Revising the Precambrian Timescale: a new look at an old story Martin J. Van Kranendonk School of Biological, Earth and Environmental Sciences, University of New South Wales Kensington, NSW 2052 Australia

E: martin.vankranendonk@unsw.edu.au

The nearly 4 billion year period of Earth history represented by the Precambrian is currently divided by chronometric boundaries derived from a 1980’s review of data compiled at the very start of the zircon geochronological revolution (Fig. 1a: Plumb, 1991). This scheme is unsatisfactory primarily because:

• Eon and era boundaries are defined as whole numbers that are disassociated from the actual rock record (Fig. 1b);

• No formal definition of early Earth history, despite widespread use of “Hadean” for rocks >c. 4 Ga.

Since the current timescale was devised, 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).

Australian Centre for Astrobiology

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Figure 1: A) Current Precambrian timescale; B) Stratigraphic column of the Hamersley Basin, Western Australia, showing ages and position of current Archean-Proterozoic boundary (2500 Ma).

Such changes are exemplified across the period of Earth’s adolescence, from 2.8–2.0 Ga (Fig. 3), during which time the Earth system was flung out of equilibrium by the largest crustal growth episode in history (2.78–2.63 Ga), followed by unprecedented deposition of banded iron-formation (2.63–2.42 Ga) and then by cooling of the atmosphere, rise of atmospheric oxygen, and global deposition of glacial rocks (2.42–2.22 Ga) during global shutdown of the magmatic system (Condie et al., 2009). These changes were accompanied by chaos in the biosphere, reflected in the largest anomalies of stable isotopes in Earth history (Fig. 4). Subsequent restart of the global mantle engine at 2.2–2.06 Ga was accompanied by deposition of the first widespread Ca-sulfate deposits (Fig. 5) and redbeds, rise of eukaryotes, and by the Lomagundi–Jatuli isotopic excursion of carbon isotopes, followed by re-appearance of iron-formations and worldwide deposition of organic-rich black shales (Shungites: Melezhik et al., 2005). This linear succession of events, exquisitely preserved in the rock record (Fig. 6), forms the basis of a revised Precambrian timescale across this period.

B)

The Precambrian Subcommission of the International Commission on Stratigraphy is currently reviewing the Precambrian timescale with the aim of establishing chronostratigraphic divisions of the Precambrian, with Global Stratotype Section and Points (GSSPs, or “Golden Spikes”) in rock successions, wherever possible (Fig. 2).

The proposed scheme follows the rationale of Cloud (1972):

“…we seek trend-related events that have affected the entire Earth over relatively short intervals of time and left recognizable signatures in the rock sequences of the globe. Such attributes are more likely to result from events in atmospheric, climatic, or biologic evolution than plutonic evolution and hence should be more characteristic of the sedimentary record than of the igneous or metamorphic record, although the latter must be included in any meaningful global assessment.”

Figure 2: A prototype for a revised Precambrian timescale, based on the data presented in Van Kranendonk (2012). Clocks represent chronometric boundaries ; Spikes represent potential GSSPs.

References citedCloud, P., 1972. A working model of the primitive earth. American Journal of Science, 272: 537–548.Condie, K.C., O’Neill, C., and Aster, R.C., 2009. Evidence and implications for a widespread magmatic shutdown for 250 My on Earth. Earth and Planetary Science Letters, 282: 294–298.Melezhik, V.A., Fallick, A.E., et al., 2005. Emergence of the aerobic biosphere during the Archean- Proterozoic transition: Challenges of future research. GSA Today, 15: 4–11.Plumb, K.A., 1991. New Precambrian time scale. Episodes, 14: 139–140.Van Kranendonk, M.J. (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.

Figure 4: Temporal variations through the Precambrian: a) Δ33S of sedimentary sulphides (orange bar, MDF = range of mass-dependent fractionation); PAL = % of present atmospheric level of oxygen (logarithmic scale; b) δ34S of sedimentary sulphides (red circles) and of seawater sulphate (blue lines); c) δ13C of kerogens; d) δ13C of carbonates (black triangles denote time of Paleoproterozoic glaciations); e) δ56Fe of diagenetic sediments; f) relative abundance of banded iron-formation. Grey shade indicates time of instability in the biosphere.

Figure 5: Polished rock slab showing gypsum crystals in carbonate from the c. 2.2 Ga Yerrida Basin, Western Australia.

Figure 3: A causative, linked series of events across the Archean-Proterozoic boundary transition: 1) radiogenic heat flow decreases to below the rate of oceanic heat flow, resulting in cooling of oceanic lithosphere and onset of modern-style plate tectonics; 2) major peak in juvenile crustal growth, releasing huge volumes of CO2 into the atmosphere and causing a highly anoxic atmosphererise and intense chemical weathering of continents, resulting in; 3) precipitation of huge volumes of banded iron-formations (BIFs); 4) mantle cooling due to widespread subduction, a decrease in volcanic CO2 emmissions, and a bloom of cyanobacteria results in atmospheric oxidation; 5) microbial bloom resulting from delivery of increased nutrients to the oceans following deglaciation, combined with increased atmospheric pCO2 from renewed volcanism, results in disequilibrium in the biosphere – the Lomagundi-Jatuli isotopic excursion.

Figure 6: Field photograph of the conformable depositional contact between banded iron-formation (BIF) and the transitional chert unit (TC) of the Hamersley Group and overlying glacial mudstones and sandstones of the Turee Creek Group; author’s finger points to the site of a possible GSSP for a revised Archean-Proterozoic boundary at the first appearance of glaciogenic rocks.

Indeed, analysis of global datasets shows that Precambrian Earth evolved through five main cycles (3.2–2.8 Ga; 2.8-2.22 Ga; 2.22-1.7 Ga; 1.7-0.9 Ga; 0.9 Ga-542 Ma), driven by changes in mantle temperature and rate of convection, and reflected in the geological record by pulses of crustal growth tied to the supercontinent cycle, and by changes in atmospheric conditions and biological activity (Fig. 7: Van Kranendonk, 2012). These cycles, and the major atmospheric and biological changes that accompanied them, form the basis for a revised Precambrian timescale changes.

A working group is currently investigating available data towards formally establishing a Hadean Eon, to reflect the period of early planetary formation, the Moon-forming Giant Impact, crystallisation of the magma ocean, and solidification of the first differentiated continental crust. The next step will be to erect a candidate GSSP section for a revised Archean-Proterozoic boundary at, or near, the transition to a cooler, more oxidized atmosphere, at roughly 2420 Ma (revised from the current 2500 Ma). Conformable successions are being investigated in South Africa and Australia. Following that, era boundaries for the Proterozoic and Archean will be re-examined.

Figure 7: Major cycles of crustal growth and biospheric response through the middle part of the Precambrian, showing relative peaks of activity, whose sharp boundaries may be used as GSSPs (spikes at top of diagram). Vertical dotted line represents the position of a potentially revised Archean-Proterozoic boundary.