PALEOCLIMATIC INSIGHTS ON THE CENOMANIAN-TURONIAN OCEANIC ANOXIC EVENT (OAE2) FROM NORTHERN IRAQ BASED ON CALCAREOUS NANNOFOSSILS AND GEOCHEMICAL DATA

Calcareous nannofossils, mineralogical and geochemical investigations are carried out on the Cenomanian-Turonian boundary of the Gulneri Formation in the Azmir, Dokan and Degala sections, northern Iraq. Regarding the calcareous nannofossil data CC11 and CC12 biozones were identified. A detailed investigation was carried out to identify calcareous nannofossils species. On the basis of their stratigraphic distribution, two biozones were proposed in this study; Quadrum gartneri Interval Biozone (CC11) part and Lucianrhabdus maleformis Interval Biozone (CC12) part. Correlation with other calcareous nannofossils biozones from regional schemes led to conclude that the age of the Gulneri Formation in the studied sections is the early Turonian. Mineralogical (X-ray diffraction XRD) and trace elements geochemistry (X-ray fluorescence XRF) data along with the dominance of calcareous nannofossil Watznauria that indicates warm and arid climatic condition prevailing in the Cenomanian-Turonian transition.


INTRODUCTION
Oceanic anoxic events (OAEs) are distinct features of the Cretaceous ocean with widespread marine anoxia events accompanied with accumulation of large amounts of organic carbon on the ocean floor in an oxygen-depletion conditions (Schlanger andJenkyns, 1976 andSchlanger et al., 1987).OAE2 "Bonarelli event", which occurred approximately at the Cenomanian/Turonian boundary is the most widespread event in the mid-Cretaceous (Jenkyns, 2010).The Gulneri Formation represents one of the main Cretaceous shale formations deposited in euxinic and small relic basin in Iraq (Buday, 1980).Several sedimentological, biostratigraphical and geochemical studies have been subjected on the The study area locally is located in the Low and High Folded Zones of the Western Zagros Fold-Thrust belt of Iraq according to Jassim and Goff (2006) and the modified tectonic subdivisions of Iraq (Fig. 1).The three studied sections lie in the northeastern part of the Arabian Plate which form a part of the long and wide northern passive margin of Gondwana bordering the Paleo-Tethys Ocean (Sengör, 1990).

MATERIALS AND METHODS
A total of 15 samples were selected for the calcareous nannofossil study using the simple smear slide technique using the method of Armstrong and Brasier (2005).The prepared slides were studied under a transmitted-light microscope (Olympus BH2).Thirty-two samples are analyzed using X-Ray Diffraction (XRD), (

Geochemistry
Several elemental ratios are used to determine paleoclimatic conditions of the studied rocks of the Gulneri Formation (Table 2 and Figs. 5, 7).The ratio of Ga/Rb in the Gulneri Formation in Azmir section ranges between 0.11 -0.14 with an average of 0.1, in Degala section, it ranges between 0.04-0.09with an average of 0.07 while in Dokan section it ranges between 0.05-0.16with an average of 0.12.In warm, humid climatic conditions salinity reduced, while hot and arid climatic conditions lead to rapid and abrupt elevation in salinity (Roy and Roser, 2013).
The Ga / Rb ratio can be used to obtain information about the ancient climate (Roy and Roser, 2013;Yandoka et al., 2015).In general, the Rb is associated with the illite which indicates the cold and arid climate.Therefore, the deposit that has a low value for the ratio of Ga/Rb vs Sr/Cu is displayed in Fig. 8. Based on this plot, the studied samples of the Gulneri Formation were deposited in a hot arid environment Roy and Roser, 2013;Yandoka et al., 2015).The Rb/Sr and the Sr/Cu ratios commonly are used as paleoclimatic indications (Sinha et al., 2006;Xu et al., 2010).Under warmer climatic conditions, the Rb/Sr ratio decreases while the Sr/Cu ratio values increase.The Rb/Sr ratio in Azmir section ranges between 0.023 and 0.195 with an average of 0.064.In Degala section it ranges between 0.022 to 0.050 with an average of 0.035, whereas, the range in Dokan section is between 0.019 to 0.06 with an average of 0.039.
The Sr/Ba ratio is useful for estimating the paleosalinity of calcareous rocks.Ratio greater than 1 indicates marine environment, while ratio between 0.6-1.0indicates transitional from marine to continental, while those less than 0.6 refers to continental environments (Zheng and Liu, 1999).In general, the high Sr/Ba ratio reflects the high salinity or arid climate, while the low ratio indicates low salinity or humid climate (Wang et al., 2017).The Sr/Ba ratio in Azmir section is between 0.083-10.05 at an average of 4.09, in Degala section the ratio is 5.495 -12.71with an average of 8.3 while in Dokan section it ranges between 0.63-8.28 at an average of 3.06 (Table 2).These ratios indicate an arid climate and high salinity conditions.The Sr/Cu ratio, which ranges from (1.3-5.0),indicates a warm humid climate, while the ratio greater than 5.0 indicates a hot and arid climate (Yandoka et al., 2015;Song et al., 2016).The values of the Sr/Cu ratio in Azmir section is 14.7-97.9with an average of 28.14, in Degala section is 28-59 with an average of 36.096, in Dokan, it ranges between 13.9-28.4with an average of 19.85.These values are all above 5, which indicates that the climate is arid and hot.

