Pulsed oxidation and biological evolution in the Ediacaran Doushantuo Formation

McFadden et al. 10.1073/pnas.0708336105.

Supporting Information

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SI Figure 3
SI Figure 4
SI Figure 5
SI Figure 6
SI Figure 7
SI Table 1
SI Text

SI Figure 3

Fig. 3. (A) Maps showing the general location of the Yangtze Platform. The black dot denotes the Yangtze Gorges area. (B) Geological map of the Huangling anticline in the Yangtze Gorges area showing outcrops of Ediacaran strata around the Huangling anticline and locations of the five sections mentioned in the main text. The Huangling granite in the center of the anticline has been dated at 819 ± 7 Ma (1).

1. Zhao Z, Xing Y, Ma G, Chen Y (1985) Biostratigraphy of the Yangtze Gorge Area, (1) Sinian (Geological Publishing House, Beijing).

SI Figure 4

Fig. 4. Stratigraphic columns of the Jiulongwan (A), Xiaofenghe (B), and Zhangcunping (C) sections (refer to Fig. 3 for localities) of the Yangtze Gorges area. Sections were logged at 0.5-m intervals and drafted by using standard Dunham classification (1). M, mudstone; W, wackestone; P, packstone; G, grainstone. The left edge shows a dominant percentage of mineralogy between calcite, dolomite, and terrigenous clay.

1. Dunham RJ (1962) in Classification of Carbonate Rocks, ed. Ham WE (Am. Assoc. Petroleum Geol.), pp. 108-121.

SI Figure 5

Fig. 5. Field photos of the Doushantuo Formation at the Jiulongwan section. (A) Base of the cap dolostone and underlying Nantuo diamictite (NT Fm). (B) Alternating black shale and argillaceous dolomite in lower member II. Note the hammer, provided for scale (30 cm long). (C) Chert nodules in member II dolostone. (D) Phosphatic packstone from sheet chert in lower member III. (E) Scours of dolomitic caps (arrows) intercalated with limestone in upper member III ribbon rock. (F) Lithologic contact (line) between member III limestone and member IV black shale.

SI Figure 6

Fig. 6. Chemostratigraphic data for the Jiulongwan section, Yangtze Gorges area. Isotopic data are the same as that in Fig. 2, with additional profiles of d¹⁸O, total organic carbon (TOC), pyrite, and carbonate-associated sulfate (CAS) concentrations. Refer to SI Table 1 for data values.

SI Figure 7

Fig. 7. Geochemical cross-plots of the Doushantuo Formation with the four members color-coded. (A) Cross-plot of d¹³C_carb against d¹⁸O, with extreme negative d¹³C_carb isotopic values (less than -20‰) interpreted as methane gas hydrate release (1) removed to better show the isotopic spread. (B) wt% pyrite-total organic carbon (TOC) cross-plot. (C) wt% pyrite-CAS crossplot. (D) d³⁴S_CAS-carbonate-associated sulfate (CAS) cross-plot. (E) d³⁴S_CAS-wt% pyrite cross-plot. (F) d³⁴S_CAS-d³⁴S_py cross-plot. Correlation coefficients (r² values) are noted. PDB, Vienna-PeeDee belemnite.

1. Jiang G, Kennedy MJ, Christie-Blick N (2003) Stable isotopic evidence for methane seeps in Neoproterozoic postglacial cap carbonates. Nature 426:822-826.

SI Text

Regional Geology and Lithostratigraphy of the Doushantuo Formation

Regional Context. The Doushantuo Formation was deposited as part of a passive margin succession that developed on the Yangtze Block after the early Neoproterozoic breakup of the Rodinia supercontinent (1, 2). It is underlain by Marinoan-equivalent glaciogenic diamictite of the Nantuo Formation (<663 Ma, >635 Ma) and overlain by the Denying Formation (551-542 Ma) (3, 4). Thickness varies from 40 to 260 m, and lateral facies change from shallow-water phosphatic dolostone to deeper-water black shale and siliceous rocks (5). Although the Doushantuo Formation outcrops widely in South China, this study focuses on the Jiulongwan and several nearby sections in the Yangtze Gorges area, located in the southern limb of the Huangling anticline, Hubei Province (SI Fig. 3).

