Objective: Here we propose to recalibrate the Paleogene time scale using 40Ar/39Ar dating to provide tight age constraints on the astronomical tuning of cyclic pre-Neogene successions. The new age data will be directly combined with the results of projects II.1 and II.3 as well as with existing literature to construct a new astronomical time scale with an unprecedented accuracy and resolution for key stratigraphic intervals in the Paleogene. 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.
Background: In the past decades improvements in analytical techniques and instrument design in isotopic dating have led to high precision radio-isotopic dates at the 0.1% uncertainty level, but also has revealed biases often exceeding internal analytical precision (e.g. with 40Ar/39Ar dates systematically younger than U/Pb dates). Here, we focus on the intercalibration of 40Ar/39Ar and U/Pb dating with the aim to provide the highest possible quality radio-isotopic ages (<150 kyr) to constrain the astronomical tuning of the Paleogene.
Research strategy and methodology: Several initial target intervals have been identified for this project: the Cretaceous-Paleogene (K-T) boundary and early Paleocene, the Paleocene-Eocene (P-E) boundary and the Eocene-Oligocene (E-O) boundary. Bentonites in magnetostratigraphically well-constrained sections the Western Interior Basin (USA and Canada) spanning the K-T boundary and early Paleocene will be (re-)dated to confirm and/or improve existing astronomical tunings (e.g. Zumaia, Spain, and ODP leg 198 and 208 sites). Ash layers associated with the North Atlantic Igneous Province (the flood volcanism provides a possible causative mechanism for climate change at the Paleoceone-Eocene transition, Storey et al., 2007), and in magnetostratigraphically well-constrained terrestrial successions in Wyoming and Nebraska (USA) and from the E-O boundary stratotype section at Massignano (Italy) will be dated to constrain the astronomical tuning of the P-E and E-O boundary intervals and to intercalibrate the 40Ar/39Ar and U/Pb dating methods. Selected ash layers will be sampled during joined field-campaigns. Samples will be prepared at the VUA and distributed between GTSnext 40Ar/39Ar laboratories for interlaboratory bias assessment.
Feasibility: Selected ash layers ideally meet the criteria of being 1) intercalated in astronomical tuned cyclic records and 2) air fall ashes where it is justified to assume a zero time lag between lock-in in the sediments and time of eruption, and 3) containing large sized K-feldspar crystals - allowing single crystal 40Ar/39Ar dating - and 4) zircon for U/Pb dating and preferably also titanite to constrain magma residence times. Due to the nature of the rock record intercalibration of all three methods is not likely to be achieved in a single stratigraphic section and isotopic methods will therefore be applied on samples from untuned but magnetostratigraphic, chemostratigraphic and/or biostratigraphic well-constrained sections. The isotopic ages will then be exported to tuned sections to independently test and constrain the tuning of the selected key intervals in order to develop a truly integrated geological time scale for the Paleogene.
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 world best laboratories. The outcome is expected to be implemented in the next versions of the GTS.
Link with other projects: This project is strongly linked to projects I.1 and II.3 and will be partly carried out on the same volcanic ash layers selected for project II.3.