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Copyright © 2003 by The National Speleological Society Journal of Cave and Karst Studies, August 2003 • 111
Sandro Galdenzi and Teruyuki Maruoka - Gypsum deposits in the Frasassi Caves, central Italy. Journal of Cave and Karst Studies 65(2): 111-125.
Caves formed by sulfuric acid from the oxidation of H2S
are found in many different parts of the world and contain con-
spicuous gypsum deposits. A review of the concepts can be
found in The Caves of the Guadalupe Mountains Research
Symposium (DuChene et al. 2000). Oxidation generally
involves bacterial activity, and these bacteria may represent the
main source of organic matter inside the cave.
Many caves in carbonate bedrock contain small gypsum
deposits formed by evaporation of sulfate-rich water on cave
fills or walls. Water seeping into the cave picks up gypsum
from oxidation of pyrite in the bedrock, which precipitates
upon reaching the cave, or anhydrite (gypsum) is dissolved
along the flow path and carried to the cave wall, where evapo-
ration causes precipitation of gypsum. Large-size (up to m-
scale) gypsum deposits are less common in carbonate caves,
and are generally considered the result of H2S-rich water cir-
culation inside the cave. Such gypsum deposits are known in
North America (Guadalupe Mountains: Hill 1987), in South
America (Las Brujas Cave: Forti et al. 1993), and in Europe
(Galdenzi & Menichetti 1995; Galdenzi 1990, 1997).
Unfortunately, gypsum is not actively forming in most of these
caves.
Some caves do include actively forming gypsum deposits
(Egemeier 1981; Galdenzi 1990; Sarbu & Kane 1995; Hose et
al. 2000), but these caves are generally short, and gypsum is
forming only on the cave walls above the water table. No large
bedded gypsum deposits have been found similar to the ones
found in the Guadalupe Mountains caves (New Mexico) or in
the Frasassi Caves (Italy).
The Frasassi Caves are unique in that they include both
active and relict meter scale gypsum deposits. Therefore, we
can compare both types of gypsum deposits directly from a
single cave system, which is a great advantage in understand-
ing how the gypsum forms. In this study, we will discuss the
depositional setting and the sulfur isotopic compositions of the
gypsum in the Frasassi Caves in order to understand how and
where the gypsum forms.
GEOLOGIC SETTING
The Frasassi Caves make up one of the most famous Italian
karst systems. They are the most visited show caves in Italy,
and about 350,000 tourists visit the caves every year. The
caves are in central Italy, on the eastern side of the Apennine
Mountains, 40 km from the Adriatic Sea. This area is charac-
terized by a mountainous landscape, with altitudes ranging
between 200 m at the bottom of the valleys to ~1000 m in the
surrounding mountains. The climate is Apenninic subcontinen-
tal, with an annual average temperature of about 13°C and an
average annual rainfall of about 1000 mm/year. Precipitation
generally reaches a maximum in autumn and spring, whereas
evaporation exceeds precipitation in summer. About 100 caves
are known in the Frasassi area: all these caves are developed in
the small area around the step cliffs of the Sentino River
Gorge, a 2 km long and 500 m deep canyon cut in the core of
a small anticlinal ridge (Fig. 1). The major cave (i.e., Grotta
del Fiume–Grotta Grande del Vento Cave System) consists of
>20 km of cave passages located at altitudes between 200 and
360 m. Two important caves (Buco Cattivo, 5000 m long, and
the Grotta del Mezzogiorno-Grotta di Frasassi System, 3500 m
GYPSUM DEPOSITS IN THE FRASASSI CAVES,
CENTRAL ITALY
SANDRO GALDENZI
Istituto Italiano di Speleologia, Frasassi Section, Viale Verdi 10, 60035 Jesi, ITALY sagalde@tin.it
TERUYUKI MARUOKA
Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, AUSTRIA
Present address: Laboratory for Space Sciences, Physics Department, Washington University,
Campus Box 1105, One Brookings Drive, St. Louis, MO 63130-4899, USA teruyuki@wuphys.wustl.edu
The Frasassi Caves are hypogenic caves in central Italy, where H2S-rich groundwater flows in the low-
est cave level. Near the water table, the H2S is converted to sulfuric acid by biotic and abiotic process-
es, which have enhanced cave development. The sulfate generally deposits above the water table as a
replacement gypsum crust coating limestone walls or as large gypsum crystals. Although the oxidation
of sulfide also occurs below the water table, sulfate saturation is not achieved, therefore, sulfate does not
precipitate below the water table.
In the upper dry levels of the cave, three main types of ancient gypsum deposits occurs: (1) replacement
crusts, similar to the presently forming deposits of the active zone, (2) microcrystalline large and thick
floor deposits, and (3) euhedral crystals inside mud. The study of the depositional setting and the analy-
sis of sulfur isotopes in the gypsum and groundwater clearly demonstrate that all the sampled gypsum in
the cave formed by H2S oxidation above the water table. Some fraction of small sulfur isotopic differ-
ences between H2S in the water and gypsum can be explained by isotopic fractionation during abiotic
and/or biotic oxidation of H2S.
112 • Journal of Cave and Karst Studies, August 2003
GYPSUM DEPOSITS IN THE FRASASSI CAVES, CENTRAL ITALY
long) occur at a higher altitude, ranging from 360 to 500 m.
The Frasassi Gorge offers a spectacular cross-section of the
core of the anticline, where the geology is well exposed. The
caves are formed mainly in the Calcare Massiccio Formation,
a thick Jurassic (Lower Lias) limestone unit exposed in the
gorge. The Calcare Massiccio formed in an epicontinental plat-
form setting, and it is a very pure limestone (over 99% calci-
um carbonate), consisting mainly of wackestone and pack-
stone facies, without any significant clay or silica minerals. It
is a very permeable limestone, due to high syngenetic porosity
and to a well developed network of fractures.
The Calcare Massiccio makes up the lower part of the sed-
imentary sequence outcropping throughout the region (Fig. 2).
The thickness of this formation can reach ~1000 m, and it
overlies a buried 2000 m thick Upper Triassic evaporitic
sequence, consisting mainly of anhydrite and dolomite
(Burano Formation: Martinis & Pieri 1964). A 50 m thick
Triassic limestone unit, rich in organics, is interbedded
between the Calcare Massiccio and the Burano formations.
Near the Frasassi Gorge, the Calcare Massiccio is overlain by
a 60 m thick unit (Bugarone Formation) formed in the Jurassic
after drowning of the carbonate platform in the shallower
depositional areas. This condensed Jurassic unit is mainly
micritic, nodular limestone with small amounts of pyrite, and
makes up a 10 m thick interbedded marly layer. This formation
represents a low permeable bed that is thin and discontinuous
and can influence underground drainage. A Lower Cretaceous
cherty limestone (Maiolica Formation), ~300 m thick, forms
another permeable and karstified section. The Calcare
Massiccio and the Maiolica formations host the main aquifer
in the central Apennine chain. A 50 m thick Cretaceous marly
formation (Marne a Fucoidi) forms a continuous aquiclude and
isolates the lower section of the stratigraphic sequence from
overlying permeable limestone formations of Late Cretaceous
and Tertiary age.
The Frasassi Anticline was formed in the late Miocene dur-
ing a tectonic compressive phase that also caused the Apennine
uplift and emersion. The fold is asymmetric, with a main
northeast vergence, and the caves are developed mainly in the
eastern limb of the anticline, where a fault has concentrated the
groundwater flow. The surface drainage formed at the end of
the early Pleistocene, when entrenchment of the valleys cut
into a preexisting “planation surface”. At that time, the gorges
cut into the anticlinal structures, and a landscape similar to the
present one was formed (Ambrosetti et al. 1982; Ciccacci et al.
1985). During the Pleistocene,climate changes also heavily
influenced geomorphic evolution. In the mountain areas, dur-
ing glaciations, the valleys were filled with alluvial gravel
Figure 1. Block diagram of the Frasassi karst system.
