Reef FAQ

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 Part of the track of the ancient microbial reef. Picture: Abigail Allwood
Part of the track of the ancient microbial reef. Picture: Abigail Allwood

Abby's Reef: Frequently Asked Questions

by Abigail Allwood

Is the discovery the first of its kind?

This is the first discovery of a large, complex, ecosystem-scale remnant of the early biosphere. It is not the first discovery of putative fossils or fossil-like structures from Early Archaean age rocks (c. 3.0-3.5 billion years old)-- there are a small number of other possible biosignatures (microfossils, chemical fossils and stromatolites) in other c. 3.5 b.y.-old rocks of the Pilbara, but whether or not those "fossils" are unequivocally biogenic has been a longstanding topic of debate. Never before has something of this size (an entire, microbially diverse fossil reef) been found. With the discovery of the Strelley Pool Chert stromatolite reef, it is extremely difficult not to believe that life was well established by 3.43 Ga. Moreover, the scale and complexity of this biosphere remnant gives us unique and telling insights to the conditions that nurtured early life: apparently these organisms did not favour "extreme" hydrothermal environments, as has been hypothesised: they flourished during a brief pause in hydrothermal activity, preferring instead the relatively "normal" marine conditions found in the shallow parts of a tidal reef platform.

Are these the only fossils of such age? What is the current understanding of the early fossil record?

A small number of other putative fossils have been reported from rocks of similar age in the Pilbara and South Africa, yet there is uncertainty about whether any of those are actually biological in origin. Different non-biological hyhpotheses have been proposed for their formation. The Strelley Pool Chert and its stromatolites were first identified more than 20 years ago (Lowe, 1980; 1983), although the biological origin of the stromatolites has remained uncertain (e.g. Lowe, 1994; Lindsay et al., 2005) – in part due to contrasting observations of the shapes and characteristics of the structures and diverging environmental interpretations.

Why is the Strelley Pool Chert so important?

the Strelley Pool Chert is unusual among the few possible fossil-bearing rocks of such age because it is widespread and contains abundant, diverse and well preserved material. Therefore the Strelley Pool Chert offers us the best opportunity to resolve the debate about whether life existed in the Early Archaean, and to learn about the nature and habitat of early life. For this reason we decided to conduct a detailed, large-scale investigation of the stromatolites and analyse them against the backdrop of their palaeoenvironmental context.

What did we do and what did we find?

After a regional reconnaissance of the formation, we determined that the previously-discovered "Trendall Locality" (Hofmann et al., 1999) lay at centre of a 10 km-long section of particularly well preserved and "stromatolitic" outcrop of the Strelley Pool Chert. We mapped that section in detail and realized that there was not just one type of stromatolite (as previously thought) but seven distinct types. The paper defines and describes these in detail. Moreover, it became apparent that their distribution and morphological characteristics related to palaeoenvironmental factors such as water depth, rates of sediment influx and hydrothermal activity. We integrated field observations, petrography and trace element geochemistry to constrain palaeoenvironmental conditions laterally and through time—before during and after stromatolite formation. This gave us detailed insights to the conditions that stromatolite-forming organisms favoured (and didn’t favour).

The Strelley Pool Chert reef provides such a complex array of associated factors indicating biology that, when put together, provide the most compelling evidence for life's existence in the Early Archaean so far. We found strong evidence that life had a firm foothold, flourishing as soon as hydrothermal and volcanic activity died down and gave way to a quiet, shallow marine reef environment. Moreover, as soon as hydrothermal and volcanic activity resumed, the stromatolites stop do we know the age of the stromatolites?

The age of the rocks is constrained by radiometric dating of the immediately underlying and overlying rock formations, so we know deposition occurred between two dates – 3.43 and 3.35 billion yrs ago. We know the stromatolites formed during deposition of the rock formation because we can see that they stood high on the seafloor and were draped by overlying sediments.

Can you speculate on the broad implications of the research?

The research findings bear directly upon existing hypotheses for the conditions that gave rise to early life on Earth—in particular, the hypothesis that early life flourished around hydrothermal vents vs. the hypothesis that early life would have favoured relatively “normal”, “non-extreme” conditions. Although life’s actual emergence may have been long before 3.43. Ga, the Strelley Pool Chert reef—as the oldest evidence of life flourishing—is associated with relatively normal marine conditions.

The research findings can be used to inform the search for ancient biosignatures in the rocks of other planets such as Mars. The techniques and approaches can be adapted for Mars exploration, but most importantly, studies such as ours help to identify the types of biosignatures we should look for, in what type of geological setting they are likely to be found, and what observations and experiments must be made in order to test whether a fossil-like structure is in fact biological.

For the story of Abby's discovery, click here

* Abby’s paper ‘Stromatolite reef from the Early Archaean Era of Australia’ is published in the June 8 issue of Nature, pp 714-718.




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