The rise of ocean and athmospheric oxygen and the evolution of animal life
The evolution of the modern oxygen-rich atmosphere and oceans apparently occurred in two major steps. The first major oxygenation step to levels higher than 10-5 present atmospheric level (PAL) was discovered by the diminishing of mass-independent fractionation signals (MIF) of sulphur isotopes in sediments younger than 2.45 Ga. The second major oxygenation step took place in the late Neoproterozoic 0.8 – 0.6 Ma ago. It was until then, that deep ocean ventilation was not completely achieved and bio-essential but redox-sensitive metals bound in anoxic deep ocean sediments were not as available as in modern oceans. This restraint of bio-essential metals may also have restricted the evolution of animals before the late Neoproterozoic. Some striking geochemical results from global occurring terminal Proterozoic marine sediment records suggest that the oceans of this period finally underwent a stepwise and protracted oxidation resulting in a stratified water column with an anoxic deep oceanic and an oxic upper oceanic realm (Kurzweil et al., 2015). The reasons for the oxygenation in the late Neoproterozoic are still debated. One possible hypothesis suggests that glacial melting increased nutrient influx and postglacial phosphate flux to the oceans, triggering primary productivity and subsequent burial of organic matter. Recent publications point out that the weathering environment on the continents changed from a solely physical to a biochemical weathering due to the evolution of fungi and lichen leading to the formation of fine grained soils and entrapment of the organic matter in the elevated deep sea sediment accumulation. The organic matter burial finally resulted in a net increase in pO2 as less oxygen is consumed for its oxidation. An alternative hypothesis by is the establishment of a positive feedback system of evolution of complex eukaryotes, benthic filter feeding, phosphorous removal and deep ocean oxygenation (Lenton et al. 2014). However, while more and more geochemical studies point out that oxygenation and coeval emergence of higher organisms occurred in the terminal Proterozoic (e.g. Chen et al., 2015), Sperling et al. (2015) even argue, on the base of a statistical evaluation of iron speciation data through Earth’s history that the terminal Proterozoic oxygenation was limited and significant rise in ocean-atmosphere oxygen to PAL did not happen before the Palaeozoic era. The evidence for the appearance of multicellular life (presumably as a result of the ocean oxygenation) was so far limited to the fossil record of the terminal Proterozoic. However, new geochemical proxies nowadays broaden the toolbox for pinpointing the onset of the biological radiation. Recent studies of marine sediments focus on non-traditional metal isotopes as proxies for the fractionation of bio-available nutrients in the ocean. The first outcomes of these analytical advantages are the findings of zinc isotope variations in Neoproterozoic rocks from Namibia and variations of cadmium in Permian rocks (Gorgiev et al. 2015) to infer bio-productivity shifts from ancient rock records.