In situ Determination of Pore-water pH in Reducing Sediments near Methane Seeps and Vents by Laser Raman Spectroscopy

Abstract:

Sediments near methane vents and seeps are often anoxic in nature due to the microbial oxidation of organic matter. When the pore-water oxygen is consumed, the microbial population resorts to using sulfate as the terminal electron receptor. For the anaerobic oxidation of methane, the net reaction is:

CH₄ + SO₄^2- = HCO₃^- + HS^- + H₂O.

Hydrogen sulfide produced by this reaction dissociates into bisulfide in proportion to the pore-water pH. Since the first pK of H₂S is about 7 and close to the in situ pore-water pH, it satisfies the criteria for a useful pH indicating dye. Although the two forms of hydrogen sulfide are not visually discernable by the human eye, these two forms have distinct Raman spectra and thus can be easily quantified using an in situ spectrometer. The relative Raman cross-sections of the hydrogen sulfide species were determined in the laboratory across a range of relevant pH values and at the approximate salinity (ionic strength) and temperature of deep-sea pore waters. With this calibration, it is simple to compute the pore-water pH from the relative abundance of the two sulfide species:

pH = pK₁ + log₁₀([HS^-]/[H₂S]).

Pore-water profiles were investigated at several sites in the Santa Monica basin around methane mounds, gas vents and cold seeps. A titanium pore-water probe with a stainless steel frit was used to filter and collect pore-water samples at 5-10 cm intervals in the top 50-60 cm of sediment. Filtration and collection of the pore-water samples was usually accomplished in 5-10 minutes, with acquisition of the laser Raman spectra requiring only 2-4 minutes additional time. Vertical profiles of sulfate, total sulfide (H₂S + HS^-), methane and pH were collected simultaneously using the laser Raman spectrometer and pore-water profile sampler. Sulfate was observed to decrease from seawater concentrations to below detection limits while both methane and total sulfide increased proportionally to the sulfate loss. Once total sulfide concentrations exceeded about 10mM, the H₂S and HS^- peaks were of sufficient size that their ratio could be determined with sufficient precision to allow determination of the in situ pH. For the several profiles observed, pH ranged from 7.5 to 7.9 near the sediment water interface and with generally decreasing profiles down-core to pH values as low as 7.0.