Cadmium enrichment in the Gulf of California

Abstract

Abstract We are reporting on the first dissolved cadmium (Cd) distributions for the upper 1000-m water column in the Gulf of California for the summer of 1996. The highest surface Cd concentrations were found at the midriff island region (0.21–0.35 nmol l −1 ), while the lowest were detected at the mouth (0.08–0.16 nmol l −1 ). The Gulf's surface waters were enriched with Cd compared to the adjacent open North Pacific Ocean (0.002–0.003 nmol l −1 ). Temperature, salinity, phosphate and Cd distributions showed intense vertical mixing at the midriff island region. In particular, the archipelago region is a source of Cd and nutrients. The enhanced vertical mixing near the island produces a less pronounced vertical Cd profiles than those for the Pacific Ocean, but are otherwise comparable to those of the California upwelling areas. Cd distribution at the island region can be partially explained by the mixing of Cd-enriched Pacific Intermediate Water, Subtropical Subsurface Water and Gulf of California Water mass. Vertical mixing near the islands provides with a mechanism for out-cropping of deep water, and a route for nutrient and Cd-enrichment to the surface water in the Gulf. The Cd-salinity analysis also revealed that some surface samples were cadmium-depleted. When Gulf of California Water is advected southward, Cd is removed from the dissolved phase and incorporated into particulate organic tissue, suggesting a relatively larger influence of biologically mediated processes. Between 500 and 1000 m depth, Cd concentration in the Gulf falls within a narrow range of 0.93–1.15 nmol l −1 with an average concentration of 1.04±0. 08 nmol l −1 . The deep Cd profiles in the Gulf are not different from the same water mass in the North Pacific Ocean. This indicates that deeper water masses flowing into the Gulf are not significantly changed in its Cd content. It also suggests that most organic matter remineralization mainly occurs above the 750 m of the Gulf of California. Thus, the dissolved Cd distribution in the Gulf of California is being controlled by a combination of biological cycling, thermohaline circulation, and the mixing processes at the midriff region.