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

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<span classCombining double low line"inline-formula">ĝ&circ; </span>&thinsp;S in the Southern Ocean and north of 40<span classCombining double low line"inline-formula">ĝ&circ; </span>&thinsp;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 (<span classCombining double low line"inline-formula">δ11</span>B) of diatom opal. We apply it for the first time to evaluate the relationship between seawater pH, <span classCombining double low line"inline-formula">δ11</span>B and B concentration ([B]) in the frustules of the diatom Thalassiosira weissflogii, cultured across a range of carbon dioxide partial pressure (<span classCombining double low line"inline-formula">pCO2</span>) 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). <span classCombining double low line"inline-formula">δ11</span>B 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-<span classCombining double low line"inline-formula">CO2</span> reconstruction in high-latitude marine sediments that have, up until now, eluded study due to the lack of suitable carbonate material.

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