The role of methane oxidation in the marine and estuarine environment

3.1 Introduction

Marine systems (oceans, coastal seas and estuaries) are dynamic and changeable environments with methane being constantly produced and utilised within these systems. Thus, methane plays an important role in the global carbon budget (Schlesinger, 1991; see Chapter 1). Within the marine environment, methane is formed by either biogenic or abiogenic (thermogenic) processes (Ivanov et al., 1993).

There are two forms of biogenic process. Firstly, methane may be produced as a by-product of fossil fuel production and/or combustion or secondly by methanogenic Archaea (see Chapter 1). Coal and petroleum are created from the decomposition of organic carbon matter and may therefore be categorised as a biogenic medium for natural gas production (Ourisson et al., 1984). However, while coal gas is formed purely from thermogenic processes, petroleum may also form biogenic gas due to its susceptibility to bacterial decomposition (Schoell, 1980; Coleman et al., 1981). These gases also differ intensely in composition. While thermogenic natural gas contains a balanced mixture of methane and higher alkanes, its biogenic counterpart has 99% methane purity. Another factor, effecting methane composition, is that approximately 20% of natural gas stores are biogenic (Rice and Claypool, 1981). Other ways in which fossil fuels may produce methane include the combustion of higher alkanes and the incomplete combustion of methane itself.

Abiogenic methane may be produced within the Earth's crust as the result of a high temperature reduction reaction between hydrogen and carbon dioxide (Schoell, 1980). This newly formed gas is then released under high pressure through volcanoes, geothermal vents or via “pockmarks” in the ocean floor. These “pockmarks” have fairly recently been found in many locations (Hovland and Judd, 1988). The continental shelf itself is dotted with these depressions. This phenomenon is formed by expulsion of fluids that often contain methane. Juhl and Taghon (1993) also observed that methane concentrations were higher downstream from a seep (relative to upstream area), thus suggesting a potential source of methane to the marine environment. This is known as thermogenic (or abiogenic) methane (see Chapter 1) but is thought to be of little importance in terms of global atmospheric methane when compared to biogenic processes (Welhan and Craig, 1979). This may be due to rapid bacterial oxidation within the water column (Faber, 1994).

Methane sources in the marine environment are largely biogenic (Scranton et al., 1993) and being a non-conservative gas, its concentration, distribution and fluxes in the water column are determined by marine processes (Seiler and Schmidt, 1974). Research on methane cycling has suggested that freshwater wetlands and rivers are much more important than the marine environment, due to bacterial substrate competition (Scranton et al., 1993). However the concentrations and possible turnover rates of methane in the marine environment are still significant due to the potential of the ecosystems to act as both a source and sink for this gas (Conrad and Seiler, 1988; Jones, 1991). Methane which does not encroach upon the atmosphere has succumbed to bacterial consumption by methane-oxidising bacteria which may decompose methane by either aerobic (Rudd, 1980) or anaerobic oxidation (Alperin and Reeburgh, 1984).

3.1.1 The methane paradox

‘Typical’ oceanic methane profiles show a pronounced maximum concentration, associated with the pycnocline (Swinnerton et al., 1969; Brooks and Bernard, 1981; Seiler and Schmidt, 1974; Scranton and Brewer, 1977; Scranton and Farrington, 1977; Traganza et al., 1979; Ward et al., 1987, 1989; Owens et al., 1991) (figure 3.0). The upper part of the water column has been found to be generally highly supersaturated with methane, with respect to the atmosphere, which may indicate that the worlds oceans may be an important source of methane to the atmosphere (Lamontagne et al., 1973; Sieburth et al., 1987; Sieburth, 1991; Owens et al., 1991). This excess methane found in the surface waters suggests that some other source of methane, rather than the atmospheric source must be considered.