Guo Hua, Du Yuansheng, , Zhou Lian, Yang Jianghai, Huang Hu

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), va?nadium (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 concentra?tions 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 enrich?ment of Zr?normalized redox?sensitive elemental abundances and a mild decrease in the val?ues 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 condi?tions 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 compat?ible 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.

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

1 Introduction

Oxygenation of the Earth′s surface is increasingly thought to have occurred in two major steps. The first large increase in atmospheric oxygen levels occurred about 2.4 billion years ago, whenPo2rose from less than 10-5of the present atmospheric level (PAL) to more than 0.01 PAL(Goldblattet al., 2006; Holland, 2006). A further increase took place during the Late Neoproterozoic period, this increase may have led to oxygenation of the deep ocean and therefore the evolution of metazoans and multicellular algae in the Ediacaran period (Des Maraiset al., 1992;Knoll and Carroll, 1999; Rothmanet al., 2003; Fikeet al.,2006). Such a two-staged oxidation implies a unique ocean chemistry for much of the Proterozoic eon, in which most Mesoproterozoic surface-ocean was oxygen-rich due to active oxygenic photosynthesis, whereas the deep-ocean remained anoxic, and likely sulfidic because of the activity of sulfate-reducing microorganisms (Canfield, 1998;Johnstonet al., 2008). The redox-stratification may have played a role in biological evolution, but much more detailed constraints on the redox conditions remain scarce and are worthy of further study (Shenet al., 2003).

Redox-sensitive trace and rare earth element (REE) concentrations recorded in chemical sediments have been used to obtain information on marine environments (Frimmel,2009). The distribution of these elements is very sensitive to water depth, salinity and oxygen level. Many elements, such as U, V, Mo, Cr, Co, and Ce, can display somewhat different abundances and valences under various redox conditions, a synthesis of redox-sensitive trace elements and REEs can provide important constraints on the oxygen concentration in bottom water (Wrightet al., 1987; Geet al., 2010).

In the current study, we conduct trace and rare earth elemental geochemistry analyses on carbonate samples from ～80 m thick marine sequences developed within the Mesoproterozoic Gaoyuzhuang Formation (～1.56 Ga) at Pingquan Section, northeastern margin of the North China Platform. This sequence spans depositional settings from deep and quiet water outer shelf to more energetic intertidal zone, thus serving as a good source to determine the redox structure in the Mesoproterozoic ocean.

2 Geological setting

The studied section located in Pingquan County, Hebei Province, about 300 km northeast of Beijing (Fig. 1). The Mesoproterozoic strata from the Changzhougou Formation to Wumishan Formation were well developed in the study area. The Gaoyuzhuang Formation consists predominantly of microcrystalline dolostones, dolomitic limestone, argillaceous limestone and silicified dolostones interpreted to have been deposited in tidal and shallow ocean environments (Fig. 1). The detailed description of the lithology,depositional environments and habitat types of the Gaoyuzhuang Formation in Pingquan Section is presented by Guoet al.(2010). Recent zircons from a tuff bed in the upper part of this formation yielded U-Pb ages of 1559±12 Ma (SHRIMP) and 1560±5 Ma (LA-MC-ICPMS) (Liet al., 2010). Lu and Li (1991) reported U-Pb ages of 1625±6 Ma for zircons separated from the underlying Dahongyu Formation volcanic ash layer. Thus, the depositional age of the Gaoyuzhuang Formation was speculated within a time span of 1600 Ma to 1550 Ma.

Our samples were collected from a given succession from the middle part of the Gaoyuzhuang Formation with a vertical thickness nearly 80 m. Based on the analyses of macro- and micro-facies features, the succession was divided into four intervals from bottom to top as M1, M2,M3 and M4, respectively, and record a shallowing-upward trend from quiet-water outer shelf below the storm wave base level to more energetic intertidal zone. The characteristics of the lithofacies associations in the four units that constituted the succession and their interpreted palaeoenvironmental settings are summarized in Figure 2.

