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

Authors: Kuang  Hongwei; Liu Yongqing; Li Jiahua; Peng Nan; Luo Shunshe; Cen Chao
Journal: International Journal of Oceanography
 (2011)

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## Abstract

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.


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## Body

## 1. 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 (Figure1). 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.Figure 1
Locations of sections and stratigraphic distribution of Mesoproterozoic in Yanshan, North China (modified after Zhu et al. [13]). (1) Gaoyuzhuang Formation and Yangzhuang Formation section in Jianshanzi, Kuancheng, Northern Hebei; (2) Wumishan Formation section in Weizhangzi, Lingyuan, Western Liaoning; (3) Hongshuizhuang -Tieling Formation section in Beizhangzi, Kuancheng, Northern Hebei; (4) Mesoproterozoic section at Guzifang, Chicheng, Northern Hebei (i.e., Xuanlong section); (5) Mesoproterozoic section at Jixian,Tianjin. (a) Mesoproterozoic strata distribution; (b) coastline; (c) locality; (d) section.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 [4–12]. 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 (1.6–1.4 Ga) and regional, or global, Mesoproterozoic stratigraphic correlation, this study examined oxygen and carbon isotopes in the Mesoproterozoic stratigraphy of Yanshan, North China.


## 2. 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 Figure1. 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.Figure 2
Mesoproterozoic stratigraphic columns and regional correlation in Yanshan, North China. (1) Dolostone; (2) muddy dolostone; (3) dolomitic limestone; (4) mud bearing dolostone; (5) sandy dolostone; (6) stromatolitic dolostone; (7) mudstone; (8) breccio-dolostone; (9) shale; (10) chert dolostone; (11) calcite dolostone; (12) nodular limestone; (13) siliceous dolostone; (14) MT limestone; MT-Molar-tooth structure.Recent studies have advanced understanding of Mesoproterozoic   geochronology in the Yanshan area. Wan et al. [14] studied the Ming Tombs in Beijing and obtained an age range of 1.75–1.80 Ga (SHRIMP age) for the bottom of the Changzhougou Formation. Gao et al. [15–17] reported SHRIMP zircon ages of 1.37 Ga for the Xiamaling Formation and 1.63 Ga for volcanic rocks in the Dahongyu Formation at Jixian, Tianjin [17]. On the basis of these dates, Qiao et al. [18] and Gao et al. [15–17] suggested the following geochronological framework for the Mesoneoproterozoic: the Changcheng system (Changzhougou Formation, Chuanlinggou Formation, Tuanshanzi Formation, and Dahongyu Formation) is <1.75 Ga; the Jixian system (Gaoyuzhuang Formation, Yangzhuang Formation, Wumishan Formation, Hongshuizhuang Formation, and Tieling Formation) is 1.6–1.4 Ga; the Xishan system (Xiamaling Formation) is 1.4–1.2 Ga the top unnamed unit is 1.2–1.0 Ga.Lithology in the research area can be subdivided into three types: (1) primarily micrite carbonate, such as micritic limestone and dolomite; (2) grained carbonate such as algae clastics dolomite, sparry oolitic dolomite, calcarenite, and calcirudite; (3) terrigenous rocks and some siliceous rocks (Table1).Table 1
Brief lithologies of Mesoproterozoic strata in the research area.
SectionsFormation and lithological associationThickness/mSection  3Tieling FormationLower part is grey purple, micritic dolostone, rudite dolostone bearing Mn, and thin bedded calcirudite with dolomitic limestone rhythm in the middle-upper, stromatolite calcareous dolostone in the top211.11Hongshuizhuang FormationDark grey shale and argillaceous dolomite in the lower, muddy dolostone interbedded with silty and sand dolomite in the upper part.101.66Section  2Wumishan FormationDolomicrite with stromatolites and ribbon silica, rhythmic oolitic dolostone, and oncolitic dolostone, and dolomitic limestone in the upper part2947.15Section  1Yangzhuang FormationDolomicrite and dolostone with terrigenous clastics and dolostone with robbon chert322.37Gaoyuzhuang FormationStromatolite dolostone in the lower part, dolomicrite with Mn, nodular limestone in the middle part and thick bedded calcareous dolomite or dolomicrite with ricontaining ribbon chert and breccia dolostone in the upper part938.62


## 3. 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) (Figure3). 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 (δ13C and δ18O, 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.Scatter-plots of Mn, Fe, Mg/Ca, Sr versusδ18O, Mn versus Fe, Fe versus δ13C, and δ13C versus δ18O for the Mesoproterozoic carbonate samples.


