The pH dependency of the boron isotopic composition of diatom opal (Thalassiosira weissflogii)

The high latitude oceans are key areas of carbon and heat exchange between the atmosphere and the ocean. As such, they are a focus of both modern oceanographic and palaeoclimate research. However, most palaeoclimate proxies that could provide a long-term perspective are based on calcareous organisms, such as foraminifera, that are scarce or entirely absent in deep-sea sediments south of 50° latitude in the Southern Ocean and north of 40° in the North Pacific. As a result, proxies need to be developed for the opal-based organisms (e.g. diatoms) that are found at these high latitudes, and which dominate the biogenic sediments that are recovered from these regions. Here we present a method for the analysis of the boron (B) content and isotopic composition (δ11B) of diatom opal. We also apply it for the first time to evaluate the relationship between seawater pH and δ11B and B concentration ([B]) in the frustules of the diatom Thalassiosira weissflogii, cultured at a range of pCO2/pH. In agreement with existing data, we find that the [B] of the cultured diatom frustules increases with increasing pH (Mejia et al., 2013). δ11B shows a relatively well-defined negative trend with increasing pH; a completely distinct relationship from any other biomineral previously measured. This relationship not only has implications for the magnitude of the isotopic fractionation that occurs during boron incorporation into opal, but also allows us to explore the potential of the boron-based proxies for palaeo-pH and palaeo-CO2 reconstruction in high latitude marine sediments that have, up until now, eluded study due to the lack of suitable carbonate material.

Donald H. K., Foster G. L., Fröhberg N., Swann G. E. A., Poulton A., Moore C. M. & Humphreys M. P., 2019. The pH dependency of the boron isotopic composition of diatom opal (Thalassiosira weissflogii). Biogeosciences Discussions. Article.

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