EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

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

Inga Labuhn1,2, Franziska Tell1,3, Ulrich von Grafenstein4, Dan Hammarlund2, Henning Kuhnert3, and Bénédicte Minster4
Inga Labuhn et al.
  • 1Institute of Geography, University of Bremen, Bremen, Germany
  • 2Department of Geology, Lund University, Lund, Sweden
  • 3MARUM, University of Bremen, Bremen, Germany
  • 4LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, Gif-sur-Yvette, France

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.

How to cite: Labuhn, I., Tell, F., von Grafenstein, U., Hammarlund, D., Kuhnert, H., and Minster, B.: A modern snapshot of the isotopic composition of lacustrine biogenic carbonates: Records of seasonal water temperature variability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13178,, 2021.