### 3.2. Analysis Results

#### 3.2.1. 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 areδ18O 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: theδ18O-Mg/Ca ratio, δ18O-Sr, δ18O-Mn, δ18O-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. δ18O is a very sensitive indicator of diagenetic processes. Mineralogical examinations of δ18O values show that dolostone is isotopically heavy and limestones have low δ18O.Previous results indicated thatδ18O values in Precambrian carbonates generally ranged from approximately −6 to −9‰  except in samples formed in evaporated marine environments  [20, 21]. The current δ18O results reveal a higher trend in dolomite than in limestone. Because of evaporation 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.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 (Table2), suggesting poor postdiagenetic alteration.Table 2
Isotopic and elemental compositions of Mesoproterozoic carbonates from Yanshan, North China.
FormationSamplesStratigraphic depth/mLithologiesδ13C/‰  (PDB)δ18O/‰  (PDB)Mn/SrSr/ppmMg/CaTieling Fm.Ktl-46-T1536.03Micritic limestone0.0−5.93.1863.30.03Ktl-40-T1563.33Limestone1.0−5.83.1559.00.07Ktl-36-T1581.47Argillaceous limestone0.6−5.45.3756.30.72Ktl-29-T2606.44Micritic dolostone−0.2−5.2Ktl-29-T1607.44Argillaceous dolostone−0.1−5.31.89Ktl-28-T1650.17Micritic dolostone−0.6−6.8271.2843.52.17Ktd-16-T1720.81Breccia dolostone−1.2−6.8Ktd-12-T2737.24Micritic dolostone−1.7−5.70.68Ktd-12-T1737.34Micritic dolostone−1.6−6.929.444.30.99Hongshuizhuang  Fm.KH-11-T1738.04Micritic dolostone−1.7−5.584.0140.62.01Wumishan  Fm.LW-409-T1857.77Micritic dolostone1.3−4.0LW-395-T1963.96Micritic dolostone1.1−7.7LW-356-T11213.39dolostone bearing MT0.7−6.6LW-347-T11265.3Micritic dolostone0.3−5.26.1110.20.6LW-331-T11364.92Calcareous dolostone0.8−7.8LW-329-T11375.58Dolomitic limestone1.5−8.60.4994.90.61LW-324-T11446.5Micritic limestone1.5−8.61.1786.10.49LW-321-T11467.67MT limestonein situ1.3−8.4LW-315-T11531.43Micritic limestone−0.2−11.50.54115.00.6LW-299-T11676.2MT micriticlimestone0.6−8.8LW-295-T11694.04Dolomitic limestone−0.8−10.2LW-293-T11702.94Micritic dolostone0.2−7.1LW-287-T11745.94Powder dolostone0.5−6.22.7234.20.6LW-286-T11752.03Powder dolostone0.7−7.12.2627.50.57LW-278-T11795.39Powder dolostone0.6−5.72.9720.90.61LW-270-T11840.77Dolostone0.2−5.26.7331.10.59LW-265-T11866.38Algal dolostone0.0−6.1LW-257-T11930.82Dolostone−2.1−5.2LW-249-T11973.53Algal dolostone0.4−6.72.4918.70.59LW-240-T12027.2Sparry dolostone0.5−7.2LW-238-T12046.83Stromatolite dolostone0.4−7.1LW-233-T12079.2Powder dolostone0.7−6.717.9219.50.48LW-227-T12117.68Powder dolostone0.7−7.717.3820.10.58LW-223-T12150.28Algal dolostone0.3−9.215.0426.30.6LW-218-T12184.96Powder dolostone−0.5−6.3LW-213-T12217.08Dolostone−0.8−6.417.218.00.61LW-207-T12271.37Powder dolostone−0.1−6.67.8319.80.6LW-201-T12321.32Stromatolite olostone0.6−7.8LW-196-T12375.19Dolostone0.3−7.6LW-192-T12405.43Powder dolostone−0.5−7.0LW-184-T12451.98Dolostone0.4−7.6LW-175-T12559.8Powder dolostone0.2−6.1LW-167-T12617.95Algal dolostone0.1−5.5LW-162-T12684.71Siliceous dolostone0.6−5.01.6823.10.61LW-158-T12749.17Powder dolostone0.5−7.0LW-155-T12767.03Powder