A team of researchers has found dolomite crystals where there shouldn’t have been any.
Until recently, researchers who found the mineral dolomite in sea and lake sediments had assumed that it was formed mainly at high salt contents and high temperatures between 20 and 40 degrees Celsius. Findings of the mineral in sedimentary boreholes were therefore interpreted as a sign of a corresponding dry and hot climate in the past.
Geologists from the Ruhr-Universität Bochum (RUB) in Germany have now surprisingly found dolomite in relatively young samples from the Vanseeboden, at times of a lower salt content and a constantly low water temperature of three degrees Celsius. The researchers therefore believe that environmental reconstructions based on dolomite finds should be revised, as reported in the journal Geophysical Research Letters.
The Dolomite Puzzle
Dolomite has been a geological mystery since its discovery in 1791. The mineral is often found in old rocks. The largest dolomite deposits date from more than 542 million years ago and indicate a marine origin. Whether the mineral precipitated directly from the water or during post-sedimentary processes remains controversial.
Nowadays, it occurs only in comparatively small quantities and predominantly under very special conditions in very warm and salty water such as in lagoons and shallow salty lakes. Many laboratory experiments to experimentally cultivate the mineral have failed. “The spectrum of conditions required for dolomite formation is therefore based on observations and assumptions that are not always easy to verify,” says author Jeremy McCormack.
McCormack and Prof. Dr. Ola Kwiecien from the Department of Sediment and Isotope Geology were surprised when they analysed sediment cores from the Vanseeboden in Turkey: they were able to detect dolomite crystals in relatively young sediments using X-ray diffraction and scanning electron microscopy.
“The sediments are only up to 150,000 years old, which is not much on the geological time scale,” explains Jeremy McCormack. “They weren’t even lithified, that is, not petrified, but still wet mud.”
Cold, low-salt water
The dolomite crystals were located in the layers of sediments that had deposited at times when the lake had a high water level. A high water level is accompanied by a lower salt content. Moreover, the water in the depth of the lake was constantly cold, around three degrees Celsius.
“This means that many of the environmental conditions that have been considered essential for dolomite precipitation cannot be that important,” explains Ola Kwiecien. “Reconstructions of the climate and environment of the past based on the presence of dolomite must therefore be revised.”
Dolomite may have been overlooked
The researchers also suspect that younger dolomite deposits may not be as rare as previously assumed. They may have been overlooked in environments where they were not expected.
The examined sediments originate from the soil under Lake Van in Turkey. They were obtained in 2010 from a floating drilling platform and were very well investigated and dated for various environmental indicators.
“Therefore, we can time the dolomite occurrences. By comparing it with previously measured climate and hydrological data, we can also determine which conditions are actually critical for dolomite precipitation and which are not,” explains Ola Kwiecien.