A comprehensive multi-proxies study based on sedimentology, biostratigraphy, mineralogy, stable isotopes, phosphorus and elemental geochemistry provides crucial information on the environmental changes associated with two major Cretaceous events, the late Cenomanian Oceanic Anoxic Event 2 (OAE2) in open marine and shallow shelf environments of Egypt and Morocco and the Cretaceous-Tertiary (KT) mass extinction in Texas, Brazil and India.
Results show that in deep-water environments of the Tarfaya Basin, Morocco, the late Cenomanian OAE2 is marked by a positive carbon isotope excursion associated with black shale deposited under anoxic conditions. In contrast, in shallow-water inner shelf environments of Azazoul, Morocco, and Wadi El Ghaib, Egypt, a synchronous positive carbon isotope excursion also marks OAE2, but deposition occurred under dysoxic, brackish and mesotrophic conditions. Ocean anoxia characteristic of OAE2 in open marine environments did not reach into shallow-water environments until the maximum transgression of the early Turonian. During OAE2, climate decoupling, which is attributed to variations in the Intertropical Convergence Zone (ITCZ) and the North-East trade winds in tropical areas, emphasizes the role of climate fluctuations in the tropics and their possible cause-and-effect relationship with the formation of OAE2.
Results from the geographically and environmentally diverse KT sequences analyzed yield important new information that contributes significantly to the ongoing controversy over the cause of the KT mass extinction. Of the five KT identifying criteria (mass extinction in planktic foraminifera, first appearance of Danian species, negative carbon isotope excursion, Ir anomaly, thin red clay layer), the Ir anomaly and the red clay layer are absent in Brazos, Texas, although these sequences, analyzed from outcrops and newly drilled cores, are complete and comparable to the El Kef KT stratotype section. Minor Ir enrichments observed in a sandstone complex below the KT boundary are likely linked to low sedimentation rates or sediment starvation, whereas the absence of Ir at the KTB is likely due to dilution effects caused by high sedimentation rates.
In the Poty Quarry of NE Brazil, a near-KT clastic unit was interpreted as impact-generated tsunami deposit based on minor Ir concentrations and calcite spherules interpreted as similar to Chicxulub impact spherules. This study of outcrops and new cores reveals a major KT unconformity at the base of a micro-conglomerate, which was deposited during the early Danian zone P1a during a sea-level fall that is correlative with global sea-level trends in the Danian. The calcite spherules were found throughout the Maastrichtian and determined to be chamber infillings by secondary calcite of the benthic foraminifer Dentalina. Poor preservation subsequently dissolved the foraminiferal shells, but preserved the spherules. The relatively minor multiple Ir concentrations are all concentrated in the early Danian in thin condensed dark shale layers, which suggests low sedimentation rates resulting in minor concentrations of PGEs.
In Meghalaya, NE India, the Um Sohryngkew section provides a unique shallow marine sequence to investigate the role of Deccan volcanism in the KT mass extinction. The Um Sohryngkew section records the only known KT boundary sequence in India that contains all the defining and supporting criteria of the KT stratotype section (GSSP), El Kef, Tunisia. The KT red layer records very high PGE enrichments, together with peaks in major (Fe, P) and trace elements, that suggests combined effects of condensed sedimentation, sediment starvation and redox variations. Both volcanism and extraterrestrial impacts are likely sources for these enrichments. Environmental consequences of Deccan volcanism during the terminal Maastrichtian reflect increasing stress conditions, as indicated by blooms of Guembelitria cretacea associated with Deccan-related acid rains dissolution and eutrophic conditions linked to high continental runoff.
The study of the late Cenomanian OAE2 in shallow-shelf environments contributes largely to the global understanding of the OAE2 by revealing the globally synchronous carbon isotope excursion, but also the delayed anoxic-dysoxic effects of OAE2 associated with the sea-level transgression (e.g. water depth). The investigations on geographically separated locations spanning the KT transition reflects the complex interactions of numerous parameters affecting extraterrestrial markers in the sedimentary record, the importance of sea-level fluctuations in the depositional regime and the potentially major environmental consequences associated with the Deccan volcanism.