The lowest interval (M1) is 19 m thick. It is composed of dark-grey bitumen-rich finely laminated micrite, containing ～1.5 m thick layer that contains oncolite-like carbonate concretions in the middle part of this interval.These concretions, 2-4 cm in diameter, display spheroidal shapes with faint concentric layers in the interior and are coated with 1-3 mm thick shells, but no nuclei in the center (Fig. 3a). The shells consist of aragonite fibers most likely originated from microbially mediated precipitation. On bedding planes, they are often expressed as small domes with 2-6 mm positive relief. These oncolite-like concentrations are generally recognized as a special kind of microbial induced sedimentary structure (MISS) related to anaerobic oxidation of methane (AOM) under anoxic/euxinic conditions (CH4+SO42-+Ca2+→CaCO3+H2S+H2O)(Shiet al., 2008). Framboidal pyrites, possible products of AOM, are found within the concretions and surrounding host rocks (Fig. 3b). Densely fine laminates are well developed and no sedimentary structures indicative of scouring are observed in this interval, suggesting a relatively deep and quiet water environment, possibly below the storm wave base level in the outer shelf.

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.

The overlying interval (M2) is about 18 m thick and consists of dark grey, thinly laminated bituminous micrites(Figs. 3c, 3d), indicative of a relatively low energy condition. Bioclastics and intraclasts are rarely found in this interval. Thus, the interval is interpreted to have been deposited in a quiet-water environment below the fair weather wave base. It should be shallower relative to the underlain M1 interval, possibly above storm wave base level on the inner shelf.

The upper M3 interval is 25.5 m thick and is comprised predominately of massive micrites mixed with land-derived terrigenous silts and clays. Wavy lamination can be clearly observed in the outcrops (Fig. 3e). These characteristics suggest that the sediments in this interval are formed under shallow and high energy environments in a subtidal setting.

The most upper M4 interval is about 14.3 m thick. It is characterized by light grey, thick-bedded microbial mat micrite (Fig. 3f). Some microbial mat chips are observed under the microscope, indicating a relatively shallow water environment. The presence of microbial mats suggests that the interval is possibly formed in an intertidal/supratidal setting.

3 Sampling and methods

We collected 24 samples through this succession for analysis of trace and rare earth elements (Fig. 2). After removing weathered surfaces and secondary veins, the

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 and then powered in an agate mill down to a grain size smaller than 200 mesh (75 μm). Trace and rare earth elements were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. 75 mg of each sample was dissolved in 6 mL 6N HF/6N HNO3(1:2) at 180℃ for at least 12 h and up to 24 h. For ICP-MS analyses, the samples were taken up in diluted HNO3and further diluted with ultraclean H2O to a volume of 100 mL together with 20 ppb In, Re, Rh, and Bi as internal standards. Analytical precision and accuracy were evaluated by duplicate analyses of samples and two international standard reference samples BHVO-1 and AGV-1. The variation in duplicate analyses was 10% or less (Zhouet al., 2008). The concentrations of redox-sensitive trace elements and relevant ratios are shown in Table 1 and the REEs concentrations are reported in Table 2.

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.

4 Results

All of the samples from the four intervals have very low Mn/Sr ratios of less than 3, indicating a high degree of preservation of primary geochemical signatures (Veizer and Hoefs, 1976). In most samples from the M1, M2 and M4 intervals, Zr concentrations are less than 16 ppm,much lower than that of the average upper crust (nearly 210 ppm) (Nothdurftet al., 2004). Otherwise, the positive Y anomalies displayed in the shale-normalized REE+Y diagrams (Fig. 5) also suggest that only minimal continental detritus was added (Changet al., 2012). However, samples from the marly limestones (or dolomitic limestones)dominated M3 interval have relatively higher Zr concentrations (20-30 ppm), which is indicative of a greater input of continental detritus. In order to correct the effect from the terrigenous components, Zr-normalized trace elements were used in the current study (Snowet al., 2005;Zhouet al., 2008).

The similar evolutionary trend in the Zr-normalized concentrations of redox-sensitive elementse.g., U, V, Mo,Co, Cr and ratios of V/Cr, Ni/Co and Mo/U (mole ratio) is shown in Figure 4. The maximum values of Zr-normalized trace element concentrations and relevant ratios almost simultaneously appear in the M1 interval. They decrease gradually from the base of the M2 interval, but to a certain extent, still exhibit mild enrichment in the whole M2 interval relative to those in the M3 and M4 intervals, where

they approximately keep invariable at much lower values.It is worth mentioning that a few samples from the M3 and M4 intervals, showing unexpected slightly higher values in Zr-normalized U and Co concentrations and Ni/Co ratios, can possibly be recognized as temporarily dyoxic indications within the microbal mat substrates in the Mesoproteorozic ocean.

