Süsiniku ja lämmastiku isotoopsuhete analüüs: parima eeltöötlusmeetodi valimine ja selle testimine paleolimnoloogilises uuringus Ähijärve näitel [Magistritöö. Juhendaja: S. Veski; kaasjuhendaja: T. Alliksaar]

Isotope ratio of carbon and nitrogen: searching the best pre-treatment method and applying in paleolimnology at Lake Ähijärv It is important to understand the ongoing global climate and environmental change, which is a combination of natural and manmade processes that have shaped the Earth for millennia. Paleolimnology is a part of past environmental research that uses lake sediments to reconstruct past environmental history by understanding the factors that have shaped lake ecosystems and their impacts. Over the past decades stable isotope analysis has become an important technique in (paleo)ecology. This master's thesis focuses on the analysis of carbon and nitrogen isotope ratios (δ13C and δ15N) from lake sediments, the values of which indicate the origin of organic matter in lakes, changes in lake productivity and various climatic and environmental parameters. The master's thesis has two main goals: a) search for the best pre-treatment method for the analysis of organic δ13C and δ15N in lake sediments and b) application in paleolimnology at Lake Ähijärv with the aim to reconstruct the post-glacial environmental changes in the lake.

The sediment material used in the master's thesis comes from Lake Ähijärv, which is located in Võru County, Antsla Parish, Karula National Park. Previously, several analyzes have been performed from Lake Ähijärv sediments - agescale, sediment composition, analysis of pollen/spores and siliceous algae - this study contributes with the proxy of stable isotopes from the sediment. The collected material was used for interdisciplinary interpretation of the results.

Previous research has found that various pre-treatment methods have been used to analyze δ13C and δ15N in lake sediments, giving different and sometimes conflicting results. The main pretreatment problems are related to sediment homogenization, nitrogen measurements and removal of inorganic carbon from the samples to determine the isotope ratio of organic carbon. In order to determine the best pre-treatment method, different methods were tested, their analytical results were compared and the most suitable preparation method was found for each problem. The most efficient method determined by this thesis was: 1) to homogenize samples in the ball mill Eppendorf test tubes with steel balls can be used, 2) for δ15N determination it is preferable to analyze the untreated samples and 3) to get rid of inorganic carbon for δ13C measurements fumigation method with 20% HCl for 6 h should be used when carbonate content of samples is less than 30%. If carbonate content is more than 30% then a stronger concentration of acid (for example 36% HCl) should be preferred.

The best method was then applied for the analysis of δ13C and δ15N in Lake Ähijärv sediments. The obtained results showed that the values of isotope ratios correlate well with previously existing material, which reflect changes in the lake caused by climate change and human impacts. There is a particularly strong correlation between δ-value indicators related to human activities, which shows the important role of humans in rearrangement of natural processes since 1000 years AD. In Anett Kuldma, süsiniku ja lämmastiku isotoopsuhete analüüs 7 addition, the results show that paleolimnological studies help to predict the environmental conditions that may affect the lake in the future.