EFFECTS OF OCEAN ACIDIFICATION ON LARGE BENTHIC FORAMINIFERAL STABLE ISOTOPE COMPOSITION
Two species, Amphistegina gibbosa (Order Rotaliida) and Archaias angulatus (Order Miliolida), were studied to assess the effects of pH 7.6 seawater on oxygen and carbon isotope discrimination during test construction. Tests of juveniles produced during the six-week culture period showed significant differences between δ18O and δ13C values from control (pH 8.0) vs the treatments (pH 7.6) for both species. Specifically, Am. gibbosa δ18O was significantly (p<0.001) higher in specimens from the pH 7.6 treatment (–0.37±0.1‰) than from the pH 8.0 control (–1.10±0.09‰). Additionally, δ13C was significantly higher (p < 0.001) at pH 7.6 (0.57±0.25‰) than at pH 8.0 (–1.86±0.32‰). Ar. angulatus δ18O was also significantly (p<0.001) higher at pH 7.6 (0.08±0.06‰) than at ph 8.0 (–0.80±0.15‰); though the δ13C was not significantly different (p=0.23).
However, separate analysis of adult terminal chambers and of whole adult tests of both species after six weeks of pH treatment at 7.6 and 8.0 showed no significant differences. Juvenile specimens formed during the culture period did not contain pre-existing carbonate that confounds the isotopic signal of experimental pH. Precisely distinguishing and extracting newly precipitated calcite in adult specimens is difficult and mixing with pretest material likely diluted the differences in isotopic composition recorded from the different growth phases. This reveals a potential bias in ocean acidification experiments when changes in test chemistries are investigated using only adult tests. Combined, these results reaffirm that the differences in calcification mechanisms of the two species from different foraminiferal orders, control the effect on stable isotopic composition of tests and will consequently reflect the decreasing seawater pH as ocean acidification proceeds.