Denudation and weathering rates from meteoric 10Be/9Be: A versatile tool in mafic, carbonate or slate-dominated lithologies from the critical zone to very large catchments 

Abstract

Meteoric 10Be is a cosmogenic nuclide produced in the atmosphere and delivered to Earth’s surface by precipitation, whereas stable 9Be is a trace metal released from bedrock during weathering. The ratio (10Be(meteoric)/9Be) is set in the critical zone, where beryllium is either adsorbed to fine-grained sediment or is dissolved in water if the pH-dependent retentivity of Be is low. A prerequisite for calculating denudation and weathering rates over millennial timescales is knowledge of the flux of 10Be deposition to Earth surface and the concentration of 9Be in the unweathered parent bedrock. We provide a review of applications of the 10Be/9Be system to Earth surface studies that covers a variety of quartz-bearing rocks, lithologies largely void of quartz like mafic rocks, mixed lithologies, and, most recently, carbonate and marble lithologies. We show the versatility of this new approach in several case studies. These range from small creek-scale mafic basins in the European midlands, medium-sized (102-103 km2) catchments covered by slate and marble in the rapidly eroding Taiwan Island, pure carbonate rocks from the French Jura Mountains, to large catchments draining mixed lithologies in the Chilean Andean Cordillera and the Amazon and Ganga river basins. We find that i) across all spatial scales, the dissolved and sediment-bound 10Be/9Be ratios agree well, indicating equilibrium of phases. Approximately at medium-sized catchment scales, ii) the 9Be concentration from published databases from typical parent rocks (http://earthref.org/GERM) represents the local bedrock 9Be concentration, but for much smaller and carbonate catchments, a locally representative bedrock concentration has to be determined on a case-by-case basis; and iii) local measurements of the depositional 10Be flux from the 10Be inventory of independently-dated deposits like e.g. terrace profiles appear to yield the most reliable estimates. Under these constraints, derived denudation and weathering rates agree in all settings to within less than a factor of 2 with either decadal-scale denudation rates from combined suspended and dissolved river fluxes or with in situ-10Be-derived denudation rates where quartz is present. This close agreement shows that 10Be/9Be ratios from such rock types yield meaningful denudation rates obtained from sub gram-sized amounts of sediment, independent of the spatial scale of sampling ranging from a single soil profile to the scale of the Amazon river basin. Small sample amounts and the independence of the presence of quartz provide a benefit over the “sister” nuclide in situ 10Be that has been widely used in landscapes of felsic rocks. Now, with 10Be/9Be, the determination of rates of Earth surface change in quartz-poor lithologies opens up entirely new opportunities.