The meteoric cosmogenic nuclide Beryllium-10 and 10Be/9Be ratios as tracer of Earth surface processes

We can now utilise natural clocks in the form of the in-situ produced cosmogenic radionuclides like 10Be. Concentrations of these nuclides provide denudation rates. Another variety of cosmogenic nuclides, meteoric 10Be, is an excellent geochemical tracer of the way water from precipitation interacts with solid matter. When used in conjunction with its stable counterpart 9Be, we combine the advantage of meteoric 10Be, delivered at roughly uniform rates, and the 9Be flux, being higher the more 9Be is released by weathering. The 10Be/9Be ratio traces the depth at which plant roots take up their trace metals.

 

Meteoric Beryllium-10 is a cosmogenic nuclide that is produced in the atmosphere and delivered to the Earth’s surface by dry and wet deposition. There is no cosmogenic Beryllium-10 present in the lithosphere because of radioactive decay (T1/2 = 1.39 My). When the Earth surface is shaped by erosion and weathering, meteoric Beryllium-10 accumulates from precipitation. Meteoric Beryllium-10 is adsorbed to minerals and can, in contrast to its “sister” nuclide in situ - produced Beryllium-10, be determined on any fine-grained sediment, independent of the presence of quartz minerals. Its concentration is inversely proportional to the erosion rate, so that erosion rates can be measured (ref 1).

However, besides erosion, the concentration of meteoric Beryllium-10 in soil material and river sediment depends on many variables, like the grain size of the adsorbing particles, the pH of associated solutions, or the water discharge. A further development of the method by our group avoids these effects by normalising over the stable isotope Beryllium-9. The stable isotope Beryllium-9 is released from primary minerals during weathering of bedrock, where it is present in concentrations of a few ppm in most rocks. In the weathering zone (critical zone: from unweathered bedrock up to the top of the vegetation cover) both isotopes mix to a characteristic rate. Beryllium can leave the critical zone either in the dissolved form in stream waters or adsorbed to minerals and transported as bedload or suspended load in rivers. The combination of these isotopes, meteoric Be-10 that is delivered to the Earth’s surface at roughly constant flux and stable Be-9 whose flux depends on the rate of weathering, serves as an ideal proxy to study erosion and weathering. Under certain assumptions the 10Be/9Be ratio taken up or adsorbed by weathering products (reac) or dissolved in river water (diss) is equal to the atmospheric flux of meteoric 10Be (F10Bemet) divided by the denudation rate D (erosion rate plus weathering rate) times the parent rock’s 9Be concentration (9Beparent) times the fraction of 9Be released from primary minerals and partitioned into the reactive or dissolved phase (ref 2):

References

Willenbring, J. K.; von Blanckenburg, F. (2010): Meteoric cosmogenic Beryllium-10 adsorbed to river sediment and soil: applications for Earth-surface dynamics. Earth-Science Reviews, 98, 1-2, 105-122.

von Blanckenburg, F., Bouchez, J., Wittmann, H. (2012): Earth surface erosion and weathering from the 10Be (meteoric)/9Be ratio. Earth and Planetary Science Letters, 351-352, 295-305.

Wittmann, H.; von Blanckenburg, F.; Bouchez, J.; Dannhaus, N.; Naumann, R.; Christl, M.; Gaillardet, J. (2012): The dependence of meteoric 10Be concentrations on particle size in Amazon River bed sediment and the extraction of reactive 10Be/9Be ratios. Chemical Geology, 318-319, 126-138.

Hella Wittmann-Oelze
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Dr. Hella Wittmann-Oelze
Earth Surface Geochemistry
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