Determining of an oceanic anoxic event and paleoenvironmental conditions of the Gulneri Formation Kirkuk Oilfield, Northern Iraq

Stable isotopic geochemistry in relation to major and trace elements geochemistry of core samples from the Gulneri Formation at Kirkuk Oilfield Northern Iraq has been conducted to determine the occurrence of the oceanic anoxic event (OAE2) and reconstruct the paleoenvironmental conditions during this event. Positive values of the stable carbon isotopic data have revealed that temperature is high during deposition accompanied with higher organic productivity and a high rate of organic carbon burial with a decrease in dissolved oxygen in marine environments, whereas, negative oxygen isotopic values indicate deposition in hot, dry, semi-arid environments with salty marine waters and high organic productivity. These results were also supported by geochemical evidence of paleoclimate through C-value and ratios of Sr/Cu, Rb/Sr, and Ga/Rb, paleosalinity using proxies from Ca/Ca+Fe and Sr/Ba ratios, and paleoproductivity using P/Ti, P/Al and barium ratios. The paleoredox proxies from V/(V+Ni), V/Ni, and U/Th ratios and Uδ indicate dominant anoxic conditions. The recorded oceanic anoxic event (OAE2) has been determined through significant excursion in carbon and oxygen isotopic values as well as geochemical proxies within the sequences of the upper part of the formation marking the final stage of the OAE event.


Introduction
The Oceanic Anoxic Events (OAEs) are referred to extreme climatic changes and significant disruptions in the world's carbon cycle, some of them are known to have occurred in middle Cretaceous (≈ 124 to 90 Ma) (Jenkyns, 2010).They are short-lived organic carbon burial episodes identified by their wide distribution as unique black beds and notable excursions in carbon isotope.The main causes of these important events and their effects on Earth's history are still unknown, despite earlier research in this area.(Singh et al., 2022).During the Cenomanian-Turonian transition (≈ 93 Ma ago), an oceanic anoxic event 2 (OAE2) took place.This event was synchronized, short-term, and global (Jenkyns, 2010).The OAE2 was a period of global change identified by elevated carbon dioxide in the atmosphere (Witkowski et al., 2018), altered cycles of nutrition (Owens et al., 2016), and increased ocean anoxia (Owens et al., 2013(Owens et al., , 2016)), making

Geological Setting
The study area is located in northeastern Iraq within Kirkuk Governorate, specifically in the Kirkuk oilfield, which is tectonically located in the foothill Zone, Figure (1).The Kirkuk oil field is a doubleplunge, asymmetric anticline with 125 km length and 4-5 km width in a northwest-southeast direction.The Kirkuk anticline is characterized by the presence of three domes, which from the southeast to the northwest, Avanah dome, Khurmala dome and the Baba dome in which the study section (K-116) is located.The Gulneri Formation which belongs to the Turonian-Early Campanian cycle represents the main current studied sequence (Chatton and Hart, 1961).The formation is unconformably underlain by the deep marine Dokan Formation (Bellen et al., 1959) and overlies by the Kometan Formation of the upper slope to the outer shelf setting (Jassim and Goff, 2006) Figure (2).The age of the formation was determined as early Turonian (Bellen et al., 1959;Chatton and Hart, 1961;Abawi et al., 2006).The Gulneri Formation was described for the first time by (Lancoster Jones, in Bellen et al., 1959) in the Dokan region, northeastern Iraq as black fissile calcareous shale rich in glauconite and cellophane dominantly in the lower part of the formation.The studied section in well K-116 consists of calcareous shale rich in organic materials, glauconite, and pyrite.Deep basinal environment, sometimes anoxic as indicated by the presence of bituminous materials and dwarfism in some of the identified planktonic foraminifera has been mentioned for the deposition of the formation (Buday, 1980;Abawi and Hammoudi, 1997;Abawi and Hammoudi, 2005;Abawi et al., 2006;Awadh and Al-Dulaimy, 2007;Abdo, 2013).Recently, Al-Haj et al. (2023) have mentioned that the depositional model of the Gulneri Formation has been changed from the deep setting with euxinic and quiet anoxic conditions in the northeast part of the Kurdistan foreland basin to a relatively shallow setting with semi-reduced conditions towards the southwest.

