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STEADYSED1 is a multicomponent reactive transport code for steady-state early diagenesis which fully incorporates the reaction couplings among the elements, C, O, N, S, Fe, and Mn. The model is tested against extensive datasets collected by Canfield et al. (1993a,b) at three coastal marine sites that exhibit high rates of combined iron and manganese (hydr)oxide reduction. It is shown that the model provides a consistent explanation of the entire body of multicomponent multisite observations. The measured concentration profiles of twenty-eight individual porewater and solid sediment species are satisfactorily reproduced. Furthermore, the model predicts the observed distributions of sulfate reduction rates, as well as diagnostic features of the porewater pH profiles. The coupled nature of the reactive species imposes stringent constraints when fitting model-calculated distributions to the data, because a single set …