Unearthing the Sources of Cave-Forming Sulfuric Acid
New science from the Geological Society of America journal GEOLOGY
Boulder, Colo., USA: A study out this month in Geology uses
isotopes of sulfur to fingerprint the sources of sulfuric acid that have
carved unique and beautiful cave systems in the Pyrenees mountains of
southern France.
Networks of caves form when carbonate rocks like limestone dissolve, also
known as karsts. In most caves, water has trickled down through Earth’s
surface, picking up carbon dioxide and becoming slightly acidic along the
way—this is the same type of mild carbonic acid that you’ll find in a can
of soda that has carbon dioxide dissolved in it.
A rarer type of cave forms from transport of fluids up through the crust
and through fault zones, forming vertical caves that can connect with
horizontal caverns, forming large networks. In some cases, when sulfur is
present, sulfuric acid forms and acts to dissolve limestone much
faster—forming caves 10–100 times faster than its carbonic acid
counterparts. When sulfur compounds are present in water or in the minerals
in the cave walls, chemical-loving bacteria use the sulfate as an energy
source, producing hydrogen sulfide as a by-product. Oxidation of this
hydrogen sulfide then forms sulfuric acid. Sulfuric acid can also come from
hydrothermal springs or from minerals within the rock—both are true in the
northern Pyrenees.
Sulfur comes in four different isotopes—each weighing a slightly different
amount. Researchers were able to estimate the relative contributions of
sulfuric acid from different sources by using these isotopes as a marker of
where the sulfur originated. The large network of limestone caves in the
foothills of the French Pyrenees mountains were formed by a combination of
acid-forming processes that left their imprint on the minerals left behind.
Sulfur-containing minerals like gypsum and mirabilite in the caves hinted
that sulfuric acid was involved in their formation. Mirabilite is a rare
mineral that forms long, thin crystals up to 50 cm in length that radiate
out like flowers.
For the first time, researchers studying limestone caves carved out by
sulfuric acid have estimated how much of the cave-forming acid was produced
by bacteria within the cave versus how much was produced by thermochemical
processes. This innovation in separating the various sources of limestone
dissolution has also allowed them to make the first estimate of how much
carbon dioxide was emitted by the formation of the caves.
Dimitri Laurent, lead author of this study, explains, “We tried to identify
hydrothermal springs close to measured faults, and then we contacted the
local speleological clubs to visit the caves near the springs. We see that
at depth in the Northern Pyrenees, in the northern foothills, there are
Triassic evaporites that produced hydrogen sulfide through thermochemical
processes 65 million years ago.” That hydrogen sulfide then traveled
through fractures in the rock and has been trapped within the cave host
rock since then. As water began to dissolve this sulfur-rich rock, the
fossil hydrogen sulfide was liberated and oxidized to form sulfuric acid.
The Triassic evaporites have also delivered sulfates to the caves more
recently, via deep hydrothermal fluids, which are then used by bacteria
within the cave.
Combining chemistry with physical observations of the landscape, the
researchers reconstructed the history of how these spectacular caves came
to be.
FEATURED ARTICLE
Unravelling biotic versus abiotic processes in the development of
large sulfuric-acid karsts
D. Laurent; G. Barré; C. Durlet; P. Cartigny; C. Carpentier; G. Paris; P.
Collon; J. Pironon; E.C. Gaucher
Contact: Dimitri Laurent, Université de Lorraine, dimit.laurent@gmail.com
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