A mid-Cretaceous (late Albian–early Turonian) stable isotope record from southern Tibet – Pristine or diagenetically altered?

The late Albian to early Turonian interval was characterized by a pronounced greenhouse climate, marked by major perturbations of the global carbon cycle and the development of several Oceanic Anoxic Events (OAEs). These events are recognized by the widespread deposition of organic-rich black shales, typically associated with distinctive carbon isotope excursions (CIEs). While high-resolution carbon isotope records of the late Albian OAE (OAE 1d, ∼103 to 99.5 Ma) are well-documented in the western Tethys and North Atlantic, detailed studies across this event from the eastern Tethys remain limited. Here, we present new data from the Qiangdong section (Gamba area, southern Tibet), including stable carbon and oxygen isotope curves, mineralogy, Rock-Eval pyrolysis and palynofacies analysis. Our continuous δ¹³C_carb record provides detailed geochemical documentation for the upper Albian to lower Turonian, enabling robust chemostratigraphic correlation with global OAE 1d records. Negative δ¹⁸O_carb values (as low as −11.9‰) and mineralogical evidence (dominated by illite, > 50%) indicate strong diagenetic overprint. Both T_max values (432–560 °C) and highly degraded, dark-colored palynomorphs suggest that the sedimentary sequence of the Qiangdong section has undergone significant thermal alteration, with organic matter reaching mature to overmature stages. This is further supported by the dominance of Watznaueria barnesiae, consistent with previous findings indicative of significant diagenetic overprint. While the δ¹³C_carb record robustly captures the OAE 1d excursion, the characteristic CIE associated with the late Cenomanian to early Turonian OAE 2 is not well expressed. This attenuation is likely due to diagenetic overprinting and decrease in carbonate content, which may have altered or obscured primary palaeoenvironmental signals in the upper part of the Qiangdong section.