Characteristics and scaling of bubble plumes from marine hydrocarbon seepage in the Coal Oil Point seep field

Abstract

The fate of marine seep gases in the ocean and atmosphere is intimately connected with bubble and bubble‐plume processes, which are strongly size‐ and depth‐dependent. Size‐dependent flux distributions, Φ, and vertical velocity functions, VZ(r), were measured with a video bubble measurement system in the Santa Barbara Channel, California. Several distinct plume types were identified for which size distributions were measured, major, minor, intermediate, and obstructed. A further vent class is elastic (which was observed but not quantified). In addition, any plume class could be oily. Minor plumes generally produced a lower emission flux, Q, and showed narrow, peaked Φ that were well described as Gaussian. The radius of the dominant peak, RP, of minor plumes varied as Q0.40, with correlation coefficient, R2 = 0.84, in agreement with theoretical relationship of RP∼Q0.4 for Q above a critical flow rate. Oil contamination was found to affect RP and was not used in the fit. A probability distribution, Ψ(RP), for all Gaussian bubble plumes was itself well described by a combination of Gaussian functions, which were different for different seep areas. Major plumes showed a broad distribution including very small and very large bubbles and were well described by a power law with exponent a, which varied with Q according to a = 0.43 + 0.55 log(Q) with R2 = 0.77. One obstructed vent was analyzed and shared characteristics with the minor bubble plumes. Mixed bubble plume size distributions showed characteristics of both major and minor plume classes, i.e., were described by a combination of Gaussian functions and power laws, and were steeper (higher a) than major plumes for the same Q. Oily plumes produced complex, confused bubble size distributions. Upwelling velocities, VUP(r), were derived from VZ(r) and increased as VUP∼Q0.66 (R2 = 0.64); however, consideration of the more intense plumes (Q > 2 cm s−1) showed VUP∼Q0.35 in agreement with other published field measurements. Thus, the weaker bubble plumes were observed during the acceleration phase.