Development and deployment of a precision underwater positioning system for in situ laser Raman spectroscopy in the deep ocean

Abstract

The fie ld of ocean geochemistry has recently been expanded to include in situ laser Raman spectroscopic measurements in the deep ocean. While this technique has proved to be successful for transparent targets, such as fluids and gases, d iff iculty ex ists in u sin g deep sub mergence vehicle man ipulators to position and control the very small laser spot with respect to opaque samples of interest, such as many rocks, minerals, bacterial mats, and seafloor gas hydrates.We have developed, tested, and successfully deployed by remotely operated vehicle (RO V) a precision underwater positioner (P UP ) which provides the stability and precision movement required to perform spectroscopic measurements usin g the Deep Ocean In Situ Spectrometer (DORISS) instru ment on opaque targets in the deep ocean for geochemical research. The positioner is also adaptable to other sensors, such as electrodes, which require precise control and positionin g on the seafloor. P UP is capable of translating the DORI SS optical head with a precision of 0.1 mm in three dimension s over a range of at least 15 cm, at depths up to 4000 m, and under the nor mal range of oceanic conditions (T, P , current velocity). The positioner is controlled, and spectra are obtained, in real time via Ethernet by scientists aboard the surface vessel. Th is capability has allowed us to acquire high quality Raman spectra of targets such as rocks, shells, and gas hydrates on the seafloor, includ in g the ability to scan the laser spot across a rock surface in sub-millimeter increments to identify the constituent mineral gra ins. These developments have greatly enhanced the ability to obtain in situ Raman spectra on the seafloor from an enormous range of specimens.

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

“…Due to the small depth of focus of all of the sampling optics described, some form of positioner will be required to locate the laser spot on the target of interest. The three-degree-of-freedom Precision Underwater Positioner (White et al, 2005) developed for the DORISS instrument is an example of the type of system needed. For long-term Raman deployments at hydrothermal vents, positioning will be a challenge, as vent deposit topography can change rapidly over time.…”

“…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).…”

“…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). …”

“…In water, the working distance and depth of focus will be slightly greater. However, even in water, the depth of focus is small enough that some mechanism of focusing or positioning the laser spot (White et al, 2005) is needed.…”

“…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).…”