dolostone0.6−5.71.4831.40.6LW-148-T12819.09Powder dolostone0.6−5.2LW-138-T12882.64Dolostone0.0−5.259.038.80.59LW-132-T12915.65Dolostone0.3−5.7LW-128-T22941.69Powder dolostone0.0−5.8LW-128-T12943.79Dolostone0.3−4.82.9815.60.6LW-119-T13054.87Siliceous dolostone−1.7−6.12.1625.10.61LW-113-T13079.65Siliceous dolostone−1.7−6.016.9621.90.6LW-99-T13192.1Dolostone−0.8−5.2LW-73-T13334.6Powder dolostone−0.7−4.8LW-67-T13381.68Powder dolostone0.0−5.8LW-57-T13448.42Powder dolostone0.2−4.72.0922.30.03LW-52-T13483.76Dolostone0.4−4.9LW-50-T13504.88Dolostone0.0−4.4LW-44-T13543.89Dolostone0.8−5.64.0715.20.6LW-39-T13578.55Stromatolite dolostone1.4−5.43.3125.70.57LW-33-T13618.71Powder dolostone1.6−4.72.1325.40.57LW-30-T13636.91Dolostone0.9−4.72.4525.30.6LW-29-T13641.91Dolostone0.5−4.2LW-24-T13665.83Dolostone0.2−2.3LW-22-T13687.32Algal dolostone−0.2−4.31.6233.50.57LW-20-T13715.37Dolostone−0.7−2.9318.10.6LW-13-T13762.27Dolostone−1.1−4.2LW-11-T13776.1Dolostone−0.3−5.5LW-3-T13826.82Muddy dolostone−0.8−2.83.4431.50.58Yangzhuang  Fm.KY2-217-T13849.48Sandy dolostone−1.3−5.1KY2-216-T13849.98Muddy dolostone−0.9−3.81.55156.20.47KY2-214-T13861.28Muddy dolostone−0.7−4.9KY2-212-T13876.08Muddy dolostone−1.4−5.914.239.20.58KY2-207-T13897.09Muddy dolostone−0.9−3.5KY2-201-T13921.36Siliceous dolostone0.1−7.3KY2-194-T13965.2Siliceous dolostone−1.1−7.13.6347.70.6KY2-192-T13983.89Siliceous dolostone−1.4−6.7KY1-190-T14006.3Siliceous dolostone−1.1−8.25.7631.40.52KY1-187-T14016.91Siliceous dolostone−1.3−8.14.0940.30.6KY1-180-T14064.11Siliceous dolostone−0.5−6.25.9846.90.59KY1-174-T14100.75Siliceous dolostone−0.6−6.51.7247.20.59Gaoyuzhuang  Fm.Gh1-170-T14191.12Algal dolostone−0.7−9.9Gh1-168-T14217.14Calcareous dolostone−0.4−14.919.254.70.55Gz2-147-T14472.34Breccia limestone0.1−5.93.9431.00.51Gz2-141-T24519.32Dolostone−0.9−7.34.7528.90.47Gz2-141-T14519.32Nodular limestone−1.8−9.913.9622.70.48Gz2-137-T14538.71Nodular limestone−0.7−12.9Gz2-135-T24547.75Siliceous shale0.0−12.3Gz2-135-T14548.15Limestone lens−0.8−12.92.4623.20.53Gz2-134-T14552.65Siliceous limestone−0.8−8.01.6316.10.19Gz1-133-T14571.12Muddy limestone−0.9−6.4Gz1-126-T14608.43Calcareous dolostone−1.1−7.4Gz1-122-T14627.76Nodular limestone−1.3−7.1Gz1-113-T14664.23Dolomitic limestone−1.2−7.7Gs-111-T14675.53Dolostone with Mn−0.7−6.2Gs-98-T14714.52Dolostone with Mn0.0−4.11.3142.30.22Gs-96-T14744.45Dolostone with Mn−0.4−4.8Gg2-91-T14791.6Muddy dolostone−1.1−4.8Gg2-89-T14812.78Stromatolite dolostone−0.1−4.30.9773.60.23Gg2-88-T14814.78Muddy dolostone−0.9−5.1Gg2-87-T14818.92Silicite−1.9−9.6Gg2-83-T14834.27Stromatolite dolostone−0.4−6.1Gg1-72-T14866.67Stromatolite dolostone−0.2−5.7Gg1-58-T14901.87Siliceous dolostone−0.1−10.4Gg1-47-T14951.22Dolostone−0.3−6.2Gg1-35-T14994.67Dolostone−0.2−5.9Gg1-29-T15034.38Dolostone−6.5−17.34.0941.60.57Gg1-28-T25046.59Quartz sandstone−1.9−7.5Gg1-28-T15047.09Quartz sandstone−2.8−7.25.442.60.58Contents of Mn and Fe typically increase with water–rock 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 ofδ18O-Mn, δ18O-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 δ13C-Fe and δ13C-δ18O 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.


