Calculating Rates and Dates and Interpreting Contaminant Profiles in Biomixed Sediments

Abstract

Abstract Contaminant profiles in well-dated sediment cores represent extremely valuable natural archives of environmental contamination, by which contaminant sources, history and cycling may be determined and contaminant concentrations in surface sediments projected into the future. However, most marine and estuarine sediments are mixed by benthic organisms to a depth of at least 2–20 cm, which introduces significant risks of misinterpreting and misapplying these archives if biomixing is not explicitly taken into account. This chapter offers a step-by-step guide to avoiding common pitfalls and appropriately applying biomixed sediment archives to reconstructing contaminant inputs to the environment. By the use of simple models, radioisotope tracers (210Pb, 137Cs) may still be applied to establish geochronologies in biomixed cores and to validate the sedimentation rates and dates derived. Having obtained (and validated) sedimentation and mixing rates for bio- mixed cores, contaminant profiles may then be interpreted through the further use of models in a way that explicitly accounts for mixing. As with any sediment core, the main caution when one attempts to infer contaminant releases from deposition histories reconstructed from biomixed sediment cores are the unknown possible impacts of environmental variation and change. Keywords