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Late Quaternary morphodynamics in East Asia
Quaternary International 263 (2012) 1–2
Contents lists available at SciVerse ScienceDirect
Quaternary International journal homepage: www.elsevier.com/locate/quaint
Guest Editorial
Late Quaternary morphodynamics in East Asia
During the EGU Meeting in Vienna in 2010, Taiwanese and German scientists organised the session GM5.4 “Late Quaternary morphodynamics in East Asia”. The convener and co-conveners were Margot Boese (Berlin), Jiun-Chuan Lin (Taipei), Chia-Hung Jen (Kaohsiung), Christopher Lüthgens (Berlin), and Dirk Wenske (Berlin). We invited scientists from East Asia to join the session, and we decided to prepare a volume about this topic for Quaternary International. The title includes a wide timespan of more than 100 ka, with a great variety of climate and subsequently changing geomorphological processes. Topography, rock properties and, in some areas, also seismicity modify the exogenetic impact, and – last but not least – increasing human activity has influenced the morphodynamics in East Asia during the Holocene. We can make no general statement for morphodynamical processes for any timeslice of the Late Quaternary because coastal areas, for example, react differently from high mountain areas, and even loess deposits and sandy aeolian accumulations are affected in different ways by climate change and/or human-induced processes. The contributions in this volume are parts of a puzzle contributing to the many studies about geomorphological processes in time and space. The papers in this volume represent studies in very disparate environments in East Asia. To start at sea level, the paper by Dörschner et al. gives an insight into the Late Holocene development of a coastal dune area in northeast Taiwan. The timeframe is set by OSL data, but the processes are still incompletely understood because the complicated interferences of varying sea level stands, tectonic uplift, climate and singular natural events have hitherto been only partly understood. This relatively small area is unique in northeast Taiwan with its aeolian dune ridges; the timing of processes is of special interest as this coastal environment directly neighbours the construction site of a nuclear power plant to the north, located on the Pacific side of the island. Landslides are in general a prominent research topic in Taiwan because its high mountain relief, tropical storms and high seismicity, coupled with uplift rates up to 8 mm/y, intensify denudation processes. Recent slope processes are analysed in a case study from NE Taiwan in the paper by Wenske, Jen, Böse and Lin. Here, for the first time, not only satellite and air photos served as the basis for tracing slope failures during the past decades, but also seasonal measurements with a total station and terrestrial laser scans over three years on selected landslide positions gave detailed information about the processes on the surface of those landslides and the quantity of sediments eroded during the summer typhoon season and the winter season. Though human impact – here mainly 1040-6182/$ – see front matter Ó 2012 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2012.02.024
road construction – amplifies hillslope sliding processes, most of the landslides are related to fluvial processes by a direct coupling of the slope with the river bed. In most parts of the high mountains of Taiwan – except in some west-draining valleys in altitudes above 2800 m – the valley bottom is identical with the river bed; therefore hillslope processes and fluvial activity are often directly connected with each other. Landslides are often, but not exclusively, linked with tropical storms because they can also occur during the winter rainy season, as this paper shows. Data of Late Quaternary sediments are available in Taiwan but have not yet been elaborated into a stratigraphical scheme. Geomorphological interpretations based on fluvial terrace morphostratigraphy date back to the middle of the last century and are still used as a stratigraphical framework. Pollen data for a record of the late Pleistocene provide information for palaeoclimatic interpretations, but these results are not yet linked to possible changes in geomorphological processes. Systematic process analyses deriving from sediments are still lacking, but the ongoing progress of dating methods is starting to enable work on a general timeframe. The paper of Wenske, Frechen, Böse, Reimann, Tseng and Hoelzmann reports about sparse remnants of fluvial terraces which have been found in the very destructive high mountain environment, and the deposits have been mapped and dated in selected places at about 1400–1500 m asl. Besides granulometric, chemical and mineralogical analyses of the fine material, these terrace sediments have been dated for the first time by various OSL protocols and radiocarbon dating. The most astonishing sedimentological fact is a layer of silt which is not likely to have formed under present-day conditions in this environment. This silt layer – dated to the middle of the last glacial cycle – may indicate a dry phase with aeolian processes. Hence an interfingering of fluvial, aeolian and colluvial sediments in the terrace is postulated, indicating severe climatic changes. Preservation conditions of the terrace remnant as well a possible connection with the studied and dated terraces in the foreland of the mountain belt are discussed. From Taiwan, with its very special situation of strong tectonic activity and the sometimes catastrophic rainfall events during typhoons, we turn to the loess plateau in China, which shows evidence of very different morphodynamical processes of the more continental climate and effects of the circulation pattern, especially the varying monsoon. A study by Long, Lai, Fuchs, Zhang and Yang in the Qinwangchuan Basin at the western edge of the loess plateau north of Lanzhou refers to an area with strong aeolian impact. Dune sand up to 20 m thick, intercalated into loess sediments, is interpreted as evidence for a phase – dated by OSL to
