Carbon and Oxygen Isotopic Stratigraphy of Mesoproterozoic Carbonate Sequences (1.6–1.4 Ga) from Yanshan in North China

In Yanshan, located in the northern part of North China, Mesoproterozoic carbonate sequences (1.6–1.4 Ga) form a 10, 000 m thick succession in an aulacogen basin. Carbon and oxygen isotope (δ13O and δ18O, resp.) data were obtained from 110 carbonate samples across three sections of these Mesoproterozoic deposits. From the early to late Mesoproterozoic, low negative values of δ13O appear, followed by low positive variation and then a stable increase. An abrupt decrease in δ13O values, with subsequent rapid increase, is found at the end of the Mesoproterozoic. During the whole Mesoproterozoic, δ18O shows a mainly negative trend and occasional highly negative isotopic shifts (from lower to upper deposits). Whole-rock carbon and oxygen isotopic compositions and profiles must be studied to provide a paleogeochemical record that can be associated with paleocean sedimentary environments, temperature, biological productivity, and sea-level fluctuations. Results of the present study correlate well with other international carbon and oxygen isotope profiles, suggesting that a global marine geochemical system existed during the interval of 1.6–1.4 Ga under a globally united tectonic, sedimentary, and geochemical background.


Introduction
Yanshan, located in the northern part of North China, has a well-developed Mesoproterozoic marine strata-type section, that is nearly 10,000 m thick (Figure 1).These strata record information on the paleogeography, paleoenvironment, and paleocean geochemistry, providing clues to life and sedimentary evolution at the North China Craton during the Late Precambrian [1].The Mesoproterozoic stratigraphy of Yanshan displays good depositional succession, very little deformation and alteration of metamorphism, and is widely known nationally and internationally.
Since establishment of the strata type in Yanshan in the 1930s [2], numerous studies have examined the stratigraphy, sedimentology, paleontology, geochronology, and geochemistry [3] and reported on carbon and oxygen isotopes [4][5][6][7][8][9][10][11][12].Zhong and Chen [4] examined the isotopic composition of the Gaoyuzhuang Formation and the Yangzhuang Formation based on oxygen and carbon isotopic data (152 values) collected from the Gaoyuzhuang to Yangzhuang Formations at the Ming Tombs, Beijing and in Jixian, Tianjin.Wang and Chen [5] studied oxygen-carbon isotopic changes of the Tieling Formation using isotopic data (94 values).Zhao [6] obtained 17 samples mainly from Mesoproterozoic carbonates in Jixian, Tianjin.Zhao et al. [7] analyzed 29 samples from Mesoproterozoic carbonates at the Ming Tombs, Beijing, while Xiao et al. [8] reported oxygen and carbon isotope findings from Mesoproterozoic carbonates from Jixian, Tianjin.Li et al. [9] studied the oxygen and carbon isotopic composition and characteristics of the Wumishan Formation at the Ming Tombs, Beijing.Li et al. [11] and Chu et al. [12] also studied the oxygen and carbon isotopic compositions, respectively, of Mesoneoproterozoic samples from Jixian, Tianjin.
Previous studies have focused mainly on Mesoproterozoic samples from the Ming Tombs in Beijing and Jixian, Tianjin.Among these studies, Chu et al. [12] were particularly systematic, although they only represented the Mesoneoproterozoic oxygen and carbon isotope compositions in Jixian and the Ming Tombs and their relation to paleoclimate and sea-level fluctuations, biotic alternation, and tectonic events.To better understand paleoceanic geochemistry of the Yanshan Mountains during the Mesoproterozoic (1.6-1.4Ga) and regional, or global, Mesoproterozoic stratigraphic correlation, this study examined oxygen and carbon isotopes in the Mesoproterozoic stratigraphy of Yanshan, North China.

