The origin and nature of life during the first four billion years of Earth's history is of fundamental importance to astrobiology. Western Australia has much of the world's oldest and best-preserved geological evidence for ancient life, and the Australian Centre for Astrobiology is a leader in the field of Precambrian research. Current research projects are investigating evidence for significant geological and biological changes preserved in Paleoarchean (3.6-3.2 billion years ago) and Neoarchean (2.8-2.5) rocks of the Pilbara region, Western Australia.
Our projects include interdisciplinary, local and international collaborations, and have produced new and innovative science outreach activities, including Virtual Field Trips to remote and significant field sites, which are produced in collaboration with NASA.
Below is a summary of Precambrian Research projects headed by Professor Martin Van Kranendonk.
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1) Climate and biological change across the Archean-Proterozoic boundary, 2.5-2.2 Ga
Perhaps the most complete and best exposed geological section across the transition from early, reducing Earth conditions (>2.5 Ga) to more modern, oxidized and cooler Earth (2.2 Ga) 鈥 known as the Great Oxidation event, or GOE 鈥 is exposed in the southern Pilbara region of Western Australia as the Turee Creek Group. Research on this section to date has uncovered a spectacular section across the GOE transition, from Archean banded iron-formation to Paleoproterozoic glacial diamictites across a transitional unit of jaspilitic- to grey-laminated chert, with conformable contacts that you can put your fingernail on (Photo 1). We are currently investigating the nature of this transitional unit in detail through geochemical analysis, Fe isotopes, and redox-sensitive elements (Mo and PGE). A more expanded section has also been found in which the transition is manifest as eight cherts interbedded with iron-formations that grade to shale.
The rest of the Turee Creek Group is also under detailed investigation, particularly in regards to its sedimentology and geochemistry. Preliminary results clearly indicate at least two, and possibly three, glacial cycles, where previously only one had been recorded (Photo 2). Facies analysis indicates that each of the cycles is associated with sea level drawdown, which can be used to infer that they were global in scope and that major climate change was associated with the rise of atmospheric oxygen.
In addition, we are investigating biological changes across the GOE. We have discovered a well-preserved assemblage of microfossils permineralized in black chert in deepwater carbonates from the upper part of the Turee Creek Group (Photo 3). These microfossils are morphologically and geochemically distinct from photosynthetic cyanobacterial communities of shallow-water stromatolites. Given the deepwater setting and through comparison with modern analogues, we are able to conclude that these represent a sublittoral sulfuretum (sulphur-cycling community) 鈥 the first to be recorded in the geological record.
Collaborators: Pascal Philippot (Institute de Physique du Globe de Paris); Clark Johnson (U. Wisconsin-Madison); Ken Williford (Jet Propulsion Laboratory); Kliti Grice and Anais Pages (Curtin University); Malcolm Walter and Rajat Mazumder (UNSW); Bill Schopf (UCLA); Aivo Lepland (Geol. Survey Norway).
2) Planetary driver of environmental change
The Earth has evolved dramatically over time, from a hot, molten ball (magma ocean) immediately following the giant Moon-forming Impact at c. 4.5 Ga, to the cool planet of today with a dozen or so large tectonic plates that are created at mid-ocean ridges and partly recycled back into the mantle across steep subduction zones. My research on early Earth has suggested that this 鈥渕odern鈥 (i.e. steep) style of subduction commenced at c. 3.1 Ga, due to a crossover point in time when the amount of conductive heat emanating from the mantle first declined to values beneath the capacity of the crust to lose that heat. This allowed the tectonic plates to grow and to cool and thicken away from mid-oceanic ridges, resulting in the onset of steep subduction (through plate sinking).
Ongoing research into the proposed Mesoarchean change in the tectonic style of the planet includes:
- Analysis of geochemical proxies of subduction through time, specifically high field strength elements in magmatic rocks, and oxygen and Hf isotopes in zircons. Preliminary results show that onset of steep subduction is accompanied by the start of the supercontinent cycle, which evolved and changed over time, peaking at c. 1 Ga and declining in intensity thereafter (Fig. 1).
- Analysis of a proposed Paleoarchean suture zone in South Africa, where high-P metamorphic mineral assemblages have been used to infer a suture zone. Detailed mapping in 2012 will be combined with geochronology and geochemistry to test this claim. Preliminary results suggest a more complex situation than previously reported.
- Analysis of oxygen isotopes in zircons from dated samples from Pilbara, Western Australia. Samples were specifically identified to test the subduction-accretion model developed for this area. Preliminary results show evidence for both high and low oxygen isotope values in zircons dated at exactly the age identified independently for subduction and accretion of the West Pilbara Superterrane, and not from zircons of any other age.
Collaborators: Chris Kirkland (Geol. Survey Western Australia); John Cliff (U. Western Australia)
Figure 1: Schematic diagram of Earth evolution through time, showing steps in crustal growth following on from pulses of mantle heat arising from the aftermath of subduction avalanches during supercontinent amalgamation that were accompanied by changes to Earth tessellation (T1-T4).
3) The North Pole volcanic caldera: habitat of Earth鈥檚 oldest stromatolites
The North Pole Dome in the Pilbara Craton, Western Australia, is famous for hosting Earth鈥檚 oldest stromatolites within a well-preserved succession of sedimentary and volcanic rocks of the Dresser Formation, Warrawoona Group. Research has indicated a link between life and hydrothermal vents that formed at the tips of growth faults developed within an active volcanic caldera. However, only a small part of the formation has been mapped in detail and it is not known whether life is solely restricted to vent environments or is more broadly spread and possibly even diverse. Ongoing mapping at North Pole will document the life signatures preserved in this unique environment in 4-D and unravel the series of events within the caldera, number of hydrothermal circulation cells and variations along and across strike in the chemistry of the system.
Figure 2: a) Zoned hydrothermal barite-chert vein: b) Hydrothermal barite curling over stromatolite at vent mouth; c) wrinkly laminated and domical stromatolites; d) diagenetic barite in silicified carbonate; e) hydrothermally-altered pillow basalt (now kaolinite) in caldera footwall.听
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Collaborators: Ian Graham and Malcolm Walter (UNSW), Franco Pirajno (Geol. Survey Western Australia).
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4) Crust formation and tectonics of the Archean Yilgarn Craton.
The Archean Yilgarn Craton of Western Australia is widely considered to have formed primarily through accretion of a series of Neoarchean terranes (now the Eastern Goldfields Superterrane) onto an older (Mesoarchean) foreland (Youanmi Terrane). However, new data from the Youanmi Terrane and analysis of ultramafic-mafic volcanic components of the Eastern Goldfields Superterrane show a commonality of events and compositions across so-called terrane boundaries, throwing into question the subduction-accretion model of crust formation. Together with colleagues at the Geological Survey of Western Australia and CSIRO, we are pursuing this new research front through continued mapping and geoscience analysis, including interpretation of seismic refraction lines across the Youanmi Terrane and geochemical modelling.
Collaborators: Tim Ivanic, Ivan Zibra, Stephen Wyche, Chris Kirkland (Geol. Survey Western Australia), Steve Barnes (CSIRO).
Figure 3: Schematic evolutionary model of Yilgarn Craton development in the Neoarchean.