In addition to the action of carbonic acid, which slowly eats away at the limestone rocks, some caves are shaped by the presence of a much more powerful acid: sulfuric acid. A new study explains the origin of this acid in the case of the vast cavities of the French Pyrenees.
Massif limestonelimestone Many underground networks and cavities are often crossed, dug over time by the action of water. Water… but not only. Because the caves are indeed the result of the chemical attack of the limestone rocks by the rainwater slightly loaded with carbonic acid. And it is the latter that will nibble the rock, slowly but surely, until it produces huge underground cavities. During his crossing of theatmosphereatmosphere and the superficial layers of the ground, in particular the vegetated layer, the rainwater is indeed charged by gazgaz carbonic. And a single chemical equationchemical equation allows to show that H2O + CO2 gives H2CO3in other words thecarbonic acidcarbonic acid.
The caves of the Pyrenees corroded by sulfuric acid
By seeping into the cracks of the limestone massifs, this acidic water will gently dissolve the rock, enlarging the fractures until they form chasmschasms and caves in which underground rivers will flow, also participating in the expansion of the network. Incidentally, the limestone dissolved in the infiltration water could be redeposited on the walls of the cavities in the form of calcitecalcitea process leading to the formation of concretions such as stalagmites, stalactites or other draperies.
So much for the generality. Let’s move on to the particular case. Because some caves do not work quite the same. However, the principle remains the same: rainwater loaded with carbonic acid infiltrates through the cracks in the rock mass. But it can happen that another acid, much more aggressive, comes into play: it issulfuric acidsulfuric acid (H2SO4). This will dissolve the rock 10 to 100 times faster than carbonic acid, producing very large cavities much more quickly, like those found in the French Pyrenees. The analysis of some mineralsminerals present in these immense caves indeed shows the presence of sulphidessulphides. This is the case of mirabilite, a strange mineralmineral flower-shaped forming long white fibrous crystals.
The question of the origin of sulfuric acid
But where does this sulfuric acid come from? This is a question that has plagued scientists for a long time. Because there can be two types of sources: one results from thermochemical reactions between water and rocks, and the other… from the activity of certain bacteria. Indeed, some bacteriabacteria use the sulphates present in the minerals constituting the walls of the caves as a source ofenergyenergy. In return, they produce Hydrogen sulfideHydrogen sulfide (H2S), which will then oxidize to form sulfuric acid.
To understand the formation of these caves, it is therefore important to define which process, biotic or abioticabiotic, is at work for the production of sulfuric acid. In a new study, published in Geologya team of French scientists therefore took an interest in the quantificationquantification some issues isotopesisotopes of sulfursulfur in the concretions of two caves in the Pyrenees: the Nébélé chasm and the Azaleguy cave.
Sulfur has indeed four different isotopes, which are not present in the same quantities depending on the sources. They are therefore good markers for defining the origin of sulfuric acid. The data collected by Dimitri Laurent and his colleagues reveal that in the two caves studied, the sulfuric acid would have a rather complex and multiple origin! A part would indeed come from the presence of fossil hydrogen sulphide trapped for 65 million years within the carbonates jurassicjurassic in which the caves developed. ” Au CretaceousCretaceous, the Pyrenees are experiencing a phase of crustal hyper-extension, associated with a rise in temperatures at depth. Under the effect of heatheatold levels of evaporitic rocks, sulphates, then react with the hydrocarbonshydrocarbons trapped in the rocks (notably the CH4) to produce hydrogen sulfide. This gas will then be trapped in the surrounding carbonate rocks dating from the Jurassic “, explains Dimitri Laurent, first author of the study. Millions of years later, the formation of the cave with the alteration of carbonates by rainwater charged with carbonic acid will come to release this hydrogen sulphide, which will be transformed into sulfuric acid. ” It is therefore an in situ source. “, he comments.
But the isotopic results also show that hydrogen sulphide was also produced by the action of bacteria on the sulphates contained in the hot springs which result from the leachingleaching of the evaporitesevaporites always present in depth. A process that is still ongoing.
This new multidisciplinary approach therefore makes it possible to better understand the formation of these astonishing caves in the Pyrenean context.
