What controls the copper isotopic composition in oceanic waters?

Takano and co-workers (2014, see reference below) strongly suggest that the isotopic composition of dissolved copper (δ⁶⁵Cu) in surface seawater is mainly controlled by supply from rivers, the atmosphere and deep seawater. This is the conclusion of a study involving six vertical profiles of copper (Cu) concentration and isotopes measured in the Indian (1) and North Pacific Oceans (5). The finding contradicts previous interpretations suggesting a strong role of the biological activity in δ⁶⁵Cu fractionation.

At depth, δ⁶⁵Cu  values are becoming heavier with the age of deep seawater, likely due to preferential scavenging of the light isotope (⁶³Cu). The authors built a box-model to quantify the oceanic budgets of both Cu concentrations and δ⁶⁵Cu. Imbalance in this model suggests that Cu fluxes from continental shelf sediment might affect Cu distribution in the open ocean.

Figure:  A box-model of Cu in the ocean based on both Cu concentration and isotopic composition. Click here to view the figure larger.

Reference:

Takano, S., Tanimizu, M., Hirata, T., & Sohrin, Y. (2014). Isotopic constraints on biogeochemical cycling of copper in the ocean. Nature Communications, 5, 5663. doi:10.1038/ncomms6663 Click here to view the paper.