Table 1 The Zr-normalizedredox-sensitive trace element concentrations and geochemical ratios of carbonate samples from the studied succession in the Gaoyuzhuang Formation, Pingquan Section

Table 2 The REEs (including yttrium) concentrations (ppm) of carbonate samples from the studiedsuccession in the GaoyuzhuangFormation, Pingquan Section

The PAAS-normalized REE+Y pattern diagrams are presented in Figure 5, where the anomalies are calculated as:Ce/C e*=CeN/(2 PrN- NdN),La/L a*=LaN/(3 P rN- 2NdN),Gd/G d*=GdN/(2 TbN- DyN)(McLennan, 1989). The samples from the M1 and M2 intervals are overall relatively enriched in REEs. However, the REE+Y distributions deviate from that of PAAS by displaying markedly positive Y anomalies (mean(Y/H o)SN= 1.45 for M1 interval and 1.48 for M2 interval), slightly positive La and Gd anomalies (meanLa/L a*= 1.25 and 1.17 for M1 and M2 intervals, respectively; meanGd/G d*= 1.16 for both M1 and M2 intervals) and weakly negative Ce anomalies(meanCe/C e*= 0.81 and 0.84 for M1 and M2 intervals, respectively) (Figs. 5a, 5b). The total REE contents have no significant correlation with Zr concentrations(R2= 0.24, not shown). Therefore, the REE+Y distributions in these samples are likely to be a primary feature of the precipitating water rather than local terrigenous influence. In contrast, samples from the M3 interval have higher total REE and Zr contents, with a good correlation with each other (most∑REE＞50ppm; Zr concentrations of 20-30 ppm;R2= 0.59, not shown), suggesting a considerable infusion of continental material, although positive Y anomalies are also observed in these samples(Fig. 5c). Samples in the M4 interval display obviously different REE+Y patterns from that of the former three intervals (Fig. 5d). They bear features typical of seawater,such as LREE depletion (meanNdN/Y bN= 0.53), markedly positive La and Y anomalies (meanLa/L a*= 1.52;mean(Y/HO)N= 1.99), negative Ce anomalies as well asa weakly positive Gd anomaly (meanCe/C e*= 0.76; mean Gd/G d*= 1.22), representative of the original water composition.

5 Discussion

5.1 Redox sensitive element and REE applications for palaeoenvironmental analyses

Oxygen levels in the water column influence the oxidation state of some trace elements and control selectively their solubility in seawater and consequently their degree of enrichment in marine sediments (Arnaboldi and Meyers, 2007; Azmyet al., 2009). So, trace element abundances in sediments and sedimentary rocks allow us to estimate the likely oxygenation state of the bottom water and sediments during sediment deposition.

Mo has very high seawater concentrations relative to crustal values and can record seawater redox conditions even with significant clastic input. Generally, molybdenum is enriched in more reducing sediments, especially in the presence of free H2S (Crusiuset al., 1996; Algeo and Maynard, 2004). Uranium is also among the redoxsensitive elements. In oxidizing environments, uranium is present as U6+in the conservative form of uranyl ions that bind to carbonate ions, forming UO2(CO3)34-, which is soluble in water. Under reducing conditions, however,U6+ions are reduced and formed lower valence compounds as UO2, U3O7or U3O8and exported into marine carbonates (Wignall and Twitchett, 1996). Similarly, vanadium is present as V5+in the form of vanadate oxyanions (HVO42-and H2VO4-) in oxic waters, it is reduced to V4+and forms insoluble VO(OH)2under mildly reducing conditions. If euxinic conditions are present, V5+will be further reduced to V3+and form solid oxide V2O3or hydroxide V(OH)3.In the last two cases, it will lead to V enrichment in marine sedimentary rocks (Breit and Wanty, 1991; Wanty and Goldhaber, 1992). Other redox-sensitive elements like Co and Cr behave in a similar way, in which they tend to be more soluble under oxidizing conditions and less soluble under reducing conditions (Algeo and Maynard,2004). Recent research suggests that the geochemical ratios of Mo/U (mole ratio), V/Cr and Ni/Co are important indicators of redox conditions in the bottom water (Mc-Manuset al., 2006). Jones and Manning (1994) pointed that the oxic-dysoxic and dysoxic-anoxic boundaries correspond respectively to V/Cr ratios of 2 and 4.25, or Ni/Co ratios of 5 and 7, by quantificationally analyzing and comparing several redox-sensitive element geochemical parameters. Algeo and Tribovillard (2009) considered Mo/U mole ratios larger than 7.9 as an indicator of anoxic/sulfidic environments by studying sediments from modern marine environments. Because these trace elements and geochemical ratios can exhibit somewhat different values to redox conditions along an oxic to sulfidic gradient, a synthesis of trace elements can provide important information on changes in bottom-water oxygen levels and redox gradations in some sedimentary systems (Tribovillardet al., 2006; Arnaboldi and Meyers, 2007).

