Reconstructing paleoceanographic conditions during the middle Ediacaran: Evidence from giant ooids in South China
Introduction
The Ediacaran (~635 to 541 Ma) was a period of intense biogeochemical change in early Earth history (Condon et al., 2005, Zhou et al., 2018). Oceanic oxygen levels are interpreted to have experienced significant variations, which impacted the emergence and evolution of complex life at that time (Fig. 1; Canfield et al., 2008, Cui et al., 2015, Sawaki et al., 2010, Sperling et al., 2015). The Shuram Excursion, which was one of the most significant negative shifts in carbonate δ13C values in geological time, has been observed globally during this time period (Fike et al., 2006, Grotzinger et al., 2011, Halverson et al., 2005, McFadden et al., 2008). Although recent studies suggest that this event could have been initiated by a globally synchronous diagenetic event (Cui et al., 2017, Grotzinger et al., 2011, Schrag et al., 2013), the utility of carbon isotopic shifts as a chemostratigraphic tool remains controversial as questions about emerging environmental conditions are addressed in Ediacaran successions. In particular, the initial timing and duration of the oxygenation of shallow marine environments in the Ediacaran are under debate due to inconsistent results obtained from varied petrographic records (Bergmann et al., 2011, Bristow et al., 2009, Cui et al., 2015, Wen et al., 2016) and geochemical proxies (Ader et al., 2014, Canfield et al., 2008, Ling et al., 2013, Sawaki et al., 2010) (Fig. 1). Therefore, it is necessary to seek a faithful seawater record that has the potential to provide a better understanding of the paleoceanographic redox conditions through space and time.
In order to be useful for the reconstruction of the paleoenvironmental conditions and oxygenation status of the shallowest seawater, a reliable sedimentary archive should preserve the geochemical signatures that reflect the original redox state of the surface ocean in which they formed, and it should be resistant to diagenetic alterations. Rare-earth element and yttrium (REY) distributions are one potential proxy for tracking the redox evolution of the surface waters of the ocean through time (Wallace et al., 2017, Webb and Kamber, 2000). Modern oceanic REY compositions are generally characterized by (1) depleted light rare earth elements (LREEs) relative to high rare earth elements (HREEs); (2) positive La and Gd anomalies and negative Ce anomalies; and (3) superchondritic Y/Ho ratios (>44) (Bau et al., 1996, Nozaki, 2001). The rare-earth element (REE) characteristics of modern marine non-skeletal carbonate sediments (e.g., microbialites and ooids) conform well to ambient seawater REE compositions in their forming processes (Li et al., 2019b, Webb and Kamber, 2000). Skeletal carbonates are not recommended for paleoseawater REE reconstruction because their relatively low concentrations are easily overprinted, and the potential effect of biological fractionation (Nothdurft et al., 2004, Webb and Kamber, 2000, Weiner and Dove, 2003). Therefore, non-skeletal carbonates have been proposed as an alternative archive of the ancient dissolved oxygen level and of the REY distributions (e.g., Nothdurft et al., 2004, Wallace et al., 2017).
The cerium (Ce) anomaly in marine carbonates is a proxy frequently used to indicate aqueous redox conditions, but when applied to ancient carbonate rocks, this method need to adequately account for the impacts of diagenesis and terrigenous contamination through different extraction techniques (e.g. Cao et al., 2020, Li et al., 2019b, Tostevin et al., 2016a, Zhang et al., 2015a). The oxidation of Ce3+ to insoluble Ce4+ decreases the amount of dissolved Ce in aqueous systems, which accordingly promotes the generation of negative Ce anomalies, i.e., a lower concentration than that of the neighboring trivalent REEs (i.e., La3+ and Pr3+), due to their similar geochemical behaviors (Henderson, 1984). However, some of the diagenetic processes that occur in porewaters or that are related to the input of exogenous impurities may lead to the alteration of the original Ce anomaly in marine carbonates (Della Porta et al., 2015, Li et al., 2019b, Ritter et al., 2015, Sholkovitz et al., 1989). Porewater REE characteristics are changeable and can be influenced by the different phases of organic matter mineralization in early diagenesis (Abbott et al., 2015, Haley et al., 2004). Carbonate REE signatures are relatively conservative during neomorphism in meteoric diagenesis (Webb et al., 2009), while in other diagenetic processes, e.g., recrystallization and dolomitization, the preservation of the primary REY distributions may be more complicated (open vs. closed systems) (Hood et al., 2018, Pederson et al., 2019). In addition, impurities such as clays and oxides in carbonates generally have flat or positive Ce anomalies, and these minerals may be extracted by different leaching sequences, depending on the acid species and molarity (Tostevin et al., 2016a). A laser-based sample introduction technique coupled with cathodoluminescence can precisely locate a target area with the least possible visible diagenetic alteration (Hood and Wallace, 2015, Kamber and Webb, 2007, Li et al., 2019b). Thus, it is particularly important to use the suitable strategy to analyze carbonate rocks with as little as possible contamination and diagenetic alternation in order to acquire the primary seawater Ce anomalies and the REEs in general.
