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

Abstract. Carbonate shells and encrustations from lacustrine organisms provide proxy records of past environmental and climatic changes. The carbon isotopic composition (δ¹³C) of such carbonates depends on the δ¹³C of dissolved inorganic carbon (DIC). Their oxygen isotopic composition (δ¹⁸O) is controlled by the δ¹⁸O of the lake water and on water temperature during carbonate precipitation. Lake water δ¹⁸O, in turn, reflects the δ¹⁸O of precipitation in the catchment, water residence time and mixing, and evaporation. A paleoclimate interpretation of carbonate isotope records requires a site-specific calibration based on an understanding of these local conditions.

For this study, samples of different carbonate components and water were collected 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. Samples from living organisms and sub-recent samples in surface sediments were taken from the calcifying alga Chara hispida, mollusks from the genus Pisidium, and adult and juvenile instars of two ostracod species, Candona candida and Candona neglecta.

Neither the isotopic composition of carbonates nor the δ¹⁸O of water vary significantly with water depth, indicating a well-mixed epilimnion. The mean δ¹³C of Chara hispida encrustations is 4 ‰ higher than the other carbonates. This is due to fractionation related to photosynthesis, which preferentially incorporates 12^C in the organic matter and increases the δ¹³C of the encrustations. A small effect of photosynthetic ¹³C enrichment in DIC is seen in contemporaneously formed valves of juvenile ostracods. The largest differences in the mean carbonate δ¹⁸O between species are caused by vital offsets, i.e. the species-specific deviations from the δ¹⁸O of inorganic carbonate which would have been precipitated in isotopic equilibrium with the water. After subtraction of these offsets, the 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 hispida 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 water temperature leads to a decrease in fractionation between carbonate and water, and therefore to a decrease in carbonate δ¹⁸O. At the same time, an increase in air temperature leads to an increase in the δ¹⁸O of lake water through its effect on precipitation δ¹⁸O and on evaporation from the lake, and consequently to an increase in carbonate δ¹⁸O, opposite to the effect of increasing water temperature on oxygen-isotope fractionation. However, the seasonal and inter-annual variability in lake water δ¹⁸O is small (~0.5 ‰) due to the long water residence time of the lake. Seasonal changes in the temperature-dependent fractionation are therefore the dominant cause of carbonate δ¹⁸O differences between species when vital offsets are corrected.

Temperature reconstructions based on paleotemperature 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 paleotemperature reconstructions.