Jef Vandenberghe

Timescales, climate and river development

Abstract In the long term (glacial/interglacial sequences), river development follows general climatic evolution. However, at the scale of one cold-warm cycle, it appears that the geomorphological effect of river activity is best expressed at times of climatic change. Such periods of pronounced fluvial action last only a short time. In more detail, the internal evolution in a drainage basin after a climatic event results in a delayed response of the morphology and sedimentation pattern. Finally, on the shortest timescale local thresholds may play a decisive role in the reactions of river systems. It is concluded that the relation between fluvial development and climate is not a simple one, but is fundamentally dependant on the timescale.

Asian monsoons in a late Eocene greenhouse world

The strong present-day Asian monsoons are thought to have originated between 25 and 22 million years (Myr) ago, driven by Tibetan–Himalayan uplift. However, the existence of older Asian monsoons and their response to enhanced greenhouse conditions such as those in the Eocene period (55–34xa0Myr ago) are unknown because of the paucity of well-dated records. Here we show late Eocene climate records revealing marked monsoon-like patterns in rainfall and wind south and north of the Tibetan–Himalayan orogen. This is indicated by low oxygen isotope values with strong seasonality in gastropod shells and mammal teeth from Myanmar, and by aeolian dust deposition in northwest China. Our climate simulations support modern-like Eocene monsoonal rainfall and show that a reinforced hydrological cycle responding to enhanced greenhouse conditions counterbalanced the negative effect of lower Tibetan relief on precipitation. These strong monsoons later weakened with the global shift to icehouse conditions 34xa0Myr ago.

Linking coarse silt production in Asian sand deserts and Quaternary accretion of the Chinese Loess Plateau

The Chinese Loess Plateau (CLP) is a large, spatially well defined and persistent zone of loess accumulation developed near the fluctuating northwest margin of the East Asian monsoon. Many studies have analyzed its loess sediments to provide insights into paleoclimatic conditions. Although spatial and temporal variations in the grain sizes of CLP sediments are fundamental to this effort, controversy over the origin of the dominant coarse quartz silt has limited interpretations. Reexamination of the spatial pattern of grain-size distribution across the CLP and a field-scale experiment conducted in the Gobi Desert revealed a genetic association between the coarse silt fraction of the loess and primary production of coarse silt through eolian abrasion of sand in the proximal Mu-Us, Tengger, and Badain Jaran sandy deserts. Our results demonstrate the effectiveness of eolian abrasion of quartz sand in primary coarse silt production in Central Asia and identify this process as the most consistent with the well-recognized systematic northwest-southeast depositional pattern of the CLP. We suggest that only abraded coarse quartz grains transported short distances by long-term persistent eolian activity can build up thick loess sequences to form a massive and spatially well defined loess plateau. These results decouple the production and transport of coarse silt and finer silt and clay particles, which have a more distant and wider provenance, changing the constraints on previous paleoclimatic reconstructions.

Aeolian process and climatic changes in loess records from the northeastern Tibetan Plateau: Response to global temperature forcing since 30 ka

The response of surface processes to global climatic changes since the last glacial is critical to understanding the mechanism of climatic changes on the Tibetan Plateau. In this study, loess from the northeastern Tibetan Plateau (NETP) was closely spaced dated to provide an independent high-resolution record of dust accumulation processes and millennial-scale climatic changes. Optically stimulated luminescence ages reveal episodes of rapid dust deposition at approximately 12.3u2009ka, 16u2009ka, 21–23u2009ka, 25–28u2009ka, and possibly also at 30–33u2009ka. These episodes are broadly correlated with high-latitude and marine records cold climatic events, such as the Younger Dryas, Heinrich event 1, and the Last Glacial Maximum. This correlation implies that dust storms in the NETP represent episodic aridification and wind strengthening, which is ascribed to the southward shift of the polar front and the intensification of the Siberian high-pressure cell that was forced by millennial-scale cooling in high northern latitudes.

Climatic and tectonic controls on the fluvial morphology of the Northeastern Tibetan Plateau (China)

The geomorphological evolution of the Northeastern Tibetan Plateau (NETP) could provide valuable information for reconstructing the tectonic movements of the region. And the considerable uplift and climatic changes at here, provide an opportunity for studying the impact of tectonic and monsoon climate on fluvial morphological development and sedimentary architecture of fluvial deposits. The development of peneplain-like surface and related landscape transition from basin filling to incision indicate an intense uplift event with morphological significance at around 10–17 Ma in the NETP. After that, incision into the peneplain was not continuous but a staircase of terraces, developed as a result of climatic influences. In spite of the generally persisting uplift of the whole region, the neighbouring tectonic blocks had different uplift rates, leading to a complicated fluvial response with accumulation terraces alternating with erosion terraces at a small spatial and temporal scale. The change in fluvial activity as a response to climatic impact is reflected in the general sedimentary sequence on the terraces from high-energy (braided) channel deposits (at full glacial) to lower-energy deposits of small channels (towards the end of the glacial), mostly separated by a rather sharp boundary from overlying flood-loams (at the glacial-interglacial transition) and overall soil formation (interglacial). Pronounced incision took place at the subsequent warm-cold transitions. In addition, it is hypothesized that in some strongly uplifted blocks energy thresholds could be crossed to allow terrace formation as a response to small climatic fluctuations (103–104 year timescale). Although studies of morpho-tectonic and geomorphological evolution of the NETP, improve understanding on the impacts of tectonic motions and monsoonal climate on fluvial processes, a number of aspects, such as the distribution and correlation of peneplain and the related morphological features, the extent and intensity of tectonic movements influencing the crossing of climatic thresholds, leading to terrace development, need to be studied further.

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.