Stromatolites of the Strelley Pool Formation — Crystal World Australia

The Strelley Pool Formation is a sub-section of depositional environments occurring across 11 greenstone belts and spanning 30,000 square kilometers in the east of the Pilbara Craton, Western Australia¹. A craton is a portion of a continent, in this case an Archean age sub-continent, of geological stability. The Strelley Pool Formation is located between two volcanic units and is comprised predominantly of sedimentary rocks. It is now widely considered to have been a shallow marine environment within the craton². This interpretation of the palaeoenvironment of the Strelley Pool is relatively recent and it had previously been interpreted as a beach environment³.

Although material from the Strelley Pool Formation is commonly referred to as Strelley Pool Chert, this is a misleading simplification. There are four geological members that comprise it⁴. The oldest of these overlays the volcanic rock characteristic of the majority of the Pilbara Craton, dating to 3.43 billion years. The youngest of these is overlain by basalts dating to 3.35 billion years. The Strelley Pool Formation was formed continuously over the intervening 80~ million years⁴. In order from oldest to youngest, the four members are:

1. Basal silicified sandstone and conglomerate, overlaying the volcanic material that forms the surface of the Craton.
2. Silicified cherty carbonate containing laminated flat stromatolites, conical stromatolies and silicified evaporates.
3. A straitified unit of laminated chert, stromatolite structures, silicified evaporite and a layer of evaporitic by-products.
4. Upper clastics composed of coarse conglomerate and breccia.

The oldest 3 members contain some of the most remarkable fossilised evidence of Archean life in the geological record⁵. However, as with all proposed microfossils, claims as to whether they are truly biological in origin rather than formed purely by geological and geochemical processes are fraught with controversy. There is often fierce debate regarding the description of any proposed microbial fossil as to whether it is a genuine stromatolite or a pseudofossil and the Strelley Pool locality is no exception^1,2,6,7. A compelling claim for evidence of early life cannot merely rely on morphological evidence⁸. Professor Martin Brasier outlines 3 mutually supporting lines of evidence to reject a non-biological origin for any candidate structure⁸:

1. A well constrained age and geological context supporting biological origin.
2. Evidence for morphology unique to or highly indicative of biology.
3. Geochemical evidence of metabolic cycling.

Over the past 2 decades the evidence that many of the structures contained within the Strelley Pool Formation are genuine stromatolites has become overwhelming. Presented here are the arguments against a biological origin as well as the most recent research in support of it.

An Abiological Interpretation

Donald R. Lowe is one of the pivotal early researchers of the Strelley Pool Formation and he can be credited with bringing the formation to prominence with his research in 1980. He originally interpreted the laminae distributed widely throughout the formation as being the result of microbial-mediated evaporation (that is, as stromatolite structures)⁹. However he quickly, and controversially, walked back this claim leading to a decades long debate about the biological origin (or lackthereof) of the structures found in the Strelley Pool Formation⁶. His arguments are summarised in a 1994 publication, at which point the structures found in the area were small conical structures interspersed by a predominance of flat carbonaceous laminations⁶. Lowe correctly pointed out that, though these structures can form through microbial mat activity, they can also form through non-biological processes such as depositions from currents and cyclical sedimentation, both of which are consistent with the environment the Strelley Pool Formation was, at the time, thought to have been formed in^3,6.  

The conical structures provided a problem for Lowe and were what lead to his initial interpretation that these were indeed the product of microbial activity⁹. Three dimensional, vertical structures potentially suggest bacterial motility and even phototaxis (movement towards or away from light)¹⁰. Phototaxis implies photoautrophy (the ability to convert light into energy in conjunction with the metabolism of carbonaceous material the bacteria inhabited)¹⁰. This is seen in modern, living stromatolites. However, Lowe correctly points out that the behaviour found in living stromatolites cannot be used to infer the behaviour of microbial communities of the past without further evidence³. The main cause of Lowe’s reversal of opinion is that the three dimensional conical structures were, at the time, a rarity and the laminations were largely extremely fine and smooth even over distances of meters⁶. They lacked the wavy or crinkly characteristics of most stromatolitic laminations⁶. Additionally, similar conical structures in the geological record and in carbonate sequences have been shown to be formed by the evaporation of water without biological assistance⁶. He also noted a lack of evidence of material worked into surrounding sediment, which results from microbes cementing detritus and rock grains together⁶. He found no evidence of gas-filled cavities or the tell-tale signs of decay such as sulfuric signatures that one would expect from microbial metabolism⁶.

