Oxidation of the Ediacaran Ocean

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Oxidation of the Ediacaran Ocean Vol 444 | 7 December 2006 | doi:10.1038/nature05345 LETTERS Oxidation of the Ediacaran Ocean D. A. Fike1, J. P. Grotzinger1{, L. M. Pratt2 & R. E. Summons1 Oxygenation of the Earth’s surface is increasingly thought to have Here we present carbon and sulphur isotope data from the Huqf occurred in two steps. The first step, which occurred 2,300 mil- Supergroup (Fig. 1; see Supplementary Information for detailed dis- lion years (Myr) ago, involved a significant increase in atmo- cussion of the data). All data were collected from cuttings in the spheric oxygen concentrations and oxygenation of the surface Miqrat-1 well (Supplementary Fig. S1), previously established as ocean1,2. A further increase in atmospheric oxygen appears to have one of the most representative subsurface sections of Huqf strata16. taken place during the late Neoproterozoic period3,4 ( 800– The Shuram Formation preserves a .15% negative excursion in 13 542 Myr ago). This increase may have stimulated the evolution d Ccarb, reaching a minimum of about 212%. The excursion spans of macroscopic multicellular animals and the subsequent radi- ,500 m of stratigraphic section16 and is the largest known perturba- ation of calcified invertebrates4,5, and may have led to oxygena- tion to the global carbon cycle in Earth history. tion of the deep ocean6. However, the nature and timing of The Shuram excursion is fundamentally different from all known 13 Neoproterozoic oxidation remain uncertain. Here we present d Ccarb excursions (for example the Marinoan cap carbonate, the high-resolution carbon isotope and sulphur isotope records from Ediacaran/Cambrian boundary and the Permian/Triassic boundary), 13 the Huqf Supergroup, Sultanate of Oman, that cover most of the in that d Ccarb reaches well below the mantle value of about 26%. 13 Ediacaran period ( 635 to 548 Myr ago). These records indicate Because d Ccarb values below 26% cannot readily be explained by that the ocean became increasingly oxygenated after the end of the changes in organic carbon burial or isotope fractionation during Marinoan glaciation, and they allow us to identify three distinct carbon fixation4, the Shuram excursion was initially attributed to 13 16,21 stages of oxidation. When considered in the context of other diagenetic alteration of the d Ccarb signal . However, standard records from this period7–15, our data indicate that certain groups methods of assessing diagenesis (see Supplementary Fig. S2a, b, of eukaryotic organisms appeared and diversified during the sec- and discussion in Supplementary Information) indicate that these 13 ond and third stages of oxygenation. The second stage corresponds samples retain primary d Ccarb values. Furthermore, the Shuram with the Shuram excursion in the carbon isotope record16 and excursion is now documented in more than 30 sections in Oman, seems to have involved the oxidation of a large reservoir of organic covering a region in excess of 105 km2 with little variation in either carbon suspended in the deep ocean6, indicating that this event magnitude or duration16,21,23. Additionally, potentially correlative may have had a key role in the evolution of eukaryotic organisms. excursions have been found across the globe: the Doushantuo Our data thus provide new insights into the oxygenation of the Formation, China9; the Wonoka Formation, Australia13; the Ediacaran ocean and the stepwise restructuring of the carbon6,16,17 Johnnie Formation, USA14; and the Nama Group, Namibia15 (Fig. 2). and sulphur cycles3,18,19 that occurred during this significant per- Taken together, they indicate strongly that the Shuram excursion iod of Earth’s history. is a primary record of an unprecedented perturbation to the global The Huqf Supergroup provides one of the best-preserved, most carbon cycle. continuous sections of Ediacaran-age strata20 (Fig. 1). The Abu Unlike younger carbon isotopic anomalies that are expressed by 13 13 Mahara Group contains Marinoan-equivalent (,635-Myr-old) gla- d Ccarb in carbonate sections and d Corg (organic carbon) in silici- cial deposits (Fiq Formation and associated Hadash cap carbonate) clastic sections, the Shuram excursion has been identified only in that overlie ,800-Myr-old crystalline basement7. Nafun Group sedi- carbonate sections9,13–16,24. Given the absence of covariation in 13 13 ments were deposited in a regionally extensive sag basin under open, d Corg with d Ccarb during the Shuram excursion reported here shallow marine conditions, and each formation can be traced and from the Wonoka Formation in Australia13, the excursion would laterally for several hundred kilometres21. Nafun strata (see be undetectable in Shuram-equivalent siliciclastic strata, which 13 Supplementary Information for a detailed discussion of lithostrati- require d Corg for chemostratigraphy. This may explain why the graphy) comprise two clastic-to-carbonate shallowing-upward suc- Shuram excursion has not been found in more Ediacaran sections cessions (Masirah Bay Formation–Khufai Formation; Shuram (for example siliciclastic-dominated sections such as the Formation–Buah Formation) with an unconformity across the Windermere Supergroup, Canada). Khufai–Shuram boundary that probably includes the interval of The data presented here indicate progressive oxidation of the Gaskiers glaciation ,580 Myr ago (ref. 22). Global correlation of Ediacaran ocean in three stages after the end of Marinoan glaciation. 