An insight into the Pila Spi Formation and the speleothems of KunaBa Cave in this formation located in Iraqi Kurdistan based on isotopic findings

Introduction

Caves are a product of karstification, during which relatively soluble rocks such as limestone are dissolved by downward-penetrating meteoric waters that have interacted with a soil horizon containing high levels of CO2. Speleothems are secondary carbonates formed in caves, such as stalactites and stalagmites. Speleothems, which are predominantly calcite in composition, form when carbonate-saturated groundwater percolates downward into a cave at a CO2 partial pressure higher than the cave atmosphere and becomes supersaturated with respect to calcium carbonate by degassing or evaporation (Harmon et al, 2004). Their carbon and oxygen isotope compositions are among the most important tracers in paleoclimate studies and reconstruction of the paleogeological environment (Valley and Cole, 2001). Due to the simple geometry, relatively rapid growth rate, and tendency to precipitate near isotopic equilibrium with dripwater, stalagmites are the subject of most isotopic studies. Oxygen isotopes reflect the δ18O of the meteoric water dripping into the cave and the temperature dependence of the water-calcite isotopic fractionation.

Materials and methods

The KunaBa Cave is located in northeastern Iraq, in the Sulaymaniyah Governorate of the Kurdistan Region, northwest of the Darbandikhan city. This area is located on the High Folded Zagros Belt, based on the subdivision of Iraqi structural units (Fouad and Sissakian, 2011). The development of this belt began in the Late Cretaceous with the subduction of the Arabian Plate margin crust from the Campanian to the Paleocene and culminated in the Neogene with the continental collision between the Arabian block and central Iran (Saura et al, 2015). The Pila Spi Formation sequence, which forms the two limbs of the Golan anticline, represents the upper part of the stratigraphic supersequence of the Arabian Plate, deposited in the Middle and Late Eocene on an uplifted zone during the final stage of subduction and closure of the remnants of the Neotethys Ocean (Al-Banna et al., 2015).

During the field observation, samples were taken from the Pila Spi Formation and speleothems, including two well-layered stalagmites, from inside the KunaBa Cave. XRD analysis was performed on one sample to determine the mineralogical composition of the speleothems. In order to determine the δD of the stalagmite-forming fluid, two samples of the fluid inclusins were analyzed using the Cavity-Ring-Down spectroscopy method. To determine the carbon and oxygen isotope values, 14 samples from the Pila Spi Formation and 2 samples from two distinct layers of each stalagmite were analyzed. For dating, the isotopic values and ratios of U and Th were determined in 2 stalagmite samples using thermal ionization mass spectrometry (TIMS).

Results and discussion

The Pila Spi Formation consists of two units in its type section. The upper unit, 57 m thick, consists of white crystalline layered bituminous limestone with bands of pale green marl or chalky marl containing chert nodules with good fossil traces. The lower part, which is 28 meters thick, consists of well-bedded white or porous bituminous limestone with weak fossil traces. The petrographic study of carbonate units in the Pila Spi Formation shows the presence of skeletal and non-skeletal grains. The main carbonate matrix of the Pila Spi Formation is carbonate mud (micrite), which has been heavily dolomitized and transformed into microspar by neomorphism. The abundance of micrite and benthic foraminifera in the facies of the Pila Spi Formation indicates its deposition in a shallow marine environment (Ali and Mohamed, 2013). KunaBa cave is located on the Golan Anticline at 45°38′47″E and 35°09′32″N. This anticline, which is composed of the Pila Spi Formation, is a narrow structure about 1 km wide and 10 km long with a northeast-southwest trend. The entrance to the KunaBa cave is very narrow and small, but it then opens into halls covered with beautiful deposits including stalactites, stalagmites and limestone waterfalls. There is no clear information about the main passages of this cave, less than one kilometer of which has been explored. XRD analysis revealed that the speleothems were composed of calcite. The samples have a layered structure in microscopic thin sections, indicating annual calcite deposition.

