Nils Broothaerts

Legacy of human-induced C erosion and burial on soil–atmosphere C exchange

Carbon exchange associated with accelerated erosion following land cover change is an important component of the global C cycle. In current assessments, however, this component is not accounted for. Here, we integrate the effects of accelerated C erosion across point, hillslope, and catchment scale for the 780-km2 Dijle River catchment over the period 4000 B.C. to A.D. 2000 to demonstrate that accelerated erosion results in a net C sink. We found this long-term C sink to be equivalent to 43% of the eroded C and to have offset 39% (17–66%) of the C emissions due to anthropogenic land cover change since the advent of agriculture. Nevertheless, the erosion-induced C sink strength is limited by a significant loss of buried C in terrestrial depositional stores, which lagged the burial. The time lag between burial and subsequent loss at this study site implies that the C buried in eroded terrestrial deposits during the agricultural expansion of the last 150 y cannot be assumed to be inert to further destabilization, and indeed might become a significant C source. Our analysis exemplifies that accounting for the non–steady-state C dynamics in geomorphic active systems is pertinent to understanding both past and future anthropogenic global change.

Fluvial architecture of Belgian river systems in contrasting environments:implications for reconstructing the sedimentation history

Accurate dating is necessary to get insight in the temporal variations in sediment deposition in floodplains. The interpretation of such dates is however dependent on the fluvial architecture of the floodplain. In this study we discuss the fluvial architecture of three contrasting Belgian catchments (Dijle, Geul and Ambleve catchment) and how this influences the dating possibilities of net floodplain sediment storage. Although vertical aggradation occurred in all three floodplains during the last part of the Holocene, they differ in the importance of lateral accretion and vertical aggradation during the entire Holocene. Holocene floodplain aggradation is the dominant process in the Dijle catchment. Lateral reworking of the floodplain sediments by river meandering was limited to a part of the floodplain, resulting in stacked point bar deposits. The fluvial architecture allows identifying vertical aggradation without erosional hiatuses. Results show that trends in vertical floodplain aggradation in the Dijle catchment are mainly related to land use changes. In the other two catchments, lateral reworking was the dominant process, and channel lag and point bar deposits occur over the entire floodplain width. Here, tracers were used to date the sediment dynamics: lead from metal mining in the Geul and iron slag from ironworks in the Ambleve catchment. These methods allow the identification of two or three discrete periods, but their spatial extent and variations is identified in a continuous way. The fluvial architecture and the limitation in dating with tracers hampered the identification of dominant environmental changes for sediment dynamics in both catchments. Dating methods which provide only discrete point information, like radiocarbon or OSL dating, are best suited for fluvial systems which contain continuous aggradation profiles. Spatially more continuous dating methods, e.g. through the use of tracers, allow to reconstruct past surfaces and allow to reconstruct reworked parts of the floodplain. As such they allow a better reconstruction of past sedimentation rates in systems with important lateral reworking.

Sensitivity of floodplain geoecology to human impact: A Holocene perspective for the headwaters of the Dijle catchment, central Belgium

Floodplain deposition rates have increased markedly under influence of human impact throughout the late Holocene in many western and central European catchments. Consequently the geomorphology and ecology of many floodplains changed. In this study we discuss this human impact and its influence on the floodplain geoecology during the middle and late Holocene for the headwaters of the Dijle catchment, located in the Belgian loess belt. The floodplain geoecology and the regional vegetation was reconstructed from sedimentological and palynological analyses. An age–depth model for the studied sequences was obtained using 17 radiocarbon dates. Statistical analyses of the pollen data (cluster analysis and canonical correspondence analysis) were used to detect changes in the pollen record. Our data show that until c. 2500 cal. BP, human impact was nearly absent or localized with no discernible influence on the floodplain geoecology. The floodplain was in a stable phase and consisted of a marshy environment where organic material could accumulate, which is interpreted as the natural state of the floodplain. From c. 2500 cal. BP onwards, human impact gradually increased. However, only when human impact in the catchment crossed a threshold around 500 cal. BP, the floodplain geoecology changed with clearing of the Alder carr forest, the establishment of a single channel river and the dominance of minerogenic overbank sedimentation. Spatial variability in the coupling between increasing human impact and changes in floodplain geoecology can be attributed to differences in hillslope–floodplain connectivity and local differences in human impact.

River Landscapes in the Dijle Catchment: From Natural to Anthropogenic Meandering Rivers

In this chapter the changing river landscapes in the Dijle catchment since the Late Glacial are discussed in terms of climatic and anthropogenic changes. Climatic changes in the Late Glacial triggered the incision of large meanders in Weichselian braided-river deposits in the downstream part of the study area, which still form one of the most prominent landforms in this region. During the early Holocene period, peat accumulation, which initiated in the now-abandoned meanders, characterized floodplains lacking a clearly defined river channel. The landscape was covered with a deciduous forest, while floodplains were mainly covered with an alder carr forest. Although agriculture started in the region in the Neolithic (~6200 cal BP), it is only from the Bronze Age onwards that anthropogenic deforestation on the hilly loess slopes caused an increase in soil erosion and sediment delivery. In the upstream part of the Dijle catchment, this resulted in accelerated floodplain sedimentation that halted peat growth and covered the Late Glacial landforms entirely. Rivers developed again a meandering channel pattern with much smaller amplitude compared to the Late Glacial meanders, whilst also levees and colluvial fans were formed. During the past 250 years, many of these channels were artificially or naturally cut off, and they remain visible in the current floodplain as oxbow lakes or shallow depressions. In this chapter we demonstrate how this evolution is the result of a change from a climate-dominated to a human-dominated fluvial landscape.

Widespread global peatland establishment and persistence for the last 130,000 years

Claire C. Treat, Thomas Kleinen, Nils Broothaerts, April S. Dalton, René Dommain, Thomas A. Douglas, Judith Drexler, Sarah A. Finkelstein, Guido Grosse, Geoff Hope, Jack Hutchings, Miriam C. Jones, Peter Kuhry, Terri Lacourse, Outi Lähteenoja, Julie Loisel, Bastiaan Notebaert, Richard Payne, Dorothy Peteet, A. Britta K. Sannel, Jonathan M. Stelling, Jens Strauss, Graeme T. Swindles, Julie Talbot, Charles Tarnocai, Gert Verstraeten, Christopher J. Williams, Zhengyu Xia, Zicheng Yu, Minna Väliranta, Martina Hättestrand, Helena Alexanderson, Victor Brovkin