The Global Carbon Cycle with Seafloor Methane

Jerry Dickens
Rice University, Houston, Texas ([email protected])

Abstract

Large quantities of methane occur as dissolved gas, gas hydrate and free gas in the pore space of sediment along continental margins. This methane is habitually omitted from discussions of carbon cycling, despite obvious fluxes to and from the ocean. At the most basic level, carbon enters the seafloor methane cycle through the breakdown of solid organic compounds, and leaves as dissolved bicarbonate, solid carbonate or methane.

Organic carbon landing on the seafloor passes through a gauntlet of microbially mediated reactions during burial. In regions with a sufficiently high flux of organic carbon, large quantities reach a methanogenic zone where approximately equal portions of dissolved bicarbonate and methane are produced. The bicarbonate is enriched in ¹³C while the methane is extremely depleted in ¹³C. With continual production of methane, gas concentrations can surpass solubility conditions to precipitate gas hydrate or free gas, depending on local pressure, temperature and salinity conditions. Gas hydrate overlies free gas in many deep ocean settings because of the geotherm. Carbon cycles between dissolved gas, gas hydrate and free gas through several pathways, including burial of all three phases, upward advection of dissolved gas and free gas, diffusion of dissolved gas, dissociation of gas hydrate, and dissolution of gas hydrate. Carbon leaves the seafloor methane cycle as bicarbonate or methane, the latter of which can involve reaction with sulfate in the sediment or venting into the water column. Excess production of bicarbonate, from either methane production or consumption, can lead to formation of authigenic carbonate, which provides another means to remove carbon.

All of the fluxes depend on external conditions such as temperature or seawater chemistry. Given that oceanographic conditions have changed significantly over time, it is speculated that the amount and fluxes of various components of the seafloor methane cycle have also varied through time substantially.

AAPG Search and Discovery Article #90086 © 2008 AAPG Foundation Distinguished Lecturer Series 2008-2009