The coordination and distribution of B in foraminiferal calcite

• Published in: Earth and Planetary Science Letters
• Main Authors: Branson, Oscar, Kaczmarek, Karina, Redfern, Simon A. T., Misra, Sambuddha, Langer, Gerald, Tyliszczak, Tolek, Bijma, Jelle, Elderfield, Henry
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2015
• Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems; 03 - Mineral Sciences; Ocean acidification
• Online Access: http://eprints.esc.cam.ac.uk/3268/; http://eprints.esc.cam.ac.uk/3268/1/branson.pdf; http://eprints.esc.cam.ac.uk/3268/2/1-s2.0-S0012821X15000849-gr001.jpg; http://eprints.esc.cam.ac.uk/3268/3/mmc1.pdf; http://eprints.esc.cam.ac.uk/3268/4/mmc2.pdf; http://eprints.esc.cam.ac.uk/3268/5/mmc3.pdf; http://eprints.esc.cam.ac.uk/3268/6/mmc4.pdf; http://www.sciencedirect.com/science/article/pii/S0012821X15000849; https://doi.org/10.1016/j.epsl.2015.02.006

The isotopic ratio and concentration of B in foraminiferal calcite appear to reflect the pH and bicarbonate concentration of seawater. The use of B as a chemical proxy tracer has the potential to transform our understanding of the global carbon cycle, and ocean acidification processes. However, discrepancies between the theory underpinning the B proxies, and mineralogical observations of B coordination in biomineral carbonates call the basis of these proxies into question. Here, we use synchrotron X-ray spectromicroscopy to show that B is hosted solely as trigonal BO3 in the calcite test of Amphistegina lessonii, and that B concentration exhibits banding at the micron length scale. In contrast to previous results, our observation of trigonal B agrees with the predictions of the theoretical mechanism behind B palaeoproxies. These data strengthen the use of B for producing palaeo-pH records. The observation of systematic B heterogeneity, however, highlights the complexity of foraminiferal biomineralisation, implying that B incorporation is modulated by biological or crystal growth processes.