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

• Main Authors: Labuhn, Inga, Tell, Franziska, Grafenstein, Ulrich, Hammarlund, Dan, Kuhnert, Henning, Minster, Bénédicte
• Format: Text
• Language: English
• Published: 2021
• Subjects: Chara hispida
• Online Access: https://doi.org/10.5194/bg-2021-235; https://bg.copernicus.org/preprints/bg-2021-235/

Carbonate shells and encrustations from lacustrine organisms provide proxy records of past environmental and climatic changes. The carbon isotopic composition ( δ 13 C) of such carbonates depends on the δ 13 C of dissolved inorganic carbon (DIC). Their oxygen isotopic composition ( δ 18 O) is controlled by the δ 18 O of the lake water and on water temperature during carbonate precipitation. Lake water δ 18 O, in turn, reflects the δ 18 O 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 δ 18 O of water vary significantly with water depth, indicating a well-mixed epilimnion. The mean δ 13 C 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 δ 13 C of the encrustations. A small effect of photosynthetic 13 C enrichment in DIC is seen in contemporaneously formed valves of juvenile ostracods. The largest differences in the mean carbonate δ 18 O between species are caused by vital offsets, i.e. the species-specific deviations from the δ 18 O 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 δ 18 O between species can mainly be attributed to seasonal water temperature changes. The lowest δ 18 O 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 δ 18 O 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 δ 18 O. At the same time, an increase in air temperature leads to an increase in the δ 18 O of lake water through its effect on precipitation δ 18 O and on evaporation from the lake, and consequently to an increase in carbonate δ 18 O, opposite to the effect of increasing water temperature on oxygen-isotope fractionation. However, the seasonal and inter-annual variability in lake water δ 18 O 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 δ 18 O differences between species when vital offsets are corrected. Temperature reconstructions based on paleotemperature equations for equilibrium carbonate precipitation using the mean δ 18 O of each species and the mean δ 18 O of lake water are well in agreement with the observed seasonal water temperature range. The high carbonate δ 18 O 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.