| 1. |
- Abbuehl, Luca M., et al.
(författare)
-
El Niño forcing on 10Be-based surface denudation rates in the northwestern Peruvian Andes?
- 2010
-
Ingår i: Geomorphology. - : Elsevier BV. - 0169-555X .- 1872-695X. ; 123:3-4, s. 257-268
-
Tidskriftsartikel (refereegranskat)abstract
- High magnitude precipitation events provide large contributions to landscape formation and surface denudation in arid environments. Here, we quantify the precipitation-dependent geomorphic processes within the Rio Piura drainage basin located on the Western Escarpment of the northern Peruvian Andes at 5 degrees S latitude. In this region, monsoonal easterly winds bring precipitation to the >3000 m asl high headwaters, from where the annual amount of precipitation decreases downstream toward the Pacific coast. Denudation rates are highest in the knickzones near the headwaters, similar to 200-300 mm ky(-1), and sediment discharge is limited by the transport capacity of the channel network. Every few years, this situation is perturbed by westerly, wind-driven heavy precipitation during El Nino events and results in supply-limited sediment discharge as indicated by bedrock channels. The detailed analysis of the stream-long profiles of two river basins within the Rio Piura catchment reveals a distinct knickzone in the transition zone between the easterly and westerly climatic influences, suggesting an En Nino forcing on the longitudinal channel profiles over at least Holocene timescales. Measured trunk stream catchment-wide denudation rates are up to ca. 300 mm ky(-1) and decrease successively downstream along the river profiles. Denudation rates of tributary rivers are ca. 200 mm ky(-1) near the plateau and show a stronger downstream decreasing trend than trunk stream rates. This suggests that the landscape is in a transient stage of local relief growth, which is driven by fluvial incision. This corroborates the results of paleoclimate studies that point towards higher El Nino frequencies during the past ca. 3000 years, leading to higher runoff and more erosion in the trunk channel compared to the hillslopes and thus growth of local relief. Downstream increases in channel gradient spatially coincide with the reaches of highest precipitation rates during El Nino events, we therefore interpret that Holocene landscape evolution has largely been controlled by climate. The ky-timescale of the Be-10 data together with the transience of the landscape implies that El Nino events in northwestern Peru have occurred since at least the Holocene, and that adjustment to channel incision is still taking place.
|
|
| 2. |
- Castillo, Miguel, et al.
(författare)
-
Knickpoint retreat and transient bedrock channel morphology triggered by base-level fall in small bedrock river catchments : The case of the Isle of Jura, Scotland
- 2013
-
Ingår i: Geomorphology. - : Elsevier BV. - 0169-555X .- 1872-695X. ; 180, s. 1-9
-
Tidskriftsartikel (refereegranskat)abstract
- A sudden drop in river base-level can trigger a knickpoint that propagates throughout the fluvial network causing a transient state in the landscape. Knickpoint retreat has been confirmed in large fluvial settings (drainage areas > 100 km(2)) and field data suggest that the same applies to the case of small bedrock river catchments (drainage areas < 100 km(2)). Nevertheless, knickpoint recession on resistant lithologies with structure that potentially affects the retreat rate needs to be confirmed with field-based data. Moreover, it remains unclear whether small bedrock rivers can absorb base-level fall via knickpoint retreat. Here we evaluate the response of small bedrock rivers to base-level fall on the isle of Jura in western Scotland (UK), where rivers incise into dipping quartzite. The mapping of raised beach deposits and strath terraces, and the analysis of stream long profiles, were used to identify knickpoints that had been triggered by base-level fall. Our results indicate that the distance of knickpoint retreat scales to the drainage area in a power law function irrespective of structural setting. On the other hand, local channel slope and basin size influence the vertical distribution of knickpoints. As well, at low drainage areas (similar to 4 km(2)) rivers are unable to absorb the full amount of base-level fall and channel reach morphology downstream of the knickpoint tends towards convexity. The results obtained here confirm that knickpoint retreat is mostly controlled by stream discharge, as has been observed for other transient landscapes. Local controls, reflecting basin size and channel slope, have an effect on the vertical distribution of knickpoints; such controls are also related to the ability of rivers to absorb the base-level fall.
|
|
| 3. |
- Ebert, Karin, et al.
