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The marine geochemistry of zirconium and hafnium McKelvey, Brad Alan


The refractory nature of Zr and Hf has limited the study of these elements in the hydrological cycle. Recent advances in analytical techniques, such as inductively coupled plasma mass spectrometry (ICP/MS), now make this possible. This thesis describes the development of an analytical technique utilizing isotope dilution and ICP/MS to determine picomolar and femtomolar concentrations of dissolved Zr and Hf in seawater and the first detailed concentration profiles of dissolved Zr and Hf in the Pacific and Atlantic Oceans. In addition, the distributions of dissolved Zr and Hfin pore waters collected from coastal sediments will be described. These data mark the first ever measurements of these two ultra-trace elements in marine interstitial waters. The chelating ion-exchange resin, Chelex-100, was found to be suitable for the extraction/concentration step required for the determination of dissolved Zr and Hf in seawater using isotope dilution analysis. The extraction was optimized for pH, flow rate, resin volume, elution volume, and the time required for isotope equilibration. The isotope ratios91 Zr/ 90 Zrand178 HE’ 177 Hf were measured using flow injection ICP/MS. The detection limits for a one litre sample are 0.21 and 0.03 pmoles/kg for Zr and Hf, respectively. The analytical precision (is) of the technique improves with increasing concentration and varies from 2.5 to 7% for Zr and 9 to 22% for Hf Results from a surface transect, samples from 25 m depth, across the North Pacific indicate an input of dissolved Zr and Hf at the ocean margins. This is thought to be a result of a flux of dissolved Zr and Hf from reducing shelf sediments and/or riverine inputs. Oceanographically consistent profiles ofZr and Hfwere obtained for Zr and Hf in the North Pacific and North Atlantic. The profiles of dissolved Zr and Hfreflect a complex combination of biogeochemical controls. Dissolved Zr and Hfhave minimum concentrations of 15-95 and 0.2-0.5 pmoles/kg, respectively, in the surface waters and increase to a maximum of 255-366 and 0.8-1.0 pmoles/kg, respectively, in the deep waters. Maintaining the large dynamic range of dissolved Zr and Hfin the water column requires a combination of surface removal and deep water input. The removal of Zr and Hf in the water column is thought to be a result of particle scavenging, and the input to the deep waters may be from a porewater flux and/or a sediment surface remineralization process. The concentrations of dissolved Zr and Hf in coastal sediment pore waters were found to be an order of magnitude higher than those found in the overlying bottom waters, The pore water profiles ofZr and Hfresembled that of dissolved Fe suggesting that the increases in Zr and Hfconcentrations at shallow depths result from the dissolution of oxide and oxyhydroxide phases during burial.

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