Paleoenvironmental Traces of Carbon and Oxygen Isotopes in Carbonate Rocks: An Example From Dengying Formation in Xichuan Area, Henan Province

The carbon and oxygen isotope composition of carbonate rocks is an important index for accurate analysis of the paleo-sea environment, which depends on Mn/Sr, δ 18 > -10‰, correlativity of δ 13 C and δ 18 O and “age effect” of δ 18 O. This study reports carbon and oxygen isotope data of carbonate rocks from the Dengying Formation in the Xichuan area. δ 13 C values range -1.58‰ to 3.76‰, with an average value of 1.55‰, and δ 18 O values are -14.91‰ to -1.88‰, with an average value of -6.95‰. The δ 18 O values of three samples are less than -10‰, so they are excluded and taken to be correlative with the cracking of the Rodinia supercontinent during the Neoproterozoic. The paleotemperature range 7.40°C to 35.05°C, with an average value of 21.09°C. Paleo-salinity range 8.38‰ to 19.30‰, with an average value of 13.89‰. Z values range 127.80 to 135.03 and thus all exceeded 120, with an average value of 131.25. These calculations indicate that the Xichuan area had deposited marine carbonate rocks, with the hot and dry tropical monsoon climate, and a transgressive process overall during the Dengying age. results show that the Xichuan area was at 10°-15° north latitude during the late Sinian.


Introduction
Composition of carbon and oxygen isotopes in carbonate rocks, which always indicates the composition of ancient oceans (Shao, 1994;Kaufman et al., 1997Kaufman et al., , 2006Guo et al., 2003Guo et al., , 2007Du et al., 2018), has also been applied in the context of oceanic paleoenvironmental tracing, such as of sealevel changes, paleotemperature, paleo-salinity, and biological vicissitudes (Kaufman et  correlation between oxygen isotope and major and trace elements. In the diagenetic process, the altered fluid has different degrees of influence on the carbon and oxygen isotopes, and has little effect on the carbon isotope, but it may cause the oxygen isotope to be fractionated again. To determine whether it is affected by the diagenesis, it is necessary to consider the correlation of carbon and oxygen isotopes. Furthermore, the oxygen isotope composition in ancient strata has often changed for a long time, it also needs to think about the "age effect". Using as an example of Dengying carbonate rocks in the Xichuan area, Henan Province, analyze reliability of carbon and oxygen isotope composition, reconstruct the paleoenvironment of Dengying Formation in the Xichuan area, and uncover other information about the biological vicissitudes, paleoclimate, sea-level changed, paleo-salinity, paleotemperature and the like. Carbon and Oxygen Isotopes and Paleoenvironmental Trace δ 13 C is described by the 13 C/ 12 C ratio because 14 C has radioactivity, which is usually characterized the carbon isotope composition in natural geological mass, the relative norm is PDB (Pee Dee Belemnite from Cretaceous in South Carolina, USA). Organic carbon (reductive carbon) and inorganic carbon (oxidative carbon) reserves are present, with a difference between average δ 13 C values is 25‰ (Chen et al., 1995). Study have revealed (Zheng, 2000) that most organic carbon δ 13 C values are highly differentiated, and characterized by negative δ 13 C excursions in the different earth spheres and ecological elements. For example, δ 13 C in lake water range − 8‰ to -16‰, but in river water is -10‰-which compared with seawater (δ 13 C = 0 ± 2‰), are negatively excursed. δ 13 C value is -24‰, -25‰, -40‰ in coal, petroleum and natural gas respectively, which are more negatively excursed than others materials. Both terrestrial flora and fauna and marine flora also tend to be negative excursion (δ 13 C terrestrial = -22‰, δ 13 C marine = -24‰), perhaps relate to organisms' ability to absorb 12 C preferentially, the δ 13 C value is less negatively excursed than others' (δ 13 C lake = -5‰, δ 13 C river = -12‰, δ 13 C non−marine sedimentary rocks = -4‰, δ 13 C metamorphic rocks = -2‰, δ 13 C magmatic rocks = -6‰, δ 13 C marine sedimentary rocks = 0‰, δ 13 C flora = 0‰; Fig. 1).

