Mineralization of refractory carbons in soils and sediments: inference on the carbon biogeochemical cycle.
Résumé
Black Carbons (BC=soots and charred particles) and Geologic Carbon (GC) form a cluster which can be defined as Refractory Carbon (RC), by considering their inertia to the degradation. Despite their occurring in the different surface carbon reservoirs, their contribution to the carbon biogeochemical cycle is still poorly known. Admitting their inertia, RC could be considered as a carbon sink. But they are also subject to mineralization processes in soils, outcropping sediments and waters, and can be assimilated to a source of atmospheric CO2. A review is here presented in order to appreciate their weathering ability and subsequently their role of sink and/or source of carbon. Oxidative alteration of CR leads successively to the formation of highly aromatic humic compounds and to the release of CO2 [1,2]. However, mineralization rate depends on the nature of CR and on some various factors (chemical structural, temperature, duration of exposition). So, the schematic classification "soots-charcoals-GC", can be established and reflects a progressive increase in the weathering potential. Up to now, few studies mention the mineralization degree of BC (one mentions 60% in a turbidite [3]). On the other hand, numerous studies of weathering profiles of GC exist and can be interpreted in terms of loss of organic carbon (mineralization). For example, a degree of mineralization of 20 / 25% for bituminous coals, and even 60 / 100% for black shales have been calculated [4, 5]. Recently, we have examined two weathering profiles of semi-anthracite and estimated that 30 / 45% of GC have been mineralised during this process enduring since 10ky [6]. This overview involves that the mineralization degree of RC is not really known and that further studies are required in order to get a better knowledge of their contribution to the carbon cycle as sink or source of atmospheric CO2. [1] : Haumaier L., Zech W. 1995. Org. Geochem., 23, 191-196. [2] : Copard Y. 2002. Doctoral thesis., Univ.Orléans, 305p. [3] : Middelburg J. J., Nieuwenhuize J., Van Breugel P. 1999. Mar. Chem., 65, 245-252 [4] : Lo H. B., Cardott B. J. 1995. Org. Geochem., 22, 73-83. [5] : Petsch S. T., Berner R. A., Eglinton T. I. 2000. Org. Geochem., 31, 475-487. [6] : Copard Y. Disnar J. R., Becq-Giraudon J. F. 2002. Inter. J. Coal Geol., 49, 57-65.