Biostratigraphy and palaeoceanography of mid-Cretaceous calcareous nannofossils : studies from the Cauvery Basin, SE India; the Anglo-Paris Basin, SE England; the North Atlantic and Pacific Oceans
The applications of mid-Cretaceous (Aptian-Cenomanian) nannofossils in biostratigraphy and palaeoceanography have been advanced based on four specific studies from India, UK, the Atlantic and Pacific Oceans. A biostratigraphic study on outcrop samples from two new sections in the Cauvery Basin (SE India) has significantly improved stratigraphic resolution in the basin using the recent zonation schemes of Bown et al. (1998) and Burnett (1998). In addition to highlighting problems associated with a few marker species for the Cenomanian, the Albian/Cenomanian and Cenomanian/Turonian boundaries have been examined with respect to their nannofossil proxies. Qualitative comparisons of coeval assemblages from India with those from three other palaeogeographical settings (England, France and the Pacific) have confirmed the overall cosmopolitan nature of Albian nannofloras, in which provinces such as the Tethyan, Boreal and Austral cannot be clearly differentiated. A palaeoclimatic study of a short section in the Gault Clay (S. England) suggests a major warming event starting at the mid-/Late Albian boundary in the Weald of the Anglo-Paris Basin. The cold-water species, Repagulum parvidentatum, gives strong evidence for this warming event by showing a rapid decline in its percentage abundance, which precisely coincides with a light oxygen isotope peak and the influx of Tethyan ammonites. A sharp productivity rise based on the well-known fertility index, Zeugrhabdotus noeliae, is found to be concomitant with the warming event. A palaeoceanographic study of the Early Albian OAElb event in the western North Atlantic (Leg 171B), based on its nannofossil productivity record and geochemical data, supports the increased productivity model as a plausible mechanism for this anoxic event. A similar study on the Pacific Ocean (Leg 198, Shatsky Rise) shows a marked temporal variation in the abundance distribution of productivity- related taxa (e.g., Biscutum constans, Zeugrhabdotus noeliae) in relation to the OAEla (Early Aptian) and OAElb (Early Albian) events. Possible explanations for this variation have been proposed, in light of the heightened submarine volcanism in the Pacific during the mid-Cretaceous. Watznaueria is found to be the most abundant taxon in all mid-Cretaceous assemblages and its dominance is considered to be independent of preservation, indicating its broad palaeoecological tolerance rather than resistance to dissolution. On the basis of taxonomic observations, four new species have been erected: Calculites karaiensis, Loxolithus bicyclus, Manivitella fibrosa and Tranolithus simplex.