Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
9530384 | Geochimica et Cosmochimica Acta | 2005 | 20 Pages |
Abstract
Compared to the first valid oceanic Pb data for samples collected in 1976, Pb at ALOHA in 1997-1999 shows decreases in surface waters and waters shallower than 200 m. Pb concentrations in central North Pacific surface waters have decreased by a factor of 2 during the past 25 yr (from â¼65 to â¼30 pmol kgâ1); surface water Pb concentrations in the central North Atlantic and central North Pacific are now comparable. We attribute the surface water Pb decrease to the elimination of leaded gasoline in Japan and to some extent by the U.S. and Canada. We attribute most of the remaining Pb in Pacific surface waters to Asian emissions, more likely due to high-temperature industrial activities such as coal burning rather than to leaded gasoline consumption. A 3-year mixed-layer time series from the nearby HALE-ALOHA mooring site (1997-1999) shows that there is an annual cycle in Pb with concentrations â¼20% higher in winter months; this rise may be created by downward mixing of the winter mixed layer into the steep gradient of higher Pb in the upper thermocline (Pb concentrations double between the surface and 200 m). From 200 m to the bottom, Pb concentrations decrease to levels of 5-9 pmol kgâ1 near the bottom; for most of the water column, thermocline and deepwater Pb concentrations do not appear to have changed significantly during the 23-yr interval.
Related Topics
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Authors
Edward A. Boyle, Bridget A. Bergquist, Richard A. Kayser, Natalie Mahowald,