Early-diagenetic methane from various tropical freshwater sediments: molecular and carbon isotope variations in one dial cycle
Abstract
Wczesnodiagenetyczny metan z różnych osadów słodkowodnych klimatu tropikalnego: zastosowanie chromatografii gazowej i analizy stosunków izotopowych w obserwacjach cyklu dobowego
In the tropical climate of Southern Thailand, on September 5 and 6, 1994 dial emissions of CO2 and CH4, and dial variations in bubble CO2 and CH4 concentrations and the (13)C/(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 δ(13)CH4 values are lowest in the night, and highest in the daytime, ranging from a minimum of-63.8%o at 02:20 (canal) to a maximum of-47.6%o at 6:45 (pit). High δδ(13)CH4 values of daytime methane occur when the concentration of CO2 decreases and acetic acid fermentation becomes relatively more important. Minimum δ(13)CH4 values are apparently caused by an enhanced CO2-H2 methanogenesis pathway. Irregular diurnal variation of the δ(13)CH4 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, δ(13)CH4 values in the pit and the pond were about 10%o 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 δ(13)CH4 values in the canal suggest an enhanced CO2 reduction pathway. The maximal δ(13)CH4 value accompanied by the highest bubble CO2 concentration is caused by strong oxidation of methane when the production rate of methane in early morning is lowest.
In the tropical climate of Southern Thailand, on September 5 and 6, 1994 dial emissions of CO2 and CH4, and dial variations in bubble CO2 and CH4 concentrations and the (13)C/(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 δ(13)CH4 values are lowest in the night, and highest in the daytime, ranging from a minimum of-63.8%o at 02:20 (canal) to a maximum of-47.6%o at 6:45 (pit). High δδ(13)CH4 values of daytime methane occur when the concentration of CO2 decreases and acetic acid fermentation becomes relatively more important. Minimum δ(13)CH4 values are apparently caused by an enhanced CO2-H2 methanogenesis pathway. Irregular diurnal variation of the δ(13)CH4 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, δ(13)CH4 values in the pit and the pond were about 10%o 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 δ(13)CH4 values in the canal suggest an enhanced CO2 reduction pathway. The maximal δ(13)CH4 value accompanied by the highest bubble CO2 concentration is caused by strong oxidation of methane when the production rate of methane in early morning is lowest.