino et al., 2010;Brennecka et al., 2011;Dahl et al., 2014;Elrick et al., 2016;Lau et al., 2016), where anoxic water masses expanded over larger areas of the seafloor and caused negative δ 238 U excursions. Here, we use uranium isotopes to identify a transient global oxygenation episode during the radiation of animals in the Cambrian.Our new δ 238 U data of carbonate-associated uranium comes from limestones collected from the Siberian Platform across the provisional Cambrian Stage 2-3 boundary (~521 to 520 million years ago) (Fig. 1), when animals that shed their exoskeleton (ecdysozoa) began to diversify (Maloof et al., 2010;Kouchinsky et al., 2012). A perturbation in the marine carbon cycle is expressed at this time as a large positive carbon isotope excursion recognised globally and in all studied sections;...

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

“…Another mechanism to account for progressive water‐column oxygenation at Silikou is secular expansion of the oxygenated surface layer of early Cambrian oceans (Figure ). 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, , ).…”

The effects of marine eukaryote evolution on phosphorus, carbon and oxygen cycling across the Proterozoic–Phanerozoic transition

“…Evidence that mid-depth waters of the Yangtze Platform oxygenated from Cambrian stages 2-4 has been used to question the early bioturbation model [106], but is actually consistent with the original predictions [100], which show that ocean oxygenation should accompany the initial onset of deep bioturbation (Stages 2-4), followed by a much slower deoxygenation (governed by the slow timescale of atmospheric pO 2 decline). Wider evidence shows an ocean oxygenation event ∼520 Ma (broadly coincident with the 'Cambrian explosion') followed by deoxygenation [13,107,108].…”

“…The Cambrian evolution of large zooplankton would also have increased the efficiency of the biological pump [36], transferring organic matter to sediments, lowering the ocean P inventory and tending to oxygenate the ocean [1,107].…”

Coupling of ocean redox and animal evolution during the Ediacaran-Cambrian transition

“…While some simple animals, such as soft-bodied sponges, which may have their origins in the Tonian 7 , could have had low oxygen requirements (0.5–4.0% of present-day levels) 8 , macroscopic animals of the Ediacaran–Cambrian transition likely required oxygen levels above the physiological thresholds for those energetically expensive behaviors 9 , 10 . A gradual rise in oceanic oxygen levels has been proposed based on substantial geochemical evidence, e.g., the high abundance and isotope values of redox-sensitive trace metals (Mo and U) in euxinic sediments, and indeed seems to occur contemporaneously with animal evolution 11 – 14 . However, iron speciation data also suggest a high degree of redox heterogeneity, and possibly even dominantly anoxic conditions with scant measurable change in oxygenation through the Ediacaran to Cambrian interval 15 – 17 .…”