An isotope falling from the sky. New method for quantifying sediment load in rivers

Sampling of Amazon River sediment for cosmogenic nuclide analysis (photo: F. von Blanckenburg, GFZ).

11.12.2015: With a new geochemical method the quantification of sediment load in rivers is easier than before. An international team of researchers under the lead of Dr. Hella Wittmann-Oelze, GFZ section Earth Surface Geochemistry, introduces this methodology in the Journal of Geophysical Research taking the example of the Amazon River.

On a geologic timescale, large rivers transport enormous amounts of suspended sediments and sand to the oceans. The quantification of such mass fluxes over long time scales is important for assessing the variability in sediment load linked to climate change or changes in land use. A new geochemical method on the basis of a cosmogenic isotope of Beryllium (10Be) now simplifies this long-term quantification of sedimentary loads and their variability.

Cosmogenic nuclides, which are isotopes of an element produced from bombardment of cosmic rays, allow for the quantification of sediment loads over geological meaningful time scales. For several years, researchers at GFZ have been using the “in situ”-produced variety of 10Be that is produced in the mineral quartz of the sediment itself to constrain erosion and sediment fluxes of drainage basins. However, one disadvantage of this in situ method is that large amounts of sampling material are needed, from which quartz minerals are tediously separated using time-consuming mineral separation procedures.

A newly developed geochemical method reduces the amount of sampling material needed to less than half a gram of fine-grained sediment of any composition, such that no quartz is needed anymore. The solution is provided by the “sister” nuclide of the in situ 10Be measured on quartz sand: meteoric 10Be, the nuclide that "falls from the sky" - i.e. that is produced in the atmosphere and deposited on Earth’s surface from rain and dust. This meteoric 10Be adsorbs on the surface of any fine-grained material and can be extracted using geochemical techniques. Together with the trace element 9Be, contained in many minerals, and using mass balance approaches, weathering and erosion rates of Earths surface can be determined. Sediment fluxes of drainage basins up to the size of the Amazon basin that integrate over several thousand years can therefore be measured and can be compared to present-day, highly variable sediment export.

This new erosion and weathering proxy has now successfully been tested in Earth’s largest sand factory- the Amazon basin. Hardly any change in sediment load is detected from the Andes to the Atlantic, resulting in no net sediment deposition observed and very little weathering taking place in the Amazon floodplain itself.

H. Wittmann, F. von Blanckenburg, N. Dannhaus, J. Bouchez, J. Gaillardet, J.L.Guyot, L. Maurice, H. Roig, N. Filizola, M. Christl, 2015. A test of the cosmogenic 10Be(meteoric)/9Be proxy for simultaneously determining basin-wide erosion rates, denudation rates, and the degree of weathering in the Amazon basin. Journal of Geophysical Research – Earth Surface, Advance Online Publication, doi:10.1002/2015JF003581