Palaeobiogeography of early cretaceous calcareous nannoplankton

Studies of Recent calcareous nannoplankton distributions in the Atlantic and Pacific Oceans (McIntyre and Be, 1967; Okada and Honjo, 1973) have revealed five distinct, latitudinally bounded, biogeographic coccolithophore zones. These zones are dynamic, and consequently their recognition in sediments has aided palaeoceanographic reconstructions of water masses through the Quaternary. Observations on Early Cretaceous nannoplankton palaeobiogeography have been limited to the recognition of nannofossil provinciality and a tentative attempt to assign taxa to biogeographic realms based largely on coeval macrofossil distributions. Assessments of environmental preferences of certain species have been hazarded (e.g. the use of Biscutum cons tans and Zeugrhahdotus erectus as high fertility indices; Roth and Bowdler, 1981), but a general lack of clear taxonomic concepts and quantitative control has hindered the spatial and temporal precision and applicability of these observations. In order to more clearly determine Early Cretaceous nannoplankton biogeography, a series of sites were studied, which provide a north-south transect through the Atlantic Ocean, supplemented by sections from the North Sea Basin, Barents Sea, Falkland Plateau, Weddell Sea (Antarctica), Argo Abyssal Plain (NW Australia) and Neuquen Basin (Argentina). Quantitative assemblage data were gathered from these sites for seven-time slices (Berriasian to Barremian), each horizon being determined by a nannofossil datum. Statistical analyses of assemblages were completed for species richness, evenness and diversity, and an attempt was made to quantify the variation observed using Principal Components Analysis. These analyses have revealed a relatively well defined, broad, low-mid latitude zone 45-50°N to 45-50°S, which is characterised by stable species compositions and high diversity. This is flanked in both northern and southern hemispheres by distinct high latitude zones. The change in assemblage abundance and composition is marked across these sharp 'biogeographic fronts'. High latitude assemblages are lower in species richness and diversity and are characterised by the presence of abundant, typically 'bipolar' taxa (e.g. Crucihiscutum salehrosum), however, 'sub-arctic' and 'sub-antarctic' realms can be distinguished from one another by the presence of 'endemic' species (e.g. B. matalosa in the southern hemisphere). A less distinct biogeographic boundary at ~40°N is distinguished by the presence/absence of rarer but biogeographically significant taxa, many of which have previously been labelled boreal and tethyan; the biogeographic divisions recorded here are consistent with these classically defined realms. In addition, continental shelf sites are characterised by lower diversity assemblages with common to dominant diagnostic taxa which vary with latitude: Nannoconus and Micrantholithus at low latitudes, and B. constans and Zeugrhabdotus spp. at higher latitudes. The latter two taxa are considered to be indicative of elevated surface water fertility and the former two may have been similarly adapted. The genus Watznaueria is ubiquitously dominant, giving the populations an unevenness which appears to be a common feature throughout coccolithophore history. Some taxonomic revisions have been attempted including four re-combinations of species and an emendation of the genus Haqius. Five new species are erected: Braarudosphaera archaeodiscula, Diazomatolithus delicatus, Micrantholithus astrionis, Micrantholithus magnus and Tubodiscus parvus