Boron incorporation in the foraminifer &amp;lt;i&amp;gt;Amphistegina lessonii&amp;lt;/i&amp;gt; under a decoupled carbonate chemistry

K Kaczmarek,M Janse,S Misra,G Langer,G Nehrke,I Horn,J Bijma

doi:10.5194/bg-12-1753-2015

Abstract

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.

Highlights

The oceans’ carbonate system comprises six co-varying parameters ([CO2], [HCO−3 ], [CO23−], pH, total alkalinity (TA), and dissolved inorganic carbon (DIC))
Since the B isotopic composition of biogenic carbonates precipitated at a certain pH value is similar to that of B(OH)−4, Hemming and Hanson (1992) concluded that only B(OH)−4 is incorporated into biogenic carbonates
Therewith, the B isotopic composition can be used as a proxy to infer the pH that prevailed during the formation of the biogenic carbonate

Summary

Introduction

The oceans’ carbonate system comprises six co-varying parameters ([CO2], [HCO−3 ], [CO23−], pH, total alkalinity (TA), and dissolved inorganic carbon (DIC)). A number of studies have shown that the boron isotopic composition (δ11B) and the B / Ca ratio of biogenic carbonates (mostly foraminifers) may serve as proxies that can provide these two parameters. Several studies show a deviation between the B isotopic composition of the biogenic carbonates and B(OH)−4 of the sea water (Sanyal et al, 1996, 2001; Foster, 2008; Rae et al, 2011) This deviation is often explained by physiological processes like photosynthesis and respiration of symbionts (e.g. dinoflagellates) which modify the pH in the microenvironment around the foraminifera (Zeebe et al, 2003) leading to shifts in the B equilibria. Another explanation for the observed deviation is that B(OH)−4 is incorporated

Methods

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Conclusion