Materials and Methods
The methodology includes two steps, the first was high-resolution sampling for ten cores from the laboratories of the Iraqi Northern Oil Company, Kirkuk, Iraq with detailed descriptions of the studied core samples.The second step represents geochemical analyses for the major and trace elements and stable carbon and oxygen isotopic geochemistry.The samples collected consist of six from the Gulneri Formation, which correspond to the OAE2 event.Additionally, two samples were taken from both the upper part of the underlying Dokan Formation (D1 and D2) and the lower part of the overlying Kometan Formation (K1 and K2).These samples were used to compare the geochemical findings of the event sequence with samples obtained from below and above the OAE2 in the study area.

X-Ray Fluorescence (XRF)
The XRF analysis was conducted on a representative sample of ten utilising a Bruker Tracer 5i XRF equipment located at the Premier Corex Group Laboratory in Houston, Texas, U.S.A.The device uses a Rh tube and performs three distinct phases of analysis to measure the complete range of components.Tables 1 and 2 display the concentrations of both the major and trace elements.

Stable Isotopic analysis
At Ruhr-Universität Bochum in Germany, carbon and oxygen stable isotopes were analysed in eleven samples using a Thermo Fisher Scientific mass spectrometer model MAT253, equipped with a ConFlo IV and a GasBench II.In order to generate carbon dioxide (CO2), carbonate samples weighing 0.02 grammes were subjected to a reaction with 100% phosphoric acid for a duration of 90 minutes at a temperature of 90°C.The isotope composition was determined by applying the calcite acid fractionation factor.According to VPDB, the isotope ratios measured for NBS18 (δ 13 C = -5.00,δ 18 O = -22.96‰)and NBS19 (δ 13 C = 1.95, δ 18 O = -2.20‰)international calcite standards were used to calibrate the measured isotope ratios.For both δ 13 C and δ 18 O values.

Geochemistry: Geochemical proxies for oxygen abundance conditions (paleoredox proxies)
V/(V+Ni).The ratio serves as a crucial indicator for reconstructing paleoredox conditions, continuously pointing to low oxygen environments (Rimmer, 2004).If the value of this ratio exceeds 0.84, it signifies euxic conditions.A value between 0.54 and 0.84 suggests anoxic conditions.A value between 0.54 and 0.46 corresponds to dysoxic conditions, while a value below 0.46 indicates an oxic condition.(Hatch and Leventhal, 1992).By applying this ratio to the studied Gulneri Formation samples (Table 3), the value ranges from 0.569 to 0.871 with an average of 0.716.This indicates that the formation was deposited in anoxic environments, while those values from the underlying Dokan and overlying Kometan formations are less than 0.46, which refer to an oxic condition.V/Ni.In general, this ratio is high in anoxic conditions and low in oxic conditions.A ratio greater than (3) indicates anoxic sedimentary conditions during organic matter deposition in marine origin, and if the ratio ranges between 1.9 and 3, then it indicates sedimentary conditions between dysoxic-oxic, and organic matter consists of a mixture of continental and marine.As for the ratio less than 1.9, it refers to oxic sedimentary conditions and the organic matter is of continental origin (Galarraga et al., 2008;Jia et al., 2013).In the current study, the ratio in the Gulneri Formation samples has ranged from 1.320 to 6.758 with an average of 3.498, (Table 3).This value indicates that the studied Gulneri sequence was deposited generally under anoxic conditions with marine sources for the organic matter as compared to oxic conditions for the underlying and overlying formations where values are less than 1.9 (Table 3).
U/Th.Uranium and thorium elements have the same chemical properties, as the ratio between them is used to estimate the state of oxidation and reduction should behave differently under reducing and oxidizing conditions, so they are used to differentiate the paloeredox.If the value is higher than 1.25, it indicates an anoxic environment, while values that are between 1.25 and 0.75 indicate a hypoxic environment, and if it is less than 0.75, it refers to the oxygenic environment (Jones and Manning, 1994;Zhang et al., 2013).The current study showed that the U/Th range in the studied samples is 1.128 to 2.848 with an average of 1.587 (Table 3).Based on these ratios, the environment of deposition was anoxic and higher than those in the underlying and overlying formation which may refer hypoxic to anoxic environments.
Uẟ.The anomaly of uranium, or the so-called uranium coefficient, depends on the relationship between uranium and thorium to determine the redox environment.To determine the anomaly coefficient, the following equation (Wignall, 1994 in Wu et al., 2017) is used: Anomaly factor values that are higher than (1) represent an oxygen-deficient environment, while a value that is less than 1 represents normal marine waters, where the ratio (U+Th/3) represents the relative value of localized uranium origin.By calculating the anomaly factor for uranium in the studied samples (Table 3), it was found that the average anomaly of the studied Gulneri samples reached 20.281, and this indicates the anoxic paleoenvironment of deposition.

