Trace and Rare Earth Elemental Geochemistry of Carbonate

Journal of Palaeogeography 2013, 2(2): 209-221 DOI: 10.3724/SP.J.1261.2013.00027

Geochemistry and sedimentaryPalaeogeography environment

Trace and rare earth elemental geochemistry of carbonate succession in the Middle Gaoyuzhuang Formation, Pingquan Section: Implications for Early Mesoproterozoic ocean redox conditions

Guo Hua1, Du Yuansheng1, *, Zhou Lian2, Yang Jianghai1, Huang Hu1 1. State Key Laboratory of Biogeology and Environmentary Geology, China University of Geosciences (Wuhan), Wuhan 430074, China 2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan), Wuhan 430074, China*

Abstract The concentrations of redox-sensitive trace elements, such as uranium (U), vanadium (V), molybdenum (Mo), cobalt (Co), chromium (Cr) and rare earth elements (REE+Y) were determined in a given carbonate succession in the Gaoyuzhuang Formation (~1.56 Ga), which spans depths from outer shelf to intertidal, to explore the Early Mesoproterozoic ocean redox conditions. The values of the Zr-normalized redox‑sensitive trace element concentrations and some relevant ratios show obvious changes from bottom to top in the succession. Samples from the outer shelf setting (M1 interval) demonstrate significantly enhanced values in Zr-normalized redox‑sensitive trace element concentrations and relevant ratios (the peaks of Mo/U, V/Cr and Ni/Co ratios larger than 8, 4.25, and 7, respectively). Authigenic framboidal pyrites were also found within oncolite-like carbonate concretions and surrounding host rocks in this interval. These all indicate a euxinic state in the outer shelf environment. Less enrichment of Zr-normalized redox-sensitive elemental abundances and a mild decrease in the values of geochemical ratios were present in the inner shelf environment (M2 interval) (the V/Cr and Ni/Co ratios fall into a range of 2.5-4.25 and 4-5, respectively), suggesting dysoxic conditions dominant in the inner shelf setting. Samples from the shallower subtidal and intertidal settings (M3 and M4 intervals) are mostly invariable with much lower values of Zr-normalized redox-sensitive elements and relevant ratios, with the V/Cr and Ni/Co ratios typically near or less than 2 and 5 respectively, indicative of oxic conditions in the high-energy subtidal/ intertidal zones. A remarkable negative Ce anomaly exhibited in the shale-normalized REE+Y diagram in the M4 interval may provide evidence in support of the hypothesis. Taken together, our results suggest a relatively shallow chemocline in the Early Mesoproterozoic ocean: the transitions between euxinic, dysoxic and oxic may occur in quiet-water outer shelf and high- energy subtidal zone, respectively. The presence of euxinic ocean bottom waters is compatible with low concentrations of seawater sulfate and reduced levels of atmospheric oxygen during this period. The extreme environmental conditions induced by these anoxic oceans could have been responsible for the delayed oxygenation of the biosphere and hindered the evolution of multicellular life.

* Corresponding author. Email: [email protected]. Received: 2012-08-13 Accepted: 2012-12-15 210 JOURNAL OF PALAEOGEOGRAPHY Apr. 2013

Key words trace elements, rare earth elements, Mesoproterozoic, redox, Gaoyuzhuang Formation, Pingquan

