The expression of the Cenomanian-Turonian oceanic anoxic event in Tibet

The Gongzha section of Tibet, China is located at the northern margin of the Indian Plate (SE Tethys) and is characterized by hemipelagic grey marls and marly limestones, light grey limestones and silty limestones, but no organic-rich sediments. High-resolution biostratigraphy reveals an expanded Cenomanian–Turonian (CT) boundary interval and the δ13C record includes the main features of the classical positive carbon-isotope excursion that characterizes the CT oceanic anoxic event. The biotic response inferred from the foraminifera suggests that oxic to dysoxic conditions prevailed, except for a short interval marked by peak abundance of Heterohelix that indicates a significantly dysoxic environment during the δ13C “b” peak excursion. The overall decreasing trend in redox-sensitive trace elements (RSTE) during the maximum δ13C excursion confirms the absence of significant longer-lasting anoxia in the Gongzha section. Enrichments in RSTE are linked to phases of increased detrital input. Chemical weathering indices suggest that the upper Cenomanian sediments accumulated under an increasingly hot and humid climate that culminated near the CT boundary. In the early Turonian lower weathering indices suggest a warm, drier climatic regime with reduced continental runoff. Phosphorus mass-accumulation rates show a significant peak at the onset of the positive δ13C excursion, followed by a decrease up to the basal Turonian. This pattern is positively correlated with the long-term decrease in detrital index as also observed in numerous other CT boundary sections (e.g., Eastbourne, Pueblo, and Whadi El Ghaib, Sinaï). Long-term phosphorus accumulation in the Gongzha section is therefore associated with changes in detrital input. The overall decreased detrital input can be explained by the increasingly remote continental sources due to the major transgression at the end of Cenomanian, coupled with changes in continental weathering intensity linked to increasingly more arid climate conditions.

Late Barremian-Early Aptian palaeoenvironmental change: the Cassis-La Bedoule section (SE France)

The well-dated section of Cassis-La Bédoule in the South Provencal Basin (southern France) allows for a detailed reconstruction of palaeoenvironmental change during the latest Barremian and Early Aptian. For this study, phosphorus (P) and clay-mineral contents, stable-isotope ratios on carbonate (δ13Ccarb) and organic matter (δ13Corg), and redox-sensitive trace elements (RSTE: V, U, As, Co, and Mo) have been measured in this historical stratotype. The base of the section consists of rudist limestone, which is attributed to the Urgonian platform. The presence of low P and RSTE content, and content of up to 30% kaolinite indicate deposition under oligotrophic and oxic conditions, and the presence of warm, humid climatic conditions on the adjacent continent. The top of the Urgonian succession is marked by a hardground with encrusted brachiopods and bivalves, which is interpreted as a drowning surface. The section continues with a succession of limestone and marl containing the first occurrence of planktonic foraminifera. This interval includes several laminated, organic-rich layers recording RSTE enrichments and high Corg:Ptot ratios. The deposition of these organic-rich layers was associated with oxygen-depleted conditions and a large positive excursion in δ13Corg. During this interval, a negative peak in the δ13Ccarb record is observed, which dates as latest Barremian. This excursion is coeval with negative excursions elsewhere in Tethyan platform and basin settings and is explained by the increased input of light dissolved inorganic carbon by rivers and/or volcanic activity. In this interval, an increase in P content, owing to reworking of nearshore sediments during the transgression, is coupled with a decrease in kaolinite content, which tends to be deposited in more proximal areas. The overlying hemipelagic sediments of the Early Aptian Deshayesites oglanlensis and D. weissi zones indicate rather stable palaeoenvironmental conditions with low P content and stable δ13C records. A change towards marl-dominated beds occurs close to the end of the D. weissi zone. These beds display a long decrease in their δ13Ccarb and δ13Corg records, which lasted until the end of the Deshayesites deshayesi subzone (corresponding to C3 in Menegatti et al., 1998). This is followed by a positive shift during the Roloboceras hambrovi and Deshayesites grandis subzones, which corresponds in time to oceanic anoxic event (OAE) 1a interval. This positive shift is coeval with two increases in the P content. The marly interval equivalent to OAE 1a lacks organic-rich deposits and RSTE enrichments indicating that oxic conditions prevailed in this particular part of the Tethys ocean. The clay mineralogy is dominated by smectite, which is interpreted to reflect trapping of kaolinite on the surrounding platforms rather than indicating a drier climate.