Seawater pH control on the boron isotopic composition of calcite: evidence from inorganic calcite precipitation experiments

Abstract

Abstract-Experiments involving boron co-precipitation with calcite have been carried out inorganically under controlled pH conditions (7.9 Ϯ 0.05, 8.3 Ϯ 0.05 and 8.6 Ϯ 0.05) to determine the dependence of the boron isotopic composition (␦ 11 B) of calcite on the pH of seawater. Another purpose of these experiments was to estimate the magnitude of the biogenic influence on the ␦ 11 B value of foraminifera by comparing their boron isotopic composition with that of the inorganic calcite over a common pH range. The results show a clear relationship between ␦ 11 B of inorganic calcite and the pH of artificial seawater. The variation of boron isotopic fractionation between seawater and calcite with pH, estimated from these experiments, is similar to that estimated for cultured O. universa and the theoretically predicted trend. The results also support the hypothesis that B(OH) 4 Ϫ is the dominant species incorporated into the calcite structure. However, the boron isotopic fractionation between seawater and inorganic calcite is lower than that estimated for O. universa indicating the presence of a biogenic effect on the boron isotopic composition at least of this species of foraminifera. Most importantly, the results imply that in spite of a small biogenic influence on the boron isotopic composition of foraminifera, the variation in ␦ 11 B of foraminiferal shells with pH (at least for O. universa) is comparable to that for inorganic calcite, supporting the potential of this isotopic signature in foraminifera as a reliable paleo pH proxy.

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

“…381 Although pH measurements have never been con-382 ducted on this species, it is well known that the 383 addition of respiratory CO 2 decreases pH in sym-384 biont-bearing foraminifera by up to 0.3 units 385 (JÖrgensen et al, 1985;Rink et al, 1998;Wolf-386 and is therefore expected to 387 in£uence G. bulloides similarly. Comparison of 388 this datum point with data from inorganic precip-389 itation experiments (Sanyal et al, 2000) demon-390 strates that G. bulloides falls slightly below the 391 inorganic precipitation curve (Fig. 2).…”

“…Comparison of the boron isotopic composition of the 2 symbiont-bearing foraminifera Orbulina universa and the 3 symbiont-barren Globigerina bulloides (¢lled circles) taken 4 from plankton tows and inorganic carbonates (open circles 5 and solid line; Sanyal et al, 2000). As the inorganic carbon-6 ate was also precipitated in arti¢cial seawater, the N 11 B val-7 ues of Sanyal et al (2000) were converted to the natural sea-8 water scale according to Zeebe and Wolf-Gladrow (2001, The N 11 B of symbiont-barren Globigerina bul-373 loides (19.0 þ 0.9x) was 1.4x lower than that 374 of the Orbulina universa shells collected from the 375 same plankton tows. Because this is the ¢rst N 11 B 376 datum ever measured on a symbiont-barren spe-377 cies, it cannot be compared to literature data.…”

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.…”

“…The incorporation of Mg into coral skeleton is controlled by varying factors. Mg can exist in the form of brucite in corals (Smith and Delong, 1978;Nothdurft et al, 2005) and Mg content in scleractinian corals varies between different genera and localities (Fallon et al, 1999). Nothdurft et al (2005) studied brucite in living scleractinian corals colonies (Acropora, Pocillopora, Porites) from subtidal and intertidal settings in the Great Barrier Reef, Australia, and subtidal Montastraea from the Florida Keys, United States.…”

“…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.…”