Microfacies and geochemistry of Kimmeridgian limestone strata in the Eastern Pontides (North-East Turkey): Palaeoclimate and palaeoenvironmental influence on organic matter enrichment

Kimmeridgian limestone (KL) strata are one of the best examples of peritidal carbonates that record palaeogeographical and palaeoclimate conditions in the Eastern Pontides, Eastern Black Sea region. These Kimmeridgian limestone strata consist of well-preserved dark grey lime-mudstones that serve as essential archives for various geochemical proxies. This study presents new data on trace elements, rare earth elements and stable isotopes (δ¹⁸O, δ¹³C) to enhance our understanding of the palaeoclimate, weathering patterns, salinity, redox conditions and productivity during the deposition of these strata. Furthermore, organic chemistry data, including total organic carbon, Rock-Eval (S1, S2, S3, HI, OI) and production index, are presented to discuss their hydrocarbon potential. The Kimmeridgian limestone strata exhibit relatively high total organic carbon content (0.16–0.22%) with an average of 0.19%, δ¹⁸O values ranging from −2.12 to −0.69‰, and δ¹³C values ranging from 1.42 to 2.09‰. Additionally, they display distinct rare earth element characteristics such as low La/Yb_N (0.64–1.00) ratios, varying Gd*/Gd (0.54–1.16) ratios, high Eu/Eu* (1.19–1.84) ratios and Ce/Ce* (0.91–1.16) ratios. The Kimmeridgian limestone strata also exhibit chondritic Y/Ho (30.48) and Zr/Hf (40.35) ratios, relatively high redox-sensitive element values, low Ga/Rb (0.10–1.10; an average of 0.29) ratios and high K₂O/Al₂O₃ ratios (0.2–0.40; an average of 0.27). According to the geochemical results of this study that are integrated with published stratigraphy and palaeontological data, the Kimmeridgian limestone strata are interpreted as having been deposited in a shallow subtidal lagoon environment or a partly protected interior ramp setting. Although sea-level fluctuations influenced the palaeoenvironmental changes, ocean circulation along the inner platform supplied nutrients to the basin. The possible occurrence of low-temperature water/rock interaction under reduced oxygen conditions contributed to additional element flux. Palaeoclimate indices suggest the existence of an aridification event, wherein arid climate played an important role in low weathering input, organic matter source, productivity, evaporation and salinity enhancement. The climate, accompanied by sedimentary factors, facilitated enhanced productivity, a balanced sedimentation rate and the preservation of organic matter under reducing conditions. Contrary to the anticipated outcome, considering the high productivity and anoxic depositional conditions suggested by microfacies and inorganic geochemical results, the over-maturation processes have resulted in a notable decline in the organic matter content and hydrocarbon potential of the samples.