Changes in carbon isotope distribution in sediments of Lake Plateliai, Lithuania, over the last 130 years

The radiocarbon (¹⁴C) dating of lake sediments is widely used to estimate the so-called reservoir age (RA), i.e. the ¹⁴C age difference between the atmospheric and lake ecosystem carbon reservoirs. However, lake sediments are a mixture of autochthonous and various allochthonous carbon sources having distinct ¹⁴C specific activities. The RA depends on the catchment bedrock, CO₂ exchange rates between water and the atmosphere, which are affected by organic carbon production and decomposition rates, inflow/outflow of organic and inorganic mater, water residence time, water level fluctuations, climate change, and other environmental factors impacting the lake’s catchment area. Every disturbance affecting carbon exchange between the water ecosystem, the terrestrial environment, and the atmosphere impacts carbon isotope distribution in the lake ecosystem.

Lake Plateliai is the largest lake in the north-western part of Lithuania (Samogitia). It is located on the territory of Samogitia National Park. The absence of cultivated fields on the park’s territory conditioned the lake to remain one of the cleanest in Lithuania. The present study focusses on sediment records from Lake Plateliai over the last 130 years. This time period is related to dam-induced lake’s water level fluctuations, increase/decrease in primary productivity due to intensive agricultural development since the 1960s, and its decline in the 1990s.

The aim of this work was to estimate how environmental factors have influenced the carbon cycle within the lake and how these impacts are recorded in sediments, i.e., changes in sedimentation rate, carbon isotope distribution among organic sediment fractions.

During the last 130 years, the radiocarbon reservoir age of the of the alkali soluble and alkali insoluble fractions of lake sediments has been reduced by 872.4 ±80 years, and a decreasing trend of ¹⁴C concentration values is recorded/observed in the upper layers. The¹⁴C specific activity values in both sediment organic fractions coincided during the last ten years and 1885-1932. However, changes in the water level during the period 1963-1976 and unknown events in 1939-1940 led to the introduction of allochthonous origin matter into the lake ecosystem, resulting lower ¹⁴C concentrations in the alkali soluble fraction compared to the alkali insoluble fraction.