Coal weathering and the geochemical carbon cycle

One of the key unsolved questions in the geochemical carbon cycle concerns the weathering rate of sedimentary organic matter in the continental surficial environment. Sedimentary organic carbon is mainly in the form of kerogen, or finely disseminated, solvent-insoluble organic matter. It has been believed that the overall weathering rate of kerogen is controlled by erosion which is in turn controlled by uplift and tectonism. Therefore complete oxidation to carbon dioxide of kerogen exposed to Earth's surficial environments has become one of the major assumptions in the studies of geochemical cycles. The rate of kerogen weathering is especially important because it is one of the major controls on atmospheric oxygen level through geologic time.; In this study laboratory experiments were performed to understand the chemistry of the aqueous oxidation of sedimentary organic matter in the form of coal. Coal was used as a substitute for kerogen because of the difficulty in obtaining pyrite-free kerogen for laboratory study. Two major topics of this study are, first, quantification of the effects of physical and chemical conditions on the oxidation kinetics of coal and second, determination of the chemical nature of the oxidation products in terms of CO{dollar}sb2,{dollar} dissolved organic carbon (DOC) and solid oxidation products.; The aqueous oxidation rate obtained in the present study for air saturated water (270{dollar}mu{dollar}M O{dollar}sb2){dollar} is approximately {dollar}rm2times10sp{lcub}-12{rcub}moles Osb2/msp2/sec{dollar} at 25{dollar}spcirc{dollar}C. The reaction order with respect to oxygen level is 0.46 to 0.48 on a several thousand hours time scale for both 24{dollar}spcirc{dollar}C and 50{dollar}spcirc{dollar}C experiments. Activation energies were found to be 39-43 KJ/mole O{dollar}sb2{dollar} in the 24-50{dollar}spcirc{dollar}C range indicating that the oxidation reaction is surface reaction controlled. CO{dollar}sb2{dollar} production rate is equal to 30 to 50% of O{dollar}sb2{dollar} consumption rate. This means that 30 to 50% of the oxygen consumed is eventually used for CO{dollar}sb2{dollar} production. A drastic increase in oxygen containing functional groups (mainly carboxylic group, esters and other carbonyl groups) was detected in the oxidized coal indicating substantial amounts of solid oxidation products were formed and retained on the coal surface. The results of batch experiments performed in aerated water show an increase of DOC with time (up to 7 years) which is interpreted as the dissolution of these oxidation products. Therefore, the oxidation products leaving coal, and probably kerogen, during sub-aqueous weathering are in the form of both CO{dollar}sb2{dollar} and DOC.