(1) Calcare Massiccio Formation; (2) cherty limestone; (3)
Cretaceous marl: a – section; b – outcrops; (4) faults; (5)
stratigraphic boundary; (6) contour intervals; (7) sulfidic
spring. Caves: (A) Grotta del Fiume-Grotta Grande del
Vento Cave System; (B) Buco Cattivo; (C) Grotta del
Mezzogiorno-Grotta di Frasassi Cave System.
Figure 2. Simplified stratigraphic succession of the
Frasassi area.
Journal of Cave and Karst Studies, August 2003 • 113
GALDENZI AND MARUOKA
deposits, while, during interglacials, the alluvial deposits and
the bedrock were eroded by the river (Bisci & Dramis 1991).
GROUNDWATER
Groundwater in the Frasassi area consists of two types:
Carbonate and sulfidic, which can be characterized by their
chemical compositions and origin. The carbonate water is
derived from diffuse infiltration of surface meteoric water
through the limestone. It characterizes all the vadose zone and
some small aquifers perched on interbedded marls (Fig. 3).
This water has a low salinity (~200-400 mg/L: Cocchioni
2002) with a very low sulfate content and high dissolved oxy-
gen (~0.32 mM/L). The sulfidic water characterizes the main
aquifer, developed in the Calcare Massiccio and Maiolica for-
mations at the core of the anticline (Fig. 1). This sulfidic
groundwater is cold (~13°C), but shows a higher salinity, up to
2 g/L, than the carbonate water. It is enriched in sodium and
chloride, and contains a high amount of sulfate (up to
2.5mM/L), but it is undersaturated with respect to gypsum.
The most significant feature of this water is the presence of
hydrogen sulfide. The H2S concentrations reach up to 0.5
mM/L. These dissolved components are probably acquired as
groundwater flows upward through the underlying anhydrite
formation. Isotopic data on δ18O, δD, and tritium suggested a
meteoric origin for the sulfidic groundwater (Tazioli et al.
1990). These authors estimated a recharge area located at alti-
tudes of 600-1000 m, with a relatively brief residence time in
the aquifer.
The sulfidic aquifer occurs at the core of the anticline in the
Calcare Massiccio and Maiolica formations, where the miner-
alized groundwater can rise through the deep faults at the east-
ern limb of the anticline. Here groundwater flow is concentrat-
ed, and the main springs are located (Fig. 1). The water table
can be reached in the lower section of the cave, at the same
level as the river. The groundwater flow is generally very slow,
and flowing water is only found in the eastern part of the cave.
The water levels are controlled by rainfall events, although
river water enters the cave directly in narrow restricted zones
near the spring. The conductivity and temperature of the sul-
fidic stream are also correlated with precipitation (Sarbu et al.
2000). These observations indicate that fresh water recharge,
derived from surface precipitation, dilutes the sulfidic ground-
water (Sighinolfi 1990; Tazioli et al. 1990).
The very low water flow in a large part of the cave leads to
groundwater stratification. Fresh water seepage stays near the
surface of the water table due to its lower salinity (Fig. 3). This
surface layer can be rich in dissolved O2, without any measur-
able H2S. The thickness of this freshwater layer ranges from 20
cm up to 5 m (Galdenzi 2001). On the contrary, in some nar-
row zones the groundwater can have a higher salinity, because
there is less dilution by the descending fresh water (Cocchioni
2002).
Hence the underground flow path is complex. The recharge
area in the surrounding limestone is about 5 km². There infil-
tration quickly reaches the water table, dilutes the mineralized
groundwater and flows toward the spring. Some meteoric
water could reach the underlying evaporitic sequence, where it
could pick up sodium chloride, sulfate, and sulfide (Fig. 4).
Figure 3. Hydrologic setting in the Frasassi Gorge. The sig-
nificant recharge of O2-rich freshwater increases water
aggressiveness when it reaches H2S at the water table. The
very low water flow induces the stratification of ground-
water in the inner parts of the cave.
Figure 4. Sketch showing the redox processes involving sul-
fur in the Frasassi aquifer. Reduction of sulfate from
underlying evaporites prevails in the deep phreatic zone,
while sulfide oxidation occurs near the water table, causing
cave development and limestone replacement with gyp-
sum.
114 • Journal of Cave and Karst Studies, August 2003
GYPSUM DEPOSITS IN THE FRASASSI CAVES, CENTRAL ITALY
Sulfate reduction could occur in the underlying Triassic lime-
stone rich in organic matter. Near the water table, oxygen in
the fresh water causes oxidation of the sulfide in the sulfidic
water. A small amount of sulfate could be reduced to sulfide in
the shallow phreatic zone, mainly in the organic-rich mud cov-
ering the floor of the submerged passages.
CAVE FEATURES AND ORIGIN
The Frasassi Caves (Fig. 5) consist of a network of ramify-
ing horizontal passages, where wide rooms alternate with
smaller tubes and also with spongework zones. The major
room, Abisso Ancona ,has a volume of ~106m³ (Fig. 6). The
system is clearly developed in several superimposed levels that
are interconnected by short shafts or inclined passages. The
genesis of the cave levels can be related to tectonic uplift and
climatic changes that occurred during the Pleistocene. At least
4 main horizontal, often overlapping levels occur in the caves,
whereas some further levels occur even lower (Fig. 1). The
water table is in the lowest sections of the cave, where active
sulfidic water is mainly in flooded passages.
The 2 lowest main levels, corresponding to the III and V
level in Bocchini & Coltorti (1990), are between 200 and 300
m msl and occur mainly in the Fiume-Vento System (Fig.1).
These passages developed in settings similar to the present,
during the deposition of the surface alluvial gravel terraces in
the Sentino River Valley, in the middle to late Pleistocene
(Bocchini & Coltorti 1990). Ages were obtained by uranium
series dating of speleothems (Taddeucci et al. 1992). In level
V, those authors obtained stalagmite ages of up to 200 ka; in
level III some stalagmites are 80 ka old, while a collapsed sta-
lagmite has a range of ages between 170 ka and 120 ka. In the
less-developed levels near the water table (e.g., I and II levels
in Bocchini & Coltorti 1990), only Holocene dates were docu-
mented.
Each cave level has typical phreatic features with mainly
horizontal tubes (1-10 m in diameter) that can form complex
mazes or can alternate with large rooms characterized by flat,
erosional rock surfaces at the floor and by rounded ceilings
(Fig. 7). Shafts and fissures in the floor of the cave represent
the original sources of H2S-rich waters that formed the cave.
Cupolas of different sizes are developed in the walls and ceil-
Figure 5. Map of Grotta del Fiume-Grotta Grande del
Vento Cave System, with sample locations.
Figure 6. View of Abisso Ancona, the main room in Fiume-
Vento Cave System. All photos by S. Galdenzi.
Figure 7. Sala del Limone, a wide room in Grotta del Vento,
with a shape similar to half a lemon. The rounded ceiling
was formed by condensation corrosion. The flat rock floor
is cut by shafts and fissures that formed below the sulfidic
groundwater level.
Journal of Cave and Karst Studies, August 2003 • 115
GALDENZI AND MARUOKA
ings of the cave. They could have formed either in the phreat-
ic or vadose zones as a result of condensation corrosion.
Bubble trails (Fig. 8) are common in many phreatic environ-
ments, indicating rising corrosive gas in the shallow phreatic
zone. Some areas are covered by gypsum replacement crusts,
while some thick floor deposits are common in the main
rooms.
The two upper main levels are developed mainly in the
Buco Cattivo and Mezzogiorno-Frasassi Caves at altitudes of
350 to 500 m (Fig. 1). The features of these caves are slightly
different from those in the Fiume-Vento System. A few gyp-
sum deposits indicate that sulfidic water circulation also
occurred in this caves, but some important branches clearly
developed in a deep phreatic zone. It may be supposed that
during the formation of these upper caves, the hydrogeologic
setting was different from the present one.
The oxidation of H2S is considered the main cave-forming
process (Galdenzi 1990). The oxidation of hydrogen sulfide to
sulfate [1] can occur in the presence of oxygen from drip
water, and can occur both in the shallow phreatic zone and dur-
ing vadose conditions, causing the dissolution of limestone [2]
and cave development.
