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Direct measurement of the boron isotope fractionation factor: Reducing the uncertainty in reconstructing ocean paleo-pH

Publication ,  Journal Article
Nir, O; Vengosh, A; Harkness, JS; Dwyer, GS; Lahav, O
Published in: Earth and Planetary Science Letters
March 5, 2015

The boron isotopic composition of calcium carbonate skeletons is a promising proxy method for reconstructing paleo-ocean pH and atmospheric CO2 from the geological record. Although the boron isotope methodology has been used extensively over the past two decades to determine ancient ocean-pH, the actual value of the boron isotope fractionation factor (εB) between the two main dissolved boron species, 11B(OH)3 and 10B(OH)-4, has remained uncertain. Initially, εB values were theoretically computed from vibrational frequencies of boron species, resulting in a value of ~19‰. Later, spectrophotometric pH measurements on artificial seawater suggested a higher value of ~27‰. A few independent theoretical models also pointed to a higher εB value. Here we provide, for the first time, an independent empirical fractionation factor (εB=26.0±1.0‰25 °C), determined by direct measurements of B(OH)3 in seawater and other solutions. Boric acid was isolated by preferential passage through a reverse osmosis membrane under controlled pH conditions. We further demonstrate that applying the Pitzer ion-interaction approach, combined with ion-pairing calculations, results in a more accurate determination of species distribution in aquatic solutions of different chemical composition, relative to the traditional two-species boron-system approach. We show that using the revised approach reduces both the error in simulating ancient atmospheric CO2 (by up to 21%) and the overall uncertainty of applying boron isotopes for paleo-pH reconstruction. Combined, this revised methodology lays the foundation for a more accurate determination of ocean paleo-pH through time.

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Published In

Earth and Planetary Science Letters

DOI

ISSN

0012-821X

Publication Date

March 5, 2015

Volume

414

Start / End Page

1 / 5

Related Subject Headings

  • Geochemistry & Geophysics
  • 51 Physical sciences
  • 37 Earth sciences
  • 04 Earth Sciences
  • 02 Physical Sciences
 

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Nir, O., Vengosh, A., Harkness, J. S., Dwyer, G. S., & Lahav, O. (2015). Direct measurement of the boron isotope fractionation factor: Reducing the uncertainty in reconstructing ocean paleo-pH. Earth and Planetary Science Letters, 414, 1–5. https://doi.org/10.1016/j.epsl.2015.01.006
Nir, O., A. Vengosh, J. S. Harkness, G. S. Dwyer, and O. Lahav. “Direct measurement of the boron isotope fractionation factor: Reducing the uncertainty in reconstructing ocean paleo-pH.” Earth and Planetary Science Letters 414 (March 5, 2015): 1–5. https://doi.org/10.1016/j.epsl.2015.01.006.
Nir O, Vengosh A, Harkness JS, Dwyer GS, Lahav O. Direct measurement of the boron isotope fractionation factor: Reducing the uncertainty in reconstructing ocean paleo-pH. Earth and Planetary Science Letters. 2015 Mar 5;414:1–5.
Nir, O., et al. “Direct measurement of the boron isotope fractionation factor: Reducing the uncertainty in reconstructing ocean paleo-pH.” Earth and Planetary Science Letters, vol. 414, Mar. 2015, pp. 1–5. Scopus, doi:10.1016/j.epsl.2015.01.006.
Nir O, Vengosh A, Harkness JS, Dwyer GS, Lahav O. Direct measurement of the boron isotope fractionation factor: Reducing the uncertainty in reconstructing ocean paleo-pH. Earth and Planetary Science Letters. 2015 Mar 5;414:1–5.
Journal cover image

Published In

Earth and Planetary Science Letters

DOI

ISSN

0012-821X

Publication Date

March 5, 2015

Volume

414

Start / End Page

1 / 5

Related Subject Headings

  • Geochemistry & Geophysics
  • 51 Physical sciences
  • 37 Earth sciences
  • 04 Earth Sciences
  • 02 Physical Sciences