Analysis of stable carbon and oxygen isotope ratios is a widely used method in paleoenvironmental studies, as these ratios reflect the depositional environment and usually vary across stratigraphic boundaries (Guo et al, 2010). In seawater, the amount of δ18O increases with increasing salinity (Wang et al, 2014). Because 16O preferentially evaporates and becomes atmospheric precipitation, the remaining seawater, which is now higher in salinity, becomes enriched in 18O. Using the empirical equation of Keith and Weber (1964) (Z = 2.048 × (δ13C(PDB) + 50) + 0.498 × (δ18O(PDB) + 50)), which is a criterion for distinguishing between marine and non-marine carbonates using δ13C and δ18O values in limestones, it was determined that the Pila Spi Formation is of marine origin (Z> 120). The δ13C and δ18O values of the Pila Spi Formation samples are negative, with mean values of −0.34‰ and −0.5‰, respectively. The oxygen isotope values in this Formation are heavier than those in marine carbonate sediments. The heavier oxygen isotope values in the Pila Spi Formation could be due to brine associated with an evaporite basin, during which the oxygen isotopic content of the basin becomes heavier than that of seawater. There is a significant correlation between the δ13C and δ18O values of carbonate rocks in a closed saline environment. The more closed the system, the higher the correlation coefficient (Wang et al, 2014). The correlation coefficient in the carbonate of the Pila Spi Formation (r = 0.921) indicates a remarkably strong correlation and a closed system.

Oxygen and carbon isotopes provide the primary basis for reconstructing the temperature or precipitation history of a site from speleothems. When the movement of air and water in a cave is relatively slow, a thermal equilibrium is established between the temperature of the bedrock and the cave air (Bradley, 2015). As a result when speleothems are deposited under isotopic equilibrium conditions, the δ18O of speleothem calcite reflects both changes in the δ18O of its dripwater and changes in cave air temperature. As a result, this principle can be used to reconstruct cave air temperature, which in many caves is related to the annual surface air temperature (Wigley and Brown, 1976). Paleotemperature determinations based on isotopic studies are only reliable if calcite (or aragonite) precipitates in isotopic equilibrium with the dripping water. This can be assessed by determining whether δ18O values are constant throughout a growth layer. If the values are different for the same layer, it indicates that the sediment has been affected by evaporation, not just slow CO2 degassing, and this changes the simple temperature-dependent fractionation relationship (Bradley, 2015). The acceptable limit for speleothems deposited in isotopic equilibrium is 0.5‰ for δ18O variations and a maximum of 0.7 for the linear correlation coefficient between δ18O and δ13C along a layer (Lauritzen, 1995; Linge et al, 2001). The results of the isotopic analysis of the stalagmites indicate that they formed under equilibrium conditions and during a slow CO2 degassing process.

The key advantage of speleothems in the field of paleoclimate studies is the possibility of accurately dating them to half a million years using U–Th-based methods (Cheng et al, 2013). The age of the stalagmites has been estimated to be 30 ± 1 and 25 ± 1 thousand years using the values and isotopic ratios of U and Th in two stalagmites. The ages of the samples were calculated using Isoplot/Ex (version 3.0) (Ludwig, 2003), a plotting and regression program designed for radioisotope data.

Conclusion

Given that stalagmites formed under isotopic equilibrium conditions, their oxygen isotope data can be used to determine the cave temperature at two time intervals obtained from the U–Th results. For this purpose, the Sharp equation (2007) was used, which is based on the oxygen isotope fractionation between speleothem (δ18Oc) and dripwater (δ18Ow) based on the ambient temperature (T, °C):

T (°C) =15.75 – 4.3(δ18Ocalcite(PDB) – δ18Owater(SMOW)) + 0.14(δ18Ocalcite(PDB) – δ18Owater(SMOW))2

Since this equation contains two unknowns (T and δ18Owater) and only one measured value (δ18Ocalcite), isotopic data from the water droplets at the time of stalagmite formation are needed to obtain the temperature. The δD value of the fluid inclusions was used to calculate its δ18O value using the equation δD = 7.68  δ18O + 6.26 (Affolter et al, 2025). Since the δ18O content of the fluid inclusion may have undergone isotopic exchange with the surrounding calcite, the δD values obtained from the fluids inclusions in the two stalagmites were −54.21 and −57.17, respectively. Thus, the δ18O values of the fluid of the two stalagmites were determined to be −7.87 and −8.26, respectively. By inserting the values into the Sharpe's equation, the cave temperature during the formation time of the two stalagmites was obtained as 10.9 and 12.1, respectively. Currently, the average annual air temperature in the Darbandikhan region has been recorded as 22.41°C over the past two decades between 2000 and 2020 (Kalloshy and Sharbazhery, 2023). The global average temperature during this period was 0.72°C (NOAA, 2024). It seems that despite limited data, the calculated annual mean temperatures between 25 and 30 thousand years ago for the study area are in acceptable with the global mean temperature of about -8°C (Petit et al, 1999) in these two time periods.