IMP. simposio danilo

GYPSUM KARST AREAS IN THE WORLD:
their protection and tourist development
Istituto Italiano di Speleologia - Memoria XVI, s.II, pp.
LUMINESCENCE OF SPELEOTHEMS IN ITALIAN
GYPSUM CAVES: PRELIMINARY REPORT
Yavor Y. Shopov1, Diana Stoykova1, Paolo Forti2
Riassunto
Utilizzando una lampada ad impulsi di Xenon si sono ottenuti gli spettri di luminescenza di tre campioni di concrezioni carbonatiche del sistema carsico dell’Acquafredda e di uno della Grotta Novella (Parco dei Gessi Bolognesi,
Italia). Si ritiene che tutte queste concrezioni carbonatiche si siano formate utilizzando esclusivamente la CO2 dell’atmosfera, dato che la roccia in cui si è sviluppata la grotta (i Gessi messiniani) non contiene carbonati. Le immagini ottenute rappresentano praticamente la registrazione delle variazioni climatiche verificatesi durante lo sviluppo
degli speleotemi. Le datazioni U/Th e 14C hanno evidenziato come le concrezioni abbiano iniziato a crescere circa
5000 anni fa. Lo studio di dettaglio degli spettri di luminescenza è ancora in corso.
Parole chiave: Luminescenza, Concrezionamento, Carsismo in gesso, Variazioni climatiche, Bologna, Italia.
Abstract
The luminescence of 3 speleothem samples from the Acquafredda karst system and 1 from the Novella Cave (Gessi
Bolognesi Natural Park, Italy) has been recorded using excitation by impulse Xe- lamp. All these carbonate speleothems
are believed to be formed only from active CO2 from the air, because the bedrock of the cave consist of gypsum and does
not contain carbonates. The obtained photos of luminescence record the climate changes during the speleothem growth.
U/Th and 14C dating proved that studied speleothems started to grow since about 5,000 years ago. The detailed analyses
of the luminescence records is still in progress.
Keywords: Luminescence, Speleothem records, Gypsum karst, Climate changes, Bologna, Italy
Introduction
Calcite speleothems frequently display luminescence that is produced by calcium salts of
humic and fulvic acids derived from soils
above the cave (SHOPOV, 1989a, 1989b;
WHITE & BRENNAN, 1989). These acids are
released by the roots of living plants, and by
the decomposition of dead vegetative matter.
Root release is modulated by visible solar
radiation via photosynthesis, while rates of
decomposition depend exponentially on soil
temperature. The soil temperature depends
mainly on solar infrared and visible radiation
(SHOPOV et al., 1994) in case the cave is covered only by grass, or on air temperatures in
case the cave is covered by forest or bush. In
the first case, microzonality of luminescence
1 University Center for Space Research, Faculty of Physics, University of Sofia, James Baucher 5, Sofia 1164, Bulgaria. E-mail: [email protected]
2 Dipartimento di Scienze della Terra e Geologico Ambientali, Via Zamboni 67, 40127, Bologna, Italy.
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of speleothems can be used as an indirect
Solar Activity (SA) index (SHOPOV et al.,
1990, 1991), but in the second case it can be
used as a paleotemperature proxy (SHOPOV et
al., 1996).
Luminescence organics in speleothems can be
divided in four types:
1) Calcium salts of fulvic acids;
2) Calcium salts of humic acids;
3) Calcium salts of huminomelanic acids;
4) Organic esters.
All these four types are usually present in a
single speleothem with hundreds of chemical
compounds with similar chemical behaviour,
but of different molecular weights.
Concentration distribution of these compounds (and their luminescence spectra)
depends on type of soils and plants over the
cave, so the study of luminescent spectra of
these organic compounds can give information about paleosoils and plants in the past
(WHITE & BRENNAN, 1989). Changes in visible colour of luminescence of speleothems
suggesting major changes of plant society are
rarely observed.
Most known luminescent centres in calcite
are inorganic ions of Mn, Tb, Er, Dy, U, Eu,
Sm and Ce (SHOPOV, 1997). Minerals contain many admixtures. Usually several centres
activate luminescence of the sample and the
measured spectrum is a sum of the spectra of
two or more of them. Luminescence of minerals formed at normal cave temperatures is
normally due mainly to molecular ions and
sorbed organic molecules. But luminescence
of uranil-ion is also very common in such
speleothems.
be potentially really interesting for high resolution luminescence analyses.
