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

[摘要] 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, theB incorporation mechanism into marine carbonates is still not fullyunderstood and analyses of field samples show species-specific andhydrographic effects on the B proxies complicating their application.Identifying the carbonate system parameter influencing boron incorporationis difficult due to the co-variation of pH, CO₃^2-andB(OH)₄^-. To shed light on the question which parameter of thecarbonate system is related to the boron incorporation, we performed cultureexperiments with the benthic symbiont-bearing foraminifer Amphistegina lessonii using a decoupledpH–CO₃^2- chemistry. The determination of the δ¹¹Band B / Ca ratios was performed simultaneously by means of a new in situ techniquecombining optical emission spectroscopy and laser ablation MC-ICP-MS. Theboron isotopic composition in the tests gets heavier with increasing pH andB / Ca increases with increasing B(OH)₄^- / HCO₃^- of theculture media. The latter indicates that boron uptake of A. lessonii features acompetition between B(OH)₄^- and HCO₃^-. Furthermore, thesimultaneous determination of B / Ca and δ¹¹B on single specimensallows for assessing the relative variability of these parameters. Amongdifferent treatments the B / Ca shows an increasing variability withincreasing boron concentration in the test whereas the variability in theisotope distribution is constant.

[学科分类] 地球化学与岩石