Determination of dissolved trace metals in the western North Pacific
Creators: name: Yang, Lu
Publisher: name: University of British Columbia
Year: 2008

description: A method for analyzing dissolved cadmium, zinc, nickel, copper, lead, aluminum, manganese, cobalt, cerium, and gallium in seawater was developed using a Chelex-100 resin for preconcentration/separation of these metals from seawater matrix, followed by ICP-MS or GFAAS detection. This method was used to investigate trace metals in the western North Pacific, a region for which little trace metal data are available. Concentrations of these elements are found to be in the range of 0.7 pmol/kg to 12nmol/kg, analytical precision is better than 7%, and the seawater detection limits are in the low picomolar range (2-40 pmol/kg) after a 125-fold preconcentration. The accuracy of this technique was verified by the analysis of standard reference materials from the National Research Council of Canada for Cd, Ni, Zn, Cu, Pb, Mn, and Co. Results are in good agreement with the certified values, within 99% confidence limit. Determining the spatial distribution of trace elements in new oceanic regions helps to elucidate the processes which control the chemical composition of seawater. Data from 6vertical profiles in the western and central North Pacific Ocean and 14 surface samples along a transect from Japan to Hawaii, collected in the spring of 1991, are reported here. Vertical profiles for dissolved Cd, Ni, Zn, Cu, Pb, Mn, and Co from the central gyre station (28°N, 175°E), close to Hawaii, agree well with previous results (Bruland, 1980;Landing and Bruland, 1987; Flegal and Patterson, 1983; Bruland, et al., 1993),demonstrating reliable sampling and analytical procedures. Remarkably similar trace metal concentrations and vertical distributions are found in the Kuroshio Current station (35°N,155°E), about 3500 km west of the central gyre station. In the coastal region near Japan (38°N, 146°E), enriched trace metal concentrations are observed in the upper water column, with a shallower maximum for those elements with a mid-depth maximum. Dissolved Al, Ce, and Ga in the central North Pacific show similar major trends to other studies (Orians and Bruland, 1985, 1988; de Barr, et al., 1985), with the exception of maximum Al concentrations found at 100-250 m rather than at the surface, and higher intermediate values. The sub-surface dissolved maximum observed in this study could be due to advection, via the North Equatorial Current, of low Al waters from the coast of North America to this area. The higher mid-depth Al concentrations could be due to inputs from the Karin Ridge. The surface transect (25m depth) for dissolved Cd, Ni, Zn, Cu, Pb, and Mn in the western North Pacific generally shows high coastal values and low concentrations in the open ocean. However, at a station midway along the transect from Japan to Hawaii (34°N,157°E), high values are observed for all elements. This is interpreted as the advection of coastal waters being carried out into the central gyre via a cold-core ring, broken off from the Kuroshio Current. This feature is observed in both salinity and temperature as well as in the trace metal data. Station HS-2, west of the Kuroshio Current, shows upwelling. Nutrient-type elements which have a surface minimum (Cd, Zn, Cu, Ni) are elevated, while scavenging-type elements, such as Mn, which have a surface maximum are depleted at this station relative to the coastal station, HS-1. The data from surface waters show a higher degree of variability than observed in vertical profiles. Since good agreement was found between vertical profiles of these metals in this study and other previous results (Bruland, 1980; Landing and Bruland, 1986; Flegal and Patterson, 1983), the surface data are believed to reflect the real variation of these metals in surface seawater, not analytical or sampling errors. One of the most surprising features of the surface transect is the lack of a strong gradient from increased eolian inputs closer to the coastal region. Dust and lead fluxes are both thought to vary by nearly two orders of magnitude across this transect, yet the range observed for the trace elements is only 2 to 4 fold. Much of the expected input appears to be balanced by increased scavenging in the coastal region. It is also likely that sampling closer to the coast of Japan (samples are not available from this area) would have shown the effects of these increased inputs.