Radiometric dating of rocks and minerals to constrain the age of the Earth, timing of geological events and paleobiological histories has its roots in the works of nuclear physicists of the early Nineteenth Century during the period of discovery of radioactivity and investigations into the nature of the atom. The intervening years since have seen great progress in using the long-lived radioactive elements to constrain the origin and evolution of the Earth and to place the rock and fossil record into a consistent, numerically quantifiable temporal framework.

U-Th-Pb and 40Ar/39Ar dating methods have emerged as the primary tools for calibrating most of Earth history. It is important for all geoscientists to appreciate the physical basis underlying these methods and to have the ability to evaluate dates by means of currently accepted practices of data presentation. This introduction, along with the accompanying chapters, is intended to help the consumers of radiometric dates to understand better the uses and limitations of radiometric dating methods in an effort to tailor methods and techniques to address specific geochronologic needs, including calibration of the geologic time scale.

The ultimate goal of a fully calibrated rock record remains an on-going endeavor. The 2004 ICS geologic time scale is the latest compilation of those efforts. The numerical age calibration is constrained by only 213 radiometric dates, the vast majority of which are U-Pb and 40Ar/39Ar dates. Radiometric age control is not evenly distributed through geologic time. There are virtually no radiometric dates in the late Cenozoic where magnetostratigraphy and cyclostratigraphic methods are more precise and applicable. Radiometric dating efforts are concentrated on biostratigraphically important segments of the rock record such as the Permian-Triassic and Cretaceous-Paleocene boundary events, and this is reflected in high-precision calibration of these boundaries. Large segments of geologic time, however, are constrained by either a few radiometric dates per chronostratigraphic unit (most of the Paleozoic) or none at all (Upper Triassic). The current status of radiometric age control on the rock record largely reflects real, underlying scientific issues in biostratigraphy and geochronology, and thus can help point the way to fruitful lines of collaboration between paleontologists, stratigraphers, and geochronologists.

High-precision geochronological techniques have improved in the past decade to the point where volcanic ash beds interstratified with fossil-bearing rocks can be dated to a precision of 0.1% or better. The integration of high-precision U-Pb zircon geochronology with bio/chemo-stratigraphic data brings about new opportunities and challenges toward constructing a fully calibrated time scale for the geologic record, which is necessary for a thorough understanding of the distribution of time and life in Earth history. Successful implementation of geochronology as an integral tool for the paleontologist relies on a basic knowledge of its technical aspects, as well as an ability to properly evaluate and compare geochronologic results from different methods. This paper summarizes the methodology and new improvements in U-Pb zircon geochronology by isotope dilution thermal ionization mass spectrometry, specifically focused on its application to the stratigraphic record.

K-Ar and more recently the 40Ar/39Ar variant are well established dating methods. The 40Ar/39Ar method requires irradiation with neutrons, posing some complications that are greatly outweighed by the benefits. The 40Ar/39Ar method is particularly powerful due to the availability of internal reliability criteria, the ability to analyze single crystals, and the amenability of the analyses to automation. 40Ar/39Ar dating has the capability for unsurpassed precision and is applicable to the broadest range of geologic environments and time scales of any radioisotope dating technique. For chronostratigraphic applications, 40Ar/39Ar is most important in the Cenozoic, becoming progressively less useful into the early Phanerozoic due to alteration and loss of radiogenic argon. Precision and accuracy of 40Ar/39Ar dating have been improved considerably in recent years, but an uncertainty of about 1% in the decay constant for 40K, probably mainly in the electron capture decay branch, still limits accuracy at about this level. Inconsistent use of standards (neutron fluence monitors) and attribution of variable ages to standards is still a source of confusion, but straightforward recalculation procedures can overcome the underlying problems provided that appropriate standards are used.

Detrital zircon geochronology is rapidly evolving into a very powerful tool for determining the provenance and maximum depositional age of clastic strata. This rapid evolution is being driven by the increased availability of ion probes and laser ablation ICP mass spectrometers, which are able to generate age determinations rapidly, at moderate to low cost, and of sufficient accuracy for most applications. Improvements in current methods will probably come from enhanced precision/accuracy of age determinations, better tools for extracting critical information from age spectra, abilities to determine other types of information (e.g., REE patterns, O and Hf isotope signatures, and/or cooling ages) from the dated grains, and construction of a database that provides access to detrital zircon age determinations from around the world.

This contribution seeks to provide a summary of radiometric dating techniques that are currently applicable to Phanerozoic sedimentary deposits and therefore relevant to paleontologists. First we overview the assumptions necessary for radiometric dating and introduce isochrons and concordia diagrams as methods for calculating ages and evaluating isotope systematics. We then focus on the four most promising systems: U-Pb dating of carbonates, Re-Os dating of black shales, and U-Pb and Lu-Hf dating of phosphates. We review expected geochemical behavior of daughter and parent isotopes in the context of both marine and terrestrial depositional environments. A critical evaluation of previously published ages and potential directions for future work are provided.

