Mercury anomalies across the Ediacaran–Cambrian boundary: Evidence for a causal link between continental erosion and biological evolution
Introduction
The Ediacara biota (570 Ma – 539 Ma?) consists mainly of tubular and frond-shaped multicellular eukaryotes and was an enigmatic group of soft-bodied organisms without modern analog (Xiao and Laflamme, 2009, Boag et al., 2016, Zhu et al., 2017, Darroch et al., 2018). Their disappearance at the end of the Ediacaran was followed by the relatively sudden appearance of most major animal phyla in the early Cambrian (c.f. Darroch et al., 2018, Wood et al., 2019). This biological evolution has been causally related to environmental perturbations across the E–C boundary, where the Ediacara biota might have been extinct due to ocean anoxia/euxinia (Kimura and Watanabe, 2001, Schroder and Grotzinger, 2007, Wille et al., 2008, Jiang et al., 2009), and/or outcompeted by the Cambrian fauna because of ocean oxygenation (c.f. Laflamme et al., 2013, Zhu et al., 2017, Darroch et al., 2018). Possible triggers for those redox variations of E–C seawaters include ocean upwelling (Banerjee et al., 1997), continental denudation led by tectonic movement and sea-level changes (Peters and Gaines, 2012, Shields and Mills, 2017, Wei et al., 2019), and the emplacement of large igneous provinces (LIPs; Wang et al., 2020). Recently, a growing number of studies considered the enhanced terrestrial input as one of the major causes for the long-term atmospheric and oceanic oxygenation from the middle Ediacaran to the Cambrian (e.g., Wei et al., 2019, Fan et al., 2020). However, geochemical clues that can reflect the linkage among the enhanced terrestrial input, environmental changes, and biological evolution across the E–C boundary are still deficient.
Mercury (Hg) concentration and its isotopic composition in marine sediments have recently been developed as novel proxies for the understanding of biological evolutions, large-scale volcanism, the enhanced terrestrial input, and ocean anoxic events in the Earth’s history (Zheng et al., 2018, Grasby et al., 2019, Pruss et al., 2019, Zerkle et al., 2020). The emplacement of LIPs has been considered as a direct trigger for most globally comparable Hg anomalies in marine sediments, particularly for those in the period of mass extinction (Ernst and Youbi, 2017, Grasby et al., 2019). It increases the Hg input into the ocean through directly increasing atmospheric Hg deposition (Thibodeau et al., 2016, Grasby et al., 2017, Jones et al., 2019) and/or inducing large-scale surface weathering to cause an enhanced watershed runoff of Hg in the soil (Grasby et al., 2017, Shen et al., 2019, Them et al., 2019, Shen et al., 2020). Except for LIPs, a high terrestrial Hg input can also be caused by the enhanced continental erosion in the period of rapidly tectonic movement and sea-level changes (Fan et al., 2020). Mercury isotopes, especially the mass-independent fractionation (MIF) signature (denoted as Δ199Hg), provide clear constraints on whether the Hg anomalies are sourced from atmospheric deposition or crustal erosion (Blum et al., 2014, Grasby et al., 2019, Them et al., 2019). Besides, there are also Hg and Δ199Hg anomalies in sulfide-rich sediments deposited in the euxinic photic zone (Zheng and Hintelmann, 2009, Zheng et al., 2018), and Hg content in these sediments are positively correlated to their total sulfur contents (TS).