Geochemical proxies for paleoclimate
C-value.The distribution of some trace elements can be used to infer the ancient climate (Ding et al., 2018).Some studies indicated that relative enrichment of some elements such as manganese (Mn), iron (Fe), vanadium (V), chromium (Cr), cobalt (Co), and nickel (Ni) existed in humid climatic conditions.On the contrary, the rise of the alkalinity of the water due to evaporation under arid climatic conditions facilitates the precipitation of salt minerals and thus enrichment of other elements such as calcium (Ca), magnesium (Mg), potassium (K), sodium (Na), strontium (Sr) and barium (Ba) (Cao et al., 2012;Wang et al., 2017;Ding et al., 2018).As a result of the geochemical behavior of these two groups of elements, the ratio between them is known as the C-value, which is an important proxy for the climate, and can be calculated by applying the following equation: A C-value within the range of 0.0 to 0.2 signifies an arid climate.A numerical range of 0.2 to 0.4 signifies a semi-arid climate.The range from 0.4 to 0.6 represents conditions that are partially dry and partially humid, whereas the range from 0.6 to 0.8 signifies a climate that is somewhat humid, and the range from 0.8 to 1.0 shows a climate that is predominantly humid (Zhao et al., 2007).By applying the abovementioned equation, the C-value for the Gulneri Formation samples has ranged from 0.109-0.251with an average of 0.151, Table (4), which points to deposition mostly under arid to semi-arid climatic conditions, while prevailing conditions of the underlying and overlying formations generally are arid (Table 4).
Sr/Cu.The ratio of strontium to copper can be used as a good proxy for paleoclimate, as this ratio increases in warm climatic conditions (Cao et 4), which indicates that the formation was deposited under an arid and hot climate.Same conditions are indicated for the underlying and overlying formations (Table 4).4).The prevailing conditions for the underlying and overlying formations are similar to those from the Gulneri Formation (Table 4).
Ga/Rb.The gallium to rubidium ratio can serve as an indicator of paleoclimatic conditions, with a low ratio suggesting a hot and arid climate (Yandoka et al., 2015).The Ga/Rb ratio varies between 0.127 and 0.141, with an average value of 0.134 (Table 4).These low values also suggest that the deposition occurred in a hot and arid climate.In order to verify the climatic conditions of the depositional environment of the Gulneri Formation, we utilised the link between the ratio of strontium to copper (Sr/Cu) and the ratio of gallium to rubidium (Ga/Rb).

Geochemical proxies for paleosalinity
Ca/(Ca+Fe).The proportions of some of the major elements in the rocks and their relationship with each other can reflect the paleosalinity in ancient environments, such as the ratio (Ca/(Ca+Fe)) that is sensitive to changes in the salinity of ancient marine waters (Nelson, 1967).If the value of this ratio exceeds 0.8, it reflects saline water, and the range between 0.4 and 0.8, points to mixed water, but if it is less than 0.4, it reflects fresh-water conditions (Lan et al.,1987 in Zhang et al., 2013).The studied Gulneri Formation samples range from 0.966 to 0.994 with an average of 0.984 (Table 5) which indicates that the formation was deposited within saline marine waters.Same conditions are indicated for the underlying and overlying formations (Table 5).5) which refers to the dominating saline water environment.An abrupt change to fresh water environment is recorded from the first sample of the overlying Kometan Formation with value of 0.283 for Sr/Ba.

Determining water quality based on isotopes
Limestone and calcareous shale samples of the Cenomenian-Turonian sequences were studied.A distinction was made between marine water and fresh water from the Z values calculated by the isotopes (δ 18 O) and (δ 13 C) through the equation proposed by (Keith and Weber, 1964): Z=a(δ 13 C+50) +b(δ 18 O+50) whereas: a= 2.048, b= 0.498, δ 18 O: oxygen isotopes of limestone (vPDB), δ 13 C: isotopic composition of carbon in limestone (vPDB).Samples with a Z value higher than 120 will be classified as marine, Z value lower than 120 are freshwater limestone.The results show that Z value is higher than 120 and represent marine environment (Table 6): The temperature of water during deposition of the limestone rocks was deduced using the isotopes (δ 18   6).
The higher temperature at sample G2 from the Gulneri Formation coincides with the lowest value of δ 18 O.

Geochemical proxies for paleoproductivity.
The relationships and proportions between some of the major elements such as (P, Al, Ti) and trace elements such as (Ba, Cu, Zn) are used to estimate the paleoproductivity.

Use of Phosphorous (P).
Phosphorus is a vital element for all life forms on earth and serves as a crucial nutrient for all living things.It plays a significant part in numerous metabolic processes and is the primary constituent of soft skeletal materials that assimilate phosphorus to build their tissues.The phosphorus level in sediments is positively correlated with productivity, as it can be sequestered by organisms upon their death.(Tribovillard et al., 2006;Shen et al., 2014).The ratios of phosphorous to aluminum (P/Al) and phosphorus to titanium (P/Ti) are used for the purpose of evaluating productivity instead of the absolute value of phosphorus in order to reduce the impact of continental clastic materials (Algeo et al., 2011), as shown below: P/Ti.The values of this ratio give good information about the paleoproductivity.If the ratio is less than 0.34, it indicates low productivity while ranges between 0.34 and 0.79, indicate medium productivity, and a higher ratio than 0.79, refers to high productivity (Algeo et al., 2011).By applying this ratio to the studied samples of the Gulneri Formation, the average value is 4.51 while the range was 0.783 to 11.190 (Table 7), which generally indicates high productivity.The result from the underlying and overlying formations show same result of high productivity (Table 3).

P/Al.
The ratio of phosphorous to aluminum values can also be used to indicate primary productivity, as Hetzel et al., (2009) stated that if the ratio values ranged between 0.01 and 0.03, it indicates very high productivity.The average value of this ratio for the studied Gulneri Formation is 0.08, (Table 7), indicating that the paleoproductivity generally was high.Use of Barium (Ba).Barium was found in the oceans either in the form of the barite mineral or in the structures of organisms and organic materials, as a result of the activity of organisms through metabolic absorption or through adsorption of barium on the structures of carbonate or silicic organisms.It also may be of detrital origin through transportation with organic materials.There is a strong relationship between Ba and organic carbon (Dymond et al., 1992;Tribovillard et al., 2006).Barium in relationship with some major elements such as Ba/Al has been frequently used as an indicator for increasing productivity.If the value of this ratio is higher than 100, it indicates the high paleoproductivity, if it is around 70, then the paleoproductivity is intermediate, while the value of less than 50 denotes low paleoproductivity (Dean et al., 1997).By applying this ratio to the Gulneri Formation (Table 7), the average value was 256.575 which indicates high paleoproductivity.

Relation between stable isotopes and diagenesis
The values of the carbon (δ 13 C) and oxygen (δ 18 O) isotopes in the ancient limestone rocks reflect their sedimentary environment, and their composition may change as a result of their influence by the diagenetic processes after deposition (Hoefs, 2009).The carbon isotope has a relatively higher resistance to diagenetic processes as compared to the oxygen isotope, which is more sensitive to the changes resulting from the diagenesis.The oxygen isotope is used to estimate the ancient temperatures of marine and oceanic waters because it is more affected by the water temperature resulting in great thermal breakage compared to the carbon isotope, this effect appears when rocks are exposed to atmospheric water with the presence of carbon dioxide gas in an open ecosystem.Therefore, in order to assess the degree of diagenesis influence on rocks and, consequently, ascertain the degree to which oxygen and carbon isotopes reflect the original imprint of the environment in which those rocks were deposited, it is imperative to ascertain the degree to which rocks are impacted by diagenetic processes prior to analyse and interpret the values of stable isotopes (Magaritz, 1991;Fisher et al., 2005;Azmy et al., 2009).There are several geochemical methods to determine the extent to which diagenetic processes affect the initial carbon and oxygen signature: 1-The positive and strong relationship between the stable carbon isotope (δ 13 C) and the stable oxygen isotope (δ 18 O), indicates the influence of diagenetic processes.However, if the relationship is weak or negative, the isotope values indicate their primary fingerprint and lack of influence by early or late diagenetic processes (Fisher et al., 2005;Li et al., 2015;Hennhoefer et al., 2018).By applying this method to the current study samples of the Gulneri Formation Figure (4), it is evident that the relationship between the isotopes is negative, indicating that the formation sequences are minimally affected by diagenetic processes.Thus, the isotope values represent their primary fingerprint in the studied sequence.

Figure 4.
The relationship between the stable carbon isotope (δ 13 C) and the stable oxygen isotope (δ1 8 O) in samples of the Gulneri Formation.
2-The strong and positive relationship between the stable oxygen isotope (δ 18 O) and the strontium element values (Sr) indicates the influence of alteration processes on sedimentary rocks in an open alteration environment.However, if the relationship is weak or inverse, the rocks are not affected by diagenetic processes (Nagarajan et al., 2013).Using this relationship on samples of the Gulneri Formation (Figure 5), the relationship is inverse.This provides further evidence that the oxygen isotope values serve as the key indicator in the examined rocks from the Gulneri Formation.

Stable Isotopes:
The data of the stable C and O isotopes are also used as paleoenvironmental proxies in the studied formation; Carbon Isotope (δ 13 C): The fractionation of carbon and consequent enrichment of the ocean in 13 C is evidence of the rise in productivity at the beginning of OAE2.The enrichment is viewed as a + 2 to 2.5 shift in (δ 13 C) values, which, when evaluated through a window covering the Upper Cretaceous, is considered a significant change (Jarvis et al., 2006).The values of the carbon isotope (δ 13 C) in samples of the Gulneri Formation range between 3 ‰ and 2.05‰ with an average of 2.4‰ (Table 6, Figure 6), and it is noted that they are positive values in all samples.The carbon isotope values exhibit a significant excursion at the upper limit of the Gulneri Formation (Figure 6).
Oxygen Isotope (δ18O): Measurements of (δ 18 O) isotopes have often been used to create the paleotemperature records in the ocean water column as well as to infer diagenetic modification of sediments and variations in freshwater input (Keller and Parado, 2004).The values of the oxygen isotope (δ 18 O) in samples of the Gulneri Formation range between -3.8‰ and -4.71‰ with an average of -4.33‰ (Table 6, Figure 6), and it is noted that they are negative values in all samples.The carbon isotope values exhibit a significant excursion at the upper part of the Gulneri Formation.

