Formation of seep bubble plumes in the Coal Oil Point seep field

摘要

The fate of marine seep gases (transport to the atmosphere or dissolution, and either bacterial oxidation or diffusion to the atmosphere) is intimately connected with bubble and bubble-plume processes, which are strongly size-dependent. Based on measurements with a video bubble measurement system in the Coal Oil Point seep field in the Santa Barbara Channel, California, which recorded the bubble-emission size distribution (I broken vertical bar) for a range of seep vents, three distinct plume types were identified, termed minor, major, and mixed. Minor plumes generally emitted bubbles with a lower emission flux, Q, and had narrow, peaked I broken vertical bar that were well described by a Gaussian function. Major plumes showed broad I broken vertical bar spanning very small to very large bubbles, and were well described by a power law function. Mixed plumes showed characteristics of both major and minor plume classes, i.e., they were described by a combination of Gaussian and power law functions, albeit poorly. To understand the underlying formation mechanism, laboratory bubble plumes were created from fixed capillary tubes, and by percolating air through sediment beds of four different grain sizes for a range of Q. Capillary tubes produced a I broken vertical bar that was Gaussian for low Q. The peak radius of the Gaussian function describing I broken vertical bar increased with capillary diameter. At high Q, they produced a broad distribution, which was primarily described by a power law. Sediment-bed bubble plumes were mixed plumes for low Q, and major plumes for high Q. For low-Q sediment-bed I broken vertical bar, the peak radius decreased with increasing grain size. For high Q, sediment-bed I broken vertical bar exhibited a decreased sensitivity to grain size, and I broken vertical bar tended toward a power law, similar to that for major seep plumes.