GFZ German research centre for geo sciences

Global Sediment Cycles

Quantifying global sediment cycles is an important basis for understanding how landscapes develop and how quickly they change. We follow the path of sediments from their formation in the mountains through the process of erosion to their deposition in the lowlands, where they form fertile soils, and in the oceans, where nutrients are released.

Two central questions in our research are 1) how much sediment is produced over periods of thousands of years in the mountaineous highlands by erosion and how much of it is channelled through the lowlands and reaches the ocean, and 2) how long the sediment is stored in the lowlands, as it can change its chemical composition during storage. Cosmogenic nuclides are our most important tool for answering these questions. With "in situ" produced 10Beryllium (10Be) we measure the denudation rates in river sediments (question 1), and with meteoric 10Be we can determine the storage time of sediment in the lowlands. Applied on a global scale and in deep time from geological archives of large drainage systems such as the Amazon or the Ganges or in the oceans, globally relevant sediment turnover rates and their integration period can thus be determined.

The measurement of in situ-produced cosmogenic nuclides in river sediments makes it possible to determine the erosion rate over a large area for entire river catchment areas. The long-term sediment production in the Amazon Basin determined in this way amounts to approx. 0.6 billion tonnes. Within the limits of uncertainty, this number is almost identical to the amount of sediment from suspended sediment measurements that is transported into the Atlantic. The Amazon is therefore not really a storing sediment today. Surprisingly, there are also indications that the amount of sediment appears to fluctuate only slightly with major climate changes or with the increase in land use caused by humans. Researchers suspect that the Amazon's large flood plain buffers these fluctuations in sediment transport over the various time scales - the Amazon could therefore be described as the world's largest shock absorber.

To determine the storage time of the sediment, we measure meteoric 10Be, the same nuclide as in situ 10Be, except that it is produced by cosmogenic radiation in the atmosphere instead of in the mineral itself. The increase in the concentration of meteoric 10Be along river systems from the erosisonal areas in the mountains to the floodplain is proportional to the storage time. In addition, an erosion rate can be measured from the meteoric 10Be concentration measured in the delivery area. Together with the stable 9Be, a trace element that is released during the weathering of rock, a denudation rate can be calculated using the 10Be(meteoric)/9Be ratio. We define denudation as the sum of physical and chemical weathering. Meteoric 10Be is therefore a very versatile tracer for earth surface processes, which we are constantly developing further.

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