#### 3.2.2. Chemostratigraphy

Table2 lists the isotopic and elemental compositions, and Figure 4 shows secular change of δ13C and δ18O. The δ13C record of the Mesoproterozoic obtained in the current study shows an overall trend of low negative mainly within the Gaoyuzhuang and Yangzhuang Formations. Subsequently, δ13C 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 δ13C occurs from the bottom of the Tieling Formation to the top of the Mesoproterozoic, with δ13C values varying between approximately −1.6 and 1‰. Oxygen isotope values (δ18O) 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 δ13C and δ18O results are comparable to previous reports of Mesoproterozoic geochemistry  [4, 8, 12] in Jixian, Tianjin.Figure 4
Stratigraphic variation in carbon and oxygen isotopic compositions of Mesoproterozoic carbonates from Yanshan, North China.


## 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) (Figure3). 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 (δ13C and δ18O, 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.Scatter-plots of Mn, Fe, Mg/Ca, Sr versusδ18O, Mn versus Fe, Fe versus δ13C, and δ13C versus δ18O for the Mesoproterozoic carbonate samples.


## 3.2. Analysis Results

### 3.2.1. 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 areδ18O 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: theδ18O-Mg/Ca ratio, δ18O-Sr, δ18O-Mn, δ18O-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. δ18O is a very sensitive indicator of diagenetic processes. Mineralogical examinations of δ18O values show that dolostone is isotopically heavy and limestones have low δ18O.Previous results indicated thatδ18O values in Precambrian carbonates generally ranged from approximately −6 to −9‰  except in samples formed in evaporated marine environments  [20, 21]. The current δ18O results reveal a higher trend in dolomite than in limestone. Because of evaporation 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.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 (Table2), suggesting poor postdiagenetic alteration.Table 2
Isotopic and elemental compositions of Mesoproterozoic carbonates from Yanshan, North China.
FormationSamplesStratigraphic depth/mLithologiesδ13C/‰  (PDB)δ18O/‰  (PDB)Mn/SrSr/ppmMg/CaTieling Fm.Ktl-46-T1536.03Micritic limestone0.0−5.93.1863.30.03Ktl-40-T1563.33Limestone1.0−5.83.1559.00.07Ktl-36-T1581.47Argillaceous limestone0.6−5.45.3756.30.72Ktl-29-T2606.44Micritic dolostone−0.2−5.2Ktl-29-T1607.44Argillaceous dolostone−0.1−5.31.89Ktl-28-T1650.17Micritic dolostone−0.6−6.8271.2843.52.17Ktd-16-T1720.81Breccia dolostone−1.2−6.8Ktd-12-T2737.24Micritic dolostone−1.7−5.70.68Ktd-12-T1737.34Micritic dolostone−1.6−6.929.444.30.99Hongshuizhuang  Fm.KH-11-T1738.04Micritic dolostone−1.7−5.584.0140.62.01Wumishan  Fm.LW-409-T1857.77Micritic dolostone1.3−4.0LW-395-T1963.96Micritic dolostone1.1−7.7LW-356-T11213.39dolostone bearing MT0.7−6.6LW-347-T11265.3Micritic dolostone0.3−5.26.1110.20.6LW-331-T11364.92Calcareous dolostone0.8−7.8LW-329-T11375.58Dolomitic limestone1.5−8.60.4994.90.61LW-324-T11446.5Micritic limestone1.5−8.61.1786.10.49LW-321-T11467.67MT limestonein situ1.3−8.4LW-315-T11531.43Micritic limestone−0.2−11.50.54115.00.6LW-299-T11676.2MT micriticlimestone0.6−8.8LW-295-T11694.04Dolomitic limestone−0.8−10.2LW-293-T11702.94Micritic dolostone0.2−7.1LW-287-T11745.94Powder dolostone0.5−6.22.7234.20.6LW-286-T11752.03Powder dolostone0.7−7.12.2627.50.57LW-278-T11795.39Powder dolostone0.6−5.72.9720.90.61LW-270-T11840.77Dolostone0.2−5.26.7331.10.59LW-265-T11866.38Algal dolostone0.0−6.1LW-257-T11930.82Dolostone−2.1−5.2LW-249-T11973.53Algal dolostone0.4−6.72.4918.70.59LW-240-T12027.2Sparry dolostone0.5−7.2LW-238-T12046.83Stromatolite dolostone0.4−7.1LW-233-T12079.2Powder