A modern snapshot of the isotopic composition of lacustrine biogenic carbonates: Records of seasonal water temperature variability
Authors: Labuhn Inga , Tell Franziska , von Grafenstein Ulrich , Hammarlund Dan , Kuhnert Henning , Minster Bénédicte 
Published: March 2021

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