2
Guest Editorial / Quaternary International 263 (2012) 1–2
the later part of MIS 3 – of either drier conditions or stronger winds with varying sediment supply: a factor that, in my opinion, should not be underestimated because sand dunes also formed in humid climate preconditioned enough sand supply was possible from areas which were, at least seasonally, without water (like dry river beds) or vegetation cover. A stronger seasonality may be one of the clues. The discussion of the results in comparison with apparently contradictory results from lake highstands and nearby palaeolakes during MIS 3a reflects the still sometimes incompatible results from different archives and the hitherto partial understanding of the whole system, including short-term changes in monsoonal wind activity and the resulting processes. The study by Huang, Pang, Zha, Su, and Zhou refers to a study site further southeast on the loess plateau, and its timeframe goes back even to the mid-Pleistocene. Erosional features such as gullies are studied in an area where agriculture has been practised for the past 8000 years, so a long-lasting human impact has also influenced the present-day morphological appearance of the landscape. A hierarchical system of steep-flanked gullies offers insight into the loess-palaeosol sequences. In the outcrops, palaeogullies can be
identified, which are interpreted in relation to the palaeosols. The palaeorelief therefore provides the basis for interpreting the variation of geomorphological processes (times with or without gully incision), mainly influenced by climate in the sense of monsoon variability. The authors relate gullying in former times to an increase in moisture due to strengthening of the maritime summer monsoon. Further impulses for gully erosion by tectonic uplift are discussed, and tectonics is considered to be an obligatory second factor. In general, the mutual influences of climatic changes on different spatial scales, tectonics and, if applicable, human impact are still a challenge to be identified by the resulting geomorphological processes. Margot Böse Freie Universität Berlin, Dept. of Earth Sciences, Malteserstr. 74-100, D-12249 Berlin, Germany E-mail address: [email protected] Available online 16 February 2012
Contents lists available at SciVerse ScienceDirect
Quaternary International journal homepage: www.elsevier.com/locate/quaint
Guest Editorial
Late Quaternary morphodynamics in East Asia
During the EGU Meeting in Vienna in 2010, Taiwanese and German scientists organised the session GM5.4 “Late Quaternary morphodynamics in East Asia”. The convener and co-conveners were Margot Boese (Berlin), Jiun-Chuan Lin (Taipei), Chia-Hung Jen (Kaohsiung), Christopher Lüthgens (Berlin), and Dirk Wenske (Berlin). We invited scientists from East Asia to join the session, and we decided to prepare a volume about this topic for Quaternary International. The title includes a wide timespan of more than 100 ka, with a great variety of climate and subsequently changing geomorphological processes. Topography, rock properties and, in some areas, also seismicity modify the exogenetic impact, and – last but not least – increasing human activity has influenced the morphodynamics in East Asia during the Holocene. We can make no general statement for morphodynamical processes for any timeslice of the Late Quaternary because coastal areas, for example, react differently from high mountain areas, and even loess deposits and sandy aeolian accumulations are affected in different ways by climate change and/or human-induced processes. The contributions in this volume are parts of a puzzle contributing to the many studies about geomorphological processes in time and space. The papers in this volume represent studies in very disparate environments in East Asia. To start at sea level, the paper by Dörschner et al. gives an insight into the Late Holocene development of a coastal dune area in northeast Taiwan. The timeframe is set by OSL data, but the processes are still incompletely understood because the complicated interferences of varying sea level stands, tectonic uplift, climate and singular natural events have hitherto been only partly understood. This relatively small area is unique in northeast Taiwan with its aeolian dune ridges; the timing of processes is of special interest as this coastal environment directly neighbours the construction site of a nuclear power plant to the north, located on the Pacific side of the island. Landslides are in general a prominent research topic in Taiwan because its high mountain relief, tropical storms and high seismicity, coupled with uplift rates up to 8 mm/y, intensify denudation processes. Recent slope processes are analysed in a case study from NE Taiwan in the paper by Wenske, Jen, Böse and Lin. Here, for the first time, not only satellite and air photos served as the basis for tracing slope failures during the past decades, but also seasonal measurements with a total station and terrestrial laser scans over three years on selected landslide positions gave detailed information about the processes on the surface of those landslides and the quantity of sediments eroded during the summer typhoon season and the winter season. Though human impact – here mainly 1040-6182/$ – see front matter Ó 2012 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2012.02.024