Geologic Setting and Samples
The Yanshan area is tectonically attributed to a Mesoproterozoic aulacogen basin, which extends on an east-west axis and contains thick deposits from the Mesoproterozoic.In ascending order, these deposits are labeled the Mesoproterozoic Changzhougou, Chuanlinggou, Tuanshanzi, Dahongyu, mainly clastic rocks, Gaoyuzhuang, Yangzhuang, Wumishan, Hongshuizhuang, and Tieling Formations.The latter five formations are the stratigraphic interval of focus here and are chiefly carbonates except for the Hongshuizhuang Formation shale.
The samples for carbon and oxygen isotopic analysis (110 samples), as well as analysis of trace elements and oxides, were collected from three individual sections, numbered (1), (2), and (3) in Figure 1.Together, these three sections make up one complete stratigraphic succession of the Mesoproterozoic in northern Hebei Province (Figure 2); Figure 2 shows the regional stratigraphic correlation.

Methods and Analysis Results
3.1.Methods.A total of 110 hand samples of carbonate were collected from three sections at Beizhangzi, Kuancheng (no.3), Weizhangzi, Lingyuan (no.2), and Jianshanzi, Kuancheng (no. 1) (Figure 3).These sections represent a successive stratigraphic column of the Mesoproterozoic in ascending order.The carbonate samples were first washed and cleaned and then powdered into 75 um grain size.The powdered carbonate samples were reacted with phosphoric acid in an on-line carbonate preparation system (SY/T 6039-94) [SY].[19]).Carbon and oxygen isotope (δ 13 C and δ 18 O, resp.)ratios of bulk rocks were analyzed at the Institute of Geology and Mineral Resources (Tianjin, China) using a mass spectrometer (MF-ISOPRIME).All values are reported as per mil (‰) relative to Vienna Pee Dee Belemnite (V-PDB) in a standard of GBW04405, GBW04406, and TB2GB04417.The precision and accuracy of the isotopic analyses are σ < 0.1‰, respectively.Concentrations of Mg, Sr, Mn, Fe, and Ca in bulk rocks were also analyzed by inductively coupled plasma mass spectrometry (ICP-MS) at the same institute in Tianjin.

