Abstract
The volume-integrated pH of seawater can be determined from the frequency and depth dependence of wind-generated ambient noise in the ocean. Over the 1–10 kHz frequency band, three main processes contribute to the acoustic attenuation in seawater: the chemical relaxation of boric acid and magnesium carbonate (<3 kHz, related to pH) and of 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, which exhibits a depth-independent directionality and weak frequency and depth-dependent intensity. By measuring the depth dependence of the spectral slope, the pH may be estimated from a comparison of the experimental data with an analytical model of ambient noise. Wideband (5 Hz–30 kHz) vertical ambient sound profiles were recorded using two- and four-channel free-falling acoustic profilers at depths ranging from 500 m to 10 km during nine deployments in the Philippine Sea, Mariana Trench, and Tonga Trench from 2009 to 2021. Two analytical models of the depth dependence of ambient noise were developed: a simplified linear model valid at depths <1,500 m and a full nonlinear model valid for the deep ocean. Estimates of pH were found by minimizing the mean absolute percent error between the measurements and the models. This method of passive acoustic absorption spectroscopy demonstrates the potential and sources of uncertainty in determining the depth-averaged value of pH. The method could be suitable for the long-term passive acoustic monitoring of ocean acidity.
Plain Language Summary
In this work, we demonstrate that ambient sound in the ocean can be used to measure local, depth-averaged ocean pH. This is possible because the absorption of sound in seawater depends on chemical processes, including the relaxation of boric acid and magnesium carbonate, and has a frequency-dependent sensitivity to the pH. By analyzing the depth dependence of ambient sound over the wind-driven noise-dominated band (1–10 kHz), we can estimate pH through a comparison of measured power spectral slopes with an analytical model. Using measurements from the Philippine Sea, Mariana Trench, and Tonga Trench, carried out from 2009 to 2021 with a free-falling autonomous instrument platform, Deep Sound, we estimated the depth-averaged pH in each location. This technique can be used for long-term passive acoustic monitoring of ocean acidity.
Key Points
- The depth dependence of the spectral slope of wind-generated noise provides a measurement of the differential acoustic attenuation between 1 and 10 kHz
- The differential attenuation is used to estimate the depth-integrated pH
- The proposed method enables long-term volumetric (order of km3) monitoring of ocean acidity
Uzhansky E. M., Barclay D. R. & Buckingham M. J., 2025. On the measurement of ocean acidity with ambient sound. Journal of Geophysical Research: Oceans 130(10): e2025JC022575. doi: 10.1029/2025JC022575. Article.
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