Cd in the western equatorial Pacific

Abstract

Abstract Dissolved Cd was investigated in relation to PO4 in January 1999 at three stations in the western equatorial Pacific. The vertical distribution of Cd, as is typical with nutrients, was depleted in the surface water, while the concentration increased with depth to approximately 0.9 nM below 1000 m. At a depth of approximately 150 m, a thermocline developed, and surface and nutrient-rich deep water separated from each other. Cd was inserted into a Redfield-type stoichiometric equation as well as PO4. Preformed Cd and PO4 were shown as the intercept of the Cd or PO4 axis in the Cd, PO4-AOU (Apparent Oxygen Utilization) plot, and less than 0.04 nM and 0.5 μM for Cd and PO4, respectively, were estimated in the shallow layer (above 150 m). Preformed Cd and PO4 increased with depth below 200 m and reached approximately 0.7 nM and 1.2 μM for Cd and PO4, respectively, in deep water (below 1000 m). Above 150 m, the molecular ratios of regenerated Cd to consumed O2 were calculated from the Cd-AOU plot, and “1:1,220,000,” “1:1,180,000,” and “1:1,150,000” were obtained at the three stations. The ratio of regenerated PO4 to consumed O2 was adapted from the “Redfield Ratio” as “1:138,” so the ratios of regenerated Cd to PO4 were estimated as 0.113, 0.117, and 0.120 (nM/μM) above 150 m at the three stations. In the relationship between Cd and PO4, a clear deviation was observed where the concentration of PO4 was about 1.1 μM. Above 150 m, the concentration of PO4 was less than 1.1 μM, and the slopes of the regression line were 0.100, 0.127, and 0.156 (nMCd/μMphosphate). These values were quite similar to the regeneration ratio of Cd to PO4 at each station, which suggests that the regeneration processes might be a main factor that regulates the correlation of Cd and PO4 in this layer. Below 200 m, a relatively steep slope (average, 0.421) and obvious negative intercept in the Cd axis were shown, and the Cd/PO4 ratios increased gradually with depth to an almost constant value of 0.314 (nM/μM) below 1000 m, which is considered to be attributable to the apparent conservative mixing between water masses with an originally different Cd/PO4 ratio (different preformed values) in deep water and just below the thermocline.