The Importance of Reversible Scavenging for the Marine Zn Cycle Evidenced by the Distribution of Zinc and Its Isotopes in the Pacific Ocean
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AbstractThe North Pacific has played an important role in ongoing discussions on the origin of the global correlation between oceanic dissolved Zn and Si, while data in the North Pacific have remained sparse. Here, we present dissolved Zn and 66Zn data from the US GEOTRACES GP15 meridional transect along 152W from Alaska to the South Pacific. In the south (<20N) Zn and Si exhibit a tight linear correlation reflecting strong Southern Ocean influence, while in the north (>20N) an excess of Zn relative to Si in upper and intermediate waters is due to regeneration of Zn together with PO4. Using a mechanistic model, we show that reversible scavenging is required as an additional process transferring Zn from the upper to the deep ocean, explaining the deep Zn maximum below the PO4 maximum. This mechanism applied for reversible scavenging also provides an explanation for the observed isotope distribution: (a) fractionation during ligand binding and subsequent removal of residual heavy Zn in the upper ocean, drives the upper ocean toward lower 66Zn, while (b) release of heavy Zn then coincides with the PO4 maximum where carrier particles regenerate, causing a middepth 66Zn maximum. In the upper ocean, seasonal physical stratification is an additional important process influencing shallow 66Zn signals. At the global scale, this mechanism invoking fractionation during ligand binding coupled with reversible scavenging offers a global explanation for isotopically light Zn at shallow depths and corresponding elevated middepth 66Zn signals, seen dominantly in ocean regions away from strong Southern Ocean control.
