Boron incorporation in the foraminifer Amphistegina lessonii under a decoupled carbonate chemistry

K. Kaczmarek, G. Langer, G. Nehrke, I. Horn, S. Misra, M. Janse, J. Bijma
2015 Biogeosciences  
<p><strong>Abstract.</strong> A number of studies have shown that the boron isotopic composition (&amp;delta;<sup>11</sup>B) and the B / Ca ratio of biogenic carbonates (mostly foraminifers) can serve as proxies for two parameters of the ocean's carbonate chemistry, rendering it possible to calculate the entire carbonate system. However, the B incorporation mechanism into marine carbonates is still not fully understood and analyses of field samples show species-specific and hydrographic effects
more » ... ydrographic effects on the B proxies complicating their application. Identifying the carbonate system parameter influencing boron incorporation is difficult due to the co-variation of pH, CO<sub>3</sub><sup>2-</sup> and B(OH)<sub>4</sub><sup>-</sup>. To shed light on the question which parameter of the carbonate system is related to the boron incorporation, we performed culture experiments with the benthic symbiont-bearing foraminifer <i>Amphistegina lessonii</i> using a decoupled pH–CO<sub>3</sub><sup>2-</sup> chemistry. The determination of the &amp;delta;<sup>11</sup>B and B / Ca ratios was performed simultaneously by means of a new in situ technique combining optical emission spectroscopy and laser ablation MC-ICP-MS. The boron isotopic composition in the tests gets heavier with increasing pH and B / Ca increases with increasing B(OH)<sub>4</sub><sup>-</sup> / HCO<sub>3</sub><sup>-</sup> of the culture media. The latter indicates that boron uptake of <i>A. lessonii</i> features a competition between B(OH)<sub>4</sub><sup>-</sup> and HCO<sub>3</sub><sup>-</sup>. Furthermore, the simultaneous determination of B / Ca and &amp;delta;<sup>11</sup>B on single specimens allows for assessing the relative variability of these parameters. Among different treatments the B / Ca shows an increasing variability with increasing boron concentration in the test whereas the variability in the isotope distribution is constant.</p>
doi:10.5194/bg-12-1753-2015 fatcat:zdkzyvs4zzcolin7vw7i2crdvm