1 Introduction Province, about 300 km northeast of Beijing (Fig. 1). The Mesoproterozoic strata from the Changzhougou Formation Oxygenation of the Earth′s surface is increasingly to Wumishan Formation were well developed in the study thought to have occurred in two major steps. The first large area. The Gaoyuzhuang Formation consists predominantly increase in atmospheric oxygen levels occurred about 2.4 of microcrystalline dolostones, dolomitic limestone, argil- -5 billion years ago, when Po2 rose from less than 10 of the laceous limestone and silicified dolostones interpreted to present atmospheric level (PAL) to more than 0.01 PAL have been deposited in tidal and shallow ocean environ- (Goldblatt et al., 2006; Holland, 2006). A further increase ments (Fig. 1). The detailed description of the lithology, took place during the Late Neoproterozoic period, this depositional environments and habitat types of the Gaoy- increase may have led to oxygenation of the deep ocean uzhuang Formation in Pingquan Section is presented by and therefore the evolution of metazoans and multicellu- Guo et al. (2010). Recent zircons from a tuff bed in the lar algae in the Ediacaran period (Des Marais et al., 1992; upper part of this formation yielded U-Pb ages of 1559±12 Knoll and Carroll, 1999; Rothman et al., 2003; Fike et al., Ma (SHRIMP) and 1560±5 Ma (LA-MC-ICPMS) (Li et 2006). Such a two-staged oxidation implies a unique ocean al., 2010). Lu and Li (1991) reported U-Pb ages of 1625±6 chemistry for much of the Proterozoic eon, in which most Ma for zircons separated from the underlying Dahongyu Mesoproterozoic surface-ocean was oxygen-rich due to Formation volcanic ash layer. Thus, the depositional age active oxygenic photosynthesis, whereas the deep-ocean of the Gaoyuzhuang Formation was speculated within a remained anoxic, and likely sulfidic because of the activ- time span of 1600 Ma to 1550 Ma. ity of sulfate-reducing microorganisms (Canfield, 1998; Our samples were collected from a given succession Johnston et al., 2008). The redox-stratification may have from the middle part of the Gaoyuzhuang Formation with played a role in biological evolution, but much more de- a vertical thickness nearly 80 m. Based on the analyses tailed constraints on the redox conditions remain scarce of macro- and micro-facies features, the succession was and are worthy of further study (Shen et al., 2003). divided into four intervals from bottom to top as M1, M2, Redox-sensitive trace and rare earth element (REE) con- M3 and M4, respectively, and record a shallowing-upward centrations recorded in chemical sediments have been used trend from quiet-water outer shelf below the storm wave to obtain information on marine environments (Frimmel, base level to more energetic intertidal zone. The character- 2009). The distribution of these elements is very sensitive to istics of the lithofacies associations in the four units that water depth, salinity and oxygen level. Many elements, such constituted the succession and their interpreted palaeoen- as U, V, Mo, Cr, Co, and Ce, can display somewhat different vironmental settings are summarized in Figure 2. abundances and valences under various redox conditions, a The lowest interval (M1) is 19 m thick. It is composed synthesis of redox-sensitive trace elements and REEs can of dark-grey bitumen-rich finely laminated micrite, con- provide important constraints on the oxygen concentration taining ~1.5 m thick layer that contains oncolite-like in bottom water (Wright et al., 1987; Ge et al., 2010). carbonate concretions in the middle part of this interval. In the current study, we conduct trace and rare earth These concretions, 2-4 cm in diameter, display spheroi- elemental geochemistry analyses on carbonate samples dal shapes with faint concentric layers in the interior and from ~80 m thick marine sequences developed within the are coated with 1-3 mm thick shells, but no nuclei in the Mesoproterozoic Gaoyuzhuang Formation (~1.56 Ga) at center (Fig. 3a). The shells consist of aragonite fibers most Pingquan Section, northeastern margin of the North China likely originated from microbially mediated precipita- Platform. This sequence spans depositional settings from tion. On bedding planes, they are often expressed as small deep and quiet water outer shelf to more energetic inter- domes with 2-6 mm positive relief. These oncolite-like tidal zone, thus serving as a good source to determine the concentrations are generally recognized as a special kind redox structure in the Mesoproterozoic ocean. of microbial induced sedimentary structure (MISS) related to anaerobic oxidation of methane (AOM) under anoxic/ 2- 2+ 2 Geological setting euxinic conditions (CH4+SO4 +Ca →CaCO3+H2S+H2O) (Shi et al., 2008). Framboidal pyrites, possible products of The studied section located in Pingquan County, Hebei AOM, are found within the concretions and surrounding Guo Hua et al.: Trace and rare earth elemental geochemistry of carbonate succession in the Middle Gaoyuzhuang Formation, Pingquan Section: Vol. 2 No. 2 Implications for Early Mesoproterozoic ocean redox conditions 211

Fig. 1 a-Lithological column of the Gaoyuzhuang Formation in Pingquan Section, Hebei Province; the studied succession here is marked with grey shadow in the middle part of the Gaoyuzhuang Formation; b-Location of the studied section. host rocks (Fig. 3b). Densely fine laminates are well devel- clearly observed in the outcrops (Fig. 3e). These character- oped and no sedimentary structures indicative of scouring istics suggest that the sediments in this interval are formed are observed in this interval, suggesting a relatively deep under shallow and high energy environments in a subtidal and quiet water environment, possibly below the storm setting. wave base level in the outer shelf. The most upper M4 interval is about 14.3 m thick. It The overlying interval (M2) is about 18 m thick and is characterized by light grey, thick-bedded microbial mat consists of dark grey, thinly laminated bituminous micrites micrite (Fig. 3f). Some microbial mat chips are observed (Figs. 3c, 3d), indicative of a relatively low energy condi- under the microscope, indicating a relatively shallow tion. Bioclastics and intraclasts are rarely found in this in- water environment. The presence of microbial mats sug- terval. Thus, the interval is interpreted to have been depos- gests that the interval is possibly formed in an intertidal/ ited in a quiet-water environment below the fair weather supratidal setting. wave base. It should be shallower relative to the underlain

