Empirical relationship between pH and the boron isotopic composition of Globigerinoides sacculifer: Implications for the boron isotope paleo‐pH proxy

Abstract

Abstract. Culture experiments were carried out with the planktonic foraminifera Globigerinoides sacculifer at three different pH values (7.6+0.05, 8.20+0.05, and 8.60+0.05) in order to establish the pH dependence of the boron isotopic fractionation between the calcitic tests of G. sacculifer and seawater. Additional objectives were to compare the pH dependences of boron isotopic composition of G. sacculifer with that of Orbulina universa and inorganic calcite in order to evaluate the robustness of the boron isotope paleo-pH proxy. The relationship between the boron isotopic composition of G. sacculifer and the culture water pH shows heavier boron isotopic values at higher pH. This empirical relationship is similar but offset (by-3%0) from that of O. universa, indicating the presence of biogenic influence ("vital effect") on the boron isotopic composition of foraminifera. However, comparison of the boron isotopic composition of O. universa and G. sacculifer with that of inorganic calcite grown over a common pH range shows that inorganic processes are the dominant control on the boron isotopic composition of foraminifera and the "vital effects" play a small but not insignificant role. The results imply that in spite of a small biogenic influence the variation in boron isotopic composition of foraminiferal shells with pH (at least for O. universa and G. sacculifer) is consistent with that for inorganic calcite, demonstrating the potential of this isotopic signature in foraminifera as a reliable paleo pH proxy.

Origin of Boron and Brine Evolution in Saline Springs in the Nangqen Basin, Southern Tibetan Plateau

“…Several studies have shown that the boron isotopic composition is controlled by pH [10,58,73,74]. According to Palmer et al [75], pH can restrict the boron isotopic characteristics of a water body to a certain extent.…”

<i>δ</i><sup>11</sup>B as monitor of calcification site pH in divergent marine calcifying organisms

“…For example, prior studies have shown that for a given pH SW , δ 11 B CaCO 3 of the coral species Porites cylindrica and Acropora nobilis are moderately elevated relative to δ 11 B CaCO 3 of the foraminifera Globigerinoides sacculifer and substantially elevated relative to δ 11 B CaCO 3 of the mussel Mytilus edulis ( Fig. 2; Heinemann et al, 2012;Sanyal et al, 2001). One possible explanation for these differences is that corals are maintaining their calcifying fluids at higher pH than the calcifying fluids of foraminifera, which are in turn elevated relative to the pH CF of mussels.…”

“…Although early studies assumed that δ 11 B CaCO 3 was indeed equivalent to seawater δ 11 B B(OH) 4 − (e.g., Hemming and Hanson, 1992), Sanyal et al (2000Sanyal et al ( , 2001 observed that empirically derived δ 11 B CaCO 3 -pH curves of biogenic and abiogenic calcites were parallel but vertically offset from the theoretical δ 11 B B(OH) − 4 -pH curve, which led them to conclude that paleo-seawater pH cannot always be directly calculated from δ 11 B CaCO 3 using the theoretical δ 11 B B(OH) − 4 -pH relationship (i.e., δ 11 B CaCO 3 -pH relationships must be empirically calibrated for the species hosting the paleo-pH proxy).…”

“…)-pH curve (see also Anagnostou et al, 2012;Honïsch et al, 2003;Sanyal et al, 1996Sanyal et al, , 2001Trotter et al, 2011). Instead, the species used for paleo-seawater pH reconstructions may require calibration through controlled laboratory experiments and/or core-top calibrations that empirically define the species-specific relationship between seawater pH (pH SW ) and δ 11 B CaCO 3 .…”

Heavy metal incorporation in foraminiferal calcite: results from multi-element enrichment culture experiments with <i>Ammonia tepida</i>

“…Nägler et al, 2000) preserved in foraminiferal test carbonate have also been used to reconstruct sea water temperature. Other geochemical proxies useful in environmental reconstructions include B isotopes, which are used to determine pH changes in the ocean water (Sanyal et al, 2001;Yu et al, 2007), and Cd, which is a proxy for nutrient concentrations (Boyle, 1988;Rickaby and Elderfield, 1999;Martin and Lea, 1998).…”