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Interpreting the role of pH on stable isotopes in large benthic foraminifera

April 21, 2016

Large benthic foraminifera (LBF) are prolific producers of calcium carbonate sediments in shallow, tropical environments that are being influenced by ocean acidification (OA). Two LBF species, Amphistegina gibbosa (Order Rotaliida) with low-Mg calcite tests and Archaias angulatus (Order Miliolida) with high-Mg calcite tests, were studied to assess the effects of pH 7.6 on oxygen and carbon isotopic fractionation between test calcite and ambient seawater. The δ18O and δ13C values of terminal chambers and of whole adult tests of both species after 6 weeks were not significantly different between pH treatments of 8.0 and 7.6. However, tests of juveniles produced during the 6-week treatments showed significant differences between δ18O and δ13C values from control (pH 8.0) when compared with the treatment (pH 7.6) for both species. Although each individual's growth was photographed and measured, difficulty in distinguishing and manually extracting newly precipitated calcite from adult specimens likely confounded any differences in isotopic signals. However, juvenile specimens that resulted from asexual reproduction that occurred during the experiments did not contain old carbonate that could confound the new isotopic signals. These data reveal a potential bias in the design of OA experiments if only adults are used to investigate changes in test chemistries. Furthermore, the results reaffirm that different calcification mechanisms in these two foraminiferal orders control the fractionation of stable isotopes in the tests and will reflect decreasing pH in seawater somewhat differently. .

Publication Year 2016
Title Interpreting the role of pH on stable isotopes in large benthic foraminifera
DOI 10.1093/icesjms/fsw056
Authors Lisa L. Robbins, P. O. Knorr, J.G. Wynn, P. Hallock, P. Harries
Publication Type Article
Publication Subtype Journal Article
Series Title ICES Journal of Marine Science
Index ID 70176146
Record Source USGS Publications Warehouse
USGS Organization St. Petersburg Coastal and Marine Science Center