The EARTHTIME initiative: A review of accomplishments and promise

The EARTHTIME initiative is a community-based effort focused on the calibration of at least the last 800 million years of Earth history. This requires a unified, multi-chronometer (radio-isotopic and cyclostratigraphic) approach integrated with paleo-biological and -climatic proxy datasets, enabling earth scientists to pose questions based on precise rates of biological, geological, and climatic change. This will facilitate, for example, assessment of links between marine and terrestrial realms, synchroneity/ordering of events and determination of rates of change.

During the first stage of EARTHTIME, significant progress was made in forging links between geochronologists, paleontologists, and stratigraphers, identifying key intervals of Earth history for which higher resolution temporal constraints are essential. Priority issues, such as inter-laboratory and inter-decay scheme biases were addressed and an aggressive plan was proposed to deal with them.

The U-Pb community has initiated ongoing experiments to assess interlaboratory bias, which is pre-requisite to any quantitative understanding of inter-decay scheme bias. The effective elimination of interlaboratory bias is being addressed via the distribution of an EARTHTIME mixed (²⁰²Pb-) ²⁰⁵Pb-²³³U-²³⁵U tracer solution (eliminating the largest source of lab-specific systematic uncertainty) and of synthetic zircon solutions to monitor long-term analytical reproducibility. In addition, a new open-source data reduction and age calculation software called U-Pb_Redux is under development for transparent reduction, analysis, and archiving of all U-Pb analyses with links to global databases. U-Pb_Redux is being designed also as a teaching tool capable of following all steps in data reduction.

The Ar-Ar community has undertaken two interlaboratory comparison experiments involving more than twenty labs. Under controlled experimental conditions, variability between laboratories for relative ages of commonly used fluence standards is significantly outside individually quoted precision. The present experiments have reached the EARTHTIME goal of increased synergy amongst the worldwide Ar-Ar community and will ultimately lead to better reproducibility. A subset of the EARTHTIME Ar-Ar group is coordinating with EarthChem to establish better standards for data collection, reduction and reporting.

Our ultimate goal is to be able to seamlessly integrate U-Pb, Ar-Ar, and cyclostratigrphic techniques to construct high-fidelity records of earth history. We have initiated a "Proof of Concept" experiment using two stratigraphic sections that span important astronomically calibrated boundaries (Cretaceous-Paleocene and Cenomanian-Turonian) and contain a number of volcanic ash beds. Zircon and sanidine separates are now ready for distribution to participating labs.

Ultimately, the level of community involvement will determine the degree to which the goals of EARTHTIME are realized.