Original paper

Recent progress in the standardization and calibration of the Cenozoic Time Scale

Hilgen, Frederik J.

Abstract

Since the publication of GTS2004, significant progress has been made in the standardization and calibration of the Geological Time Scale for the Cenozoic (= Paleogene + Neogene), especially as far as the older Paleogene part is concerned. This progress is especially connected with the direct astronomical tuning of significant portions of the Paleogene and with - informal - proposals for defining the Selandian, Thanetian and Chattian GSSPs. Based on ODP Site 1218 in the equatorial Pacific, an astronomical calibration was established for the entire Oligocene, resulting in a directly tuned age of ∼33.79 Ma for the Eocene-Oligocene (E-O) boundary when linked to the established age of 23.03 Ma for the Oligocene-Miocene boundary in GTS2004. The tuned age is corroborated by the tuning of the E-O boundary stratotype section at Massignano section but is at odds with recalculated 40Ar/39Ar ages for ash layers in continental record of North America. However the age is consistent with recalculated alternative ages for the ash beds and with a revised age estimate for the boundary based on the ignimbrite succession in New Mexico. Three options - with 405-kyr offsets - were presented for the tuning of the entire Paleocene, resulting in ages of 65.28, 65.68 and 66.08 Ma for the K/Pg boundary. Application of a new astronomically calibrated age for the Fish Canyon sanidine dating standard yields a 40Ar/39Ar age close to ∼66.0 Ma for the boundary, indicating that the oldest tuning option is correct. However, a discrepancy is found in the number of 405-kyr cycles in the older portion of the Paleocene, preventing the construction of a tuned and stable Paleocene time scale at this time. The Neogene part of the time scale remained relatively unchanged as might be expected from an Astronomical Time Scale once the underlying tuning is basically correct. Nevertheless considerable progress has been made in establishing a standard marine isotope stratigraphy and astrochronology. In addition, the Serravallian GSSP was formally defined coincident with the Mi3b isotope shift marking the final phase of the Middle Miocene climate transition. Starting from a Neogene perspective, a re-introduction of unit stratotypes for global stages and a formal definition of orbital-induced cycles as formal chronostratigraphic units of minor rank (chronozones) were proposed. Clearly the progress made in the Cenozoic time scale since 2004 holds bright prospects for establishing a fully tuned Cenozoic time scale with an unprecedented accuracy, precision and resolution in the coming years.

Keywords

geological time scalecenozoicpaleogeneneogeneradio-isotopic datingastronomical dating