Boron isotope pH calibration of a shallow dwelling benthic nummulitid foraminifera
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The boron isotope palaeo-pH/CO2 proxy is one of the key quantitative tools available to reconstruct past changes in the concentration of CO2 in the atmosphere. In particular, marine calcifying organisms have been shown to be useful archives of this proxy, enabling quantitative variations in pH/CO2 to be reconstructed throughout the Cenozoic. In order to provide an alternative proxy archive to the widely used planktonic foraminifera, we investigated the symbiont-bearing, high-Mg, shallow-dwelling, tropical large benthic foraminifera (LBF) species Operculina ammonoides and present a calibration of the relationship between the shell boron isotopic composition and seawater pH. We investigated specimens collected from both several reefs as well as grown in laboratory culture experiments in which pH and DIC were decoupled from each other, measuring newly-formed chambers using laser-ablation as a sample introduction technique. Based on our laboratory culture samples, the resulting linear relationship between the in situ boron isotopic composition of aqueous borate ion (B(OH)4−) and the shells of O. ammonoides is characterised by a gradient of 0.38-0.10+0.12. In contrast, the boron isotopic composition of the field collected samples displays a near 1:1 relationship with B(OH)4−. We suggest that the shallow slope of the laboratory culture regression is the result of the difference between their micro-environment carbonate chemistry and that of the surrounding seawater driven by a pH dependence of the relative rates of calcification and photosynthesis. Based on a model of the effect of these processes on the diffusive boundary layer, we show that this effect is expected in laboratory culture experiments free from micro-turbulence, but not in the foraminifer's natural environment. As such, we demonstrate the utility of these organisms as proxy archive, while also highlighting how laboratory experimental design has the potential to drive important changes in the micro-environment and resulting shell chemistry of organisms of this size. Given that the genus Operculina originated in the late Palaeocene, this work paves the way towards deep-time palaeo-pH/CO2 reconstructions using foraminifer species which have a very closely related modern representative.
| Original language | English |
|---|---|
| Journal | Geochimica et Cosmochimica Acta |
| Volume | 378 |
| Pages (from-to) | 217-233 |
| Number of pages | 17 |
| ISSN | 0016-7037 |
| DOIs | |
| Publication status | Published - 2024 |
Bibliographical note
Funding Information:
We thank Richard Zeebe and Dieter Wolf-Gladrow for providing their code for their diffusion-reaction model and Oscar Branson for providing the Python frontend. We thank Max Fursman for putting Puspita Kanna in touch with us for this collaboration. The foraminifera culturing was funded by NSF-BSF project 1634573 and by ISF project 790/16 and 1886/20 to Jonathan Erez. FIERCE is financially supported by the Wilhelm and Else Heraeus Foundation and by the Deutsche Forschungsgemeinschaft (DFG: INST 161/921-1 FUGG, INST 161/923-1 FUGG and INST 161/1073-1 FUGG), which is gratefully acknowledged. This research was funded through the VeWA research consortium (Past Warm Periods as Natural Analogues of our high-CO 2 Climate Future) by the LOEWE programme of the Hessen Ministry of Higher Education, Research and the Arts, Germany. We would like to thank the editor, editor in chief, and two anonymous reviewers for taking the time and effort necessary to review the manuscript. We sincerely appreciate all valuable comments and suggestions, which resulted in a substantially improved version. This is FIERCE contribution No. 168.
Funding Information:
We thank Richard Zeebe and Dieter Wolf-Gladrow for providing their code for the diffusion-reaction model and Oscar Branson for providing the Python frontend. We thank Max Fursman for putting Puspita Kanna in touch with us for this collaboration. The foraminifera culturing was funded by NSF-BSF project 1634573 and by ISF project 790/16 and 1886/20 to Jonathan Erez. FIERCE is financially supported by the Wilhelm and Else Heraeus Foundation and by the Deutsche Forschungsgemeinschaft (DFG: INST 161/921-1 FUGG, INST 161/923-1 FUGG and INST 161/1073-1 FUGG), which is gratefully acknowledged. This research was funded through the VeWA research consortium (Past Warm Periods as Natural Analogues of our high-CO2 Climate Future) by the LOEWE programme of the Hessen Ministry of Higher Education, Research and the Arts, Germany, and also by the RSE, Saltire International Collaboration Award number 1964. We would like to thank the editor, editor in chief, and two anonymous reviewers for taking the time and effort necessary to review the manuscript. We sincerely appreciate all valuable comments and suggestions, which resulted in a substantially improved version. This is FIERCE contribution No. 168.
Publisher Copyright:
© 2024 The Authors
- Boron isotope, Foraminifera, LA-MC-ICPMS, Laboratory calibration, pH proxy, Symbiont-bearing
Research areas
ID: 399570943