M1 interval, possibly above storm wave base level on the 3 Sampling and methods inner shelf. The upper M3 interval is 25.5 m thick and is comprised We collected 24 samples through this succession for predominately of massive micrites mixed with land-de- analysis of trace and rare earth elements (Fig. 2). After rived terrigenous silts and clays. Wavy lamination can be removing weathered surfaces and secondary veins, the 212 JOURNAL OF PALAEOGEOGRAPHY Apr. 2013

Fig. 2 Sedimentary log of the studied succession from the Gaoyuzhuang Formation of the Pingquan Section, northeastern North China Platform, showing the vertical changes of lithofacies and palaeoenvironments. The sampling locations corresponding to the vertical succession are displayed in the right side of the lithology column. samples were coarsely crushed with a steel jaw crusher versity of Geosciences, Wuhan. 75 mg of each sample was and then powered in an agate mill down to a grain size dissolved in 6 mL 6N HF/6N HNO3 (1:2) at 180℃ for at smaller than 200 mesh (75 μm). Trace and rare earth ele- least 12 h and up to 24 h. For ICP-MS analyses, the sam- ments were analyzed by inductively coupled plasma mass ples were taken up in diluted HNO3 and further diluted spectrometry (ICP-MS) at the State Key Laboratory of with ultraclean H2O to a volume of 100 mL together with Geological Processes and Mineral Resources, China Uni- 20 ppb In, Re, Rh, and Bi as internal standards. Analytical Guo Hua et al.: Trace and rare earth elemental geochemistry of carbonate succession in the Middle Gaoyuzhuang Formation, Pingquan Section: Vol. 2 No. 2 Implications for Early Mesoproterozoic ocean redox conditions 213

Fig. 3 Characteristic features of the studied section of the Gaoyuzhuang Formation in the Pingquan Section, northeastern North China Platform. a-Field photo of the oncolite‑like carbonate concretions from the middle part of the M1 interval; b-Photomicrograph showing oxidized pyrite framboids within the oncolite‑like carbonate concretions and surrounding host rocks; c-Field photo of the thinly laminated bituminous micrites from the M2 interval; d-Horizontal layers in thin section of micrites from the M2 interval; e-Wavy lamination is visible in the M3 interval; f-Microbial mat micrites in the M4 interval. Pen is 16 cm in length. precision and accuracy were evaluated by duplicate analy- diagrams (Fig. 5) also suggest that only minimal continen- ses of samples and two international standard reference tal detritus was added (Chang et al., 2012). However, sam- samples BHVO-1 and AGV-1. The variation in duplicate ples from the marly limestones (or dolomitic limestones) analyses was 10% or less (Zhou et al., 2008). The con- dominated M3 interval have relatively higher Zr concen- centrations of redox-sensitive trace elements and relevant trations (20-30 ppm), which is indicative of a greater in- ratios are shown in Table 1 and the REEs concentrations put of continental detritus. In order to correct the effect are reported in Table 2. from the terrigenous components, Zr-normalized trace elements were used in the current study (Snow et al., 2005; 4 Results Zhou et al., 2008). The similar evolutionary trend in the Zr-normalized All of the samples from the four intervals have very concentrations of redox-sensitive elements e.g., U, V, Mo, low Mn/Sr ratios of less than 3, indicating a high degree Co, Cr and ratios of V/Cr, Ni/Co and Mo/U (mole ratio) is of preservation of primary geochemical signatures (Veiz- shown in Figure 4. The maximum values of Zr-normalized er and Hoefs, 1976). In most samples from the M1, M2 trace element concentrations and relevant ratios almost and M4 intervals, Zr concentrations are less than 16 ppm, simultaneously appear in the M1 interval. They decrease much lower than that of the average upper crust (nearly gradually from the base of the M2 interval, but to a certain 210 ppm) (Nothdurft et al., 2004). Otherwise, the positive extent, still exhibit mild enrichment in the whole M2 in- Y anomalies displayed in the shale-normalized REE+Y terval relative to those in the M3 and M4 intervals, where 214 JOURNAL OF PALAEOGEOGRAPHY Apr. 2013 Ni/Co Mo/U (mole ratio) V/Cr Mo/Zr Cr/Zr Co/Zr 2.72 0.26 0.97 0.39 2.81 4.20 2.84 V/Zr 0.23 0.06 0.96 0.02 0.36 0.06 2.69 3.63 0.77 3.930 30.93 zhuang Formation, Pingquan Section 83.94 4438 20.24 0.19 1.33 0.11 1.74 0.08 0.35 0.06 4.95 6.56 1.82 sensitive trace element concentrations and geochemical ratios of carbonate samples from the studied succession in the Gaoyu - succession in the studied from the samples of carbonate ratios and geochemical concentrations element trace sensitive ‑ 111.6 normalized redox normalized - 2 ‑ Zr The

7.50 M1 - 3 10.0 M1 - 4 148.1 95.66 7430 16.06 0.05 1.55 0.12 2.66 0.21 0.39 0.10 6.89 6.62 4.48 12.2 M1 - 5 114.5 68.13 2512 4.986 0.03 1.68 0.06 1.43 0.14 3.28 0.17 0.44 10.2 5.80 15.0 M1 - 6 117.2 65.90 3183 4.006 0.05 1.78 0.09 1.88 0.31 1.12 0.21 1.67 5.38 8.94 19.0 M2 - 1 120.0 80.77 4425 37.93 0.03 1.49 0.04 0.44 0.04 0.07 0.03 6.30 5.66 2.31 33.1 M2 - 627.2 187.424.5 M2 - 421.5 M2 - 3 106.8 126.4 M2 - 2 6804 211.0 71.49 14.04 146.3 92.34 6344 0.07 75.75 6610 10.63 1.75 4618 11.35 0.03 0.11 16.24 1.17 0.04 1.77 0.09 0.10 0.06 2.29 0.39 1.47 0.08 1.93 0.11 0.10 1.89 0.06 2.99 0.41 0.13 1.36 5.21 0.11 0.39 0.08 3.59 0.11 0.31 3.15 4.91 2.03 0.09 4.66 4.37 4.03 2.51 4.10 3.28 37.0 M3 - 130.0 162.6 M2 - 5 81.83 105.0 6029 58.54 25.21 6714 0.27 5.080 1.55 0.05 0.34 0.88 1.75 0.04 0.20 0.26 2.01 0.08 0.12 3.35 0.69 3.12 0.15 2.89 4.33 1.85 1.51 39.5 M3 - 2 80.41 224.8 4294 25.84 0.03 0.36 0.05 0.85 0.03 0.30 0.06 2.86 3.29 1.24 41.0 M3 - 3 48.046.5 M3 - 5 M3 - 4 192.7 144.5 91.39 77.65 3847 6903 10.16 21.24 0.04 0.25 2.11 0.09 1.86 1.37 0.05 0.12 0.79 0.42 0.03 0.11 0.36 3.27 0.09 4.89 2.18 2.57 3.40 1.63 52.0 M3 - 6 127.3 73.85 4700 21.94 0.24 1.72 0.04 1.02 0.03 0.36 0.10 2.85 2.27 1.54 57.8 M3 - 7 104.0 70.96 6072 28.53 0.35 1.46 0.04 0.77 0.02 0.33 0.05 2.38 2.98 1.07 62.5 M4 - 1 52.22 176.3 1964 5.022 0.04 0.30 0.07 0.84 0.04 0.61 0.13 1.38 3.12 1.41 0.90 M1 - 1 149.3 115.6 2817 2.586 0.02 1.29 0.38 5.02 0.23 1.31 0.33 3.84 3.16 1.50 69.265.7 M4 - 3 M4 - 2 49.60 110.5 364.8 67.54 1682 4854 4.032 9.098 0.03 0.04 0.14 0.11 1.64 1.27 0.06 0.03 0.77 1.09 0.03 0.12 0.30 1.16 0.06 5.85 2.59 2.79 0.76 1.33 5.50 M1 73.1 M4 - 4 59.37 481.6 5138 2.980 0.03 0.12 0.15 1.23 0.05 0.85 0.23 1.45 3.80 0.79 76.8 M4 - 5 56.75 296.0 1653 3.310 0.02 0.19 0.14 1.51 0.05 0.73 0.13 2.09 3.83 0.96 Table 1 Table M1 M2 M3 M4 Interval Height (m) Samples Mn (ppm) Sr (ppm) Fe (ppm) Zr (ppm) Mg/Ca Mn/Sr U/Zr Guo Hua et al.: Trace and rare earth elemental geochemistry of carbonate succession in the Middle Gaoyuzhuang Formation, Pingquan Section: Vol. 2 No. 2 Implications for Early Mesoproterozoic ocean redox conditions 215 0.03 3.19 - 4 19.6 27.4 3.67 13.6 2.44 0.42 2.33 0.32 1.86 0.41 1.14 0.16 0.97 0.16 15.7 The REEs (including yttrium) concentrations (ppm) of carbonate samples from the studied succession in Gaoyuzhuang Formation, Pingquan Section

10.0 M1 76.8 M4 - 5 6.17 5.97 0.95 3.92 0.76 0.19 1.01 0.16 1.06 0.27 0.83 0.12 0.75 0.12 15.5 7.50 M1 - 3 11.4 15.8 2.05 7.49 1.35 0.26 1.39 0.19 1.15 0.25 0.70 0.11 0.62 0.10 10.3 73.1 M4 - 4 3.16 3.02 0.46 1.78 0.37 0.08 0.42 0.06 0.42 0.11 0.32 0.05 0.31 0.05 6.10 5.50 M1 - 2 14.4 20.9 2.74 10.4 1.95 0.39 1.95 0.29 1.81 0.39 1.08 0.17 1.00 0.16 13.9 69.2 M4 - 3 3.61 3.73 0.54 2.18 0.41 0.08 0.47 0.07 0.48 0.11 0.36 0.06 0.37 0.06 6.60 65.7 M4 - 2 7.35 10.2 1.39 5.41 1.05 0.18 1.14 0.18 1.16 0.29 0.88 0.13 0.90 0.15 15.3 0.90 M1 - 1 6.92 7.07 0.90 3.23 0.57 0.15 0.61 0.09 0.58 0.14 0.41 0.06 0.41 0.06 5.99 62.5 M4 - 1 4.65 5.30 0.82 3.04 0.57 0.13 0.54 0.08 0.49 0.13 0.38 0.05 0.35 0.05 6.01 57.8 M3 - 7 10.0 14.5 1.85 6.94 1.41 0.26 1.32 0.20 1.25 0.29 0.86 0.13 0.86 0.13 13.7 52.0 M3 - 6 11.3 16.1 2.06 7.79 1.54 0.29 1.50 0.22 1.39 0.31 0.87 0.13 0.88 0.13 12.6 48.0 M3 - 5 8.01 11.7 1.53 5.65 1.01 0.20 0.96 0.14 0.79 0.16 0.42 0.07 0.36 0.06 6.09 46.5 M3 - 4 10.2 14.3 1.80 6.86 1.33 0.25 1.31 0.19 1.11 0.25 0.70 0.11 0.72 0.10 10.2 41.0 M3 - 3 8.66 14.6 1.92 7.39 1.57 0.31 1.55 0.23 1.42 0.31 0.87 0.14 0.85 0.13 11.0 39.5 M3 - 2 10.7 15.2 1.93 7.35 1.44 0.29 1.39 0.21 1.27 0.27 0.79 0.12 0.67 0.10 10.4 37.0 M3 - 1 10.5 16.2 1.98 7.58 1.49 0.27 1.40 0.21 1.23 0.25 0.71 0.10 0.64 0.09 9.35 33.1 M2 - 6 21.0 31.0 3.82 14.0 2.47 0.39 2.11 0.28 1.49 0.29 0.79 0.11 0.70 0.10 12.1 30.0 M2 - 5 14.7 17.7 2.23 8.14 1.44 0.31 1.51 0.20 1.24 0.27 0.74 0.11 0.65 0.10 13.7 27.2 M2 - 4 7.78 11.2 1.44 5.31 0.97 0.16 0.84 0.11 0.62 0.13 0.34 0.05 0.32 0.05 5.04 24.5 M2 - 3 8.64 12.6 1.63 6.14 1.14 0.21 1.06 0.15 0.91 0.19 0.54 0.08 0.48 0.08 7.28 21.5 M2 - 2 9.70 14.1 1.82 6.58 1.23 0.23 1.10 0.15 0.88 0.18 0.50 0.08 0.44 0.07 7.10 19.0 M2 - 1 12.4 20.8 2.59 9.43 1.90 0.31 1.53 0.24 1.45 0.31 0.86 0.13 0.83 0.12 9.85 15.0 M1 - 6 7.79 12.2 1.71 6.69 1.20 0.22 1.08 0.15 0.84 0.18 0.48 0.07 0.45 0.07 6.14 12.2 M1 - 5 4.54 5.81 0.77 2.88 0.50 0.10 0.49 0.06 0.38 0.08 0.23 0.03 0.20 Table 2 Table M4 M3 M2 M1 Interval Height (m) Samples La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y 216 JOURNAL OF PALAEOGEOGRAPHY Apr. 2013 they approximately keep invariable at much lower values. of enrichment in marine sediments (Arnaboldi and Mey- It is worth mentioning that a few samples from the M3 ers, 2007; Azmy et al., 2009). So, trace element abundanc- and M4 intervals, showing unexpected slightly higher val- es in sediments and sedimentary rocks allow us to estimate ues in Zr-normalized U and Co concentrations and Ni/Co the likely oxygenation state of the bottom water and sedi- ratios, can possibly be recognized as temporarily dyoxic ments during sediment deposition. indications within the microbal mat substrates in the Mes- Mo has very high seawater concentrations relative to oproteorozic ocean. crustal values and can record seawater redox conditions The PAAS-normalized REE+Y pattern diagrams are pre- even with significant clastic input. Generally, molybde- sented in Figure 5, where the anomalies are calculated as: num is enriched in more reducing sediments, especially * * Ce// Ce= CeNN(2 Pr - NdN), La// La= LaNN()32Pr - NdN , in the presence of free H2S (Crusius et al., 1996; Algeo * Gd// Gd= GdNN()2Tb - DyN (McLennan, 1989). The sam- and Maynard, 2004). Uranium is also among the redox- ples from the M1 and M2 intervals are overall relatively sensitive elements. In oxidizing environments, uranium enriched in REEs. However, the REE+Y distributions de- is present as U6+ in the conservative form of uranyl ions 4- viate from that of PAAS by displaying markedly positive that bind to carbonate ions, forming UO2(CO3)3 , which

