Assessing the anthropogenic impact on soil redistribution processes using radionuclides (10Be, 210Pb, 137Cs and 7Be) : a case study from the Christina River Basin, USA

Anthropogenic driven soil erosion has increased drastically within the last few centuries. Accelerated removal of topsoil degrades soil characteristics, decreasing the agricultural capacity to sustain food production for future human generations. In order to reduce sediment erosion to a minimum, soil movement dynamics have to be understood over relevant timescales. Emphasis is required on human land use, which has changed over time, altering erosion and deposition processes in the landscape. This thesis aims to understand the development of erosion with time and studies the effects of the anthropogenic impact on the Earth’s surface. A case study was conducted in the Christina River Basin in south-eastern Pennsylvania (USA) to evaluate the anthropogenic impact on sediment redistribution before and during European colonisation of the US East Coast. The Christina River Basin is focus of the Christina River Basin Critical Zone Observatory, where researchers of many disciplines (e.g. hydrology, geochemistry, geology, biology, geomorphology, soil science) study the interactions of the water, mineral and carbon cycle in relation to different land uses. This site is particularly suitable for this study due to the sudden change in land use with the arrival of European Settlers and the well-recorded history of land use in the area since. Besides sediment processes related to the European colonisation, special emphasis was placed on the last century and the particular impact of land management practices on the landscape. For the approach of this study, upland and depositional sediment profiles, as well as suspended sediment collected from rivers at different times were analysed for radionuclide activity. In particular, a combination of the short-lived radionuclides 7Be, 210Pb, and 137Cs with the long-lived meteoric nuclide 10Be was used to allow detection of sediment erosion as well as tracing of sediment source to streams over different time frames. Natural sediment processes were studied by investigating meteoric 10Be in different upland and valley hollow soil profiles in the Christina River Basin. Natu-ral sediment erosion rates of 17 to 18 mm per 1000 years and soil residence time of 26,000 to 57,000 years in upland sites classify the region to be characterised by slowly eroding soils. Valley hollow study sites indicate an alteration in sediment supply due to climate change within the last 80,000 years. Analysis of meteoric 10Be on suspended sediment in rivers with time (from pre-colonial to present times) and a comparison to the historical background of land use management indicated drastic changes in the sediment sources to the streams during the colonisation of the US East Coast. Such profound changes in the soil movement dynamics can be tied to a complete deforestation of the watershed during that time. However, a slow recovery of sediment sources to pre-colonial conditions within the last 100 years was observed, which may be related to the afforestation in parts of the watershed and change in farming practises. Examination of floodplains in recent history (last 100 years) using 210Pb and 137Cs identified a correlation between changes in land use and floodplain development. In particular, deforestation led to channel migration within the watershed, whereas afforestation reduced the flood magnitude. A discrimination of different sediment sources (agricultural and forest surface, channel migration) was detected on present suspended sediment by using a combination of tracers: 7Be, 137Cs, 210Pb and 10Be. About 50 % of sediment in the stream was attributed to channel migration classifying the stream as relatively unstable, whereas the second major sediment contribution was agricultural soil (32 %). As both sources are closely related to human activity in the study area, appropriate land management practices and stream channel stabilisation are of great importance for a hydro- and morphodynamic balance in future times. Results presented in this study provide evidence of the powerful impact anthropogenic land use has had on sediment movement in the Christina River Basin. More importantly, this study has shown the usefulness of combining long- and short-lived radionuclides to identify soil redistribution at different time periods and scales. Radionuclide data does not only reflect the drastic impact during European colonisation but also detects gradual changes that occur due to human action to limit soil erosion within the last century. This study indicates that some of the best management practices may minimise anthropogenic induced soil erosion, whilst simultaneously signifying the need to further improve land use management and reduce sediment erosion.