Oceanic Anoxic Event 2 (OAE2)
The major ocean basins have experienced a significant accumulation of sediments rich in organic material.There has been a rapid cycling of marine life, as well as notable increases in carbon isotope levels in both carbonate rocks and organic matter.Additionally, there are other obvious abnormalities in the chemical composition.(e.g.S, Li, Hg, Os, and Cr isotopes) are indicative of dramatic changes in ocean conditions during OAE2 (Gomes et al., 2016;Jenkyns, 2010;Jenkyns et al., 2017).By combining geochemical and isotopic data collected from the Gulneri Formation, valuable insights can be gained into the paleoceanographic changes that occurred during the OAE2 period in the southern Tethys region, the best stratigraphic tool currently used can be considered isotopic geochemistry (δ 13 C and δ 18 O) (Frijia ).The positive δ 13 C shift, which has been observed in the Gulneri Formation, suggests that the main palaeoceanographic characteristics associated with OAE2 include increased productivity of marine surface waters and greater preservation and burial of organic matter.The δ 18 O fluctuation around the Cenomanian -Turonian boundary is attributed to the difference in δ 18 O in ocean water due to temperature change and/or influx of freshwater.The most noticeable feature comprises variable readings up until the carbon peak connected to a negative excursion for oxygen and subsequent recovery to values as high as -4.2.An apparently negative excursion in the δ 18 O profile that occurs along with the positive carbon excursion may indicate warming throughout the CTB into the early Turonian (Figure 5).The uppermost Gulneri Formation deposits exhibited the OAE2 geochemical signature, characterised by elevated carbon isotope values due to increased organic carbon burial.This occurred prior to a worldwide sea level increase at the end of the Cenomanian period.(Haq, 2014).Low values were present in the lowest part of the section, with values varying between 2.06 and 2.17 (Figure 6).The values rapidly rise to a maximum of 3.0 We believe that the latest peak in the isotope δ 13   2023), marks the conclusion of OAE2. Figure (7) shows that this anomaly in stable isotope values relates to the anomaly in environmental proxy values reported for the studied sequence, which correspond to the environmental conditions that prevailed during the OAE2 event.Kump and Arthur, 1999).The geochemical evidence was also used to estimate the paleotemperature by evaluating and studying the proportions and relationships between the major and trace elements for the Gulneri Formation samples in the study section.
The oxygen isotope data (Table 6), reveals that all values are negative, indicating that temperatures increased throughout the deposition of the Gulneri Formation.The values in the lower and middle parts of the formation are significantly higher than the values in the upper part, which is shown by a negative shift in the vertical distribution (Figure 6), which confirms an increase in temperatures with the upper part of the formation.Geochemical data of the paleoenvironment conditions using C-value, Sr/Cu, Rb/Sr, and Ga/Rb proxies suggest deposition mainly in hot and dry conditions.

Paleosalinity:
Paleosalinity can be established by employing the oxygen isotope (δ 18 O) measurements.As salinity increases, its values rise as well.When carbonate rocks are deposited in fresh-water, their average values for this isotope are -8.66‰,whereas when they are deposited in salty marine seas, their typical values are -5.25‰(Keith and Weber, 1964; Longinelli and Nuti 1973; Kolodny et al., 1983;Steuber, 1999).Geochemical data is also helpful in establishing paleosalinity by using certain relationships between the major or trace elements.In general, the Gulneri Formation sequences negative oxygen isotope values indicate their deposition in saline marine waters.This was in accordance with the results of the geochemical analysis of paleosalinity using the proxies Ca/Ca+Fe and Sr/Ba as mentioned previously and also demonstrates deposition in saline marine waters (Figures 6, 7).