dolostone0.7−6.717.9219.50.48LW-227-T12117.68Powder dolostone0.7−7.717.3820.10.58LW-223-T12150.28Algal dolostone0.3−9.215.0426.30.6LW-218-T12184.96Powder dolostone−0.5−6.3LW-213-T12217.08Dolostone−0.8−6.417.218.00.61LW-207-T12271.37Powder dolostone−0.1−6.67.8319.80.6LW-201-T12321.32Stromatolite olostone0.6−7.8LW-196-T12375.19Dolostone0.3−7.6LW-192-T12405.43Powder dolostone−0.5−7.0LW-184-T12451.98Dolostone0.4−7.6LW-175-T12559.8Powder dolostone0.2−6.1LW-167-T12617.95Algal dolostone0.1−5.5LW-162-T12684.71Siliceous dolostone0.6−5.01.6823.10.61LW-158-T12749.17Powder dolostone0.5−7.0LW-155-T12767.03Powder dolostone0.6−5.71.4831.40.6LW-148-T12819.09Powder dolostone0.6−5.2LW-138-T12882.64Dolostone0.0−5.259.038.80.59LW-132-T12915.65Dolostone0.3−5.7LW-128-T22941.69Powder dolostone0.0−5.8LW-128-T12943.79Dolostone0.3−4.82.9815.60.6LW-119-T13054.87Siliceous dolostone−1.7−6.12.1625.10.61LW-113-T13079.65Siliceous dolostone−1.7−6.016.9621.90.6LW-99-T13192.1Dolostone−0.8−5.2LW-73-T13334.6Powder dolostone−0.7−4.8LW-67-T13381.68Powder dolostone0.0−5.8LW-57-T13448.42Powder dolostone0.2−4.72.0922.30.03LW-52-T13483.76Dolostone0.4−4.9LW-50-T13504.88Dolostone0.0−4.4LW-44-T13543.89Dolostone0.8−5.64.0715.20.6LW-39-T13578.55Stromatolite dolostone1.4−5.43.3125.70.57LW-33-T13618.71Powder dolostone1.6−4.72.1325.40.57LW-30-T13636.91Dolostone0.9−4.72.4525.30.6LW-29-T13641.91Dolostone0.5−4.2LW-24-T13665.83Dolostone0.2−2.3LW-22-T13687.32Algal dolostone−0.2−4.31.6233.50.57LW-20-T13715.37Dolostone−0.7−2.9318.10.6LW-13-T13762.27Dolostone−1.1−4.2LW-11-T13776.1Dolostone−0.3−5.5LW-3-T13826.82Muddy dolostone−0.8−2.83.4431.50.58Yangzhuang  Fm.KY2-217-T13849.48Sandy dolostone−1.3−5.1KY2-216-T13849.98Muddy dolostone−0.9−3.81.55156.20.47KY2-214-T13861.28Muddy dolostone−0.7−4.9KY2-212-T13876.08Muddy dolostone−1.4−5.914.239.20.58KY2-207-T13897.09Muddy dolostone−0.9−3.5KY2-201-T13921.36Siliceous dolostone0.1−7.3KY2-194-T13965.2Siliceous dolostone−1.1−7.13.6347.70.6KY2-192-T13983.89Siliceous dolostone−1.4−6.7KY1-190-T14006.3Siliceous dolostone−1.1−8.25.7631.40.52KY1-187-T14016.91Siliceous dolostone−1.3−8.14.0940.30.6KY1-180-T14064.11Siliceous dolostone−0.5−6.25.9846.90.59KY1-174-T14100.75Siliceous dolostone−0.6−6.51.7247.20.59Gaoyuzhuang  Fm.Gh1-170-T14191.12Algal dolostone−0.7−9.9Gh1-168-T14217.14Calcareous dolostone−0.4−14.919.254.70.55Gz2-147-T14472.34Breccia limestone0.1−5.93.9431.00.51Gz2-141-T24519.32Dolostone−0.9−7.34.7528.90.47Gz2-141-T14519.32Nodular limestone−1.8−9.913.9622.70.48Gz2-137-T14538.71Nodular limestone−0.7−12.9Gz2-135-T24547.75Siliceous shale0.0−12.3Gz2-135-T14548.15Limestone lens−0.8−12.92.4623.20.53Gz2-134-T14552.65Siliceous limestone−0.8−8.01.6316.10.19Gz1-133-T14571.12Muddy limestone−0.9−6.4Gz1-126-T14608.43Calcareous dolostone−1.1−7.4Gz1-122-T14627.76Nodular limestone−1.3−7.1Gz1-113-T14664.23Dolomitic limestone−1.2−7.7Gs-111-T14675.53Dolostone with Mn−0.7−6.2Gs-98-T14714.52Dolostone with Mn0.0−4.11.3142.30.22Gs-96-T14744.45Dolostone with Mn−0.4−4.8Gg2-91-T14791.6Muddy dolostone−1.1−4.8Gg2-89-T14812.78Stromatolite dolostone−0.1−4.30.9773.60.23Gg2-88-T14814.78Muddy dolostone−0.9−5.1Gg2-87-T14818.92Silicite−1.9−9.6Gg2-83-T14834.27Stromatolite dolostone−0.4−6.1Gg1-72-T14866.67Stromatolite dolostone−0.2−5.7Gg1-58-T14901.87Siliceous dolostone−0.1−10.4Gg1-47-T14951.22Dolostone−0.3−6.2Gg1-35-T14994.67Dolostone−0.2−5.9Gg1-29-T15034.38Dolostone−6.5−17.34.0941.60.57Gg1-28-T25046.59Quartz sandstone−1.9−7.5Gg1-28-T15047.09Quartz sandstone−2.8−7.25.442.60.58Contents of Mn and Fe typically increase with water–rock 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 ofδ18O-Mn, δ18O-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 δ13C-Fe and δ13C-δ18O 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.


