1. Journal of the Geological Society
Geochemistry of Ediacaran cap dolostones across the Yangtze Platform, South China: implications for diagenetic modification and seawater chemistry in the aftermath of the Marinoan glaciation
by Simon V. Hohl, Harry Becker, Shao-Yong Jiang, Hong-Fei Ling, Qingjun Guo, and Ulrich Struck
Journal of the Geological Society
Volume 174(5):
September 1, 2017
© 2017 The Author(s)‏

2. Localities of studied cap dolostone sections on the Yangtze Platform.
Localities of studied cap dolostone sections on the Yangtze Platform. Palaeo-depositional setting after Jiang et al. (2011). Proximal platform sections: 1, Xiaofenghe; 2, Huajipo; 3, Jiuqunao–Jijiawan; 4, Jiulongwan (all Hubei Province); slope section: 5, Huanglianba (Guizhou Province); distal basin sections: 6, Longbizui; 7, Yanwutan (both Hunan Province). Stratigraphic column of Yangtze Platform cap dolostones modified after Jiang et al. (2003).
Simon V. Hohl et al. Journal of the Geological Society 2017;174:
© 2017 The Author(s)‏

3. Petrographic features of cap dolostones.
Petrographic features of cap dolostones. First row, thin-section scans (polarized light images), second row, thin-section microphotographs; third row, SEM images using a back-scatter electron detector (BSD). (a) Calcite bands in dolomite of Jiuqunao section. (b) Calcite veins cutting through organic carbon-rich dolomite at Jiuqunao section. The dolomite shows local dedolomitization into calcite (bright region). (c) Framboidal pyrite grain (15 µm) in dolomite and silicate matrix at Jiulongwan section. (d) Dolomite rhombs in calcite matrix at Xiaofenghe section; dedolomitization leaves ‘ghosts’ of previous dolomite rhombs. (e) Fe-rich dolomite rhombs in Mg dolomite matrix at Longbizui; the centre of some dolomites show oxidation of Fe. (f) Subhedral sulphide grains in Huanglianba (slope) cap dolostone float in a calcitic matrix with few dolomite rhombs preserved.
Simon V. Hohl et al. Journal of the Geological Society 2017;174:
© 2017 The Author(s)‏

4. PAAS-normalized REE + Y patterns of cap dolostones from proximal intra-platform (top) to distal basin (bottom) sections on the Yangtze Platform.
PAAS-normalized REE + Y patterns of cap dolostones from proximal intra-platform (top) to distal basin (bottom) sections on the Yangtze Platform. REE + Y patterns generally display HREE enrichments relative to the LREE. However, the variations in REE concentrations of the samples as well as the extent of HREE enrichment are highly variable. Whereas negative Ce anomalies occur in only some samples (left grey bar), Y/Ho fractionation as an indicator of open-marine saline waters occurs in most samples (right grey bar). A sample from the platform section at Huajipo is highlighted, as two different leaching steps were performed on the dolostone host rock and a micro-drilling into a 2 mm thick Ba-rich calcitic vein: the dotted line represents the dolomite leachate and the dashed line the calcite leachate. It should be noted that Eu abundances were corrected for Ba16O+ interferences on Eu masses in the ICP-MS analysis. The apparent negative Eu anomaly (middle grey bar) may reflect a possible overcorrection of Eu because the sample is rich in Ba (100 µg g−1). The dashed line in the Yanwutan sample set represents a duplicate of YAW 503.
Simon V. Hohl et al. Journal of the Geological Society 2017;174:
© 2017 The Author(s)‏

