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Mission | |||||||
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Leg
5 Leg
Summary: Leg 5 will consist of a series of ROV dives designed to investigate the ways that fluids and gases escape from the seafloor and into the water column at different depths and temperatures. These studies are critical to understanding the formation of gas hydrates and the influence of vented gases on biological communities. The majority of operations will take place in the Guaymas Basin, where the ROV will sample the sediments using a variety of coring devices, and will also collect fluid and gas samples in the water column. Instrumented probes on the ROV will be used for measurements of physical and chemical parameters in the sediments and in the water column. Three additional ROV dives will be made in the Salsipuedes Basin to the northwest to measure the characteristics of methane bubbles and carbon dioxide droplets as they rise from the seafloor in warm water. The leg 5 coordinator is Dr. Charles Paull from MBARI, and the Mexican collaborator is Dr. Juan Carlos Herguera from CICESE. History
& Purpose: This
cruise represents a coordinated effort between the Paull, Brewer and
DeLong groups at MBARI and Dr. Herguera from CICESE. We are planning for
12 operational days with the R/V Western Flyer and
ROV Tiburon
in the Guaymas basin of the Gulf of California. An additional three days
will be spent in the warm water basins of the Sea of Cortez. Our
common objectives are to investigate the pathways and processes by which
fluids and gases escape from the seafloor through the gas hydrate
stability zone. The Guaymas Basin is one of the few areas in the world
where gases are reported to be venting from seafloor at oceanic depths
(≥2,000 m). This area provides an opportunity to investigate the
differences between gas hydrate-bearing and gas hydrate-free sediments,
the biogeochemical processes that happen on and in the seafloor around
active gas vents, and to trace the gas upward into the water column. Usually
at the depths and temperatures associated with the Guaymas Basin, the gas
hydrate stability field extends for hundreds of meters into the
subsurface. However, the sharp lateral thermal gradients around the hot,
young, oceanic crust, cause the lower limit of the gas hydrate stability
to intersect the seafloor. At the same time, the high sedimentation rates
of organic-rich sediment and very high temperature gradients in the
sediments stimulate rapid methane and associated light hydrocarbon
production, providing large quantities of methane and other
gas-hydrate-forming gases. Thus, sharp lateral transitions from gas
hydrate-bearing to free gas-bearing sediments should occur within the near
surface sediments of the Guaymas Basin. Such transitions are usually only
accessible in several hundred meter deep bore-holes that pass through the
phase boundary associated with the base of the gas hydrate stability
field. We will sample along seafloor transects that extend from gas
hydrate-bearing to gas hydrate-free sediments within similar sediments
along the same bed. The unique geometry provides an unusual control for
assessing the physical and biogeochemical effects gas hydrate presence has
on sediment and the flow of carbon into the surrounding environment and
biota.
Our science plan for the warm water basins in the Sea of Cortez is to investigate the behaviour of rising methane bubbles and carbon dioxide droplet plumes outside of the hydrate stability fields. We will release these gases/liquids at various depths, proceeding upwards from the maximum depth of the basin, chasing the bubbles. |