A chronostratigraphic division of the Precambrian: possibilities and challenges
|Location||International Geological Congress,oslo 2008|
|Author||Van Kranendonk, Martin|
|Holding Date||08 September 2008|
Since Earth formed from the accumulation of dust and gas, our planet has undergone secular change as a result of slow cooling and compositional differentiation. Early in this history, Earth acquired a biological community of living organisms. These two components — the geotectonic and the geobiologic — have co-evolved until the present day, with the additional influence of extra-terrestrial events.
The Precambrian (from T0= 4.567 Ga to 542 ± 1 Ma) is marked by a sparse and poorly-preserved fossil record, a decreasing volume of supracrustal rocks, and increasing degree of metamorphism and tectonic disturbance with increasing age, making the establishment of a chronostratigraphic time scale more challenging. As a result, subdivisions of the Archean and Proterozoic eons have been defined chronometrically, in terms of a round number of millions of years before present (Ma) or Global Standard Stratigraphic Ages (GSSAs), based on a global geotectonic compilation in the 1980s.
Since that time, however, there has been an explosion of new geoscientific information about Precambrian geotectonic and geobiological events. Consequently, the current Precambrian Subcommission is striving to establish a more "natural" set of subdivisions of Precambrian time that incorporates major biologic, hydro/atmospheric, and tectonic events, with an emphasis on geobiological events wherever possible. GSSAs will necessarily remain, but a major aim is to establish Global Standard Section and Points (GSSPs), wherever possible.
The following revisions of Precambrian time are being considered:
• A GSSA boundary to define the base of the Archean and top of the Hadean eons, at the age of Earth’s oldest dated rock (4.03 Ga).
• An Eoarchean—Paleoarchean boundary GSSP at the base of the ~3.49 Ga Dresser Formation in the Warrawoona Group, the first definitively fossiliferous horizon that is preserved within a continuous stratigraphic succession.
• A ~2.45 Ga Archean-Proterozoic boundary, reflecting the transition from chemical weathering to physico-mechanical weathering under higher pO2 conditions, with a GSSP in the Hamersley Basin, Australia.
• Potential GSSPs on either side of the 2.32-2.06 Ga Lomagundi-Jatuli C-isotopic event, at the disappearance of the S-MIF signature and of highly positive ä13C values, respectively; the former reflecting the onset of the Great Oxidation Event of the atmosphere through microbial processes within a series of global glaciations.
• GSSPs at the onset and termination of sulphidic oceans at ~1.84 Ga, and ~0.8 Ga, respectively, the latter approaching the onset of global glaciations in the Cryogenian that are associated with the second great rise in atmospheric oxygen.
The next step is consultation with the geoscientific community. Formal proposals will be developed, discussed and circulated within the Precambrian community for ratification by the Precambrian Subcommission.