Photos on fig.1-4 are negative photographs of
integral phosphorescence of 2-5mm thick
polished sections of the speleothem samples
under excitation by impulse Xe-lamp in the
entire UV and visible spectra. So darker parts
of the image correspond to brighter luminescence, which in turn indicates an higher concentration of humic and fulvic acids, and perhaps to warmer climate (SHOPOV, 1997).
The most interesting among the studied samples is the calcite stalagmite SP1 (Fig. 1) from
the Spipola Cave (Acquafredda karst system).
The U/Th and 14C datings of the first layers
of carbonate speleothems within the cave (but
not of this sample) proved that all the
speleothems in this karst system started to
grow since about 5 thousand years ago
(FORTI, 2003). Moreover all the carbonate
speleothems from the gypsum karst of
Experimental
The visible luminescence of two calcite
speleothems from the Acquafredda karst system and of one calcite speleothem from
Novella Cave (Gessi Bolognesi Natural Park)
has been analysed. All the samples proved to
88
Fig. 1 - Luminescence of sample SP1 from the
Spipola Cave (Acquafredda karst system).
Fig. 2 - Luminescence of the sample NO1 from
the Novella Cave.
Fig. 3 - Luminescence of sample SP2 from the
Spipola Cave (Acquafredda karst system).
Bologna are believed to be formed only from
active CO2 from the air, because the bedrock
of the cave consist of gypsum and does not
contain any significative amount of carbonate
(FORTI & RABBI, 1981). Luminescence of
this sample is very strong and exhibits many
luminescence bands, which in turn make evident many variations in the direction of the
growth axis of the speleothem, therefore this
speleothem has been utilised for detailed
reconstruction of earthquakes of the past
(FORTI & POSTPISCHL, 1986). It was taken
from the cave about 50 years ago (19501955). At that time the speleothem was
active. The cave is covered only by thin layer
(less than 50cm in average) of soil and a large
portion of it is covered by an oak forest. The
location of the main entrance is Lat. 44° 26’
47” / Long. 11° 22’ 58”.
A calcite flowstone from the Inner Doline in
the Spipola Cave, which was some half meters
thick has been analysed. U/Th and 14C dating of it (FORTI, 2003) defined that its bottom is 5,000 and the top 2,000 years old. It
has white colour with long fine crystals along
the growth axes. Photos of luminescence of
two pieces of it (SP2 and SP3) are presented
on Fig. 3 and 4. Luminescence of the SP2
sample (Fig. 3) is strong and exhibits many
fine luminescence bands. Because the half
meters thick flowstone formed in 3,000 years
or less, these bands seems to be annual.
Luminescence of the SP3 sample (Fig. 4) is
stronger but exhibits few luminescence bands.
Finally a calcite flowstone (NO1) from
Novella Cave, 3.5km far from Spipola (Fig. 2)
was analysed: it was sampled about 30 years
ago when it was still active. Luminescence of
this sample is strong and exhibits several thick
luminescence bands with relatively stable
intensity of luminescence. They are separated
by hiatuses. This suggests that this flowstone
was growing relatively fast, but only during
short periods of time (with stable climate).
Such growth pattern suggests that the
speleothem growth was possible only in a very
89
Fig. 4 - Luminescence
of sample SP3 from the
Spipola Cave
(Acquafredda karst system).
small range of climatic conditions, so it did
not grow most of the time. The location of
the main entrance of the Novella Cave is at
Lat. 44° 25’ 38” / Long. 11° 24’ 54”. The situation of this cave is similar to that of
Spipola: completely covered by oak forest.
The average temperature inside the two caves
is about 10-11°C. Both caves are developed in
Messinian gypsum. Further dating of the
studied samples is necessary in order to convert obtained luminescence images into paleoclimatic records.
Acknowledgements
The research has been made under grant
NZ811 of Bulgarian Scientific Foundation to
Y. Shopov and funded by project MURSTCofin 2000, responsible Prof. Ugo Sauro,
University of Padova.
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