The fossilized remains of marine creatures like foraminifera are essential archives for understanding the evolution of Earth’s climate and oceans over geological time. These tiny fossils can reveal details about ancient sea temperatures, but their usefulness depends on how well the original chemical and isotopic composition of the shells has been preserved over millions of years. A new study has found that even ancient, well-preserved fossil seashells remain susceptible to subtle chemical changes, challenging assumptions about the reliability of these paleoclimate records.
Preserving the Past in Fossil Seashells
The oxygen isotope paleothermometer is one of the most valuable tools for reconstructing past ocean temperatures from the fossilized shells of marine creatures like foraminifera, bivalves, and brachiopods. The ratio of oxygen isotopes in their shells reflects the temperature of the seawater when the shell was formed, allowing scientists to estimate ancient sea surface and deep-water temperatures dating back millions of years.
However, the accuracy of these reconstructions depends on the assumption that the original isotopic composition of the shells has been preserved despite the ravages of time and the chemical changes that occur during the fossilization process, known as diagenesis. As organisms die and their remains are buried, the shells can undergo significant alterations, with the original mineral and chemical signatures replaced by new materials.
Exploring the Diagenetic Susceptibility of Fossil Seashells
In this new study, researchers set out to investigate how susceptible fossil seashells are to diagenetic changes, even when they appear well-preserved on the outside. They compared the internal structure and chemical composition of modern Click Here
’>Ammonia</a> foraminifera shells with 14-million-year-old fossil Ammonia shells from Poland.</p>
<p><figure class="wp-block-image size-large"><img decoding="async" src="https://famefreq.s3.us-east-2.amazonaws.com/1729685736_41598_2024_75588_Fig1_HTML.png" alt="figure 1" class="wp-image-1" /><figcaption class="wp-element-caption">Fig. 1</figcaption></figure>
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<p>Using a suite of advanced imaging techniques, including <b>scanning electron microscopy</b>, <b>atomic force microscopy</b>, <b>cathodoluminescence</b>, and <b>electron backscatter diffraction</b>, the researchers found that the fossil shells were remarkably well-preserved. They had a similar glassy, translucent appearance to modern shells, and their internal nanostructure was also comparable.</p>
<p>However, the researchers also found some key differences. The fossil shells had partially degraded organic structures that normally act as pathways for diagenetic fluids to penetrate the shell. Additionally, the fossil shells showed evidence of secondary calcite precipitation within their pores and chambers.</p>
<p><figure class="wp-block-image size-large"><img decoding="async" src="https://famefreq.s3.us-east-2.amazonaws.com/1729685742_41598_2024_75588_Fig2_HTML.png" alt="figure 2" class="wp-image-2" /><figcaption class="wp-element-caption">Fig. 2</figcaption></figure>
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<p>To further investigate the susceptibility of the fossil shells to diagenetic changes, the researchers conducted controlled experiments, exposing both modern and fossil Ammonia shells to a highly <b>18O-enriched seawater solution</b> at high temperature for several days. This allowed them to track the extent of oxygen isotope exchange between the shells and the surrounding fluid.</p>
<p><h3>Fossil Seashells Remain Vulnerable to Diagenesis</h3>
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<p>The results were striking. Even though the fossil shells had lost some of their organic structures, they still exhibited extensive <b>oxygen isotope exchange</b> with the surrounding fluid, reaching about half the level of exchange observed in the modern shells.</p>
<p><figure class="wp-block-image size-large"><img decoding="async" src="https://famefreq.s3.us-east-2.amazonaws.com/1729685747_41598_2024_75588_Fig3_HTML.png" alt="figure 3" class="wp-image-3" /><figcaption class="wp-element-caption">Fig. 3</figcaption></figure>
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<p>This demonstrates that fossil seashells, even those that appear well-preserved, are not immune to diagenetic alteration. The organic matter that helps facilitate fluid penetration and isotopic exchange in modern shells may degrade over time, but the underlying nanostructure of the shells remains reactive, allowing diagenetic fluids to continue to interact with the shell material.</p>
<p><h3>Implications for Paleoclimate Reconstructions</h3>
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<p>These findings have important implications for the reliability of paleoclimate reconstructions based on fossil seashell geochemistry. While the fossil shells may appear pristine, their susceptibility to diagenetic alteration means that the original isotopic signatures may have been partially or even completely overprinted by later chemical changes.</p>
<p><figure class="wp-block-image size-large"><img decoding="async" src="https://famefreq.s3.us-east-2.amazonaws.com/1729685752_41598_2024_75588_Fig4_HTML.png" alt="figure 4" class="wp-image-4" /><figcaption class="wp-element-caption">Fig. 4</figcaption></figure>
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<p>“The best (and perhaps only) way to remove this bias from paleo-ocean temperature records calculated from oxygen isotope compositions of carbonate fossils, is to better understand the diagenetic susceptibility of the biogenic carbonates most used in paleoclimate studies and to apply the necessary correction factors to existing data,” the researchers conclude.</p>
<p>This study highlights the need for continued research into the complex diagenetic processes that can affect fossil seashells, in order to improve the accuracy and reliability of paleoclimate reconstructions that rely on these ancient archives.</p>
<p>Author credit: This article is based on research by Deyanira Cisneros-Lazaro, Arthur Adams, Jarosław Stolarski, Sylvain Bernard, Damien Daval, Alain Baronnet, Olivier Grauby, Lukas P. Baumgartner, Torsten Vennemann, Jo Moore, Claudia Baumgartner, Cristina Martin Olmos, Stéphane Escrig, Anders Meibom.</p>
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