Baars, Oliver (2011) Distribution and speciation of zinc,cadmium and cobalt in the Southern Ocean. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, Germany, 151 pp.

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Abstract

Iron is the main control on phytoplankton productivity and macro-nutrient biogeochemistry in the Southern Ocean. However other micro-nutrients may also influence aspects of phytoplankton productivity and macro-nutrient cycling in this region. Three prime candidates have been identified for this study: zinc (Zn), cadmium (Cd) and cobalt (Co). Zn and Co are metal centres in important enzymes, transcription factors and vitamins (B12). Cd, long thought to be solely a toxic element, has recently been found as a metal centre in marine carbonic anhydrases – key enzymes needed for the acquisition of inorganic carbon. Deep water formation in the Southern Ocean directly impacts the other ocean basins. In this area Zn and Cd incorporation in opal and calcite shells have been used as indicators to reconstruct past nutrient conditions and ocean circulation but many questions remain open with regard to the cycle of these elements in the water column. In this work I report the first basin wide field study of distribution and speciation for dissolved Zn and Cd in the Atlantic sector of the Southern Ocean. The study area covered two north-south transects along the Zero Meridian and across the Drake Passage. Free Zn and Cd concentrations increased by 2-3 orders of magnitude in surface waters south of the Subantarctic Front linked to upwelling deep waters. Metal specific organic ligands (logK’M’=9.0-10.5) were detected throughout the study area for Zn (L < 4 nM) and Cd (L < 1.2 nM). For Zn, these ligands were however nearly always saturated due to the presence of excess concentrations of dissolved Zn in the high nutrient waters south of the Polar Front. For Cd, a small ligand excess remained but in contrast to measurements in the Subantarctic Zone, no source of ligands were detected in surface waters south of the Polar Front. These observations strongly suggest that a major fraction of ligands in the Antarctic Ocean were not actively produced for acquisition but rather consisted of degradation products. Based on these results, limitation by Zn was unlikely south of the Polar Front. The findings have also clear implications for Cd:PO43- uptake ratios in the Southern Ocean and for the interpretation of paleo-records utilizing Zn or Cd. Vertical profiles of Zn and Cd were associated with local minima and maxima in the upper water column consistent with significant uptake by phytoplankton and fast release by zooplankton grazing. Zn (Cd) und Si (PO4 3-) were strongly correlated throughout the study area. Using the speciation data for Zn and Cd it was possible to constrain the possible causes of observed variations in trace metal:macro-nutrient uptake ratios with iron or light limitation being an important control. Accurate isotope dilution measurements for Cd revealed that high algal Cd uptake relative to PO43- in combination with fast remineralisation resulted in high and constant Cd/PO43- ratios in deep waters of the Southern Ocean. Antarctic Intermediate Water contained almost the same Cd/PO43- ratio as Antarctic Bottom Water. Together with previous observations in lower latitudes, these new findings are combined in a conceptual model to explain slopes and intercepts of Cd vs. PO43- correlations and the “kink” in the combined Cd vs. PO43- dataset of the Atlantic Ocean whereby Fe limitation in the Southern Ocean was the key-control. Co is also a bio-essential element but has a much shorter residence time than Zn and Cd. Therefore it Co is present at low pM concentrations and because of the extreme analytical challenges for its detection and speciation only a few measurements are available from the Southern Ocean. During this study I developed a catalytic adsorptive cathodic stripping voltammetry method with an excellent detection limit (0.9 pM) that significantly facilitates future routine measurements of Co distribution and speciation. The work gives also new insights into the catalysis mechanism underlying the analytical application of this method. In this work, central questions concerning the interrelations between Zn/Cd, biology and macronutrients could be answered. Iron and/or light are a key control on the biogeochemistry of Zn and Cd and notably their uptake relative to macro-nutrients. In the consequence the availability of Zn and Cd relative to macro-nutrients in other oceans is also affected with implications for the Cd/PO43- and Zn/Si relation in the deep glacial ocean. Ubiquitous specific organic deep water ligands may affect the bioavailability of Zn and Cd upon upwelling or the incorporation in the hard parts of benthic foraminifera. Unravelling the identity and reactivity of these compounds are interesting targets of future research. The new development for the electrocatalytic detection of Co will help to illuminate the role of Co distribution and speciation in future studies.

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