Climatic Factor "C"
In the present work, the value of ' c' as an important climate factor (Ding et al., 2018)    Calcareous nannofossil data indicate that the studied intervals are early Turonian in age.This period was marked by major environmental changes at regional to global scale including the early Turonian Oceanic Anoxic Event (OAE2).Several multidisciplinary researches were focused on OAEs which relate the event to the excess of CO2, intense volcanism and altered climate and oceanic chemistry (Corbett andWatkins, 2013 andAguado et al., 2016).
Calcareous nannoplanktons are extremely sensitive to changes in surface waters parameters such as; temperature, nutrient content and interacts with the carbon cycle through biological processes and production of calcareous oozes.Therefore, they are used as a tool to understand the marine ecosystem dynamics of the past.Calcareous nannofossil from the Tethys Ocean mentioned to climatic fluctuations and changes in ocean fertility during the early Turonian.
Over this time interval, the Tethys Ocean was characterized by phases of black shale deposition controlled by orbital forcing (Jarvis et al., 2011).Calcaroeus nannofossil -68 53 (2C), 2020: et Watznauria is commonly used as a paleoclimatic warm water indicator (Giraud et al., 2006).
The abundance of Watznauria in the studied sections among other calcareous nannofossils may indicate warm conditions.The clay minerals (kaolinite and smectite) are considered a proxy for chemical weathering (hydrolysis) which are prevalent in the most humid climate, while illite is considered as physical weathering mineral index that prevails in arid climatic conditions (Kunhardt 2016).
Whereas, high ratio of mixed clay (illite/smectite) is produced from dry and arid climatic conditions (Chamley, 1989 andDera et al. 2009).The abundance of illite and chlorite and the low content of kaolinite indicate high salinity and may be related to transgression while abundance of kaolinite and low content of illite and chlorite indicates low salinity (Cao et al., 2015).Low amounts of kaolinite accompanied with relatively abundance of illite and illite/smectite mixed layers clay minerals in the Gulneri Formation may refer to the semi-arid or arid climatic of the source region.

Geochemical Constraints
The Ga/Rb ratio can be used to obtain information about the paleoclimate (Roy andRoser, 2013 andYandoka et al., 2015).In general, the Rb is associated with the illite which indicates the cold and arid climate.Therefore, the deposit that has a low value for the ratio of Ga/Rb indicates the cold and dry climate.On the other hand, the Ga has a significant enrichment in kaolinite, which indicates the warm climate.Other used ratios, Rb/Sr, Sr/Ba and Sr/Cu indicate that the studied rocks were deposited in hot, arid climates.The relative content and distribution of some trace elements in fine-sized rocks can be used to derive their climatic conditions during deposition (Ding et al., 2018).Elements of manganese, iron, vanadium,

Fig. 4 .
Fig. 4. X-ray diffractograms for two selected samples showing the common clay and non-clay minerals Ga/Rb indicates the cold and dry climate.On the other hand, the Ga has a significant -, which indicates the warm climate as Ga/Rb ratio.Cross plot between is used and the formula for "c" is as follows:C = Σ (Fe + Mn + Cr + Ni + V + Co) / Σ (Ca + Mg + Sr + Ba + K + Na)Values between 0-0.2 indicate an arid climate and those between 0.2-0.4indicate a semiarid climate and the value between 0.4-0.6 refer to a semiarid-semi-humid climate semi-arid-semihumid and values between 0.6-0.8indicate a semi humid climate and the value of 0.8-1.0indicate a humid climate.The value of "c" in Azmir section is 0.041, and in the Dokan section is at a rate of 0.017 and in Degala section is at the range of 0.001.These values show that the studied Gulneri Formation was deposited in an arid climate condition.
Fig. 5. Chart showing stratigraphic variation in trace elements ratios in the Azmir section

Fig. 7 .Fig. 8 .
Fig. 7. Chart of stratigraphic variation in trace elements ratios in the Degala section

Lucianrhabdus maleformis interval biozone (CC12)
Regardingthese data the age of the Gulneri Formation in Azmir and Degala sections is late Cenomanian while in Dokan section is middle Turonian.
Azmir and Degala sections must be considered late Cenomanian as Helenea chiastia which is a marker for the top of UC5 (UC5c) with the age of late Cenomanian is observed.In Dokan section E. eximius is recorded Eiffellithus eximius is recorded which is a marker for the base of UC8a (middle Turonian), but we cannot find L. septenarius and L. maleformis.