Diagenesis.
Because of the possible complex postdepositional history of carbonates in the study area, we expected that all samples would show geochemical evidence and nonalteration; however, many carbon isotopic studies of Paleozoic and Proterozoic carbonates have shown that carbon isotopic composition is less sensitive to diagenetic alteration than are δ 18 O or trace element (Fe, Mn, Sr) compositions, as long as diagenetic fluids are relatively carbon-poor.Tests of diagenesis in Mesoproterozoic carbonates are expected to identify the most chemically altered samples as those that are least likely to preserve meaningful carbon isotopic signatures.Identification of diagenetic alteration was based on the following: the δ 18 O-Mg/Ca ratio, δ 18 O-Sr, δ 18 O-Mn, δ 18 O-Fe, and Fe-Mn (Figure 3).Ratios of Mg/Ca cover a range of 0.1 to 0.7, mainly 0.6.The Mg/Ca ratio is >0.5 in dolomite and 0.1-0.5 in limestone.δ 18 O is a very sensitive indicator of diagenetic processes.Mineralogical examinations of δ 18 O values show that dolostone is isotopically heavy and limestones have low δ 18 O.
Previous results indicated that δ 18 O values in Precambrian carbonates generally ranged from approximately −6 to −9‰ except in samples formed in evaporated marine environments [20,21].The current δ 18 O results reveal a higher trend in dolomite than in limestone.Because of evaporation Trace element content also provides a powerful tool for assessing the degree of diagenesis.The trace elements Sr, Mn, and Fe exhibit predictable behavior during diagenetic alterations.In general, the Sr content decreases through interaction with relatively Sr-poor fluids.However, examination of the Sr content in more than 50 carbonates samples showed low Sr content, mainly 5-50 ppm (Table 2), suggesting poor postdiagenetic alteration.
Contents of Mn and Fe typically increase with waterrock interaction in the presence of reduced fluids and therefore generally reflect either carbonate precipitation in the presence of Mn and Fe-rich waters, or postdepositional alteration under burial conditions.Relationships of δ 18 O-Mn, δ 18 O-Fe, and Fe-Mn reveal that Fe and Mn present distinct and similar trends for limestone and dolostone in the Mesoproterozoic (Table 2 and Figure 3).In most of the samples, Mn is <600 ppm and Fe is <8000 ppm.Relationship of δ 13 C-Fe and δ 13 C-δ 18 O also suggests that the carbonate samples underwent poor postdepositional diagenetic alterations and well-preserved information on synsedimentary marine oxygen and carbon isotopes in the Mesoproterozoic. 2 lists the isotopic and elemental compositions, and Figure 4 shows secular change of δ 13 C and δ 18 O.The δ 13 C record of the Mesoproterozoic obtained in the current study shows an overall trend of low negative mainly within the Gaoyuzhuang and Yangzhuang Formations.Subsequently, δ 13 C displays low positive variations and stable increase, although with occasional negative values, in the Wumishan Formation; this is followed by a decrease to a negative level of approximately −1.7‰ in the Hongshuizhuang Formation.A rapid increase of δ 13 C occurs from the bottom of the Tieling Formation to the top of the Mesoproterozoic, with δ 13 C values varying between approximately −1.6 and 1‰.Oxygen isotope values (δ 18 O) from this study (Table 2 and Figure 4) show a distinctly negative trend varying from −2.1 to −10‰, excluding diagenetically altered values lower than −10‰.A few highly negative oxygen isotopic shifts occur within the middle Gaoyuzhuang and Yangzhuang Formations and the middle and upper Wumishan Formation.These δ 13 C and δ 18 O results are comparable to previous reports of Mesoproterozoic geochemistry [4,8,12] in Jixian, Tianjin.

Discussion
The carbon and oxygen isotopic compositions display regular variations that correspond to the evolutionary sedimentary environments in a tidal setting from the Gaoyuzhuang to Yangzhuang Formations.The δ 13 C values vary from approximately 0 to 1.5%.Micritic and stromatolitic dolomite in the Gaoyuzhuang Formation yields a δ 13 C value of approximately −0.5‰.The relatively low δ 13 C value of nodular limestone (−0.5 ∼ 1.5‰) is consistent with a deep water environment and low biological productivity [22,23].Although showing good continuity with δ 13 C values of the Gaoyuzhuang Formation, the Yangzhuang Formation reveals a level of slightly negative δ 13 C values, with a minimum of −1.35‰ in the middle of the formation, possibly due to postdiagenetic alternation, dolomitization, silicification, or terrigenous sediments input.For the Gaoyuzhuang and Yangzhuang Formations, excluding diagenetically altered samples (< −10‰), δ 18 O shows highly negative values, mostly ranging from approximately −4 to −7‰ with a minimum of −9.89‰.The Wumishan Formation is interpreted as peritidal deposits in an epicontinental sea [6] and characterized by dolomite with stromatolitic laminations.A majority of stromatolites in the Wumishan Formation suggest high biological productivity.The δ 18 O value in this formation varies between −8.8‰ and −2.3‰, displaying a slight decrease upwards.Except for two positive drifts of δ 13 C, reaching 1.56 at the bottom and 1.49 at the top, the other values vary around zero, probably induced by high-frequency sea-level changes, palaeotemperature, or productivity.The Hongshuizhuang Formation is composed of dark-gray shale rich in organic material with dolomitic intercalations and a lower negative δ 13 C value (−1.7‰) and a moderate δ 18 O value (−5.5‰).The Tieling Formation shows a lower negative trend of δ 13 C varying between −1.7‰ and 1.18‰ in the lower part, with an increase of positive values (−0.6 ∼ 1‰) upwards.The δ 18 O of this unit maintains a stable level of −6.9 ∼ 5.3‰, possibly reflecting a response to the sedimentary environmental transition from supratidal to intertidal-subtidal and an increase of biological activity.
Previous studies on carbon isotopes of the Mesoproterozoic [7,8,11,12] showed that δ 13 C values during 1.8-1.6Ga were mainly negative (−3 to 0‰) and increased slowly, reaching a level of zero, with abrupt oscillation occurring at the beginning of 1.6 Ga.Zhai and Piao [24] and Chu et al. [12] concluded that the evolutionary trend of carbon isotopes was a geochemical record that corresponded to break up of the supercontinent in the Early Mesoproterozoic.Li et al. [11] suggested, based upon geochemistry, that high salt concentration of the paleocean spurred the evolution of the microorganism community, which resulted in the variation of δ 13 C isotopes in sediments during this period.
Knauth and Lowe [25], De La Rocha [26], and Robert and Chaussidon [27] suggested that during the interval of 1.4-1.2Ga, ocean temperature reached around 70 • C based on study of global average δ 18 O and δ 30 Si isotopes in carbonates.The δ 13 C values of Mid-Riphean carbonate from the southern Urals of Russia (1.4-1.2Ga) show a low positive range of 0-3‰ and a negative anomaly below −20‰ (Bartley et al., [28]) in black shale.The δ 13 C in the Mesoproterozoic Belt Supergroup (1.45-1.20 Ga) in North America and Canada, consisting mainly of carbonate-bearing molar-tooth structure, varies from −0.2 to 1.4‰ from down to up [20], while δ 13 C in carbonates of 1.70-1.60Ga in the McArthur and Mount Isa basins in northern Australia displays stable variation of −0.6 to 1‰ [29].A carbon isotope curve for ca.1.6-0.6Ga reported

