Abstract
Ocean acidification is an ongoing concern due to its impact on the marine ecosystem. The volume integrated pH of sea water can be determined from the depth-dependence of ambient sound, which depends on the acoustic absorption properties of seawater. For a wind-driven noise in the ocean over the band 1–10 kHz, two main contributions to sound attenuation are associated with the ionic relaxation of boric acid (<3 kHz), related to pH, and magnesium sulfate (>3 kHz), unrelated to pH. When local winds are strong (>10 m/s), the ambient noise is dominated by locally generated surface noise and has a depth-independent directionality and a weakly frequency and depth-dependent intensity, due to sound absorption. By measuring the attenuation of sound in a wide frequency band, it is possible to estimate pH by comparing the experimentally measured attenuation with an analytical theory of passive acoustic absorption spectroscopy. Measurements of the depth-dependent ambient sound field were carried out in the Philippine Sea, Mariana Trench, and Tonga Trench throughout 2009—2021. The wideband (5 Hz—30 kHz) acoustic data were recorded with untethered free-falling autonomous recording systems carrying two or four hydrophones.
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