The Experts below are selected from a list of 4338 Experts worldwide ranked by ideXlab platform

Hansherbert Brintzinger - One of the best experts on this subject based on the ideXlab platform.

Stefan Beck - One of the best experts on this subject based on the ideXlab platform.

Jacques Schott - One of the best experts on this subject based on the ideXlab platform.

  • Theoretical isotopic fractIonatIon between structural boron in carbonates and aqueous boric acid and Borate Ion
    Geochimica et Cosmochimica Acta, 2017
    Co-Authors: Etienne Balan, Johanna Noireaux, Vasileios Mavromatis, V Montouillout, Giuseppe Saldi, Marc Blanchard, Fabio Pietrucci, Christel Gervais, James Rustad, Jacques Schott
    Abstract:

    The 11B/10B ratio in calcite and aragonite is an important proxy of oceanic water pH. However, the physico-chemical mechanisms underpinning this approach are still poorly known. In the present study, we theoretically determine the equilibrium isotopic fractIonatIon properties of structural boron species in calcium carbonates, BO33-, BO2(OH)2- and B(OH)4- anIons substituted for carbonate groups, as well as those of B(OH)4- and B(OH)3 species in vacuum. Significant variability of equilibrium isotopic fractIonatIon properties is observed among these structural species which is related to their contrasted coordinatIon state, B-O bond lengths and atomic-scale environment. The isotopic compositIon of structural boron does not only depend on its coordinatIon number but also on its medium range environment, i.e. farther than its first coordinatIon shell. The isotopic fractIonatIon between aqueous species and their counterparts in vacuum are assessed using previous investigatIons based on similar quantum-mechanical modeling approaches. At 300K, the equilibrium isotope compositIon of structural trigonal species is 7 to 15 ‰ lighter than that of aqueous boric acid molecules, whereas substituted tetrahedral Borate Ions are heavier than their aqueous counterparts by 10 to 13 ‰. Although significant uncertainties are known to affect the theoretical predictIon of fractIonatIon factors between solids and solutIons, the usually assumed lack of isotopic fractIonatIon during Borate incorporatIon in carbonates is challenged by these theoretical results. The present theoretical equilibrium fractIonatIon factors between structural boron and aqueous species differ from those inferred from experiments which may indicate that isotopic equilibrium, unlike chemical equilibrium, was not reached in most experiments. Further research into the isotopic fractIonatIon processes at the interface between calcium carbonates and aqueous solutIon as well as long duratIon experiments aimed at investigating the kinetics of equilibratIon of boron environment and isotopic compositIon are therefore required to refine our understanding of boron coprecipitatIon in carbonates and thus the theory behind the use of boron isotopes as an ocean pH proxy.

  • crystallographic control on the boron isotope paleo ph proxy
    Earth and Planetary Science Letters, 2015
    Co-Authors: Johanna Noireaux, Jacques Schott, Jérôme Gaillardet, Pascale Louvat, Vasileios Mavromatis, V Montouillout, Claire Rollionbard, Daniel R Neuville
    Abstract:

    When using the boron isotopic compositIon (δ11B) of marine carbonates as a seawater pH proxy, it is assumed that only the tetrahedral Borate Ion is incorporated into the growing carbonate crystals and that no boron isotope fractIonatIon occurs during uptake. However, the δ11B of the calcium carbonate from most modern foraminifera shells or corals skeletons is not the same as the δ11B of seawater Borate, which depends on pH, an observatIon commonly attributed to vital effects. In this study, we combined previously published high-field 11B MAS NMR and new δ11B measurements on the same synthetic calcite and aragonite samples precipitated inorganically under controlled environments to avoid vital effects. Our results indicate that the main controlling factors of δ11B are the solutIon pH and the mineralogy of the precipitated carbonate mineral, whereas the aqueous boron concentratIon of the solutIon, CaCO3 precipitatIon rate and the presence or absence of growth seeds all appear to have negligible influence. In aragonite, the NMR data show that boron coordinatIon is tetrahedral (BO4), in additIon, its δ11B is equal to that of aqueous Borate, thus confirming the paleo-pH hypothesis. In contrast, both trigonal BO3 and tetrahedral BO4 are present in calcite, and its δ11B values are higher than that of aqueous Borate and are less sensitive to solutIon pH variatIons compared to δ11B in aragonite. These observatIons are interpreted in calcite as a reflectIon of the incorporatIon of decreasing amounts of boric acid with increasing pH. Moreover, the fractIon of BO3 measured by NMR in calcite is higher than that inferred from δ11B which indicates a coordinatIon change from BO4 to BO3 upon boron incorporatIon in the solid. Overall, this study shows that although the observed differences in δ11B between inorganic and biological aragonite are compatible with a pH increase at calcificatIon sites, the B speciatIon and isotope compositIon of biological calcites call for a more complex mechanism of boron incorporatIon.

Claire Rollionbard - One of the best experts on this subject based on the ideXlab platform.

