A numerical model for early diagenetic processes in the sediments of the north-western continental shelf of the Black Sea is presented. The north-western shelf area is influenced by nutrient, organic matter and reactive iron inputs from major rivers such as the Danube, Dnieper and Dniester. Low oxygen concentrations in the bottom water and high carbon deposition rates have resulted in a dominance of anoxic mineralization processes including manganese, iron and sulphate reduction. The model includes six solid phase variables (labile and less labile organic carbon, iron-and manganese oxides, iron monosulphide and pyrite) and eight solutes (oxygen, nitrate, ammonium, manganese, ferrous iron sulphate, hydrogen sulphide and methane). The cycling of iron and sulphur is explicitly incorporated in the model since these elements play an important role in the mineralization of organic matter on the continental shelf. The model is capable of performing both steady state and dynamic simulations and can be coupled to biogeochemical models for water-column processes. The output of the model is composed of depth profiles of the state variables and their biogeochemical reaction rates and fluxes of the solute substances across the sediment-water interface. The model has been applied to solid-phase and porewater data from a station near the Danube delta. A steady state stimulation reasonably reproduced the main features, but not the distribution of iron and the concentrations of oxygen and total organic carbon in the first 2cm. A dynamic simulation revealed that a pulse flux of organic matter resulting from the spring bloom could be responsible for the deviation from steady state in the near-surface distribution of total organic carbon and oxygen. The effect of carbon loading on organic matter degradation pathways has been investigated. There is a gradual increase in the relative importance of anoxic mineralization pathways as carbon loadings increase, but the transitions from manganese-based respiration to iron-based respiration and from iron-based respiration to sulphate reduction are rather abrupt. The first jump is due to the positive feedback between manganese reduction coupled to ferrous iron oxidation and iron oxide reduction. The second swap is due to the formation of iron sulphides. This will consume part of the reduced iron, which is consequently not available for re-oxidation to iron oxide, and the iron-reducing bacteria become iron-oxide limited. The cycling efficiency of iron changes abruptly, and a larger part of the iron will be fixed as iron sulphides. [KEYWORDS: diagenetic modelling; iron cycling, sulphur cycling; Black Sea continental shelf]
Original languageEnglish
Pages (from-to)403-421
JournalEstuarine Coastal and Shelf Science
Issue number3
StatePublished - 2002

ID: 222797