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

Abstract

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 (Φ) 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 Φ that were well described by a Gaussian function. Major plumes showed broad Φ 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 Φ that was Gaussian for low Q. The peak radius of the Gaussian function describing Φ 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 Φ, the peak radius decreased with increasing grain size. For high Q, sediment-bed Φ exhibited a decreased sensitivity to grain size, and Φ tended toward a power law, similar to that for major seep plumes.

Appendix

A (−):

Peak value of F in Gaussian functional fit

C (−):

Circularity of bubble outline. Used in image processing

D_G (mm):

Equivalent spherical diameter of sediment grains

F (# µm⁻¹ m⁻¹):

Function that represents either Φ or Ψ

N (−):

Number of bubbles analyzed

Q (L min⁻¹):

Flow (corrected to STP)

Q_Layer (L min⁻¹):

Layer flux (total volume of bubbles in a layer)

R² (−):

Correlation coefficient

R_G (mm):

Equivalent spherical radius of sediment grains

R_P (µm):

Radius of peak concentration in Φ

V_B (cm s⁻¹):

Bubble rise velocity in stagnant fluid

V_UP (cm s⁻¹):

Upwelling velocity

V_V (cm³):

Porosity or sediment void volume

V_Z (cm s⁻¹):

Bubble vertical velocity

dBD (g cm⁻³):

Dry bulk density

g (m s⁻¹ s⁻¹):

Gravity

r (µm):

Equivalent spherical bubble radius

r_cap (µm):

Capillary tube orifice radius opening

s (−):

Power law exponent in Φ

t (s):

Time

ϕ (cm³ cm⁻³):

Porosity

ζ (−):

Relative density

ρ_G (g cm⁻³):

Bubble gas density

ρ_L (g cm⁻³):

Water density

τ (µm):

Gaussian function half-width

Φ (# µm⁻¹ s⁻¹):

Bubble flux size distribution

Ψ (# µm⁻¹ m⁻¹):

Bubble-layer population size distribution