In situ Raman analyses of deep‐sea hydrothermal and cold seep systems (Gorda Ridge and Hydrate Ridge)

Abstract

[1] The Deep Ocean Raman In Situ Spectrometer (DORISS) instrument was deployed at the Sea Cliff Hydrothermal Field and Hydrate Ridge in July 2004. The first in situ Raman spectra of hydrothermal minerals, fluids, and bacterial mats were obtained. These spectra were analyzed and compared to laboratory Raman measurements of standards and samples collected from the site. Spectra of vent fluid ($294°C at the orifice) at $2700 m depth were collected with noncontact and immersion sampling optics. Compared to spectra of ambient ($2°C) seawater, the vent fluid spectra show changes in the intensity and positions of the water O-H stretch bands due to the elevated temperature. The sulfate band observed in seawater spectra is reduced in vent fluid spectra as sulfate is removed from vent fluid in the subseafloor. Additional components of hydrothermal fluid are present in concentrations too low to be detected with the current Raman system. A precision underwater positioner (PUP) was used to focus the laser spot on opaque samples such as minerals and bacterial mats. Spectra were obtained of anhydrite from actively venting chimneys, and of barite deposits in hydrothermal crusts. Laboratory analysis of rock samples collected in the vent field also detected the presence of gypsum. Spectra of bacterial mats revealed the presence of elemental sulfur (S 8 ) and the carotenoid beta-carotene. Challenges encountered include strong fluorescence from minerals and organics and insufficient sensitivity of the instrument. The next generation DORISS instrument addresses some of these challenges and holds great potential for use in deep-sea vent environments.

Laser Raman spectroscopy as a technique for identification of seafloor hydrothermal and cold seep minerals

“…Pure sulfur has little fluorescence under either green or red excitation. The spectral data obtained in this study correspond well to data reported in the literature for elemental sulfur analyzed in the lab (Edwards et al, 1997) and filamentous sulfur produced by bacterial mats analyzed in the lab (Pasteris et al, 2001) and in situ on the seafloor (White et al, 2006b). …”

“…It should be noted that all spectra in this study were obtained in air (unless otherwise noted). Previously collected in situ spectra did not show any effects from seawater, temperature or pressure on the Raman spectra of hydrothermal minerals (White et al, 2005;White et al, 2006b). …”

“…Laboratory Raman studies of individual shells have shown the presence of calcite and/or aragonite. In situ laser Raman spectroscopy measurements have also identified calcite and aragonite in shells on the seafloor (White et al, 2005;White et al, 2006b).…”

“…DORISS has been used to identify and analyze gas mixtures (White et al, 2006a), gas hydrates (Hester et al, 2006;Hester et al, 2007), and minerals. Naturally occurring minerals identified in situ by Raman spectroscopy to date include hydrothermally produced barite and anhydrite, calcite and aragonite in shells, and bacterially produced sulfur (White et al, 2005;White et al, 2006b).…”

“…Laboratory Raman studies by Pasteris et al (2001) have identified elemental sulfur in the S 8 configuration in the bacteria Thioploca and Beggiatoa. In situ laser Raman spectroscopic measurements of bacterial mats at the Hydrate Ridge site have also detected the presence of S 8 sulfur (White et al, 2006b). Sulfur is also produced hydrothermally and in volcanic eruptions (Nakamura et al, 2006).…”