Effect of pH on Classical (δ18O and δ13C) and Clumped (Δ47) Isotopic Measurements of Laboratory-Cultured Large Benthic Foraminifera (LBF), Amphistegina lobifera
Supervisors: Marion Peral, Julie Meilland, Philip Claeys (VUB, Brussels)
Reef-dwelling calcifiers represent an important group of marine calcifiers that face numerous environmental stresses associated with anthropogenic carbon dioxide emissions leading to ocean acidification. Photosymbiont-bearing calcifiers, such as large benthic foraminifera (LBF), are particularly sensitive to climate change. However, its sensitivity to global change is still unclear. Foraminiferal stable isotopes (δ18O and δ13C) are crucial palaeoceanographic proxies for reconstructing past climate conditions. However, the interpretation of these proxies can be complicated by the “pH effect”, whereby changes in seawater pH influence isotope incorporation, an issue that has been well-studied in planktonic but not in LBF. While advancements in clumped isotope analysis (Δ47) have solved most of the uncertainties (influence of oxygen isotope composition of the seawater), the impacts of pH on Δ47 in LBF remain an important open question. Therefore, to gain insight into their responses to near-future conditions and to investigate the effect of pH on classical (δ18O and δ13C) and clumped (Δ47) isotopes to improve palaeoceanographic reconstructions, Amphistegina lobifera from the Red Sea was cultured under five pH conditions (pH 7.54, 7.78, 8.05, 8.23, and 8.55) with constant temperature (25 ± 0.5 °C) and salinity (38) for 41 days. Our results show that A. lobifera displays the most significant shell growth in pH treatments (pH 7.78 and 8.23) close to ambient levels (pH 8.05), whereas notably lower rates are found in treatments further from 8.05 (pH 7.54 and 8.55). In all cases, there were no signs of dissolution or cracks, no noticeable changes in pore parameters, minimal mortalities, and a higher level of observed metabolic trade-off driving their fast adaptive response to the fluctuating pH conditions. The stable carbon and oxygen isotopes of LBF exhibited a negative correlation with pH, showing 0.62 ‰ and 0.52 ‰ reductions in δ13C of calcite (δ13Ccalcite) and δ18O of calcite (δ18Ocalcite) with a unit increase in pH between pH 7.54 and pH 8.55. Most noteworthy, this report explores the first-ever report on the impact of pH on clumped (Δ47) isotopes (1 unit increase in pH corresponds to 0.02 ‰ decrease in Δ47) of LBF. The underlying cause of these isotopic variations (classical and clumped) is linked with modifications in the metabolic and dissolved inorganic carbon (DIC) species, which are then followed by adjustments in the pairing of 13C and 18O isotopes within the CO₃²⁻ lattice. This pH correction in Δ47 will allow for more accurate Δ47 measurements and the reconstruction of bottom ocean temperatures, while also gaining an understanding of ice sheet volume changes when combined with δ18O of seawater (δ18Osw). To improve the clumped isotope technique that accounts for these effects, further research is needed to determine the extent to which pH influences both classical (δ18O) and clumped (Δ47) values of LBF with more data points between pH 8.25 to 7.80 (projected shift from 1751 to 2100) and with different species.

Keywords: Amphistegina lobifera, Large benthic foraminifera, Ocean acidification, Classical and clumped isotopes, pH.