13 d Ccarb (carbonate, see Supplementary Information for carbon These successive stages occur well after the last of the extreme and sulphur isotope nomenclature) anomalies provides two age con- Neoproterozoic glaciations9 and before the extinction and sub- straints for the Buah Formation (Fig. 1): ,550 Myr for the mid-Buah sequent evolutionary radiation across the Ediacaran/Cambrian (correlation with Doushantuo Formation, China7,9), and ,548 Myr boundary20. The first stage of oxidation occurs in the Khufai and for the upper Buah (correlation with Nama Group, Namibia7,8). Masirah Bay formations above the Marinoan-equivalent Hadash These correlations are supported by multiple ages7 spanning 541– cap carbonate. In the Hadash cap, Dd34S, the difference between 547 Myr ago obtained from the overlying Ara Group. 34 34 coeval d SSO4 (carbonate-associated sulphate) and d Spyr (pyrite) 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. 2Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405, USA. {Present address: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA. 744 © 2006 Nature Publishing Group NATURE | Vol 444 | 7 December 2006 LETTERS (Fig. 1e), ranges from 1% to 12%. These low values, consistent with unconformity prevents meaningful comparison between data from those found above the coeval Marinoan diamictite in Namibia19, the base of the Shuram and those from the top of the Khufai. The 13 indicate marine sulphate concentrations ([SO4]) of less than absence of a parallel negative excursion in d Corg suggests that pools 200 mM (ref. 25), which prevents the expression of significant iso- of organic carbon and dissolved inorganic carbon in the Ediacaran 13 topic fractionation during bacterial sulphate reduction (BSR). ocean were effectively decoupled, such that excursions in d Ccarb, Throughout the Masirah Bay and Khufai formations above the cap which reflects the dissolved inorganic carbon pool—the source for carbonate, Dd34S increases gradually to ,35%, recording an increase carbon fixation of coeval organic matter—were not recorded in 13 6 to [SO4] . 200 mM and unlimited expression of BSR fractionation. d Corg of syndepositional sediments . Our trends agree with the 6 13 Although it is difficult to gauge the magnitude of the increase in observation of an overall absence of covariation between d Corg 18,19 13 [SO4], reports of Ediacaran and early Cambrian enrichment and d Ccarb in a global compilation of Neoproterozoic data from 34 and variability in d SSO suggest that [SO4] did not exceed ,730 to 555 Myr ago. With decoupled carbon reservoirs, negative 4 13 ,5 mM (ref. 26), which is appreciably lower than the modern value excursions in d Ccarb can result from the oxidation of part of a much of 28 mM. Two key fluxes for regulating [SO4] are riverine sulphate larger DOC reservoir represented by the deep ocean, and their ampli- 13 derived from pyrite oxidation (source) and marine reduction of tude is limited by d Corg (about 230%) rather than mantle com- sulphate to sulphide (sink). Because these fluxes both depend directly position (about 26%) as in a steady-state model. (Here and on oxygen concentrations, increased [SO4] probably correlates with throughout the text, the term DOC is used to refer to a large reservoir increased oxygen availability. It is likely that the increase in [SO4] of organic carbon (not the result of coeval primary production) above the cap carbonate is in part the recovery from a glacially suspended in the deep ocean; use of the term DOC is not intended induced drawdown of sulphate concentrations19. However, on the to distinguish between truly ‘dissolved’ organic carbon and sus- basis of ,610-Myr-old detrital zircons in the upper Khufai7,27 the pended colloidal or fine particulate organic carbon.) This vast 34 13 minimum time represented by the increase in Dd S in stage I strata organic reservoir would effectively buffer coexisting d Corg through overlying the ,635-Myr-old Marinoan cap carbonate is ,25 Myr adsorption on siliciclastic particles and/or during carbonate precip- and therefore probably records an increase in atmospheric oxygen itation in significant excess of the influx of detritus from primary in addition to any effects associated with deglaciation. This increase production. At the initiation of the Shuram excursion, this adsorp- in Dd34S constitutes the first stage (Fig. 1, stage I) of Ediacaran oxida- tion would have to contribute $90% of TOC to mask the signal of tion observed in the Huqf sediments. Throughout stage I strata, d13C-depleted primary organic matter. However, because the organic 13 13 d Ccarb and d Corg were not coupled.
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