Revising the Precambrian Timescale: a new look at an old story
Martin J. Van Kranendonk
Such changes are exemplified across the period of Earth’s
School of Biological, Earth and Environmental Sciences,
University of New South Wales
Kensington, NSW
2052 Australia
adolescence, from 2.8–2.0 Ga (Fig. 3), during which time the
E: [email protected]
Ga) and then by cooling of the atmosphere, rise of atmospheric
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
oxygen, and global deposition of glacial rocks (2.42–2.22 Ga)
A)
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);
•
B)
Archean-Proterozoic boundary
during global shutdown of the magmatic system (Condie et al.,
The nearly 4 billion year period of Earth history
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 reappearance of iron-formations and worldwide deposition of
organic-rich black shales (Shungites: Melezhik et al., 2005).
No formal definition of early Earth history, despite
This linear succession of events, exquisitely preserved in the
widespread use of “Hadean” for rocks >c. 4 Ga.
rock record (Fig. 6), forms the basis of a revised Precambrian
timescale across this period.
Since the current timescale was devised, knowledge of the
Precambrian Earth system has exploded, with precise UPb zircon ages and new isotopic geochemical techniques
Figure 1: A) Current Precambrian
timescale; B) Stratigraphic
column of the Hamersley Basin,
Western Australia, showing ages
and position of current ArcheanProterozoic boundary (2500 Ma).
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).
Figure 5: Polished rock
slab showing gypsum
crystals in carbonate from
the c. 2.2 Ga Yerrida Basin,
Western Australia.
The Precambrian Subcommission of the International
Commission on Stratigraphy is currently reviewing the
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 ironformations (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 ironformation (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.
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.
Indeed, analysis of global datasets shows that Precambrian Earth
Precambrian timescale with the aim of establishing
evolved through five main cycles (3.2–2.8 Ga; 2.8-2.22 Ga;
chronostratigraphic divisions of the Precambrian, with
2.22-1.7 Ga; 1.7-0.9 Ga; 0.9 Ga-542 Ma), driven by changes in
Global Stratotype Section and Points (GSSPs, or “Golden
mantle temperature and rate of convection, and reflected in the
Spikes”) in rock successions, wherever possible (Fig. 2).
geological record by pulses of crustal growth tied to the
supercontinent cycle, and by changes in atmospheric
The proposed scheme follows the rationale of Cloud
conditions and biological activity (Fig. 7: Van Kranendonk,
(1972):
2012). These cycles, and the major atmospheric and biological
“…we seek trend-related events that have affected the
changes that accompanied them, form the basis for a revised
entire Earth over relatively short intervals of time and left
Precambrian timescale changes.
recognizable signatures in the rock sequences of the
A working group is currently investigating available data
globe. Such attributes are more likely to result from
towards formally establishing a Hadean Eon, to reflect the
events in atmospheric, climatic, or biologic evolution
period of early planetary formation, the Moon-forming Giant
than plutonic evolution and hence should be more
Impact, crystallisation of the magma ocean, and solidification
characteristic of the sedimentary record than of the
of the first differentiated continental crust. The next step will be
igneous or metamorphic record, although the latter must
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.
to erect a candidate GSSP section for a revised Archean-
be included in any meaningful global assessment.”
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
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.
Australian Centre for Astrobiology
investigated in South Africa and Australia. Following that, era
boundaries for the Proterozoic and Archean will be reexamined.
References cited
Cloud, 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
ArcheanProterozoic 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.