Fate and behaviour of marine organic matter during burial of anoxic sediments: Testing CH2O as generalized input parameter in reaction transport models

• Organic matter composition is a major control in reaction transport models.
• The C-cycle of anoxic marine sediments during burial is modelled.
• The often used “CH2O” in reactive transport models is valid as an approximation.
• The modelled marine C-cycle is determined by the pre-assigned organic matter type.
• The nominal oxidation state of input organic C controls carbon species distribution.

The precise modelling of organic matter degradation is key in unravelling the marine carbon cycle and to constrain methane and carbonate formation within sediments. So far, different model approaches for calculating organic matter degradation have been developed. The term “organic matter” is considered in different ways in the diverse models and the simplification “CH2O” is often used. In this work, we test the common approach of “CH2O” as a simplification for total degradable organic matter. The focus of our study is on shallow to 500 m deep anoxic marine sediments. We performed hydrogeochemical modelling in different ocean drilling program (ODP) case studies with single organic compounds and with a mixture of organic compounds that considers the most reactive compounds in marine sediments. The modelling results indicate that the distribution of carbon species (carbonates, methane, CO2, and alkalinity) generated by the degradation of organic carbon is dependent on the type of organic matter that is given in the model for remineralization. The closer the nominal oxidation state of the organic carbon that is remineralized in the model calculations is to the oxidation state of carbon in methane (− 4), the more methane is produced and less carbonates (carbon oxidation state of + 4) precipitate. Simulation results suggested that the commonly and often used “CH2O” as initial input in reaction transport models is a valid simplification of complex organic matter. However, the better the model input parameters are constrained, the better will be the conclusions on the marine carbon cycle and methane formation.