WHAT'S IN A DATE? INTEGRATING GEOCHRONOLOGY AND STRATIGRAPHIC STUDIES (THE EARTHTIME INITIATIVE)

Precise and accurate knowledge of geological time is central to understanding Earth history. While geologists depend on published timescales, many are not aware of the details of the age data which underpin its calibration. Most timescales are necessarily based on interpolation between geochronological tie points of variable quality that are often determined by averaging dates obtained using different techniques. Recent advances in geochronology and correlation methods allow us to develop a calibrated geological timescale with significantly improved accuracy and precision. Ultimately this allows us to address the rates of geological, biological, and climate change in geologic time. All who use the geological timescales should understand the details of how they are constructed, considering the power and limitations of geochronological and correlation techniques.

The EARTHTIME initiative (http://earth-time.org) has been developed as a community-based effort to focus attention on the calibration of at least the last 800 million years of Earth history. This requires using a unified, multi-chronometer (radio-isotopic and cyclostratigraphic) approach integrated with paleo-biological and paleo-climatic proxy datasets. This in turn will permit Earth scientists to realize and address a new series of questions that rely on knowledge of precise rates of biological, geological, and climatic change, allowing for example, assessment of links between marine and terrestrial realms, or synchroneity/ordering of events for intervals of interest.

New developments in geochronological techniques have enabled dating of ash beds intercalated with fossil-bearing sedimentary rocks at unprecedented levels of precision (0.1-0.05%). This high level of precision has in turn identified issues of accuracy such as interlaboratory and/or inter-decay scheme bias. A major goal of EARTHTIME is to address and resolve these issues.

Progress must still be made on the intercalibration of high-precision U-Pb datas with timescales based upon 40K decay scheme and/or astronomical cyclicity to achieve the goal of seamless calibration of geological time. Ultimately, the level of involvement of the geologic community, geochronologists and paleontologists in particular, will determine the degree to which the goals of EARTHTIME are realized.