Abstract. 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∘ S in the Southern Ocean and north
of 40∘ N in the North Pacific. As a result, proxies need to be
developed for the opal-based organisms (e.g. diatoms) found at these high
latitudes, which dominate the biogenic sediments 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 apply it for
the first time to evaluate the relationship between seawater pH, δ11B and B concentration ([B]) in the frustules of the diatom
Thalassiosira weissflogii, cultured across a range of carbon dioxide partial pressure (pCO2) and
pH values. In agreement with existing data, we find that the [B]
of the cultured diatom frustules increases with increasing pH (Mejía et al.,
2013). δ11B shows a relatively well defined negative trend with
increasing pH, completely distinct 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.
Supplementary material to "The pH dependency of the boron isotopic composition of diatom opal (<i>Thalassiosira weissflogii</i>)"