Microbial Mats in Archaen

Palaeontology: Modern life in ancient mats

Michael M. Tice1
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Abstract

Microbial communities seem to have inhabited tidal sediments 2.9 billion years ago much as they do today — but what organisms were involved, and how they made their living, remain intriguing questions.

Establishing from fossil records how microbes and microbial ecosystems evolved is not an easy task: although 'microfossils' have been used to infer the presence and identity of microbes in particular environments1, 2, their simple shapes and comparative rarity limit what they can tell us. An alternative approach is to look for traces of products from communities of microorganisms3, 4. Research by Noffke et al.5, reported in Geobiology, illustrates this possibility. These authors have discovered evidence of 'microbial mats' in 2.9-billion-year-old sedimentary rocks from South Africa — a find that significantly augments the record of such structures from the Archaean eon, which ended 2.5 billion years ago.

Microbial mats are communities of microorganisms that grow in or on otherwise loose sediments, giving their substrate cohesiveness and tensile strength. Their consolidating effect means that they can produce a trace fossil record in sandstones and mudstones — even when no organic matter or microfossils are preserved. Structures that owe their existence to the stabilizing influence of mats can thus be important markers of ancient microbial ecosystems that would otherwise remain undetected.

Noffke et al.5 describe sandstones of the Sinqueni Formation, part of the Pongola Supergroup of eastern South Africa. These rocks were formed from sandy sediments deposited in a tidal environment 2.9 billion years ago, but several features preserved in them demonstrate a cohesiveness not seen in unconsolidated sand. Three structures seem to point particularly conclusively to an overlying microbial mat (Fig. 1). First, anomalously coherent, deformed chunks of sandy bed would have originated as chips of mat ripped up by energetic tidal currents and subsequently redeposited (Fig. 1a). Second, overfolded chips of rock (clasts) indicate where pieces of mat were rolled over on themselves, unexpected behaviour for layers of loosely associated sand grains (Fig. 1b). Third, oscillation cracks are present; these features would have been formed above the normal tidal range when pockets of gas periodically accumulated under, and escaped from, the mats (Fig. 1c). Such processes caused the mats' surfaces to expand and contract, forming cracked beds with upturned edges.

Just an fyi.

Oh, and bacterial/microbial mats are hardly strictly in the past. Ward strongly implies as much in The Medea Hypothesis. Next post on that relatively soon.