EFFECTS OF OCEAN ACIDIFICATION ON LARGE BENTHIC FORAMINIFERAL STABLE ISOTOPE COMPOSITION

Large benthic Foraminifera (Protista), which live predominantly in shallow tropical seas, are prolific producers of carbonate sediments and thus may be impacted by ocean acidification. Ocean acidification models project that the global average pH of the surface ocean will decline from a current value of 8.1 in 2015 ([CO_2atm] ~400 ppm) to 7.6 in 2140 ([CO_2atm] ~1,300 ppm). At pH 8.0, 88% of total carbon (C_T) is in the form of HCO₃^– and 12% is present as CO₃^2–; while at pH 7.6, 94% is HCO₃^– and 4% is CO₃^2–.

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.