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UBC Theses and Dissertations

Sources of Mn, Al, Cd and Cu to coastal waters of the California Current system Merrin, Claire

Abstract

Dissolved Mn, Al, Cd and Cu concentrations were determined across a series of horizontal surface transects along the Northeast Pacific Ocean from the coast off Big Sur, California to Vancouver Island, British Columbia. This region of coastline allows the relative importance of different modes of input of trace metals to California Current surface waters to be assessed. In particular, the impact on dissolved concentrations of fluvial input from the Columbia River, diffusion from reducing shelf sediments and/or sediment resuspension coupled with upwelling, and the width of the continental shelf were examined. In addition, Mn, Al, Cd and Cu distributions were compared with Fe and Ga data from the same region. Background California Current surface water values were determined for Mn (3- 5nmol/kg), Al (0.38-0.65mnol/kg), Cd (0.08-0.30nmol/kg) and Cu (0.79-1.51nmol/kg). These data agree well with previous work. According to this study, the Columbia River is a considerable source of Mn and Al to California Current surface waters, with the highest levels of Mn and Al for all transects observed coincident with the lowest salinity waters sampled (Mn up to 117nmol/kg; Al up tol.lnmol/kg). Metal versus salinity plots for both Mn and Al indicate non-conservative mixing and removal as the water ages. Al is removed significantly faster than Mn and Ga. This agrees well with previous estimates of surface ocean residence times for these elements. In contrast to Mn and Al, Cd does not show enrichment in low salinity waters. However, it is possible that the Columbia may well be providing small quantities of Cd to California Current waters, but in this area the input is masked by relatively higher background signals. No data for Cu were available for the Columbia River area. Summer upwelling off the coasts of Oregon, Washington and California supplied significant quantities of Mn (4.6-39.5nmol/kg), Cd (0.32-0.92nmol/kg) and Cu (1.66- 4.52nmol/kg) to California Current surface waters. For Mn and Cu, diagenic remobilization and/or sediment resuspension coupled with upwelling explains the elevated signals for these metals. For Cd, the elevation is attributable to transport of enriched bottom water to the surface by upwelling. For Al, either sediments in this area are not a significant source of this metal, or Al is indeed supplied by sediments but is then stripped out of the water at a dramatic rate. Mn and Cu (as well as Cd and Al to a lesser extent) showed high elevation coincident with transects overlying areas where the continental shelf is wide, and significantly lower elevation coincident with transects overlying areas where the continental shelf is narrow. This again emphasizes the potential impact of sediments on trace metals concentrations in surface waters.

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