engleski

How to measure the passage of time in karst?

The only way of documenting the time is by monitoring karstification and measurement of its products. Subaerially exposed limestones are 'decorated' by various types of grikes which generally follow the rule: the larger - the older, but how old? In stabile climate it should be easy to associate the size of karren with its age, provided we know the mass of dissolved calcite. Kamenitzas are common all over the karst world and document the volume of dissolved calcite, so it should be possible to use them as a proxy for dating of the exposed surfaces. We have tested the possibility of determining the age of kamenitzas as a function of limestone dissolution rate and their volume. In our experiment we have: a) measured the volume of several closed kamenitzas, b) analyzed mineralogical composition of their substrate, and c) calculated minimal age of kamenitzas as a function of their volume and dissolution rate of limestone. The type of limestone is determined by calcimetry and XRD analyses of insoluble fraction, whereas the limestone dissolution rate was measured at various localities by other researchers by dissolution of 'standard' limestone tablets. To account for difference of local limestone and 'standard' tablets, we have tested their solubility under controlled laboratory conditions. Smaller kamenitzas were apparently formed in more-or-less stabile climatic conditions, in considerably shorter time compared with very large kamenitzas which must have been formed in time spans when climate change provided significant influence on the rate of their growth. Theoretically a kamenitza of 100 cm3 was formed by dissolution of 271 g. of calcite in 5.593, 2 years at dissolution rate of 0, 04845 g/y [1]. Consequently, measured small kamenitza on Rab Island with volume of 837, 4 cm3 would be formed in 46.839, 5 years. However, it is very unlikely that the corrosion started in Late Pleistocene at the time of very low temperature and CO2 levels [2]. Consequently, extrapolation of modern corrosion rates to geological history is misleading, and relationship of karstification with paleoclimate should be further studied. [1] Krklec, K. (2011): Korozija karbonatnih stijena i geneza reljefa na otoku Visu – korelacija s odabranim priobalnim lokalitetima. Ph.D. Thesis, University of Zagreb. [2] Petit et al. (1999): Nature 399, 429-436.

Karstification; grikes; limestone dissolution; paleoclimate; dating

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