Regional-scale lake-sediment sampling and analytical protocols with examples from the Geological Survey of Canada

Abstract

Regional-scale lake sediment surveys have been successfully used since the 1970s as a means for reconnaissance geochemical exploration. Lake sediment sampling is typically performed in areas with a lack of streams and an overabundance of small-sized (=5 km across) lakes. Lake sediments are known to have major, minor and trace element concentrations that reflect the local geology. Overall, lake sediment surveys are planned and conducted following four distinct stages: 1) background research, 2) orientation survey, 3) regional survey, and 4) detailed survey. At the Geological Survey of Canada, samples are usually collected from a helicopter with floats. Sample density ranges from 1 sample per 6 - 13 km2. Samples are collected from the centre of the lake using a gravity torpedo sampler which corresponds to a hollow-pipe, butterfly bottom-valved sampler attached by a rope to the helicopter. Collected sediment samples are then placed in labelled bags and left to air dry. Detailed field notes and additional samples (field duplicates), for the purpose of an adequate quality assurance and quality control program, are also taken. Samples are then milled and sent to analytical laboratories for element determination. Commonly used analytical methods include: X-ray fluorescence (XRF), atomic absorption spectroscopy (AAS), inductively coupled plasma-atomic emission spectrometry (ICP-AES) and -mass spectrometry (ICP-MS), instrumental neutron activation analysis (INAA), and/or determination of volatile compounds and organic carbon using Loss on Ignition (LOI). Analytical data is first evaluated for quality (contamination, accuracy and precision). Numerous options for the analysis of lake sediment data exist, ranging from simple basic element concentration maps and statistical graphical displays together with summary statistics, to employing multivariate methodologies, and, more recently, using machine learning algorithms. By adopting the set of guidelines and examples presented in this manual, scientific researchers, exploration geologists, geochemists and citizen scientists will be able to directly compare lake sediment datasets from anywhere in Canada.