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The Boron Isotope pH-proxy - The Foster Lab

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The Boron Isotope pH-proxy – The Foster Lab

boron isotope geochemistry
plasma on the neptune mc-icpms – as hot as the surface of the sun!
Culturing foraminifera G. ruber at the IUI in Israel (image credit: Michael Henehan, Yale)
It was recognised in the early 1990’s that the boron isotopic composition of marine carbonates, such as foraminifera and coral, was determined largely by ocean pH (Hemming and Hanson, 1992) development of the boron isotope-pH proxy rapidly followed and has occured hand-in-hand with advances in analytical cause boron has only two isotopes, getting the required analytical precision is therefore and his group has been at the forefront of using the latest generation of mass spectrometers (multi-collector inductively coupled plasma mass spectrometers, MC-ICPMS for short) to make the tricky boron isotope measurements for the past decade (e. g. see Foster, 2008 and Foster et al., 2013) continue to make analytical developments and are currently exploring a number of novel systems to aid in the measurement and preparation (e. ESI’s prepFAST) and micro-analysis (e. laser ablation) of samples side these analyitical efforts, we are also exploring the groundtruthing of the proxy with culture experiments (growing coral and foraminifera at different pH) and laboratory experiments with inorganic calcite/ ultimate aim is to make boron isotope measurements routine and strengthen the theoretical and experimental basis of the d11B-pH PublicationsJoseph A. Stewart, Russell D. Day, Steven J. Christopher, John R. Kucklick, Louise Bordier, Thomas B. Chalk, Arnaud Dapoigny, Eric Douville, Gavin L. Foster, William R. Gray, Rosanna Greenop, Marcus Gutjahr, Freya Hemsing, Michael J. Henehan, Philip Holdship, Yu-Te Hsieh, Ana Kolevica, Yen-Po Lin, Elaine M. Mawbey, James W. B. Rae, Laura F. Robinson, Rachael Shuttleworth, Chen-Feng You, Shuang Zhang (2020) NIST RM 8301 Boron Isotopes in Marine Carbonate (Simulated Coral and Foraminifera Solutions): Interlaboratory d11B and trace metal ratio value assignment, Geostandards and Geoanalytical Research,, M., Bordier, L., Douville, E., Farmer, J., Foster, G. L., Hathorne, E. C., Hönisch, B., Lemarchand, D., Louvat, P., Mcculloch, M., Noireaux, J., Pallavicini, N., Rae, J. W. B., Rodushkin, I., Roux, P., Stewart, J. A., Thil, F., & You, C. (2020). Sub‐Permil interlaboratory consistency for solution‐based boron isotope analyses on marine carbonates. Geostandards and Geoanalytical Research., H. K., Foster, G. L., Frohberg, N., Swann, G., Poulton, A. J., Moore, C. M., Humphreys, M. P. (2020) The pH dependency of the boron isotopic composition of diatom opal (Thalassoiosira weissflogii), Biogeosciences, Biogeosciences, 17, 2825–2837, 2020, de la Vega, E., Foster, G. L., Martinez-Boti, M. A., Anagnostou, E., Field, P. M., Kim, M. H., Watson, P. Wilson, P. (2020) Automation of boron chromotographic purification for d11B analysis of coral aragonite, Rapid Communications in Mass Spectrometry, 34:e8762, doi:10. 1002/rcm. 8762Badger, M. S, Chalk, T. B., Foster, G. L., Bown, P. R., Gibbs, S. J., Sexton, P. F., Schmidt, D. N., Palike, H., Mackensen, A., Pancost, R. D. (2019) Insensitivity of alkenone carbon isotopes to atmospheric CO2 at low to moderate CO2 levels, Climate of the Past, 15, 539-554, Anagnostou, E., Williams, B., Westfield, I., Foster, G. L., Ries, J. (2019) Calibration of the pH-d11B and temperature-Mg/Li proxies in long-lived high-latitude crustose coralline red alga Clathromorphum compactum via controlled laboratory experiments, Geochimica et Cosmochimica Acta, 254, 142-155,, C. D., Chalk, T. B., Babila, T. L., Milton, J. A., Palmer, M. R., Foster, G. L. (2019) The effect of matrix interferences in situ boron isotope analysis by laser ablation MC-ICP-MS, Rapid Communications in Mass Spectrometry, Hain, M. P., Foster, G. L., CHalk, T. (2018) Robust constraints on past climate forcing by CO2 from novel boron isotope proxy systematics, Paleoceanography and Paleoclimatology, 33, doi:10. 1029/2018PA003362. Donald, H. K., Ries, J. B., Stewart, J. A., Fowell, S. E., & Foster, G. (2017). Boron isotope sensitivity to seawater pH change in a species of Neogoniolithon coralline red alga. Geochimica et Cosmochimica Acta, 217, 240-253. DOI: 10. 1016/, R., Foster, G. L., Sosdian, S. M., Hain, M. P., Oliver, K. I. C., Goodwin, P., Chalk, T. B., Lear, C. H., Wilson, P. A. (2017) A record of Neogene seawater δ11B reconstructed from paired δ11B analyses on benthic and planktic foraminifera, Climate of the Past, 13, 149-170(doi:10. 5194/cp-13-149-2017), G. L., and Rae, J. B (2016) Reconstructing ocean pH with boron isotopes in foraminifera. Annual Review of Earth and Planetary Sciences, 44, 207-237 (doi:10. 1146/annurev-earth-060115-012226)Martin, P., Goodkin, N. F., Stewart, J. A., Foster, G. L., Sikes, E. L., White, H. K., Hennige, S., Roberts, J. M. (2016) Deep-sea coral d13C: A tool to reconstruct the difference between seawater pH and d11B-derived calcification site pH. Geophysical Research Letters, 43, 299-308 (doi:10. 1002/2015GL066494), Kirsty M., Anagnostou, Eleni, Pearson, Paul N. and Foster, Gavin L. (2015) Assessing the impact of diagenesis on δ11B, δ13C, δ18O, Sr/Ca and B/Ca values in fossil planktic foraminiferal calcite. Geochimica et Cosmochimica Acta (doi:10. 1016/), Gavin L., Hönisch, Bärbel, Paris, Guillaume, Dwyer, Gary S., Rae, James W. B., Elliott, Tim, Gaillardet, Jérôme, Hemming, N. Gary, Louvat, Pascale and Vengosh, Avner (2013) Interlaboratory comparison of boron isotope analyses of boric acid, seawater and marine CaCO3 by MC-ICPMS and NTIMS. Chemical Geology, 358, 1-14. (doi:10. 1016/emgeo. 2013. 08. 027). Henehan, Michael J., Rae, James W. B., Foster, Gavin L., Erez, Jonathan, Prentice, Katherine C., Kucera, Michal, Bostock, Helen C., Martínez-Botí, M. A., Milton, J. A., Wilson, P. A., Marshall, Brittney J. and Elliott, Timothy (2013) Calibration of the boron isotope proxy in the planktonic foraminifera Globigerinoides ruber for use in palaeo-CO2 reconstruction. Earth and Planetary Science Letters, 364, 111-122. 1016/).
Boron Isotopes - Palaeocean-CO2