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Because 17 O only occurs in the solar system, describe the oxygen isotope composition in natural materials by using δ 18 O, which equates to 18  Different sedimentary rocks have different isotope composition, and because they underwent less post-sedimentation change than magmatic rocks and metamorphic rocks, their carbon and oxygen isotope composition may still convey partial paleoenvironment information.

C With Biological Evolution, Climate, and Sea-Level Vicissitudes
It is necessary to research the carbon isotope composition of marine carbonate rocks for studying biological evolution, climate, and sea-level vicissitudes (Chappell and Shackleton, 1986;Joachimski et al. ,2006). Carbon isotope composition is usually affected by biological activities, specifically (1)Biological evolution: In suitable conditions, 13 C ocean water will increase when large quantities of flora absorb light isotopes 12 C, so that δ 13 C value tends to positively excurse in sedimentary carbonate rocks, whereas δ 13 C value tends to negatively excurse. Thus, positive δ 13 C excursion indicates a high level of marine productivity.
(2)Climate change: The maximum temperature that an organism can withstand varies from 1 °C to 5 °C (Wang and Xia, 2000). During the glacial epoch, temperatures decreased rapidly, so that partially stenothermal organisms perished because of an inability to adapt the environment. The 12 C was increased accompany with decreased of marine productivity, so that δ 13 C value showed negative 5 excursion.
(3)Sea-level vicissitudes: When sea-level rises, paleo-land erosion and oxidation area decreases, and reductive organic carbon diminishes. Ocean water tends to concentrate 13 C, has positive δ 13 C excursion.
The aforementioned biological evolution, climate, and sea-level changes not only affect organic carbon burial rates but also correspond to and influence one another. In glacial epoch, temperature and sea-level diminish, making organisms likely to perish; 12 C is released from organisms to seawater; 13 C content is lower than 12 C; and negative δ 13 C excursion is seen. As a result, negative δ 13 C excursion indicates reductions of sea-level and temperature, declines of organisms, and decrescence of organic carbon burial rate, whereas positive δ 13 C excursion means a raise in sea-level and temperature, prosperity of organisms, and growth of the organic carbon burial rate.
In the Jiangshan area, Zhengjiang Province, Dengying carbonate rocks' δ 13  Compared with carbon isotope in carbonate rocks, the temperature has a sizable effect on oxygen isotope, creating a positive correlation between oxygen isotope and temperature (Zhang, 1997). It is thus possible to calculate the paleotemperature of water using δ 18 O (Zhang, 1985).  Shao (1994) suggested an adjustment to Shackleton's formula: where δ 18 O H2O means that δ 18 O (SMOW) value in ancient seawater, which equals 0‰: As a result, the empirical formula has to be adjusted: Based on various research findings, the formula is more effective when uses after the Jurassic, primarily for reasons associated with post-diagenesis and hydrothermal activity as well as tectonic activities. Such activities may lead to oxygen isotopic fractionation, so that oxygen isotope does not preserve original information about isotope composition. Srinivasan et al. (1994) found that minimum temperature of saddle dolomite when mineralized can also be calculated based on hydrothermal replacement:

Data Reliability Analysis of Carbon and Oxygen Isotopes
Compared with carbon isotope, oxygen isotope is more likely to be affected by tectonic movements, hydrothermal activities and freshwater leaching. Accordingly, before using carbon and oxygen isotope composition to analyze paleo-ocean environment, it must be determined whether samples have undergone isotope exchange, or still retain the original characteristics of their carbon and oxygen isotopes. If not, samples are not significance for the paleo-geological environment. There are main four methods for analyzing the reliability of such data: (1)Mn/Sr. Sr prefers to host in the marine carbonate rocks. Kaufman and Knoll (1995)  (2) δ 18 O value. Oxygen isotope in carbonate rocks is very sensitive to diagenetic alteration and is also used for discriminating carbonate rock alterative level and testing data reliability (Derry et al., 1994;Kaufman and Knoll, 1995): when δ 18 O > -5‰, carbonate rocks have not suffered alteration, whereas δ 18 O range − 5‰ to -10‰ indicates light alteration and δ 18 O < -10‰ indicates alteration is so strong that the data will not convey paleoenvironmental information and data should be abandoned. The older has undergone stronge diagenesis, original information will have been largely lost. Because oxygen isotope is sensitive to temperature, its level of changeability is higher than carbon isotope, in what Shao (1994) called the "age effect".
To eradicate the "age effect", Shao (1994) proposed to use the relationship between δ 18 O values and geological age. In this approach, the difference between the average δ 18

