Evaluation of bacterial glycerol dialkyl glycerol tetraether and <sup>2</sup>H–<sup>18</sup>O biomarker proxies along a central European topsoil transect
<p>Molecular fossils, like bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs), and the stable isotopic composition of biomarkers, such as <span class="inline-formula"><i>δ</i><sup>2</sup>H</span> of leaf wax-derived <span class=&...
| Main Authors: | , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-02-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/17/741/2020/bg-17-741-2020.pdf |
| Summary: | <p>Molecular fossils, like bacterial branched glycerol dialkyl glycerol
tetraethers (brGDGTs), and the stable isotopic composition of biomarkers,
such as <span class="inline-formula"><i>δ</i><sup>2</sup>H</span> of leaf wax-derived <span class="inline-formula"><i>n</i></span>-alkanes (<span class="inline-formula"><i>δ</i><sup>2</sup>H</span><span class="inline-formula"><sub><i>n</i>-alkane</sub></span>) or <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> of hemicellulose-derived
sugars (<span class="inline-formula"><i>δ</i><sup>18</sup>O</span><span class="inline-formula"><sub>sugar</sub></span>), are increasingly used for the
reconstruction of past climate and environmental conditions. Plant-derived
<span class="inline-formula"><i>δ</i><sup>2</sup>H</span><span class="inline-formula"><sub><i>n</i>-alkane</sub></span> and <span class="inline-formula"><i>δ</i><sup>18</sup>O</span><span class="inline-formula"><sub>sugar</sub></span> values
record the isotopic composition of plant source water (<span class="inline-formula"><i>δ</i><sup>2</sup></span>H<span class="inline-formula"><sub>source-water</sub></span> and <span class="inline-formula"><i>δ</i><sup>18</sup>O</span><span class="inline-formula"><sub>source-water</sub></span>), which
usually reflects mean annual precipitation (<span class="inline-formula"><i>δ</i><sup>2</sup></span>H<span class="inline-formula"><sub>precipiation</sub></span> and <span class="inline-formula"><i>δ</i><sup>18</sup>O</span><span class="inline-formula"><sub>precipiation</sub></span>), modulated
by evapotranspirative leaf water enrichment and biosynthetic fractionation
(<span class="inline-formula"><i>ε</i><sub>bio</sub></span>). Accuracy and precision of respective proxies
should be ideally evaluated at a regional scale. For this study, we analysed
topsoils below coniferous and deciduous forests as well as grassland soils
along a central European transect in order to investigate the variability
and robustness of various proxies and to identify effects related to
vegetation. Soil pH values derived from brGDGTs correlate reasonably well
with measured soil pH values but are systematically overestimated (<span class="inline-formula">Δ</span>pH <span class="inline-formula">=</span> <span class="inline-formula">0.6±0.6</span>). The branched vs. isoprenoid tetraether index (BIT)
can give some indication whether the pH reconstruction is reliable.
Temperatures derived from brGDGTs overestimate mean annual air temperatures
slightly (<span class="inline-formula">Δ<i>T</i><sub>MA</sub>=0.5</span> <span class="inline-formula"><sup>∘</sup></span>C <span class="inline-formula">±</span> 2.4). Apparent
isotopic fractionation (<span class="inline-formula"><i>ε</i><sub><i>n</i>-alkane/precipitation</sub></span> and
<span class="inline-formula"><i>ε</i><sub>sugar∕precipitation</sub></span>) is lower for grassland sites than
for forest sites due to signal damping; i.e. grass biomarkers do not
record the full evapotranspirative leaf water enrichment. Coupling <span class="inline-formula"><i>δ</i><sup>2</sup>H</span><span class="inline-formula"><sub><i>n</i>-alkane</sub></span> with <span class="inline-formula"><i>δ</i><sup>18</sup>O</span><span class="inline-formula"><sub>sugar</sub></span> allows us to
reconstruct the stable isotopic composition of the source water more
accurately than without the coupled approach (<span class="inline-formula">Δ<i>δ</i><sup>2</sup>H</span> <span class="inline-formula">=</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M39" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>∼</mo><mo>-</mo><mn mathvariant="normal">21</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="32pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="e82a4d8b4a959976b462fc4efa2646e9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-17-741-2020-ie00001.svg" width="32pt" height="10pt" src="bg-17-741-2020-ie00001.png"/></svg:svg></span></span> ‰ <span class="inline-formula">±</span> 22 ‰ and <span class="inline-formula">Δ<i>δ</i><sup>18</sup>O</span> <span class="inline-formula">=</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M43" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>∼</mo><mo>-</mo><mn mathvariant="normal">2.9</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="35pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="fa28c201b7bdb30ed9d566e4ac581654"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-17-741-2020-ie00002.svg" width="35pt" height="10pt" src="bg-17-741-2020-ie00002.png"/></svg:svg></span></span> ‰ <span class="inline-formula">±</span> 2.8 ‰).
Similarly, relative humidity during daytime and the vegetation period
(RH<span class="inline-formula"><sub>MDV</sub></span>) can be reconstructed using the coupled isotope approach
(<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M46" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="normal">Δ</mi><msub><mi mathvariant="normal">RH</mi><mi mathvariant="normal">MDV</mi></msub><mo>=</mo><mo>∼</mo><mo>-</mo><mn mathvariant="normal">17</mn><mo>±</mo><mn mathvariant="normal">12</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="107pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="d823af5a8ee2834d6d10a3ec7bc50932"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-17-741-2020-ie00003.svg" width="107pt" height="12pt" src="bg-17-741-2020-ie00003.png"/></svg:svg></span></span>). Especially for
coniferous sites, reconstructed RH<span class="inline-formula"><sub>MDV</sub></span> values as well as source water
isotope composition underestimate the measured values. This can likely be
explained by understorey grass vegetation at the coniferous sites
contributing significantly to the <span class="inline-formula"><i>n</i></span>-alkane pool but only marginally to the
sugar pool in the topsoils. Vegetation-dependent variable signal damping
and <span class="inline-formula"><i>ε</i><sub>bio</sub></span> (regarding <span class="inline-formula"><sup>2</sup>H</span> between <span class="inline-formula"><i>n</i></span>-alkanes and leaf
water) along our European transect are difficult to quantify but likely
contribute to the observed underestimation in the source water isotope
composition and RH reconstructions. Microclimate variability could cause the
rather large uncertainties. Vegetation-related effects do, by contrast, not
affect the brGDGT-derived reconstructions. Overall, GDGTs and the coupled
<span class="inline-formula"><i>δ</i><sup>2</sup>H</span><span class="inline-formula"><sub><i>n</i>-alkane</sub></span>–<span class="inline-formula"><i>δ</i><sup>18</sup>O</span><span class="inline-formula"><sub>sugar</sub></span> approach have
great potential for more quantitative paleoclimate reconstructions.</p> |
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| ISSN: | 1726-4170 1726-4189 |