“Both the insignificant and the extraordinary are the architects of the natural world.”
Carl Sagan in “Cosmos – Heaven & Hell”
After geologists could finally answer how the spectacular peaks of the Dolomites formed, the next urgent questions was if the dolomite rock (or dolostone) was a primary product of marine deposition or a secondary product of alteration of common limestone. One of the first explanations was formulated by the Italian mining engineer Giovanni Arduino in 1779 – who imaged that common limestone (also found in the region of the Dolomites) reacted with the dissolved chemicals in hydrothermal solutions to form dolostone. A hypothesis still popular during the 19th century, as volcanic dikes can in fact be found in some (but unfortunately not all) outcrops with dolomite rock. An alternative hypothesis assumed that percolating groundwater, saturated with magnesium, reacted with the primarily deposited limestone. However this secondary modification of limestone alone could not explain the large amounts of dolomite rock found in the Dolomites and other regions of the world.
Fig.1. “Esquisse d´une carte geologique de la parte meridionale du Trentino” (1822), by geologist Leopold von Buch, showing the distribution of dolostone (dark blue – No.IV) and limestone (light blue – No.V) in the Dolomites (image in public domain).
The U.S. geologist James Dwight Dana (1813-1895) studied the reefs and atolls in the South Pacific and discovered that dolomite can be found in the sediments of the lagoon of uplifted atolls. This was an important observation, as it demonstrated that dolomite forms from the water of the sea and under “low” tropical temperatures. Could therefore the rocks, composing the Dolomites, be formed in the past in a similar environment and depositional setting?
A possible answer for this problem came from the study of a characteristic rock-formation in the Dolomites – the appropriately denominated Hauptdolomit, the “main dolostone” – Formation, was defined in the Bavarian Alps by geologist VON GUEMBEL 1857 and introduced in the stratigraphic nomenclature of the Alps in 1876 by geologist LEPSIUS. The Hauptdolomit Formation is found in the Dolomites, as well as in a very similar development in the Northern Calcareous Alps, the Apennines, Dinarides and Sicily.
Fig.2. 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, with almost no transport of the former soft sediments by waves or currents.
During the Upper Carnian and the Norian stage (geological time intervals of the Triassic, some 216,5 – 203,6 million years ago) the area of the future Dolomites and the other mountain ranges was situated on the shelf area of the Tethys Sea. In the shallow sea a large carbonate platform developed. The lagoons and muddy flats of the Triassic carbonate platform were colonized by algae, bacteria and 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 Hauptdolomite 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: no or only a limited formation of dolomite is today observed in such an environment. 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 (1867-1940) 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 for decades into obscurity. New impulses 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. Also in the Hauptdolomit-formation the fossil remains of similar mats can be found. Bacterial activity seems to be therefore 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. The 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 significantly the range of dolostone formation, showing that also in deep sea sediments there are bacteria that can induce the precipitation of dolomite from seawater.
Despite these insights, still 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?
Fig.4. The Dachstein- formation is a geological unit overlying the Hauptdolomit, despite the appearance and probably similar depositional setting it is formed of common limestone; 200 million years ago the precipitation of dolomite suddenly ceased – the cause is still unknown.
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