Results
The Dengying dolomites are mainly distributed in the Yangtze platform but are also scattered in the 9 Qinling, Huaxia and Tarim plates. The Xichuan area is located in the intersect zone of the Yangtze platform and Qinling orogen, and the Dengying Formation is exposed completely, with great thickness (about 200-400 m), is a perfect location to study the sedimentary environment and geochemical characteristics of Dengying Formation. To reconstruct the paleo-environment of the Xichuan area and tectonic evolution of the Qinling orogen, and research biological vicissitudes, climate, paleo-sea-level change, paleotemperature, paleo-salinity, it is necessary to know the carbon and oxygen isotope composition of the Dengying dolomites.

Geological Conditions
The Xichuan area belongs to the south Qinling stratigraphic division, in the north Yangtze platform, with east Qinling orogen, and is mainly controlled by the Zijingguan-Shigang regional synclinorium, Xiaodouling-Tianguan fracture, and Jianhuanzhai-Huangfengya fracture, with a NWW-SEE strike. The area exposed upper Sinian to Paleozoic stratigraphy entirely-a series of deep to shallow marine continental clastic rocks and carbonate rocks (Yan et al.,1992). Ordovician, Silurian, early and late Devonian and middle-late Carboniferous deposits (Fig. 2).
The Dengying carbonate rocks are dolomites, with the east area is thicker than the west. The first and third members are white to creamy white and the second is gray-black (Zheng et al., 2017(Zheng et al., , 2018. It is integrated with the underlying Doushantuo Formation and is fault or parallel unconformity with the 10 above Shuigoukou Formation.

Composition of Carbon and Oxygen Isotopes and Reliability Analysis
In this study, we choose 14 samples from the Xichuan area of southwestern Hanan Province, their compositions of oxygen and carbon isotopes are listed in Table 1 Table 2, using corrected data to plot curve graphs of paleotemperature, paleo-salinity, and paleo sea-levels are shown in Fig. 4. During the Dengying age, corresponding to δ 13 C values' changeability, the Xichuan area featured a warm climate, abundant biomass, active biological activities, and a high organic carbon burial rate.
Then, prompted by negative δ 13 C excursion, temperatures began to fall, biological activities also began to diminish, and the organic carbon burial rate fell in the middle Dengying. In the middle-late Dengying, temperatures began to rise, organism developed, and organic carbon burial rate increased until the late Dengying, when biological variety was effected by falling temperatures and organic carbon burial rate.

Conclusions
(1) Carbon and oxygen isotope composition is usually used to quantitatively or qualitatively characterize ancient ocean sea-level changes, paleotemperature, paleo-salinity, and the like. Certain data are need to analyze reliability: Mn/Sr is less than 10, δ 18 O exceeds − 10‰, δ 13 C and δ 18 O have 13 not positively correlation, and correct "age effect" of δ 18 O values.
(2) The changeable characteristics of δ 13 C indicate that it experienced potentially two transgressregression cycles, with sea-level changing frequently. δ 18 Tables   Due to technical limitations, Tables 1 & 2 are only available for download from the Supplementary Files section. Figure 1 The distribution characteristics of carbon, oxygen isotopes (From Zheng, 2000).

Figures
21 Figure 1 The distribution characteristics of carbon, oxygen isotopes (From Zheng, 2000).  Scatter plot of Dengying carbonate rocks in the Xichuan area.

Figure 3
Scatter plot of Dengying carbonate rocks in the Xichuan area.
25 Figure 4 Curve graph of carbon, oxygen isotopes from the Xichuan area. Curve graph of carbon, oxygen isotopes from the Xichuan area.

Supplementary Files
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