In situ Raman measurement of HS− and H2S in sediment pore waters and use of the HS−:H2S ratio as an indicator of pore water pH

The smell of hydrogen sulfide upon recovery of deep sea cores from areas of reducing sediments is familiar to most ocean scientists and is simple testimony to the problem of dissolved gas loss during core recovery and processing. For this reason methods of in situ measurement of sulfide and other gases are keenly sought, and both microelectrode and spectroscopic measurements have been widely used. We report here on the use of a robust 50 cm long titanium pore water probe combined with a laser Raman sensing system for in situ measurement of the sulfide concentration and sulfide speciation status of deep sea sediment pore waters. Uniquely the Raman signal spectroscopically senses and simultaneously resolves both the H2S (υ1 2592 Δcm− 1) and HS− (υ1 2573 Δcm− 1) species. We have calibrated the Raman response factors as a function of pH and found that the Raman cross section for HS− is a factor of 1.563 greater than for the H2S form. We have carried out stepwise profiling of the pore water chemistry (CH4, SO42 −, H2S, HS−) of highly reducing sediments in the Santa Monica Basin and from the combined sulfide species we can assess total dissolved sulfide. Since the ratio of HS−:H2S is a well-defined function of pH with the pK for this equilibrium being close to 7.0, and since the pH of many sediment pore waters is also close to 7 then the observed HS−:H2S ratio provides an elegant pH sensitive “dye”. We have solved the required Raman-equilibrium relationships and show that the in situ pH of sulfide rich pore waters can be determined rapidly and directly from these observations. Several field examples are provided.