Paleoredox:
The level of oxygen in the sedimentary environment of the Gulneri Formation strata in the study area was determined using the carbon isotope values (δ 13 C).In contrast, a decline in these values (toward negative values) suggests an increase in ocean oxygen due to greater oxidation of organic carbon.A rise in these values (toward positive values) indicates a decrease in ocean oxygen (Faure and Mensing, 2005;Hoefs, 2009;2021).The abundance of oxygen (oxidation and reduction) for the formation sequences was also determined using geochemical data.As a result, those conditions are reflected in trace element ratios, including the V/(V+Ni), V/Ni and U/Th ratios and Uẟ as previously shown in this study that indicates dominantly anoxic conditions for the Gulneri Formation, while they shift to oxic conditions in the underlying and overlying formations.According to Table 6 and Figures 6 and 7, the vertical distribution of carbon isotope in the Gulneri Formation values is uneven, as it will be seen that the values are identical in the Gulneri Formation while abrupt shift existed in the lower part of the overlying Kometan Formation.Low oxygen conditions (anoxic/dysoxic) in combination with upwelling-controlled paleoproductivity (organic carbon fluxes) which is generally high as mentioned previously using P/Ti, P/Al, and Barium have major and favorable controls over organic matter preservation and enrichment (Al-Sagri, 2015; Sajid et al., 2020).The data of the present study are consistent with the paleontological evidence of anoxic deficiency recorded by Abdo (2013) along with the presence of shale bed and pyrite as well as the absence of benthic foraminifera and the widespread (Heterohelicid) and calcispheres, which indicates anoxic events (Kuhnt et al., 1986).The oxygen depletion recorded in the current study also is consistent with similar events recorded in other regions of the world, such as the Pacific Ocean (Jenkyns, 2010)

Conclusions
 The examination of the correlation between stable isotopes and diagenetic processes revealed that diagenetic processes have a minor impact on the formation sequences.Therefore, the stable isotope values in these sequences show their original signature. Positive carbon isotope (δ 13 C) values indicate increasing temperatures, higher organic productivity, higher rate of organic carbon burial, and a decrease of dissolved oxygen in marine environments. The oxygen isotope (δ 18 O) values were negative, indicating that the formation was deposited in hot, dry, semi-arid environments with salty marine waters and high organic productivity.This also was supported by geochemical evidence of a paleoclimate. Geochemical indicators for ancient oxygen levels have shown that the formation was formed under oxygen-depleted circumstances, with the organic materials originating from the sea (marine origin). Geochemical evidence of paleosalinity and paleoproductivity also indicated that the formation was deposited in salty, highly productive marine waters. Within the studied area, there is evidence of an oceanic anoxic event (OAE2).The upper part of the formation shows a notable change in stable isotope values of carbon and oxygen, as well as geochemical proxies.This indicates the final phase of the OAE event.

Figure 3 .
Figure 3.The relationship between the ratio (Sr/Cu) and the ratio (Ga/Rb) showing the paleoclimatic conditions of the Gulneri Formation in the study area, according to (Ding et al., 2018).

14 Figure 5 .
Figure 5.The relationship between the stable oxygen isotope (δ 18 O) and the strontium element values (Sr) for the Gulneri Formation.
al. 2015;Brčić et al. 2017).The use of δ13 C as an effective correlation tool along with paleoenvironmental proxies for various sedimentary environments, The excursion into higher stable carbon isotope levels during OAE2, lasting for a period of 2-4, is considered one of the most consistent isotope curves in the history of the Earth.(Jarvis et al., 2011; Jenkyns et al., 2017; O'Connor et al., 2020 C curve seen in this sequence, correlates with peak C as reported by Jarvis et al. (2011); Brčić et al. (2021), and peak D (plateau) as reported by Melinte-Dobrinescu et al. (

Figure 6 .
Figure 6.The vertical distribution values of the carbon (δ 13 C) and oxygen (δ 18 O) isotopes in the studied Gulneri Formation.