[1] H2S + 2O2⇔H++ HSO4–⇔2H++ SO4=
[2] 2H++ SO4=+ CaCO3⇔Ca++ + SO4=+ H2O + CO2
Cave development by O2-rich infiltrating meteoric water
plays only a secondary role, and it is limited to a network of
narrow passages formed in the vadose zone in the Frasassi
karst area and in the surrounding mountains (Galdenzi 1996).
Here, infiltrating meteoric water descends quickly to the water
table. Similar networks can also facilitate sulfuric acid speleo-
genesis, by quickly delivering O2-rich meteoric water to the
groundwater, where H2S oxidation can proceed (Fig. 3).
Recent investigations pointed out the role of microbial
activity in speleogenesis. Chemoautotrophic microorganisms
live near the redox interface between the sulfidic groundwater
and the oxygen present in the atmosphere and in the seepage
water, using the chemical energy resulting from the oxidation
of H2S. C and N stable isotopic results showed that the organ-
ic matter produced in situ by these microbial communities rep-
resents the trophic support for the rich community of inverte-
brates that inhabit the sulfidic sections of the caves (Sarbu et
al. 2000). Biologic activity can significantly accelerate the oxi-
dation of H2S, causing the production of H2SO4as a by-prod-
uct of their metabolism. This has an important role in the cave
development, increasing the water aggressiveness on the cave
walls and accelerating the dissolution of the limestone.
Therefore, cave development can be considered, at least part-
ly, a consequence of bacterial activity (Galdenzi et al. 1999).
PRESENT GYPSUM DEPOSITION
The morphologic effects of the oxidation of H2S can be
directly observed in the lower parts of the Frasassi Caves,
where the corrosive processes on the limestone are still active
in the sulfidic water. Here, bacterial colonies cover the bottom
Figure 8. A bubble trail in Grotta del Vento. It consists of a
rill that originates below the water table inside a pocket, a
fissure or in a small side passage on the cave wall and rises
upward. It can be some meters long and few decimeters
deep, and can have also a meandering pattern.
Figure 9. Microbial mats covering limestone walls in the
sulfidic groundwater of Frasassi Caves.
116 • Journal of Cave and Karst Studies, August 2003
GYPSUM DEPOSITS IN THE FRASASSI CAVES, CENTRAL ITALY
of the flooded galleries (Fig. 9) and cause the oxidation of H2S,
but gypsum deposition cannot occur in the water because sat-
uration of sulfate is not achieved. In some pools and streams,
H2S and CO2are released to the cave atmosphere from the
water and can diffuse to the nearby rooms. The concentration
of H2S and CO2reached peaks at 8 ppm and 5800 ppm, respec-
tively, in the cave air near the sulfidic streams (Galdenzi 2001).
The gas can rise toward the upper cave levels due to the small
differences in the temperature (~1°C), but the H2S concentra-
tion decreases quickly away from the water table (Fig. 10).
The gas concentration that can build up where air exchange
is low was simulated by creating an air bell floating on the
water table. Here the concentration of H2S exceeded 120 ppm,
while O2decreased to 7% (Galdenzi 2001). On the other hand,
rapid air flow in the open cave disperses the H2S and keeps the
concentration low.
The limestone walls exposed to H2S vapors are highly cor-
roded and partially or completely covered by gypsum crusts
(Fig. 11), sometimes associated with small amounts of ele-
mental sulfur. The gypsum crust generally consists of white,
finely crystalline gypsum, whereas some large crystals grow
on the gypsum crusts or directly on the limestone. The lime-
stone surface under the gypsum crust is severely corroded,
with hemispheric corrosion pockets some cm deep. The inten-
sity of limestone corrosion was measured using limestone
tablets (80 x 40 x 10 mm), exposed to acidic vapors in the cave
atmosphere for 5 years (Galdenzi et al. 1997). At the end of the
experiment, these tablets were completely covered with
replacement gypsum, and the limestone surface under the gyp-
sum crust was irregularly corroded, with incipient erosional
pockets. The average weight loss, measured after gypsum
removal, was about 15 mg/cm²/a, with significant variation
due to small variations in the location of each tablet. This
weight loss can correspond to an average loss of about 0.05
mm/a at the limestone surface. In the same experiment, quite
similar values were obtained for limestone tablets placed in the
sulfidic groundwater, where gypsum deposition did not occur.
Galdenzi et al. (1997) also discovered a biofilm at the lime-
stone-gypsum interface. This means that bacterial communi-
ties played an important role in the H2S oxidation. The micro-
bial biofilm can grow and cover the walls with a layer of
organic mucous matter, that forms organic “stalactites”, secret-
ing acidic drops, rich in H2SO4, with a pH < 1 (Galdenzi et al.
1999) (Fig. 12). Microbiologic studies of these biofilms found
sulfur-oxidizing bacteria that play an important role both in
limestone corrosion and in the cave food web (Vlasceanu et al.
2000). These organic formations are quite similar to those
described in the Cueva de Villa Luz, Mexico, by Hose and
Pisarowicz. (1999).
Figure 10. Schematic profile through the Frasassi sulfidic
section. Small thermal differences cause water vapor to
rise and condense at higher levels. CO2diffuses into the
upper levels, while H2S is present only near the water table.
Gypsum replacement crusts are growing on the limestone
walls directly exposed to H2S (after Galdenzi 2001).
Figure 11. White, finely crystalline gypsum, growing on
limestone pockets, Ramo Sulfureo, Grotta del Fiume.
(Image width = 30 cm).
Figure 12. Organic “stalactites” growing on gypsum crys-
tals coating the cave walls in Grotta del Fiume. They con-
sist of mucous glycocalyx, secreted by sulfur-oxidizing bac-
teria to protect themselves from the very acidic environ-
ment. The droplets are rich in H2SO4and their pH is
always < 1.
Journal of Cave and Karst Studies, August 2003 • 117
GALDENZI AND MARUOKA
ANCIENT GYPSUM DEPOSITS
Gypsum deposition in limestone caves is fairly common in
central Italy (Galdenzi & Menichetti 1995). Some large and
interesting gypsum deposits are found particularly in the
Faggeto Tondo Cave (Forti et al. 1989) and in the Monte
Cucco Cave (~20 km to the west). Large gypsum deposits also
occur in the dry, upper levels of Frasassi Cave. Three main
types of deposits can be observed: (1) replacement crusts, (2)
large floor deposits, and (3) gypsum crystals inside mud.
REPLACEMENT CRUSTS
This is the only type of ancient gypsum deposit that can be
compared with gypsum presently forming in the cave. The old
gypsum replacement crusts are quite common in many hori-
zontal passages and rooms. Later seepage water often dis-
solved these old crusts, exposing small, rounded corrosion
pockets on the limestone walls (Fig. 13). Commonly these cor-
rosion pockets represent the only evidence of a preexisting
gypsum crust. The replacement gypsum is generally recrystal-
lized on the surface, but its characteristics are similar to the
gypsum crusts that are forming in the sulfidic section of the
cave, and they are believed to be a product of subaerial
replacement of limestone (Bertolani et al. 1977; Ciarapica &
Passeri 1978; Cigna & Forti 1986; Galdenzi 1990). In some
passages, Galdenzi (1990) also reported that their lower parts
show typical phreatic features such as rounded cupolas and
bubble trails, while in the upper parts corrosion pockets or
residual gypsum crusts cover the limestone walls (Fig. 14).
The distribution and thickness of gypsum forming today in
the Frasassi Caves is much more limited than it was in the past.
This may be due to the present hydrologic setting, because a
free interface between groundwater and cave air generally
exists only at the bottom of shafts or descending passages,
where gypsum is forming in restricted areas (Fig. 3).
Moreover, in most of these places, a layer of infiltration water
overlies the sulfidic groundwater, preventing the release of
H2S and the subaerial growth of gypsum replacement crusts. In
the past, the cave was an almost ideal water table cave, with
many partly flooded rooms and passages formed as a result of
degassing (Galdenzi 1990). At that time, condensation-corro-
sion could have been more important in enlarging the cave, as
the wide distribution of old replacement crusts testifies.
