Constraints on barium isotope fractionation during aragonite precipitation by corals. (December 2015)

• Record Type: Journal Article
• Authors: Pretet, Chloé; van Zuilen, Kirsten; Nägler, Thomas F.; Reynaud, Stéphanie; Böttcher, Michael E.; Samankassou, Elias
• Is Part Of: Depositional record. Volume 1:Number 2(2015)
• Page Start: 118
• Page End: 129
• Publication Date: 2015-12
• Subjects: Aragonite -- Ba isotope fractionation -- Barium isotopes -- in situ growth experiment -- isotope proxy -- Mediterranean seawater -- natural/cultured scleractinian corals; Sediments (Geology) -- Periodicals; Sedimentology -- Periodicals; Sedimentation and deposition -- Periodicals; 552.5
• DOI: 10.1002/dep2.8
• Languages: English
• ISSNs: 2055-4877

Abstract

Abstract: We 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 (δ 137/134 Ba between +0·16 ± 0·12‰ and +0·41 ± 0·12‰) were isotopically identical or lighter than that of the ambient Mediterranean surface seawater (δ 137/134 Ba = +0·42 ± 0·07‰, 2SD), which corresponds to an empirical maximum value of Ba isotope fractionation of Δ 137/134 Bacoral‐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 δ 137/134 Ba 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. Abstract : The Ba isotopic composition of the cultured, aragonitic scleractinian corals is isotopically lighter or close to that of the ambient seawater. Coral samples from natural settings show a wide range in δ 137/134 Ba values (+0·14 to +0·77‰), most probably due to spatial and/or temporal seawater heterogeneity.