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Earth and Planetary Science Letters a Pulse of Oxygen Increase in The Earth and Planetary Science Letters 527 (2019) 115797 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl A pulse of oxygen increase in the early Mesoproterozoic ocean at ca. 1.57–1.56 Ga ∗ ∗ Mohan Shang a,b, Dongjie Tang a,c, , Xiaoying Shi a,b, , Limin Zhou d, Xiqiang Zhou e, Huyue Song f, Ganqing Jiang g a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Beijing), Beijing 100083, China b School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China c Institute of Earth Sciences, China University of Geosciences (Beijing), Beijing 100083, China d National Research Center of Geoanalysis, Beijing 100037, China e Key Lab of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China f State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan), Wuhan 430074, China g Department of Geoscience, University of Nevada, Las Vegas, NV 89154-4010, USA a r t i c l e i n f o a b s t r a c t Article history: The relationship between oxygen and evolution of early eukaryotes including algae and primitive animals Received 2 January 2019 in geological history has been debated, partly due to the varying estimates of oxygen levels in the mid- Received in revised form 25 July 2019 Proterozoic (ca. 1.8–0.8 Ga) ocean and atmosphere. The upper part of the Gaoyuzhuang Formation (ca. Accepted 29 August 2019 1.60–1.54 Ga) in North China hosts decimeter-scale multicellular eukaryotic fossils and is documented Available online xxxx with a decrease in cerium anomaly indicative of ocean oxygenation. However, the atmospheric oxygen Editor: I. Halevy level across this interval and its subsequent oxidation state require further investigation using additional + 13 13 Keywords: redox proxies. Here we report I/(Ca Mg) ratios, carbonate/organic carbon isotopes (δ Ccarb and δ Corg), boring billion and phosphorous (P) contents across the ca. 1.57–1.56 Ga fossil-bearing interval in the North China oxygenation episode Platform. High I/(Ca+Mg) ratios (≥2.6 μmol/mol; up to 3.8 μmol/mol) from shallow-water carbonates Gaoyuzhuang Formation of the Gaoyuzhuang Formation suggest an episode of significant oxygen increase up to ≥4% PAL (present I/(Ca+Mg) atmospheric level). The I/(Ca+Mg) ratios return back to ≤0.5 μmol/mol shortly after the peak values North China Platform without evidence for increasing water depth or diagenetic alteration, implying a short-lived oxidation 13 13 event. The increase of I/(Ca+Mg) ratios is associated with a −3.5h negative δ Ccarb and δ Corg anomaly and an increase in P/Al ratios that are best explained by oxidation of dissolved organic carbon (DOC) in the ocean. Oxygen consumption through oxidation of DOC may have quickly lowered marine and atmospheric O2 levels to the early mid-Proterozoic (1.8–1.4 Ga) background oxygen concentration of ≤0.1–1% PAL. Short-lived oxidation events in an overall anoxic mid-Proterozoic ocean and atmosphere best explain the existing geochemical data and evolutionary stasis of eukaryotes during the “Boring Billion”. © 2019 Elsevier B.V. All rights reserved. 1. Introduction basins at ca. 1.85 Ga (Planavsky et al., 2018a), ca. 1.57–1.54 Ga (Tang et al., 2016; Zhang et al., 2018), ca. 1.4 Ga (Cox et The mid-Proterozoic (ca. 1.8–0.8 Ga) witnessed the emergence al., 2016; Hardisty et al., 2017;Mukherjee and Large, 2016; but slow diversification of eukaryotes (Butterfield, 2015; Knoll, Sperling et al., 2014; Yang et al., 2017; Zhang et al., 2016), and 2014). Multi-proxy geochemical studies suggested that the mid- ca. 1.1 Ga (Gilleaudeau et al., 2016). The overall weakly oxy- Proterozoic ocean was mostly ferruginous with euxinic wedges genated condition was possibly caused by low primary produc- on shelf margins (e.g., Luo et al., 2014; Planavsky et al., 2011; tion and low organic carbon burial flux (Crockford et al., 2018; Poulton and Canfield, 2011) and perhaps episodes of increased Ozaki et al., 2019). Estimates of mid-Proterozoic atmospheric O2 oxygen or spatially-limited oxygenation in some sedimentary levels also vary significantly. Earlier assessments on the basis of iron retention in paleosols suggested atmospheric O2 con- centrations of ≥1–3% PAL [present atmospheric levels] (Rye and * Corresponding authors at: State Key Laboratory of Biogeology and Environmen- tal Geology, China University of Geosciences (Beijing), Beijing 100083, China. Holland, 1998;Zbinden et al., 1988; but see Planavsky et al., E-mail addresses: [email protected] (D. Tang), [email protected] (X. Shi). 2018b for a different view). Paleoenvironment and diagenetic mod- https://doi.org/10.1016/j.epsl.2019.115797 0012-821X/© 2019 Elsevier B.V. All rights reserved. 