2022 CSDMS meeting-058


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Generating geochemical maps from river sediment samples: an inverse modelling approach

Alex Lipp, (He/Him),Imperial College London London , United Kingdom. a.lipp18@imperial.ac.uk
Gareth Roberts, Imperial College London , United Kingdom.
Alexander Whittaker, Imperial College London , United Kingdom.
Charles Gowing, British Geological Survey , United Kingdom.
Patrice De Caritat, Geoscience Australia , Australia.

[[CSDMS meeting abstract::Geochemical maps are an essential geoscience data product used for generating environmental baselines, identifying mineral deposits, and aiding wider geologic understanding. However, surveying large continental-scale areas remains challenging as there is a perception that large numbers of samples must be gathered and analysed. This issue has resulted in only a small portion of Earth’s surface being mapped geochemically at any scale. We resolve this challenge by presenting a methodology to map large areas by unmixing the composition of sediment contained within rivers or floodplains, which integrate large upstream areas [1]. Our method works by modelling sediment geochemistry, assuming it to be a conservative mixture of the geochemistry upstream. Next, we invert this model by seeking the distribution of upstream geochemistry that best fits a suite of observations downstream. This inversion is performed using an optimisation algorithm and by imposing spatial smoothness. We test the method in the Cairngorms, UK, where the upstream geochemistry is very well constrained by the independent G-BASE geochemical survey [2]. We gather 67 fine-grained sediment samples from carefully selected sample sites along five rivers draining the Cairngorms. We then use these samples, along with the inverse methodology, to produce geochemical models (maps) of the region. Comparing these maps to the independent survey data we find that they successfully reconstruct the source region chemistry for a range of elements, but with up to 100 times fewer samples. Having validated the approach, we next apply it to alluvial geochemical data gathered as part of the Northern Australia Geochemical Survey [3]. The maps produced from this dataset compare favourably against published lithological maps. We conclude that this novel methodology is an effective and practical approach to analyse river sediment geochemistry, which allows continental-scale areas to be surveyed efficiently and accurately.


  1. Lipp, A. G., et al. (2021), Geochemistry, Geophysics, Geosystems; DOI: 10.1029/2021GC009838
  2. Johnson, C. C., et al. (2005), Geochemistry: Exploration, Environment, Analysis; DOI: 10.1144/1467-7873/05-070
  3. Bastrakov, E. N. & Main, P. T. (2020), Exploring for the Future, Extended Abstracts; DOI: 10.11636/134367]]