Maardu järve setete orgaanilise aine põhine paleokeskkonna rekonstruktsioon [Magistritöö. Juhendajad: A. Heinsalu; M. Liiv]

Based on the composition of lake sediments, it is possible to study the history of the lake and its catchment area and find the reasons for the changes. The lake sediment organic matter is a mixture of formerly living limnic organisms from the lake itself and terrestrial biotic remains from its catchment. The research is based on post-glacial lake sediment sequence from Lake Maardu. It was chosen as the object of this work, because earlier research had recorded distinct changes in the history of the lake. The master's thesis had three main objectives: (1) to determine the variability of organic matter in Lake Maardu during the Holocene; (2) comparison of the results of organic matter in lake sediments with other bio-, climate- and magnetostratigraphy proxies and (3) reconstruction of the development of Lake Maardu over the last 10,700 years.

Maardu lake sedimentary record was first analysed by the loss-on-ignition method to determine the sediment organic matter, mineral matter and calcium carbonate content. Secondly, the sediment organic carbon and total nitrogen content were determined and used to calculate the C/N ratio. The multi-proxy approach was applied to reconstruct the evolution of the lake and its catchment area including also the stratigraphic evidence from previous studies such as chronology, pollen, magnetic parameters, diatoms and green algae. In addition, statistical analyses were implemented in the paleoreconstruction.

Based on the cluster analysis, the development of Lake Maardu can be divided into five different stages. As a result of the regression of the Yoldia Sea, the basin of Lake Maardu became terrestrial at 10,700-10,600 years before present (BP). The Ancylus Lake transgression at 10,600 years BP caused the formation of the lake. The establishment of vegetation and soil stabilisation in the lake catchment area occurred until 9,300 years BP and the lake water had relatively high mineral content. In between 9,300-3,700 years BP, the lake was in its natural status, climate was warmer than at present, the catchment area was covered with broad-leaved forest, no soil erosion occurred and the lake water had low nutrient concentrations. Since 3700 years BP early farming activities in the vicinity of the lake are recorded as increased catchment topsoil erosion and higher nutrient content in the lake. At the end of the 19th century, the lake was drained and the bottom of the lake became dry. In the 20th century Lake Maardu was restored and the water from phosphorite quarries was discharged into the lake, which led to a deterioration of the water quality.

Current study indicated that the sedimentary variables of Lake Maardu record such as organic matter, organic carbon and total nitrogen content integrate the signals of lake evolution, climatic and hydrological changes throughout the Holocene as well as long-term rural land use practices in the surrounding of the lake. However, sediment organic matter C/N ratio captured only extraordinary events, such as regression of the Yoldia Sea and terrestrialisation of the lake depression as well as the man-made drainage of the lake. Long-term natural processes and also millennia-long agricultural land use in the catchment area were not reflected in the sediment C/N ratio, possibly due to large lake surface area.