The Discovery of the Trendall Locality

Based on the evidence at hand, Lowe was right to proceed with caution in interpreting the Strelley Pool Formation as containing evidence of microbial mat activity. However, his sample size was limited. A section of the formation in the Panorama Greenstone Belt, discovered by A. F. Trendall in 1984 and referred to now as the Trendall Locality, was examined in detail in 1999 by H. J. Hofmann¹. Far larger conical and branched columnar structures, with far greater morphological diversity than any previously described at the formation, shed new light on the debate¹. The co-existence of columnar forms with conical forms suggested that they were likely caused by biologically mediated processes as the abiological processes required to create both forms occurring together, in such great numbers and density, were too complex to be considered seriously¹. Additionally, many of the forms resembled widely accepted stromatolites such as Jacuphyton¹.

Martin van Kranendonk continued Hofmann’s research in 2003, discovering geochemical evidence for an anoxic, shallow marine depositional environment inconsistent with Lowe’s proposed hypothesis of an abiological hydrothermal formation². Van Kranendonk also discovered evidence of detrital cementation in the form of thin, flat pebbles and sand-sized carbonate sediment, trapped and bound by the proposed microbial mats^2,7. The absence of such material formed one of Lowe’s main objections to a biological origin for the Strelley Pool stromatolitic structures⁶.

Seven Different Morphotypes Discovered

In a series of papers published between 2006 and 2009, Abigail Allwood and colleagues described seven distinct stromatolite morphologies occurring over several kilometers of outcrop around and along the Trendall Locality^4,10,11. The changes in morphology are inferred to be related to changes in sediment structure and water depth¹¹. Among the morphologies identified were an abundance of large conical structures very similar to those described by Hoffmann in 1999 and discussed above⁴.

Allwood presented 3 arguments to support the claim that the seven distinct structures were indeed created with microbial assistance¹⁰.
1. The conical and columnar structures indicate a preference for vertical growth that is most plausibly explained by upward migrating microbial colonies, building the structures towards the sediment-water interface.
2. The texture of the laminae within the cones and the areas between the cones show significant differences. Intercone areas bare the hallmarks of mechanical (abiological) deposition whereas the cone lamine display signs of chemical deposition typical of microbial mat activity.
3. There is a significant enrichment of rare-earth-elemnts in the carbonate laminae. These rare-earth-elements occur as a result of the microbial trapping and binding of grains in a marine environment during stromatolite accretion. Although cementation of detritus can be produced abiologically, it has never been observed to do so to this extent, in such a concentrated area.

The perceived scarcity of stromatolite-like structures and the lack of diversity that lead Lowe to dismiss a biological interpretation was completely overturned by Allwood’s research. Her great contribution to this field of study was to examine in detail the structures over an area spanning several kilometers. As such, she discovered not only many previously undescribed stromatolite morphologies from the area, but an abundance of them¹⁰.

In classifying a structure as a stromatolite, it is important to establish that microbial activity is partially responsible for building the structure rather than microbial activity influencing the structure after its construction⁵. All seven of the structures described by Allwood show significant evidence of being mediated by microbial colonies during their construction. 6 of these occur within a layer of carbonate and chert ranging between 10 and 20 metres thick (the second of the geological members described above)⁴. The 7^th, an iron-rich laminae structure of domal shape, does not form the focus of Allwood’s research thus far and formed during a geological period after the 6 described below (in the third of the geological members), but we mention it for completeness¹⁰.

1. Large Complex Cones – These are found in areas of the Strelley Pool Formation extending beyond the Trendall Locality and were, prior to Allwood’s work, considered not just the only stromatolitic morphology in the area but also to be rare and, as such, potentially abiological. They bare a striking resemblance to modern conical stromatolites, which are formed as previously mentioned by upward-migrating microbial colonies at the sediment-water interface. The trace chemical signatures Allwood detected in these structures are only known to occur in biologically mediated sediments and there is no abiological analogue either in nature or experimentally⁴.

2. Encrusting/Domical Laminates – These display a granular sediment most likely due to the trapping and binding of grains by microbial mats. This activity is known in many modern and widely-acknowledged fossil stromatolite taxa such as Colonnella and Conusella⁴.

3. Wavy Laminates – Superficially, these forms are the closest of the seven in resemblance to naturally occurring phenomena. They appear as climbing ripples, typical of sediments in medium-energy shallow marine environments. However, close inspection reveals that the laminae have conical shapes and steep slopes indicative of microbial influence. They too have analogues in the stromatolite taxon Irregularia⁴.

4. Small-Crested/Conical Laminates – These are the most widely observed structures of the Strelley Pool Chert and have lead to the vast majority of debate. Their fine laminations are morphologically similar to laminations that occur from natural sediment deposition. However, microscopic and chemical analysis shows evidence of evaporative activity likely caused by microbial metabolic activity⁴.

5. Egg-Carton Laminates – As with the Large Complex Cone morphotypes, these too are similar to modern conical stromatolites and are morphologically similar to the Large Complex Cones albeit smaller, more rounded and less steep. Their formation is likely to be caused by the same mechanism as the Large Complex Cone morphotypes although in an environment of different energy levels and available materials. They are analogous to the stromatolite taxon Conophyton⁴.