Yangtze Gorges Stratigraphy. The Doushantuo Formation in the Yangtze Gorges area is generally subdivided into four lithologic members (SI Fig. 4A). Member I is represented by a cap carbonate (SI Fig. 5A) that contains sheet cracks, tepee-like structures, macropeloids, and barite fans (6). The cap dolostone is dated at 635.2 ± 0.6 Ma (4). It is conformably overlain by member II, which is characterized by alternating black shale and dolomicrosparite beds (SI Fig. 5B), with abundant pea-sized chert nodules at certain horizons (SI Fig. 5C). An ash bed 5 m above the cap carbonate is dated at 632.5 ± 0.5 Ma (4). Sedimentary structures are sparse and include parallel laminations and rare intraclasts, indicating that the mode of sedimentation was dominated by suspension settling. Member III consists of dolostone and ribbon rock deposited under relatively shallower-water conditions periodically at or above wave base, as evidenced by intraclastic packstones-grainstones (SI Fig. 5D), scour marks (SI Fig. 5E), small-scale cross-bedding, and thick microbial mats. The black shale of member IV (SI Fig. 5F) is thinner than, yet lithologically comparable to, the lower Doushantuo (member II) shale. An ash bed near the contact with the overlying Dengying Formation is dated at 551.1 ± 0.7 Ma (4). Although a sharp lithofacies change is present at the base of member IV (SI Fig. 5F), there is no evidence for subaerial exposure or erosional unconformity at the base of member IV or anywhere else in the Doushantuo Formation at Jiulongwan (1). Therefore, the Doushantuo Formation at the studied section seems to represent a very condensed Ediacaran succession with likely small-scale hiatuses associated with very slow deposition.

Although these lithostratigraphic members have been used informally to describe the Doushantuo Formation as a whole, they are only easily recognizable in the southern limb of the Huangling anticline. Correlation of these lithostratigraphic members to Doushantuo successions elsewhere in South China is challenging, in part because of drastic thickness and facies variability. For example, the Doushantuo Formation at the Xiaofenghe section (see SI Fig. 3 for location) (7), located ≈60 km north of the Jiulongwan section on the northeastern limb of the Huangling anticline (SI Fig. 4B), is composed of a basal cap carbonate that is overlain by a succession of quartzose shale, phosphatic packstone-grainstone, and cherty dolostone, which, in turn, passes upward into argillaceous limestone. The dominance of phosphatic and quartz clasts and the larger grain size suggest that the depositional environment at Xiaofenghe was likely shallower-water, higher-energy, and closer to a terrestrial source. Farther north, the Zhangcunping section (SI Fig. 4C) consists of terrigenous shale and phosphorite that passes upward into shallow-water phosphatic dolomite with breccias, cross-bedding, and abundant scours. Erosional unconformities are recognized above a 5-m phosphorite deposit in the middle of the Doushantuo formation and at the Doushantuo-Dengying boundary in the Zhangcunping section.

Although regional correlation between sections is challenging because of such variations in lithostratigraphy and thickness, the mid-Doushantuo sequence boundary at the Zhangcunping section might be correlated with the top of the cycle 2 at the member II-III transition at the Jiulongwan section (8).

Several hypotheses surround the depositional environment of the Doushantuo Formation in the Yangtze Gorges area: (i) open marine conditions (9); (ii) an intrashelf basin that developed as a result of faulting and rapid transgression during deglaciation of the Nantuo glaciation (10); or (iii) member II in the Yangtze Gorges area representing a long-lasting alkaline lake that progressively became open marine in member IV (11). Although we cannot dismiss the possibility that the Doushantuo sediments were accumulated in a partially restricted basin, it is likely that the basin was connected to the open ocean. A marine origin of member II in the Yangtze Gorges area is supported by the following arguments. First, member II black shale is widely distributed in South China and is traceable beyond the Yangtze Gorges region (8, 12). Second, the stratigraphic contact between member II black shale and the underlying cap carbonate (member I) is conformable and traceable beyond the Yangtze Gorges region (3, 6). The Doushantuo cap carbonate is similar to equivalent cap carbonates elsewhere and is likely of marine in origin (6, 13). Third, the macroalgal fossil Enteromorphites occurs in member II black shale at the Jiulongwan section (14), as well as supposedly marine shales in member IV at the Miaohe section in the Yangtze Gorges area (15) and at the Jiangkou section in the more offshore central Guizhou region (16). Fourth, member II in the Yangtze Gorges area contains a number of acritarch taxa that have a wide geographic distribution in and beyond South China (5, 17). Finally, bio-, chemo-, and sequence-stratigraphic evidence supports the correlation between member II in the Yangtze Gorges area and lower Ediacaran successions in northern India and elsewhere (1, 5, 8, 12, 13).

Additional Description and Discussion of Geochemical Data

Total Organic Carbon, Pyrite, and Carbonate-Associated Sulfate. SI Fig. 6 and SI Table 1 show all geochemical trends of the Doushantuo Formation, including total organic carbon (TOC), pyrite concentrations, carbonate-associated sulfur (CAS) concentrations, d¹⁸O_carb, d¹³C_org, d¹³C_carb, Dd¹³C, d³⁴S_py, d³⁴S_CAS, and Dd³⁴S.