Recently, extremely high Hg concentrations (up to several ppm) have been sparsely reported in early Cambrian phosphorites and black shales in South China (Yin et al., 2017, Wang et al., 2020). But the origin of Hg and its relationship with the E–C biological evolution are not well constrained due to 1) the lack of Hg data for the late Ediacaran sediments and 2) the lack of Hg data for sediments related to other cratons except South China. There was a strong palaeogeographic affinity between the Indian Craton and South China during the Ediacaran and Cambrian period (Fig. 1), owing to the correlative lithological sequences (Jiang et al., 2003) and fossil records, e.g., Wengania exquisite (Hughes et al., 2015) and Shaanxilithes ningqiangensis (Tarhan et al., 2014). Paleocurrent reconstruction and detritus signals indicate that marginal E–C basins of the northern Indian Craton were fertilized by the erosion of giant orogens after the collisions between Eastern and Western Gondwana (Wang et al., 2019). In this paper, the concentration, and isotopic compositions of Hg in the E–C sedimentary rocks from the Mussoorie section, Lesser Himalaya, India are presented (N 30°27′06.8″; E 78°09′13.1″; Fig. 1, Fig. 2). In combination with previous works on early Cambrian sediments, South China, our new dataset is used to investigate a possible linkage among the intensified continental erosion, ocean biogeochemical cycling, and the E–C biological evolution.
Section snippets
Geological background
Ediacaran–Cambrian marine sediments in the Lesser Himalaya of India were exposed as a result of the ongoing continent–continent collision between the Indian and Eurasian plates (McKenzie et al., 2011, Colleps et al., 2018, Wang et al., 2021). The Ediacaran Krol Group, dominated by carbonates, has been sub-divided into the Krol A, B, C, D and E members according to textural and compositional variations (Jiang et al., 2002) (Fig. 2, Fig. 3 and A1). The D and E members of the Krol Group are coeval
Scanning electron microscope
Petrographic data were collected on freshly broken chips of samples, using a Hitachi S-1530 variable pressure scanning electron microscope (SEM) at the Electron Microscope Unit, the University of Hong Kong (Liu and Zhou, 2017). Iron and P distributions in a Fe-(hydro)oxide grain were obtained on a polished thin section (Fig. 4), using a JXA-8230 electron microprobe at the University of Hong Kong. The elemental mapping was performed in wavelength dispersive mode (WDS) with an accelerating
Results
Dolomites from the basal Krol D Member to lower Krol E Member (Interval I; Fig. 3) generally have low contents of Hg (2 – 18 ppb) (Fig. 5), Al (<0.52 wt%), TOC (<0.6 wt%) and TS (<0.03 wt%) (Table 1). A 0.25-m-thick siltstone from the lower Krol D Member with an Al2O3 content of 17.7 wt% has a Hg content of 16 ppb; and a 1-m-thick carbonaceous layer from the upper Krol D Member with a TOC content of 1.62 wt% has a Hg content of 39 ppb (Fig. 5). The ratio of Rb/Sr in dolomites from Interval I
Hg enrichment in marine sediments across the E–C boundary
Ediacaran–Cambrian marine sediments from the Mussoorie section, Lesser Himalaya, record a positive excursion of Hg (Fig. 5), in that the concentration of Hg increases from several ppb in dolomites of the basal Krol D Member (Interval I) to hundreds of ppb in silty dolomites of the upper Krol E Member (Interval II), and to thousands of ppb in the cherty phosphorites of the lower Tal Group (Interval III) (Fig. 5). The concentration of Hg keeps high (>6000 ppb) in a ∼0.6-m black shale layer
Conclusions
The Hg anomalies in marine sediments from the Mussoorie section indicate that continental erosion had a profound influence on the ocean biogeochemistry and redox states at the E–C transition. Intensified crustal erosion had increased the primary productivity and nutrient in the E–C ocean. Weathering of organic-rich sediments on land and upwelling of nutrient-rich water led to a suboxic-anoxic condition in the ocean surface across the E–C boundary, which potentially was the dominant cause for
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was financially supported by the National Natural Science Foundation of China (41772087 and 41972242) and the Fok Ying Tung Education Foundation (171013). We thank Rajesh Sharma, Nitesh Mishra and Junhong Zhao for their assistance in the field and Runsheng Yin for the Hg analysis. We thank Prof. Xiangdong Li for reading an early draft of this paper and for his insightful comments. We gratefully acknowledge the support of the University Research Facility in Chemical and
Research data
Research Data associated with this article can be accessed at https://doi.org/10.6084/m9.figshare.13509597.v1.
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