The primary isotopic signatures are readily altered during the diagenetic processes of the skeletal and non-skeletal components of the carbonate sediments (Armstrong-Altrin et al. 2011). Multiple methods have been developed to assess δ18O and δ13C variations during diagenetic influences involving correlation between δ18O and δ13C (Armstrong-Altrin et al. 2011; Oehlert and Swart 2014; Ali and Sa’ad 2018). The δ18O isotope is more prone to diagenetic overprint and highly sensitive to temperature changes, especially in the bulk carbonate (Schobben et al. 2016). Therefore, a positive correlation between δ18O and δ13C reflects primary marine isotopic signals. As shown in Fig. 4, the stable isotopes result of the Jeribe Formation is displaying a strong correlation between δ13C and δ18O (r = 0.85). Generally, it refers to the lack of significant influence of diagenesis on the stable isotopic composition. Bivariate plots of δ18O and δ13C are another convenient method. It is used to distinguish the depositional and/or diagenetic environments of carbonate rocks. (Hudson 1977; Nelson and Smith 1996) had used a δ18O and δ13C cross-plot to identify the origins of different types of carbonate rocks. Therefore, this approach has been adopted by many workers, such as (Bathurst 1983; Choquette and James 1987; Moore 1989; Morse and Mackenzie 1990). Despite significant effaces of evaporation, meteoric dilution, and carbon transfer in the shallow marine relative to platform margin, slope, and basinal environments, the isotopes composition in the investigated succession exhibits primary stable isotope values and can be used as a direct proxy for the composition of seawater in the Kirkuk- Dezful Embayment during the Middle Miocene (Figs. 4 and 5).
Carbonates of the Jeribe Formation show vertical variation in δ18O and δ13C composition; however, most samples have positive values. At the bottom of the succession, the boundary of the Early-Middle Miocene is marked by prominent negative values of the δ18O and δ13C. However, both isotopes display enrichment after depletion, which is marking the beginning of transgression in the Middle Miocene, denotate Ng30 date of the final closure of Neo-Tethys by (Sharland et al. 2004). Enrichment of both δ13C and δ18O isotopes has occurred in two main zones. It is separated by depletion in the middle unit, on both of the δ13C and δ18O. It is clearly expressed by a vertical change in the environmental conditions from the lagoon to the reef environment and shoal facies in the uppermost at the contact with the overlying Fatha Formation.
Mi zones have been identified and defined using the criteria of Miller, Wright, et al. (1991) based on the maximum δ18O value. The current study represents a single-point zone recorded with more details about Mi2, Mi3, zones, where Mi3 has been subdivided into Mi2b, Mi2c, and Mi2d, while Mi3 zone is subdivided into Mi3a, Mi3b, and Mi3c. These discernible deviations represent a minor step in the major cooling trend started with Mi 3 and Mi2, that gradual increase in glaciation has a profound and immediate impact on the depositional environment of the Jeribe Formation.
Coralline algae are used to assess the paleo-climate, paleo-latitudes, and paleo-depth (Mccoy and Kamenos, 2015). They have wide temperature range tolerate and live at depths of 125 m or more in moderate wave energy areas (Scholle and Ulmer-Scholle 2003). They are sensitive to water depth and terrigenous input. Corallines, in general, prefer decreased siliciclastic loading and shallow to mildly deeper bathymetric (Sarkar et al. 2016). Coralline red algae of the corallinales order (Archaeolithothamnium, sp and Mesophyllum, sp) are identified as autochthonous form. They comprise the major type in the middle part of the Jeribe Formation associated with coral and benthic foraminifera.
Dasycladaceae is one of the large calcareous green algae families, which are not abundant in the Jeribe Formation. The green and the red algae are observed in the onset of the middle unit of the studied sections. Dasycladaceae green algae commonly occur in quiet, shallow, or very shallow, normal salinity, and warm water in the reef-lagoon environment (Flugel and Flügel, 2004). This environment is characterized by abundant green algae at depths of 50–100 m in the reef and near-back reef areas. It is more significant of modern and ancient carbonate deposits of warm water regions (Scholle and Ulmer-Scholle 2003). Green algae are mostly indicative of back reef-lagoon facies. The distribution of small encrusting green and red algae can be used to estimate the depth-related zones (Flügel 2012).
The presence of red and green algae together indicates that the sedimentation had occurred in the upper part of the photic zone. The absence of green algae in the lower part within the lagoonal environments and the middle part within the coral reef facies reflect cold water relatively; while the uppermost part within the shoal facies reflects high energy. They observed together within the back-reef to reef facies.
The recognized coral occurs in the middle part of the successions. It represents tropical environments, normal salinity, and waters at depths from the intertidal zone to a few tens of meters. It is mainly in areas with a relatively low influx of terrigenous sediments (Scholle and Ulmer-Scholle 2003). Corals contributed significantly to the formation of large and extended reef-structures in the deeper- and cold-water settings. However, it is also in relatively shallow warm-temperate environments (Flugel and Flügel 2004). Corals that occur in situ were found in a shelf sequence formed during a relatively sea-level highstand (Scholle and Ulmer-Scholle 2003). Observed coral reefs and the absence of green algae indicated sea level rise and/or subsidence occurrences. In cause subsidence’s occurrence that relevant for brecciate outcropping, indicate changes in nutrient supply are regarded as a major control on productivity. Slumping and sliding are controlled and triggered by high rates of sedimentation creating oversteep slopes and differential compaction (Flugel and Flügel 2004).
