Variability of gas composition and flux intensity in natural marine hydrocarbon seeps

Abstract

The relationship between surface bubble composition and gas flux to the atmosphere was examined at five large seeps from the Coal Oil Point seep field (Santa Barbara Channel, CA, USA). The field research was conducted using a flux buoy designed to simultaneously measure the surface bubbling gas flux and the buoy's position with differential GPS, and to collect gas samples. Results show that the flux from the five seeps surveyed a total of 11 times ranged from 800-5,500 m 3 day −1 . The spatial distribution of flux from the five seeps was well described by two lognormal distributions fitted to two flux ranges. The seafloor and sea surface composition of bubbles differed, with the seafloor bubbles containing significantly more CO 2 (3-25%) and less air (N 2 and O 2 ). At the sea surface, the mole fraction of N 2 correlated directly with O 2 (R 2 = 0.95) and inversely with CH 4 (R 2 = 0.97); the CO 2 content was reduced to the detection limit (<0.1%). These data demonstrate that the bubble composition is modified by gas exchange during ascent: dissolved air enters, and CO 2 and hydrocarbon gases leave the bubbles. The mean surface composition at the five seeps varied with water depth and gas flux, with more CH 4 and higher CH 4 /N 2 ratios found in shallower seeps with higher flux. It is suggested that the CH 4 /N 2 ratio is a good proxy for total or integrated gas loss from the rising bubbles, although additional study is needed before this ratio can be used quantitatively.

Sonar gas flux estimation by bubble insonification: application to methane bubble flux from seep areas in the outer Laptev Sea

“…In the shallow Coal Oil Point seep field, most of the CH 4 reaches the atmosphere directly (Clark et al, 2005) from mixing in the near field (Clark et al, 2000) and in the far (down-current) field when winds strengthen as typically occurs diurnally for coastal California. The same is true for the shallow ESAS where virtually all seabed CH 4 (dissolved and gaseous) is emitted in the WWML and escapes to the atmosphere directly by bubbles or through air-sea gas exchange by frequent storms (Shakhova et al, 2014).…”

A low‐cost automated trap to measure bubbling gas fluxes

“…An automated funnel-type trap equipped with a differential pressure sensor has previously been used to measure gas fluxes in marine hydrocarbon seeps Clark et al 2009), although we are unaware of any long-term or extensive deployments of this type of trap. The electronic setup in the device also appears to be expensive, bulky, and time-consuming to build.…”

Reduced Methane Emissions from Santa Barbara Marine Seeps

“…The simulations were performed applying a vertically-integrated Gaussian plume model [27] and emission rates of 18 kt CH 4 y −1 [4], while assuming an average composition of 60% CH 4 at the sea surface, accounting for dissolution in the water column [8]. We took a simplified approach by assuming that all emissions from the seep field originated from the two main seep areas, Trilogy and La Goleta, which were enlarged to 1500 m in diameter.…”

“…Hydrocarbon release mechanisms and emissions have been investigated using various methodologies, including sonar [4,6], in situ methane and other hydrocarbon measurements [5,[7][8][9], as well as underwater flow meters and optical bubble counters [6,10,11]. Airborne hyperspectral imaging spectroscopy has been used to map CH 4 plumes from a range of the larger seeps [12][13][14].…”

“…The marine seeps located offshore of Coal Oil Point were reported to emit about 18 kt CH 4 y −1 in total to the atmosphere, estimated using data from 1994-1996 [4] and assuming an average composition of 60% CH 4 for the direct emissions [8]. According to Bradley et al [9], based on long-term atmospheric measurements at Coal Oil Point, emissions generally decreased until 1997 and afterwards increased at least until 2008 (the end of the analyzed time series).…”