Articles | Volume 12, issue 6
Biogeosciences, 12, 1753–1763, 2015
Biogeosciences, 12, 1753–1763, 2015

Research article 18 Mar 2015

Research article | 18 Mar 2015

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

K. Kaczmarek1, G. Langer2, G. Nehrke1, I. Horn3, S. Misra2, M. Janse4, and J. Bijma1 K. Kaczmarek et al.
  • 1Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, am Handelshafen 12, 27570 Bremerhaven, Germany
  • 2Department of Earth Science, University of Cambridge, Dowing Site, CB2 3EQ Cambridge, UK
  • 3Institute of Mineralogy, Leibniz University, Callin street 3, 30167 Hanover, Germany
  • 4Burgers Zoo, Antoon van Hooffplein 1, 6816 SH Arnhem, the Netherlands

Abstract. A number of studies have shown that the boron isotopic composition (δ11B) 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 on the B proxies complicating their application. Identifying the carbonate system parameter influencing boron incorporation is difficult due to the co-variation of pH, CO32- and B(OH)4-. 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 Amphistegina lessonii using a decoupled pH–CO32- chemistry. The determination of the δ11B 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)4- / HCO3- of the culture media. The latter indicates that boron uptake of A. lessonii features a competition between B(OH)4- and HCO3-. Furthermore, the simultaneous determination of B / Ca and δ11B 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.

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Short summary
Culture experiments based on a decoupled pH and CO32- chemistry indicate that the δ11B of the test of A. lessonii is related to pH whereas the B/Ca of the foraminiferal shells show a positive correlation with B(OH)4-/HCO3-. The latter observation suggests a competition between B(OH)4- and HCO3- of the culture media for B uptake into the test.
Final-revised paper