The role of microorganisms in rock formation

The way in which microorganisms affect rock-forming processes is one of the most vital geological questions. Bacteria activate crystallization and sedimentation of many minerals,¹ and sediments, formed through weathering, have been reported in structures dating back to the Archean period. In all geological settings, weathering profiles represent reliable evidence of continental regimes, while in Precambrian structures they often provide unique information about continental features that may have led to the formation of life.

The existence of Early Precambrian cratons (old and stable parts of the continental crust) on land has been suggested solely on the basis of elemental and isotopic correlations of carbon, hydrogen, nitrogen, and phosphorous in rock. The presence of microbial material on land as early as 2.7–2.6 billion years ago has thus been affirmed.² Yet, fossilized remains of Precambrian land microorganisms have not yet been discovered.

Our study focuses on paleoweathering profiles in the Fenno-Scandinavian shield (Karelian) with an age of 2.4–2.1 billion years. We have found remains of a diverse range of fossil microorganisms,³ including filaments or threads (see Figure 1), coccoidal forms (globules with diameters of up to 5μm: see Figure 2), rather large (diameter >10μm) spherical forms, and fossil biofilms. Rocks often consist almost completely of destroyed cocci, dumbbell-like forms, and shreds of filaments or threads (see Figure 3).

Figure 1. Filamental microfossils. (left) Rock consisting of filament structures. (right) Single filament resembling a crumpled cyanobacteria Microcoleus envelope.⁴

Figure 2. Coccoidal form with uneven surface. Numerous traces of globules resembling cocci can be seen.

Figure 3. Fragments of rock containing remains of fossil microorganism.

In all cases, the chemical composition of microfossils is identical to that of the rock matrix, including the main rock-forming oxides of silicon, aluminum, iron, potassium, and magnesium. This provides indirect confirmation that the microbiological complex is coeval with the host rock. It is very likely that the microorganisms contained in the rock acted as catalysts. Decay (decomposition) of minerals and their transformation into clay-like material occurred through bacterial participation. The weathered Early Precambrian crusts, for example, were perhaps formed through interactions between the peculiar specific composition of these microorganisms and the conditions prevailing at the time of hypergene (secondary) transformations.

We have shown that a morphological diversity of bacterial life forms existed as early as the Early Precambrian period. Bacterial finds in objects of hypergene origin testify to their exogenous nature. Early Precambrian microorganisms, bacteria, and perhaps cyanobacteria (blue-green bacteria that obtain their energy through photosynthesis) and even eukaryots (organisms whose cells contain complex structures enclosed within membranes) accompanied and promoted the formation of weathering crusts. We, therefore, suggest that colonization of land by microbes happened at this time and that weathering crusts (paleosoles) eventually (and directly) generated real soils.

As our next step, we will study one of the most ancient weathering crusts of the Fenno-Scandinavian shield (2.8 billion years old) and take the first steps towards understanding the role of some lanthanoids and rare-earth metals in the ancient fossilization of microorganisms and ancient weathering-crust formation.

This study was carried out within the framework of the program of basic research of the Presidium of the Russian Academy of Sciences, ‘Origin of Biosphere and Evolution of Geobiological Systems’ (subprogram II). It was supported by the Russian Foundation for Basic Research through projects 08-04-00484 and SS-4207.2008.5.