Abstract
AbstractWe present a barium (Ba) isotope fractionation study of marine biogenic carbonates (aragonitic corals). The major aim is to provide first constraints on the Ba isotope fractionation between modern surface seawater and coral skeleton. Mediterranean surface seawater was found to be enriched in the heavy Ba isotopes compared to previously reported values for marine open ocean authigenic and terrestrial minerals. In aquarium experiments with a continuous supply of Mediterranean surface water, the Ba isotopic composition of the bulk sample originating from cultured, aragonitic scleractinian corals (δ137/134Ba between +0·16 ± 0·12‰ and +0·41 ± 0·12‰) were isotopically identical or lighter than that of the ambient Mediterranean surface seawater (δ137/134Ba = +0·42 ± 0·07‰, 2SD), which corresponds to an empirical maximum value of Ba isotope fractionation of Δ137/134Bacoral‐seawater = −0·26 ± 0·14‰ at 25°C. This maximum Ba isotope fractionation is close and identical in direction to previous results from inorganic precipitation experiments with aragonite‐structured pure BaCO3 (witherite). The variability in measured Ba concentrations of the cultured corals is at odds with a uniform distribution coefficient, D(Ba/Ca), thus indicating stronger vital effects on isotope than element discrimination. This observation supports the hypothesis that the Ba isotopic compositions of these corals do not result from simple equilibrium between the skeleton and the bulk seawater. Complementary coral samples from natural settings (tropical shallow‐water corals from the Bahamas and Florida and cold‐water corals from the Norwegian continental shelf) show an even wider range in δ137/134Ba values (+0·14 ± 0·08 to +0·77 ± 0·11‰), most probably due to additional spatial and/or temporal seawater heterogeneity, as indicated by recent publications.
Highlights
The earth alkaline element barium (Ba) has attracted significant attention in the biogeochemistry community
The purpose of this study is to evaluate (i) if Ba isotope fractionation in corals is on the same order as that of abiogenic precipitation or if significant vital effects are observable; and (ii) if corals from natural settings show larger variations than explainable by precipitation, that is, are there indications for isotopically heterogeneous seawater
As (Ba/Ca)solution is constant for the cultured corals (Ba/Ca)carbonate ratios bear the same information on relative variations as D(Ba/Ca)
Summary
The earth alkaline element barium (Ba) has attracted significant attention in the biogeochemistry community. Ba/Ca ratios in the shells of foraminifers are used to reconstruct past ocean alkalinity and circulation patterns (Lea et al, 1989; Hall & Chan, 2004; Weldeab et al, 2007a) or act as a a 2016 The Authors. The Depositional Record published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists.
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