Early and mid-holocene environmental conditions in the eastern Adriatic recorded in speleotherms from Mala Špilja cave and Velika Špilja cave (Mljet island, Croatia)

Abstract

This study presents high resolutionoxygen and carbon isotopic record of two U-Th dated stalagmites from the Eastern Adriatic caves. The stalagmites were collected from Mala spilja and Velika spilja caves situated on Mljet Island in the southern part of the Croatian Adriatic. Dripwater samples were collected from Medvjeđa spilja, Strasna pec, Spilja u Vrdolje, Kraljicina spilja, Velika spilja and Mala spilja caves. All caves formed in well-stratified Cretaceous limestones. The average value of deuterium excess of the dripwater is 9.3 ‰, indicating that the atmospheric conditions over the Atlantic Ocean have greater influence on the isotopic composition precipitation in the Adriatic region than the Eastern Mediterranean Sea. The longest isotopic record of the speleothems is of stalagmite MSM-1 from Mala spilja Cave dated from 119.2 ±3.3 ka to 5.6±0.6 ka, although deposition was not continuous. During the Holocene the speleothem grew between 7 ka and 4 ka. That interval is characterized by several δ18Oc and δ13Cc fluctuations reflecting changes in the environment. Superimposed on these fluctuations, there is an increasing trend in the 7.0−6.5 ka interval, evident in elevated δ18O values, which primarily refers the trend to drier conditions in the eastern Adriatic, which were occasionally interrupted by wet intervals. Humid conditions are particularly pronounced by low δ18Oc and δ13Cc values of speleothem MSM-1 between 7.3 and 6 ka, as a reflection of increased precipitation and lower temperatures. The transition to today's Mediterranean climate (Cs) occurred between 6 ka and 5 ka, while the driest conditions are recorded at ~4.7 − 4.2 ka. Comparisons between the eastern Adriatic δ18Oc and δ13Cc records of the speleothem from Mala spilja Cave (MSM-1) and Velika spilja Cave (Mljet Island) with Soreq Cave (Israel) and Corchia Cave (Italy) together with other proxies such as lake sediments demonstrate that the speleothems from the Eastern Adriatic caves record the local and the regional climatic changes. Key words: Holocene, speleothem; stable isotopes; Eastern Adriatic, Croatia. Rekonstrukcija okoljskih razmer v zgodnjem in poznem Holocenu na podlagi zapisa v sigi jam Mala Spilja in Velika Spilja (otok Mljet, Hrvaska) Raziskava temelji na visoko locljivih zapisih kisikovih in ogljikovih izotopov, pridobljenih iz dveh stalagmitov, datiranih z uran-torijevo metodo. Kapnika sta bila vzorcena v jamah Mala spilja in Velika spilja na otoku Mljet, v južnem delu hrvaskega Jadrana. Vzorcena je bila tudi prenikla voda v jamah Medvjeđa spilja, Strasna pec, Spilja u Vrdolje, Kraljicina spilja, Velika spilja in Mala spilja. Vse omenjene jame so v plastovitem krednem apnencu. Povprecna vrednost presežka devterija v prenikli vodi je 9,3 ‰, kar kaže na prevladujoc podnebni vpliv Atlantskega oceana v primerjavi z vplivom vzhodnega Sredozemlja. Najdaljsi izotopski zapis smo pridobili iz kapnika MSM-1 iz Male spilje, katerega starost je med 119,2 ±3,3 ka in 5,6±0,6 ka. Za kapnik je znacilnih vec prekinitev izlocanja. V holocenu je kapnik rastel v obdobju med 7 ka in 4 ka. V tem obdobju je vec izrazitih nihanj δ18Oc in δ13Cc, kar kaže na spremembe okolja. Med 7,0 in 6,5 ka je v zapisu trend narascanja δ18O, kar kaže na relativno suhe razmere v vzhodnem Jadranu, ki so jih obcasno prekinila vlažna obdobja. Med 7,3 in 6 ka je vec obdobij z nizkim vrednostmi δ18Oc in δ13Cc v vzorcu MSM-1, kar kaže na veliko padavin in nizke temperature. Prehod v danasnjo sredozemsko klimo (Cs) se je zgodil med 6 ka in 5 ka, najbolj suha obdobja smo zabeležili med 4,7 ka in 4,2 ka. Primerjava izotopskih zapisov δ18Oc on δ13Cc iz Male spilje (MSM-1) in Velike spilje (Mljet) z zapisi v jami Soreq (Izrael) in v sistemu Corchia (Italija) ter drugimi kazalci nekdanjih okolij, npr. jezerskimi sedimenti, kažejo, da sige vzhodne ga Jadrana hranijo pomembne zapise o lokalnih in regionalnih podnebnih spremembah. Kljucne besede: Holocen, siga, stabilni izotopi, vzhodni Jadran, Hrvaska.