engleski

Carbon isotope composition in karst aquatic mosses and possibility of using it in tufa dating

Hygrophyte (true aquatic and wet-rock) moss and mesophyte (land) moss species were collected at the Plitvice Lakes and along the Zrmanja and Krupa rivers water path. Most of the aquatic moss species were tuficulous – tufa forming. Tufa is precipitated on the surface and on the base of the plant when plant uses dissolved inorganic carbon (DIC) from water. The carbon isotope concentrations (a14C and δ13C) were measured in the plants and in DIC. Aquatic mosses can use carbon for the photosynthesis from two carbon reservoirs: atmospheric CO2 and DIC. Fraction of the atmospheric carbon in moss (ωatm. C) was calculated from a14C of the atmosphere, of DIC and of mosses. From their δ13C values and the calculated ωatm. C, 13C fractionation from CO2 (as a part of DIC) to moss tissue (εmoss/g-aq) was also calculated. The recent data were compared with the published data on a14C and δ13C values of moss and DIC from ≈30 years ago (Srdoč et al. 1985 ; Marčenko et al. 1989), when a14C of the atmosphere was 30 % higher. All collected moss samples were identified as C3 photosynthetic cycle plants. 13C fractionation factor from atmospheric CO2 to terrestrial plant tissue for C3 types of plants is well studied and amounts to ≈-20 ‰, however εmoss/g-aq for aquatic plants depends on numerous conditions (e.g., concentrations of HCO3— and CO2, pH, amount of available nutrients, water depth, temperature, water flow velocity). Currently, it is known that the carbon isotope composition in aquatic plants growing in hard-water karst lakes and rivers is strongly influenced by the compositions of DIC, however, εmoss/g-aq has never been determined although different εmoss/g-aq values could point to the difference in mosses carbon photosynthesis pathway. A significant positive correlation of moss a14C and δ13C was found in both periods at Plitvice Lakes and at both Plitvice Lakes and Zrmanja-Krupa locations. It was also found that three species of mosses (true aquatic Cinclidotus aquaticus and wet-rock Hymenostylium recurvirostrum and Ptychostomum pseudotriquetrum) had calculated ωatm. C ≈0 % or even negative implying that they incorporate carbon only from DIC, i.e., that they turn to anabiosis during dry periods. For all other submerged mosses ωatm. C ranged up to 66 %. Calculated εmoss/g-aq values ranged from -43 ‰ to -19 ‰. The anabiotic species, as well as a wet-rock Gimnostomum aeruginosum had the highest εmoss/g-aq, -21.1 ± 1.9 ‰ on the average, which was similar to the fractionation factor for C3 plants from atmospheric CO2. Three groups of moss species were reviled after correlating εmoss/g-aq with ωatm. C: (i) mosses with εmoss/g-aq ≈-21 ‰, (ii) other true aquatic mosses (Cinclidotus riparius, Rhynchostegium riparioides and Fontinalis antipyretica) and (iii) amphiphyte mosses (Palustriella commutata and Eucladium verticillatum). The groups (ii) and (iii) had lower εmoss/g-aq values than -20 ‰. Group (ii) showed statistically significant correlation between εmoss/g-aq and ωatm. C. Amphiphyte mosses (iii) had more intense 13C fractionation from DIC with higher water flow rates. This grouping clearly shows that by having different εmoss/g-aq values, mosses have different photosynthetic mechanisms and different adjustments to the same environment. A statistically significant correlation between δ13C of moss and ωatm.C was observed for the other true aquatic and amphiphyte mosses. This feature opened a possibility of using δ13C of moss remains found in inactive tufas for determination of true tufa 14C age, which is often compromised with uncertain 14C initial activity. ωatm.C from measured δ13Cmoss of a fossil moss with unknown age could be calculated form the δ13Cmoss - ωatm.C correlation. Knowing a14C of moss and approximating a14C of tufa with a14C of DIC at the time of tufa formation, the “a14C of the atmosphere” could be calculated. This value is the percentage of non-decayed 14C atoms from the day of tufa formation which can then be used in a regular formula for 14C age determination. However, there are numerous limitations to this age determination, among others is the very large standard uncertainty of tufa ages derived in this way. This can be overcome by more moss and DIC sampling and δ13C and a14C measurements in order to reduce the uncertainty of δ13Cmoss -ωatm.C correlation line. Nevertheless, with this current cognition, this procedure could be applied for the moss species Palustriella commutata, the most abundant one at the Plitvice Lakes, which introduced the highest amount of the atmospheric carbon and so had the lowest uncertainty.

project REQUENCRIM ; workshop ; quaternary ; isotope methods ; moss ; isotope fractionation

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