Katharina Perch-Nielsen

Paleogene magnetic stratigraphy in Umbrian pelagic carbonate rocks: The Contessa sections, Gubbio

The Umbrian sequence of pelagic carbonate rocks provides an opportunity for precise correlation between Paleogene biostratigraphy and geomagnetic polarity history. The red-to-pink, Paleocene to middle Eocene Scaglia Rossa limestone, the varicolored upper Eocene Scaglia Variegata limestone, and the gray-green Oligocene Scaglia Cinerea marlstone form a 250-m-thick, continuous exposure in the Contessa Valley near Gubbio, Italy. Magnetostratigraphic, lithostratigraphic, and biostratigraphic investigations in three sections covering the entire Paleogene have confirmed and dated the geomagnetic reversal sequence for most of this period. The ferromagnetic mineral in the Scaglia Cinerea is magnetite and AF demagnetization to peak fields of 20 mT is sufficient to define the characteristic remanent magnetization. The Scaglia Variegata and pink Scaglia Rossa samples contain an additional hematite component which is very pronounced in dark red Paleocene Scaglia Rossa samples. Changes of magnetic mineralogy take place in these limestones during heating, especially above 500 °C. However, thermal demagnetization is effective in isolating the characteristic remanence vectors, which form almost antipodal clusters of directions representing normal and reversed polarities. The directions are rotated counterclockwise, partly due to post-Oligocene tectonism. Paleontological zonations of planktic foraminifera and calcareous nannofossils were hampered by poor preservation, but the major epoch and stage 29, as in the Gubbio Bottaccione section; Paleocene-Eocene, just above anomaly 25; Eocene-Oligocene, between anomalies 13 and 15; Oligocene-Miocene, just below anomaly 6C. These correlations require slight modifications to previous conclusions on Paleogene sea-floor spreading rates.

Mass Mortality and Its Environmental and Evolutionary Consequences

The latest Mesozoic and earliest Tertiary sediments at Deep Sea Drilling Project site 524 provide an amplified record of environmental and biostratographic changes at the end of Cretaceous. Closely spaced samples, representing time intervals as short as 102 or 103 years, were analyzed for their bulk carbonate and trace-metal compositions, and for oxygen and carbon isotopic compositions. The data indicate that at the end of Cretaceous, when a high proportion of the oceans planktic organisms were eliminated, an associated reduction in productivity led to a partial transfer of dissolved carbon dioxide from the oceans to the atmosphere. This resulted in a large increase of the atmospheric carbon dioxide during the next 50,000 years, which is believed to have caused a temperature rise revealed by the oxygen-isotope data. The lowermost Tertiary sediments at site 524 include fossils with Cretaceous affinities, which may include both reworked individuals and some forms that survived for a while after the catastrophe. Our data indicate that many of the Cretaceous pelagic organisms became extinct over a period of a few tens of thousands of years, and do not contradict the scenario of cometary impact as a cause of mass mortality in the oceans, as suggested by an iridium anomaly at the Cretaceous-Tertiary boundary.

Major calcareous nannofossil and foraminiferal events between the middle eocene and early miocene

Abstract The more significant biostratigraphic events during the time interval from beginning Middle Eocene to beginning Miocene with special emphasis on the Eocene-Oligocene boundary are discussed. The microfossils considered are calcareous nannofossils, planktic foraminifera and smaller and larger benthic foraminifera. Areas taken into account include particularly the Caribbean (Trinidad, Barbados), DSDP South Atlantic sections (Cape Basin Site 360, Walvis Ridge Sites 362A, 363, Angola Basin Site 364), the equatorial North Atlantic (DSDP Site 354) and the Southern Alps (Possagno). In the larger foraminifera the treatment is somewhat different, with selected families and genera being considered for their distribution in the eastern and western hemisphere.

New calibration of Neogene radiolarian events in the North Pacific

Abstract New age models for twelve Deep Sea Drilling Project sites in the North Pacific have been produced, based on (in order of importance in our dataset) a recompilation of previously published diatom, calcareous nannofossil and foraminifer first and last occurrences, and magnetostratigraphy. The projected ages of radiolarian first and last occurrences derived from the line of correlation of the age/depth plots have been computed from these sites, and 28 radiolarian events have thereby been newly cross calibrated to North Pacific diatom and other stratigraphy. Several of the North Pacific radiolarian events are older than in previously published equatorial Pacific calibrations, and some may be diachronous within the North Pacific. These patterns may be due to complex latitudinal patterns of clinal variation in morphotypes within lineages, or to migration events from the North Pacific towards the Equator.