Michal Gradzinski (firstname.lastname@example.org)
Institute of Geological Sciencess, Jagiellonian University, Oleandry Str. 2a, 30-063 Krakow, Poland
Cave pearls have been known since a long time from many caves. Typical cave pearls are characterised by regular internal lamination and smooth to polished external surfaces. Most of authors ascribed their origin to physico-chemical precipitation of calcium carbonate in highly agitated waters.
Some authors described also other type of cave pearls (in this paper called irregular cave pearls) which are built of micrite, display rough external surfaces and irregular internal lamination. Origin of these pearls still remains open question. Field observations suggest that irregular pearls grow in calm waters.
Several hundreds of cave pearls have been studied by means of standard petrographic microscope and scanning electron microscope coupled with EDS analyser. They were collected in caves of Slovakia (the Slovakian Karst, the Lower Fatra Mts.), Slovenia and southern Poland (the Western Tatra Mts., the Cracow-Wieluñ Upland, the Holy Cross Mts.). Some pearls collected in 1961 in the Cuban caves (the Sierra de los Organos and the Caguanes peninsula) were studied as well.
The biggest pearls reach a few centimetres in diameter while the smallest ones do not exceed few millimetres. Small gastropod shells, clastic grains, bat bones are nuclei of the pearls. Some pearls lack nuclei especially that, which are characterised by soft and pasty consistency, similar to fresh moonmilk speleothems.
Various microbial structures can be observed in the pearls under SEM. Detailed investigation reveals that they are of globular, filamentous and rod shape. The sizes of globular and rod-shaped structures reach from 0.1 µm to several micrometers. The filamentous structures are several microns long and their diameters range from 0.1 µm to more than 1 µm. In some cases they form dense web wrapping pearls. The discussed structures are calcified replicas of microbial (bacterial or fungal) cells. They are most common in irregular cave pearls. SEM investigation indicates that in pearls found in statu nascendi some living micro-organisms occur as well. The above observations are strongly supported by microbiological analysis of the sampled pearls.
It should be stressed that almost all of the examined microbial replicas are characterised by three dimensional morphology without evidence of collapse. Another important feature is the similarity of shape and size of the living organisms to their mineral replicas. These features suggest that calcification took place when the microbes were alive, during their death or shortly after their death.
Frequent occurrence of microbial structures in irregular cave pearls proves that the more regular cave pearls, the less role of microbes in it growth. It can be suggested that low energy of water promotes microbial activity causing cave pearls growth. Rare occurrence of microbial fabrics within the regular, smooth pearls confirmed their non-biological origin.
This study was supported by KBN (State Commitee for Scientific Research) grant no 6P04D 019 14.
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