5. Initial 87Sr/86Sr v. oxygen and carbon isotope compositions, and Sr and Ba concentrations of cap dolostones from the Yangtze Platform.
Initial 87Sr/86Sr v. oxygen and carbon isotope compositions, and Sr and Ba concentrations of cap dolostones from the Yangtze Platform. We modelled two fluid compositions (Fluids I and II) for open-system fluid–rock interaction following Jacobsen & Kaufman (1999) to fit the observed trends in our dataset. Starting compositions (grey fields) are represented by the presumably least altered rocks in the sample suite, with trace element compositions taken from a Phanerozoic marine calcite standard (CAL-S) and the likely 87Sr/86Sr ratio of early Ediacaran seawater proposed by Jacobsen & Kaufman (1999) and Halverson et al. (2007). For most dolostones, Sr isotopic compositions increase with decreasing δ18Ocarb values and Sr and Ba concentrations. δ13Ccarb does not show well-correlated behaviour with these parameters, presumably because of decoupling. Slope and basin samples may have seen multiple stages of overprinting by radiogenic, Sr-rich sedimentary basin brines (c, fluid paths A and B). Samples marked with a cross show elevated Al concentration (>0.35 wt%) and are probably affected by clay mineral leaching.
Simon V. Hohl et al. Journal of the Geological Society 2017;174:
© 2017 The Author(s)‏

6. O v.
O v. C stable isotope compositions of the carbonate fraction in cap dolostones from the Yangtze Platform in comparison with data from Huang et al. (2011). The literature data show a clear positive correlation of the two isotope systems (especially for samples from the Jiulongwan section on the platform; dashed trend line through open circles, R2 = 0.87) suggesting a diagenetic overprint of some of the samples. With the exception of samples from Jiulongwan, data obtained for this study show no clear correlation of the two isotope systems. Symbols are as in Figure 4.
Simon V. Hohl et al. Journal of the Geological Society 2017;174:
© 2017 The Author(s)‏

7. Sr isotopic compositions and Y/Ho ratios v
Sr isotopic compositions and Y/Ho ratios v. total REE concentrations, Al concentrations, REE ratios and calcite molar percentage in carbonate leachates.
Sr isotopic compositions and Y/Ho ratios v. total REE concentrations, Al concentrations, REE ratios and calcite molar percentage in carbonate leachates. The grey dashed lines represent thresholds for possible contamination resulting from clay mineral leaching (Al >0.35 wt%, REE >12 µg g−1, PrN/YbN <1) whereas the black dashed lines in CeN/CeN* plots reveal the discrimination between positive (anoxic) and negative Ce anomalies (oxic water conditions). The horizontal line represents a threshold for Y/Ho ratios that are not clearly marine. We prefer the threshold of Y/Ho <27 (grey lower bar) as the real discriminative for continental crust values. The grey upper bar represents the lowest Sr isotopic composition proposed for early Ediacaran seawater and modern open-marine surface water Y/Ho ratios respectively. Symbols are as in Figure 4.
Simon V. Hohl et al. Journal of the Geological Society 2017;174:
© 2017 The Author(s)‏

8. Chemostratigraphic columns of cap dolostones from different localities on the Yangtze Platform.
Chemostratigraphic columns of cap dolostones from different localities on the Yangtze Platform. For comparative purposes and clarity, stratigraphic positions of all samples have been normalized to a model thickness preserving the original relative distances between sampled rocks at the outcrops (see data in Tables 2 – 4). (a) Initial 87Sr/86Sr ratios of platform, slope and basin cap dolostones from the Yangtze Platform; the vertical dashed black line represents the lowest known 87Sr/86Sr in Post-Marinoan cap dolostones from the Maiberg Formation, Namibia (Halverson et al. 2007). The right-hand dashed line represents data from a drill core of the intra-platform Wuhe–Gaojiaxi section (Sawaki et al. 2010). (b) Except for one sample from the Longbizui section all δ18Ocarb are <0. (c) Except for one sample from the Yanwutan section, almost all δ13Ccarb are <0, apart from those of the top of the Jiulongwan section, which is in accordance with recently published data for that section (dash–dot line; Huang et al. 2013). One extremely light δ13Ccarb value (−30.6) of the Huajipo section lies outside the range shown. (d) Y/Ho ratios in cap dolostones are nearly always superchondritic (dotted line indicates the PAAS value), but lower than in modern seawater (shaded area; Bau et al. 1995); data points possibly influenced by leaching of clay minerals have been marked with a cross. (e) BaO-interference-corrected and PAAS-normalized Eu/Eu* ratios show no clear trend with stratigraphic or palaeogeographical location. The positive Eu anomalies are similar to previously published data from Zhongling (dash-dot line; Huang et al. 2011). (f) Dolostones deposited on the platform generally display CeN/CeN* <1 (resembling earlier data from Huang et al. (2011); dash–dot line) and display shifts to values >1 at the top. CeN/CeN* ratios of basin-facies cap dolostones (diamonds) are ≥1, whereas samples from the lower part of the Huanglianba slope section display values <1 and a shift to positive values in the upper part (grey triangles).
Simon V. Hohl et al. Journal of the Geological Society 2017;174:
© 2017 The Author(s)‏

