Marine Hydrocarbon Seeps

Over the last century the atmospheric concentration of the potent greenhouse gas methane, CH₄, has risen approximately 1% per year. The reasons for this trend, are poorly understood because global CH₄ sources and sinks still need quantification. Recent estimates suggest marine seep contributions to atmospheric CH₄ could play an important role. Marine seeps also release oil, impacting the local marine environment. Seeps are home to numerous creatures from specialized bacteria to tube worms to mussels and fish. Marine hydrocarbon seeps are cold and (distinguished from volcanic, hot seeps) have two major sources, biogenic - i.e., bacterial production of gas, and petrogenic or thermogenic - i.e., relating to subsurface petroleum researvoirs that "leak" to the surface. Some seep gas arises from CH₄ hydrate dissociation, a -water ice that is stable at great depths and low temperatures. Most seeps release CH₄, with trace gases including n-alkanes (ethane, propane, etc.) and hydrogen sulfide. Carbon dioxide can be important or even dominant.

Seeps release gas as bubbles or oil and gas as oily bubbles, or oil as droplets that that rise through the water column. Areas of research include quantifying emissions and relating emissions and fate to environmental variables, and the environmental impact of seeps.

Seep Plume Sonar Image

The UCSB Seep Group has a wide range of current projects using diverse tools. These include:

Bubble measurements

acoustic system

To understand the impact of seeps on the environment and global climate requires significant interdisciplinary advances in a range of areas. These include spatial and temporal distributions of seep emissions globally, bubble and bubble plume processes, the interaction of currents and winds and waves with seep plumes and surface oil slicks, multiphase migration through fractured rocks and faults, and improvement in numerical modeling efforts. Additionally, seeps provide a bioavailable energy source, which supports chemosynthetic communities, but the carbon cycle pathways are just beginning to be understood. Fortunately, recent studies are rapidly increasing our knowledge and understanding.

Scuba diver at Shane Seep (22-m depth). Sea anemonae is a matridium, yellow line is transect line.

Due to the difficulty of planned oil release studies and the impossibility of realistic duplication of oceanic processes in the laboratory, perennial seeps provide an ideal natural laboratory for the study of hydrocarbons in the marine environment. Escaping bubble streams provide an easily identifiable locator of active seepage, both visually and acoustically (sonar). The sea surface, although challenging environment for work, allows complete mobility in any direction. Moreover, where the sediment overburden is thin, the seabed provides a roughly horizontal, planar transect through a fracture network, thereby allowing non-destructive study of multiphase migrational processes. Because seeps respond to external forcing (tidal, swell , and potentially others, such as earthquakes and earth tides) the seep response to these perturbations probes subsurface processes.