Boron Isotopes in Carbonates as a Proxy for Paleo-pH: Applications and Limitations

N. G. Hemming Lamont-Doherty Earth Observatory, Rt. 9W, Palisades, NY 10964, USA

Abhijit Sanyal Lamont-Doherty Earth Observatory, Rt. 9W, Palisades, NY 10964, USA

W. S. Broecker Lamont-Doherty Earth Observatory, Rt. 9W, Palisades, NY 10964, USA

G. N. Hanson Dept. of Earth and Space Sciences, SUNY Stony Brook, Stony Brook, NY 11794, USA

Recent studies have shown that the B isotopic composition of many marine carbonates is very close to the B isotopic composition of the B(OH)4- species in sea water, and thus is controlled by pH. Most modern open marine carbonates have a relatively narrow range of about 22±2” (Hemming and Hanson, 1992). This 18” fractionation from sea water is exactly the same as the fractionation between bulk sea water and the B(OH)4- aqueous species at pH=8.2. Synthetic calcite and aragonite grown at constant laboratory conditions (pH=8.0) and known fluid B isotopic composition also have B isotopic compositions equal to the calculated isotopic composition of the B(OH)4- species (Hemming et al., 1995). These observations have been used to infer paleo-pH from the B isotopic composition of foraminifera (Spivack et al., 1993; Sanyal et al., 1995). The fractionation factor for the incorporation into the mineral must be known precisely, and the B isotopic composition of the precipitating fluid must be known. Thus far, this information has come from empirical observations from modern marine (Hemming and Hanson, 1992; Sanyal et al., 1995), synthetic (Hemming et al., 1995) and cultured (Sanyal et al., subm.) carbonate samples.

Modern core-top planktic foraminifera (Sacculifer) have B isotopic compositions consistent with the known pH of surface sea water at the collection site. Benthic foraminifera from the same site record a lower B isotopic composition than the planktonic species, consistent with the lower pH of the bottom water. This observation was taken as evidence that foraminifera B isotopic compositions have a predictable relationship to the pH of the ambient sea water, and so was applied to glacial age planktic and benthic foraminifera from cores (Sanyal et al., 1995). The interpreted pH values from this data show that surface and bottom waters during glacial times have a similar difference in pH as is seen today, but there is a pH offset towards higher values in sea water during the glacial. The offset of about 0.3 pH units is consistent with the decrease in pCO2 of the atmosphere during glacial times as measured directly from air trapped in ice cores (Neftel et al., 1988). It was assumed that foraminiferal species variability was minimal, and mixed benthic species were used due to the paucity of benthic foraminifera in the core samples. This is a reasonable assumption based on the results with modern benthics at known pH (Sanyal et al., 1995). However, the foraminifera (Orbulina universa) cultured in the laboratory at various pH conditions (7.9, 8.2, 8.4, and 8.8) have isotopic compositions about 4” below that calculated for the B(OH)4- species (Sanyal et al., subm.). This important observation shows that certain species of foraminifera have a constant offset from the predicted value. In this case, the B isotope data at the four pH values define a curve of the same shape as that for the B(OH)4- species,
indicating that indeed the offset is constant over the pH range found in nature. This highlights the importance of establishing calibrations for the species of interest, but suggests that as long as the offset is known, B isotope data can still be used for pH determinations.

In order to calculate glacial sea water pH from B isotopic compositions, it must be assumed that the bulk sea water
B isotopic composition of the glacial oceans was the same as today. This is a reasonable assumption as B has a residence time in the ocean of over 16 million years. However, this may not be a reasonable assumption for samples older than the residence time. We estimate the B isotopic composition of Mississippian ocean water to be about 4” lower than present day, based on the B isotopic composition of pristine Mississippian brachiopods.


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