Holocene alluvial sediment storage in a small river catchment in the loess area of central Belgium

Abstract

Soil erosion and sediment deposition widely affect landscape development, particularly in erosion-prone areas with loessderived soils. Nevertheless, until now, few attempts were made to quantify soil losses and sediment storage over long (centennial or millennial) timescales. In this study, the Holocene alluvial sediment storage in a small river catchment (52 km 2 ) of the Belgian loess belt is estimated, and a preliminary sediment budget for the catchment is presented.In the valley of the Nethen River (c. 13 km long), a detailed survey of the alluvial sediment archive was conducted. Hand augerings and percussion drillings were made along cross-valley transects at 12 locations in the catchment. AMS 14 C dating of peat samples provided a temporal framework for the interpretation of the cores.Results show that the thickness of Holocene sediment deposits in the Nethen valley is 4 to 6m, which corresponds to a total clastic sediment mass of ∼14 × 10 6 t stored in the valley bottom. Three alluvial units could be distinguished and associated with deposition phases from 9600 to 2900 B.C., 2900 B.C. to A.D. 1000 and A.D. 1000 to present. In contrast to the older sediments (units 1 and 2), deposits from the last 1000 year (unit 3) contain little organic matter. They are seldom intercalated with peat layers, and devoid of tufa. Unit 3 reaches a thickness of c. 2m, thereby representing 50% of the Holocene sediment mass stored in the alluvial plain. The mean sedimentation rate in the alluvial plain for this last phase is ∼26t ha − 1 a − 1 , which is about tenfold larger than the sedimentation rates calculated for the older Holocene sediment units. Sediment supply towards the alluvial plain has therefore increased tremendously since Medieval times.These results are in contrast to dating results obtained for colluvial sediments in a nearby dry valley within the catchment of the Nethen, where soil erosion and sediment deposition started in the early Iron Age and was already substantial during the Roman Age. This means that there is a time lag of about one millennium between the onset of high sedimentation rates in the upstream area and high deposition rates in the alluvial plain. This is probably caused by a change in coupling (sediment connectivity) between the plateau, slopes, and rivers. As soil erosion proceeds, first the dry zero-order valleys in the catchment act as sediment traps, and only after these are filled sediment reaches the floodplains. The preliminary sediment budget for the Nethen catchment illustrates that 50% of the sediment that was eroded during the Holocene was stored in colluvial deposits, which are mainly located on footslopes and in dry valley bottoms. Another 29% of the sediment mass is stored in the alluvial plain.

Fluvial terrace formation in the northern Upper Rhine Graben during the last 20000 years as a result of allogenic controls and autogenic evolution

“…Large-scale deforestation during the Bronze Age, Iron Age, and Roman times in the hinterland (e.g., Friedmann, 2000;Singer, 2004;Lechner, 2005;Bos et al, 2008) resulted in significant changes in sediment availability. Initially, this led to increased deposition on hillslopes as colluvium (e.g., Lang and Hönscheidt, 1999;Lang, 2003) and in smaller catchments upstream as alluvium, which is quantified by several studies (e.g., Lang and Nolte, 1999;Houben, 2003;Rommens et al, 2006;De Moor, 2007;Seidel and Mäckel, 2007). In the following Iron Age, human-induced sedimentation is also recognised in the trunk valley, for example in the southern Upper Rhine Graben (Mäckel and Friedmann, 1999;Mäckel et al, 2003;Seidel and Mäckel, 2007).…”

Streams, Valleys and Floodplains in the Sediment Cascade

“…accretion only with a large sample and error analysis (Rommens et al, 2006;Trimble, 2009). Long trenches (e.g.…”

Deposition and fate of organic carbon in floodplains along a tropical semiarid lowland river (Tana River, Kenya)

“…The OC that is deposited with sediments may therefore be stored for several centuries, if not millennia in floodplain environments. Studies in the colluvial and alluvial stores in temperate environments suggest that OC may indeed be preserved over considerable periods but that a large fraction of the buried OC will eventually be mineralized [Galy et al, 2007;Rommens et al, 2006;Van Oost et al, 2012]. This mineralization process may take a long time.…”