Figure 13. Limestone pockets on the cave walls after gyp-
sum removal by dripping water and gravity, Abisso
Ancona, Grotta del Vento. (Image length = 60 cm)
Figure 14. Evidence of the old water table in a dry passage
of Fiume-Vento Cave System. Note the rough limestone
surface above the water level and the rounded features in
the flooded zone. A bubble trail is near the speleologist’s
head.
118 • Journal of Cave and Karst Studies, August 2003
GYPSUM DEPOSITS IN THE FRASASSI CAVES, CENTRAL ITALY
LARGE FLOOR DEPOSITS
Some large deposits of massive gypsum occur on the floor
of the main rooms, where they are generally associated with
replacement crusts that cover the limestone walls (Fig. 15).
These floor deposits consist of white, finely crystalline gyp-
sum, very similar to the replacement crusts. The floor deposits
form mounds several meters thick, often below wide cupolas
(Fig. 16), or form small gypsum “glaciers”, similar to the ones
described in Lechuguilla Cave (Davis 2000). Maximum thick-
ness reaches 5 m, and volume exceeds 1000 m³ (Fig.17).
The large gypsum deposits in the Frasassi Caves were for-
merly thought to be the by-product of sulfate-saturated phreat-
ic water (Bertolani et al. 1977; Ciarapica & Passeri 1978).
Ciarapica & Passeri (1978) proposed that the massive gypsum
deposition could be produced by the rapid cooling of warm
water. Bertolani et al. (1977), based on cave mineral associa-
tions, excluded the possibility of thermal water flow in the
cave, but believed the gypsum deposition was a result of super-
saturated groundwater. However, based on depositional set-
tings and characteristics, Galdenzi (1990) concluded that these
large gypsum deposits were produced above the water table.
Figure 15. Gypsum floor deposit below a wall covered by
corrosional limestone pockets near Lago Cristallizzato,
Grotta del Vento.
Figure 16. Piles of gypsum on the cave floor overlying mud
with gypsum crystals, Sala Duecento, Grotta del Vento.
Figure 17. Profiles through some of the most significant
gypsum deposits in Grotta del Vento.
Figure 18. Gypsum crystals growing inside mud in Frasassi
Caves.
Journal of Cave and Karst Studies, August 2003 • 119
GALDENZI AND MARUOKA
Galdenzi showed that these deposits lack any sedimentary
structure or texture that can be attributed to a subaqueous envi-
ronment. In particular, the gypsum is never interbedded with
mud layers, while in the entire cave, mud deposits that origi-
nate below the water table are very common. Clear evidence of
an origin above the water table also includes: (1) rare breccias
of fallen gypsum crusts and (2) the corrosion runnels on
bedrock below the floor deposit formed by flowing corrosive
water. However, the main evidence for deposition above the
water table is the depositional setting of these gypsum deposits
in the western parts of the cave where recent gypsum is not
forming. Here gypsum deposits in the lower part of the main
rooms, below the old water table, are missing. Therefore, these
large floor deposits were interpreted to have formed by the
detachment and flow of slushy gypsum produced on the lime-
stone walls as replacement crusts in zones exposed to intense
H2S vapors.
GYPSUM CRYSTALS INSIDE MUD
The last type of gypsum deposit typically occurs inside
mud layers that, in places, underlie the large floor deposits.
The quartz-rich mud (Table 1), contains authigenic, euhedral
gypsum crystals that grew inside the sediments (Fig. 18).
Ciarapica & Passeri (1978) proposed a phreatic origin for this
type of gypsum via seepage and trapping of sulfate in solution
inside mud sediments.
ISOTOPIC ANALYSIS: PURPOSE AND METHODS
Sulfur isotope ratios (34S/32S) can vary as a result of bio-
logic and inorganic reactions involving the chemical transfor-
mation of sulfur species. Sulfur isotope studies have been use-
ful in understanding the processes of sulfur cycling in many
sulfur-related systems (see Canfield 2001, for a recent review).
Here we use sulfur isotopes of gypsum to see whether gypsum
formed by oxidation of H2S or by precipitation of sulfate from
saturated water.
Isotopic compositions of sulfur were measured in sulfides
and sulfates using a helium-gas continuous-flow isotope-ratio
mass spectrometer (CF-IR-MS: Micromass Optima; Maruoka
et al. 2002, 2003). The samples were weighed into 12 x 5 mm
tin capsules, together with a mixture of V2O5and SiO2to pro-
mote full combustion (Yanagisawa & Sakai 1983). The sulfur
isotopic compositions are expressed in terms of δ34S (‰) rela-
tive to the Canyon Diablo standard. Results of three IAEA sil-
ver sulfide standards (IAEA-S-1, -0.30‰; IAEA-S-2, 22.67‰;
IAEA-S-3, -32.55‰: Ding et al. 2001) were compared to con-
strain the δ34S values. The isotopic compositions of sulfur were
determined at a precision of ± 0.2‰ (1σ).
SAMPLING SITES
SULFIDIC WATER
The isotopic composition of sulfide and sulfate in the water
can be used to discuss the origin of gypsum deposits inside the
cave. Tazioli et al. (1990) also analyzed δ34S in the water and
in a gypsum sample, without discussing its characteristics and
location, and confirmed its derivation from the H2S dissolved
in the water. Some water samples at different sites were col-
lected November 11, 2000, at the end of a dry period when
freshwater recharge to the groundwater and discharge were
low. We sampled sulfate and sulfide in two different springs
and in two cave pools with different water salinities (Fig. 5).
The concentration of the oxidized and reduced sulfur in these
water samples was determined by Cocchioni (2002).
The sulfide in the Main Spring water was collected from
the main surface spring along the river, in the gorge. Here,
many small springs have the same temperature and conductiv-
ity, about 13.5°C and 1600 µS, respectively. They can be con-
sidered to represent the “normal” sulfidic groundwater in the
shallow phreatic zone, formed by the mixing of the rising min-
eralized water and the descending meteoric freshwater in the
cave.
The Fissure Spring is a small emergence, very near the
Main Spring. The sulfidic water in the Fissure Spring is sig-
nificantly different compared to the “normal” sulfidic spring
water. Throughout the year the temperature is ~0.5°C higher
and the conductivity is ~30% higher. This spring is probably
supplied by water rising from a deeper phreatic zone, with a
lower dilution of descending freshwater (Cocchioni 2002).
The sulfide and sulfate of a sulfidic stream were sampled
in the most studied cave room (Ramo Sulfureo: Galdenzi et al.
1997, 1999; Sarbu et al. 2000). Here, a direct influence from
meteoric fresh water is well documented (Sarbu et al. 2000)
and the chemical characteristics of the groundwater are similar
to the Main Spring water (Cocchioni 2002). A large surface
area of flowing sulfide water allows the release of H2S into the
cave air. The sulfide and sulfate of a stagnant sulfidic pool,
Lago Verde, was also sampled. Here, groundwater has a chem-
ical composition similar to the Fissure Spring (Cocchioni
2002).
The concentrations of sulfide in total sulfur range between
5.5 and 17.9% (Table 2). The Fissure Spring and the Lago
Verde sulfidic pool, supplied by deeper water, appear to be
more enriched in H2S and have a higher ratio of sulfide/sulfate.
The Main Spring and the Ramo Sulfureo water, on the con-
trary, has an higher value of oxidized sulfur owing to recharge
by oxygen-rich freshwater. Sulfide δ34S values in the water
range between –13.30‰ and –15.03‰, while sulfate δ34S val-
Table 1. Mineralogical composition of sediment samples
inside cave.
dominant abundant present scarce or trace
1- sand calcite quartz feldspar, mica illite
2- mud quartz calcite, illite, chlorite, feldspar,
gypsum mica montmorillonite
4- mud quartz calcite feldspar kaolinite, amesite
6- mud quartz illite mica, chlorite,
feldspar cristobalite
7- mud quartz illite, mica chlorite
120 • Journal of Cave and Karst Studies, August 2003
GYPSUM DEPOSITS IN THE FRASASSI CAVES, CENTRAL ITALY
ues range between +20.11 and +22.17‰ (Table 2). The sulfate
δ34S is lower in the Main Spring water than in the Fissure
Spring water. As the Main Spring water mixed with the
descending oxygen-rich freshwater, part of the H2S, depleted
in 34S, in the Main Spring water would be oxidized to sulfate
before degassing from the surface. This could cause the low
sulfate δ34S values in the Main Spring water.
