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

<p>Carbonate shells and encrustations from lacustrine organisms provide proxy records of past environmental and climatic changes. The oxygen isotopic composition (&#948;¹⁸O) of such carbonates depends on water temperature during carbonate precipitation, and on the &#948;¹⁸O of the lake water. Lake water &#948;¹⁸O, in turn, is controlled by the &#948;¹⁸O of precipitation in the catchment, water residence time and mixing, and by evaporation. A paleoclimate interpretation of carbonate &#948;¹⁸O records requires a site-specific calibration based on an understanding of the local conditions.</p><p>For this study, carbonates and water were sampled in the littoral zone of lake Locknesj&#246;n, central Sweden (62.99&#176;N, 14.85&#176;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 <em>Chara hispida</em>, the mollusk <em>Pisidium</em>, and adult and juvenile instars of two ostracod species, <em>Candona candida</em> and <em>Candona neglecta</em>.</p><p>We show that neither the &#948;¹⁸O of carbonates nor the &#948;¹⁸O of water vary significantly with water depth, indicating a well-mixed epilimnion. The largest differences in the mean carbonate &#948;¹⁸O between species are caused by vital offsets, i.e. the species-specific deviation from the &#948;¹⁸O 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 &#948;¹⁸O between species can mainly be attributed to seasonal water temperature changes. The lowest &#948;¹⁸O values are observed in <em>C</em><em>h</em><em>ara</em> 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 &#948;¹⁸O 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 &#948;¹⁸O. An increase in temperature also leads to an increase in the &#948;¹⁸O of lake water through its effect on precipitation &#948;¹⁸O and on evaporation, and consequently to an increase in carbonate &#948;¹⁸O, opposite to the temperature effect on fractionation. However, the seasonal and inter-annual variability in lake water &#948;¹⁸O is small (0.5&#8240;) due to the long water residence time. Seasonal changes in the temperature-dependent fractionation are therefore the dominant cause of carbonate &#948;¹⁸O differences between species.</p><p>Temperature reconstructions based on &#8220;paleo-temperature&#8221; equations for equilibrium carbonate precipitation using the mean &#948;¹⁸O of each species and the mean &#948;¹⁸O of lake water are well in agreement with the observed seasonal water temperature range. The high carbonate &#948;¹⁸O 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.</p>