TY - JOUR
T1 - The effect of carbonate chemistry on trace element incorporation in high-Mg calcitic foraminifera
AU - Hauzer, Hagar
AU - Evans, David
AU - Müller, Wolfgang
AU - Rosenthal, Yair
AU - Erez, Jonathan
N1 - Publisher Copyright:
© 2024 Elsevier Ltd
PY - 2025/2/1
Y1 - 2025/2/1
N2 - The sodium-to-calcium ratio (Na/Ca) of biogenic CaCO3 has recently been introduced as a proxy for past seawater Ca2+ concentrations ([Ca2+sw]) as demonstrated by a positive correlation between seawater and shell Na/Ca with a minor influence of salinity. In the present study, we investigate the effect of carbonate chemistry on the Na/Ca proxy by conducting a set of experiments in which pH and the concentration of dissolved inorganic carbon (DIC) were independently varied. In addition to Na+, the incorporation of Li+, Mg2+, and Sr2+ into the shells of the large benthic high-Mg calcitic foraminifer Operculina ammonoides was assessed by culturing under constant DIC (∼2170 µmol kg−1) with varying pH (7.5–8.4 NBS scale), and under varying DIC (830–2470 µmol kg−1) with constant pH (∼7.9). Foraminiferal growth rate correlates linearly with calcite saturation state (Ω) of the experimental seawater (SW). The lowest pH and DIC experiments were characterized by low population growth rates, and some of these specimens died and their shells partially dissolved. Na/Cashell and Li/Cashell in O. ammonoides are positively correlated with SW [CO32–] and Ω, whereas Sr/Cashell and Mg/Cashell are much less sensitive to these parameters. The relative sensitivity of Na/Cashell to Ω in O. ammonoides is ∼ 4 % per Ω unit. However, given that past changes in surface water Ω were probably small relative to changes in [Ca2+sw] the correction for this secondary effect over the Cenozoic is likely to be small. Therefore, we conclude that the sensitivity of O. ammonoides Na/Ca to the carbonate system is unlikely to compromise the use of this proxy to reconstruct past [Ca2+sw]. In the case of the low-Mg planktic and benthic foraminifera, a data compilation exercise indicates that no resolvable carbonate chemistry effect exists on Na/Ca. Thus, the Na/Ca proxy in benthic nummulitid and planktic foraminifera can be utilized for past [Ca2+sw] reconstructions. Furthermore, coupling this information with the distribution coefficients of other elemental and isotopic systems (e.g., Li+, Sr2+, Mg2+, K+, B, δ11B) may allow the reconstruction of wider aspects of past ocean chemistry. Finally, comparison of trace and minor element incorporation into low and high-Mg foraminiferal species, coccolithophores, inorganic calcite, and amorphous CaCO3 (ACC), we propose a modified biomineralization model for hyaline foraminifera centered on SW vacuolization. Foraminiferal data can be explained by a biomineralization process in which high-Mg species utilize a precursor phase (ACC) to produce high-Mg calcite whereas low-Mg species actively remove Mg2+ from the site of calcification.
AB - The sodium-to-calcium ratio (Na/Ca) of biogenic CaCO3 has recently been introduced as a proxy for past seawater Ca2+ concentrations ([Ca2+sw]) as demonstrated by a positive correlation between seawater and shell Na/Ca with a minor influence of salinity. In the present study, we investigate the effect of carbonate chemistry on the Na/Ca proxy by conducting a set of experiments in which pH and the concentration of dissolved inorganic carbon (DIC) were independently varied. In addition to Na+, the incorporation of Li+, Mg2+, and Sr2+ into the shells of the large benthic high-Mg calcitic foraminifer Operculina ammonoides was assessed by culturing under constant DIC (∼2170 µmol kg−1) with varying pH (7.5–8.4 NBS scale), and under varying DIC (830–2470 µmol kg−1) with constant pH (∼7.9). Foraminiferal growth rate correlates linearly with calcite saturation state (Ω) of the experimental seawater (SW). The lowest pH and DIC experiments were characterized by low population growth rates, and some of these specimens died and their shells partially dissolved. Na/Cashell and Li/Cashell in O. ammonoides are positively correlated with SW [CO32–] and Ω, whereas Sr/Cashell and Mg/Cashell are much less sensitive to these parameters. The relative sensitivity of Na/Cashell to Ω in O. ammonoides is ∼ 4 % per Ω unit. However, given that past changes in surface water Ω were probably small relative to changes in [Ca2+sw] the correction for this secondary effect over the Cenozoic is likely to be small. Therefore, we conclude that the sensitivity of O. ammonoides Na/Ca to the carbonate system is unlikely to compromise the use of this proxy to reconstruct past [Ca2+sw]. In the case of the low-Mg planktic and benthic foraminifera, a data compilation exercise indicates that no resolvable carbonate chemistry effect exists on Na/Ca. Thus, the Na/Ca proxy in benthic nummulitid and planktic foraminifera can be utilized for past [Ca2+sw] reconstructions. Furthermore, coupling this information with the distribution coefficients of other elemental and isotopic systems (e.g., Li+, Sr2+, Mg2+, K+, B, δ11B) may allow the reconstruction of wider aspects of past ocean chemistry. Finally, comparison of trace and minor element incorporation into low and high-Mg foraminiferal species, coccolithophores, inorganic calcite, and amorphous CaCO3 (ACC), we propose a modified biomineralization model for hyaline foraminifera centered on SW vacuolization. Foraminiferal data can be explained by a biomineralization process in which high-Mg species utilize a precursor phase (ACC) to produce high-Mg calcite whereas low-Mg species actively remove Mg2+ from the site of calcification.
KW - Biomineralization
KW - Carbonate chemistry
KW - Foraminifera
KW - Proxies
KW - Trace elements
UR - http://www.scopus.com/inward/record.url?scp=85211198035&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2024.11.022
DO - 10.1016/j.gca.2024.11.022
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AN - SCOPUS:85211198035
SN - 0016-7037
VL - 390
SP - 105
EP - 116
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -