In situ Raman probe for quantitative observation of sediment pore waters in the Deep Ocean — Development and applications

Using the well developed Deep Ocean Raman In Situ Spectrometer (DORISS), we have developed an in situ deep-sea marine sediment pore water probe for geochemical studies. This novel device allows detailed measurement of pore water profiles. Our particular interest is in dissolved CH₄, SO₄⁼, H₂S, and pH in situ without incurring artifacts due to the degassing that typically occurs with core recovery. A second generation DORISS instrument was developed at the Monterey Bay Aquarium Research Institute (MBARI) by engineering and science staff in 2005. The instrument is a plugin module for either of remotely operated vehicle (ROV) platforms at MBARI. DORISS has successfully performed in situ measurements on targets of scientific interest including high-temperature hydrothermal vent fluids, complex gas hydrates, and numerous other targets. Previously Raman techniques had been avoided to analyze sediment pore water geochemistry because of experience with the pore-waters strongly fluorescing. Simple experiments conducted with extracted pore-waters had previously demonstrated that fluorescence overwhelmed the relatively weak laser Raman signal. The in situ Raman pore water probe has worked around the fluorescence problem. The Raman pore water probe has been deployed a number of times at gas hydrate sites such as Hydrate Ridge, Barkley Canyon, and in the Santa Monica basin over the last two years. This paper will discuss the technique details and the exploration of expanding these same techniques into longer probes and looking at other difficult to obtain chemistries.