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要旨:
We present coupled cadmium (Cd) concentrations and Cd isotopic compositions (expressed as δ114Cd) in seawater profiles along the northeast-southwest GEOTRACES GA11 section in the tropical Atlantic Ocean. The GA11 section encompasses three contrasting regions including the Amazon River plume, the North African dust plume, and the Guinea Dome oxygen deficient zone (ODZ). Given the long oceanic residence time of Cd (104 to 105 yr), local inputs such as atmospheric and riverine sources are generally considered to be of little importance for the open ocean Cd budget, and the limited Cd isotope dataset available thus far has prevented any unambiguous conclusions on the importance of these processes. The GA11 section is ideally located for assessing the influence of external, natural and anthropogenic riverine and eolian inputs, as well as internal processes, on the Cd mass balance in the tropical Atlantic Ocean. As in previous Cd isotope studies, this dataset documents that both surface consumption–regeneration of micronutrients within the water column and deep water mass mixing exert the prime control on the cycling of Cd. However, we do observe some near-surface samples with lower-than-expected δ114Cd signatures along the western section of the transect. Surface waters sampled at the margin of the Amazon freshwater plume show no resolvable difference in [Cd] and δ114Cd compared to waters outside of the plume, suggesting that the Amazon River is not an important source of Cd to the open ocean. Although the eastern GA11 transect is directly downwind of the Saharan dust plume, atmospheric Cd deposition accounts for <1% of the inferred upwelling flux, indicating that atmospheric inputs to the surface North Atlantic Ocean, even within the main Saharan dust plume, only have a relatively minor influence on the Cd budget. In the subsurface tropical Atlantic (100–200 m water depth), there is a clear deviation from a tight linear Cd-PO4 correlation toward lower Cd concentrations for a given PO4 concentration. Our new Cd data show that this Cd-PO4 decoupling is likely a feature of the entire tropical Atlantic and may possibly reflect PO4 enrichment via preferential re-mineralization of organically-bound phosphorus. Alternatively, the decoupling may arise from subsurface Cd depletion caused by precipitation of Cd sulphide within sinking organic particulate microenvironments, as has been suggested in recent studies.
