Multi-proxy assessment of brachiopod shell calcite as a potential archive of seawater temperature and oxygen isotope composition
<p>Most of our knowledge of past seawater temperature history is based on <span class="inline-formula"><i>δ</i><sup>18</sup></span>O values of calcium carbonate fossil shells. However, the determination of past temperatures using this proxy require...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-04-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/20/1381/2023/bg-20-1381-2023.pdf |
| Summary: | <p>Most of our knowledge of past seawater temperature history is based on
<span class="inline-formula"><i>δ</i><sup>18</sup></span>O values of calcium carbonate fossil shells. However, the
determination of past temperatures using this proxy requires the knowledge
of past seawater <span class="inline-formula"><i>δ</i><sup>18</sup></span>O values, which is generally poorly
constrained. Other paleothermometers using carbonate archives, such as <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mg</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="8d51dfa4941e3d7b79a8ce41d2626020"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1381-2023-ie00001.svg" width="37pt" height="14pt" src="bg-20-1381-2023-ie00001.png"/></svg:svg></span></span>
ratios and clumped isotopes (<span class="inline-formula">Δ<sub>47</sub></span>), have been developed to allow for paleotemperatures to be estimated independently and to allow past ocean <span class="inline-formula"><i>δ</i><sup>18</sup></span>O values to be calculated using various groups of calcifying
organisms. Articulated brachiopod shells are some of the most commonly used
archives in studies of past oceanic geochemistry and temperature. They are
abundant in the fossil record since the Cambrian, and for decades, their low
Mg–calcite mineralogy has been considered relatively resistant to diagenetic
alteration. Here, we investigate the potential of brachiopod shells as
recorders of seawater temperatures and seawater <span class="inline-formula"><i>δ</i><sup>18</sup></span>O values
using new brachiopod shell geochemical data by testing multiple well-established or suggested paleothermometers applied to carbonate archives.</p>
<p>Modern articulated brachiopod shells covering a wide range of temperatures
(<span class="inline-formula">−1.9</span> to 25.5 <span class="inline-formula"><sup>∘</sup></span>C), depths (5 to 3431 m) and salinities (33.4 to
37.0 PSU) were analysed for their stable isotope compositions (<span class="inline-formula"><i>δ</i><sup>13</sup></span>C, <span class="inline-formula"><i>δ</i><sup>18</sup></span>O and <span class="inline-formula">Δ<sub>47</sub></span>) and their elemental ratios
(<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mg</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="93d9205225782a1f8841d5931bc9119e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1381-2023-ie00002.svg" width="37pt" height="14pt" src="bg-20-1381-2023-ie00002.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Sr</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="33pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="88479e7fe5ac264051dbc20ed4e5c89d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1381-2023-ie00003.svg" width="33pt" height="14pt" src="bg-20-1381-2023-ie00003.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Na</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="36pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="8b148d1480ad20f0f48f68de630bcb57"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1381-2023-ie00004.svg" width="36pt" height="14pt" src="bg-20-1381-2023-ie00004.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Li</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="31pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="308a6af7980b11d9d6b6ebcebc073fe7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1381-2023-ie00005.svg" width="31pt" height="14pt" src="bg-20-1381-2023-ie00005.png"/></svg:svg></span></span>). Our data allowed us to propose a revised
oxygen isotope fractionation equation between modern-brachiopod shell
calcite and seawater:
</p><div class="disp-formula" content-type="numbered" id="Ch1.E1"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="block" overflow="scroll" dspmath="mathml"><mtable><mlabeledtr><mtd><mtext>(1)</mtext></mtd><mtd><mrow>
<mi>T</mi>
<mo>=</mo>
<mo>-</mo>
<mn mathvariant="normal">5.0</mn>
<mo>(</mo>
<mo>±</mo>
<mn mathvariant="normal">0.2</mn>
<mo>)</mo>
<mo>(</mo>
<msup>
<mi mathvariant="italic">δ</mi>
<mn mathvariant="normal">18</mn>
</msup>
<msub>
<mi mathvariant="normal">O</mi>
<mi mathvariant="normal">c</mi>
</msub>
<mo>-</mo>
<msup>
<mi mathvariant="italic">δ</mi>
<mn mathvariant="normal">18</mn>
</msup>
<msub>
<mi mathvariant="normal">O</mi>
<mi mathvariant="normal">sw</mi>
</msub>
<mo>)</mo>
<mo>+</mo>
<mn mathvariant="normal">19.4</mn>
<mo>(</mo>
<mo>±</mo>
<mn mathvariant="normal">0.4</mn>
<mo>)</mo>
<mo>,</mo>
</mrow></mtd></mlabeledtr></mtable></math><div><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="416pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="dad0b18f89b626f929e4e5d67e76f980"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1381-2023-e_1.svg" width="416pt" height="15pt" src="bg-20-1381-2023-e_1.png"/></svg:svg></div></div><p>
where <span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>c</sub></span> is in ‰ VPDB, <span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>sw</sub></span> is in ‰ VSMOW, and <span class="inline-formula"><i>T</i></span> is in <span class="inline-formula"><sup>∘</sup></span>C. Our results strongly support the use of clumped isotopes as an
alternative temperature proxy but confirm significant offsets relative to
the canonical relationship established for other biogenic and abiogenic
calcium carbonate minerals. Brachiopod shell <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mg</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="5494b56141ea31f11a218bfc4b38b461"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1381-2023-ie00006.svg" width="37pt" height="14pt" src="bg-20-1381-2023-ie00006.png"/></svg:svg></span></span> ratios show no
relationship with seawater temperatures, indicating that this ratio is a
poor recorder of past changes in temperatures, an observation at variance
with several previous studies. Despite significant correlations with
brachiopod living temperature, brachiopod shell <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Sr</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="33pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="3d09db9d2ca4d4697a1ddefedd348745"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1381-2023-ie00007.svg" width="33pt" height="14pt" src="bg-20-1381-2023-ie00007.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M25" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Na</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="36pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="eab88b5bbd423223e0768ab4cead9897"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1381-2023-ie00008.svg" width="36pt" height="14pt" src="bg-20-1381-2023-ie00008.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M26" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Li</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="31pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="2e539cc754540033ff93fd92d3e31cbb"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1381-2023-ie00009.svg" width="31pt" height="14pt" src="bg-20-1381-2023-ie00009.png"/></svg:svg></span></span>
values indicate the influence of environmental and biological factors
unrelated to temperature, which undermines their potential as alternative
temperature proxies. Kinetic effects (growth rates) could explain most of the
deviation of brachiopod shell calcite from expected isotopic equilibrium
with seawater and part of the distribution of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M27" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Sr</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="33pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="fcb882a35b990a83e39588a10abaf6f8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1381-2023-ie00010.svg" width="33pt" height="14pt" src="bg-20-1381-2023-ie00010.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M28" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Na</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="36pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="36b471cad79683553095440c8036f8c3"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1381-2023-ie00011.svg" width="36pt" height="14pt" src="bg-20-1381-2023-ie00011.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M29" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Li</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="31pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="c6b229fe0d2ca5ac742a1f2d1a430b98"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1381-2023-ie00012.svg" width="31pt" height="14pt" src="bg-20-1381-2023-ie00012.png"/></svg:svg></span></span> ratios.</p> |
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| ISSN: | 1726-4170 1726-4189 |