Reply to comment by A. Kopf on ‘‘Methane emission from the mud volcanoes of Sicily (Italy)’’, and notice on CH4 flux data from European mud volcanoes

Abstract

The paper ‘‘Methane emission from the mud volcanoes of Sicily (Italy)’’ by Etiope et al. [2002] represents the first report ever done on experimental CH4 output data from subaerial mud volcanoes (MV). A review of available CH4 flux data and detailed discussion about the global implications of mud volcanic CH4 emission has been made elsewhere [Etiope and Klusman, 2002; Morner and Etiope, 2002]. [2] The comment by Kopf [2003] contributes to open discussions and to make the readership aware on how important this subject is. In this reply we wish to clarify that precise data of CH4 flux from geologic sources are beginning to be available only now. It would be opportune that the MV-expert community could agree in using a common unit for the gas flux. We propose t y 1 and Mt y 1, and not metres cubed, consistently with the data reported for the methane sources/sinks budget by the IPCC. [3] Sicilian MVs, the first to be measured in detail, are considerably much smaller than the Azeri Ashgil MV, mentioned by Kopf [2003], and it is therefore obvious to expect a lower gas flux. Anyway the Dashgil mud volcano flux data are not based on exact measurements but only on visual estimates of the bubbles [Hovland et al., 1997]. In order to fully reply to Kopf [2003], hereafter we briefly discuss the problem of how to estimate the total number of MVs in the world and present new data from other European MVs, recently investigated. Finally, we outline the global importance of mud volcanic CH4 emission, as Kopf [2003] and recent literature is stressing.