Y anomalies (mean (/YHo).SN = 145 for M1 interval and is soluble in water. Under reducing conditions, however, 1.48 for M2 interval), slightly positive La and Gd anoma- U6+ ions are reduced and formed lower valence compounds * lies (mean La/. La = 125 and 1.17 for M1 and M2 in- as UO2, U3O7 or U3O8 and exported into marine carbon- tervals, respectively; mean Gd/. Gd* = 116 for both M1 ates (Wignall and Twitchett, 1996). Similarly, vanadium is 5+ 2- and M2 intervals) and weakly negative Ce anomalies present as V in the form of vanadate oxyanions (HVO4 * - 4+ (mean Ce/. Ce = 081 and 0.84 for M1 and M2 inter- and H2VO4 ) in oxic waters, it is reduced to V and forms vals, respectively) (Figs. 5a, 5b). The total REE contents insoluble VO(OH)2 under mildly reducing conditions. If have no significant correlation with Zr concentrations euxinic conditions are present, V5+ will be further reduced 2 3+ (R = 02. 4, not shown). Therefore, the REE+Y distribu- to V and form solid oxide V2O3 or hydroxide V(OH)3. tions in these samples are likely to be a primary feature In the last two cases, it will lead to V enrichment in ma- of the precipitating water rather than local terrigenous in- rine sedimentary rocks (Breit and Wanty, 1991; Wanty and fluence. In contrast, samples from the M3 interval have Goldhaber, 1992). Other redox-sensitive elements like higher total REE and Zr contents, with a good correlation Co and Cr behave in a similar way, in which they tend with each other (most /REE 2 50 ppm; Zr concentra- to be more soluble under oxidizing conditions and less tions of 20-30 ppm; R2 = 05. 9, not shown), suggesting soluble under reducing conditions (Algeo and Maynard, a considerable infusion of continental material, although 2004). Recent research suggests that the geochemical ra- positive Y anomalies are also observed in these samples tios of Mo/U (mole ratio), V/Cr and Ni/Co are important (Fig. 5c). Samples in the M4 interval display obviously indicators of redox conditions in the bottom water (Mc- different REE+Y patterns from that of the former three in- Manus et al., 2006). Jones and Manning (1994) pointed tervals (Fig. 5d). They bear features typical of seawater, that the oxic-dysoxic and dysoxic-anoxic boundaries cor- such as LREE depletion (mean NdNN/0Yb = .53), mark- respond respectively to V/Cr ratios of 2 and 4.25, or Ni/ edly positive La and Y anomalies (mean La/. La* = 152; Co ratios of 5 and 7, by quantificationally analyzing and mean (/YHON).= 199), negative Ce anomalies as well as a comparing several redox-sensitive element geochemi- weakly positive Gd anomaly (mean Ce/. Ce* = 076; mean cal parameters. Algeo and Tribovillard (2009) considered Gd/. Gd* = 122), representative of the original water com- Mo/U mole ratios larger than 7.9 as an indicator of anoxic/ position. sulfidic environments by studying sediments from modern marine environments. Because these trace elements and 5 Discussion geochemical ratios can exhibit somewhat different values to redox conditions along an oxic to sulfidic gradient, a synthesis of trace elements can provide important informa- 5.1 Redox sensitive element and REE applications tion on changes in bottom-water oxygen levels and redox for palaeoenvironmental analyses gradations in some sedimentary systems (Tribovillard et Oxygen levels in the water column influence the oxida- al., 2006; Arnaboldi and Meyers, 2007). tion state of some trace elements and control selectively Besides, Ce anomalies have been used to determine dep- their solubility in seawater and consequently their degree ositional redox conditions, due to Ce valence and solubility, Guo Hua et al.: Trace and rare earth elemental geochemistry of carbonate succession in the Middle Gaoyuzhuang Formation, Pingquan Section: Vol. 2 No. 2 Implications for Early Mesoproterozoic ocean redox conditions 217 sensitive elements and their geochemical ratios for the studied succession in the Gaoyuzhuang Formation, Pingquan Section, ‑ normalized abundances of the redox ‑ of the Zr Profiles

