Pulsed oxidation and biological evolution in the Ediacaran Doushantuo Formation

Abstract

Recent geochemical data from Oman, Newfoundland, and the western United States suggest that long-term oxidation of Ediacaran oceans resulted in progressive depletion of a large dissolved organic carbon (DOC) reservoir and potentially triggered the radiation of acanthomorphic acritarchs, algae, macroscopic Ediacara organisms, and, subsequently, motile bilaterian animals. However, the hypothesized coupling between ocean oxidation and evolution is contingent on the reliability of continuous geochemical and paleontological data in individual sections and of intercontinental correlations. Here we report high-resolution geochemical data from the fossil- acritarchs ͉ isotopes ͉ redox ͉ Neoproterozoic ͉ early animals T he Ediacaran (635-542 Ma) Earth witnessed profound changes in the aftermath of widespread and potentially global ice ages, including the evolution and radiation of complex megascopic life and major perturbations of the global carbon cycle that accompanied oxygenation of the deep ocean (1-9). These biological and environmental events have been speculatively linked, yet their temporal relationships have not been accurately documented in relatively continuous and fossil-rich sections that span a range of well-documented depositional settings. For example, geochemical data from siliciclasticdominated Ediacaran successions in Newfoundland (5) and the western United States (7) are incomplete, and those from the early Ediacaran interval in Oman (4) are of low stratigraphic resolution. Furthermore, paleontological data from these successions are limited to macroscopic Ediacara fossils and the biomineralizing animal Cloudina (10).To further test the proposed linkages between redox changes and biological evolution (4, 5), we carried out a high-resolution chemostratigraphic and biostratigraphic investigation of the fossiliferous Doushantuo Formation in the Yangtze Gorges area, South China. Our data reveal pulsed oxidation events that coincide with the origination and diversification of acanthomorphic acritarchs and other multicellular life forms in the basin. In combination with available data from other Ediacaran successions, our results indicate that oxidation of terminal Proterozoic oceans may have been episodic (4), with the final and permanent oxidation occurring Ϸ551 Ma. Sedimentological, Paleontological, and Geochemical DataThe Doushantuo Formation in the Yangtze Gorges area, constrained between 635.2 Ϯ 0.6 and 551.1 Ϯ 0.7 Ma (11), is divided into four lithostratigraphic members (Fig. 1). At the Jiulongwan section [supporting information (SI) Fig. 3], member I represents an Ϸ5-m-thick cap dolostone overlying the Nantuo glacial diamictite and contains a suite of enigmatic sedimentary structures and textures (12, 13). Member II is characterized by Ϸ70 m of alternating organic-rich shale and dolostone beds with abundant pea-sized chert nodules. Member III is Ϸ70 m thick and is composed of dolostone and bedded chert in the lower part that passes up-section into alternating limestone-dolostone ''ribbon rocks.'' Me...

Rare earth elements and carbon isotope geochemistry of the Doushantuo Formation in South China: Implication for middle Ediacaran shallow marine redox conditions

“…Previous studies indicate that in the Yangtze Gorges area Ediacaran acanthomorphs appear shortly after the termination of the Nantuo glaciation and disappear in Member III of the Doushantuo Formation, just below the pronounced negative δ 13 C carb excursion EN3 (Figure 1) that is correlated with the Shuram excursion and represents the largest negative δ 13 C carb excursion in Earth history [2,3,7,11]. After EN3, Ediacara-like megafossils appear in the Dengying Formation [12][13][14][15].…”

“…Carbon and sulfur isotope data indicate that deep oceans were episodically oxygenated and deep ocean DOC progressively remineralized during the Ediacaran Period [1,2]. These remineralization events likely involved upwelling of anoxic DOC-laden water to shallow shelves, impacting on the shallow-water redox conditions and biological evolution.…”

Neoproterozoic microfossils from the margin of the East European Platform and the search for a biostratigraphic model of lower Ediacaran rocks

“…Post-Sturtian but pre-Marinoan biology remains poorly documented, so it is hard to know whether inferred extinctions accompanied Sturtian or Marinoan glaciation. Available data also suggest that the major biological reorganization represented by ECAP microfossils occurred well after Marinoan deglaciation, in association with mid-Ediacaran redox change (Fike et al, 2006;Canfield et al, 2007;McFadden et al, 2008), animal radiation (Peterson and Butterfield, 2005;Yin et al, 2007), or the Acraman impact event (Grey et al, 2003). To the extent that at least some ECAP fossils preserve egg or diapause cysts of early metazoans (Yin et al, 2007), the ECAP radiation may signal the expansion of animals with resting stages in their life cycles (Marcus and Boero, 1998).…”

“…Grey (2002) noted that earlier Ediacaran microfossil assemblages "are poorly known but are similar to pre-glacial ones except that there are fewer species." Similarly, in China, diverse acanthomorphic acritarchs of the middle and upper Doushantuo Formation are preceded by simpler and less diverse microfossils, with uncommon acanthomorphs appearing just below an ash bed dated by U-Pb on zircons as 632.5±0.5 million years (Condon et al, 2005;McFadden et al, 2006McFadden et al, , 2008Zhou et al, 2007;Yin et al 2007). …”

Geochemical constraints on the origin of Doushantuo cap carbonates in the Yangtze Gorges area, South China

“…Most Marinoan glacial deposits are directly overlain by "cap carbonates" with peculiar sedimentary features and negative carbon isotope signatures (Kennedy, 1996;Hoffman et al, 1998;James et al, 2001;Hoffman and Schrag, 2002;Jiang et al, 2003a;Frimmel and Folling, 2004;Shields et al, 2007a, b;Zhou and Xiao, 2007;Shen et al, 2008), implying severe and rapid climatic changes which are thought to serve as an 'environmental filter' for biological evolution (Hoffman et al, 1998;Runnegar, 2000;Hoffman and Schrag, 2002). Strata above these postglacial cap carbonates contain the Earth's earliest multi-cellular organisms interpreted as early Metazoans (Xiao et al, 2002;Xiao, 2004;Yin et al, 2007;McFadden et al, 2008;Yuan et al, 2011). These cap carbonates have been of special interest, understandably, as they may provide significant hints about the link between the earliest diversification of animals and the most severe glaciation in Earth's history.…”