Oceanic pH control on the boron isotopic composition of foraminifera: Evidence from culture experiments

Abstract

Culture experiments were carried out with the species Orbulina universa at four different pH values (7.70±0.05, 8.15±0.05, 8.60±0.05, and 9.00±0.10) in order to establish the pH‐dependence of boron isotope fractionation between seawater and foraminifera. A clear relationship between the boron isotopic composition of the foraminifera and the pH of the seawater culture solutions was found, showing heavier boron isotopic composition at higher pH. This finding supports the viability of boron isotopes as a paleo‐pH tool. It is important to note that Orbulina universa cultured in natural seawater, as well as those obtained from coretop samples, have a significantly lighter boron isotopic ratio than Globigerinoides sacculifer from coretop samples, suggesting that at least for this species, a vital effect is active.

The influence of symbiont photosynthesis on the boron isotopic composition of foraminifera shells

“…Although the plankton tow results 21 are preliminary, they support the hypothesis that respiration and photosynthesis are the key physiological parameters 22 responsible for species-specific vital effects. , 1996Spivack et al, 1993). …”

Boron isotope fractionation during brucite deposition from artificial seawater

“…Many studies have indicated that only B(OH) − 4 is incorporated preferentially into corals marine carbonates (Vengosh et al, 1991;Hemming and Hanson, 1992;Hemming et al, 1995;Spivack et al, 1993;Sanyal et al, 1996Sanyal et al, , 2000. Under this circumstance, all the δ 11 B values of carbonates are considered to be paralleled with the theoretical δ 11 B sw3 and are lower than that of the parent seawater; α carb−sw between carbonates and seawater are lower than 1 and increase with pH.…”

“…But when T r = 20 h, the maximum [B] d may appear at pH ≤ 9.5, while the highest value at pH 9.5 is 1859 ppm, which is much higher than 951.5 ppm at pH 10. Considering the slight difference in the (Sanyal et al, 1996). The variation trend of δ 11 B for T r = 0 h and T r = 20 h are basically same and all the δ 11 B d values are enriched in the heavy isotope relative to the δ 11 B fsw (Fig.…”

“…In addition, the influence of brucite in corals on the δ 11 B-pH proxy is more severe. Because one of the essential hypotheses for the δ 11 B-pH proxy is that only the B(OH) − 4 incorporated into corals, that is 10 B incorporated into corals preferentially (Vengosh et al, 1991;Hemming and Hanson, 1992;Sanyal et al, 1996). Under this circumstance, the boron isotope fractionation factor between coral and seawater is less than one.…”

“…A series of inorganic calcite precipitation experiments have shown that B(OH) − 4 is the only or dominant species incorporated into calcite (Hemming et al, 1995;Sanyal et al, 1996Sanyal et al, , 2000. However, Klochko et al (2009) recently found that both trigonal and tetrahedral coordinated boron existed in biogenic and hydrothermal carbonates.…”