Abstract
Tufa is a fresh-water surface calcium carbonate deposit precipitated at or near ambient temperature, and commonly contains the remains of macro- and microphytes. Many Holocene tufas are found along the Zrmanja River, Dalmatian karst, Croatia. In this work we present radiocarbon dating results of older tufa that was found for the first time at the Zrmanja River near the Village of Sanaderi. Tufa outcrops were observed at different levels, between the river bed and up to 26 m above its present level. Radiocarbon dating of the carbonate fraction revealed ages from modern, at the river bed, up to 40 kBP ~20 m above its present level. These ages fit well with the hypothesis that the Zrmanja River had a previous surface connection with the Krka River, and changed its flow direction toward the Novigrad Sea approximately 40 kBP (Marine Isotope Stage 3). Radiocarbon AMS dating of tufa organic residue yielded a maximum conventional age of 17 kBP for the highest outcrop position indicating probable penetration of younger organic material to hollow tufa structures, as confirmed by radiocarbon analyses of humin extracted from the samples. Stable carbon isotope composition (δ13C) of the carbonate fraction of (−10.4 ± 0.6)‰ and (−9.7 ± 0.8)‰ for the Holocene and the older samples, respectively, indicate the autochthonous origin of the carbonate. The δ13C values of (−30.5 ± 0.3)‰ and (−29.6 ± 0.6)‰ for organic residue, having ages <500 BP and >5000 BP, respectively, suggest a unique carbon source for photosynthesis, mainly atmospheric CO2, with an indication of the Suess effect in δ13C during last centuries. The oxygen isotopic composition (δ18O) agrees well with deposition of tufa samples in two stages, the Holocene (−8.02 ± 0.72‰) and “old” (mainly MIS 3 and the beginning of MIS 2) (−6.89 ± 0.34‰), suggesting a ~4 °C lower temperature in MIS 3 compared to the current one.
Highlights
Tufa from a period pre-dating the Holocene, was found in the Zrmanja River area and this is the first time that tufa from Marine Isotope Stage 3 (MIS 3) and the beginning of MIS 2 has been found in Croatia
The carbon stable isotope composition of the carbonate fraction indicates that tufa formation in all periods was authigenic, without incorporation of old detrital limestone and suitable for paleoenvironmental research
Radiocarbon dating of tufa samples found at the river-bed level revealed similar ages of both organic and carbonate fractions
Summary
Tufa is a secondary carbonate wide spread within the karst areas worldwide. It is often the only available secondary carbonate within an area of important interest for paleoenvironmental investigation. Dating of tufa and its use as paleoenvironmental tool has always been connected with difficulties. Researchers have attempted to obtain reliable dates from tufa samples using various approaches, such as establishment of the direction of tufa growth [4–8], radiocarbon dating after correction for the effects of “dead” carbon, or using U-Th dating. Tufa may erode, incorporate some allochthonous terrestrial material and “dead” carbonate, and experience recrystallization [9–11]. Reliable dating of tufa enables interpretation of the physico-chemical proxies and can provide many answers on Holocene and Pleistocene palaeoclimate [1,4,6,7,12–25]. It is essential that the results obtained by different analyses can be put into a correct time frame
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