Effect of Dissolved Organic Carbon and Alkalinity on the Density of Arctic Ocean Waters

Abstract

At constant temperature, the density of deep waters in the oceans is higher than that of surface waters due to the oxidation of plant material that adds NO₃, PO₄, and Si(OH)₄, and the dissolution of CaCO₃(s) that adds Ca²⁺ and HCO₃. These increases in the density have been used to estimate the absolute salinity of seawater that is needed to determine its thermodynamic properties. Density (ρ), total alkalinity (TA), and dissolved organic carbon (DOC) measurements were taken on waters collected in the eastern Arctic Ocean. The results were examined relative to the properties of North Atlantic Waters. The excess densities (Δρ = ρ_Meas − ρ_Calc) in the surface Arctic waters were higher than expected (maximum of 0.008 kg m⁻³) when compared to Standard Seawater. This excess is due to the higher values of the normalized total alkalinity (NTA = TA * 35/S) (up to ~2,650 μmol kg⁻¹) and DOC (up to ~130 μmol kg⁻¹) resulting from river water input. New measurements are needed to determine how the DOC in the river waters contributes to the TA of the surface waters. The values of Δρ in deep waters are slightly lower (−0.004 ± 0.002 kg m⁻³) than that in Standard Seawater. The deep waters in the Arctic Ocean, unlike the Atlantic, Pacific, Indian, and Southern Oceans, do not have significant concentrations of silicate (maximum ~15 μmol kg⁻¹) and that can affect the densities. Since the NTA of the deep Arctic waters (2,305 ± 6 μmol kg⁻¹) is the same as Standard Seawater (2,306 ± 3 μmol kg⁻¹), the decrease in the density may be caused by the lower concentrations of DOC in the deep waters (44–50 μmol kg⁻¹ compared to the Standard Seawater value of 57 ± 2 μmol kg⁻¹). The relative deficit of DOC (7–13 μmol kg⁻¹) in the deep Arctic waters appears to cause the lower densities (−0.004 kg m⁻³) and Absolute Salinities (S _A, −0.004 g kg⁻¹). The effect of increases or decreases in Δρ and δS _A due to DOC in other deep ocean waters may be hidden in the correlations of the changes with silicate. Further work is needed to separate the effects of SiO₂ and DOC on the density of deep waters of the world oceans.