Project II.3: New U/Pb constraints for the Paleogene Time Scale (Diana Sahy supervised by Dan Condon)

As outlined above, due to uncertainties with the astronomical solutions in sections older than ca. 40 Ma, developing a highly resolved geological time scale for the Paleogene will require the integration of astronomical-tuning and radio-isotopic dating applied in key stratigraphic intervals. Due to the nature of the rock record this intercalibration can not be achieved in a single stratigraphic section and isotopic methods will therefore be applied on samples from untuned but magnetostratigraphic, chemostratigraphic and/or biostratigraphical well-constrained sections. The isotopic ages can then exported to tuned sections to independently test and constrain the tuning and permit development of truly integrated geological time scales. In this project three target regions have been identified:

  1. the Western Interior Basin (USA and Canada) focussing upon climate records and biozonesthe North Atlantic Igneous Province focussing upon flood volcanism (possibly a causative mechanism of climate change at the Paleocene-Eocene transition) and calibration of magnetozones (chrons)
  2. The northern Apennines (Italy) where cyclic Eocene-Oligocene  successions contain ash beds with the potential for direct dating of cyclic sedimentation.

Objective: The major objective of this project is the calibration of the Paleogene time scale using the U-Pb systematics of zircons, and to combine these results with 40Ar/39Ar dating (Project II.2) and astronomical tuning of cyclic Palaeogene successions (Project II.1) in order to obtain a highly resolved and robust time scale. This new time scale can then directly be applied to assess fundamental aspects of Earth's history such as seafloor spreading rate histories and the potential role of long-period orbital climate forcing. It will also be useful to correlate marine and terrestrial stratigraphic sequences in order to relate continental with marine processes and vice versa. Since the Paleogene is the key interval for the evolution of birds and mammals, precise correlations between terrestrial and marine records will provide invaluable insights into the influence of continuous and/or catastrophic environmental changes on their evolution.

Background: In the last 10 years the analytical techniques for the precise and accurate analysis of U and Pb in small zircon samples (individual grains or a few grains) have improved significantly resulting in uncertainties that reach the 100 kyrs mark. In addition the understanding of the behaviour of U and Pb in zircon during alteration (weathering, metamictisation, leaching) has improved dramatically. Due to the small sample size needed and deeper understanding of the internal structures of zircons and their bearing on U and Pb mobility in zircons, it is possible to select specific grains that can be expected to yield robust growth ages. Careful treatment of these individual grains by acid leaching or gentle heat treatment improves the reliability of the ages obtained with the U-Pb method from zircons considerably (e.g. Mattinson, 2005).

Research strategy and methodology: The zircons will be selected from the same units that will also be used for 40Ar/39Ar dating (Project II.2). This strategy is essential to obtain independent geochronologic information on the same unit in order to be able to evaluate the quality and reliability of the two methods and the robustness of the absolute age for the individual layers.

Feasibility: The samecriteria apply as outlined in Project II.2. Due to improvements in analytical techniques (lower blanks, smaller samples sizes) it is possible to date single zircons of Paleogene age with uncertainties of a few kyrs. After a careful preselection of the most suitable grains from a bentonite or ash layer it is very likely to obtain undisturbed grains that grew in a short time interval and do not show any inherited components or parts that have suffered Pb-loss. Such grains will yield reliable ages with high precision and high resolution needed to correlate the absolute ages with magnetostratigraphic, chemostratigraphic and/or biostratigraphic constrained sections.

Innovative aspects and relevance: It will be for the first time that all techniques will be truly integrated to establish a much improved Paleogene Time Scale. Of crucial importance is the effort to perform detailed interlaboratory comparisons of key samples involving some of the best laboratories in the world. The outcome is expected to be implemented in the next version of the GTS.

Link with other projects: This project is strongly linked to projects I.1 and II.2 and will be partly carried out on the same volcanic ash layers selected for project II.2