The influence of bioturbation on geochemical cycles at Blakeney salt marsh, Norfolk: an analysis using lipid biomarkers

Abstract

A redox dichotomy is seen in the pond sediments of the salt marshes in East Anglia, Norfolk, U.K. At Blakeney salt marsh, ponds contain two sediment types: bioturbated sediments are iron-rich and contain burrowing organisms, while unbioturbated sediments are sulfidic with no evidence of burrowing organisms. Such end-member pond types with distinct sediment geochemistry occur despite ponds being located within metres of each other. Differences in pond geochemistry have been linked to the presence or absence of bioturbation, due to the ventilation and reworking of water and sediment in the substratum. In this study, the sediments of one bioturbated and one unbioturbated pond were collected and analysed through the extraction of lipid biomarkers. Biomarkers, the traces of previously living organisms, which can be found in sediments, as well as rocks and petroleum, can be used to look for similarities or differences between bioturbated and unbioturbated sediments, which may further be linked to the activities of organisms as well as sediment geochemistry. This study employed a multi-proxy approach, by analysing fatty acids, GDGTs, and polar lipids. As such, the contributions of algae, bacteria, higher plants, higher animals, and archaea were considered using these biomarkers. The delivery and burial of organic matter was evaluated using bulk organic carbon data, with potential organic matter sources further investigated using compound-specific stable isotope analysis (CSIA) of fatty acid methyl esters (FAME). The influence of burrowing organisms is seen in the biomarker results of the bioturbated pond sediments. Organic matter delivery to both ponds is similar, however the processing of organic matter in the substratum differs between the pond types. Microbial activity is seen in the upper 10 cm of the bioturbated pond, while aeration and ventilation linked with burrowing is evident in the uniform vertical profiles of GDGT and polar lipid concentrations, as well as the depth profile of organic carbon content. With no burrowing organisms in the unbioturbated pond sediments, organic matter instead accumulates and is degraded.