Calcareous nannofossil biostratigraphy and assemblages of the Cenomanian‐Turonian boundary interval: Implications for the origin and timing of oceanic anoxia

Abstract

A detailed nannofossil zonation has been derived for the Cenomanian-Turonian boundary interval consisting of three zones, two subzones, and nine additional biohorizons. This biostratigraphy, based on the investigation of numerous sections from Europe, North America, and Africa, allows accurate correlation of sections from shelf and deep-sea environments. The increased resolution helps solve several pertinent paleoceanographic and sedimentologic problems in this time period. Facies patterns are variable in this interval for numerous reasons. A widespread hiatus occurred in the latest Cenomanian and earliest Turonian in much of northwest Europe and may have been an indirect result of peak marine transgression. In parts of the deep Tethys, dissolution of carbonate took place in undersaturated bottom waters or during early diagenesis. Only a few boundary sections, mostly within the Western Interior, United States, are characterized by continuous carbonate deposition. In many sections a 313 C excursion (which has been associated with the burial of large amounts of marine organic matter) coincides with a facies change, lower ratios of coccolith carbonate to micfite, and poorer nannofossil preservation. This excursion therefore may be strongly overprinted by diagenesis. Numerous nannofossil events indicate that the carbon excursion is slightly diachronous between sequences of the Western Interior, and that the shift occurred distinctly later in this region than in northwest Germany. The data obtained do not invalidate the concept of a Cenomanian-Turonian ocean-wide anoxic event but show how local tectonic, climatic, and oceanographic perturbations have masked the global scenario. In particular, they indicate that carbon shifts may be basinal and not always oceanic phenomena, a conclusion which can be rationalized with overall sluggish middle Cretaceous circulation. Nannofossil assemblages in almost all sequences observed contain exceptionally high abundances of two dissolution-resistant taxa in particular samples: Eprolithus floralis and Broinsonia sp.. However, the overall composition of assemblages does not change dramatically in the boundary interval, in contrast to the macrofossils and foraminifera, which suffered widespread extinctions.

Upper Cretaceous and Cenozoic Calcareous Nannofossils of the Kerguelen Plateau (Southern Indian Ocean) and Prydz Bay (East Antarctica)

“…Core 119-738C-31R (467.6-476.3 mbsf) is assigned lower Turonian to Coniacian age (NCI3-15) because it includes the first occurrence of Micula staurophora (Thierstein, 1976; equivalent to Quadrum gartneri of Manivit et al, 1977). The assemblages in Core 119-738C-31R lack that species, but document the first occurrence of Kamptnerius magnificus and the last occurrence of Gartnerago nanum which can be correlated to earliest Turonian assemblages elsewhere (Bralower, 1988). The lowermost chalk recovered in Core 119-738C-31R is in depositional contact with volcanic basement and is considered to be of early Turonian age.…”

The importance of diazotrophic cyanobacteria as primary producers during Cretaceous Oceanic Anoxic Event 2

“…Detailed description of geology of this region and the Bonarelli sediments were provided in Arthur and Premoli Silva (1982) and Kuroda et al (2005), respectively. The paleo-continental configuration at that time was shown elsewhere (e.g., Bralower, 1988;Erbacher et al, 2005;Kuroda and Ohkouchi, 2006). At the Gorgo Cerbara outcrop, forty-three blocks were sampled from across the 104-cm interval of Bonarelli.…”

Site 763

“…Sample 122-763C-2R-1, 78-79 cm, represents the last occurrences of the nannofossil species Axopodorhabdus albianus, Corollithion kennedyi, Microstaurus chiastius, and Lithraphidites acutum. The extinction levels of these taxa are spaced out over a 5-m interval in sequences from the Western Interior Basin of North America (Bralower, 1988). No true Turonian assemblages were observed in Hole 763C, although the glauconitic claystone contains a very etched nannoflora characteristic of the Cenomanian/Turonian boundary interval.…”