Raman Studies of
Seafloor Materials: 
In-situ and in the Laboratory

Laser Raman Spectroscopic Facility

Washington University in St. Louis

The deep ocean is an "extreme aqueous environment ," permeated by fine particles of degraded organic matter and characterized by high pressures (about 360 atmospheres at 3.6 km depth), low temperatures (~ 2° C), and a high concentration (3.5 wt. %) of potentially corrosive salts.  Such conditions make it difficult to analyze interesting geologic materials and dynamic processes on the ocean floor, such as the fluids and solids that issue from hydrothermal vents, the rocks that cool from undersea lava eruptions, the skeletons and shells of reef-building animals such as corals and clams, and ice-like clathrates that form when natural gas seeps upward through the ocean-floor sediments and into the water column above.   Moreover, because many of the phases of interest are not stable once they are brought to the surface and exposed to ambient pressure and temperature, only an in situ analytical technique can enable detailed investigation of the ocean environment.

Raman spectroscopy is well suited to meet the challenges of analysis on the ocean floor: the technique is very amenable to materials that reside in an aqueous environment; analysis is possible on solids, liquids, gases, and dissolved species; and modern Raman instrumentation has fiber-optically-coupled components that can be encapsulated in pressure-resistant housings for operation underwater. Our group has been collaborating with Dr. Peter Brewer and other oceanographic researchers at the Monterey Bay Area Research Institute (MBARI) in the development of a deep ocean Raman system .

Our group helped to select and specify an appropriate vendor of a small, portable Raman probe, which was outfitted by MBARI's engineering staff with a pressure housing so that it could be operated underwater. Laboratory simulation measurements are being made with a duplicate KOSI Raman instrument (not enclosed in pressure housings, but with the same laser, gratings, and CCD detector as the MBARI deep ocean Raman system ) in our laboratory at Washington University in St. Louis in a 38-liter aquarium tank using probe-head optics identical to the two types deployed on the seafloor   (dry optic and immersion probe optic), except for the lack of a  "fish-eye lens " in front of the dry optic. 

Since the Spring of 2002, the deep ocean Raman system has been successfully deployed multiple times by MBARI's remotely operated vehicles (ROVs). One of the seafloor minerals of particular interest in this study is methane clathrate hydrate. Clathrate hydrates are cage-like minerals in which water forms an ice-like cage structure in which gas molecules, such as methane, reside. The fact that huge amounts of methane clathrate exist under the high pressures and cold temperatures of the seafloor make this an important mineral to consider as both a future source of natural gas (methane) and a major reservoir of carbon-bearing "greenhouse" gas that otherwise could be in our atmosphere. MBARI's in-situ deep ocean Raman instrument will be deployed to study the structure and composition of this important seafloor mineral.

Raman spectroscopy in-situ on the sea floor (at 3600 m ocean depth): Shown here is the 532 nm laser beam exiting the Raman probe head of MBARI's deep ocean Raman system during spectral acquisition from liquid CO2 in a glass cylinder. In the foreground is a 4-liter glass beaker filled with solid CO2 clathrate. The experiments were done as part of MBARI's CO2 sequestration studies .

Mineralogy clearly is not what it used to be, and "getting out into the field" has taken on a whole new meaning for some geologists!

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You may also want to check out the work by our colleagues in the Washington University
Planetary Surface Materials Raman Group