How Microorganisms Helped To Make Mountains

This underwater “column” is actually a natural formation. (Credit: University of Athens)

Both the insignificant and the extraordinary are the architects of the natural world.

Carl Sagan

Research published recently has shown that what were thought to be the columns of a sunken city found in the sea of the Greek island of Zakynthos were actually made by microbial activity, not ancient humans. It turns out that microorganisms living around natural occurring methane leaks force the precipitation of minerals like dolomite and pyrite in the sediment. Then the resulting mineral-concretions, once eroded out from the ground, look like pipes and can be mistaken for columns.

Microorganisms are one of the architects of the natural world – able, despite their minute size, to “build” massive mountains. Dolostone is a common variety of limestone, characterized by its high content of calcium and magnesium. Calcium and magnesium are commonly found in seawater, however in experiments dolomite is chemically very hard to precipitate directly from water. U.S. geologist James Dwight Dana studied the reefs and atolls in the South Pacific and discovered that dolomite can be found in the local sediments. This was an important observation, as it demonstrated that dolomite can form directly from tropical seawater.

However, Dana couldn’t explain the mechanism of dolomite formation, nor could he explain why experiments failed to replicate the naturally occurring precipitation. One possible answer for this problem came from the study of a characteristic rock-formation in the Dolomites – the appropriately-named Hauptdolomit, the “main dolostone” formation. During the Triassic (some 216.5 – 203.6 million years ago) the area of the future Dolomites was situated in the Tethys Sea. In the shallow sea, a large carbonate platform developed, like what’s today found in the Bahamas.

The Hauptdolomit – formation forms characteristic steep cliffs, here the Sass dla Crusc (Hl. Kreuz Kofel), 2.907m (with some locals). The well layered structure demonstrates that these rocks were deposited in a shallow lagoon, photo by the author.

The lagoons and muddy flats of this carbonate platform were colonized by algae and bacteria for millions of years, forming a 1,000 meter (almost 3,300 feet) thick succession of dolomite rock. The area had a species-poor faunal community of invertebrates, dominated by gastropods and bivalves. Sometimes dinosaurs crossed the tidal flat. Their tracks have been preserved in some locations of the Dolomites – a clue that there were islands large enough to sustain such large vertebrates.

The top of the Hauptdolomit is characterized by the development of fossil soils, reflecting a major sea-level fall. The extreme shallow water conditions continued uninterrupted for millions of years and led to deposition of an up to 1,000 meter thick succession of dolomite rock. This reconstruction seemed to fit the observations of naturalists of modern reefs, atolls and carbonate platforms – there was only one problem: only very limited formations of dolomite have been observed in such an environment today.

In the modern sea, only aragonite and calcite are stable minerals and therefore can form directly by precipitaton from the water. Dolomite forms only locally, in pools of warm and salty water. The inorganic formation of dolomite alone is too inefficient to explain the importance of dolomitic rocks in the stratigraphic record.

However, (micro-)organisms can significantly increase the precipitation of dolomite from seawater. In the early 20th century scientists started to experiment with microorganisms and sedimentation. The Russian microbiologist Georgi A. Nadson observed the nucleation of dolomite in cultures of bacteria and published his observation in a paper entitled “Microorganisms as geological factor” (1903). Despite these promising results, the difficulties in observing bacteria and the formation of the crystals prevented further research and the idea faded into obscurity for decades.

New impulses to this idea were provided by the discovery of microbial mats in coastal lagoons along the shores of Brazil. Today, we know that many modern tidal flats are covered by a community of algae and bacteria. These organisms secrete a sort of mucus (extracellular polymeric substances – EPS), which acts as sediment trap and provides favorable conditions for some minerals, forming laminated microbial mats.

Microbial mats in dolostone, photo by the author.

In the Hauptdolomit-formation, the fossil remains of similar mats can be found. Bacterial activity therefore seems to be an important factor to explain the deposition of dolomite rock.

There are two main ways in which organisms can contribute to the formation of minerals. Biotically-controlled precipitation happens when an organisms controls the extent, the kind and the rate of mineral formation, for example to form shells or skeletal elements. Biotically-induced precipitation occurs indirectly, as the presence of organic matter or byproducts of the metabolism (like the EPS) of a life form causes chemical reactions and favorable conditions for the formation of minerals.

A recently published study (KRAUSE et al. 2012) expanded the range of dolostone formation significantly, showing that in deep sea sediments there are bacteria that can induce the precipitation of dolomite from seawater. Despite these insights, many questions remain unanswered. Why are there phases in earth history when dolostone formation was so common? Why not today, despite microbial activity? Were these phases controlled by the evolution of the microbial communities or changes in the seawater chemistry? If so, what caused these changes in the oceans of the past?