Rise to modern levels of ocean oxygenation coincided with the Cambrian radiation of animals

Abstract

The early diversification of animals (∼630 Ma), and their development into both motile and macroscopic forms (∼575–565 Ma), has been linked to stepwise increases in the oxygenation of Earth's surface environment. However, establishing such a linkage between oxygen and evolution for the later Cambrian ‘explosion' (540–520 Ma) of new, energy-sapping body plans and behaviours has proved more elusive. Here we present new molybdenum isotope data, which demonstrate that the areal extent of oxygenated bottom waters increased in step with the early Cambrian bioradiation of animals and eukaryotic phytoplankton. Modern-like oxygen levels characterized the ocean at ∼521 Ma for the first time in Earth history. This marks the first establishment of a key environmental factor in modern-like ecosystems, where animals benefit from, and also contribute to, the ‘homeostasis' of marine redox conditions.

Marine redox variations and nitrogen cycle of the early Cambrian southern margin of the Yangtze Platform, South China: Evidence from nitrogen and organic carbon isotopes

“…This major anoxic event may have resulted in the extinction of many SSF-secreting organisms (Zhu et al, 2006). However, we infer that this was a local anoxic event occurring in this basin while the open ocean was oxygenated to modern-like levels, evidenced by high ␦ 98 Mo values (∼+2.4‰) (Chen et al, 2015). Mo bulk concentrations and Mo/TOC bulk ratios in the studied sections of this stage are high ([Mo] bulk > 100 ppm, mean Mo/TOC bulk = ∼23).…”

“…This expansion was developed widely from the shelf margin to the slope-basin areas of the South China basin as evidenced by iron speciation and Mo chemistry data (Canfield et al, 2008;Chen et al, 2015;Feng et al, 2014;Guo et al, 2007a;Och, 2011;Och et al, 2013) (Fig. 6b).…”

“…Previous studies argued that the Ni-Mo enrichments in the sediments could originate from hydrothermal input (Lott et al, 1999;Steiner et al, 2001), enrichment from seawater (Lehmann et al, 2007;Mao et al, 2002), or more likely the combination of both processes (Pašava et al, 2008). Recent studies demonstrated that the Ni-Mo sulfide ore layer, which is usually less than half meters in thickness (Jiang et al, 2006), has relatively lower ␦ 98 Mo values (<1.6‰) (Lehmann et al, 2007;Xu et al, 2012) (low-temperature hydrothermal fluids ␦ 98 Mo = +0.8‰, McManus et al, 2002), while black shales above and below this Ni-Mo ore layer have high ␦ 98 Mo values (∼1.9-2.4‰) (Chen et al, 2015) (modern seawater ␦ 98 Mo = +2.34‰, Goldberg et al, 2013), suggesting that hydrothermal input of Mo may have contributed only to the Ni-Mo ore layer, but not to the adjacent black shales (Chen et al, 2015). Thus Mo enrichment in the black shales resulted from redox processes and could provide information about the ocean Mo reservoir.…”

“…Currently available Fe speciation data indicate that euxinic waters were not widespread during the latest Ediacaran -the earliest Cambrian interval and that the water column was stratified with an oxygenated upper layer overlying a ferruginous deepwater column in the basin of South China (Canfield et al, 2008;Chen et al, 2015;Feng et al, 2014;Goldberg et al, 2007;Och, 2011;Och et al, 2013;Wang et al, 2012a) (Fig. 6a).…”

“…Based on published Fe speciation data (Canfield et al, 2008;Chen et al, 2015;Feng et al, 2014;Jin et al, 2014;Och, 2011;Och et al, 2013;Wang et al, 2012a), the Liuchapo and Niutitang formations (and correlative formations) were mostly deposited under anoxic conditions. Anaerobic degradation of organic matter during early diagenesis retains the initial ␦ 15 N value or decreases ␦ 15 N only to a slight extent (less than 2‰) (Freudenthal et al, 2001;Möbius et al, 2010;Robinson et al, 2012).…”