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

Abstract

Abstract. The boron isotope composition (δ 11 B) of marine biogenic carbonates has been predominantly studied as a proxy for monitoring past changes in seawater pH and carbonate chemistry. However, a number of assumptions regarding chemical kinetics and thermodynamic isotope exchange reactions are required to derive seawater pH from δ 11 B biogenic carbonates. It is also probable that δ 11 B of biogenic carbonate reflects seawater pH at the organism's site of calcification, which may or may not reflect seawater pH. Here, we report the development of methodology for measuring the δ 11 B of biogenic carbonate samples at the multicollector inductively coupled mass spectrometry facility at Ifremer (Plouzané, France) and the evaluation of δ 11 B CaCO 3 in a diverse range of marine calcifying organisms reared for 60 days in isothermal seawater (25 • C) equilibrated with an atmospheric pCO 2 of ca. 409 µatm. Average δ 11 B CaCO 3 composition for all species evaluated in this study range from 16.27 to 35.09 ‰, including, in decreasing order, coralline red alga Neogoniolithion sp. (35.89 ± 3.71 ‰), temperate coral Oculina arbuscula (24.12 ± 0.19 ‰), serpulid worm Hydroides crucigera (19.26 ± 0.16 ‰), tropical urchin Eucidaris tribuloides (18.71 ± 0.26 ‰), temperate urchin Arbacia punctulata (16.28 ± 0.86 ‰), and temperate oyster Crassostrea virginica (16.03 ‰). These results are discussed in the context of each species' proposed mechanism of biocalcification and other factors that could influence skeletal and shell δ 11 B, including calcifying site pH, the proposed direct incorporation of isotopically enriched boric acid (instead of borate) into biogenic calcium carbonate, and differences in shell/skeleton polymorph mineralogy. We conclude that the large inter-species variability in δ 11 B CaCO 3 (ca. 20 ‰) and significant discrepancies between measured δ 11 B CaCO 3 and δ 11 B CaCO 3 expected from established relationships between abiogenic δ 11 B CaCO 3 and seawater pH arise primarily from fundamental differences in calcifying site pH amongst the different species. These results highlight the potential utility of δ 11 B as a proxy of calcifying site pH for a wide range of calcifying taxa and underscore the importance of using species-specific seawater-pH-δ 11 B CaCO 3 calibrations when reconstructing seawater pH from δ 11 B of biogenic carbonates.

Seawater pH reconstruction using boron isotopes in multiple planktonic foraminifera species with different depth habitats and their potential to constrain pH and <i>p</i>CO<sub>2</sub> gradients

“…Results for the isotopic composition of the NEP coral are shown in Table S2, average values are δ 11 B NEP = 25.70 ± 0.93 ‰ (2SD, n = 22) over seven different analytical sessions with each number representing an ab initio processed sample. Our results are within error of published values of 26.20 ± 0.88 ‰ (2SD, n = 27) and 25.80 ± 0.89 ‰ (2SD, n = 6) by Holcomb et al (2015) and Sutton et al (2018), respectively. Chemically cleaned JCp-1 samples were measured at 24.06 ± 0.20 (2SD, n = 6) and are within error of published values of 24.37±0.32 ‰, 24.11±0.43 ‰, and 24.42 ± 0.28 ‰ by Holcomb et al (2015), Farmer et al (2016, and Sutton et al (2018), respectively.…”

“…We also observed in our study that N. dutertrei, G. menardii, P. obliquiloculata, and G. tumida record a lower pH than ambient seawater, with δ 11 B carbonate lower than expected δ 11 B borate , and we suggest the results are consistent with lower photosynthetic activity compared to the mixed-layer dwelling species. These observations, based on δ 11 B carbonate measurements, are in line with direct observations from Takagi et al (2019) that show dinoflagellatebearing foraminifera (G. ruber, T. sacculifer, and O. universa) tend to have a higher symbiont density and photosynthesis activity while P. obliquiloculata, G. menardii, and N. dutertrei have lower symbiont density and P. obliquiloculata and N. dutertrei have the lowest photosynthetic activity. In the same study, P. obliquiloculata exhibited minimum symbiont densities and levels of photosynthetic activity, which may explain why P. obliquiloculata exhibited the lowest microenvironment pH as recorded by δ 11 B.…”

“…T. sacculifer has the potential to support more photosynthesis due to its higher symbiont density, and higher photosynthetic activity compared to other species, which may support higher symbiont-host interactions (Takagi et al, 2019). These results would be consistent with a greater sensitivity of T. sacculifer's photosynthetic activity with changes in insolation-water depth.…”

“…Based on the observations of Takagi et al (2019), we can assume that the low δ 11 B of O. universa and T. sacculifer (without sacc) from the WEP is explained by low photosynthetic activity. It has been shown for T. sacculifer and O. universa that symbiont photosynthesis increases with higher insolation (Jørgensen et al, 1985;Rink et al, 1998) and the photosynthetic activity is therefore a function of the light level the symbionts received.…”

“…7b). It can also be noted that T. sacculifer exhibits the largest variation in symbiont density versus test size (Takagi et al, 2019), suggesting that lower size fraction reported for the WEP (250-400 µm) compared to the 300-400 µm at the other sites can be related to a decrease in photosynthetic activity and a lower δ 11 B. Unfortunately, no weight-per-shell data were determined on foraminifera samples to constrain whether test size was significantly different across sites.…”