Conclusions
The bulk-rock carbon and oxygen isotopic compositions and profiles obtained from carbonates from central and northern Yanshan provide a paleogeochemical record of ocean depositional environments, temperature, biological productivity, and sea-level fluctuations.From the early to late Mesoproterozoic, low negative δ 13 C values appear in both the Gaoyuzhuang and Yangzhuang Formations.This is followed by low positive variations and a stable increase with occasional negative value in the Wumishan Formation.An abrupt decrease in δ 13 C values, down to −1.7‰, occurs in the Hongshuizhuang Formation; δ 13 C values then rapidly increase from the bottom of the Tieling Formation to the end of the Mesoproterozoic, varying between −1.6‰ and 1‰.
The main δ 18 O trend is negative value, varying from −2.3‰ to −10‰.A few highly negative oxygen isotopic shifts occur within the middle Gaoyuzhuang Formation, the Yangzhuang Formation, and the middle and upper Wumishan Formation.
The carbon and oxygen isotope profiles of this study correspond well with results of other international studies, suggesting that a global marine geochemical system existed during the past 1.4 Ga interval and revealing a globally united tectonic, sedimentary, and geochemical background.

Figure 4 :
Figure 4: Stratigraphic variation in carbon and oxygen isotopic compositions of Mesoproterozoic carbonates from Yanshan, North China.

Table 1 :
Brief lithologies of Mesoproterozoic strata in the research area.
and salinization, peritidal (intertidal-supratidal) carbonates have higher values than coeval subtidal carbonates; the former have values of −2‰ in dolomite, and the latter have values of −8‰ in limestone and dolomitic limestone or dolomite.

Table 2 :
Isotopic and elemental compositions of Mesoproterozoic carbonates from Yanshan, North China.

Table 2 :
Continued.Kah et al. [21]presents a slow increase in δ13C values above 0‰, comparable to the δ13C profiles of the present study and suggesting that a global marine geochemical system prevailed and was universally correlated during the interval of 1.6-1.2Ga, in response to the global tectonic background. by