engleski

Determining environmental conditions that shape shallow, partly submerged coastal cave today – case of Y-cave, Dugi otok, Croatia

Last late Pleistocene–Holocene sea transgression submerged hundreds of caves of Dinaridic Classical karst along the Croatian coast. From the total number of >235 investigated caves, more than half host completely marine conditions (euhaline) with the belonging biocenoses (b. of semidark caves and b. of caves and ducts in total darkness) whilst other (mainly coastal) experience different environmental conditions depending on the depth, tide and seaward freshwater discharge, and there species of organisms and biocenoses are distributed accordingly. One of such is Y-cave, shallow (down to -12 m), over 80 m long horizontal cave developed in Turonian (K2) limestone, located on Dugi otok Island (Croatia), where impact of these different environmental conditions could be recognised within only a few tens of meters distance. Geochemical condition produced by the interaction of freshwater and seawater, known as mixing corrosion, is responsible for dissolutional effects that influence the cave carbonate regardless if it is secondary deposited flowstone or limestone bedrock. Since pronounced dissolution and freshwater influence (corroded speleothems and bedrock, lack of sessile marine organisms) were visually noticed only in particular areas of the Y-cave, sets of limestone tablets coupled with temperature data loggers were placed along the cave and left exposed for 1 year, in order to asses dissolutional effect within different cave conditions. Results obtained by measuring the mass difference of carbonate tablets coincide with organisms' distribution and abundance. Moreover, the results, although preliminary, indicate that in spite of submarine position, the environment in particular areas of the cave is not saturated with respect to CaCO3, as expected in the shallow sea, but aggressive. The distribution of living communities along the cave together with the results of measured abiotic parameters clearly reflects complex environmental conditions that shape this shallow, partly submerged coastal cave today. Hypogene caves are lately defined as structures dissolutionally enlarged by the upwelling flow, independent from the surface influence, i.e. the most pronounced is hydrogeological aspect. But if we take into consideration geochemical aspect of the cave formation being induced not only by meteoric freshwater, but by the other sources as well (seawater in this case), Y-cave can be regarded as a cave with at least one phase of speleogenesis ascribed to hypogene settings.

submerged karst ; marine caves ; mixing corrosion ; environmental complexity

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