Increase of seawater Mo inventory and ocean oxygenation during the early Cambrian
Abstract The early Cambrian radiation of macroscopic animals, commonly referred as the ‘Cambrian Explosion’, is one of the most important biological innovations in Earth history. Because oxygen is a requirement for complex animal life, it is generally accepted that ocean oxygenation may have been the environmental trigger for animal evolution, but direct geochemical evidence supporting oxygen increase prior to the ‘Cambrian Explosion’ remains sparse and insufficient. To better understand the casual relationship between ocean oxygenation and animal evolution, we have carried out a systematic analysis of redox sensitive trace element (RSTE) concentrations, total organic carbon (TOC) contents and pyrite morphology for the early Cambrian Niutitang Formation of the Hejiapu section, northeastern Guizhou Province, South China. The new data, in combination with previously published data from other sections, show strong Mo enrichments (average of 88 ppm) and high Mo/TOC ratios (average of 14 ppm/wt.%) in the euxinic black shales of the lower Niutitang Formation (~ 520 Ma), indicating increase of the oceanic Mo reservoir. In the light of the positive correlation between sediment Mo/TOC and aqueous Mo in modern basins, the early Cambrian oceanic Mo concentration is estimated as ~ 50 nmol, which is about half of the modern ocean Mo concentration (~ 105 nmol) but is significantly higher than that of the Mesoproterozoic oceans (10–20 nmol). The marked increase in oceanic Mo reservoir is therefore interpreted as recording significant rise of atmospheric oxygen and pervasive ocean oxygenation, which may have facilitated the diversification of macroscopic metazoans during the early Cambrian.