Measuring ocean pH with ambient noise

The depth-dependent, volume-integrated pH of seawater over can be measured directly from the profiles of ambient noise using passive acoustic absorption spectroscopy. Acoustic absorption in seawater is frequency dependent and caused by the chemical relaxation of three constituents in the band 1 – 10 kHz: boric acid and magnesium carbonate below 3 kHz and magnesium sulfate above 3 kHz.  The concentrations of boric acid and magnesium carbonate are directly tied to seawater pH, while the concentration of magnesium sulfate is not, causing the wideband acoustic absorption curve to be sensitive to pH.  Under strong local wind conditions, ambient noise is dominated by locally generated surface noise caused by wind-driven breaking of surface gravity waves. This noise field has been shown to have a depth-independent directionality and weak frequency and depth-dependent intensity.  However, comparisons of the depth-dependent frequency changes of the ambient noise spectrum to an analytical model can be used to infer the depth-integrated pH.  This sensing method has been demonstrated using wideband (5 Hz – 30 kHz) vertical ambient sound profiles recorded using free-falling acoustic recording platforms that have been deployed in the Philippine Sea, Mariana Trench, and Tonga Trench from 2009 to 2021. These recorders capture the ambient noise field the surface to depth up to 10 km, along with direct measurements of temperature, salinity, pressure, and sound speed. Estimates of pH were found by minimizing the mean absolute percent error between the measurements and an analytical model of the depth-dependence of ambient noise. 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.