Fig. 4 Fig. northeastern North China Platform. 218 JOURNAL OF PALAEOGEOGRAPHY Apr. 2013

Fig. 5 PAAS‑normalized REE+Y patterns of carbonate samples from the M1, M2, M3 and M4 intervals in the Gaoyuzhuang Forma- tion, respectively, Pingquan Section, northeastern North China Platform. Guo Hua et al.: Trace and rare earth elemental geochemistry of carbonate succession in the Middle Gaoyuzhuang Formation, Pingquan Section: Vol. 2 No. 2 Implications for Early Mesoproterozoic ocean redox conditions 219 which varies as a function of redox potential. Under oxidiz- for Y, in addition, no significant negative Ce anomalies are ing conditions, Ce3+ is oxidized to Ce4+, resulting in decou- observed (Fig. 5a). The geochemical characteristics imply pling of Ce from the other REEs due to formation of less a possible euxinic condition in the interval.The sedimen- soluble Ce4+ species and/or preferential adsorption of Ce4+ tary features from this unit do not contradict our geochem- species on particle surfaces. These processes will induce a ical results. The oncolite-like carbonate concretions that pronounced negative Ce anomaly (Bau and Dulski, 1996). developed in the interval are generally recognized as being

Based on this, negative Ce anomalies in marine carbonates related to CH4 gas release, which may be generated either have been recognized as reflecting oxidizing conditions ei- from anaerobic decomposition of buried organic matter ther in the water column or at the water-sediment interface. (2CH2O→CH4+CO2) or from bacterial methanogenesis (CO +4H →CH +2H O) during shallow burial (Shi et 5.2 Redox condition of the Early Mesoproterozoic 2 2 4 2 al., 2008). Part of the methane escapes to the atmosphere, ocean whereas another part is consumed by methanotrophs in The disappearance of banded iron formations from the consortium with sulfate‑reducing bacteria, resulting in geological record marks the end of a 2.5 Gyr period domi- the production of 13C-depleted authigenic carbonates and 2- 2+ nated by anoxic and iron-rich deep oceans. However, the hydrogen sulfide (CH4+SO4 +Ca →CaCO3+H2S+H2O). chemistry of the oceans in the following Mid-Proterozoic Some authigenic carbonate minerals, such as rosette sider- interval does not appear to have changed to the condi- ites, dumbbell-shaped aragonites, ankerites and botryoidal tions similar to our oxygen-rich modern oceans. Numer- carbonate cements are observed within the layers contain- ous studies based on iron speciation (Shen et al., 2002), ing oncolite-like concretions layers and its equivalents, sulfur isotopes (Shen et al., 2003; Sarkar et al., 2010), Mo providing supports for the AOM hypothesis (Shi et al., isotopes (Arnold et al., 2004), and organic geochemistry 2008). The sedimentary features and the corresponding en- (Brocks et al., 2005) demonstrate a stratified world, with richments of redox sensitive trace elements reflect together strongly reducing (possibly sulfidic) deep-ocean condi- the presence of euxina in the water column or the water- tions overlain by an oxygenated surface-ocean and atmos- sediment interface in the quiet-water outer shelf setting. phere (Canfield, 1998; Poulton et al., 2004; Johnston et Samples from the M2 interval have moderate enrich- al., 2008; Johnston et al., 2009). Marine sulfate concentra- ments of Zr-normalized redox-sensitive trace elements, tions may have remained extremely low during this period although an obvious decrease is present relative to the un- (possibly <2.4 mM), perhaps induced by low atmospheric derlying M1 interval. Most V/Cr and Ni/Ci ratios fall into oxygen concentrations or high intensity of bacterial sulfate a range of 2.5-4.25 and 4-5 respectively, which indicate reduction (Shen et al., 2002; Kah et al., 2004). Recently, that dysoxic conditions may be dominant in this interval. Brocks et al. (2005) found biomarkers of phototrophic Dysoxic conditions are in agreement with sedimentary green and purple sulphur bacteria in 1.64 Gyr sedimen- features, where