TY - JOUR
T1 - Experimental Measurement of Boron Isotope Fractionation in Seawater
AU - Klochko, Kateryna
AU - Kaufman, Alan J.
AU - Yao, Wengsheng
AU - Byrne, Robert H.
AU - Tossell, John A.
PY - 2006/1/1
Y1 - 2006/1/1
N2 - The boron isotopic composition of marine carbonates is considered to be a tracer of seawater pH. Use of this proxy benefits from an intimate understanding of chemical kinetics and thermodynamic isotope exchange reactions between the two dominant boron-bearing species in seawater: boric acid B(OH) 3 and borate ion B(OH) 4 − . However, because of our inability to quantitatively separate these species in solution, the degree of boron isotope exchange has only been known through theoretical estimates. In this study, we present results of a spectrophotometric procedure wherein the boron isotope equilibrium constant ( 11–10 K B ) is determined empirically from the difference in the dissociation constants of 11 B(OH) 3 and 10 B(OH) 3 in pure water, 0.6 mol kg − 1 H 2 O KCl and artificial seawater. Within experimental uncertainty, our results show no dependence of 11–10 K B on temperature, but 11–10 K B at 25 °C in pure water was statistically different than results obtained in solutions at high ionic strength. 11–10 K B of the seawater ( S = 35, B T = 0.01 mol kg − 1 H 2 O) at 25 °C is 1.0272 ± 0.0006. This result is significantly larger than the theoretical value used in numerous paleo-pH studies ( 11–10 K B = 1.0194).
AB - The boron isotopic composition of marine carbonates is considered to be a tracer of seawater pH. Use of this proxy benefits from an intimate understanding of chemical kinetics and thermodynamic isotope exchange reactions between the two dominant boron-bearing species in seawater: boric acid B(OH) 3 and borate ion B(OH) 4 − . However, because of our inability to quantitatively separate these species in solution, the degree of boron isotope exchange has only been known through theoretical estimates. In this study, we present results of a spectrophotometric procedure wherein the boron isotope equilibrium constant ( 11–10 K B ) is determined empirically from the difference in the dissociation constants of 11 B(OH) 3 and 10 B(OH) 3 in pure water, 0.6 mol kg − 1 H 2 O KCl and artificial seawater. Within experimental uncertainty, our results show no dependence of 11–10 K B on temperature, but 11–10 K B at 25 °C in pure water was statistically different than results obtained in solutions at high ionic strength. 11–10 K B of the seawater ( S = 35, B T = 0.01 mol kg − 1 H 2 O) at 25 °C is 1.0272 ± 0.0006. This result is significantly larger than the theoretical value used in numerous paleo-pH studies ( 11–10 K B = 1.0194).
KW - boron isotope equilibrium constant
KW - marine carbonates
KW - seawater pH proxy
UR - https://digitalcommons.usf.edu/msc_facpub/1749
UR - https://doi.org/10.1016/j.epsl.2006.05.034
U2 - 10.1016/j.epsl.2006.05.034
DO - 10.1016/j.epsl.2006.05.034
M3 - Article
VL - 248
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
ER -