Comment on bg-2021-235

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Carbonate shells and encrustations from lacustrine organisms provide proxy records of past environmental and climatic changes. The carbon isotopic composition (<span class="inline-formula"><i>δ</i>¹³C</span>) of such carbonates depends on the <span class="inline-formula"><i>δ</i>¹³C</span> of dissolved inorganic carbon (DIC). Their oxygen isotopic composition (<span class="inline-formula"><i>δ</i>¹⁸O</span>) is controlled by the <span class="inline-formula"><i>δ</i>¹⁸O</span> of the lake water and by water temperature during carbonate precipitation. Lake water <span class="inline-formula"><i>δ</i>¹⁸O</span>, in turn, reflects the <span class="inline-formula"><i>δ</i>¹⁸O</span> of atmospheric precipitation in the catchment area, water residence time and mixing, and evaporation. A paleoclimatic interpretation of carbonate isotope records requires a site-specific calibration based on an understanding of these local conditions. For this study, samples of different biogenic carbonate components and water were collected in the littoral zone of Lake Locknesjön, central Sweden (62.99<span class="inline-formula">^∘</span> N, 14.85<span class="inline-formula">^∘</span> E, 328 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">m</mi><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">a</mi><mo>.</mo><mi mathvariant="normal">s</mi><mo>.</mo><mi mathvariant="normal">l</mi><mo>.</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="36pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="4bfbe43a0c86958fccfe62f96625904c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-19-2759-2022-ie00001.svg" width="36pt" height="10pt" src="bg-19-2759-2022-ie00001.png"/></svg:svg></span></span>) along a water depth gradient from 1 to 8 <span class="inline-formula">m</span>. Carbonate samples of living organisms and subfossil remains in surface sediments were taken from the calcifying alga <i>Chara hispida</i>, from bivalve mollusks of the genus <i>Pisidium</i>, and from adult and juvenile instars of two ostracod species, <i>Candona candida</i> and <i>Candona neglecta</i>. Our results show that neither the isotopic composition of carbonates nor the <span class="inline-formula"><i>δ</i>¹⁸O</span> of water vary significantly with water depth, indicating a well-mixed epilimnion. The mean <span class="inline-formula"><i>δ</i>¹³C</span> of <i>Chara hispida</i> encrustations is 4 <span class="inline-formula">‰</span> higher than the other carbonates. This is due to fractionation related to photosynthesis, which preferentially incorporates <span class="inline-formula">¹²C</span> into the organic matter and increases the <span class="inline-formula"><i>δ</i>¹³C</span> of the encrustations. A small effect of photosynthetic <span class="inline-formula">¹³C</span> enrichment in DIC is seen in contemporaneously formed valves of juvenile ostracods. The largest differences in the mean carbonate <span class="inline-formula"><i>δ</i>¹⁸O</span> between species are caused by vital offsets, i.e., the species-specific deviations from the <span class="inline-formula"><i>δ</i>¹⁸O</span> 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 <span class="inline-formula"><i>δ</i>¹⁸O</span> between species can mainly be attributed to seasonal water temperature changes. The lowest <span class="inline-formula"><i>δ</i>¹⁸O</span> values are observed in <i>Chara hispida</i> 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 <span class="inline-formula"><i>δ</i>¹⁸O</span> values. The seasonal and interannual variability in lake water <span class="inline-formula"><i>δ</i>¹⁸O</span> is small (<span class="inline-formula">∼</span> 0.5 <span class="inline-formula">‰</span>) due to the long water residence time in the lake. Seasonal changes in the temperature-dependent fractionation are therefore the dominant cause of carbonate <span class="inline-formula"><i>δ</i>¹⁸O</span> differences between species when vital offsets are corrected. Temperature reconstructions based on paleotemperature equations for equilibrium carbonate precipitation using the mean <span class="inline-formula"><i>δ</i>¹⁸O</span> of each species and the mean <span class="inline-formula"><i>δ</i>¹⁸O</span> of lake water are well in agreement with the observed seasonal water temperature range. The high carbonate <span class="inline-formula"><i>δ</i>¹⁸O</span> 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.