### 3.2.2. Chemostratigraphy

Table2 lists the isotopic and elemental compositions, and Figure 4 shows secular change of δ13C and δ18O. The δ13C record of the Mesoproterozoic obtained in the current study shows an overall trend of low negative mainly within the Gaoyuzhuang and Yangzhuang Formations. Subsequently, δ13C 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 δ13C occurs from the bottom of the Tieling Formation to the top of the Mesoproterozoic, with δ13C values varying between approximately −1.6 and 1‰. Oxygen isotope values (δ18O) 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 δ13C and δ18O results are comparable to previous reports of Mesoproterozoic geochemistry  [4, 8, 12] in Jixian, Tianjin.Figure 4
Stratigraphic variation in carbon and oxygen isotopic compositions of Mesoproterozoic carbonates from Yanshan, North China.


## 3.2.1. 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 areδ18O 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: theδ18O-Mg/Ca ratio, δ18O-Sr, δ18O-Mn, δ18O-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. δ18O is a very sensitive indicator of diagenetic processes. Mineralogical examinations of δ18O values show that dolostone is isotopically heavy and limestones have low δ18O.Previous results indicated thatδ18O values in Precambrian carbonates generally ranged from approximately −6 to −9‰  except in samples formed in evaporated marine environments  [20, 21]. The current δ18O results reveal a higher trend in dolomite than in limestone. Because of evaporation 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.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 (Table2), suggesting poor postdiagenetic alteration.Table 2
Isotopic and elemental compositions of Mesoproterozoic carbonates from Yanshan, North China.
FormationSamplesStratigraphic depth/mLithologiesδ13C/‰  (PDB)δ18O/‰  (PDB)Mn/SrSr/ppmMg/CaTieling Fm.Ktl-46-T1536.03Micritic limestone0.0−5.93.1863.30.03Ktl-40-T1563.33Limestone1.0−5.83.1559.00.07Ktl-36-T1581.47Argillaceous limestone0.6−5.45.3756.30.72Ktl-29-T2606.44Micritic dolostone−0.2−5.2Ktl-29-T1607.44Argillaceous dolostone−0.1−5.31.89Ktl-28-T1650.17Micritic dolostone−0.6−6.8271.2843.52.17Ktd-16-T1720.81Breccia dolostone−1.2−6.8Ktd-12-T2737.24Micritic dolostone−1.7−5.70.68Ktd-12-T1737.34Micritic dolostone−1.6−6.929.444.30.99Hongshuizhuang  Fm.KH-11-T1738.04Micritic dolostone−1.7−5.584.0140.62.01Wumishan  Fm.LW-409-T1857.77Micritic dolostone1.3−4.0LW-395-T1963.96Micritic dolostone1.1−7.7LW-356-T11213.39dolostone bearing MT0.7−6.6LW-347-T11265.3Micritic dolostone0.3−5.26.1110.20.6LW-331-T11364.92Calcareous dolostone0.8−7.8LW-329-T11375.58Dolomitic limestone1.5−8.60.4994.90.61LW-324-T11446.5Micritic limestone1.5−8.61.1786.10.49LW-321-T11467.67MT limestonein situ1.3−8.4LW-315-T11531.43Micritic limestone−0.2−11.50.54115.00.6LW-299-T11676.2MT micriticlimestone0.6−8.8LW-295-T11694.04Dolomitic limestone−0.8−10.2LW-293-T11702.94Micritic dolostone0.2−7.1LW-287-T11745.94Powder dolostone0.5−6.22.7234.20.6LW-286-T11752.03Powder dolostone0.7−7.12.2627.50.57LW-278-T11795.39Powder dolostone0.6−5.72.9720.90.61LW-270-T11840.77Dolostone0.2−5.26.7331.10.59LW-265-T11866.38Algal dolostone0.0−6.1LW-257-T11930.82Dolostone−2.1−5.2LW-249-T11973.53Algal dolostone0.4−6.72.4918.70.59LW-240-T12027.2Sparry dolostone0.5−7.2LW-238-T12046.83Stromatolite dolostone0.4−7.1LW-233-T12079.2Powder dolostone0.7−6.717.9219.50.48LW-227-T12117.68Powder dolostone0.7−7.717.3820.10.58LW-223-T12150.28Algal dolostone0.3−9.215.0426.30.6LW-218-T12184.96Powder