Jurikova, Hana (2018) Boron element and isotope composition of Phanerozoic brachiopods - implications for the marine carbonate cycle. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, 144 pp.

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Abstract

CO2-induced ocean acidification and associated decrease of seawater carbonate saturation state contributed to multiple environmental crises in Earth’s history and currently poses a major threat for marine calcifying organisms. Owing to their high abundance and good preservation in the Phanerozoic geological record, brachiopods present an expedient taxon of marine calcifiers for palaeo-proxy applications as well as studies on biological mechanism to cope with environmental change. The boron isotope composition of marine calcifiers is regarded as one of the most reliable palaeo-pH proxies, yet until now hardly applied to brachiopods. To investigate the geochemical and physiological responses of brachiopods to prolonged low-pH conditions we cultured Magellania venosa, Terebratella dorsata and Pajaudina atlantica under controlled experimental settings over a period of two years. Our experiments demonstrate that brachiopods form their calcite shells under strong biological control, which enables them to survive and grow under low-pH conditions and even in seawater strongly undersaturated with respect to calcite (pH = 7.35, Ωcal = 0.6). Using boron isotope (δ11B) systematics including MC-ICP-MS as well as SIMS analyses, validated against in vivo microelectrode measurements, we show that this resilience is achieved by strict regulation of the calcifying fluid pH between the epithelial mantle and the shell. We provide a culture-based δ11B−pH calibration, which as a result of the internal pH regulatory mechanisms deviates from the inorganic borate ion to pH relationship, but confirms a clear yet subtle pH dependency for brachiopods. At a micro-scale level, the incorporation of δ11B appears to be principally driven by a physiological gradient across the shell, where the δ11B of the innermost calcite records the internal calcifying fluid pH while the composition of the outermost layers is also influenced by seawater pH. This study details for the first time the functioning of the boron isotope proxy in brachiopods and lays the ground for its application in palaeo-reconstructions (Chapter 3)...

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