  • crystallographic control on the boron isotope paleo ph proxy
    Earth and Planetary Science Letters, 2015
    Co-Authors: Johanna Noireaux, Jacques Schott, Jérôme Gaillardet, Pascale Louvat, Vasileios Mavromatis, V Montouillout, Claire Rollionbard, Daniel R Neuville
    Abstract:

    When using the boron isotopic compositIon (δ11B) of marine carbonates as a seawater pH proxy, it is assumed that only the tetrahedral Borate Ion is incorporated into the growing carbonate crystals and that no boron isotope fractIonatIon occurs during uptake. However, the δ11B of the calcium carbonate from most modern foraminifera shells or corals skeletons is not the same as the δ11B of seawater Borate, which depends on pH, an observatIon commonly attributed to vital effects. In this study, we combined previously published high-field 11B MAS NMR and new δ11B measurements on the same synthetic calcite and aragonite samples precipitated inorganically under controlled environments to avoid vital effects. Our results indicate that the main controlling factors of δ11B are the solutIon pH and the mineralogy of the precipitated carbonate mineral, whereas the aqueous boron concentratIon of the solutIon, CaCO3 precipitatIon rate and the presence or absence of growth seeds all appear to have negligible influence. In aragonite, the NMR data show that boron coordinatIon is tetrahedral (BO4), in additIon, its δ11B is equal to that of aqueous Borate, thus confirming the paleo-pH hypothesis. In contrast, both trigonal BO3 and tetrahedral BO4 are present in calcite, and its δ11B values are higher than that of aqueous Borate and are less sensitive to solutIon pH variatIons compared to δ11B in aragonite. These observatIons are interpreted in calcite as a reflectIon of the incorporatIon of decreasing amounts of boric acid with increasing pH. Moreover, the fractIon of BO3 measured by NMR in calcite is higher than that inferred from δ11B which indicates a coordinatIon change from BO4 to BO3 upon boron incorporatIon in the solid. Overall, this study shows that although the observed differences in δ11B between inorganic and biological aragonite are compatible with a pH increase at calcificatIon sites, the B speciatIon and isotope compositIon of biological calcites call for a more complex mechanism of boron incorporatIon.

  • boron isotopes as ph proxy a new look at boron speciatIon in deep sea corals using 11b mas nmr and eels
    Geochimica et Cosmochimica Acta, 2011
    Co-Authors: Claire Rollionbard, Dominique Blamart, Julien Trebosc, Gregory Tricot, Alexandre Mussi, Jeanpierre Cuif
    Abstract:

    Abstract Dissolved boron in modern seawater occurs in the form of two species, trigonal boric acid B(OH)3 and tetrahedral Borate Ion B ( OH ) 4 − . One of the key assumptIon in the use of boron isotopic compositIons of carbonates as pH proxy is that only Borate Ions, B ( OH ) 4 − , are incorporated into the carbonate. Here, we investigate the speciatIon of boron in deep-sea coral microstructures (Lophelia pertusa specimen) by using high field magic angle spinning nuclear magnetic resonance (11B MAS NMR) and electron energy-loss spectroscopy (EELS). We observe both boron coordinatIon species, but in different proportIons depending on the coral microstructure, i.e. centres of calcificatIon versus fibres. These results suggest that careful sampling is necessary before performing boron isotopic measurements in deep-sea corals. By combining the proportIons of B(OH)3 and B ( OH ) 4 − determined by NMR and our previous Ion microprobe boron isotope measurements, we propose a new equatIon for the relatIon between seawater pH and boron isotopic compositIon in deep-sea corals.

  • intra shell boron isotope ratios in the symbIont bearing benthic foraminiferan amphistegina lobifera implicatIons for δ11b vital effects and paleo ph reconstructIons
    Geochimica et Cosmochimica Acta, 2010
    Co-Authors: Claire Rollionbard, Jonathan Erez
    Abstract:

    Abstract The boron isotope compositIon of marine carbonates is considered to be a seawater pH proxy. Nevertheless, the use of δ11B has some limitatIons such as the knowledge of the fractIonatIon factor (α4-3) between boric acid and the Borate Ion and the amplitude of “vital effects” on this proxy that are not well constrained. Using secondary Ion mass spectrometry (SIMS) we have examined the internal variability of the boron isotope ratio in the shallow water, symbIonts bearing foraminiferan Amphistegina lobifera. Specimens were cultured at constant temperature (24 ± 0.1 °C) in seawater with pH ranging between 7.90 and 8.45. Intra-shell boron isotopes showed large variability with an upper limit value of ≈30‰. Our results suggest that the fractIonatIon factor α4-3 of 0.97352 ( Klochko et al., 2006 ) is in better agreement with our experiments and with direct pH measurements in seawater vacuoles associated with the biomineralizatIon process in these foraminifera. Despite the large variability of the skeletal pH values in each cultured specimen, it is possible to link the lowest calculated pH values to the experimental culture pH values while the upper pH limit is slightly below 9. This variability can be interpreted as follows: foraminifera variably increase the pH at the biomineralizatIon site to about 9. This increase above ambient seawater pH leads to a range in δ11B (Δ11B) for each seawater pH. This Δ11B is linearly correlated with the culture seawater pH with a slope of −13.1 per pH unit, and is independent of the fractIonatIon factor α4-3, or the δ11Bsw through time. It may also be independent of the pKB (the dissociatIon constant of boric acid) value. Therefore, Δ11B in foraminifera can potentially reconstruct paleo-pH of seawater.

