Stable Isotopes Carbon C 13 and Oxygen O 18 as Indicators of Triassic Limestone Sedimentation Environment and Diagenesis

The results of researches of the stable isotopes, carbon 13 C and oxygen 18 O, measured in Triassic limestones of Opole Silesia in Poland were presented in this article. The study was carried out to obtain data for interpretation of the environment of these rocks formation. Moreover, it was possible to form the theory about diagenetic processes which inuenced on the mineral composition of limestone and some of their carbonte phases. The results of study show a general differentiation of δ 13 C and δ 18 O contents in carbonate minerals. All δ 18 O values are less than 0 ‰ . It indicates that the origin oxygen isotope composition could be probably reset by diagenesis. The crystallization temperatures of low-Mg calcite and high-Mg, calculated on the basis of δ 18 O values are greater than 25 o C. They are higher than typical for sea basin and are also not be related to the presence of hydrothermal solutions. The increased temperatures of calcites crystallization are related to diagenetic processes that took place after the deposition and burial of carbonate material. The preservation of high-Mg calcite, an ustable carbonate phase, which is usually trasformed into low-Mg calcite during diagenesis, is probably connected with the increased salinity of the sea basin in which studied limestones were formed.


Introduction
Stable isotopes, carbon 13 C and oxygen 18 O, are usually measured for interpretation of the environment of rock formation (Migaszewski, 1986(Migaszewski, , 1989Migaszewski, 1990a (Migaszewski, 1989;Tripati et al., 2010). It is de ned as the deviation of the isotope ratio from the standard value. The standards used for determining 13 C and 18 O isotopes are PDB (Pee Dee Belemnite) standard (a chalk belemnost rostrum from Pee Dee Formation in South Carolina) or V-SMOW standard (Standard Mean Ocean Water -average oceanic water) (Migaszewski et al., 1995).
Stable isotope geochemistry has been also used as an indicator of palaeoclimate. The potential oxygen isotope compositions could be used for palaeotemperature reconstruction (McCrea, 1950;Epstein et al., 1951Epstein et al., , 1953Urey et al., 1951;Friedman and O'Neil, 1977;Tucker and Wright, 1990; Romanek et al., 1992;Leng and Marschal, 2004;White, 2013;Henkes et al., 2014). A problem in the quantitative interpretation of the climate geochemical factors is that the variables that can be measured in the sediments, especially δ 18 O and δ 13 C in carbonates, are in uenced by a wide range of interlinked environmental processes and not a single factor (McCrea, 1950;Urey et al., 1951;Leng and Marschal, 2004). A change in temperature will be able to produce a shift in the equilibrium oxygen isotope composition of carbonates forming in lakes, will affect the isotope composition of the rainfall and may also affect rates of evaporation, both in the lake and in the catchment (Leng and Marschal, 2004;Ghosh et al., 2006).
Limestones are one of the most important of all the sedimentary rocks used in different branches of industry. They are composed mostly of calcite mineral (CaCO 3 -low magnesium calcite). Sometimes includes other carbonate phases, like high-Mg calcite, protodolomite, ordered dolomite or huntite. These carbonate phases were identi ed in Triassic limestones of Polish part of Germanic Basin (South-West part of Poland -area of the Opole Silesia) (Szulc, 1990(Szulc, , 2000Stanienda, 2005Stanienda, , 2006Stanienda, , 2011Stanienda, , 2013a2016a,b;Stanienda-Pilecki, 2018, 2019. There are numerous Triassic limestone deposits in this territory. These limestones are Lower Muschelkalk (Middle Triassic) sediments. The Muschelkalk pro le contains Gogolin Beds (the bottom of pro le -initial sea transgressive stage), Górażdże Beds (advanced sea transgression stage), Dziewkowice (Terebratula) Beds (sea transgression peak stage) and Karchowice Beds (the upper formation of pro le -sea regression stage). The formation names are regional. They come from the names of nearby towns. The limestones of these units are built of carbonate phases different in Ca and Mg content.
The results of researches of the stable isotopes Carbon C 13
The measurements were carried out in the Mass Spectrometry Laboratory in Lublin (Institute of Physics of the Maria Curie-Skłodowska University) using a mass spectrometer on CO 2 gas samples prepared from original geological samples.
The quantities of 13

Results Of Study
Seventeen samples taken from all four limestone formations were selected for studies of stable isotopes δ 13 C and δ 18 O. The values of 13 C/ 12 C and 18 O/ 16 O were determined in these samples and calculated on the PDB δ 13 C and δ 18 O scale of low magnesium calcite (Fig. 2a), high magnesium calcite (Fig. 2b) and dolomite ( Fig. 2c) (Stanienda, 2011(Stanienda, , 2013a (Stanienda, 2013a).
In samples taken from the Karchowice Unit these values are: δ 13 C, from -8. 12  The results were presented in the Table 2. Analyzing the results presented in Table 2, the tendency of an increase (in the case of 9 samples) in the content of δ 13 C SMOW from "pure" calcite to dolomite was obserwed.
The exceptions are:

Discussion
The δ 13 (Migaszewski, 1989). Values of δ 13 C, in the range from -1.21 to 3.17‰ and δ 18 O, from -6.7 to 0.23‰, are characteristic for Triassic dolomites from the area of Chrzanów (Migaszewski, 1986 Figures 3 to 5). Therefore, a general differentiation of the δ 13 C content in carbonate minerals of studied samples was observed. However, there is no relations between the value of δ 13 C and belonging the samples to any of the four units occuring in the analyzed pro le. Samples from each formation vary in content of δ 13 C. Therefore, there is no relations between δ 13 C content and the type of the stage of sea transgression or regression.  Results of researches show that there is no relationship between the value of δ 13 C of low-Mg calcite and high-Mg calcite and the crystallization temperature of these carbonate phases (Figures 9 and 10). However, a relationship between the value of δ 18 O of low-Mg calcite and high-Mg calcite and the crystallization temperature of these carbonates was observed (Figures 11 and 12). The crystallization temperature of lowmagnesium calcite and high-magnesium calcite increases as the of δ 18 O value decreases. This relationship is almost linear (Figures 11 and 12).
The results of the ealier conducted research show that the isotope exchange of oxygen takes place between minerals and seawater at elevated temperatures (Epstein et al., 1953;Tucker and Wright, 1990;Hałas et al., 1993). Therefore, the value of δ 18 O of primary carbonates depends on the δ 18 O of seawater, and also on the temperature at which their crystallization takes place and the isotope exchange is completed (Migaszewski, 1989 Due to the research results it is necessary to consider the conditions of the basin environment in which the studied limestones were formed. It is also necessary to analyse the conditions in which the diagenetic processes took place. During these processes despite the in uence of high temperatures and pressures (characteristic of advanced diagenetic processes) on the sediment, the unstable carbonate phase -highmagnesium calcite was preserved. Therefore, it can be concluded that the studied limestones underwent to diagenetic processes, during which the isotopic composition changed, and carbonate minerals at elevated temperatures underwent recrystallization, agradation and neomorphic processes. High-Mg calcite, an unstable carbonate phase, which, such as aragonite, is usually transformed during burial into low-Mg calcite. The preservation of this carbonate phase is probably connected with the increased salinity of sea basin in which studied limestones were formed. Analyzing the processes, which in uence on the Muschelkalk limestones of the Opole Silesia (South-West part of Poland) formation it is necessary to remember that the Triassic sediments of the South-West part of Poland are the sediments of the East part of the Epicontinental Germanic Basin, shallow sea basin, probably characterized by increased salinity.

Conclusions
Stable isotopes, carbon 13 C and oxygen 18 O, were measured in Triassic (Muschelkalk) limestones from the area of Opole Silesia in Poland. Research was carried out to obtain data for interpretation of the environment of these rock formation and diagenetic processes which in uenced on the present mineral composition of limestone and some properties of carbonate phases.
The results of δ 13 C study show a general differentiation of the δ 13 C content in carbonate minerals.
Moreover, there is no relations between the δ 13 C value and belonging the samples to any of the four units occuring in the pro le. A general differentiation of δ 18 O content in carbonate minerals of studied samples was also observed. However all values are less than 0‰. It can indicate that the primary oxygen isotope composition of the studied carbonates could be reset by diagenesis. There is also no relations between δ 13 C and δ 18 O contents and the type of the stage of sea transgression or regression. The interpretation and analysis of the results of the research were done by myself.

Ethical statements
Funding-This research did not receive any speci c grant from funding agencies in the public, commercial, or not-for-pro t sectors.
Availability of data and material-The data presented in this manuscript can be found in the cited articles and in the Author's database.

Con icts of interest/Competing interests-No con ict of interest.
Ethics approval-The results of this manuscript are presented clearly, honestly and without fabrication, falsi cation or inappropriate data manipulation. I have followed editorial ethics and academic ethics in this manuscript. I have not used any unethical languages in any part of the manuscript.    Relation between the low-Mg calcite crystallization temperature and δ 13 C VPDB Figure 10 Relation between the high-Mg calcite crystallization temperature and δ 13 C VPDB Page 20/20

Figure 11
Relation between the low-Mg calcite crystallization temperature and δ 18 O VPDB Figure 12 Relation between the high-Mg calcite crystallization temperatures and δ 18 O VPDB