In the upper part, the red algae, stromatolites, and benthic foraminifera are identified. Corallinacean algae and abundant foraminiferal types reflect the shallow open marine environments (Flügel 2012). Red algae and the benthic foraminifera emphasize deposition in the upper photic zone to the upper part of the lower photic zone. Stromatolites were observed in the upper part of the succession within reef environments. It represents the range from subtidal to intertidal settings (Scholle and Ulmer-Scholle 2003). They are commonly set in tidal and subtidal carbonates as well as in subtidal shelf carbonates or shallow subtidal environments. They take their energy from the decomposition of organic material to inorganic components by redox processes in shallow-water settings (Flugel and Flügel, 2004).
In the lagoonal environments, mudstone facies exhibit the lowest values on both δ13C and δ18O, while wackestone facies display shifts towards high values. (CM 1) the event formed over wackestone facies, that occurrences as the interval between two major cooling Mi 2a and Mi 2b. Suggests the differentiation between the mudstone and wackestone facies due to different depositional energy, which appears to be the major controlling factor to the heaviest and linked to glacial-eustatic.
High energy in packstone and grainstone facies within the back reef environments display depleted on δ13C within major glacial zones Mi 2b and Mi 2c. Enrichment by δ18O and absences of green algae indicative glacially climate. According to (Purkis et al. 2015), grainstone and boundstone facies are indicative of water depths < 5 m while wackestone at depths of > 20 m. Suggests sea-level fall in this interval caused to the glacial event. In shallow open marine, moderately energy represent by wackestone facies amongst back reef environments exhibit highest values of 13C and formed (CM 2), associated with miner step in the major cooling. Hasenclever et al. (2017) suggests that growing ice-sheets as a result of reduced volcanic emissions have been affected on the global decrease in sea level. Glacio-eustatic sea-level rose has been controlling on facies change.
Reef facies showed substantial variation in the δ13C and δ18O associated with a change in skeletal grain. Reef environments represent by boundestone and framestone facies display the highest values on both the δ13C and δ18O and formed CM 3 to 6 events, synchronous with major glacial zones Mi 3a, b, and c. Shifted to depletion on the 13C and 18O value at the onset of the reef facies with significant differences in the types of the skeletal grain to coral mat compared to other fossils. Coral reefs are important proxy data due to their sensitivity to temperature. It’s affected by cooling episodes that prevented coral reef growth. This shifted to depletion reflected short-term inter-glacial events. According to Immenhauser et al. (2002), restricted-platform water displays relatively low 13C values due to remineralized organic carbon during the long residence time of seawater on the platform. Breccia records in these facies emphases tectonic event may be uplift with the shallowest setting occurrence, reflects oceanographic changes.
These events coupled to short-term events records indicated by depleted in 18O relatively near 0‰ and display the more negative value in 13C as interval events between CM 3 and 4 and CM 5 and 6, suggesting interglacial periods and oceanographic changes within reef environments.
Relatively high energy in the packstone facies within the second cycle of shallow open marine shows, relatively, lowest values in 13C. Continual enhancing of the 18O values by upwelling at this facies reflect cooling of the surface waters; while decreases of the δ13C values, reflect variations in carbonate production and accumulation rates, because the carbonate production is lower in the cool water, suggesting this period as minor step of major cooling. High energy in the garinstone facies of the shoal environments display more negative on both 13C and 18O value. Shoaling reflects climatic and oceanographic changes that impacted the integral closure of the Tethys seaway during the middle Miocene.
On these bases, the variability of 13C came from depositional energy in the shallowest setting that linked to glacial-eustatic within Mi 2 zone, as interglacial periods indicated by coincidentally with high carbonate production. According to the Grossman (2008), sea level falls, as continental ice sheets and glaciers expand is a primary force that influences reef initiation, development and enabling coral-reef animals, and plants to settle and growth. Mi 3 zone is controlled by Glacio-eustatic sea-level fall. Local tectonic events indicated by breccia records at the onset of Mi 3 zone reflects oceanographic changes, association with coral reef growth indicates major climatic changes into the greenhouse as a short term.
According to Mason, Edmonds, and Turchyn (2017), carbon is originated from volcanic arc, represent an essential source in the marine environment. Subduction-related volcanic arcs, particularly, continental arcs, are the main source of heavier carbone due to reworking of crustal limestone that is often associated with the closing of ocean basins. Weissert and Erba (2004) mentioned that the major positive C-isotope anomalies in the Early Cretaceous coincide with episodes of volcanic activity. Moreover, Volcanic arc occurred during the thrusting and later closing of the ocean, is combined with increased 13C, suggests high δ13C concentration as CM-events in shallow marine carbonates linked to the structural location of Kirkuk- Dezful Embayment, which is lying adjacent to the volcanic activity and coincide to the platform destruction during the final phase of Tethys sea closure. Many published papers are mentioned to the Mi 3b Mi 3b is marking the final major cooling step and ended of the Monterey Excursion (Miller et al. 1991a; Zachos et al. 2001, 2008a; Shevenell et al. 2004, 2008). In the current study, Mi 3c marking final major cooling coincide with the ended of the Monterey Excursion (Fig. 6).