Johanna Noireaux - One of the best experts on this subject based on the ideXlab platform.

  • Theoretical isotopic fractIonatIon between structural boron in carbonates and aqueous boric acid and Borate Ion
    Geochimica et Cosmochimica Acta, 2017
    Co-Authors: Etienne Balan, Johanna Noireaux, Vasileios Mavromatis, V Montouillout, Giuseppe Saldi, Marc Blanchard, Fabio Pietrucci, Christel Gervais, James Rustad, Jacques Schott
    Abstract:

    The 11B/10B ratio in calcite and aragonite is an important proxy of oceanic water pH. However, the physico-chemical mechanisms underpinning this approach are still poorly known. In the present study, we theoretically determine the equilibrium isotopic fractIonatIon properties of structural boron species in calcium carbonates, BO33-, BO2(OH)2- and B(OH)4- anIons substituted for carbonate groups, as well as those of B(OH)4- and B(OH)3 species in vacuum. Significant variability of equilibrium isotopic fractIonatIon properties is observed among these structural species which is related to their contrasted coordinatIon state, B-O bond lengths and atomic-scale environment. The isotopic compositIon of structural boron does not only depend on its coordinatIon number but also on its medium range environment, i.e. farther than its first coordinatIon shell. The isotopic fractIonatIon between aqueous species and their counterparts in vacuum are assessed using previous investigatIons based on similar quantum-mechanical modeling approaches. At 300K, the equilibrium isotope compositIon of structural trigonal species is 7 to 15 ‰ lighter than that of aqueous boric acid molecules, whereas substituted tetrahedral Borate Ions are heavier than their aqueous counterparts by 10 to 13 ‰. Although significant uncertainties are known to affect the theoretical predictIon of fractIonatIon factors between solids and solutIons, the usually assumed lack of isotopic fractIonatIon during Borate incorporatIon in carbonates is challenged by these theoretical results. The present theoretical equilibrium fractIonatIon factors between structural boron and aqueous species differ from those inferred from experiments which may indicate that isotopic equilibrium, unlike chemical equilibrium, was not reached in most experiments. Further research into the isotopic fractIonatIon processes at the interface between calcium carbonates and aqueous solutIon as well as long duratIon experiments aimed at investigating the kinetics of equilibratIon of boron environment and isotopic compositIon are therefore required to refine our understanding of boron coprecipitatIon in carbonates and thus the theory behind the use of boron isotopes as an ocean pH proxy.

  • crystallographic control on the boron isotope paleo ph proxy
    Earth and Planetary Science Letters, 2015
    Co-Authors: Johanna Noireaux, Jacques Schott, Jérôme Gaillardet, Pascale Louvat, Vasileios Mavromatis, V Montouillout, Claire Rollionbard, Daniel R Neuville
    Abstract:

    When using the boron isotopic compositIon (δ11B) of marine carbonates as a seawater pH proxy, it is assumed that only the tetrahedral Borate Ion is incorporated into the growing carbonate crystals and that no boron isotope fractIonatIon occurs during uptake. However, the δ11B of the calcium carbonate from most modern foraminifera shells or corals skeletons is not the same as the δ11B of seawater Borate, which depends on pH, an observatIon commonly attributed to vital effects. In this study, we combined previously published high-field 11B MAS NMR and new δ11B measurements on the same synthetic calcite and aragonite samples precipitated inorganically under controlled environments to avoid vital effects. Our results indicate that the main controlling factors of δ11B are the solutIon pH and the mineralogy of the precipitated carbonate mineral, whereas the aqueous boron concentratIon of the solutIon, CaCO3 precipitatIon rate and the presence or absence of growth seeds all appear to have negligible influence. In aragonite, the NMR data show that boron coordinatIon is tetrahedral (BO4), in additIon, its δ11B is equal to that of aqueous Borate, thus confirming the paleo-pH hypothesis. In contrast, both trigonal BO3 and tetrahedral BO4 are present in calcite, and its δ11B values are higher than that of aqueous Borate and are less sensitive to solutIon pH variatIons compared to δ11B in aragonite. These observatIons are interpreted in calcite as a reflectIon of the incorporatIon of decreasing amounts of boric acid with increasing pH. Moreover, the fractIon of BO3 measured by NMR in calcite is higher than that inferred from δ11B which indicates a coordinatIon change from BO4 to BO3 upon boron incorporatIon in the solid. Overall, this study shows that although the observed differences in δ11B between inorganic and biological aragonite are compatible with a pH increase at calcificatIon sites, the B speciatIon and isotope compositIon of biological calcites call for a more complex mechanism of boron incorporatIon.