Early-diagenetic methane from various tropical freshwater sediments: molecular and carbon Isotope variations In one dial cycle

In the tropical climate of Southern Thailand, on September 5 and 6,1994, dial emissions of CO sub(2) and CH sub(4), and dial variations in bubble CO sub(2) and CH sub(4) concentrations and the super(13)C/ super(12)C ratio in bubble methane were observed in shallow freshwater sediments of a high sulphate concentration pit, a high turbulence canal, and in a pond with a high density of emerged macrophytes. Measurements show that d super(13)CH sub(4) values are lowest in the night, and highest in the daytime, ranging from a minimum of 63.8ppt at 02:20 (canal) to a maximum of 47.6ppt at 6:45 (pit). High d super(13)CH sub(4) values of daytime methane occur when the concentration of CO sub(2)decreases and acetic acid fermentation becomes relatively more important. Minimum d super(13)CH sub(4) values are apparently caused by an enhanced CO sub(2)-H sub(2) methanogenesis pathway. Irregular diurnal variation of the d super(13)CH sub(4) value in the pond was caused by oxidation and ventillation of sediments by macrophytes and the different lithologies of the sediments in separate sampling stations. Generally, d super(13)CH sub(4) values in the pit and the pond were about 10ppt higher as compared to values from the canal, suggesting an active acetic acid pathway and oxidation effect in the former two environments. Likewise, the lowest production rate of methane and the lowest d super(13)CH sub(4)values in the canal suggest an enhanced CO sub(2) reduction pathway. The maximal d super(13)CH sub(4) value accompanied by the highest bubble CO sub(2) concentration is caused by strong oxidation of methane when the production rate of methane in early morning is lowest.