This lecture reviews Geologic Time Scale 2004 (Gradstein, Ogg et al., 2004; Cambridge University Press), constructed and detailed by 40 geoscience specialists, and indicates how it will be further refined. Since Geologic Time Scale 1989 by Harland et al., many developments have taken place: (1) Stratigraphic standardization through the work of the International Commission on Stratigraphy (ICS) has greatly refined the international chronostratigraphic scale. In some cases, traditional European-based stages have been replaced with new subdivisions that allow global correlation. (2) New or enhanced methods of extracting high-precision age assignments with realistic uncertainties from the rock record. These have led to improved age assignments of key geologic stage boundaries and other global correlation horizons. (3) Orbital tuning has greatly refined the Neogene, and improved parts of Paleogene and Mesozoic. (4) Statistical techniques of compiling integrated global stratigraphic scales within geologic periods.

Anticipated advances to the Geologic Time Scale during the next 8 years include: a geologically realistic Precambrian scale, formal definition of all Phanerozoic stage boundaries, orbital tuning of polarity chrons and biostratigraphic events for entire Cenozoic and Cretaceous, a detailed database of high-resolution radiometric ages that includes “best practice” procedures, full error analysis, monitor ages and conversions, resolving age dating controversies (e.g., zircon statistics and possible reworking) across Devonian/Carboniferous, Permian/Triassic, and Anisian/Ladinian boundaries, improved and standardized dating of several ‘neglected’ intervals (e.g., Upper Jurassic – Lower Cretaceous, and Carboniferous through Triassic, and detailed integrated stratigraphy for Upper Paleozoic through Lower Mesozoic.

The geochronological science community and ICS are focusing on these issues. A modified version of the time scale to accompany the standardization (boundary definitions and stratotypes) of all stages is planned for 2008 (to be presented at the 33th International Geologic Congress in Oslo), with a totally revised version of GTS available in 2012.

The main goal is to introduce some of the technical methods of time series analysis and cyclostratigraphy, with the overall focus to enable interested scientists with little experience in time series analysis to “get their hands dirty” while analysing their own data sets. The course notes make use of example data sets, and provide pointers to additional methods and software to conduct the basic steps of analysis of geological data that appear to show evidence of cyclicity. The short course notes presented here were originally developed as teaching material for advanced undergraduate and postgraduate students at the University of Cambridge and Stockholm University. The original notes were modified and enhanced, and aim to supplement the presentation during the Paleontological Society Short Course 2006.

Species origination and extinction events far outnumber radioisotopically dated events in the ancient stratigraphic record. In order to calibrate rapid rates of Mesozoic and Paleozoic change and to estimate the ages of paleobiologic events it would be ideal to have multiple dated events in single stratigraphic sections. This condition is rarely realized and the practical alternative is to build composite sections that combine information from many different locations. The compositing process takes advantage of all available evidence of relative age to produce high resolution time lines; i.e. ordered sequences of individual events whose average spacing is much finer than the duration of biostratigraphic zones and can approach the uncertainty intervals of the highest precision radioisotopic dates. Dated events are included in the compositing process from the outset. As a result the sequencing procedure is more efficient and the dated events find their optimal positions in the time line independent of any biostratigraphic zonal schemes. The sequencing procedures follow simple logical rules that may be learned from tiny data sets. When usefully large numbers of events are involved, however, the sequencing must be undertaken by computer and there is seldom a unique solution that best fits the field data. The range of positions in sequence that an event may occupy across the full set of equally best-fit solutions is a measure of the resolving power of the event. As new high-precision dates and detailed range charts continue to become available, the quality of the time lines will improve and they will become increasingly viable alternatives to zonal time scales in the older parts of the Phanerozoic.

The advent of greatly improved radiometric dating techniques with lower uncertainties, the development of new dating and correlation techniques, including vastly expanded quantitative biostratigraphic methods, and the possibility of reliable extension of orbital cyclostratigraphy into the Paleozoic all promise a great improvement in the ability of geologists to construct high-resolution temporal frameworks far deeper into the past. Such techniques have already allowed the generation of a high-resolution temporal framework for the Ediacaran-Cambrian radiation of metazoa, helped greatly narrow the duration of the great Permo-Triassic mass extinction and eliminated several hypothesized causes, and narrowed the duration of the oceanic anoxic event at the Cenomanian-Turonian (Late Cretaceous boundary). Temporal resolution of 100 kyr (0.02%) or even better into the early Paleozoic now seems likely, opening a host of new questions for reliable investigation. Further exploiting the possibilities of these techniques will require paleontologists to improve methods to integrate these disparate techniques, improve our understanding of the analysis of evolutionary rates, and confront the challenges of settings where geochronologic resolution may be greater than paleontologic resolution.