road construction – amplifies hillslope sliding processes, most of the landslides are related to fluvial processes by a direct coupling of the slope with the river bed. In most parts of the high mountains of Taiwan – except in some west-draining valleys in altitudes above 2800 m – the valley bottom is identical with the river bed; therefore hillslope processes and fluvial activity are often directly connected with each other. Landslides are often, but not exclusively, linked with tropical storms because they can also occur during the winter rainy season, as this paper shows. Data of Late Quaternary sediments are available in Taiwan but have not yet been elaborated into a stratigraphical scheme. Geomorphological interpretations based on fluvial terrace morphostratigraphy date back to the middle of the last century and are still used as a stratigraphical framework. Pollen data for a record of the late Pleistocene provide information for palaeoclimatic interpretations, but these results are not yet linked to possible changes in geomorphological processes. Systematic process analyses deriving from sediments are still lacking, but the ongoing progress of dating methods is starting to enable work on a general timeframe. The paper of Wenske, Frechen, Böse, Reimann, Tseng and Hoelzmann reports about sparse remnants of fluvial terraces which have been found in the very destructive high mountain environment, and the deposits have been mapped and dated in selected places at about 1400–1500 m asl. Besides granulometric, chemical and mineralogical analyses of the fine material, these terrace sediments have been dated for the first time by various OSL protocols and radiocarbon dating. The most astonishing sedimentological fact is a layer of silt which is not likely to have formed under present-day conditions in this environment. This silt layer – dated to the middle of the last glacial cycle – may indicate a dry phase with aeolian processes. Hence an interfingering of fluvial, aeolian and colluvial sediments in the terrace is postulated, indicating severe climatic changes. Preservation conditions of the terrace remnant as well a possible connection with the studied and dated terraces in the foreland of the mountain belt are discussed. From Taiwan, with its very special situation of strong tectonic activity and the sometimes catastrophic rainfall events during typhoons, we turn to the loess plateau in China, which shows evidence of very different morphodynamical processes of the more continental climate and effects of the circulation pattern, especially the varying monsoon. A study by Long, Lai, Fuchs, Zhang and Yang in the Qinwangchuan Basin at the western edge of the loess plateau north of Lanzhou refers to an area with strong aeolian impact. Dune sand up to 20 m thick, intercalated into loess sediments, is interpreted as evidence for a phase – dated by OSL to
2
Guest Editorial / Quaternary International 263 (2012) 1–2
the later part of MIS 3 – of either drier conditions or stronger winds with varying sediment supply: a factor that, in my opinion, should not be underestimated because sand dunes also formed in humid climate preconditioned enough sand supply was possible from areas which were, at least seasonally, without water (like dry river beds) or vegetation cover. A stronger seasonality may be one of the clues. The discussion of the results in comparison with apparently contradictory results from lake highstands and nearby palaeolakes during MIS 3a reflects the still sometimes incompatible results from different archives and the hitherto partial understanding of the whole system, including short-term changes in monsoonal wind activity and the resulting processes. The study by Huang, Pang, Zha, Su, and Zhou refers to a study site further southeast on the loess plateau, and its timeframe goes back even to the mid-Pleistocene. Erosional features such as gullies are studied in an area where agriculture has been practised for the past 8000 years, so a long-lasting human impact has also influenced the present-day morphological appearance of the landscape. A hierarchical system of steep-flanked gullies offers insight into the loess-palaeosol sequences. In the outcrops, palaeogullies can be
identified, which are interpreted in relation to the palaeosols. The palaeorelief therefore provides the basis for interpreting the variation of geomorphological processes (times with or without gully incision), mainly influenced by climate in the sense of monsoon variability. The authors relate gullying in former times to an increase in moisture due to strengthening of the maritime summer monsoon. Further impulses for gully erosion by tectonic uplift are discussed, and tectonics is considered to be an obligatory second factor. In general, the mutual influences of climatic changes on different spatial scales, tectonics and, if applicable, human impact are still a challenge to be identified by the resulting geomorphological processes. Margot Böse Freie Universität Berlin, Dept. of Earth Sciences, Malteserstr. 74-100, D-12249 Berlin, Germany E-mail address: [email protected] Available online 16 February 2012