2 M. Shang et al. / Earth and Planetary Science Letters 527 (2019) 115797 els constructed from trace element concentrations, organic car- have been interpreted as deposits from shallow subtidal to in- bon contents and biomarkers of the Mesoproterozoic Xiamaling tertidal environments (Mei, 2008). Member II consists of man- Formation (ca. 1.40–1.35 Ga) in North China also suggested at- ganiferous dolostone in the lower part and medium- to thick- mospheric O2 levels of ≥4–8% PAL (Zhang et al., 2016, 2017). bedded dolostone in the upper part. The lower Member II con- However, the lack of observable chromium (Cr) isotope fraction- tains dark shale interbeds with manganese concretions and has ation in mid-Proterozoic marine ironstones and shales, and Ce been interpreted as subtidal deposits (Mei, 2008). The upper Mem- anomalies in carbonates implies a much lower O2 level that was ber II contains microbially induced sedimentary structures and likely <0.1–1% PAL (Bellefroid et al., 2018;Cole et al., 2016; mud cracks indicative of deposition from intertidal to suprati- Planavsky et al., 2014). In contrast, Cr isotopes from the 1.1–0.9 Ga dal environments (Fig. 2B). The lower to middle part of Mem- marine carbonates show considerable fractionations and suggest ber III is composed of thin-bedded muddy dolostone (Fig. 2C and atmospheric O2 concentrations of >0.1–1% PAL (Gilleaudeau et al., D), calcareous mudstone with cm- to dm-sized carbonate concre- 2016). Similarly, high Cr isotopes from the Shennongjia Group in tions (Fig. 2E) and some thrombolite layers (Fig. 2F). The gen- South China suggest atmospheric O2 levels of >1% PAL since ca. eral lack of wave- and tide-agitated sedimentary structures in 1.33 Ga (Canfield et al., 2018). this part suggests deposition in low-energy environments proba- The higher O2 estimates for mid-Proterozoic surface environ- bly close to storm wave base (Guo et al., 2013;Luo et al., 2014; ments would argue against oxygen limitation as an evolutionary Mei, 2008), while the layered thrombolites were likely formed barrier for eukaryotes (particularly animals) because many early in subtidal environments (Tang et al., 2013). The upper part of stem-group animals and sponges have a minimum oxygen require- Member III consists of laminated microbial dolostones with flaser ment of 0.5–4% PAL (e.g., Mills et al., 2014;Sperling et al., 2013; bedding (Fig. 2G) and interference ripple marks, suggestive of de- Zhang et al., 2016). However, most geochemical data used for position from shallow subtidal to intertidal environments. Member O2 estimation were obtained from short stratigraphic intervals in IV of the Gaoyuzhuang Formation is characterized by massive mi- geographically distinct sedimentary basins, which leads to uncer- crobial reef dolostones with a total thickness up to 450 m (Mei, tainties about the stability of O2 levels in the mid-Proterozoic. 2008). Conical stromatolites (Conophyton-like) over 2 m high and Although many have assumed that atmospheric O2 concentrations 35 cm wide and various other microbialites (cf. Bartley et al., 2015) remained relatively stable in the mid-Proterozoic, either at lower are common in this member, suggesting subtidal depositional en- or higher levels (e.g., Lyons et al., 2014; Planavsky et al., 2014; vironments at least episodically below fair-weather wave base. Pre- Zhang et al., 2016), there exists a possibility that O2 concentra- vious studies on carbonate fabrics and mineral compositions of the tions fluctuated significantly during the mid-Proterozoic. Gaoyuzhuang Formation suggested fabric-retentive early dolomiti- A particularly intriguing interval to test the oxygen stability zation (e.g., Zhang W. et al., 2016; Tang et al., 2017a), which may is the ca. 1.57–1.56 Ga oxidation event documented from the have helped preserve primary geochemical signatures. Gaoyuzhuang Formation of the North China Platform (Zhang et al., 2018). This interval hosts decimeter-scale multicellular eu- 3. Materials and methods karyotes (Zhu et al., 2016) that have been linked to oxygen increase in the early Mesoproterozoic. The associated decrease of cerium (Ce) anomaly from ∼1.0 to ∼0.8 was interpreted as In this study, 234 samples from Member II and III of the Gaoyuzhuang Formation in the Gan’gou section were analyzed. evidence for significant ocean oxygenation (Tang et al., 2016; Zhang et al., 2018), but whether it represents a pulse of oxy- Decimeter-sized multicellular eukaryotic fossils were not reported gen increase or the initiation of a permanently more oxygenated in this section, but the lithological and carbon isotope correlation Mesoproterozoic ocean requires further investigation. In this pa- indicates that the studied interval covers the fossil-bearing strata per, we report iodine-to-calcium-magnesium [I/(Ca+Mg)] ratios of of the Qianxi and Kuancheng sections (Zhu et al., 2016). The sam- 13 pling section is well exposed along the fresh road cuts at Gan’gou carbonate rocks, carbonate/organic carbon isotopes (δ Ccarb and ◦ ◦ 13 (40 39 35.05 N, 116 14 35.63 E), north of Beijing, China (Fig.
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