Ocean oxygenation in the wake of the Marinoan glaciation

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From: Nature(Vol. 486, Issue 7417)
Publisher: Nature Publishing Group
Document Type: Report
Length: 3,543 words
Lexile Measure: 1510L

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Metazoans are likely to have their roots in the Cryogenian period (1-3), but there is a marked increase in the appearance of novel animal and algae fossils shortly after the termination of the late Cryogenian (Marinoan) glaciation about 635 million years ago (4-6). It has been suggested that an oxygenation event in the wake of the severe Marinoan glaciation was the driving factor behind this early diversification of metazoans and the shift in ecosystem complexity (7,8). But there is little evidence for an increase in oceanic or atmospheric oxygen following the Marinoan glaciation, or for a direct link between early animal evolution and redox conditions in general (9). Models linking trends in early biological evolution to shifts in Earth system processes thus remain controversial (10). Here we report geochemical data from early Ediacaran organic-rich black shales (~635-630 million years old) of the basal Doushantuo Formation in South China. High enrichments of molybdenum and vanadium and low pyrite sulphur isotope values (D34S values [greater than or equal to] 65 per mil) in these shales record expansion of the oceanic inventory of redox-sensitive metals and the growth of the marine sulphate reservoir in response to a widely oxygenated ocean. The data provide evidence for an early Ediacaran oxygenation event, which pre-dates the previous estimates for post-Marinoan oxygenation (11-13) by more than 50 million years. Our findings seem to support a link between the most severe glaciations in Earth's history, the oxygenation of the Earth's surface environments, and the earliest diversification of animals.

The increase of oxygen in Earth's surface environments was protracted and is thought to have proceeded in two major steps. The ocean-atmosphere system was essentially devoid of oxygen until very early in the Proterozoic eon, when atmospheric oxygen rose to > 1% of the present atmospheric level (PAL) (14,15). The timing and dynamics of this initial oxygenation are under active investigation, but there is little doubt that a major atmospheric transition, the 'Great Oxidation Event' (GOE), occurred about 2.4 billion years (Gyr) ago (14,15). It is commonly assumed that there was a second significant oxygen rise to near PAL during the late Neoproterozoic (~750-542 Myr ago) (14,15); however, the timing and magnitude of this second oxygenation event remain elusive (9,15).

The appearance of metazoan fossils has traditionally been used as a minimum estimate for the timing of the late Neoproterozoic oxygenation event (16). Molecular clock estimates place the origin of crown-group animals in the Cryogenian period (850-635 Myr ago) (1). There are also sponge biomarkers (2) and sponge-like fossils (3) in Cryogenian or older rocks, but these are only simple metazoans with limited oxygen demands (see Supplementary Information). Novel micro-and macrofossils interpreted as early metazoans appear immediately above Marinoanage (~635 Myr ago) glacial deposits in South China (4-6) (see Supplementary Information). Further, there is a radiation in marine algae following the Marinoan glaciation, which is probably linked to a metazoan-driven shift in trophic structure and ecosystem complexity (17). It has been proposed that this biological innovation is linked to an...

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Gale Document Number: GALE|A303896235