Fig. 4 Profiles of the Zr-normalized abundances of the redox-sensitive elements and their geochemical ratios for the studied succession in the Gaoyuzhuang Formation, Pingquan Section,northeastern North China Platform.

Fig. 5 PAAS-normalized REE+Y patterns of carbonate samples from the M1, M2, M3 and M4 intervals in the Gaoyuzhuang Formation, respectively, Pingquan Section, northeastern North China Platform.

Besides, Ce anomalies have been used to determine depositional redox conditions, due to Ce valence and solubility,which varies as a function of redox potential. Under oxidizing conditions, Ce3+is oxidized to Ce4+, resulting in decoupling of Ce from the other REEs due to formation of less soluble Ce4+species and/or preferential adsorption of Ce4+species on particle surfaces. These processes will induce a pronounced negative Ce anomaly (Bau and Dulski, 1996).Based on this, negative Ce anomalies in marine carbonates have been recognized as reflecting oxidizing conditions either in the water column or at the water-sediment interface.

5.2 Redox condition of the Early Mesoproterozoic ocean

The disappearance of banded iron formations from the geological record marks the end of a 2.5 Gyr period dominated by anoxic and iron-rich deep oceans. However, the chemistry of the oceans in the following Mid-Proterozoic interval does not appear to have changed to the conditions similar to our oxygen-rich modern oceans. Numerous studies based on iron speciation (Shenet al., 2002),sulfur isotopes (Shenet al., 2003; Sarkaret al., 2010), Mo isotopes (Arnoldet al., 2004), and organic geochemistry(Brockset al., 2005) demonstrate a stratified world, with strongly reducing (possibly sulfidic) deep-ocean conditions overlain by an oxygenated surface-ocean and atmosphere (Canfield, 1998; Poultonet al., 2004; Johnstonet al., 2008; Johnstonet al., 2009). Marine sulfate concentrations may have remained extremely low during this period(possibly ＜2.4 mM), perhaps induced by low atmospheric oxygen concentrations or high intensity of bacterial sulfate reduction (Shenet al., 2002; Kahet al., 2004). Recently,Brockset al. (2005) found biomarkers of phototrophic green and purple sulphur bacteria in 1.64 Gyr sedimentary sequences, and suggested that euxinic condition may have penetrated into the photic zone. Until now, Studies of Early Mesoproterozoic ocean chemistry all indicate a very shallow chemocline during that period, when the redox boundary may have been just meters or a few tens of meters below the water surface.

The values of Zr-normalized redox-sensitive trace elemental abundances and geochemical ratios presented here show a systematic, decreasing trend along a depth gradient from the quiet-water outer shelf to the more energetic intertidal zone. Samples from the M1 interval are significantly enriched in the redox-sensitive trace elements (e.g., U, V,Mo, Co, Cr ) and geochemical ratios (Fig. 4). In this interval, the peaks of V/Cr and Ni/Co ratios typically are larger than 4.25 and 7 respectively, and the maxium Mo/U ratio can reach up to 8.94 (Fig. 4). Samples from the interval show nearly flat PAAS-normalized REE patterns except for Y, in addition, no significant negative Ce anomalies are observed (Fig. 5a). The geochemical characteristics imply a possible euxinic condition in the interval.The sedimentary features from this unit do not contradict our geochemical results. The oncolite-like carbonate concretions that developed in the interval are generally recognized as being related to CH4gas release, which may be generated either from anaerobic decomposition of buried organic matter(2CH2O→CH4+CO2) or from bacterial methanogenesis(CO2+4H2→CH4+2H2O) during shallow burial (Shiet al., 2008). Part of the methane escapes to the atmosphere,whereas another part is consumed by methanotrophs in consortium with sulfate-reducing bacteria, resulting in the production of13C-depleted authigenic carbonates and hydrogen sulfide (CH4+SO42-+Ca2+→CaCO3+H2S+H2O).Some authigenic carbonate minerals, such as rosette siderites, dumbbell-shaped aragonites, ankerites and botryoidal carbonate cements are observed within the layers containing oncolite-like concretions layers and its equivalents,providing supports for the AOM hypothesis (Shiet al.,2008). The sedimentary features and the corresponding enrichments of redox sensitive trace elements reflect together the presence of euxina in the water column or the watersediment interface in the quiet-water outer shelf setting.

