An isotope mass balance model for the correlation of freshwater bivalve shell (Unio pictorum) carbonate δ18O to climatic conditions and water δ18O in Lake Balaton (Hungary)

The oxygen isotope composition of bivalve shells (delta O-18(shell)) can potentially record environmental variability of shallow lakes and therefore it has been extensively used as a proxy in the reconstruction of past climate and environmental conditions. As delta O-18(shell) reflects - besides the water temperature - the oxygen isotope composition of lake water (delta O-18(L)), it is required to interpret the quality and impact of parameters influencing the delta O-18(L). Using the isotope mass balance model. I tested the hypothesis that Balaton lake water delta O-18 variability can be described as a result of the combined effects of three main climatic parameters such as river runoff precipitation and evaporation. 1 calculated delta O-18(L) time series for the period 1999-2008 for the whole water body at Siofok (eastern part of Lake Balaton, Hungary) based on measured precipitation, inflow and evaporation amount and measured inflow, precipitation delta O-18 and calculated vapour delta O-18 data. The comparison of the modelled delta O-18(L) time series to measured surface delta O-18(L) data revealed that delta O-18 of Balaton water is sensitive for variation of climatic parameters. This variability is most striking at the surface, while according to the results of the model, the whole water body itself is less sensitive. Monthly differences suggest that generally during summer the whole water body is mixed up, while moderate isotope stratification (0.3-0.7 parts per thousand difference between surface and whole water body) can be assumed in early spring and autumn. Predictions of shell 6180 values were made using the measured surface water delta O-18 data and the modelled delta O-18 values for the whole water body. High-resolution sampling was conducted on two Unio pictorum shells covering the period of 2001-2008, and both predictions were compared to measured shell delta O-18 records. The results showed that the prediction for the whole water body gives a better fit to the measured shell delta O-18, suggesting that the whole water body better describes the isotope variability of shell carbonate. As a practical application of the isotope mass balance model was used to present the effect of precipitation and evaporation on delta O-18(shell). The relationship between intra-shell delta O-18-variability and precipitation amount (precipitation/evaporation ratio) were determined, which allows the quantified prediction the impact of meteorological parameters affecting the oxygen isotope composition of shell carbonate.