Carbon Isotopic Ratios of Methane in Marine Atmosphere Over the Arabian Coast
D.K. Rao and R.A. Jani
Physical Research Laboratory, Ahmedabad, India; +91 079 2631426, E-mail: firstname.lastname@example.org
Systematic air sampling was done on board ‘Sagar Pachmi‘ in the costal region of the Arabian Sea along the cruise track from Cochin to Goa, during November 2010, to find the source for excess methane within the marine atmosphere over the Arabian Sea. Ambient air was collected into 10L SS cylinders at 7bar pressure from a height of ~5 meters above sea surface at different latitude intervals. The carbon isotopic composition (δ13C) of CH4 in these samples were measured using dual inlet Isotope Ratio Mass Spectrometer after CH4 was converted to CO2. The CH4 concentrations vary from 1880 to 1943 ppbv where as its δ13C value vary in a narrow range of -44.9 to -46.5‰. The CH4 concentrations are more than that of tropospheric values (1754 ppbV) over the costal waters of the Arabian Sea from Kanyakumari to Mumbai during the months of Nov – Dec, 2003-2007. It is estimated that there is an excess CH4 of ~ 6 – 9.5% in these samples. In general, it is believed that CH4 concentrations in the marine atmosphere are related to emissions from the Arabian Sea due to upwelling, which brings CH4 rich water to the surface. The excess CH4 in these samples is either from surface waters or wind transport from the land surface to the sampling location. Our data suggests that excess methane must have come from land to ocean surface since the wind direction is northeast in three samples. In these cases, there is an increase of CH4 concentrations with increasing wind speed. This also indicates that CH4 emissions must have come from land surface. The values of δ13C of CH4 in these samples are enriched compared to the tropospheric value (-47.1‰), which indicates that the excess CH4 is thermogenic based on simple mass balance equation for two component mixing. The δ13C of the source for each sample is calculated separately, using the difference between estimated background values of both concentration and isotopic composition. They are in the range of -39.5 to -30.5‰ which confirms that the source for excess CH4 in these samples is thermogenic.