The densities of 124 samples of seawater from stations at 35°N in the Pacific Ocean have been measured with a vibrating flow densimeter at 25°C. The measured densities were compared with those calculated from the equation of state of Millero et al. (1976b), derived for seawaters of constant relative composition. Values for the excess density Δd(excess) = d(meas) ‐ d(calc) were found to be ±3.8 ± 3.0 × 10−6 g cm−3 from 0 to 490m, 12.5 ±4.2 × 10−6g cm−3 from 490 to 1000 m,; and 17.6 ± 2.6 × 10−6g cm−3 from 1000 to 5834 m. The excess densities for the deep waters are in good agreement with our earlier measurements: 16.1 ± 3.6 × 10−6g cm−3 (Millero et al., 1976c). The values of Δd(excess) predicted by correcting for the increase of alkalinity (ΔAT), total carbon dioxide (ΣCO2), and dissolved silica (ΔSiO2) and nitrate (ΔNO3) in the deep waters (Brewer and Bradshaw, 1975; Millero et al., 1976c) agree with the measured values on the average to ±5.2 × 10−6 g cm−3. In the deep waters the measured values of Δd(excess) are ∼6 × 10−6 g cm−3 higher than the predicted values. The values of Δd(excess) were also calculated by assuming that the changes in salinity due to the added solids affect the density by the same amount as changes in weight‐diluted standard seawater: 106Δd = 757ΔS(‰). The values of Δd(excess) calculated from 106Δd(excess) = 37.9ΔAT + 72.8ΔSiO2 + 47.7ΔNO3 agree with the measured values on the average to ±4.3 × 10−6 g cm−3, independent of the depth. These results indicate that the density changes due to small changes in the composition of deep ocean waters can be accounted for by changes in the salinity due to the mass of added dissolved solids (ΔS(‰) = ΣMi Δni, where Mi is the equivalent or molecular weight and Δni is the change in the equivalents or moles of solute i in 1 kg of seawater).
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