A solvent extraction technique for the isotopic measurement of dissolved copper in seawater

• A new sample preparation method for seawater copper isotopic analysis (δ65Cu).
• Solvent-extraction was used to pre-concentrate metals from seawater samples.
• Anion-exchange was used to purify copper from the metal-rich extract.
• δ65Cu was measured in the north Tasman Sea.
• Seawater δ65Cu may be linked to marine biological activity.

Stable copper (Cu) isotope geochemistry provides a new perspective for investigating and understanding Cu speciation and biogeochemical Cu cycling in seawater. In this work, sample preparation for isotopic analysis employed solvent-extraction with amino pyrollidine dithiocarbamate/diethyl dithiocarbamate (APDC/DDC), coupled with a nitric acid back-extraction, to concentrate Cu from seawater. This was followed by Cu-purification using anion-exchange. This straightforward technique is high yielding and fractionation free for Cu and allows precise measurement of the seawater Cu isotopic composition using multi-collector inductively coupled plasma mass-spectrometry. A deep-sea profile measured in the oligotrophic north Tasman Sea shows fractionation in the Cu isotopic signature in the photic zone but is relatively homogenised at depth. A minima in the Cu isotopic profile correlates with the chlorophyll a maximum at the site. These results indicate that a range of processes are likely to fractionate stable Cu isotopes in seawater.

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