Samples from the M2 interval have moderate enrichments of Zr-normalized redox-sensitive trace elements,although an obvious decrease is present relative to the underlying M1 interval. Most V/Cr and Ni/Ci ratios fall into a range of 2.5-4.25 and 4-5 respectively, which indicate that dysoxic conditions may be dominant in this interval.Dysoxic conditions are in agreement with sedimentary features, where the dark grey thin-bedded bituminous micrites were well developed. It is thus speculated that dysoxic conditions may have been prevalent in the inner shelf environment.

In contrast, extremely low values for Zr-normalized concentrations of redox-sensitive elements (e.g., U, V, Mo,Co and Cr) and geochemical ratios of Mo/U, V/Cr and Ni/Co are present in the overlying M3 and M4 intervals (Fig.4), with lower V/Cr and Ni/Co ratios typically near or less than 2 and 5, respectively. Negative Ce anomalies are observed in the PAAS-normalized REE+Y diagram in the M4 interval (Fig. 5d), which indicates that the subtidal/intertidal zones were oxygenated. In addition, oxic conditions were probably normally developed in the high-energy shallow water environment due to frequent exchange with the overlying oxic atmosphere.

As discussed above, the transition between euxinic, dysoxic and oxic state may occur in the quiet-water outer shelfand the high-energy subtidal zone, respectively. The redox boundary is very shallow relative to that in most modern basins (Algeo and Maynard, 2004). Our results support the interpretation of the previously proposed redox-structure in the Mesoproterozoic oceans. The presence of euxinic ocean bottom water is compatible with reduced levels of atmospheric oxygen and low concentrations of seawater sulfate. The extreme environmental conditions could have been responsible for the delayed oxygenation of the biosphere and hindered the evolution of multicellular life.

6 Conclusions

Trace and rare earth elemental analyses were conducted on the marine carbonate succession in the Mesoproterozoic Gaoyuzhuang Formation at Pingquan Section, northeastern margin of the North China Platform, to determine the redox condition of the Early Mesoproterozoic ocean.The values of Zr-normalized redox-sensitive trace elemental abundances in Mo, V, U, Co and Cr and relevant ratios(Mo/U, V/Cr and Ni/Co) show obvious fluctuations along a depth gradient from the quiet-water outer shelf to the more energetic intertidal zone. The maxima of Zr-normalized redox-sensitive elemental abundances and geochemical ratios were observed in the M1 interval, in addition, no significant negative Ce anomalies are found in the PAAS-normalized REE+Y diagram. These geochemical results suggest euxinic conditions possibly dominated the quietwater outer shelf setting below the storm wave base level.In contrast, a less significant enrichment of Zr-normalized redox-sensitive elemental abundances in the overlying M2 interval in association with a mild decrease in geochemical ratios is indicative of dysoxic conditions in the depth interval between the fair weather wave base and the storm wave base. The samples from the M3 and M4 intervals are mostly invariable at much lower values of Zr-normalized redox-sensitive elemental abundances and geochemical ratios. A pronounced negative Ce anomaly is observed in the M4 interval, suggesting the dominance oxygenated conditions in the subtidal/intertidal settings. Based on the analyses above, a very shallow chemocline is expected in the Early Mesoproterozoic ocean. The transitions between euxinic, dysoxic and oxic state may occur in the quiet-water outer shelf and the high-energy subtidal zone, respectively.

Acknowledgements

We would like to thank four reviewers for their constructive and helpful comments. This work was financially supported by National Basic Research Program of China(Grant No. 2011 CB808800).