Mystery of Viennese “fool’s gold” fossils solved

Mystery of Viennese “fool’s gold” fossils solved

What’s with those little tubes of pyrite? © NHM Vienna, Mathias Harzhauser

Millimetre-sized tubes made of the golden shimmering material pyrite: Researchers have now uncovered what these structures discovered underground in Vienna are all about: They are therefore fossil evidence of the degradation and intensive production of methane by anaerobic microorganisms. The scientists explain that this played out in the apparently deoxygenated sediments of a large lake that stretched across the region around 11 million years ago.

Research results in the course of construction work: As the international team led by senior author Mathias Harzhauser from the Natural History Museum in Vienna reports, the history of the discovery of the mysterious structures began with excavations for the expansion of the subway network in the Austrian capital. They opened up insights into the geological history of the region long before Vienna came into being. Mighty layers of clay came to light that were deposited in a huge lake about eleven million years ago. According to earlier studies, this so-called Pannon Lake stretched from what is now the Czech Republic to Serbia. Where Vienna is today is its western bank.

Mysterious Findings

Lake Pannon about eleven million years ago. © NHM Vienna, Mathias Harzhauser

In order to get clues about the development of the lake, researchers took drill cores from the sediment material and analyzed them. Numerous fossil shells of mussels and snails provided information about life in Lake Pannon, which dominated the region for more than five million years. But surprisingly, the scientists also found layered structures from around eleven million years ago in which no shells could be found. Instead, however, they found strange tubular structures a few millimeters long.

Harzhauser and his colleagues have now devoted a more detailed study to these mysterious structures. Under the microscope, it was initially shown that the tubes are composed of tiny spheres of the mineral pyrite. The material, also known as fool’s gold because of its apparent similarity to the precious metal gold, is an iron-sulphur compound (FeS2). However, fine tubular structures made of pyrite, such as those found in the drill cores, were previously unknown, say the researchers. They analyzed the enigmatic structures more closely and investigated possible scenarios of their origin.

Microbial causative agents

Scanning electron micrograph of the pyrite globules once produced by sulfur bacteria. © Zhiyong Lin, University of Hamburg

As they report, a plausible explanation finally emerged: The most important aspect is that pyrite can only form under anaerobic conditions, since otherwise the iron would preferentially combine with oxygen. The team therefore concludes that the activity of sulfur bacteria led to the formation of the globules. These bacteria fed on methane in the lake sediment. According to the researchers, their combination into the hollow structures resulted from the spatial circumstances: “The tubular structures probably formed along tiny channels through which the gas penetrated upwards through the mud,” explains first author Zhiyong Lin from the University of Hamburg.

The team concludes that the tubular structures are evidence of sulfate-driven, anaerobic oxidation of methane. The gas itself was previously formed to a significant extent by a different group of microorganisms, the scientists explain. This in turn means that anaerobic conditions had apparently developed in the sediments of the lake at certain times, under which this biocoenosis was able to develop.

The reduction of the strong greenhouse gas at that time could have prevented the already warm climate of the era from tipping over even further, the scientists say. “Eleven million years ago, the microorganisms inhibited development by eating up the methane,” says Harzhauser. However, this only worked because the sediment of the lake was apparently completely devoid of oxygen at the time. This is also the explanation for the absence of the mussel fossils in the layers with the pyrite tubes: the paradise of microbial methane producers and eaters was a dead zone for many other organisms.

Source: Natural History Museum Vienna, specialist article: Communications Earth & Environment, doi: 10.1038/s43247-023-00879-2

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