the dark grey thin-bedded bituminous mic- tary sequences, and suggested that euxinic condition may rites were well developed. It is thus speculated that dys- have penetrated into the photic zone. Until now, Studies oxic conditions may have been prevalent in the inner shelf of Early Mesoproterozoic ocean chemistry all indicate a environment. very shallow chemocline during that period, when the re- In contrast, extremely low values for Zr-normalized dox boundary may have been just meters or a few tens of concentrations of redox-sensitive elements (e.g., U, V, Mo, meters below the water surface. Co and Cr) and geochemical ratios of Mo/U, V/Cr and Ni/ The values of Zr-normalized redox-sensitive trace ele- Co are present in the overlying M3 and M4 intervals (Fig. mental abundances and geochemical ratios presented here 4), with lower V/Cr and Ni/Co ratios typically near or less show a systematic, decreasing trend along a depth gradient than 2 and 5, respectively. Negative Ce anomalies are ob- from the quiet-water outer shelf to the more energetic inter- served in the PAAS-normalized REE+Y diagram in the tidal zone. Samples from the M1 interval are significantly M4 interval (Fig. 5d), which indicates that the subtidal/ enriched in the redox-sensitive trace elements (e.g., U, V, intertidal zones were oxygenated. In addition, oxic condi- Mo, Co, Cr ) and geochemical ratios (Fig. 4). In this inter- tions were probably normally developed in the high-ener- val, the peaks of V/Cr and Ni/Co ratios typically are larger gy shallow water environment due to frequent exchange than 4.25 and 7 respectively, and the maxium Mo/U ratio with the overlying oxic atmosphere. can reach up to 8.94 (Fig. 4). Samples from the interval As discussed above, the transition between euxinic, dys- show nearly flat PAAS-normalized REE patterns except oxic and oxic state may occur in the quiet-water outer shelf 220 JOURNAL OF PALAEOGEOGRAPHY Apr. 2013 and the high-energy subtidal zone, respectively. The redox supported by National Basic Research Program of China boundary is very shallow relative to that in most modern (Grant No. 2011 CB808800). basins (Algeo and Maynard, 2004). Our results support the interpretation of the previously proposed redox-structure References in the Mesoproterozoic oceans. The presence of euxinic ocean bottom water is compatible with reduced levels of Algeo, T. J., Maynard, J. B., 2004. Trace‑element behavior and redox atmospheric oxygen and low concentrations of seawater facies in core shales of Upper Pennsylvanian Kansas‑type cyclo- sulfate. The extreme environmental conditions could have thems. Chemical Geology, 206(3-4): 289-318. been responsible for the delayed oxygenation of the bio- Algeo, T. J., Tribovillard, N., 2009. Environmental analysis of pale- oceanographic systems based on molybdenum‑uranium covaria- sphere and hindered the evolution of multicellular life. tion. Chemical Geology, 268(3-4): 211-225. Arnaboldi, M., Meyers, P. A., 2007. 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Oceanic redox tal abundances in Mo, V, U, Co and Cr and relevant ratios conditions in the Late Mesoproterozoic recorded in the upper Vazante Group carbonates of Sao Francisco Basin, Brazil: Evi- (Mo/U, V/Cr and Ni/Co) show obvious fluctuations along dence from stable isotopes and REEs. Precambrian Research, a depth gradient from the quiet-water outer shelf to the 168(3-4): 259-270. more energetic intertidal zone. The maxima of Zr-normal- Bau, M., Dulski, P., 1996. Distribution of yttrium and rare-earth ele- ized redox-sensitive elemental abundances and geochemi- ments in the Penge and Kuruman iron‑formations, Transvaal Su- cal ratios were observed in the M1 interval, in addition, no pergroup, South Africa. Precambrian Research, 79(1-2): 37-55. significant negative Ce anomalies are found in the PAAS- Breit, G. N., Wanty, R. B., 1991. Vanadium accumulation in carbona- normalized REE+Y diagram. 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