Modelling the holocene sediment budget of the Rhine system

Abstract

Rivers transport large amounts of water, sediments, nutrients and carbon from the continents to the oceans. Thus, they are important links within the global biogeochemical cycle. To understand biogeochemical fluxes in river channels, holistic system-based approaches are needed that consider river channels and their corresponding catchments. Sediment fluxes in fluvial systems change in consequence of changing external controls (land use and climate). However, the system's response to land use and climate change varies depending on internal controls (e.g. catchment size and structure). While forcing-response mechanisms of small catchments are reasonably well understood, the response of larger drainage basins is less clear. In particular, the impact of land use and climate change on the Rhine system is poorly known owing to the catchment size (185 000 km²) and the long history of human cultivation, which started approx. 7500 years ago.<br /> A sediment budget is calculated to specify the amount of alluvial sediment and total organic carbon that deposited during the Holocene and to estimate long term soil erosion rates. The focus was driven to floodplains because they act as important sinks in terms of sediment and carbon flux and therefore, provide a range of potential sites of palaeoecological data. To obtain information on the temporal development of the Rhine system, a database of 14C-ages taken from colluvial and alluvial deposits was compiled and analysed in terms of i) cumulative frequency distributions of the ages and ii) changing sedimentation rates on floodplains and in palaeochannels.<br /> The results of the sediment budget suggest that 59±14 10^9 t of Holocene alluvial sediment is stored in the non-alpine part of the Rhine catchment (South and Central Germany, Eastern France, The Netherlands). About 50% of Holocene alluvial sediment is deposited along the trunk valley and the delta (Upper Rhine, Lower Rhine, coastal plain), while the rest is stored along the tributary valleys. The floodplain sediment storage corresponds to a mean erosion rate of 0.55±0.16 t/ha/year (38.5±10.7 mm/kyr) across the Rhine catchment outside the Alps. This Holocene-averaged estimate amounts for sediments that were delivered to the channel network and is at the lower limit of erosion rates from other studies of different methodology.<br /> The statistical analysis of 1948 organic carbon measurements in different parts of the Rhine catchment suggest a strong influence of the sedimentary facies on the organic carbon content. The analysis allowed the development of a conceptual carbon budget model of fluvial systems, which was coupled with the alluvial sediment storage, to estimate the Holocene sequestration rates of carbon storage in floodplains. Averaged over the Rhine catchment the sedimentary carbon sequestration ranges between 3.4 to 25.4 g m²/year with more reasonable values between 5.3 to 17.7 g m²/year. Compared to the recent particulate carbon export, these values are in the same order of magnitude but somewhat smaller indicating that approximately the same amount of the exported carbon may be stored in floodplains. However, compared to sedimentary carbon sequestration rates obtained elsewhere, the presented values are at the lower limit, corresponding to the lower mean Holocene soil erosion and floodplain accumulation rates.<br /> Based on the cumulative frequency distributions of the 14C-ages eight periods of geomorphic activity are identified (peaking at 8.2 kyr, 7.54 kyr, 5.6 kyr, 4.2 kyr, 3.3 kyr, 2.8 kyr, 2.3 kyr and since 1075 years BP). These periods were compared with climatic, palaeohydrological and human impact proxy data. Until 4200 years BP, events of geomorphic activity are mainly coupled to wetter and/or cooler climatic phases. Between 3300 and 2770 years BP, the increased geomorphic activity cannot unequivocally be related to climate. The growing population and the intensification of agricultural activities must be considered as an additional control during the Bronze age. Since 1075 years BP the growing population density is considered as the major external forcing. Additionally, it could be shown that the newly developed approach has major advantages compared to the approach used to analyse the 14C-database of Great Britain, Spain and Poland.<br /> Concerning the changing sedimentation rates on floodplains and in palaeochannels, three phases were identified, during the last 14 000 years: i) the Late Glacial-Holocene with medium sedimentation rates, ii) the Holocene Climatic Optimum with slightly decreasing rates and iii) the last 4000 years, which are characterised by increasing sedimentation rates. The results are in good correspondence with a conceptual model of the Holocene floodplain development in Europe.<br /> Until now, the spatially distributed sediment and carbon budgets and the temporal analysis of the $^{14}$C-database are analysed based on linear relationships of causes and effects. However, to understand the nonlinear response of the Rhine system to changes of land use and climate impacts during the Holocene, it is necessary to couple the temporal and spatial approaches that were developed in this thesis.