6. Cuspate Swales – Of all seven morphotypes, these are the most problematic for an abiological interpretation. They are highly complex structures with crested ridged that have further concave-sided conical structures. They also display smaller cones often adorned with crests. They occur in high density over scales of many metres. To interpret these as abiological, a highly improbable combination of physical and chemical processes would have to interplay and be maintained across a time period of many millions of years allowing for the deposition of scores of similar structures. Further, their complex morphology is found in a known stromatolite taxon, Thesaurus, as well as other known taxa. Chemical and sedimentological analysis shows that these structures were formed in soft sediment, rather than rigid crystalline crusts, making their maintenance and widespread distribution even more unlikely if explained by natural, abiological processes. It is most parsimonious to assume a biological original for these structures⁴.

Allwood’s detailed, unprecedented work in the area resolves most of the objections to a biological intepretation of the Strelley Pool stromatolites. If the carbonate of the Strelley Pool is abiological in origin, processes leading to seven different, widespread morphological structures, each with analogues in known stromatolites, must have operated persistently over a period of tens of millions of years, sometimes occurring simultaneously with each other in the same geographic location⁴. Additionally, once the sedimentology of the locality shows evidence of increased water depth, evidence of stromatolitic structures disappears¹¹. This is consistent with the interpretation of these microbes as phototrophic. Increased water depths limit the light that could reach these organisms, thus eliminating their source of energy and explaining their disappearance.

Allwood ends one of her papers with a well earned jab at those that persist with an abiological interpretation – “Perhaps in this instance, abiogenesis is the ‘extraordinary claim’ that requires extraordinary proof” ⁴.

Glossary

Abiological – formed without the involvement of any organism.
Archean Age – the period of Earth’s history beginning 4 billion years ago and ending 2.5 billion years ago.
Chert – a fine-grained composed of small quartz crystals.
Laminae – layered structures within a rock that appear as straight or wavy lines in cross-section.
Morphotype – a fossil or structure characterised by distinct morphological characteristics that separate them from other fossils or structures.
Sedimentary Rock – rock that is built by the layering of sediment over time, as opposed to igneous rocks formed by the cooling of magma or lava.
Stromatolite – calcareous structures built, at least in part, by cyanobacteria.

References and Further Reading

1. Hofmann, H. J. (2000). Archean stromatolites as microbial archives. In Microbial sediments (pp. 315-327). Springer, Berlin, Heidelberg.
2. Van Kranendonk, M. J., Webb, G. E., & Kamber, B. S. (2003). Geological and trace element evidence for a marine sedimentary environment of deposition and biogenicity of 3.45 Ga stromatolitic carbonates in the Pilbara Craton, and support for a reducing Archaean ocean. Geobiology, 1(2), 91-108.
3. Lowe DR (1983) Restricted shallow-water sedimentation of early Archean stromatolitic and evaporitic strata of the Strelley Pool Chert, Pilbara Block, Western Australia. Precambrian Res 19:239–283
4. Allwood AC, Walter MR, Kamber BS, Marshall CP, Burch IW (2006) Stromatolite reef from the early Archaean era of Australia. Nature 441:714–718
5. Wacey D (2010) Stromatolites in the ~3400 Ma Strelley Pool Formation, Western Australia: examining biogenicity from the macro- to nano-scale. Astrobiology 10:381–395
6. Lowe DR (1994) Abiological origin of described stromatolites older than 3.2 Ga. Geology 22:387–390
7. Van Kranendonk MJ (2011) Morphology as an indicator of biogenicity for 3.5–3.2 Ga fossil stromatolites from the Pilbara Craton, Western Australia. In: Reitner J et al (eds) Advances in stromatolite geobiology, vol 131, Lecture notes in earth sciences. Springer, Germany, pp 537–554
8. Brasier, M., McLoughlin, N., Green, O., & Wacey, D. (2006). A fresh look at the fossil evidence for early Archaean cellular life. Philosophical Transactions of the Royal Society B: Biological Sciences, 361(1470), 887-902.
9. Lowe DR (1980) Stromatolites 3,400-Myr old from the Archean of Western Australia. Nature 284:441–443
10. Allwood AC, Walter MR, Burch IW, Kamber BS (2007) 3.43 billion-year-old stromatolites reef from the Pilbara Craton of Western Australia: ecosystem-scale insights to early life on Earth. Precambrian Res 158:198–227
11. Allwood AC, Grotzinger JP, Knoll AH, Burch IW, Anderson MS, Coleman ML, Kanik I (2009) Controls on development and diversity of early Archean stromatolites. Proc Natl Acad Sci 106:9548–9555