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 carbon isotopic composition (<span class="inline-formula"><i>δ</i><sup>13</sup>C</span>) of such carbonates depend...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-06-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/19/2759/2022/bg-19-2759-2022.pdf |
| Summary: | <p>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><sup>13</sup>C</span>) of such carbonates depends on the <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> of dissolved inorganic carbon (DIC). Their oxygen isotopic composition (<span class="inline-formula"><i>δ</i><sup>18</sup>O</span>) is controlled by the <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> of the lake water and by water temperature during carbonate precipitation. Lake water <span class="inline-formula"><i>δ</i><sup>18</sup>O</span>, in turn, reflects the <span class="inline-formula"><i>δ</i><sup>18</sup>O</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.</p>
<p>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"><sup>∘</sup></span> N, 14.85<span class="inline-formula"><sup>∘</sup></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>.</p>
<p>Our results show that neither the isotopic composition of carbonates nor the <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> of water vary significantly with water depth, indicating a well-mixed epilimnion.
The mean <span class="inline-formula"><i>δ</i><sup>13</sup>C</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"><sup>12</sup>C</span> into the organic matter and increases the <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> of the encrustations. A small effect of photosynthetic <span class="inline-formula"><sup>13</sup>C</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><sup>18</sup>O</span> between species are caused by vital offsets, i.e., the species-specific deviations from the <span class="inline-formula"><i>δ</i><sup>18</sup>O</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><sup>18</sup>O</span> between species can mainly be attributed to seasonal water temperature changes. The lowest <span class="inline-formula"><i>δ</i><sup>18</sup>O</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><sup>18</sup>O</span> values. The seasonal and interannual variability in lake water <span class="inline-formula"><i>δ</i><sup>18</sup>O</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><sup>18</sup>O</span> differences between species when vital offsets are corrected.</p>
<p>Temperature reconstructions based on paleotemperature equations for equilibrium carbonate precipitation using the mean <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> of each species and the mean <span class="inline-formula"><i>δ</i><sup>18</sup>O</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><sup>18</sup>O</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.</p> |
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| ISSN: | 1726-4170 1726-4189 |