Fossil coccolith morphological attributes as a new proxy for deep ocean carbonate chemistry
<p>Understanding the variations in past ocean carbonate chemistry is critical to elucidating the role of the oceans in balancing the global carbon cycle. The fossil shells from marine calcifiers present in the sedimentary record are widely applied as past ocean carbon cycle proxies. However, t...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-05-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/20/1725/2023/bg-20-1725-2023.pdf |
| Summary: | <p>Understanding the variations in past ocean carbonate chemistry is critical
to elucidating the role of the oceans in balancing the global carbon cycle.
The fossil shells from marine calcifiers present in the sedimentary record
are widely applied as past ocean carbon cycle proxies. However, the
interpretation of these records can be challenging due to the complex
physiological and ecological response to the carbonate system during an
organisms' life cycle and the potential for preservation at the
seafloor. Here we present a new dissolution proxy based on the morphological
attributes of coccolithophores from the Noëlaerhabdaceae family
(<i>Emiliania huxleyi</i> <span class="inline-formula">></span> 2 <span class="inline-formula">µ</span>m, and small <i>Gephyrocapsa</i> spp.). To evaluate the influences of
coccolithophore calcification and coccolith preservation on fossil
morphology, we measured morphological attributes, mass, length, thickness,
and shape factor (ks) of coccoliths in a laboratory dissolution experiment
and surface sediment samples from the South China Sea. The coccolith
morphological data in surface sediments were also analyzed with environment
settings, namely surface temperature, nutrients, pH, chlorophyll <span class="inline-formula"><i>a</i></span>
concentration, and carbonate saturation of bottom water by a redundancy
analysis. Statistical analysis indicates that carbonate saturation of the
deep ocean explains the highest proportion of variation in the morphological
data instead of the environmental variables of the surface ocean. Moreover,
the dissolution trajectory in the ks vs. length of coccoliths is comparable
between natural samples and laboratory dissolution experiments, emphasizing
the importance of carbonate saturation on fossil coccolith morphology.
However, the mean ks alone cannot fully explain the main variations observed
in our work. We propose that the normalized ks variation (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="italic">σ</mi><mspace linebreak="nobreak" width="0.125em"/><mo>/</mo><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">ks</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="6876e7c9ebe9eff0713ec99312c480e0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1725-2023-ie00001.svg" width="29pt" height="14pt" src="bg-20-1725-2023-ie00001.png"/></svg:svg></span></span>), which is the ratio between the standard deviation of ks (<span class="inline-formula"><i>σ</i></span>) and the mean
ks,
could reflect different degrees of dissolution and size-selective
dissolution, influenced by the assemblage composition. Applied together
with the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="italic">σ</mi><mspace width="0.125em" linebreak="nobreak"/><mo>/</mo><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">ks</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="82a57a3c576a34b62f23894f25e1b6c5"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1725-2023-ie00002.svg" width="29pt" height="14pt" src="bg-20-1725-2023-ie00002.png"/></svg:svg></span></span> ratio, the ks factor of fossil coccoliths in deep
ocean sediments could be a potential proxy for a quantitative reconstruction
of past carbonate dissolution dynamics.</p> |
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| ISSN: | 1726-4170 1726-4189 |