A modern snapshot of the isotopic composition of lacustrine biogenic carbonates: Records of seasonal water temperature variability

Carbonate shells and encrustations from lacustrine organisms provide proxy records of past environmental and climatic changes. The oxygen isotopic composition (δ¹⁸O) of such carbonates depends on water temperature during carbonate precipitation, and on the δ¹⁸O of the lake water. Lake water δ¹⁸O, in turn, is controlled by the δ¹⁸O of precipitation in the catchment, water residence time and mixing, and by evaporation. A paleoclimate interpretation of carbonate δ¹⁸O records requires a site-specific calibration based on an understanding of the local conditions.

For this study, carbonates and water were sampled in the littoral zone of lake Locknesjön, central Sweden (62.99°N, 14.85°E, 328 m a.s.l.) along a water depth gradient from 1 to 8 m. We took samples from living organisms and sub-recent samples in surface sediments of the calcifying algae Chara hispida, the mollusk Pisidium, and adult and juvenile instars of two ostracod species, Candona candida and Candona neglecta.

We show that neither the δ¹⁸O of carbonates nor the δ¹⁸O of water vary significantly with water depth, indicating a well-mixed epilimnion. The largest differences in the mean carbonate δ¹⁸O between species are caused by vital offsets, i.e. the species-specific deviation from the δ¹⁸O of inorganic carbonate which would have been precipitated in isotopic equilibrium with the water. After subtraction of these constant vital offsets, remaining differences in the mean carbonate δ¹⁸O between species can mainly be attributed to seasonal water temperature changes. The lowest δ¹⁸O values are observed in Chara encrustations, which form during the summer months when photosynthesis is most intense. Adult ostracods, which calcify their valves during the cold season, display the highest δ¹⁸O values. This is because an increase in temperature leads to a decrease in fractionation between carbonate and water, and therefore to a decrease in carbonate δ¹⁸O. An increase in temperature also leads to an increase in the δ¹⁸O of lake water through its effect on precipitation δ¹⁸O and on evaporation, and consequently to an increase in carbonate δ¹⁸O, opposite to the temperature effect on fractionation. However, the seasonal and inter-annual variability in lake water δ¹⁸O is small (0.5‰) due to the long water residence time. Seasonal changes in the temperature-dependent fractionation are therefore the dominant cause of carbonate δ¹⁸O differences between species.

Temperature reconstructions based on “paleo-temperature” equations for equilibrium carbonate precipitation using the mean δ¹⁸O of each species and the mean δ¹⁸O of lake water are well in agreement with the observed seasonal water temperature range. The high carbonate δ¹⁸O variability of samples within a species, on the other hand, leads to a large scatter in the reconstructed temperatures based on individual samples. This implies that care must be taken to obtain a representative sample size for paleo-temperature reconstructions.