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

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 delta 18O and delta 13C 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 delta 18O and delta 13C 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.在正受海洋酸化（Ocean Acidification, OA）影响的浅海热带环境中，大型底栖有孔虫（Large benthic foraminifera, LBF）是碳酸钙沉积物的主要产出类群。本研究选取两种大型底栖有孔虫：具低镁方解石壳体的 Amphistegina gibbosa（轮虫目 Rotaliida），以及具高镁方解石壳体的 Archaias angulatus（粟虫目 Miliolida），以评估pH 7.6条件下壳体方解石与周围海水之间的氧、碳同位素分馏效应。实验持续6周后，两个物种成体个体的终房室及完整壳体的δ¹⁸O与δ¹³C值，在pH 8.0与pH 7.6的处理组间均无显著差异。然而，6周处理期间产生的幼体壳体，其δ¹⁸O与δ¹³C值在两个物种的对照组（pH 8.0）与处理组（pH 7.6）间均存在显著差异。尽管本研究对所有个体的生长过程进行了拍照与测量，但从成体标本中区分并手动提取新沉淀方解石存在较大难度，这很可能掩盖了同位素信号本应存在的差异。但实验期间通过无性繁殖产生的幼体标本，并不含有会干扰新同位素信号的旧碳酸盐成分。本研究数据表明，若仅使用成体个体探究壳体化学组成的变化，海洋酸化（OA）实验的设计可能存在潜在偏差。此外，本研究结果进一步证实，这两个有孔虫目类群不同的钙化机制调控着壳体中稳定同位素的分馏过程，且对海水pH降低的响应模式存在一定差异。