GFZ German research centre for geo sciences

Cosmogenic Nuclides

Cosmogenic nuclides can be used to date landscape forms such as moraines or river terraces. In river sediment or soils, they provide information about the erosion rates of river catchment areas or how quickly soils form through the weathering of rock.

Cosmogenic nuclides, such as the radioactive isotopes 10Beryllium (Be) and 26Aluminium (Al), are nuclides formed by cosmogenic radiation. Cosmogenic radiation consists of high-energy galactic particles that constantly collide with molecules in our atmosphere and cause a cascade of so-called secondary radiation. So-called "meteoric" cosmogenic nuclides are also produced in the atmosphere during these collisions. Some of the radiation reaches the Earth's surface, where cosmogenic nuclides are formed in rocks. This type of nuclide is called "in situ"-produced nuclides. The place of formation (atmosphere versus in rocks or soils) is therefore different, but in principle the same nuclides are formed (e.g. meteoric "10Bemet" versus terrestrial "10Beinsitu"). However, meteoric nuclides are formed at a much higher production rate - up to 1 million atoms of 10Be per cm2. In rocks, the production rate of 10Beinsitu is many times lower - only a few atoms per year! It also usually requires a type of mineral - typically quartz. Quartz is common in the Earth's crust, but some rocks, such as volcanic basalt or limestone, have no quartz at all. The low production rate of "in situ" nuclides is due to the fact that the secondary radiation only penetrates into the upper few metres of the earth's crust before it is completely absorbed by interactions with the rock. The observation period of the method depends on how quickly this upper layer is removed to the absorption depth. This observation period therefore varies depending on the speed of erosion and is several millennia. This long integration period primarily records the natural erosion rates that existed before human intervention in nature.

The integration period is similar for meteoric nuclides because they are transported to the earth's surface by dust and rain, where they adsorb to sediment particles and then enter the soil with water. Due to this adsorption and the higher production rate, the measurement of meteoric nuclides is also possible in water, plants or very small quantities (< 1 g) of rock or soil, whereas "in situ" nuclides require much larger quantities of sediment (several grams of quartz). It is important to note that 10Be, for example, due to its half-life of 1.4 million years, can no longer originate from the early Earth period as this early nuclide has decayed. The stopwatch for these isotopes therefore only starts ticking again with cosmic radiation, i.e. when erosion exposes upper soil layers.

We continue to develop these types of methods in our section...

Our hobbyhorse is the development of a new method based on the ratio of 10Bemet to 9Be (10Bemet/9Be), the stable part of the element beryllium. 9Be occurs as a trace element in water, rock, soil or sediment. It is released during the chemical weathering of rock. Cosmogenic meteoric 10Be is added from the atmosphere with an inbuilt stopwatch, allowing us to determine erosion and weathering rates that are completely independent of rock or mineral type. We are currently developing the 10Bemet/9Be method for basalt and carbonate because basalt in particular can remove a lot of CO2 from the atmosphere during weathering. The weathering of carbonate also represents a short-term CO2 sink. Knowledge of the rates of weathering is essential for making accurate climate predictions.

 

back to top of main content