Chronostratigraphic terminology at the Paleocene/Eocene boundary
Integrated research over the past decade has led to the recognition of a short (150-200 k.y.) interval of Paleogene time within Chron C24r at ~55.5 Ma, formerly termed the late Paleocene Thermal Maximum (LPTM) but more recently the Paleocene-Eocene Thermal Maximum (PETM), that was crucial in the climatic, paleoceanographic, and biotic evolution of our planet. Stable isotope analysis of marine carbonates indicates that there were transient changes in surface and deep-water temperatures. These climatic changes coincided with a negative 3%-4% carbon isotope excursion (CIE), which is recorded in both marine and terrestrial deposits. It was soon realized that the CIE not only constitutes a powerful tool for long distance ("global") isochronous correlation, but even more importantly that it is coeval with notable biotic events in both marine and continental fossil records that have long been taken as criteria for the beginning of the Eocene in North America and more recently in deep sea cores. On the other hand, the conventional Paleocene/Eocene boundary level at the Thanetian/Ypresian boundary in Belgium and the London Basin has been found to be ~1 m.y. younger than the CIE, based on the association of the First Appearance Datum (FAD) of the (calcareous nannoplankton) Tribrachiatus digitalis (at ~54.4 Ma) with the base of the Ypresian in the London Basin. Although the Ypresian definition would take priority under normal circumstances, a consensus has been reached to redefine the Eocene in recognition of the worldwide significance and correlatibility of stratigraphic features associated with the PETM. Redefinition of the Eocene, however desirable, nevertheless cannot proceed in a stratigraphic vacuum, and this paper is concerned with resolving the consequences of this action. To be made coincident with the CIE at ~55.5 Ma, the Ypresian/Thanetian boundary would have to be lowered by ~1 m.y., resulting in the inflation of the span of the Ypresian by 20% and a reduction of the span of the Thanetian by 30%. At the same time, the terminology of the strata in the leapfrogged interval would be thrown into total conflict with the literature, with the substitution of one widely used stage name for the other in the conflicted interval. On the other hand, to relocate the Paleocene/Eocene boundary without moving the stage boundaries would result in the upper third of the Thanetian falling within the Eocene, demolishing a century-old consensus. We propose that the destabilizing effect of the new boundary in the classic chronostratigraphy of western Europe can best be minimized with the introduction of a pre-Ypresian Stage, to encompass the orphaned upper Thanetian interval as the basal unit of the Eocene under a separate name. To this end, we suggest the reintroduction of the Sparnacian Stage, now that its original concept has been shown to correlate essentially with the interval between the CIE and the FAD of T. digitalis.
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- Causes and consequences of globally warm climates in the early Paleogene