Many studies have focused on middle Ediacaran REY signatures; however, the oxygen levels inferred by the calculated Ce anomalies reported by these studies are inconsistent (Cui et al., 2015, Hohl et al., 2015, Ling et al., 2013, Wen et al., 2016, Xin et al., 2015, Zhou et al., 2012) (Fig. 1). Although the dissolved oxygen levels of the middle Ediacaran seawater are uncertain and the corresponding Ce anomalies may not be similar to those of modern well-oxygenated seawater, previous studies have documented an almost flattened pattern, which significantly differs from those of modern seawater and modern carbonate sediments (Frimmel, 2009, Hohl et al., 2015, Hu et al., 2016, Huang et al., 2009, Wen et al., 2016). In fact, the available data for various water-depth conditions (surface, shallow, and deep oceans in Ce-anomaly signatures) (German and Elderfield, 1990, Hood and Wallace, 2015, Planavsky et al., 2010), depositional settings (open versus restricted, and clean versus dirty) (Bayon et al., 2015, Frimmel, 2009, Zhao et al., 2009), and analytical techniques (Strong- and weak-acid digested, sequential leaching, and laser ablation) (Cao et al., 2020, Li et al., 2019b, Tostevin et al., 2016a) suggest markedly different characteristics of REE compositions and Ce anomalies in the carbonate sediments. Extracting reliable records of REE concentrations in order to interpret the redox state during this critical period in Earth history still presents a challenge.
Oolite deposits in middle Ediacaran strata may be a useful archive for palaeoceanographic reconstruction. Numerous oolites have been described in the middle Ediacaran carbonates (e.g. Bergmann et al., 2011, Trower and Grotzinger, 2010). Notably, giant ooids, with diameters > 2 mm that contain consistent geochemical and petrographic features with normal-sized ooids (Beukes, 1983, Richter, 1983, Sumner and Grotzinger, 1993), have been observed in middle Ediacaran deposits in Death Valley of California, Sadlerochit Mountains of Alaska, Huqf area of Oman, and Zhuya of Siberia (Bergmann et al., 2011, Loyd et al., 2012, Macdonald et al., 2009, Melezhik et al., 2008). Although temporal distributions of giant ooids were principally reported in Proterozoic records (Sumner and Grotzinger, 1993, Tang et al., 2015, Thorie et al., 2018, Trower and Grotzinger, 2010), and only a few intervals after the Cambrian Period (e.g., Early Triassic) (Li et al., 2013), widespread occurrences of giant ooids within specific period in Earth history may provide insights into global heterogeneity of shallow ocean temperature, chemistry, and carbonate saturation state (Sumner and Grotzinger, 1993, Thorie et al., 2018). In many of these case studies, giant ooids were interpreted to form in high-energy, open-ocean settings (Li et al., 2019a, Li et al., 2013, Thorie et al., 2018), and their early dolomitization preserved the original fabrics in pore waters similar to contemporaneous seawater (Hood et al., 2018, Mueller et al., 2020). Given the location and environmental conditions at the time of their deposition, and the retention of original fabrics, middle Ediacaran giant ooids have the potential to be a reliable archive for paleoceanographic conditions and REY compositions of contemporaneous seawater that is currently lacking.
