A role for scavenging in the marine biogeochemical cycling of zinc and zinc isotopes

- Seawater dissolved δ114Cd increases towards the surface ocean, while δ66Zn decreases.
- Degrading phytoplankton release dissolved P and Cd, while much Zn is readsorbed.
- Adsorbed Zn is isotopically heavy compared to dissolved Zn.
- Scavenging may impact both modern and ancient Zn marine biogeochemical cycles.

Zinc (Zn) and cadmium (Cd) are important biologically active trace-metals in the ocean. To date, the marine distributions of these elements have been understood primarily in terms of biological assimilation by growing phytoplankton and regeneration of sinking biological material. Initial studies of Zn and Cd concentrations and stable isotope ratios (δ66Zn and δ114Cd) have therefore focused on their use as simple tracers of assimilation and regeneration in the oceans. However, these two processes are insufficient to explain new data on the marine distribution of Zn and δ66Zn. Here, using the first high-resolution paired marine depth profiles of Zn, Cd, δ66Zn and δ114Cd, we suggest that scavenging of Zn onto organic matter plays a major, yet largely unconsidered, role in the marine cycling of Zn. This hypothesis is supported by culture experiments, which show that Zn released from degrading phytoplankton is rapidly scavenged back onto organic matter, and that adsorbed Zn is isotopically heavier than the dissolved phase by 0.58‰. In contrast, very little Cd or phosphate was scavenged and Cd isotopes were not significantly fractionated during degradation. Our hypothesis is further supported by one-dimensional modeling, which reproduces observed marine δ66Zn profiles with <1% of Zn adsorbed to particles. Understanding how Zn cycling in the oceans is a balance between assimilation, scavenging, and regeneration is necessary in order to investigate δ66Zn as a tracer of marine productivity. We anticipate that paired analyses of δ66Zn and δ114Cd will prove to be valuable new tools in constraining patterns of global primary productivity, providing key information for the marine carbon cycle during periods of past and present global climate change.

89