GYPSUM DEPOSITS
Gypsum deposits were sampled in various areas of the cave
system (Fig. 5) in order to analyze their isotopic compositions.
Here they will be briefly described (Table 3).
Recent gypsum
Some gypsum, both microcrystalline and large crystals, up
to several cm long, were collected at two different sites where
gypsum is actively forming. A sample was also obtained from
the surface of the limestone tablets described in previous
experiments (Galdenzi et al. 1997).
Grotta del Vento, level III
This cave level is well developed in the Grotta del Vento:
here a layer of freshwater over the sulfidic groundwater pre-
vents the escape of H2S, therefore all gypsum deposits can be
considered inactive. Some of the largest gypsum deposits are
located in this part of the cave.
Lago Cristallizzato. A small floor deposit in the Abisso
Ancona, the main cave room, is under a limestone wall com-
pletely covered by corrosional limestone pockets with some
residual replacement crusts (Fig. 15). Seepage water removed
a large amount of the original deposit, while the replacement
crust on the cave wall is almost entirely missing. Both the
replacement crusts and the floor deposit were sampled.
Abisso Ancona. This is the largest gypsum deposit in the
cave. It developed like a gypsum “glacier” under a high wall,
completely covered by corrosional limestone pockets with
some residual replacement crusts (Fig. 17). Both the replace-
ment crusts and the floor deposit were collected for this study.
Table 2. Isotopic composition of sulfide and sulfate in the
groundwater.
sulfide–total δ34S sulfide δ34S sulfate
sulfur ratio
Main Spring 12.1% –14.47 +20.34
Fissure Spring 17.9% –13.30 +22.06
Ramo Sulfureo 5.5% –15.03 +20.11
Lago Verde 17.2% –14.49 +22.17
Triponzo Spring – 9.09 +17.45
Table 3. Isotopic composition of sulfur in gypsum deposits in the caves.
Cave sample locality karst level depositional fine-grained gypsum crystals
setting gypsum δδ34Sδδ34S
Grotta del Fiume G4 Ramo Sulfureo active limestone tablet -19.17
G19-18 Laghi di Lucia active active wall crust -19.62 -17.64
G27-26 Ramo Sulfureo active active wall crust -18.80 -10.79
G14-13 Ramo Sulfureo partly active wall crust -15.52 -13.90
G2 old branches II wall crust -14.07
G3 Pozzo Cristalli unclear wall crust -7.82
G28 Pozzo Cristalli unclear wall crust -10.33
Grotta del Vento G5 Lago Cristallizzato III floor deposit -14.75
G6 Lago Cristallizzato III wall crust -16.06
G7 Abisso Ancona III floor deposit -12.14
G24 Abisso Ancona III floor deposit -11.69
G25 Abisso Ancona III wall crust -16.04
G8 Sala Duecento III floor mud -14.05
G9 Sala Duecento III floor deposit -14.14
G10 Sala Duecento III floor deposit -13.37
G1 Sala Duecento III floor deposit -12.19
G12 Sala Orsa III floor -11.69
G11 Sala Orsa III crevasse -12.59
G15 Piano Superiore V floor deposit -10.58
G21 Abisso Ancona V floor deposit -9.54
G22-23 Abisso Ancona V floor deposit -7.93 -7.53
Triponzo Spring T3-4 Triponzo wall crust -24.24 -22.52
T5 Triponzo wall crystals -19.09
Journal of Cave and Karst Studies, August 2003 • 121
GALDENZI AND MARUOKA
Sala Duecento. This part of the cave consists of many
interconnected rooms developed around a main large passage.
This part of the cave includes two very interesting deposits,
from which we collected samples. The first one represents the
most spectacular natural section in the cave (Fig. 19). Here a
small gypsum “glacier” is deeply dissected by dripping water.
A succession of fine sand and mud with gypsum crystals
(Table 1, samples G1 and G2), is overlain by thick, white,
microcrystalline gypsum. The other deposit in Sala Duecento
is on the floor of a wide room, cut by many shafts (Fig. 16).
The gypsum overlies gray mud including small gypsum crys-
tals (Table 1, samples G4 and G6), whereas the overhanging
walls and roof are corroded by wide cupolas and by small
pockets with a few residual gypsum replacement crusts.
Gypsum is not present inside the shafts, which acted as vents
for the sulfidic water when the cave level was forming.
Sala dell’Orsa. This room constitutes an intermediate part
of a large shaft that opens in the flat rock floor of a big room
at level V and reaches the actual water table. The lower part of
this shaft, below level III, is deeply corroded by rounded
phreatic features, while in its upper part scattered gypsum
deposits occur inside the deeply corroded limestone wall or
cover the floor. Three other cave minerals related to the sul-
fidic water (halloysite, barite, and jarosite) have been detected
in this zone (Bertolani et al. 1977). Here we sampled a wall
deposit and a nearby fissure filled with gypsum (Fig. 17).
Grotta del Vento, level V
Some samples were collected in this upper dry level in a
short, lateral branch of the Abisso Ancona Room. The gypsum
lies on the floor, under wide cupolas in the roof. It is the typi-
cal, white, finely crystalline gypsum, recrystallized on the sur-
face. It lies where a rising phreatic passage reaches the main
room. A further sample of large gypsum crystals was collected
from the top of piles in the large passage above Sala Duecento
that are deeply dissected by dripping water (Fig. 20).
Grotta del Fiume deposits
In this part of the cave system, sulfidic water flows in the
lower cave passages, therefore condensation-corrosion occurs
near the sulfidic pools and in adjacent upper level passages.
Moreover, in this zone two minor cave levels are well devel-
oped between level III and the water table. Therefore, H2S can
easily rise from the water table toward the upper dry level, and
gypsum deposition can occur in the same cave level at differ-
ent times.
Old Branch. This part of the cave represents a network of
passages developed near the surface, ~10 m above the water
table. Widespread replacement crusts can be observed on the
cave walls. A few small floor deposits and gypsum crystals
inside mud are also present (Table 1, sample G7).
Pozzo dei Cristalli. This shaft opens up below level III and
is directly connected with the lower passages and with the
water table. The walls are completely covered by a thick crust
Figure 19. Anatural section through one of the largest gyp-
sum floor deposits of Sala Duecento, Grotta del Vento.
Compare with the section in Figure 17.
Figure 20. Pile of gypsum cut by dripping fresh water in
the level V, Grotta del Vento.
122 • Journal of Cave and Karst Studies, August 2003
GYPSUM DEPOSITS IN THE FRASASSI CAVES, CENTRAL ITALY
of replacement gypsum, and the corrosional processes are still
weakly active. The sulfidic water is the same as Lago Verde
and the Fissure Spring.
Ramo Sulfureo. In this part of the cave, corrosion by the
cave air is the most intense. Some deposits are still growing,
and here the main research on the sulfidic zones was concen-
trated.
TRIPONZO SPRING: A COMPARISON
Triponzo Spring is a sulfidic thermal spring located ~60
km south of Frasassi. The deep valley of the Nera River cuts
the Calcare Massiccio Formation in the northern periclinal ter-
mination of an anticline, where the spring reaches the surface,
in a geologic setting similar to the Frasassi Gorge. Sulfide and
sulfate in the Triponzo water were sampled in August 2000.
The temperature of the water was 29-30°C, the conductivity
was 2.14 mS/cm; the sulfate δ34S value was +17.45‰, and the
sulfide δ34S was -9.09‰.
