Project III.3: U/Pb dating and the integrated Cretaceous time scale (Benjamin D. Heredia supervised by Klaus Mezger)

Objective: The aim of this task is to provide high-precision U-Pb age constraints to provide a framework for the integration of multi-proxy lithological and geochemical datasets collected from selected land-based sedimentary successions for the development of a cyclostratigraphic reconstruction of the Late Cretaceous (Cenomanian-Maastrichtian). In essence, the radio-isotopic dates will be used to 'pin-down' the long-term stable 405-kyr eccentricity and 21-kyr precessional cycles recognized in the lithological records and relative datasets, thus permitting the evaluation of astronomical-tuned time scale from the base of the Maastrichtian boundary to the base of the Cenomanian.

Background: Over the past decade improvements in analytical protocols for ID-TIMS U/Pb dating have meant that with single zircon dating of Cenozoic zircons it is possible to achieve precision of <0.1%. Accuracy of such dates is limited to ca. 0.2% at present due to decay constant uncertainties and inter-laboratory bias, however we believe this value will approach 0.1% (mainly due to decay constant uncertainties) in the next couple of years due to implementation of the protocols already being developed as part of the EARTHTIME Initiative. Furthermore, the efforts on the intercalibration of 238U, 235U and 40K based decay schemes (see task IV.3) will help improve the relative accuracy of the dates generated in tasks III.2 and III.3 to ca. 0.1% (at present this value is closer to 1%). Such levels of precision (ca. 50-kyr at 100 Ma) and accuracy (ca. 100-kyr at 100 Ma) mean that is possible to date the ca. 405-kyr eccentricity cycles and thus 'pin-down' the cyclicity-based astronomical timescales (see task III.1).

Research strategy and methodology: The strategy employed in this project is to obtain high-precision radio-isotopic dates (U/Pb) on volcanic ash beds. A major obstacle in the Cretaceous is that the dateable materials are usually confined to successions that do not record an astronomical influence, one exception being the Bridge Creek Limestone Member which spans the Cenomanian-Turonian boundary. The Cenomanian-Turonian boundary is already the focus of a 'proof of concept' project under the auspices of the EARTHTIME Initiative aimed direct calibration of the ca. 405-kyr cyclicity. This project will be complete by the end of 2008 and will provide a foundation for tasks III. Beyond this interval direct calibration has limited potential due to lack of co-occurrence of suitable materials therefore we propose a different approach: calibration of bio- and magneto-stratigraphic variation in the dateable sections (mainly occurring in the Western Interior Basin) which can then be transpose this radio-isotopic based timescale to succession Europe where cyclostratigraphic reconstruction are being developed (see Task III.1). In addition, several target bentonites have been identified in these European successions. Due to their weathered nature they are not suitable for 40Ar/39Ar dating, however they may bear primary zircon.

Feasibility: Unpublished data U-Pb ID-TIMS data obtained on zircons from bentonites in the Western Interior basin (K-T boundary section, Denver Basin and Bridge Creek Limestone Member) indicate zircons are amenable to single crystal (or fragment) analyses with precision at the 1‰ level. This task will build upon the two EARTHTIME 'proof of concept' projects to be executed during 2008: 1) The absolute age of the Cretaceous-Paleogene boundary and 2) Dating eccentricity cycles at the Cenomanian-Turonian boundary, significantly expanding the coverage. Due to the indirect calibration of the cyclostratigraphy, magneto- and biostratigraphy will play a crucial role in the integration of the datasets.

Innovative aspects and relevance: Integrated with the other two projects in this task, these data will result in a stable Late Cretaceous Time Scale with an unprecedented resolution, accuracy and precision. It will provide insight into the greenhouse mode of Earth System behaviour and the nature of oceanographic events such as the Ocean Anoxic events (OAEs).

Link with other projects: This project is strongly linked to project III.3, which will be carried out on the same ash layers when possible (i.e. if both zircon and sanidine are found together). In addition, it is also linked to project II.3 and IV.3 for U/Pb dating methodology.