CADMIUM IN THE CALIFORNIA CURRENT SYSTEM - TRACER OF PAST AND PRESENTUPWELLING

Over 100 samples were collected off the west coast of North America du ring 1991-1993 to determine the relation between wind-driven upwelling and nearshore concentrations of dissolved silicate (Si), phosphate (P ), and cadmium (Cd). Highly enriched in deep water offshore, these con stituents are sensitive indicators of upwelling. Coastal water was sam pled from the shore in January and June 1992 at 12 sites distributed b etween 36 degrees and 48 degrees N latitude. In January the compositio n of nearshore water along this transect was fairly uniform: 5-15 mu m ol/kg for Si, 0.5 to 1.0 mu mol/kg for P, and 0.1-0.3 nmol/kg for Cd. In June, elevated concentrations of Si (30 mu mol/kg), P (2.0 mu mol/k g), and Cd (0.6 nmol/kg) revealed a region of intense upwelling betwee n 38 degrees and 40 degrees N. The pattern is broadly consistent with meridional gradients in coastal upwelling calculated from the long-ter m mean of alongshore winds compiled from ship reports. Nearshore water was also collected biweekly to monthly at two sites 3 km apart near S an Francisco Bay (37.5 degrees N) during 1991-1993. The variability se en in the time series suggests that the composition of nearshore water integrates the effect of alongshore winds over timescales of several weeks. Seasonal variations in Si (5-50 mu mol/kg), P (0.5-2.5 mu mol/k g), and Cd (0.1-0.8 nmol/kg) concentrations were consistent with upwel ling during spring and summer. Maximum Si, P, and Cd concentrations re ached in May 1991 were consistent with advection to the very nearshore region from a depth of about 300 m relative to a vertical profile at a distance of 200 km from the coast. Nearshore Si, P, and Cd concentra tions were reduced relative to 1991 in 1992 and, to a lesser extent, i n 1993 due to weaker upwelling linked to the warm phase of the Fl Nino -Southern Oscillation. During periods of weaker upwelling or downwelli ng, variations in P, Si, and Cd concentrations became uncoupled. There is a good correlation between the coastal Cd time series near San Fra ncisco Bay (37.5 degrees N) and a second order polynomial function of the the upwelling index of Bakun [1975] at 35 degrees N, filtered with a 30-day running mean (r(2) = 0.71, n=39). The index is a daily estim ate of coastal upwelling calculated from 6-hourly mean atmospheric pre ssure distributions at 36 degrees N. From this function and a record o f daily upwelling indices we infer a range of annually averaged coasta l Cd concentrations of at least 0.3-0.5 nmol/kg since 1967. Cd/Ca rati os in shells of foraminifera from San Francisco Bay suggest that avera ge coastal ed concentrations 3500-4500 years ago were at the upper end of this range.