In this area, the deep karst is not well known. Only a few
small caves were explored in the steep mountain sides, but an
interesting room was encountered in a hydroelectric tunnel.
This small cave is no longer accessible, but it was described
during the drilling by an Italian geologist interested in cave
origin (Principi 1931). He understood the importance of the
cave and suggested that “the cave did not form by normal karst
dissolution, but developed as a consequence of the sulfidic
water action, that replaced limestone with gypsum, which
could be easily removed by flowing water”. The active corro-
sion of sulfidic water on the limestone walls could be observed
until recently in a partly flooded artificial tunnel, where the
sulfidic water flowed to the old thermal baths. The new ther-
mal baths, built recently, reach the sulfidic water through bore-
holes, therefore the old flooded tunnel was destroyed. A
remaining dry tunnel allowed a glimpse inside the karstified
limestone near the spring. Here a network of open fissures and
fractures is entirely covered by gypsum. In places, the lime-
stone is replaced by a thin layer of finely crystalline gypsum,
with small crystals on the surface. By turns, small gypsum
crystals can grow directly on the limestone. These gypsum
deposits were sampled to determine their sulfur isotopic com-
position (Table 3).
ORIGIN OF GYPSUM DEPOSITS
The Frasassi Caves make it possible to compare the iso-
topic content of sulfur in the water and in the gypsum deposits.
It can help in the study of gypsum deposition and is very use-
ful in deciphering the origin of the gypsum deposits in the
upper dry levels. The δ34S values of the gypsum collected in
the cave range from –7.82 to –19.60‰. These are much lower
than those of sulfate in the sulfidic groundwater (from +20.11
to +22.17‰). As it is impossible to produce such large isotopic
fractionations during precipitation from water, the gypsum
cannot have been produced by precipitation from saturationed
groundwater. These low δ34S values are related to H2S (δ34S =
-13.30‰ and -15.03‰). Moreover, the sulfide oxidation
occurred in the air. If the oxidation had occurred in the water,
the sulfate from H2S would mix with the abundant sulfate in
the water, and the lower δ34S values would not be preserved in
the gypsum. This conclusion is consistent with the sub-aerial
depositional settings of the gypsum (Galdenzi 1990).
REPLACEMENT CRUSTS
The gypsum replacement crusts in the upper dry levels
have δ34S values similar to those of sulfide now dissolved in
the water. Therefore, the sulfur in the replacement crust came
from H2S released from groundwater. This conclusion is con-
sistent with many observations, such as the gravity-controlled
shape of the corrosion pockets on cave walls, the analogies
with the present depositional setting, and the localization
above the original old water table level in many passages.
FLOOR DEPOSITS
The massive floor deposits are not forming today in the
cave, and they occur mainly in cave levels III and V, which
formed in the middle and late Pleistocene (Bocchini & Coltorti
1990; Taddeucci et al. 1992). These gypsum deposits were
considered the result of sulfate saturation in the groundwater
(Bertolani et al. 1977; Ciarapica & Passeri 1978) or the result
of collapse and flow of gypsum replacement crusts (Galdenzi
1990). In the Guadalupe Mountains caves, U.S.A., where past
gypsum deposition is well documented, both phreatic and
vadose gypsum deposition are described, although the largest
deposits are generally considered the result of sulfate supersat-
uration in the groundwater (Hill 1987; Buck et al. 1994;
Palmer & Palmer 2000). The sedimentary structures that sug-
gest a phreatic deposition for gypsum in Guadalupe Mountains
caves (Hill 1997) are not clearly developed in the Frasassi
deposits, therefore the depositional setting is not necessarily
the same.
In the Frasassi Caves, since we can compare the isotopic
composition of sulfidic water and gypsum, we can easily see
that the gypsum δ34S values clearly exclude the possibility that
these massive floor deposits formed by precipitation below the
water table in sulfate-supersaturated groundwater.
Furthermore, the δ34S of the floor deposits is similar to the
adjacent replacement crusts (i.e., Lago Cristallizzato, Abisso
Ancona, Table 3), suggesting that the growth of the replace-
ment crusts and the deposition of nearby floor deposits were
related. Judging from their thickness, we can exclude direct
limestone replacement of the cave floor by H2S oxidation in
the cave air. Therefore, these gravity piles or “gypsum glaci-
ers”, lying below walls or roofs covered with limestone pock-
ets or replacement crust, can be considered the result of the
detachment and flow of moonmilk-like replacement gypsum
produced on the cave walls over a long time, as Galdenzi
(1990) proposed.
Journal of Cave and Karst Studies, August 2003 • 123
GALDENZI AND MARUOKA
GYPSUM IN MUD DEPOSITS
The low-δ34S values of the euhedral gypsum crystals incor-
porated in mud also can be explained by the result of H2S oxi-
dation. Therefore, we can conclude that they could not form
below the water table as a consequence of sulfate saturation
inside the mud. Moreover, their values are quite similar to the
overlying massive floor deposits. So these gypsum crystals
grew by the seepage of sulfate-rich water from the overlying
massive gypsum deposits. The chemical composition of this
mud (Table 1) differs from the other mud deposits of the cave
(Bertolani et al. 1977) and also implies etching by strong acid.
The seepage of acidic water below the large gypsum deposits
is also evidenced by meandering corrosional rills on the floor,
formed where gypsum floor deposits directly overlie lime-
stone, without interposed mud layers.
SULFUR ISOTOPIC FRACTIONATION
Although the low δ34S values in the gypsum are related to
H2S, the values do not correspond exactly to those of H2S. The
δ34S in the cave gypsum differs between deposits, and values
can be lower or higher than values of present-day H2S rising in
the groundwater. The δ34S in gypsum deposits ranges between
–7.82‰ and –19.62‰, whereas in water the δ34S values of H2S
rang from –13.30‰ to –15.03‰, with an average value of
about –14.2‰. These differences in the δ34S of gypsum and
H2S could represent the depositional setting in that the gypsum
deposits formed in different places, over a period of ~200 ka.
ACTIVELY FORMING GYPSUM
The δ34S values of actively forming microcrystalline gyp-
sum are relatively constant. The δ34S values of this gypsum on
the active cave walls are ~5‰ lower than those of sulfide in
the water. This depletion of 34S may be related to kinetic iso-
topic fractionation during oxidation of sulfide (Fry et al.
1988). In the Triponzo aquifer, where gypsum growth is
presently occurring, δ34S in water sulfide is –9.09‰, while
microcrystalline gypsum in the limestone fissure network is
–24.24‰, with a depletion of about –14‰ of 34S. Similar val-
ues were also obtained in other active H2S caves: Hose et al.
(2000) reported from Cueva de Villa Luz, Tabasco, Mexico,
δ34S values of –1.7‰ for H2S and –23.4‰ for gypsum.
Isotopic fractionation of sulfur during abiotic oxidation is
generally believed smaller than that measured from these
caves (up to 5‰: Fry et al. 1988; Canfield 2001). Furthermore,
sulfur oxidizing organisms are believed capable only of a small
isotopic fractionation, even though this subject is not well
explored (Canfield 2001). Therefore, a large fractionation
should not be due only to isotopic fractionation during oxida-
tion. Part of the isotopic fractionation might be explained by
the additional production of H2S in the shallow phreatic zone,
which might be more depleted in 34S than that rising in the
aquifer. Although sulfate-reducing bacteria are not reported in
those caves, they may be responsible for producing more
depleted 34S.
Sulfur isotope compositions of associated large and micro-
crystalline gypsums are shown in Table 3. The δ34S values of
the euhedral crystals are higher and closer to those of H2S than
those of the respective microcrystals. The δ34S value of sample
G26 is even higher than that of H2S in the water. As biotic H2S-
oxidation is presently believed to cause a smaller isotopic frac-
tionation than abiotic oxidation (Canfield 2001), the δ34S val-
ues similar to those of H2S may imply that the sulfate in those
gypsums is mainly produced by sulfide-oxidizing bacteria
rather than by abiotic oxidation. Actually, actively forming
gypsum crystals are often covered with biofilms that contain
sulfide oxidizing bacteria.