dolostone−0.5−6.3LW-213-T12217.08Dolostone−0.8−6.417.218.00.61LW-207-T12271.37Powder dolostone−0.1−6.67.8319.80.6LW-201-T12321.32Stromatolite olostone0.6−7.8LW-196-T12375.19Dolostone0.3−7.6LW-192-T12405.43Powder dolostone−0.5−7.0LW-184-T12451.98Dolostone0.4−7.6LW-175-T12559.8Powder dolostone0.2−6.1LW-167-T12617.95Algal dolostone0.1−5.5LW-162-T12684.71Siliceous dolostone0.6−5.01.6823.10.61LW-158-T12749.17Powder dolostone0.5−7.0LW-155-T12767.03Powder dolostone0.6−5.71.4831.40.6LW-148-T12819.09Powder dolostone0.6−5.2LW-138-T12882.64Dolostone0.0−5.259.038.80.59LW-132-T12915.65Dolostone0.3−5.7LW-128-T22941.69Powder dolostone0.0−5.8LW-128-T12943.79Dolostone0.3−4.82.9815.60.6LW-119-T13054.87Siliceous dolostone−1.7−6.12.1625.10.61LW-113-T13079.65Siliceous dolostone−1.7−6.016.9621.90.6LW-99-T13192.1Dolostone−0.8−5.2LW-73-T13334.6Powder dolostone−0.7−4.8LW-67-T13381.68Powder dolostone0.0−5.8LW-57-T13448.42Powder dolostone0.2−4.72.0922.30.03LW-52-T13483.76Dolostone0.4−4.9LW-50-T13504.88Dolostone0.0−4.4LW-44-T13543.89Dolostone0.8−5.64.0715.20.6LW-39-T13578.55Stromatolite dolostone1.4−5.43.3125.70.57LW-33-T13618.71Powder dolostone1.6−4.72.1325.40.57LW-30-T13636.91Dolostone0.9−4.72.4525.30.6LW-29-T13641.91Dolostone0.5−4.2LW-24-T13665.83Dolostone0.2−2.3LW-22-T13687.32Algal dolostone−0.2−4.31.6233.50.57LW-20-T13715.37Dolostone−0.7−2.9318.10.6LW-13-T13762.27Dolostone−1.1−4.2LW-11-T13776.1Dolostone−0.3−5.5LW-3-T13826.82Muddy dolostone−0.8−2.83.4431.50.58Yangzhuang  Fm.KY2-217-T13849.48Sandy dolostone−1.3−5.1KY2-216-T13849.98Muddy dolostone−0.9−3.81.55156.20.47KY2-214-T13861.28Muddy dolostone−0.7−4.9KY2-212-T13876.08Muddy dolostone−1.4−5.914.239.20.58KY2-207-T13897.09Muddy dolostone−0.9−3.5KY2-201-T13921.36Siliceous dolostone0.1−7.3KY2-194-T13965.2Siliceous dolostone−1.1−7.13.6347.70.6KY2-192-T13983.89Siliceous dolostone−1.4−6.7KY1-190-T14006.3Siliceous dolostone−1.1−8.25.7631.40.52KY1-187-T14016.91Siliceous dolostone−1.3−8.14.0940.30.6KY1-180-T14064.11Siliceous dolostone−0.5−6.25.9846.90.59KY1-174-T14100.75Siliceous dolostone−0.6−6.51.7247.20.59Gaoyuzhuang  Fm.Gh1-170-T14191.12Algal dolostone−0.7−9.9Gh1-168-T14217.14Calcareous dolostone−0.4−14.919.254.70.55Gz2-147-T14472.34Breccia limestone0.1−5.93.9431.00.51Gz2-141-T24519.32Dolostone−0.9−7.34.7528.90.47Gz2-141-T14519.32Nodular limestone−1.8−9.913.9622.70.48Gz2-137-T14538.71Nodular limestone−0.7−12.9Gz2-135-T24547.75Siliceous shale0.0−12.3Gz2-135-T14548.15Limestone lens−0.8−12.92.4623.20.53Gz2-134-T14552.65Siliceous limestone−0.8−8.01.6316.10.19Gz1-133-T14571.12Muddy limestone−0.9−6.4Gz1-126-T14608.43Calcareous dolostone−1.1−7.4Gz1-122-T14627.76Nodular limestone−1.3−7.1Gz1-113-T14664.23Dolomitic limestone−1.2−7.7Gs-111-T14675.53Dolostone with Mn−0.7−6.2Gs-98-T14714.52Dolostone with Mn0.0−4.11.3142.30.22Gs-96-T14744.45Dolostone with Mn−0.4−4.8Gg2-91-T14791.6Muddy dolostone−1.1−4.8Gg2-89-T14812.78Stromatolite dolostone−0.1−4.30.9773.60.23Gg2-88-T14814.78Muddy dolostone−0.9−5.1Gg2-87-T14818.92Silicite−1.9−9.6Gg2-83-T14834.27Stromatolite dolostone−0.4−6.1Gg1-72-T14866.67Stromatolite dolostone−0.2−5.7Gg1-58-T14901.87Siliceous dolostone−0.1−10.4Gg1-47-T14951.22Dolostone−0.3−6.2Gg1-35-T14994.67Dolostone−0.2−5.9Gg1-29-T15034.38Dolostone−6.5−17.34.0941.60.57Gg1-28-T25046.59Quartz sandstone−1.9−7.5Gg1-28-T15047.09Quartz sandstone−2.8−7.25.442.60.58Contents of Mn and Fe typically increase with water–rock 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 ofδ18O-Mn, δ18O-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 δ13C-Fe and δ13C-δ18O 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.


