Assessing the Ediacaran Doushantuo Formation: Improving stratigraphic correlation of the Upper Doushantuo Black Shales via mercury content
Evaluación de la Formación ediacárica de Doushantuo: mejora de la correlación estratigráfica de las pizarras negras de Doushantuo superior a partir del contenido en mercurio

M. Nolan¹, S. Xiao², B.C. Gill¹, D. Jones¹, C. Zhou³

¹Department of Geosciences, Virginia Tech, Blacksburg, VA 2406, USA. Email: Mnolan1@vt.edu; ORCID ID: https://orcid.org/0000-0002-0582-651X, https://orcid.org/0000-0001-7402-0811, https://orcid.org/0000-0001-8478-3599

²Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA. ORCID ID: https://orcid.org/0000-0003-4655-2663

³CAS Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China. ORCID ID: https://orcid.org/0000-0002-5460-4278

IntroductionTOP

The Ediacaran Period is a critical time interval in the history of life, when environmental changes and evolutionary events set the stage for the Cambrian explosion of animal diversity (e.g. Marshall, 2006). The Ediacaran Doushantuo Formation of South China hosts exceptionally preserved fossils and has yielded geochemical data detailing major events, most importantly the Shuram Carbon Isotope Anomaly (e.g. McFadden et al., 2009).

ResultsTOP

Understanding these histories requires stratigraphic correlation between sections of the Doushantuo Formation, especially correlations among black shale units (Member IV, Miaohe Member, and Lower Black Shale). Conventionally, the Member IV is correlated to the tripartite Lower Black Shale, Upper Dolostone, and Miaohe Member; however, some δC_carb data suggests that only the Lower Black Shale correlates to the Member IV, thus the Miaohe Member correlates to the Shibantan Member and the Upper Dolostone correlates to the Hamajing Member of the overlying Dengying Formation (An et al., 2015). These correlations have significant consequences for interpreting the age and duration of the Shuram Event and the age of the fossils preserved in these units (Zhou et al., 2017). Building on current lithostratigraphic and stable carbon isotopic chemostratigraphic correlations, we use mercury [Hg] contents to refine stratigraphic correlations of the Doushantuo, as Hg contents can provide information on regional and potentially global volcanic input and local redox conditions, the former of which could facilitate stratigraphic correlation (see Selin, 2009 for discussion of the global mercury cycle, and Thibodeau et al., 2016 for an example of volcanic mercury correlation).

Samples were collected from six sections of the Doushantuo Formation around the Huangling Anticline of south China (Fig. 1G.). These sections include the Qinglinkuo, Zhimaping, Sixi, Jiulongwan, Maxi, and Yinyin roadcut sections (Fig.1A-F.). They represent both Member IV and tripartite sequences. The trends in mercury concentration are presented in figure 1A-F. The Zhimaping and Yinyin road sections have the lowest mercury concentrations, though both contain potential interval of Hg enrichments. In each of the six sections, there is at least one mercury concentration enrichment in either the Miaohe Member or the upper part of the Member IV, and another interval of Hg enrichment in the Lower Black Shale, or lower portion of the Member IV. For each section, this enrichment is of a greater magnitude than the upper one.

To improve sectional comparisons, mercury concentration is normalized with Total Organic Carbon content (TOC) to account for differential sedimentation rates between sections over time. TOC data are available only for the Jiulongwan section (McFadden et al., 2008). When normalized against TOC data, [Hg]/TOC data from the Jiulongwan (Fig. 2.) also show two peaks in enrichment in lower and upper parts of Member IV, again with a larger magnitude in the lower enrichment, consistent with the non-normalized data from the other sections. Beyond these signals, there are several TOC normalized mercury enrichments in the Member III at Jiulongwan. However, this unit generally has very low non-normalized mercury concentrations, and very low TOC values, suggesting the high and variable [Hg]/TOC ratios result from the low TOC values rather than actual increased sedimentary input of Hg during a volcanic or redox related event.

ConclusionsTOP

There are clear mercury enrichments in the black shale units, likely the result of volcanic input or anoxia-driven mercury deposition. If the Hg enrichments are volcanic in origin, or are due to synchronous regional anoxia, then the patterns of Hg enrichment are consistent with the conventional correlation of these units, with the tripartite units equivalent to Member IV black shales. Furthermore, a volcanic origin would offer targets for radiometric dating. The source of the Hg enrichment can be explored via comparison of mass-independent and mass dependent Hg. isotopic fractionation. Additionally comparing redox proxy data from the intervals of Hg enrichment could indicate if the enrichments are due to comparable changes in redox conditions, or increased Hg input under differing redox conditions, consistent with volcanic input.

ReferencesTOP