As mentioned above, the δ34S value of sample G26 is even
higher than those of H2S in the water. This cannot be explained
by only H2S oxidation. Therefore, a more complex biologic
activity affecting the gypsum should be considered. Sulfate-
reducing bacteria are known to cause high isotopic fractiona-
tion, producing sulfide depleted in 34S (Kaplan & Rittenberg
1964; Canfield 2001). Therefore, small amounts of sulfate in
the gypsum might have been re-reduced to volatile H2S. This
process could have produced the 34S-enriched sulfate because
34S-depleted H2S would have been released after the reduction.
Similar considerations could explain the high δ34S value of
gypsum in the Pozzo dei Cristalli where finely crystalline gyp-
sum is about +9‰ enriched in 34S compared with gypsum
forming in the other cave areas, and it is also +5‰ enriched in
34S compared with water sulfide.
UPPER LEVEL DEPOSITS
In the upper dry III level of Grotta del Vento, where gyp-
sum deposits probably formed before the Holocene, we
observe some variation in the δ34S values. The sulfur in the big
floor deposits is enriched in 34S compared to the associated
replacement crusts. In the Lago Cristallizzato, the difference
between the floor deposit and the nearby replacement crust is
+1.3‰, while in the Abisso Ancona it is about +4‰. These dif-
ferences between replacement crust and related floor deposit
suggest that changes in the isotopic composition of gypsum
might have occurred after gypsum formed on the walls.
Furthermore, the δ34S values in the upper levels gypsum is
generally higher than those in actively forming gypsum areas,
and also in groundwater H2S. In level V, all the 4 samples have
a δ34S value higher than –11‰; in level III the average value is
–13.5‰ (11 samples), ranging between –11.69‰ and
–16.06‰. These variations of isotopic composition might have
been induced by the isotopic compositions of the H2S released
from the water. That could be caused by the activity of sulfate-
reducing bacteria in the aquifer. As the bacterial activity should
be influenced by environmental factors, such as the groundwa-
ter temperature, the amount of fresh water recharge, and the
extension of the free interface between groundwater and cave
atmosphere, the δ34S values in the ancient gypsum may repre-
sent such factors at the time when the gypsum was produced.
124 • Journal of Cave and Karst Studies, August 2003
GYPSUM DEPOSITS IN THE FRASASSI CAVES, CENTRAL ITALY
CONCLUSIONS
The development of the Frasassi Caves can be clearly relat-
ed to the oxidation of H2S rising in the groundwater. H2S oxi-
dation can involve bacterial activity and occurs mainly in the
shallow phreatic zone, utilizing oxygen dissolved in dripwater
or diffusing from the cave atmosphere. At present, gypsum
deposits form above the water table, where crusts of slushy
gypsum including some large crystals replace the limestone
walls. Below the water table limestone corrosion occurs with-
out gypsum deposition, because sulfate saturation is not
reached in the groundwater.
Three main types of gypsum can be observed in the dry
upper levels of the cave: Replacement crusts similar to the
actively forming deposits, large and thick microcrystalline
floor deposits, and euhedral crystals in mud. The sulfur iso-
topic composition of these gypsum deposits shows that the sul-
fate was supplied by the oxidation of H2S in the cave atmos-
phere. In the Frasassi caves, phreatic sulfate precipitation are
(and were) prohibited due to the dilution of the groundwater by
sulfate-poor meteoric water. These data agree with the sedi-
mentary characteristics and the sub-aerial depositional setting
of the gypsum.
The size of the old massive deposits and their distribution
in the upper cave levels imply that there were some periods
with a gypsum formation more intense than recent one. It prob-
ably can be related to the development of widespread inter-
faces between sulfidic groundwater and the cave atmosphere,
which could exist while the cave was an almost ideal water
table cave. Similar conditions repeatedly occurred during the
cave history, depending on the surface geomorphic evolution.
Small hydrologic changes inside the cave seem capable of
influencing the solutional and depositional effects of the sul-
fidic water circulation inside the same cave system.
The sulfur isotopic data also confirm that large gypsum
floor deposits could form by the flow of slushy gypsum from
the walls and ceilings to the floor. This conclusion might be
helpful in studying similar gypsum deposits, known in other
dry caves of central Italy.
A comparison of the active and dry gypsum deposits made
it possible to show the changes in the sulfur isotopic composi-
tion of the gypsum during limestone replacement. Because the
sulfur isotope composition was related to the depositional set-
ting of the gypsum deposit, H2S caves could represent a good
natural environment in order to study isotopic fractionation of
sulfur for oxidation-reduction processes involving biologic
activity.
ACKNOWLEDGMENTS
We wish to thank Arthur and Margaret Palmer for having
kindly reworked the English and for their helpful suggestions;
we also thank Christian Koeberl for allowing us to use the
mass spectrometer for this study, Antonio Rossi who analyzed
mud samples from the Frasassi Caves, and Mario Cocchioni
who permitted us to utilize data on water chemistry. Helpful
comments during the preparation of the manuscript are also
due to Jennifer Macaledy and Alessandro Montanari. We
should like to thank Carol A. Hill and George W. Moore for
their helpful and constructive review of the paper.
REFERENCES
Ambrosetti, P., Carraro, F., Deiana, G., & Dramis, F., 1982, Il sollevamento
dell’Italia Centrale tra il Pleistocene inferiore e il Pleistocene medio:
C.N.R., Progetto Finalizzato “Geodinamica”, Pubblicazione, v. 513, n. 2,
p. 219-223.
Bisci, C., & Dramis, F., 1991, La geomorfologia delle Marche, in Marche,
Regione, ed., L’ambiente fisico delle Marche: Firenze, S.E.L.C.A., p. 81-
113.
Bertolani, M., Garuti, G., Rossi, A., & Bertolani-Marchetti, M., 1977, Motivi
di interesse mineralogico e petrografico nel complesso carsico Grotta
Grande del Vento-Grotta del Fiume: Le Grotte d’Italia, s. 4, v. 6, p. 109-
144.
Bocchini, A., & Coltorti, M., 1990, Il complesso carsico Grotta del Fiume
Grotta Grande del Vento e l’evoluzione geomorfologica della Gola di
Frasassi, in Galdenzi, S., & Menichetti, M., eds, Il carsismo della Gola di
Frasassi: Memorie Istituto Italiano Speleologia, s. II, v. 4, p. 155-180.
Buck, M.J., Ford, D.C., & Schwarcz, H.P., 1994, Classification of cave gyp-
sum deposits derived from oxidation of H2S, in Sasowsky, I.D., & Palmer,
M.V., eds., Breakthroughs in karst geomicrobiology and redox geochem-
istry: Charles Town, WV, Karst Waters Institute, Special Publication, v. 1,
p. 5-9.
Canfield, D.E., 2001, Biogeochemistry of sulfur isotopes: Reviews of
Mineralogy and Geochemistry, v. 43, p. 607-636.
Ciarapica, G., & Passeri, L., 1978, Speleotemi solfatici e fasi sedimentarie car-
siche: Congresso su Processi Neocarsici e Paleocarsici. Napoli, 1-3 aprile
1978. (unpublished)
Ciccacci, S., D’alessandro, L., Dramis, F., Fredi, P., & Pambianchi, G., 1985,
Geomorphological and neotectonic evolution of the Umbria-Marche
Ridge, Northern Sector: Studi Geologici Camerti, v. 10, p. 7-15.
Cigna, A., & Forti, P., 1986, The speleogenetic role of air flow caused by con-
vection: 1st contribution: International Journal of Speleology, v. 15, p. 41-
52.
Cocchioni, M., 2002, Chimismo delle acque del complesso ipogeo di Frasassi:
Camerino University report, Camerino, Italy, 146 p.
Davis, D.G., 2000, Extraordinary features of Lechuguilla Cave, Guadalupe
Mountains, New Mexico: Journal of Cave and Karst Studies, v. 62, n. 2,
p. 147-157.
