River sediment geochemistry as a conservative mixture of source regions: Observations and predictions from the Cairngorms, UK

Alex George Lipp , Gareth G Roberts , Charles J. B. Gowing, Alexander C Whittaker, Victoria Fernandes <...  more

Alex George Lipp , Gareth G Roberts , Charles J. B. Gowing, Alexander C Whittaker, Victoria Fernandes

The elemental composition of sediments in rivers is the product of physical and chemical erosion of rocks, which is then transported across drainage networks. A corollary is that fluvial sedimentary geochemistry can be used to understand geologic, climatic and geomorphic processes. We develop a simple methodology to predict elemental compositions of river sediments from digital elevation data and geochemical maps using erosional models. We test these models using a new sedimentary geochemical dataset from carefully chosen sample sites. Sediment compositions are predicted by formally integrating eroding substrates with known compositions acros...  more

The elemental composition of sediments in rivers is the product of physical and chemical erosion of rocks, which is then transported across drainage networks. A corollary is that fluvial sedimentary geochemistry can be used to understand geologic, climatic and geomorphic processes. We develop a simple methodology to predict elemental compositions of river sediments from digital elevation data and geochemical maps using erosional models. We test these models using a new sedimentary geochemical dataset from carefully chosen sample sites. Sediment compositions are predicted by formally integrating eroding substrates with known compositions across drainage basins. Different parameterisations of erosional models, including the Stream Power formulation and uniform incision rates, are tested. Substrate chemistry was determined from the G-BASE geochemical survey. Predictions are tested using a new suite of compositions obtained from fine grained (<150 um) sediments at 67 sites along the Spey, Dee, Don, Deveron and Tay rivers, Cairngorms, UK. Results show that sedimentary geochemistry can be predicted using simple models that include the topography of drainage networks and substrate compositions as input. Our predictions in this location are insensitive to the choice of erosional model, which we suggest is a consequence of broadly homogeneous rates of erosion throughout the study area. Principal component analysis of the river geochemical data suggests that the composition of most Cairngorms river sediments can be explained by mafic/felsic provenance and conservative mixing downstream. Successful prediction of sedimentary geochemistry suggests that inverting the composition of ancient sedimentary rock might allow quantitative reconstruction of specific past environmental conditions.