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. 

The operational plan will be to dive on sites where water column plumes have been identified or expected based on available data. Tiburon’s scanning sonar will be used to search for water column plumes or to identify areas of bottom roughness associated with seafloor seepage sites. The first few dives will be spent exploring the area and collecting thermal measurements along potential transects. Subsequent dives will be devoted to collecting various types of samples and data. The subsurface sediments (and their contained biota, fluids and gases) will be sampled using MBARI’s ROV-based vibracorer, clathrate bucket, and ROV push cores. Special emphasis will be spent on collecting cores and other samples in short transects that cross the edge of the gas hydrate stability field or extend through areas associated with active chemosynthetic biological communities that surround seafloor venting sites. Surface sediments and biota will be sampled using the ROV’s push cores and other manipulator operated sampling tools. Transects of water column samples will be conducted using the ROV-mounted Radium sampler, Niskin bottles, gas sample bottles, and the syringe sampler. Other water column samples will be collected using the CTD sampler from the Western Flyer. Various probes will be inserted into the sediment to measure the physical properties of the sediments (e.g., temperature). Other sensors will be deployed from the ROV and CTD to make measurements of the water column characteristics.  

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.