## 3.2.2. Chemostratigraphy

Table2 lists the isotopic and elemental compositions, and Figure 4 shows secular change of δ13C and δ18O. The δ13C record of the Mesoproterozoic obtained in the current study shows an overall trend of low negative mainly within the Gaoyuzhuang and Yangzhuang Formations. Subsequently, δ13C 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 δ13C occurs from the bottom of the Tieling Formation to the top of the Mesoproterozoic, with δ13C values varying between approximately −1.6 and 1‰. Oxygen isotope values (δ18O) 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 δ13C and δ18O results are comparable to previous reports of Mesoproterozoic geochemistry  [4, 8, 12] in Jixian, Tianjin.Figure 4
Stratigraphic variation in carbon and oxygen isotopic compositions of Mesoproterozoic carbonates from Yanshan, North China.


## 4. 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. Theδ13C values vary from approximately 0 to 1.5%. Micritic and stromatolitic dolomite in the Gaoyuzhuang Formation yields a δ13C value of approximately −0.5‰. The relatively low δ13C 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 δ13C values of the Gaoyuzhuang Formation, the Yangzhuang Formation reveals a level of slightly negative δ13C 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‰), δ18O 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 δ18O value in this formation varies between −8.8‰  and −2.3‰, displaying a slight decrease upwards. Except for two positive drifts of δ13C, 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 δ13C value (−1.7‰) and a moderate δ18O value (−5.5‰). The Tieling Formation shows a lower negative trend of δ13C varying between −1.7‰  and 1.18‰  in the lower part, with an increase of positive values (−0.6 ~ 1‰) upwards. The δ18O 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 δ13C values during 1.8–1.6 Ga 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 δ13C 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 1.4–1.2 Ga, ocean temperature reached around 70°C based on study of global average δ18O and δ30 Si isotopes in carbonates. The δ13C values of Mid-Riphean carbonate from the southern Urals of Russia (1.4–1.2 Ga) show a low positive range of 0–3‰  and a negative anomaly below −20‰  (Bartley et al.,  [28])  in black shale. The δ13C 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 δ13C in carbonates of 1.70–1.60 Ga 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. 1.6–0.6 Ga reported by 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 1.6–1.2 Ga, in response to the global tectonic background.


## 5. 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 negativeδ13C 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 δ13C values, down to −1.7‰, occurs in the Hongshuizhuang Formation; δ13C values then rapidly increase from the bottom of the Tieling Formation to the end of the Mesoproterozoic, varying between −1.6‰  and1‰.The mainδ18O 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.



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*Source: 410621-2011-03-06.xml*