Characterisation of marine and lacustrine sediments in a drowned thermokarst embayment, Richards Island, Beaufort Sea, Canada

A multidisciplinary study was made of lakes and lagoons in a thermokarst em bayment on Richards Island, Canadian Beaufort Sea, in order to investigate the response of the Mackenzie Delta shoreline to changing hydrological infl uences. Seismo-acoustic profiles made from an amphibious vehicle, combined with sediment core data, show that the deepest lakes contain up to 10 m of transparent or stratified silty mud overlying the Kittigazuit sand basement . Palynological and microfossil data from 21 surface samples allow delineat ion of freshwater, saltmarsh, tidal flat and subtidal assemblages. These mo dern reference data allow distinction of freshwater and marine facies in th e cores and subsequent interpretation of geochemical content (carbon, ammon ia, sulphate). Radiocarbon dates and Cs-137 show that sedimentation rates h ave increased by an order of magnitude since drowning of the embayment duri ng the past millennium, and that most of this sediment is from the Mackenzi e River plume.