Therefore, the aims of this work are (1) to introduce the petrographic and geochemical characteristics of the giant-ooid-bearing beds and associated neighboring strata in South China and compare these characteristics with global records for the middle Ediacaran; (2) to decipher the paleoceanographic conditions in the upper part of the second Ediacaran positive carbon isotopic shift (EP2) and explorer the possible mechanism of giant-ooid formation in South China; and (3) to extract reliable Ce anomalies and REY patterns from giant ooids using in situ laser ablation inductively coupled plasma-mass spectroscopy (LA-ICP-MS) for the understanding of the degree of oxygenation in the surface ocean of this period.
Section snippets
Geological setting
During the Ediacaran, the South China Craton, which comprises the Yangtze and Cathaysia blocks, was an isolated continent surrounded by oceans at the middle-low latitudes of the northern hemisphere (Sahoo et al., 2016, Zhang et al., 2013, Zhang et al., 2015b) (Fig. 2A and B). The Yangtze Block was covered by extensive carbonate and siliciclastic sediments during the Ediacaran, and it can be further divided into shelf interior, margin, and basin settings (Jiang et al., 2011) (Fig. 2B and C). The
Petrographic analyses
Bulk rock samples were collected from the Nanshanping section throughout the Doushantuo Formation with a resolution of 1.5–2 m (102 samples in total) for petrographic and geochemical analyses. Polarizing (Leica DM4500, China University of Petroleum, Beijing) and cathodoluminescence (RELIOTRON III, RELION Industries; current 5–8 kV, Institute of Geology and Geophysics, CAS) microscopes were used to examine the petrographic textures and to identify any diagenetic alterations. The thin-sections
Carbonate δ13C values
In this study, bulk rock stable δ13C and δ18O values were measured from the base to the top of the Doushantuo Formation (Fig. 3 and Table S1). The original values are listed in Table S1. Three negative and two positive shifts in δ13C values were identified in the Doushantuo Formation (Fig. 3), exhibiting a trend very similar to those of other neighboring well-studied sections, e.g. the Yangjiaping and Zhongling sections (Ader et al., 2009, Cui et al., 2015, Zhu et al., 2007). Stratigraphically,
Regional chemostratigraphic correlation of the giant-ooid bearing oolites in South China
From a chemostratigraphic perspective, three negative (EN1, EN2 and EN3a) and two positive (EP1 and EP2) δ13C shifts were identified in the Nanshanping section, which are comparable with the shifts in the well-studied neighboring sections of the inner shelf to shelf slope environments. The cap carbonate of the basal part of the Doushantuo Formation in Nanshanping corresponds to the first negative shift in δ13C values (Fig. 3), so it is generally regarded as a marker horizon in Ediacaran strata (
Conclusions
- (1)
The deposition of giant ooids was diachronous during the middle Ediacaran. The giant ooids in the Nanshanping section exhibit well-formed morphologies, with alternating dark- and light-colored laminae, which likely experienced early marine dolomitization. The occurrences of giant-ooid-bearing beds in different blocks were not synchronous, and the petrographic characteristics of giant ooids in middle Ediacaran are also different.
- (2)
As a reliable warm-water indicator, widespread oolite records
CRediT authorship contribution statement
Chaojin Lu: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Visualization. Fei Li: Conceptualization, Methodology, Resources, Validation, Writing - review & editing, Funding acquisition, Project administration, Supervision. Amanda M. Oehlert: Formal analysis, Writing - review & editing. Jie Li: Writing - review & editing. Huayao Zou: Resources, Project administration, Funding acquisition, Supervision.
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 study was supported by the Strategic Priority Research Program of the Chinese Academy of Science (No. XDA14010306), NSFC (No. 41872119), and China Petroleum and Chemical Corporation Research Program (No. P16108). Discussions with Peter K. Swart, Noah Planavsky and Francis A. Macdonald are greatly appreciated. We are also grateful for helpful comments and constructive suggestions from Huan Cui and an anonymous reviewer, as well as the editor.
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