Abstract
Abstract. In order to fully constrain paleo-carbonate systems, proxies for two out of seven parameters, plus temperature and salinity, are required. The boron isotopic composition (δ11B) of planktonic foraminifera shells is a powerful tool for reconstructing changes in past surface ocean pH. As B(OH)4− is substituted into the biogenic calcite lattice in place of CO32−, and both borate and carbonate ions are more abundant at higher pH, it was suggested early on that B ∕ Ca ratios in biogenic calcite may serve as a proxy for [CO32−]. Although several recent studies have shown that a direct connection of B ∕ Ca to carbonate system parameters may be masked by other environmental factors in the field, there is ample evidence for a mechanistic relationship between B ∕ Ca and carbonate system parameters. Here, we focus on investigating the primary relationship to develop a mechanistic understanding of boron uptake. Differentiating between the effects of pH and [CO32−] is problematic, as they co-vary closely in natural systems, so the major control on boron incorporation remains unclear. To deconvolve the effects of pH and [CO32−] and to investigate their impact on the B ∕ Ca ratio and δ11B, we conducted culture experiments with the planktonic foraminifer Orbulina universa in manipulated culture media: constant pH (8.05), but changing [CO32−] (238, 286 and 534 µmol kg−1 CO32−) and at constant [CO32−] (276 ± 19.5 µmol kg−1) and varying pH (7.7, 7.9 and 8.05). Measurements of the isotopic composition of boron and the B ∕ Ca ratio were performed simultaneously using a femtosecond laser ablation system coupled to a MC-ICP-MS (multiple-collector inductively coupled plasma mass spectrometer). Our results show that, as expected, δ11B is controlled by pH but it is also modulated by [CO32−]. On the other hand, the B ∕ Ca ratio is driven by [HCO3−], independently of pH. This suggests that B ∕ Ca ratios in foraminiferal calcite can possibly be used as a second, independent, proxy for complete paleo-carbonate system reconstructions. This is discussed in light of recent literature demonstrating that the primary relationship between B ∕ Ca and [HCO3−] can be obscured by other environmental parameters.
Highlights
Before the Anthropocene, the atmospheric CO2 concentration was governed by the surface ocean [CO2], because the carbon content of the ocean is 65 times larger than that of the atmosphere (Siegenthaler and Sarmiento, 1993)
As B(OH)−4 is substituted into the biogenic calcite lattice in place of CO23−, and both borate and carbonate ions are more abundant at higher pH, it was suggested early on that B / Ca ratios in biogenic calcite may serve as a proxy for [CO23−]
The B / Ca ratio of specimens grown under lower pHT values (7.9 and 7.7) is negatively offset from the relationship found at pHT 8.05 and the overall correlation of B / Ca and [CO23−] is very low (R2 = 0.2; Fig. 2e)
Summary
Before the Anthropocene, the atmospheric CO2 concentration was governed by the surface ocean [CO2], because the carbon content of the ocean is 65 times larger than that of the atmosphere (Siegenthaler and Sarmiento, 1993). Understanding the global carbon cycle and the evolu-. Howes et al.: Controls on the incorporation of boron into Orbulina universa tion of atmospheric pCO2 in Earth history requires knowledge of the dynamics of the oceanic carbonate chemistry. The unprecedented magnitude and rate of carbon emissions has caused both warming and acidification of the oceans (Bijma et al, 2013; Ciais et al, 2013; Gattuso and Hansson, 2011; Gattuso et al, 2015; Rhein et al, 2013). The interest in the reconstruction of seawater carbonate chemistry to identify ocean acidification in Earth history experienced an impetus (Hönisch et al, 2012; Martínez-Botí et al, 2015a)
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