Magnetic dating of the Holocene monogenetic Tkarsheti volcano in the Kazbeki region (Great Caucasus)

The radiocarbon technique is widely used to date Late Pleistocene and Holocene lava flows. The significant difference with palaeomagnetic methods is that the 14C dating is performed on the organic matter carbonized by the rock formation or the paleosols found within or below the lava flow. On the contrary, the archaeomagnetic dating allows to date the moment when the lava is cooling down below the Curie temperatures. In the present study, we use the paleomagnetic dating to constrain the age of the Tkarsheti monogenetic volcano located within the Kazbeki Volcanic Province (Great Caucasus). A series of rock-magnetic experiments including the measurement of hysteresis curves, isothermal remanence, back-field and continuous thermomagnetic curves were applied. These experiments indicated that Pseudo-Single-Domain Ti-poor titanomagnetite is responsible for remanence. A characteristic remanent magnetization was obtained for all twenty analyzed samples yielding a stable single magnetization component observed upon both thermal and alternating field treatments. Comparison of the mean directions obtained (Inc = 48.6º, Dec = 6.4º, A95 = 4.0° and K = 67) with the SCHA.DIF.14k model yielded two main time intervals (4740–4650 or 4427– 4188 BC) as the best age estimate of the Lesser Tkarsheti lava flow. These results suggest an earlier age (between approximately 200 and 700 years) for this monogenetic lava flow than expected from the estimated age provided by a former 14C dating obtained in 1973 on woody remains. This first attempt to use the archaeomagnetic technique in the Caucasus indicates that the SCHA.DIF.14k geomagnetic model may be successfully used for dating purposes in the region.Projects BU0066U16 and BU235P18 (Junta de Castilla y Leon, Spain) and the European Regional Development Fund (ERDF). AG is grateful for financial support of CONACyT 252149 and UNAM-PAPIIT project 101717. MC-R and AC acknowledge the financial support given by the Junta de Castilla y León (project BU235P18) and the European Regional Development Fund (ERD)

Using radiocarbon to determine the mycorrhizal status of fungi

Measurements of C-13 in fungal sporocarps are useful in assessing mycorrhizal or saprotrophic status. Because C-14 measurements can indicate the age of fungal carbon (C) and mycorrhizal fungi depend closely on recent photosynthate, C-14 may provide additional insight into possible mycorrhizal status. Sporocarps, needles, and litter from Woods Creek, OR, USA together with archived sporocarps were measured for C-14 content by accelerator mass spectrometry. Known mycorrhizal fungi resembled current-year needles (Amanita, Cantharellus and Gomphidius) or atmospheric CO₂ (Tuber) in C-14 and indicated an average age of 0-2 yr for incorporated C, whereas saprotrophic genera (Pleurocybella , Lepiota and Hypholoma) were composed of C at least 10 yr old. Of genera tentatively considered mycorrhizal from previous work with C-13, only Otidia and Sowerbyella appeared mycorrhizal from C-14 measurements, whereas Aleuria, Clavulina, Paurocotylis and Ramaria (sensu lato) consisted of older carbon and were presumably saprotrophic. C-14 clearly separated known mycorrhizal or saprotrophic fungi and indicated C-13 measurements should be interpreted cautiously on species of unknown status. C-14 results for needles and mycorrhizal fungi suggested that C sources other than atmospheric CO₂ may contribute small amounts of C. Possible sources include storage of carbohydrates and amino acids, organic nitrogen uptake, and incorporation of soil-respired CO₂ by anaplerotic or photosynthetic pathways