In an effort to better understand gas hydrates and
their makeup, scientists and engineers from the Center for Hydrate
Research at the Colorado School of Mines and the Monterey Bay Aquarium
Research Institute (MBARI) have for the first time obtained data
from a Raman spectrometer deployed in the deep sea. With support
from NOAA's Undersea Research Program and MBARI, scientists are
exploring the composition and fate of gas hydrates in their natural
environment using a modified bench-top Raman spectrometer at Hydrate
Ridge (off the Oregon coast) and at Barkley Canyon (off Vancouver
Island, British Columbia). This research has provided new insights
into the stability, structure, composition and heterogeneity of
these energy-rich, and potentially unstable methane hydrate deposits
found in the deep sea (Figure 1).
Figure 1. Location of Hydrate Ridge off the
coast of Oregon (A) and Barkley Canyon off the coast of Vancouver
Island, British Columbia (B).
Before this project, experimental hydrate stability
studies have been conducted in the laboratory, using synthetic materials
and, more recently, naturally formed gas hydrates that have been
recovered and returned to shore for analysis. While very valuable,
these experimental results are difficult to relate to natural gas
hydrate formed in the deep sea. There is a need for technological
advances to enable in situ hydrate studies. The development
of the undersea Raman spectrometer to directly measure in situ
characteristics of natural hydrates begins to fulfill this need.
|Figure 2. Gas hydrate from Barkley Canyon.
As the hydrate's hydrocarbons that were trapped within the hydrate
water cages escapes and burns.
What are gas hydrates?
Gas hydrates look like ice, but their chemical structure
is quite different. Under the cold, high pressure conditions of
the deep seafloor, carbon dioxide and methane gases interact with
water to form crystalline "cages" that enclose and trap
the gas molecules. The resulting "gas hydrates" can retain
a phenomenal amount of methane -- one volume of solid hydrate breaks
down at lower pressure to release 164 volumes of methane gas (Figure
Submarine gas hydrates occur within sediment deposits
at appropriate depths along most of Earth’s continental shelves.
They can also occur beneath Arctic permafrost. Most known hydrates
are white in color. They form as microbial activity changes sediment
carbon to methane.
Current estimates of hydrate methane reserves greatly
exceed all other fossil fuel reservoirs, representing a vast potential
energy resource. Because they contain vast amounts of methane, gas
hydrates are an important component of the global carbon cycle,
with the potential to contribute large volumes of greenhouse gases
and significantly impact global climate change. Because rapid chemical
and physical changes in gas hydrates could potentially trigger seafloor
collapse, followed by landslide and tsunami generation, they are
also considered a potential geohazard. These remarkable deposits
have focused the attention and imagination of scientists, resource
managers and the public.
Advanced technology development
Figure 3. MBARI ROV Ventana deploying
the Raman spectrometer (DORISS) and the micromanipulator (PUP)
off the side of the R/V Pt. Lobos in Monterey Bay
to measure the composition and structure of natural and synthetic
gas hydrates at depth. (larger
Raman spectroscopy is a well-established laboratory
tool that uses a laser beam to stimulate and measure changes in
the characteristic vibrations of individual molecules. Raman measurements
yield a distinct spectrum that is characteristic of the chemical
species present and of their state (i.e., gas, liquid, and crystalline).
The challenge for MBARI engineers was to enclose the Raman spectrometer
within a deep sea pressure vessel, while maintaining the ability
to focus the laser onto a sample several centimeters away. This
delicate instrument package also needed to be robust enough to be
launched and carried to the seafloor on an undersea robot (Figure
3). The team was able to overcome all these challenges and still
maintain the laser focus on a spot that is only a few microns across.
This precise focus allows for very detailed study of the internal
structure and heterogeneity of seafloor hydrate outcrops.
Figure 4. Raman spectra from natural hydrates
measured on the seafloor at Hydrate Ridge. (larger
Figure 5. Yellow and white hydrates were
observed at Barkley Canyon. The Raman probe (PUP) was positioned
with the ROV manipulator to conduct in situ measurements.
The first in situ measurements were made
at Hydrate Ridge off the Oregon coast (Figure 4). At this site,
gas hydrates are associated with cold seeps venting mainly methane
gas from the seafloor. The Raman instrument determined that the
bubbles from the cold seeps contain mainly methane, with small quantities
of hydrogen sulfide. With the help of the Raman instrument the science
team observed the formation of a hydrate coating around gas bubbles
that were moving through the sediment. They discovered that these
hydrate-encrusted bubbles can accumulate to form a hydrate bubble-fabric
within the seafloor, trapping small pockets of free gas. Disturbance
of a large bubble accumulation could destabilize the seafloor.
A second expedition, to Barkley Canyon, off the coast
of Vancouver Island, Canada, located massive gas hydrate accumulations,
several meters high, exposed on the seafloor at 850 meters depth.
This hydrate deposit was discovered by a fishing trawler that pulled
up over a ton of yellow, oil-stained hydrate. The yellow, oil-stained
Barkley Canyon hydrates differ from common white hydrates of microbial
origin. Yellow hydrates are formed from deeply buried organic matter
that has been heated and "cracked" at high temperatures,
perhaps derived from a petroleum reservoir deep beneath this site.
Raman instrument measurements on the seafloor showed that these
deposits contain mixtures of yellow and white hydrate. At "Double
Mound," yellow hydrate, which overlies white hydrate (Figure
5), has Raman spectra that indicate a significant amount trapped
oil within the hydrate structure. In contrast, the white hydrate
includes a rich blend of entrapped hydrocarbons, including methane,
ethane, propane, and butane.
Figure 6. The research team comprised of
scientists and engineers on the R/V Western Flyer
with the ROV Tiberon in the background.
The success of this project highlights the need for
engineers and scientists with laboratory and at-sea experience to
work together to develop new tools in order to solve tough problems
(Figure 6). Further hydrate studies are essential to evaluate whether
hydrates can alleviate growing energy demands, to understand their
potential effects on slope stability and their importance for global
carbon cycling, especially for greenhouse gas emissions. More work
is needed to understand the complex pathways by which these multi-component
hydrate systems are formed. In situ Raman spectroscopy
is an important first step in an exciting new future for deep sea