Hyperspectral temperature and salt dependencies of absorption by water and heavy water in the 400-750 nm spectral range

The temperature and salt dependencies of absorption by liquid water (H2O) and heavy water (D2O) were determined using a hyperspectral absorption and attenuation meter (WET Labs, AC-S). Sodium chloride (NaCl) was used as a proxy for seawater salts. There was no significant temperature (ΨT) or salt (ΨS) dependency of absorption at wavelengths <550 nm. At wavelengths >550 nm, ΨT exhibited peaks at ∼604, 662, and 740 nm. A small negative trough in ΨS occurred at ∼590 nm, followed by a small positive peak ∼620 nm, a larger negative trough at ∼720 nm, and a strong positive peak at ∼755 nm. The salt dependency of absorption by heavy water, ΨsH, exhibited a negative power-law shape with very low ΨsH, at wavelengths >550 nm. Our experiments with NaCl, clean open ocean seawater, and artificial seawater support the hypothesis that salts modify the absorption spectra of seawater by modifying the molecular matrix and vibrations of pure water.