A simple and rapid method for measuring total free sulfides in marine sediments

Abstract

The quantitatively most important process by which organic matter in marine sediments is mineralized is performed by sulfate-reducing bacteria, resulting in the accumulation of total dissolved (free) sulfide (S²⁻ = H₂S + HS⁻ + S²⁻) in porewater. S²⁻ is toxic to benthic animals and vascular plants and measurements serve as a proxy for the deleterious effects of organic enrichment on benthic habitat, biodiversity, and ecosystem function. Methodologies for measuring S²⁻ in water have been pursued for at least a century, and standard approaches employ colorimetry (methylene blue and iodometric titration) and potentiometry. These standard methods require between 1 and 200 mL of porewater, which can be laborious to obtain. The ion-selective electrode method is widely employed as a practical approach for sediment S²⁻ analysis but lacks analytical robustness and is highly prone to measurement biases that misinform research and environmental management decisions. A technically simple method is described, based on direct UV spectrophotometry, for the near real-time field analysis of small porewater samples. The procedure prevents known measurement biases associated with particulate sulfide interference, S²⁻ volatilization and oxidation, and represents a practical approach for monitoring organic enrichment and classifying benthic ecological quality status. Porewater concentrations between 200 and 15,000 μmol L⁻¹ can be measured and instrument calibration is highly stable. The method has the capacity to rapidly process and analyze sediment samples at low cost, which helps resolve the problem of chronic under-sampling associated with the use of traditional S²⁻ methods.