A 3.4 billion-year-old snapshot – Fossil Friday

There are amazing things to learn in the world of palaentology and archaeology and then there is this. Things that just blow it off. Indeed, in this week’s Fossil Friday we’ll be discussing about a 3.4 billion-year-old fossil. One that dates back to the times when Life appeared on Earth. And we’re not talking about some old finding, no, no, we’re talking about a discovery that was published this week.

photograph of fossil microbial mat

Photograph of the facies of the studied rock formation. On the close up on the right, thin sheets (laminations) can be observered (taken from Duda et al.).

These precise fossils are called stromatoliths, they come from Australia and are in a rock formation dated between 3.43 to 3.35 billion years old. Stromatoliths (a.k.a stromatolites) are a type of layered rock formation that can have a biological origin. In this case, they are formed by a slow accumulation of carbonate material as well as other sedimentary grains that are then cemented by microbial mats (colonies of bacteria). Stromatoliths are fairly common in the fossil record especially in the very early geological strata. The usual problem with fossil stromatoliths is that it is complicated to tell whether they are the results of bacterial activity or if they are a result of abiotic (= non-living) deposition – in other words, sediments could have been deposited this way just by chance. But for this precise example, the authors are pretty certain these fossils are the results of the existence of photosynthetic organisms.

proof of carbonate colocalised with organic matter, current microbial mat producing a stromatolite

Left: calcium carbonate (bright red), one of the base material of stromatolites, in association with organic matter (black) (Taken from Duda et al.). Right: current microbial mats producing stromatolites (Australia).

There are multiple lines of evidence that the authors have used to prove their fossils have been made by primitive bacteria. One of them is the finding of calcium carbonate (CaCO3) in very close association with organic material (see photo above). Calcium carbonate is a chemical compound that is one of the main constituent of the rocks seen in the first banner. The fact that calcium carbonate is associated with the organic material strongly suggests it has a biological origin. The scientists believe that these fossils were structures that used to be situated in a shallow marine environment, where the bacteria could photosynthesise – they were not too deep in the water – but could still be protected from UV radiations by their calcareous shell. While we do not really know what these structures would have looked like 3.4 billion years ago, it is probable that they looked similar to today’s ones. The second photo above is an example of stromaliths being formed by living microbial mats. Hence why this fossil is a snapshot of long gone ages.

Let’s explore the past, interrogate the present and dream the future.
Adrien for Sci&Fi, over and out…

Duda, J-P et al. (2016) A Rare Glimpse of Paleoarchean Life: Geobiology of an Exceptionally Preserved Microbial Mat Facies from the 3.4 Ga Strelley Pool Formation, Western Australia. PlosOne.

Visual credits:
Banner 1: figures 3B and 3C from the article.
Banner 2: figures 7A and 7D and Stromatolites in Sharkbay via Wikimedia.

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