Identification of source for excess methane in marine atmosphere over Arabian coast

Systematic air sampling has been done on board 'Sagar Pachmi' in the costal region of Arabian Sea along the cruise track from Cochin to Goa during November 2010 to find the source for excess methane in marine atmosphere over Arabian Sea. Ambient air was collected into a 10L SS cylinders at 7bar pressure and also in a 200cc evacuated glass bulbs for isotopic and concentration measurements from a height of ~5 meters above Sea surface at different latitude intervals. The carbon isotopic composition (delta13C) in these samples were measured using dual inlet IRMS after methane is converted in to CO2 using conventional method. Methane concentrations in all these samples vary from 1880 to 1943 ppbv where as its delta13C value vary in a narrow range of -44.9 to -46.5‰. The CH4 concentrations are more than that of tropospheric values (1775 ppbv) over the costal waters of Arabian Sea from Kanyakumari to Mumbai during the month of November-December from 2003-2007. It is estimated that an excess methane of ~ 7 - 11% in these samples. In general, it is believed that CH4 concentrations in marine atmosphere are related to emissions from the Arabian Sea due to upwelling which brings methane rich water to the surface. The excess methane in these samples is either from methane rich water in surface waters or wind flown from land surface to sampling location. Our data suggests that excess methane must have come from land to Ocean surface since the wind direction is NE in these samples. Also, 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 delta13C of CH4 in these samples are enriched compared to that of from tropospheric value (-47.1‰) which indicates that the excess methane is thermogenic type and probably the methane must have come from land. We estimated the delta13C of CH4 for this excess methane and they vary between -39 to -25‰. It indicates that the source for the excess methane is from leakage of methane gas from seeps or oil wells from nearby coastal locations.