REVISING THE PRECAMBRIAN TIMESCALE: A NEW LOOK AT AN OLD STORY

The nearly 4 billion year period of Earth history represented by the Precambrian is currently represented on the Geological Timescale by chronometric boundaries derived from a 1980’s review of data compiled at the very start of the zircon geochronological revolution. Since then, knowledge of the Precambrian Earth system has exploded, with precise U-Pb zircon ages and new isotopic geochemical techniques revealing a rich history that can be used to better constrain the evolution of our planet and the biosphere through deep time (Van Kranendonk, 2012).

The Precambrian Subdivision of the International Commission on Stratigraphy is currently reviewing the Precambrian timescale with the aim of establishing chronostratigraphic divisions of the Precambrian, with GSSPs in rock succession, wherever possible. A working group is currently investigating the possibility of formally establishing a Hadean Eon, to reflect the period of early planetary formation (this Eon will be represented by chronometric boundaries, as there is no preserved stratigraphy from this time). The next step will be to erect a candidate GSSP section for a revised Archean-Proterozoic boundary at, or near, the transition to an oxidized atmosphere, at roughly 2420 Ma (revised from the current 2500 Ma). Conformable successions are being investigated in South Africa and Australia. Archean and Proterozoic era boundaries that reflect global changes in the Earth system will be assessed in due course. Proterozoic examples include: 2.42-2.22 Ga widespread glaciations and magmatic shutdown; 2.22-2.06 Ga Lomagundi-Jatuli positive carbon isotopic excursion; 2.0-1.9 Ga Shunga event; 1.9-1.75 Ga accretion of the Columbia/Nuna supercontinent; 1.75-1.0 Ga period of relatively stable conditions during the Mesoproterozoic; and ~850-630 Ma widespread glacial conditions (the Cryogenian).

Van Kranendonk, MJ (2012): A chronostratigraphic division of the Precambrian: possibilities and challenges. In: Gradstein, F.M, Ogg, J.G., Schmitz, M.D., Ogg, G.J. (eds.), The Geologic Time Scale 2012; Elsevier, USA, pp. 313–406.