Meteoric ¹⁰Be/⁹Be in Carbonates: GFZ

Meteoric ¹⁰Be/⁹Be in Carbonates

The Lison spring in the French Jura mountains. Shaft height and width at the spring outlet approx. 10x10 m. *(Photo: H. Wittmann-Oelze)*

Karst in the Jura *(Photo: H. Wittmann-Oelze)*

Doubs Spring, Jura *(Photo: H. Wittmann-Oelze)*

Meteoric ¹⁰Be, produced in the atmosphere, can be used to quantify denudation rates (the sum of weathering and erosion) when normalized with the stable trace element ⁹Be. We measure this ¹⁰Be/⁹Be ratio in river sediments, soils, but also in water or even plants.

In this project, we have developed this new method for carbonate rock. We have chosen the Jura Mountains in the NE of France as a natural “laboratory” because runoff data and water chemistry are well known here, with which our new method can be compared.

Grafik — Fig. 1: Left/A: The ¹⁰Be/⁹Be method on silicate rock. Right/B: the method modified for carbonate rock. *(Figure: H. Wittmann-Oelze)*

Carbonate systems are more complex than the weathering of silicate rocks (e.g. basalt, Fig. 1A) because various new phases (e.g. secondary carbonates, Fig. 1B) can form by precipitation from the water, but these have nothing to do with weathering. Such precipitation is controlled by supersaturation of the water with Ca²⁺ cations, which are washed out of the rock. As soon as CO₂ (e.g. bound to roots in soils or at springs from air) is available, CaCO₃ (calcium carbonate) precipitates. At springs, the precipitation product is called “travertine”.

To validate our new method, we quantified secondary carbonate formation using ¹³C/¹²C (d¹³C). By measuring d¹³C on rock, sediment, soil and, above all, water, we can calculate what proportion comes from the rock (i.e. primarily formed calcium carbonate) and how much secondary carbonate has precipitated from the water. We have also calculated how much ⁹Be comes from silicate clays. Such silicate phases can be found in every carbonate rock.

Our results show that despite this complexity, the new ¹⁰Be/⁹Be proxy gives very similar denudation rates for soils to those from solute loads in rivers calculated from water chemistry measurements. A direct comparison shows that soils erode at about 300 t/km²*yr, but rates measured from sediment at springs are somewhat higher. We assume that subsurface weathering also occurs as these sediments are channeled through the karst system. Weathering rates from water chemistry, however, could only reflect the weathering in soils, because Ca²⁺ concentrations are set there.

Project Time Frame

since 2017