Stable isotope analysis of giant clam shells: layer discrepancies, temporal shift, and organic contamination

Michelle Elizabeth Gannon

doi:10.17918/00000880

Giant clams (Tridacninae) are important mollusks in Pacific and Indo-Pacific coral reefs. Like coral, they host a photosymbiotic relationship with zooxanthellae, which provide essential nutrition increasingly through life. Their shells are often isotopically assessed as [delta]18O values are used to reconstruct sea surface temperature. Yet, values of [delta]13C are not frequently reported or discussed but may provide insights to the ambient dissolved inorganic carbon. Additionally, values of [delta]15N and [delta]13C from shell-bound organic material may offer unique perspectives on the dietary shift from filtered particulate material to photosynthate from zooxanthellae. First, values of [delta]13C are compared between the inner (IL) and outer shell layers (OL) of ten Tridacna to gain an understanding of differences within the individual, those living together and across three species. Typically, [delta]13C values do not differ in the IL of contemporaneous specimens while those of the OL do. This suggests the [delta]13C values of the IL represent values closer to ambient DIC and may experience less isotopic fractionation. Next, to apply these results, profiles of [delta]13C values from the IL of ten Tridacna collected between 1857 and 1992 are presented in combination with profiles from literature through time. Since 1841, 13C has become depleted in the IL by approximately 1.5[per mille], reminiscent of that of [delta]13C from atmospheric CO₂ from Mauna Loa Observatory. A positive relationship between both [delta]13C systems suggests Tridacna are a good record of the Suess Effect. Finally, the organic shell matrix is isotopically assessed by decalcification. Bioeroding organisms present in these samples introduced isotopic contamination. It is hypothesized that sequential extraction of organic material would initially remove bioeroding organisms to ultimately reveal isotope values of the giant clam, however there was no significant isotope shift through extraction time. Bioerosion is predicted to be exacerbated by ocean acidification and global climate change, emphasizing the need for enhanced water quality management. This work highlights the untapped potential for environmental assessment and reconstruction utilizing stable isotopes in three parts of the giant clam shell (IL; OL; organic matrix). With enhanced technology and water quality management, these results provide a framework for future studies.

Stable isotope analysis of giant clam shells: layer discrepancies, temporal shift, and organic contamination

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Stable isotope analysis of giant clam shells: layer discrepancies, temporal shift, and organic contamination

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