A new microbial ecosystem existed in bubbles of 400 million year old volcanic rocks and may well be found in three billion year old rocks: That is the gist of an article by Prof. Jörn Peckmann of the MARUM - Center for Marine Environmental Sciences at the University of Bremen. The article will be published in the march issue of the journal Geobiology.
Peckmann and his team found mineral deposits in former cavities of pillow basalts that are best explained by the theory that microbes lived deep within the rocks. Their fossils were found in rocks formed from lava that flowed out of the sea floor. Now they are part of the Rheinische Schiefergebirge, about 50 kilometers west of the German town of Kassel.
"Microbes are hardy creatures, they thrive where nobody would even look for life", says geologist Jörn Peckmann, professor at the MARUM - Center for Marine Environmental Sciences at the University of Bremen. "One of these places is in the middle of a rock." Because the microbes lived inside the rock, their fossils are well protected against the ravages of time. "And since they present a possibly very ancient ecosystem, it might be possible to find specimen considerably older than three billion years", hopes Jörn Peckmann.
But how do microbes get inside the rock? When lava flows out of an underwater volcano it is cooled quickly and forms typical shapes, so called pillow basalt. "This lava often contains large amounts of gas, so that the resultant rocks are full of bubbles", explains the geologist. "Through fine pores and cracks sea water circulates in the bubbles of diameters of usually less than a few millimeters." The water transports dissolved minerals, which get deposited on the walls. On these deposits grow microbes, which also travelled with the pore water. With age this porosity lessens because all the bubbles, pores and cracks are filled by the minerals transported by the pore water.
The thin sections of rocks analysed by the team clearly show thin regular strands of three to eight microns wide, encrusted by mineral deposits. Living microbes today show similar deposits. A clear indication that the structures found in the basaltic rocks were indeed formed by living organisms. Further evidence comes from the size and form of the structures which is consistent with microbial activity. The fact that the structures in question occur inside the bubbles of the intact rock also strengthens Peckmanns theory. Would the structures occur along fracture planes or crystal borders they could be explained by physical processes that often occur there.
"The most astonishing thing is, that we found these inclusions not only in the Rheinische Schiefergebirge in Hesse, but also in similarly aged pillow basalts in Thuringia and Bavaria. It seems as if these organisms were quite common", stresses the geologist from Bremen. What the microbes did in the rocks and what nourished them is still a riddle for the MARUM-team. Iron is one of the possible energy sources discussed. "Our further steps are to investigate whether this form of life was common and whether it still exists today." A question that is not irrelevant. "We think these organisms were quite common. If that is the case, they might have had a great influence on the chemistry of the basaltic rocks at the sea floor and therefore on sea water chemistry." And this in turn would have influenced the conditions for other organisms. "This would mean that we have to rewrite some explanations of how and why life in the oceans changed."
Further information, interviews, images:Kirsten Achenbach
Kirsten Achenbach | idw
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