Late cretaceous (Maestrichtian) calcareous nannoplankton biogeography with emphasis on events immediately preceding the cretaceous/paleocene boundary

Abstract

The Maestrichtian biogeography of calcareous nannoplankton is investigated in order to characterize paleoenvironmental conditions in the marine photic zone during the latest Cretaceous. Different theories explaining the biospheric turnover at the Cretaceous/Paleocene (KIP) boundary have alternatively suggested or denied substantial environmental perturbations during the last ~500 ky of the Cretaceous. The purpose of this study is to determine whether evidence from calcareous nannoplankton supports a gradual (or stepwise) decline of the photic zone environment presaging the K/P boundary. In order to achieve this goal a detailed quantitative study of the biogeography of calcareous nannoplankton was carried out in three time slices from early and late Maestrichtian. Well preserved material was investigated from five sections: Ocean Drilling Program Site 690 in the Atlantic sector of the Southern Ocean represents the Maestrichtian high southern nannoplankton province. Indian Ocean Sites 217 and 761, South Atlantic Site 528 and the land based, epicontinental section from Millers Ferry, Alabama, represent the Maestrichtian mid-/low latitudinal bioprovince. Quantitative counts were performed on settling slides under the light microscope. Occasionally scanning electron microscopy was employed to resolve taxonomic uncertainties. A pronounced turnover from early to late Maestrichtian occurred in the nannoplankton in high southern latitudes. Numerous taxa (Biscutum boletum, B. coronum, B. dissimilis, B. magnum, Misceomarginatus spp., Monomarginatus spp., Neocrepidolithus watkinsii, Nephrolithus corystus, Octocyclus magnus, Phanulithus obscurus, Psyktosphaera firthii, and Reinhardtites spp.) that are restricted to (or most abundant in) high southern latitudes became extinct in the latest early and earliest late Maestrichtian (between ~72.4 and 70.4 Ma), resulting in a loss of about one third of the early Maestrichtian nannoplankton (corresponding to ~20-25% of the assemblage). It is argued that the extinctions are not a consequence of temperature changes alone. Instead they may be a consequence of increased surface water fertility (and only secondarily due to a temperature decrease). In addition to the extinctions, about another third of all taxa present (Biscutum constans, B. notaculum, Biscutum sp. 1, Chiastozygus garrisonii, C. amphipons, Discorhabdus ignotus, Rhombolithion rhombicum, Scapholithus fossilis, Staurolithites laffittei, Watznaueria barnesae , Zygodiscus compactus, and Z. diplogrammus) disappeared from high southern latitudes during the same time interval (~72.4 and 70.4 Ma) but persisted until the end of the Maestrichtian in lower latitudes. These geographic restrictions are interpreted as a consequence of global cooling. No comparable changes were recorded in mid- and low latitudes in the early Maestrichtian, but this may represent an artifact of sampling. While previous speculations on the paleoenvironmental preferences of some nannofossil taxa have been confirmed, several commonly accepted interpretations of the biogeographic significance of other taxa are contradicted. Micula staurophora seems to be a warm water indicator and abundance peaks of this species cannot be attributed exclusively to diagenetic effects. The biogeographic evolution of the high latitude taxon Ahmuellerella octoradiata does not correlate with temperature trends suggested from stable isotope studies implying that this taxon is not a cold water indicator. Abundance changes of other high latitude taxa (e.g. Nephrolithus frequens, Cribrosphaerella? daniae, Kamptnerius magnificus, and Gartnera&o spp.) correlate roughly with temperature changes, but seem to respond only beyond a certain threshold. No gradual or stepwise extinctions were observed during the last 500 ky of the Maestrichtian. Environmental perturbations as indicated by stable isotope studies (e.g. warming pulse, circulation changes) led to abundance fluctuations of a few taxa, but did not result in any extinctions. This supports previous observations that the extinctions of the calcareous nannoplankton at the K/P boundary were not presaged during the Maestrichtian.