The international Geologic Time Scale (Gradstein et al. 2012; Figure 1) integrates available stratigraphic and geochronology information. It provides the relational framework for the physical, chemical, and biological processes on Earth. The time scale is crucial for understanding the dynamics of the five major biosphere collapses and extinctions through deep time. Humans have now induced a new and major biosphere upheaval.
Calibration to absolute (linear) time of the succession of events recorded in the rocks on Earth has three components:
- The standard stratigraphic divisions and their correlation in the global rock record.
- The means of measuring linear time or elapsed durations from the rock record.
- The methods of effectively joining the two scales, the stratigraphic one and the linear one.
For historic reasons and convenience in communication, the rock record of Earth’s history is subdivided in a chronostratigraphic scale of standardized global stratigraphic units, such as ‘Ordovician’, ‘Miocene’, ‘Harpoceras falciferum ammonite Zone’, or ‘polarity Chron C24r’. Unlike the continuous ticking clock of the chronometric scale (measured in years before ‘Present’, which is defined as 2000 CE), the chronostratigraphic scale is based on relative time units, in which global reference points at boundary stratotypes define the limits of the main formalized units, such as ‘Ediacaran’ or ‘Devonian.’ The chronostratigraphic scale is an international-ratified convention based on the actual rock record, whereas its calibration to linear time is a matter for discovery or estimation. The suite of stage boundaries and definitions are compiled here: engineering.purdue.edu/Stratigraphy.
The radiometric (U-Pb and 40Ar/39Ar) and orbital-tuning methods that provide our geological clocks are becoming better calibrated. However, few (eight!) dates are directly on stage boundaries, hence interpolation methods are used to assign the ages to the majority of stage boundaries.
The history of life as calibrated by Geologic Time Scale 2012
In 1982, Raup and Sepkoski published a much-quoted study that identified five major biological mass extinctions. Further taxonomic data mining and improved analysis of the considerable data set (Sepkoski 2002), show the major extinctions and radiations (Figure 2). Among extinctions, the end of Ordovician, end of Permian, and end of Cretaceous show the strongest signal, although details and causes of these mass extinctions are complex and only partly understood. Among originations, the great Cambrian diversification, the Great Ordovician Biodiversification Event, and the post Cretaceous–Palaeogene boundary event recovery standout.
The major extinction events have been recognized for a very long time; in fact, they were the main reason for dividing the Phanerozoic into the Palaeozoic, Mesozoic, and Cenozoic eras. Having accurate ages for the GTS is a fundamental requirement for understanding the timing and rate of extinction, and the causal links between biotic and extra-biotic factors.
Life in the Precambrian
The Precambrian is at the dawn of a geologically meaningful stratigraphic scale. The oldest record of early life forms on Earth was recovered from the circa 3.49 Ga old Dresser Formation in the Pilbara Craton of Australia. Here, stromatolites and possible microfossils are preserved in a thin succession of carbonates, sandstones, and hydrothermal precipitates deposited under intermittently shallow-water conditions within a volcanic caldera setting. Later, circa 2.6 Ga, a significant increase in oxygenic relative to anaerobic photosynthesis is thought to have arisen, linked to a marked increase in ocean primary productivity.
The stratigraphy of the Cryogenian and Ediacaran periods of the latest Precambrian is developing rapidly, with many new chemostratigraphic, biostratigraphic, and radiometric correlation levels. The Ediacaran Period (635–541 Ma) marks a pivotal position in the history of life, between the microscopic, mostly prokaryotic assemblages that had dominated the classic ‘Precambrian’, and the large, complex, and commonly shelly animals that dominated the Cambrian and younger Phanerozoic periods. Diverse large spiny acritarchs and simple animal embryos occur immediately above the base of the Ediacaran and range through at least the lower half of the Ediacaran. The mid-Ediacaran Gaskiers glaciation (584–582 Ma) was immediately followed by the appearance of the Avalon assemblage of the largely soft-bodied Ediacara biota (579 Ma). The earliest abundant bilateral burrows and impressions (555 Ma) and calcified animals (550 Ma) appear towards the end of the Ediacaran Period.
References and acknowledgments
Gradstein, F M, J Ogg, M Schmitz, G Ogg et al. (2012). The Geologic Time Scale 2012. Elsevier, Boston. 1144 p.
McNeely, J (2001). Invasive species: a costly catastrophe for native biodiversity. Land Use and Water Resources Research, 1 (2), 1–10.
Millennium Ecosystem Assessment (2005). Ecosystems and Human Well-being: synthesis. Island Press, Washington. 137 p.
D M Raup and J Sepkoski (1982). Mass extinctions in the marine fossil record. Science 215 (4539), 1501–1503.
Sepkoski, J (2002). A Compendium of Fossil Marine Animal Genera, ed. D Jablonski, M Foote. Bulletin of the American Palaeontological Society 363, 1–560.
Øyvind Hammer kindly assisted with the update of Figure 2.