When applying the boron isotope proxy, paleoceanographers make use of the fact that higher atmospheric pCO₂ causes seawater to acidify, i.e. its pH decreases, and lower atmospheric pCO₂ turns seawater more alkaline, i.e. its pH increases. Boron chemistry in the ocean is sensitive to seawater acidity and this is recorded in the boron isotope composition of planktic foraminifera. So, by analyzing the boron isotopic composition of fossil planktic foraminifera, paleoceanographers can reconstruct ancient surface ocean acidity, and translate this information into atmospheric pCO₂ if a second parameter of the marine carbonate system can be constrained - in addition to temperature, pressure and salinity. For a general background, see also note on General background and some commonalities of paleo-CO₂ proxies.

In detail, the sensitivity of δ¹¹B in marine carbonates is based on the chemical speciation of dissolved boron in seawater. Dissolved boron occurs in two forms, boric acid (B(OH)₃) and borate ion (B(OH)₄^-), and their relative abundance depends on seawater acidity, where boric acid is more abundant at lower pH (i.e. more acidic water) and borate at higher pH (more alkaline seawater, see Figure a). There are also two isotopes of boron - ¹⁰B, which preferentially resides in borate ion, and ¹¹B, which preferentially occurs in boric acid. When seawater pH increases, more and more boric acid is converted to borate ion (Figure a), and the ¹¹B originally present in boric acid is transferred to borate ion as well. Consequently, the boron isotope ratio (¹¹B/¹⁰B) of borate ion increases at higher pH, and decreases at lower pH (Figure b). Boron isotope analyses of marine carbonates have shown that they preferentially incorporate borate ion into their shells and skeletons, such that their ¹¹B/¹⁰B ratio also increases when seawater pH is higher and decreases when seawater pH is lower (Figure b). Boron is thereby not an essential component of a calcium carbonate skeleton, but it is rather an impurity that replaces (i.e. substitutes) carbonate ion (CO₃^2-) in the CaCO₃ mineral lattice. As boron becomes a solid component of a CaCO₃ shell, it remains locked into the shell after the carrier organism dies, and therefore can be recorded in the geologic archive if the fossil shell is preserved, e.g., in ocean sediments.

As an impurity, boron is not very abundant in marine carbonates, scientists consider it a trace element, but there is enough boron present to be analyzed. Scientists report the boron isotopic composition of marine carbonates as δ¹¹B, which is the ¹¹B/¹⁰B ratio of a carbonate sample relative to the ¹¹B/¹⁰B ratio of a standard. Because the measured isotope differences are small and precise reporting of this ratio would require a cumbersome value with 4 decimal places (e.g., ¹¹B/¹⁰B_sample/¹¹B/¹⁰Bstandard =1.0204), scientists simplify this term by subtracting 1 and multiplying the ratio by 1000, i.e. δ¹¹B = (¹¹B/¹⁰B_sample/¹¹B/¹⁰Bstandard -1) x1000. The unit of δ¹¹B is ‰ (per mil or per thousand) and marine carbonate δ¹¹B values typically range between 14 and 30‰, depending on the carbonate producer (e.g., foraminifera record lower δ¹¹B than tropical corals, Figure b) and on the seawater pH under which the organisms grew.

Figure caption. Schematic presentation of boron proxy systematics in seawater. Panel a displays the pH-dependent abundance of dissolved boric acid and borate ion, panel b the corresponding boron isotopic composition (δ¹¹B) of the two dissolved boron species. The δ¹¹B of seawater (δ¹¹B_sw) is 39.6‰ in the modern ocean (but has varied in the past). ^¹¹B is preferentially present in boric acid, ¹⁰B in borate ion. At low pH, when dissolved boron exists in the form of boric acid, δ¹¹B_boric acid approaches δ¹¹B_sw; at high pH, when dissolved boron exists in the form of borate, δ¹¹B_borate approaches δ¹¹B_sw. At intermediate pH, when boric acid dissociates to borate ion and H⁺, more and more ¹¹B is present in the form of borate ion and δ¹¹B_borate consequently increases at higher pH. Because marine carbonates such as the skeletons of corals and shells of foraminifera incorporate borate ions, their isotopic composition also increases with pH as δ¹¹B_borate increases (grey and black symbols and lines in b). Because ocean uptake of atmospheric CO₂ acidifies seawater, low pH and low δ¹¹Bcarbonate correspond to high PCO₂ and high pH and high δ¹¹Bcarbonate to low PCO₂ (see green arrows at bottom of figure). In ocean regions where PCO₂ and pCO₂ are in equilibrium, atmospheric pCO₂ can be reconstructed from δ¹¹B_carbonate.

Paleo-CO₂ reconstruction from δ¹¹B typically uses shells of symbiont-bearing planktic foraminifera, which live in nutrient-poor ocean regions such as subtropical gyres. Subtropical gyres are large rotating water masses centered around 30° latitude in the northern and southern hemisphere of all ocean basins. In subtropical gyres, seawater resides at the sea surface for a long time, which allows air-sea gas exchange to reach equilibrium (i.e. PCO₂ = pCO₂). Symbiont-bearing planktic foraminifera are bound to the surface ocean, where enough sunlight is available for the symbionts to photosynthesize (e.g., Henehan et al., 2013; Hönisch and Hemming, 2005). This fortunate coincidence of a surface ocean, low nutrient habitat makes them ideal targets for reconstructing atmospheric paleo-CO₂ and alleviates some of the concerns that are central to, e.g., the phytoplankton δ¹³C proxy, which often needs to account for significant air-sea CO₂ disequilibrium in ocean regions that are productive enough to allow sufficient phytoplankton to grow and produce their biomarkers.

However, like all proxies, the boron isotope proxy is also not free of challenges. Such challenges include the uncertainty introduced by foraminifera species-specific vital effects on recorded isotope ratios, the relatively poorly constrained boron isotopic composition of past seawater (which evolved through geologic time), the need for a second parameter of the marine carbonate system (e.g., alkalinity or the total concentration of dissolved inorganic carbon, DIC), for which scientists do not yet command a specific paleo-proxy, and potential diagenetic modification of the original proxy signal via partial dissolution of fossil foraminifera shells in the sediment. For more information on the boron isotope proxy, see Hönisch et al. (2019).

Boron isotope flow chart

The figure above shows a simple flow chart of the different parameters required to calculate CO₂ from this proxy. A template for entering new paleo-CO₂ estimates from boron isotopes can be found here. For adding new data, please fill out the template and submit it to the paleo-CO₂ database at the NOAA National Centers for Environmental Information (formerly known as the National Climatic Data Center, NCDC), using email address paleo@noaa.gov.

References cited