Ding, T., Valkiers, S., Kipphardt, H., De Bie’vre, P., Taylor, P.D.P., Gon¢anti-
ni, R., & Krouse, R., 2001, Calibrated sulfur isotope abundance ratios of
three IAEA sulfur isotope reference materials and V-CDT with a reassess-
ment of the atomic weight of sulfur: Geochimica et Cosmochimica Acta,
v. 65, p. 2433-2437.
DuChene, H.R., Hill, C.A., Hose, L.D., & Pisarowicz, J.A., eds., 2000, The
caves of the Guadalupe Mountains research symposium: Journal of Cave
and Karst Studies, v. 62, n. 2, 159 p.
Egemeier, S.J., 1981, Cavern development by thermal waters: National
Speleological Society Bulletin, v. 43, p. 31-51.
Forti, P., Benedetto, C., & Costa, G., 1993, Las Brujas cave (Malargue,
Argentina): An example of the oil pools control on the speleogenesis:
Theoretical and Applied Karstology, v. 6, p. 87-93.
Journal of Cave and Karst Studies, August 2003 • 125
GALDENZI AND MARUOKA
Forti, P., Menichetti, M., & Rossi, A., 1989, Speleothems and speleogenesis of
the Faggeto Tondo Cave (Umbria, Italy), in Hazslinszky, T. & Takacsne
B.K. (eds.) Proceedings, International Congress of Speleology, 10th,
Budapest: v. 1, p. 74-76.
Fry, B., Gest, H., & Hayes, J.M., 1988, 34S/32S fractionation in sulfur cycles
catalyzed by anaerobic bacteria: Applied and Environmental
Microbiology, v. 54, p. 250–256.
Galdenzi, S., 1990, Un modello genetico per la Grotta Grande del Vento, in
Galdenzi, S., & Menichetti, M., eds, Il carsismo della Gola di Frasassi:
Memorie Istituto Italiano di Speologia, s. II, v. 4, p. 123-142.
Galdenzi, S., 1996, Il carsismo profondo nell’Appennino Umbro Marchigiano
(Italia), in Verico, P. & Zorzin, R. (eds.) Proceedings, 1992 International
Congress: Alpine Caves: Alpine karst systems and their environmental
context, Asiago, Italy, p. 229-242.
Galdenzi, S., 1997, First geological data on the caves around the Sibari Plain
(South Italy): Journal of Cave and Karst Studies, v. 59, n. 2, p. 81-86.
Galdenzi, S., 2001, L’azione morfogenetica delle acque sulfuree nelle Grotte
di Frasassi, Acquasanta Terme (Appennino marchigiano - Italia) e di
Movile (Dobrogea–Romania): Le Grotte d’Italia, s. V, v. 2, p. 49-61.
Galdenzi, S., & Menichetti, M., 1995, Occurrence of hypogenic caves in a
karst region: Examples from central Italy: Environmental Geology, v. 26,
p. 39-47.
Galdenzi, S., Menichetti, M., & Forti, P., 1997, La corrosione di placchette cal-
caree ad opera di acque solfuree: Dati sperimentali in ambiente ipogeo, in
Jeannin, P.Y. (ed.) Proceedings, International Congress of Speleology,
12th, Le Chaux-de-Fonds, Switzerland, v. 1, p. 187190.
Galdenzi, S., Menichetti, M., Sarbu, S., & Rossi, A., 1999, Frasassi caves: A
biogenic hypogean karst system? in Audra, P. (ed.) Proceedings European
Conference Karst 99, Grands Causses, Vercors, France: Cagep, Université
de Provence, Etudes de Géographie physique, travaux 1999, suppl. n. 28,
p. 101-106.
Hill, C.A., 1987, Geology of Carlsbad Cavern and other caves in the
Guadalupe Mountains, New Mexico and Texas: New Mexico Bureau of
Mines and Mineral Resources, Bulletin, v. 117, 150 p.
Hose, L.D.& Pisarowicz, J.A., 1999, Cueva de Villa Luz, Tabasco, Mexico:
Reconnaissance study of an active sulfur spring cave: Journal of Cave and
Karst Studies, v. 61, n. 1, p. 13-21.
Kaplan, I.R., & Rittenberg, S.C., 1964, Microbiological fractionation of sul-
phur isotopes: Journal of General Microbiology, v. 34, p. 195-212.
Martinis, B., & Pieri, M., 1964, Alcune notizie sulla formazione evaporitica
del Triassico superiore nell’Italia centrale e meridionale: Memorie Società
Geologica Italiana, v. 4, n. 1, p. 649-678.
Maruoka, T., Koeberl, C., Hancox, P.J., & Reimold, W.U., 2003, Sulfur geo-
chemistry across a terrestrial Permian-Triassic boundary section in the
Karoo Basin, South Africa: Earth Planetary Science Letters, v. 206, p.
101-117.
Maruoka, T., Koeberl, C., Newton, J., Gilmour, I., & Bohor, B.F., 2002, Sulfur
isotopic compositions across terrestrial Cretaceous-Tertiary boundary
successions, in Koeberl, C., & MacLeod, K.G., eds., Catastrophic events
and mass extinctions: Impact and beyond: Geological Society of America
Special Papers, v. 356, p. 337-344.
Palmer, A.N. & Palmer, M.V., 2000, Hydrochemical interpretation of cave pat-
terns in the Guadalupe Mountains, New Mexico: Journal of Cave and
Karst Studies, v. 62, n. 2, p. 91-108.
Principi, P., 1931, Fenomeni di idrologia sotterranea nei dintorni di Triponzo
(Umbria): Le Grotte d’Italia, v. 5, p. 1-4.
Queen, J.M., Palmer, A.N., & Palmer, M.V., 1977, Speleogenesis in the
Guadalupe Mountains, New Mexico: Gypsum replacement of carbonate
by brine mixing, in Ford, T.D. (ed.) Proceedings, International Congress
of Speleology, 7th, Sheffield (U.K.): p. 333-336.
Sarbu, S. M., Galdenzi, S., Menichetti, M., & Gentile, G., 2000, Geology and
biology of the Frasassi Caves in Central Italy, an ecological multi-disci-
plinary study of a hypogenic underground ecosystem, in Wilkens, H., et
al., eds., Ecosystems of the world: New York, Elsevier, p. 359-378.
Sarbu, S.M., & Kane, T.C., 1995, A subterranean chemoautotrophically based
ecosystem: National Speleological Society Bulletin, v. 57, p. 91-98.
Sighinolfi, G. P., 1990, Chimismo ed origine delle acque del sistema ipogeo
“Grotte di Frasassi” (Ancona) Implicazioni speleogenetiche ed
ambientali, in Galdenzi, S., & Menichetti, M., eds, Il carsismo della Gola
di Frasassi: Memorie Istituto Italiano di Speleologia, s. II, v. 4, p. 109-
122.
Taddeucci, A., Tuccimei, P., & Voltaggio, M., 1992, Studio geocronologico del
complesso carsico Grotta del Fiume-Grotta Grande del Vento (Gola di
Frasassi, AN) e indicazioni paleoambientali: Il Quaternario, v. 5, p. 213-
222.
Tazioli, G. S., Cocchioni, M., Coltorti, M., Dramis, F., & Mariani, M., 1990,
Circolazione idrica e chimismo delle acque sotterranee dell’area carsica di
Frasassi nelle Marche, in Galdenzi, S., & Menichetti, M., eds, Il carsismo
della Gola di Frasassi: Memorie Instituto Italiano di Speleologia, s. II, v.
4, p. 93-108.
Vlasceanu, L., Sarbu, S.M., Engel, A.S., & Kinkle, B.K., 2000, Acidic, cave-
wall biofilms located in the Frasassi Gorge, Italy: Geomicrobiology
Journal, v. 17, p. 125-139.
Yanagisawa, F., & Sakai, H., 1983, Thermal decomposition of barium sulfate-
vanadium pentaoxide-silica glass mixtures for preparation of sulfur diox-
ide in sulfur isotope ratio measurements: Analytical Chemistry, v. 55, p.
985-987.
