Natural flux of greenhouse methane from the Timor Sea to the atmosphere

Methane gas bubbles from the Cornea Seep were sampled at the sea surface in the Timor Sea continental shelf area in June 2005. Total bubble gas flux was 0.076 to 0.76 L m(-2) h(-1) during the 6 h d(-1) periods of low neap tides in June 2005. This bubble gas contained an average of 26 mmol CH4 L-1 and about 0.16 and 0.006 mmol L-1 of ethane and propane. We estimate the daily flux from the sea surface to the atmosphere to be 0.012 to 0.12 mol CH4 m(-2) d(-1) or 0.13 to 1.3 t CH4 d(-1) from an area of about 0.7 km(2). This methane flux came from a 500 x 1400 m carbonate pavement dome on the seafloor at 84 m water depth. The seep hard ground was swath mapped, and 3.5 kHz subbottom profile data indicate that the seep dome was strongly reflective with poor penetration into the subsurface, consistent with the presence of a carbonate hard ground. Carbon and deuterium isotope ratios (delta C-13 = -41 to -42 parts per thousand delta D = -157 to -158 parts per thousand) of the seep bubble gas indicate that this methane had a thermogenic origin and was in the same isotopic range as gas within the Late Cretaceous Cornea oil and gas field. We could not detect inputs of fluids containing nutrients or short-lived radium isotopes at this site, commonly associated with other cold seeps. Tens to a hundred of kilometers seaward from the Cornea seep site, water column dissolved methane concentrations in this sector of the Timor Sea shelf and slope were 100-500 times supersaturated with respect to the atmosphere, and thus the water column is expected to be degassing additional methane to the atmosphere. Perhaps there are thousands of other methane seeps (of similar magnitude to the Cornea Seep) on this shelf and slope to account for all the excess dissolved methane (similar to 86,000 t) measured in the water column. These measured and calculated fluxes provide evidence for the hypothesis that shallow sea seeps may be a significant source of atmospheric methane, in contrast to deep sea vents, where most of the methane is dissolved and oxidized in seawater and does not reach the atmosphere.