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-rich Doushantuo Formation (635-551 Ma) in South China that confirm trends from other broadly equivalent sections and highlight key features that have not been observed in most sections or have received little attention. First, samples from the lower Doushantuo Formation are characterized by remarkably stable delta(13)C(org) (carbon isotope composition of organic carbon) values but variable delta(34)S(CAS) (sulfur isotope composition of carbonate-associated sulfate) values, which are consistent with a large isotopically buffered DOC reservoir and relatively low sulfate concentrations. Second, there are three profound negative delta(13)C(carb) (carbon isotope composition of carbonate) excursions in the Ediacaran Period. The negative delta(13)C(carb) excursions in the middle and upper Doushantuo Formation record pulsed oxidation of the deep oceanic DOC reservoir. The oxidation events appear to be coupled with eukaryote diversity in the Doushantuo basin. Comparison with other early Ediacaran basins suggests spatial heterogeneity of eukaryote distribution and redox conditions. We hypothesize that the distribution of early Ediacaran eukaryotes likely tracked redox conditions and that only after approximately 551 Ma (when Ediacaran oceans were pervasively oxidized) did evolution of oxygen-requiring taxa reach global distribution.

Fig. 1.

Fossil distribution in the Doushantuo Formation. Member II fossil occurrences are from the Jiulongwan section, and member III fossil occurrences are from the nearby Tianjiayuanzi section (diamonds) in the Yangtze Gorges and the Weng'an section (bars) in Guizhou Province (6, 38). The member IV fossils are occurrences from the nearby Miaohe section (15). Acanthomorph taxonomy follows that in ref. , pending systematic revision of several taxa. Diamonds indicate actual stratigraphic occurrences; bars, stratigraphic ranges based on first and last occurrences. The Inset shows generalized fossil distribution in the Ediacaran Period (4, 11), and the shaded portion represents the Doushantuo Formation. See SI Fig. 3 for section localities and Fig. 2 for explanation of stratigraphic column and δ¹³C_carb features. PDB, PeeDee belemnite. NT, Nantuo Formation; DY, Dengying Formation.

Fig. 2.

Chemostratigraphic profiles of the Doushantuo Formation at Jiulongwan. The lithostratigraphic division (I–IV), sedimentary cycles (–3), and δ¹³C_carb features (EN1, EP1, EN2, EP2, EN3a, EN3b, and EN3c) are labeled. The left edge of the stratigraphic column shows percentages of dominant mineralogy (calcite vs. dolomite vs. terrigenous clay) estimated from thin sections, and the right edge shows dominant lithologies. M, micrite; W, wackestone; P, packstone; G, grainstone; NT, Nantuo Formation; DY, Dengying Formation.