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Title: The carbon cycle and bioactive trace metals in the oceans : constraints from zinc isotopes
Author: Zhao, Ye
ISNI:       0000 0004 2717 6823
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2011
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Zinc (Zn) is a transition metal whose stable isotopes can record information on important oceanographic processes. This dissertation has established an analytical approach for the separation and isotopic analysis of Zn in seawater, and applied it to various seawater samples to trace biogeochemical processes in the oceans. The main analytical challenges in the isotopic analysis of Zn in seawater relate to very low concentrations (< nmol/kg) in a matrix of major elements present at concentrations up to 8 - 9 orders of magnitude higher. These problems have been overcome by a pre- concentration procedure, using Al(OH)3 co-precipitation, followed by further purification and separation of Zn on multiple anion exchange columns. Since Zn is ubiquitous in the environment, it is crucial that the Zn environmental blank contribution is maintained at a low level. The procedural blank has been reduced to max 2 - 3 ng for 4 L seawater samples in this study, which enables the isotopic analysis of most Zn- depleted surface samples. A Zn double spike approach has been adopted, and further developed in this study, for the correction for mass discrimination during the separation and analysis procedure. The double spike approach, and subsequent analyses of Zn samples on MC-ICPMS, were proven successful, with tests on Zn JMC standards yielding δ66Zn = +0.03±0.09%0 (n = 178). The analytical approach has been tested using multiple analyses of samples from a homogenized tank of deep Atlantic seawater, obtaining consistent results of δ66Zn = +0.46±0.08%0 (n = 6) and [Zn] = 2.22±0.04 nmol/kg (n = 6). A few Zn standard addition experiments have been done on the post- eo-precipitation 'Zn free' seawater samples and MQ samples, the results of which further proved the robustness of the double spike approach and the chemical procedure. Data were collected in order to test this new technique, and also to serve as the first substantial dataset available to understand how Zn isotopes are fractionated in the oceans, and how they may be used as a tracer of biogeochemical cycling. Depth profiles of Zn concentrations and isotopic compositions are presented from three different locations in the world's oceans - the subtropical Atlantic Ocean, the Southern Ocean and the Northeast Pacific Ocean, demonstrating the responses of Zn isotopes to biological cycling in two contrasting biogeochemical regimes: trace-metal limited and major-nutrient limited. The new technique has produced nutrient-type Zn concentration profiles which compare reasonably well with previous measurements. Deep ocean Zn isotopic compositions are very homogeneous (at δ66Zn = +0.52±0.10%0), similar to or slightly heavier than those of the continental inputs. In contrast, surface ocean Zn isotopes show pronounced variability, with the largest variations in the trace-metal limited HNLC regions. Overall, Zn isotopes at the surface are controlled by inputs (dust, upwelling, δ66Zn ~ +0.3 to +0.5%0) and preferential removal of the light isotope by phytoplankton (pushing residual seawater δ66Zn up to over +1%0). Anomalously low ()6 Zn values have been found in sub-surface waters (around 40 - lOOm) at all locations studied, but especially the NE Pacific, reflecting either very shallow regeneration of some of the isotopically light Zn taken up by phytoplankton cells at the surface, or preferential scavenging of the heavy isotopes. This dissertation, by developing an effective analytical approach for the separation and isotopic analysis of Zn in seawater and presenting several sets of preliminary Zn isotopic data in the oceans, lays the groundwork for the potential use of Zn isotopes as a marine biogeochemical tracer.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available