Carbon and Oxygen Isotope Proxies from the Campanian-Maastrichtian Deposits of the Aktolagai Plateau (kazakhstan): Chemostratigraphy, Palaeotemperatures and Ocean Chemistry Reconstructions

The study area is located in the eastern Caspian Lowland, on the western bank of the Emba River. Here, upper cretaceous deposits are exposed in the Aktolagai plateau escarpment, overlain by lower eocene deposits. Biostratigraphic, paleomagnetic, and isotopic data (for part of the section) are published in the paper (Baraboshkin et al., 2019). We analyzed the carbon and oxygen isotopic composition of 108 bulk carbonate rock samples and 80 samples from belemnite rostra. From each belemnite rostrum, 4 to 9 samples were collected from different growth zones. Variations in δ¹³C within single rostra reached 2.4‰, likely due to recrystallization near the apical line and edge. Samples from recrystallized areas, identified under UV light, were excluded from the dataset. The averaged δ¹³C values from belemnites correspond well with those from bulk samples. Comparing our δ¹³C curve with other sections, we identified several isotopic events: the Late Campanian Event (LCaE), the Langei Event, the Middle Maastrichtian Event (MME), and the Cretaceous-Paleogene boundary event (KPgBE), in addition to the previously known Campanian-Maastrichtian Boundary Event (CaMBE) (Baraboshkin et al., 2019). The δ¹⁸O values range from -3.2‰ to -0.3‰ in bulk samples and from -0.7‰ to 1.4‰ in belemnite rostra. Paleotemperatures were calculated using the equation of Erez and Luz (1983). As the deposits are planktonic limestones, temperatures from bulk samples represent Sea Surface Temperature (SST: 15 - 26°C), while those from belemnites represent Bottom Water Temperature (BWT: 8 - 16°C). The δ¹⁸O values in belemnites are typically ~1‰ higher (equivalent to ~4°C cooler) than in bulk samples, consistent with a depth-related temperature gradient. A notable synchronous negative excursion in both δ¹³C (~1‰) and δ¹⁸O (~1.8‰) occurs in the upper Late Campanian, at the boundary of paleomagnetic chrons C32r.1r and C32n.2n. In this interval, while δ¹³C in belemnites also decreases, their δ¹⁸O shows a positive trend. The δ¹⁸O difference between bulk samples and belemnites here reaches 3.5‰ (a 16°C contrast), which cannot be explained by a normal temperature gradient. Lithological uniformity argues against localized recrystallization by meteoric waters. Research (Spero, 1997; Zeebe, 1999, 2001) shows that the δ¹⁸O recorded by planktonic foraminifera depends not only on temperature but also on seawater pH, which is controlled by dissolved CO₂ concentration. An increase in pH of 1 unit can decrease δ¹³C by ~1‰ and δ¹⁸O by ~1.5‰ (Zeebe, 2001), closely matching the excursion observed in the Aktolagai section. We propose that a decrease in atmospheric pCO₂ during the Late Campanian transition from a greenhouse to a more temperate maastrichtian climate increased surface water pH, lowering δ¹⁸O values. Concurrently, falling pCO₂ would cause global cooling, tending to increase δ¹⁸O. This opposing effect could create a paleotemperature signal suggesting surface water warming and bottom water cooling from the same event. Our data correlate well with the composite Campanian-Maastrichtian paleotemperature curve for 30–48°N latitude (O’Brien et al., 2017). Similar positive δ¹⁸O excursions in the terminal Campanian found elsewhere indicate this was a global event. The study was supported by the Russian Science Foundation Grant No. 25-77-10106, https://rscf.ru/project/25-77-10106/.