Investigating the distributions of zinc and cadmium in the subarctic northeast Pacific Ocean

Date

2017-03-22

Authors

Janssen, David

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Abstract

Zinc (Zn) and cadmium (Cd) have nutrient-type vertical distributions reflecting control driven by biological uptake in surface waters and remineralization of sinking biogenic particles at depth. Both metals show strong correlations with major algal nutrients (Cd with phosphate (PO43-) and Zn with silicic acid (Si)) in the world ocean. Through their roles as micronutrients and toxins to marine phytoplankton, Zn and Cd can influence surface biological community composition. Preserved Zn and Cd records have been employed as proxies to gain insight into nutrient distributions, circulation, and organic carbon export in the paleocean. A thorough and mechanistic understanding of the biogeochemical cycling of Zn and Cd is necessary for accurate paleoceanographic reconstructions as well as predicting alterations in metal supply to the modern surface ocean and its impacts on primary productivity due to oceanic changes. My dissertation aims to further this understanding through an investigation of Zn and Cd distributions in the subarctic northeast Pacific through samples collected along the Line P transect. A major focus of this dissertation was identifying and characterizing depletions of metals in O2-depleted waters relative to global and basin scale metal:macronutrient correlations. Dissolved Cd profiles from the subarctic northeast Pacific and the eastern North Atlantic show a deficit of Cd relative to regional Cd:PO43- relationships. Particulate Cd and Cd stable isotopes (ε112/110Cd) from low-O2 North Atlantic waters and published sedimentary data from the subarctic northeast Pacific point to a previously undocumented water-column metal removal process acting in O2-depleted waters. Metal sulphide formation, likely in association with particulate microenvironments, can explain the observed deficits. Other metals with similar sulphide coordination chemistry should also form metal sulphides if this process is occurring. Dissolved Zn from Line P showed distributions and Zn:Si relationships that are consistent with the removal of metal in O2-depleted waters through sulphide formation. A first order approximation of the Cd deficit suggests that sulphide formation may be an important sink term in the global Cd cycle. Surface and upper nutricline Zn:Si and Cd:PO43- relationships in the chronically iron (Fe)-limited subarctic northeast Pacific showed distinct trends, which differ from those seen in Fe-replete regions. Distributions suggest the formation of surface biogenic particles with high Cd:PO43- and Zn:Si, leaving surface waters depleted in metals relative to macronutrients and resulting in high metal:macronutrient ratios in the nutricline as these particles sink and are remineralized. This is consistent with understandings of phytoplankton physiology and uptake of divalent metals under Fe-limitation, and corresponds well with global data for dissolved Cd:PO43- patterns in Fe-limited regions. Subsurface high Cd:PO43- and Zn:Si may also be influenced by the advection of water enriched in trace metals. The distinct shallow remineralization horizon observed for Zn compared to Si in the subarctic northeast Pacific by this and previous work presents a fundamentally different distribution than observed in global Zn:Si compilations. Directed sampling in the subarctic northeast Pacific should help elucidate the mechanism behind the oceanographically distinct distributions in this basin. Dissolved ε112/110Cd from Line P demonstrates a remarkably uniform subarctic northeast Pacific deepwater reflecting an advected source signal. Particulate ε112/110Cd samples show an active Cd cycle, which is not imprinted upon the dissolved phase. Particulate ε112/110Cd from 200-600 m depth is among the lightest ε112/110Cd ever reported for natural telluric samples. This may be an important sink for light Cd in the global ocean, which at present is heavy with respect to known sources. Line P surface waters with very low Cd concentrations are not accurately represented by a closed-system Rayleigh model, which can describe ε112/110Cd in the Southern Ocean. This suggests spatially and/or temporally variable surface ε112/110Cd fractionation. A large difference is observed in reported dissolved ε112/110Cd at very low Cd concentrations between different instrumentations. An intercalibration is necessary to determine if this is an analytical artefact or reflects real oceanic variability.

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Keywords

Zinc, Cadmium, Cadmium isotopes, northeast Pacific, GEOTRACES, biogeochemical cycles

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