Boron Isotopes – Palaeocean-CO2

Boron isotopes in Marine Carbonates
Variation in abundance (a) and isotopic composition (b) of borate ion (B(OH)4-) and boric acid (B(OH)3) in seawater.
Seawater pH as a function of alkalinity (ALK) and dissolved inorganic carbon (DIC).
The Ph Proxy – The Basics
Seawater contains around 4. 5 ppm boron (B) and it exists in two forms as function of pH: boric acid (B(OH)3) and borate ion (B(OH)4-; top). Boron has two isotopes: 11B and 10B (80:20 ratio) and due to structual differnces there is a pronouced isotopic fractionation between the two aqueous forms. Boric acid has a higher 11B/10B ratio than borate ion by 1. 0272 (Klochko et al., 2006). As pH changes the abundance of each aqueous species changes and so too does their isotopic composition (bottom). The proxy is based on the observation that predominantly only borate ion is incorporated into marine calcium carbonate (Hemming and Hanson, 1992).
Ph to CO2
The boron isotopic composition of ancient foraminifera that lived in the surface mixed layer offer a means to reconstruct ancient seawater pH. Seawater pH, like CO2, is determined by the ratio of total dissolved carbon (DIC) to total alkalinity (ALK; top). Given the nature of the CO2-system in seawater an additional parameter of the system is however needed for quantitative estimates of seawater CO2 content. Once seawater CO2 content is reconstructed air-sea disequilibrium with respect to CO2 then needs to be accounted for (bottom). Careful selection of study sites can minimise the influence of this requirement on the final CO2 reconstructed.
Existing Validation Efforts
There have been a number of attempts to validate the boron isotope proxy against the ice-core CO2 record (summarised in Foster and Rae, 2016, see figure above). There is clearly a good agreement with the average residual for the data shown above being 23 μatm: clearly demonstrating that for the Late Quaternary the boron isotope proxy is able to accurately reconstruct atmospheric CO2 variations. These results support the use of boron isotopes to provide accurate estimates of pCO2 beyond the reach of the ice cores (the last 800 kyr).
Cenozoic CO2 using Boron isotopes
Summary of boron isotope derived CO2 for the last 65 million years. Click here (insert link) for data in the diagram above. The data clearly show that CO2 has, on the whole, decreased over this interval, with a peak of ~1400 ppm during the Early Eocene.
Rejected datasetsThe following datasets have been rejected from thecompilation due to advances in the understanding of the isotopic fractionation factor, improved estimates of the second carbonate system parameter, and advances in analytical techniques:Spivak et al. (1993); Sanyal et al. (1995); Pearson and Palmer (2000) B/Ca and why we don’t Use itBecause the concentration of borate ion is pH dependent and it is predominantly borate that is included into foraminifera, the concentration of boron (expressed as B/Ca ratio) should also be pH dependent. In benthic forams it works well as a a carbonate ion proxy (see Yu and Elderfield, 2007), but in planktics there seems to be a myriad of other controls (see Allen et al., 2012; Henehan et al., 2015; Uchikawa et al., 2015). It is therefore too early to use B/Ca as a quantitative proxy of CO2.

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