Barešić, Jadranka; Sironić, Andreja; Faivre, Sanja; Damir, Borković; Ivanka, Lovrenčić Mikelić; Krajcar Bronić, Ines
Does fresh-water tufa have potential in paleoresearch?.
In: Book of Abstracts, 3rd Radiocarbon in the Environment Conference. Giwice, Poland, Silesian University of Technology, p. 154
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Abstract
Tufa is a fresh-water surface calcium carbonate deposit precipitated at or near ambient temperature, which commonly contains the remains of macro- and microphytes. Within the project Reconstruction of the Quaternary environment in Croatia using isotope methods financed by Croatian Science Foundation (2014-2018, HRZZ-IP-11-2013-1623), several locations of fresh water tufa have been found along the Zrmanja and Krupa Rivers, Dalmatian karst, Croatia. Radiocarbon dating of the carbonate fraction of tufa revealed that majority of the samples was formed during the Holocene. However, samples from location Sanaderi found at different altitudes gave a broad range of dates, from modern to older than 50 ky. The samples with ages close to the limit of the radiocarbon method can be dated also by the U/Th method. Additionally, it was recently suggested that radiocarbon dating of organic residue may lead to more accurate ages that dating of carbonates. Therefore, we performed radiocarbon AMS dating of organic residue of 11 tufa samples from the Sanaderi location, at altitudes between 50 m and 75 m asl. The ages of organic residue of samples, that yielded carbonate conventional ages from 900 BP to 7300 BP, were from modern to 5300 BP. By applying freshwater reservoir correction of 85 pMC to carbonate fraction and 97 pMC to organic fraction (obtained from radiocarbon activity of dissolved inorganic carbon and fresh moss, respectively), both groups of dates agree well. Radiocarbon dating of tufa organic residue with carbonate fraction ages >20 ky, yielded ages from 7400 BP to 17000 BP. Difference between the organic and carbonate ages increased with the increase of age. The difference cannot be explained by including reservoir correction of 85 pMC and 97 pMC for carbonate and organic fraction, respectively. This is clear evidence that reservoir freshwater effect valid for the Holocene cannot be applied for age correction of older samples. Stable carbon isotope composition (δ 13C) of carbonate fraction ranges between -10.5‰ and -8.5‰, with the mean values of (-9.6 ± 0.5)‰ and (-9.7 ± 0.8)‰ for the Holocene and the older samples, respectively. These values indicate the autochthonous origin of the carbonate and negligible contribution of old limestone contamination. The δ 13C values in organic residue ranged from -30.9‰ to- 28.8‰, with the average values of (- 30.5 ± 0.3)‰ and (-29.6 ± 0.6)‰ for organic residue having ages 5000 BP, respectively. These values suggest a unique carbon source for photosynthesis, mainly atmospheric, i.e., tufa forming plant was the same for all dated samples, with an indication of the Suess effect in δ 13C during last centuries. Since some of the radiocarbon ages of organic residue do not seem credible, e.g., 17000 BP, we suggest a hypothesis of contamination of older organic matter by incorporation of fresh organisms. We extracted humin as the most stable organic fraction and the AMS dating results will help resolving this problem and perhaps give the answers on paleo-freshwater effect. Nevertheless, additional parallel analyses of tufa carbonate fraction and organic residues are needed, if samples of good-quality autochthonous and compact tufa can be provided.
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