There are 2 Boron Isotopes
• Two isotopes of Boron, 10B (19.9 %) and 11B (80.1 %)
• Unusual in that heavy isotope is most abundant
• Low atomic mass ‐> large mass difference (~10%) ‐> large natural fractionations Methods
• Boron isotopes are analyzed using Negative‐
Thermal Ionization Mass Spectrometry (N‐TIMS) or Multicollector Inductivly Coupled Plasma Mass Spectrometry (MC‐ICP‐MS).
• Typical precision (N‐TIMS) <1‰ (best 0.2 ‰) (2)
• Ionization efficiency and beam stability are strongly influenced by matrix ‐ measurements are nontrivial.
• Samples reported as permil deviation (11B) from SRM‐951 (and sometimes seawater).
Applications of B isotopes
• Semi‐conservative tracer in continental waters (water sources and mixing)
• Proxy for past ocean pH in biogenic carbonates
– Low temperature alteration of oceanic crust
– Fluid flow in convergent margin settings
B Isotopes in Continental Waters
• Most common application of B isotopes
– Israel – Fort Ord
– Fresh Kills
• Large variation in environmental 11B
– Seawater = 39.5 ‰
– Ground waters = 0 to 30 ‰
– Anthropogenic B = 0 to 10 ‰
δ11B values vs B concentrations for wastewater and background
groundwater reported for well-studied groundwater systems in the
U.S.A. (El Paso, Texas; data from Bassett et al., 1995) and Israel
(Coastal Plain Aquifer; data from Vengosh et al., 1994). Error bars
correspond to 2σ analytical uncertainties. Ranges in δ11B of nonmarine Na-borate minerals and commercial borax from the U.S.A. and
Turkey (data from McMullen et al., 1961; Swihart et al., 1986; Xiao et
al., 1988; Oi et al., 1989; Vengosh et al., 1994; Palmer and Helvaci,
1995) and industrial sodium perborate monohydrate and tetrahydrate
products are shown for comparison.
Davidson and Basset 1993
Widory et al., 2005
B isotopes in precipitation
B isotopes as a paleo‐pH proxy
• Surface ocean pH is important indicator of state of ocean/atmosphere carbon cycle ‐
linked to pCO2.
• B isotopes are one of the few potential methods for quantitatively assessing changes in global carbon cycle on geologic time scales. • But not easy.
Active carbon reservoirs
pCO2 (ppmv)
Age (years)
Atmosphere
750 Gt C
‐30%
CO2(atm)
HCO3‐+H+
CO2(aq) +H2O H2CO3
‐30%
Vegetation and soils
2 200 Gt C
Ocean
39 000 Gt C
+2%
CO32‐+2H+
Boron Cycle in the Ocean
Ocean Concentration = 0.450 mM
Occurs either as B(OH)3 or B(OH)4‐
Well mixed, “major minor” element
Main sources: rivers, high temp alteration of ocean crust
• Main sinks: low temp alteration of oceanic crust, CaCO3
• Residence time between 12 and 20 Myr
•
•
•
•
The boron isotope proxy for past seawater‐pH
B/Ca
CaCO3 + B(OH)4‐ 
Ca(HBO3) + HCO3‐ + H2O
70
B(OH)
60
pH (SWS)
b
3
40
B(OH)
11
B (‰)
50
30
= 27.2‰
modern marine
carbonates
20
10
4
7
7.5
8
8.5
9
pH (total scale)
Hemming & Hanson, GCA, 1992
9.5
10
CO2(atm) 
CO2(atm) 
CO2(aq) 
CO2(aq) 
pH 
11B 
pH 
11B 
11B
pH + pCO2
MC‐ICP‐MS
S. pistillata (Krief et al. 2010)
Porites sp. (Krief et al. 2010)
O. universa (Hönisch & Rae, unpubl.)
N‐TIMS
P. cylindrica (Hönisch et al. 2004)
Acropora sp. (Reynaud et al., 2004)
A. nobilis (Hönisch et al. 2004)
G. sacculifer (Sanyal et al. 2001)
inorganic calcite (Sanyal et al. 2000)
O. universa (Sanyal et al. 1996)
Benthic foraminifer calibration for 11B
Hönisch et al. 2008
Pearson and Palmer 2000
Arrow indicates inferred transient ocean acidification event.
Kasemann et al., 2010
Atmospheric carbon dioxide through the Eocene–Oligocene climate transition
Paul N. Pearson, Gavin L. Foster & Bridget S. Wade
Nature 461, 1110-1113(22 October 2009) doi:10.1038/nature08447
ice‐sheet formation might have been triggered when atmospheric carbon dioxide levels
CO2/climate relationship intact but no long‐term drawdown of CO2
2.8
3.2
4
4.4
18OLR04
3.6
4.8
5.2
350
pCO
2
300
250
200
150
0
500
1000
1500
2000
Time (ky)
Hönisch et al. 2009
Mid‐Pleistocene transition
Hönisch et al. 2009
Onset of Northern Hemisphere Glaciation
Seki et al., 2010, Hönisch et al. 2009; Bartoli et al., 2011
Cenozoic records of climate and atmospheric pCO2
1,500
1,000
500
atmospheric pCO2 (ppm)
2,000
18O (‰)
ice‐free temperature (°C)
Age (millions of years ago)
Beerling & Royer, Nature Geoscience, 2011
Zachos et al., Nature, 2008
Seasonal changes in pH I a coral reef system