Simultaneous determination of δ11B and B/Ca ratio in marine biogenic carbonates at nanogram level

Abstract

In this study we introduce a new in situ technique which allows the determination of the boron isotopic composition and B/Ca ratios simultaneously at the nanogram level using a combination of optical emission spectroscopy and multiple ion counting MC ICP-MS with laser ablation. This technique offers a new application in the paleo-field of oceanography and climatology since small samples like e.g. single foraminiferal shells can be analyzed. The simultaneous determination of the boron isotopic composition and B/Ca ratios provides two independent proxies which allow the reconstruction of the full carbonate system. To test the new technique we performed measurements on the cultured, benthic foraminifer Amphistegina lessonii. Our results yielded an average boron isotopic composition δ11B=18.0±0.83‰ (SD) with an average internal precision of 0.52‰ (RSE). The boron concentration was 53±7μg/g (SD). These results agree with the range reported in the literature. The reconstructed mean pH value is in excellent agreement with the measured pH of the seawater in which the foraminifers grew. The analysis of a foraminifer consumed approximately 1200. ng calcium carbonate containing ca. 0.06. ng boron. Compared to bulk analytical methods, this new technique requires less material and reduces the time for sample preparation.