Coupling Zn and Si through the Southern Ocean: new paradigms in oceanic nutrient cycling
From Up-Down to Side-to-Side.
The textbook view explains the ‘nutrient-type’ profiles of many elements (N, P, Si, Zn, Cd, Ba, Ni…) via biological uptake in the surface ocean and remineralisation of organic matter at depth. Overprinted on this 'Up-Down' biological cycling is deep water aging along the ‘Global Ocean Conveyor Belt’; i.e., high concentrations of nutrients in the old waters of the deep Pacific are explained as reflecting the accumulation of remineralised nutrients over time. However, the behaviour of one element, Zinc (Zn) and its isotopes (δ⁶⁶Zn), has proved extremely difficult to reconcile with this view. Zn is strongly coupled to the distribution of Si (Silica) in the global oceans. Both elements have a deeper maxima in dissolved concentrations than the classic ‘labile’ nutrients (e.g., P, N, Cd). The oceanic cycle of Si is primarily controlled by opal-secreting diatoms. The deeper maximum in dissolved Si can therefore, at least intuitively, be explained as the result of the slower dissolution of diatom shells versus the rapid remineralisation of organic material (containing the P, N, Cd etc.). Coupling Zn and Si would be easy, therefore, if significant quantities of Zn was also present in the opal of diatoms. However, both culturing experiments and x-ray based imaging of naturally-harvested diatoms have shown that only a tiny fraction of the Zn in a diatom is present in the shell. The majority is co-located with the organic soft parts of the organism, with the P, N, Cd. Sediment trap data indicate that Zn is released from sinking diatoms concurrently with the P, N and Cd, too, and not with Si.
In this talk, I will show that a new way of thinking is required to explain the Zn-Si relationship, and the distribution of δ⁶⁶Zn, in the ocean. This new view moves away from the conventional Up-Down towards 'Side-to-Side', with a key role for the Southern Ocean and its ecology. I will emphasise that this 'new’ view is, in fact, not new at all, and has been invoked to explain the oceanic Si distribution for over a decade. Like any new paradigm, it requires time, and several lines of evidence, to be established. We believe that evidence from Zn and Zn isotopes will help cement it’s place in the textbooks.