Ja Burnett

The Cretaceous-Tertiary biotic transition

Mass extinctions are recognized through the study of fossil groups across event horizons, and from analyses of long-term trends in taxonomic richness and diversity. Both approaches have inherent flaws, and data that once seemed reliable can be readily superseded by the discovery of new fossils and/or the application of new analytical techniques. Herein the current state of the Cretaceous-Tertiary (K-T) biostratigraphical record is reviewed for most major fossil clades, including: calcareous nannoplankton, dinoflagellates, diatoms, radiolaria, foraminifera, ostracodes, scleractinian corals, bryozoans, brachio-pods, molluscs, echinoderms, fish, amphibians, reptiles and terrestrial plants (macrofossils and palynomorphs). These reviews take account of possible biasing factors in the fossil record in order to extract the most comprehensive picture of the K-T biotic crisis available. Results suggest that many faunal and floral groups (ostracodes, bryozoa, ammonite cephalopods, bivalves, archosaurs) were in decline throughout the latest Maastrichtian while others (diatoms, radiolaria, benthic foraminifera, brachiopods, gastropods, fish, amphibians, lepidosaurs, terrestrial plants) passed through the K-T event horizon with only minor taxonomic richness and/or diversity changes. A few microfossil groups (calcareous nannoplankton, dinoflagellates, planktonic foraminifera) did experience a turnover of varying magnitudes in the latest Maastrichtian-earliest Danian. However, many of these turnovers, along with changes in ecological dominance patterns among benthic foraminifera, began in the latest Maastrichtian. Improved taxonomic estimates of the overall pattern and magnitude of the K-T extinction event must await the development of more reliable systematic and phylogenetic data for all Upper Cretaceous clades.

The magnetostratigraphy of Coniacian-Late Campanian chalk sequences in southern England

Abstract Magnetostratigraphic data are reported from the Coniacian to early Late Campanian chalk sequences at Culver Cliff and Scratchells Bay on the Isle of Wight and Seaford Head in East Sussex, UK. Mean natural remanent magnetisation (NRM) intensity values for individual sample sites range from 0.0013 to 1.6008 mA/m, with an overall mean of 0.0165 mA/m. Reliable determination of the remanence of the very weakly magnetic units was achieved by carrying out replicate palaeomagnetic measurements on large volume samples, using a 2-G ‘wholecore’ cryogenic magnetometer. Palaeomagnetic measurements on other, standard-sized samples were made with a CCL ‘discrete sample’ cryogenic magnetometer. Incremental thermal and AF demagnetisation treatment was used to remove magnetic overprints and to isolate the characteristic remanent magnetisation (ChRM). A reliability classification scheme has been developed and applied in this study, to provide an objective assessment of the quality of the palaeomagnetic results. IRM acquisition experiments suggest that the dominant magnetic mineral in these sediments is magnetite, but mixtures of hematite and magnetite occur also. SEM and TEM studies of magnetic mineral extracts indicate the presence of magnetite and hematite, preserved as inclusions within silicate grains, together with abundant bacterial magnetite preserved as individual grains and chains in some samples. Samples containing the greatest proportions of bacterial magnetite generally have the highest NRM intensity. The composite magnetic polarity sequence, derived by combining the data from all three sections, is characterised by three principal magnetic polarity zones which can be correlated with geomagnetic Chrons C34n, C33r and C33n, respectively. The positions of the chron boundaries have been tied to macrofossil and nannofossil biostratigraphic zones in these sections. A series of previously undocumented short magnetic polarity sub-chrons has been identified within the longer magnetochrons. These have potential value for high-resolution stratigraphic correlations and significance for theories of geodynamo behaviour.

Biostratigraphical correlation of the Campanian–Maastrichtian boundary: Lägerdorf–Hemmoor (northwestern Germany), DSDP Sites 548A, 549 and 551 (eastern North Atlantic) with palaeobiogeographical and palaeoceanographical implications

A correlation of the Campanian-Maastrichtian boundary is attempted using foraminiferal and nannoplankton data from two areas: the eastern North Atlantic and northwestern Germany. The Boreal benthic and Tethyan planktonic foraminiferal zonation schemes are applied to Site 548A, where both foraminiferal groups occur frequently. A direct comparison of both biozonations reveals that the base of the Maastrichtian, according to planktonic foraminifers, has to be placed in the Upper Campanian of the Boreal benthic foraminiferal biozonation, which concurs with the nannoplankton results. The Tethyan Middle and Upper Maastrichtian are probably equivalent to the Upper Maastrichtian in the Boreal sense. The bases of the Maastrichtian substages are thus diachronous between the Boreal and Tethyan realms. Palaeotemperatures (which were estimated using the oxygen isotopic composition of the Goban Spur chalks) indicate, in combination with palaeowind directions, that the faunal and floral distribution pattern recorded is the result of a stable, warm water outflow from the northwest European epicontinental seas through the Channel area to the Celtic Shelf sea and Goban Spur. This mechanism appears to have been a dominant separating factor of the Boreal and Tethyan bioprovinces on the western European Shelf.

Critical events in the evolutionary history of calcareous Nannoplankton

Calcareous nannofossil diversity, and rates of speciation and extinction are calculated for five million year intervals from their first appearance in the Late Triassic through to the Present Day. Important evolutionary events are as follows: first appearance in the Late Triassic, Triassic-Jurassic boundary extinctions, Tithonian radiation (and the first occurrence of nannofossil carbonates), Late Cretaceous diversity maximum, Cretaceous-Tertiary boundary extinctions, Palaeocene radiation, mid Eocene to Oligocene diversity decline, and early Miocene diversity rise. These events are related to possible causal factors of which climate appears to be the most fundamental. Other factors may include biogeographical isolation, sea level change, and the configuration of Mesozoic oceans.