TOC compositions are highly variable but are generally enriched within argillaceous units of member II (up to 4%) and member IV (up to 8%). In contrast, member III records very low TOC (<0.1%). Pyrite compositions show a similar stratigraphic pattern, with high concentrations in members II and IV (up to 12%) and low concentrations in member III (<1%). CAS concentrations vary widely throughout the Doushantuo Formation. Most samples have CAS concentrations between 50 and 600 ppm, but some in members I and II have CAS concentrations greater than 1200 ppm. There seems to be a consistent up-section increase in CAS concentrations in members III and IV, from <200 to ≈1200 ppm, which is coincident with decreasing d³⁴S_CAS and d³⁴S_py trends.

The TOC and pyrite concentration data are consistent with the geochemical model outlined in the text. High TOC and pyrite concentrations in members II and IV provide additional evidence for an anoxic depositional environment below wave base. It is possible that anoxia in member II resulted from pulses of high bioproductivity, whereas that in member IV was driven by oceanic upwelling.

Diagenetic Evaluation. Diagenetic alteration of the carbonate succession at Jiulongwan was evaluated by using petrographic observations and geochemical cross-plots. Samples for d¹³C_carb and d¹⁸O_carb analysis were carefully drilled micritic carbonates; veins, vugs, calcispars, or dolospars were avoided. Samples for d¹³C_org, d³⁴S_py, and d³⁴S_CAS analysis were carefully selected fresh rock chips to avoid surface contamination.

Despite these measures, it is possible that isotopic variability of the Doushantuo Formation may reflect some degree of diagenetic overprinting during dolomitization and early burial (18). However, the stratigraphic consistency of d¹³C_carb and d¹³C_org profiles (SI Fig. 6) suggests that the overall secular patterns are preserved. In addition, the d¹³C_carb profiles reported here are almost indistinguishable from d¹³C_carb profiles at nearby sections in the Yangtze Gorges area (8, 12, 13), indicating secular rather than completely diagenetic signatures. Furthermore, overall interregional similarity to Ediacaran d¹³C_carb profiles in India (19), Oman (20), Australia (21), and the southwestern United States (22) is difficult to explain in terms of diagenetic alteration. Finally, although the combined d¹³C_carb-d¹⁸O_carb cross-plot shows minor covariation and wide variability, there is no significant covariation with respect to cross-plots subdivided according to lithostratigraphic members (SI Fig. 7A), suggesting that diagenetic alteration likely did not play a significant role.

The sulfur data are more variable at meter scale and are viewed with caution. As expected, pyrite concentrations are positively correlated with TOC concentrations (SI Fig. 7B), suggesting that Doushantuo sedimentary pyrites are marine in origin (23). However, pyrite concentrations and isotopes could be compromised by contamination of organic-bound sulfur when organic-rich samples were analyzed by using the direct combustion method. To test the direct combustion method, organic-rich intervals from member IV were analyzed independently by using both chromium reduction and combustion methods. The results were comparable, with little variability throughout member IV despite the high TOC and pyrite concentrations.

CAS concentrations and isotopic values may also be affected by oxidation of pyrite and organically bound sulfur during laboratory preparation of pyrite-rich samples (24), despite our attempt to remove pyrite-derived sulfur by using 30% hydrogen peroxide or 5.25% NaOCl. This can be an issue, particularly in some pyrite-rich samples from lower member II and member IV. However, several pyrite-poor samples from member I also have very high CAS concentrations (up to 2000 ppm). A cross-plot of pyrite concentration against CAS concentration does not show significant correlation (SI Fig. 7C) regardless of whether the data are combined or divided according to lithostratigraphic members. Furthermore, if d³⁴S_CAS was strongly influenced by pyrite oxidation during laboratory preparation, there should be strong negative correlation between d³⁴S_CAS and CAS concentration (SI Fig. 7D), negative correlation between d³⁴S_CAS and pyrite concentration (SI Fig. 7E), and positive correlation between d³⁴S_CAS and d³⁴S_py (SI Fig. 7F). Our data do not show the expected correlations. Thus, we conclude that pyrite oxidation during laboratory preparation alone cannot explain our data.

The behavior of d³⁴S_CAS during dolomitization has not been completely understood. However, previous investigations have shown that d³⁴S_CAS of calcite seems to be buffered against diagenetic change (25, 26) and d³⁴S_CAS values of Mesoproterozoic dolostone match those of co-occurring evaporates (27), which lends support to the d³⁴S_CAS proxy for sea-water sulfate.

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