9. Enrichment factors of redox-sensitive elements in carbonate leachates of cap dolostones relative to pure Phanerozoic calcite standard CAL-S (= 1) versus their stratigraphic height.
Enrichment factors of redox-sensitive elements in carbonate leachates of cap dolostones relative to pure Phanerozoic calcite standard CAL-S (= 1) versus their stratigraphic height. For comparison we also show the relative enrichment of the recent Porites coral standard (JCp-1; red/grey dashed line). For comparative purposes the heights of all sections were normalized to a model height, preserving the original relative distances between sampled rocks at the outcrops. Redox-sensitive elements Fe, V and Mo in platform sections are depleted relative to CAL-S but enriched relative to JCp-1, whereas U is depleted compared with both reference materials and Mn is enriched. Samples from the Longbizui section (basin) show a general trace metal enrichment compared with platform and slope samples.
Simon V. Hohl et al. Journal of the Geological Society 2017;174:
© 2017 The Author(s)‏

10. Trace metal enrichments relative to the CAL-S carbonate reference standard (black dashed line).
Trace metal enrichments relative to the CAL-S carbonate reference standard (black dashed line). For comparison the relative enrichments of recent Porites coral (JCp-1) and a Permian dolomite reference material (JDo-1) are shown by dashed lines. (a) Positive correlations of Mn/Fe with positive Ce anomalies (grey bar indicates 5% uncertainty array around Ce/Ce* baseline) argue for MnO2 dissolution (black arrows) in anoxic–manganous waters (less than 10 µM O2). (b) Mn-enriched dolomites show high Mn/Fe ratios and form an Ediacaran manganous dolomite array. Highlighted data points indicate true marine samples with Y/Ho >36. (c, d) Mo enrichments with increasing (and positive) CeN/CeN* ratios in basinal cap dolostones match manganous seawater. (e, f) Basinal carbonate leachates from Longbizui section (diamonds) reveal a strong enrichment of Fe that correlates positively with CeN/CeN* and Mn enrichments.
Simon V. Hohl et al. Journal of the Geological Society 2017;174:
© 2017 The Author(s)‏

11. Schematic model of the depositional conditions on the Yangtze Platform (profile modified after Och et al. 2016) at the time of cap dolostone precipitation, following the Neoproterozoic redox model of Tostevin et al.
Schematic model of the depositional conditions on the Yangtze Platform (profile modified after Och et al. 2016) at the time of cap dolostone precipitation, following the Neoproterozoic redox model of Tostevin et al. (2016b). An intermediate layer of manganous water conditions (dissolution of MnO2) led to the release of Ce into seawater, which produced positive or no Ce anomalies in carbonate leachates. Positive Ce anomalies of each studied section correlate with the lightest C isotope compositions and Mn enrichments. In shallow-water cap dolostones Mn/Fe ratios and Mn enrichments are correlated whereas in deep basinal sections the proxies are decoupled owing to the increased incorporation of Fe into these carbonates. Meltwater influx may further suppress the Y/Ho fractionation in proximal platform samples. (Plotted values represent the median of the obtained proxies of carbonate leachates from each studied section.)‏
Simon V. Hohl et al. Journal of the Geological Society 2017;174:
© 2017 The Author(s)‏