HR: 14:55h
AN: PP33D-06    [Abstracts]
TI: Experimental determination of boron isotope fractionation in seawater
AU: * Klochko, K
EM: klochko@geol.umd.edu
AF: Department of Geology, University of Maryland, Geology Building, College Park, MD 20742
AU: Kaufman, A J
EM: kaufman@geol.umd.edu
AF: Department of Geology, University of Maryland, Geology Building, College Park, MD 20742
AU: Yao, W
EM: wyao_00@hotmail.com
AF: College of Marine Sciences, University of South Florida, 140 7th Ave. South, St.Petersburg, FL 33701
AU: Byrne, R H
EM: byrne@marine.usf.edu
AF: College of Marine Sciences, University of South Florida, 140 7th Ave. South, St.Petersburg, FL 33701
AU: Tossell, J A
EM: tossell@chem.umd.edu
AF: Department of Chemistry and Biochemistry, University of Maryland, 0107 Chemistry Building, College Park, MD 20742
AB: The boron isotopic composition of marine carbonates is believed to be a useful tracer of seawater pH, which may then be used to reconstruct atmospheric pCO₂ through time. Use of this proxy requires 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^-, which is preferentially incorporated into the carbonate lattice. However, due to our inability to quantitatively isolate these species from seawater, the magnitude of boron isotope fractionation at different temperatures and salinities has not previously been empirically measured. All paleo-pH studies have relied on the boron isotope equilibrium constant (^11-10K_b = 1.0194 at 25°C) estimated theoretically in 1977 by Kakihana and colleagues. Here we present results of empirical determination of the boron isotope equilibrium constant at different temperatures and ionic strengths. The determinations are based on titration of isotopically labeled solutions, containing either ¹⁰B(OH)₃ or ¹¹B(OH)₃, with NaOH. The pH of the titrated solutions is precisely measured using thymol blue indicator absorbance ratios. Differences in solution pH or, equivalently, borate/boric acid pK values between the isotopically substituted solutions, provides the desired equilibrium constant for the reaction: ¹⁰B(OH)₃ + ¹¹B(OH)₄^- <=> ¹¹B(OH)₃ + ¹⁰B(OH)₄^-. We have performed experiments to assess the influence of the temperature (25 and 40°C), ionic strength (0.05 and 0.7 molar) and medium composition (pure water, 0.6 M KCl, and synthetic seawater) on the isotopic equilibrium constant. Within experimental uncertainty maximum of ±0.002 (1σ), our results show only a weak dependence of the equilibrium constant on the above factors. The boron isotope equilibrium constant in seawater (S = 35) was determined to be 1.0269 ± 0.0013 at 25°C (1σ, n=6), which is in poor agreement with the theoretical basis for all previous paleo-pH estimates. Application of the new empirically derived equilibrium constant to previously published results may help to explain systematic offsets from expected δ¹¹B values in studies of modern and ancient carbonates.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 4271 Physical and chemical properties of seawater
DE: 4803 Analytical chemistry
DE: 4924 Geochemical tracers
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005