Table 1 .
Concentrations of major elements (%) for the studied samples.

Table 2 .
Concentrations of trace elements (ppm) for the studied samples, (-means not detected).

Table 3 .
Geochemical proxies for oxygen abundance conditions.Note that the maximum (Max.), minimum (Min.) and average (Avg.)values are only for the Gulneri Formation samples.
After calculating this ratio for the Gulneri Formation, it was found that the average ratio has a range of 14.880 to 22.447 with an average of 19.103 (Table al., 2015; Yin et al., 2017).The values that range between 1.3 and 5 indicate a warm-humid climate, and that range between 5 and 10 indicate a semi-humid to semi-arid climate (semi-humid-semi-arid), but if it is greater than 10, it refers to arid and hot climate (Yandoka et al., 2015; Song et al., 2016).

Table 4 :
Geochemical proxies for paleoclimate in the studied samples.Note that the maximum (Max.), minimum (Min.) and average (Avg.)values are only for the Gulneri Formation samples.
(Cao et al., 2015;Zeng et al., 2019) strontium (Rb/Sr) can also be used to determine the nature of paleoclimatic conditions, the ratio generally decreases in hot and arid climate conditions(Cao et al., 2015;Zeng et al., 2019).The ratio in the studied samples has an average of 0.038 which indicates hot and arid climate conditions (Table

Table 5 :
(Zhen et al., 2020;Sajid et al., 2020)in the studied samples.Strontium (Sr) and barium (Ba) have similar chemical properties in the alkaline earth metal, while they are quite different in their geochemical behavior.Strontium is directly deposited from marine waters, while barium is easily absorbed by clay minerals and fine clastic deposits(Xu et al., 2011;Meng et al., 2012).The ratio (Sr / Ba) can be used to evaluate the changes in the degree of salinity of the water column and the corresponding climatic conditions and is important for estimating the degree of ancient salinity of limestone rocks(Zheng and Liu, 1999).If the ratio is greater than 1, it represents the saline water environment and the range between 0.6 and 1 points to the brackish water environment, but if it is less than (0.6) it represents the fresh water environment(Zhen et al., 2020;Sajid et al., 2020).The values of the studied samples have ranged from 1.215 to 2.590 with an average of 2.033 (Table

Table 6 .
Stable C and O isotopic data for the Formation, with environment and temperature estimates for the limestone units only from the present study.

Table 7 .
Geochemical proxies for paleoproductivity in the studied samples.Note that the maximum (Max.), minimum (Min.) and average (Avg.)values are only for the Gulneri Formation samples.
Kaufman and Knoll, 1995;Steuber, 1999; values is a result of isotopic exchange between meteoric or hydrothermal solutions and carbonate rocks.Therefore, its values recorded in marine carbonate rocks are employed as markers of seawater temperature and modification processes after deposition.A higher value of the oxygen isotope indicates a lower temperature, while a lower value indicates a higher temperature (O'Neil et al., 1969;Kaufman and Knoll, 1995;Steuber, 1999; (Wang et al., 2009)Wang et al., 2009), and Morocco(Gertsch et al., 2010)which may give a global extent of the present OAE2 from Iraq.In the outcrop section from northern Iraq, the OAE2 is suggested to occur in the early to middle Turonian (Al-Lhaebi et al., 2020).The early Turonian anoxic conditions of OAE2 were also registered from the shallow coastal sequences of NE Egypt by El-Sabbagh et al. (2011) who mentioned that anoxic conditions of OAE2 begin after the first δ13C peak and end at or near the Cenomanian-Turonian boundary in open marine environments while it exist late in early Turonian in Shallower setting.The above mentioned results are consistent with previous studies on the Gulneri Formation that mentioned the environmental conditions of the Gulneri Formation during OAE2 from northern Iraq (Awadh and Al-Dulaimy, 2007; Al-Sagri, 2015; Ahmad et al., 2018; Lawa et al., 2023).