(författare)
-
DEM identification of macroscale stepped relief in arctic northern Sweden
- 2011
-
Ingår i: Geomorphology. - : Elsevier BV. - 0169-555X .- 1872-695X. ; 132:3-4, s. 339-350
-
Tidskriftsartikel (refereegranskat)abstract
- Stepped relief is a characteristic feature of many upland areas on Earth. In this study, we examine if stepped relief can be identified objectively by GIS-analysis of digital elevation models (DEMs). We specifically study the stepped relief landscapes of northern Sweden, comprising areas of contrasting topography in the mountainous northern Scandes and on the inselberg plains of the Precambrian basement east of the Caledonides.We mainly use hypsographic curves to examine the elevation distribution of the study areas and to identify palaeosurfaces. Peaks in the hypsographic curves are interpreted as palaeosurfaces, while hypsographic minima are interpreted as breaks in slope, separating these surfaces. In the northern Scandes, where only patchy remnants of palaeosurfaces remain, we use empirical cutoff values of slope angles to restrict palaeosurface areas to those identified in thefield and in air photos. In addition, air photo andfield mapping of glacially eroded areas is necessary to exclude glacially formed low relief surfaces, such as valley floors. These latter procedures introduce an unavoidable degree of subjectivity to the study. Our results indicate that in the northern Scandes, surfaces with an inclination of 11°, after glacially formed features are abstracted, correspond well with palaeosurface remnants. Breaks in slope separating the surface generations in themountains are centred around 860, 1320, and 1520 masl (above sea level), respectively. On the plains east of the northern Scandes, hypsographic data were filtered to remove inselbergs in order to analyse only the plains. Hypsographic curves of both the filtered and the unfiltered data of the inselberg plains show minima at elevations that correspond to steps separating multiple palaeosurfaces at elevations of 190, 250, and 400 masl. The steps separating the different palaeosurfaces are, in places, aligned with known geological discontinuities, but extensive remnants also transect structure. The presence of stepped relief is consistent with existing models of phased Cenozoic uplift and incision in northern Fennoscandia.
|
|
| 4. |
- Ebert, Karin, 1976-, et al.
(författare)
-
The impact of Quaternary glaciations on inselbergs in northern Sweden
- 2010
-
Ingår i: Geomorphology. - : Elsevier. - 0169-555X .- 1872-695X. ; 115:1-2, s. 56-66
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the glacial modification of inselbergs (large, isolated bedrock hills) in northern Sweden. Inselbergs are generally regarded as products of deep weathering (etching) and stripping under warm and humid climates. Unlike inselbergs found in the tropics, the inselbergs in northern Sweden were exposed to ice sheet glaciation for long periods of the Quaternary. We used DEMs to examine 794 inselbergs in our study area and they were classified according to their degree of glacial modification. Bedrock structural control was assessed using GIS data on the regional geology. Clusters of inselbergs were mapped in the field for features indicative of glacial erosion, such as glacial cliffs and stripped bedrock surfaces, and for features indicative of limited erosion, such as tors and blockfields. The results of the study indicate that inselbergs in the area were mostly modified by Quaternary ice sheets only to a low or moderate extent and that the degree of glacial erosion is dependent on their relief and location. Inselbergs with a relative relief of < 100 m and in areas of lower absolute relief experienced the strongest glacial modification, where the strongest glacial modification can result in lateral erosion of the inselberg flanks. Inselberg summits often display signs of minimal glacial erosion, such as tor-like bedrock outcrops with signs of strong weathering. In summary, we argue that inselbergs in northern Sweden have largely retained their pre-Quaternary shape despite long periods of ice sheet cover.
|
|
| 5. |
- Ellery, W. N., et al.
(författare)
-
Peat formation in the context of the development of the Mkuze floodplain on the coastal plain of Maputaland, South Africa
- 2012
-
Ingår i: Geomorphology. - : Elsevier BV. - 0169-555X .- 1872-695X. ; 141, s. 11-20
-
Tidskriftsartikel (refereegranskat)abstract
- This paper examines the geomorphological and sedimentological development of blocked-valley lakes in the Mkuze floodplain on the coastal plain of Maputaland, northern KwaZulu-Natal, South Africa. Blocked tributary valley lakes north of the floodplain become progressively shorter, broader, and less linear toward the eastern (downstream) end of the east-west oriented Mkuze floodplain. Clastic sediment forms surface sedimentary fill in tributary valleys in the west, while peat predominates tributary valley fill in the east. Two contrasting adjacent tributary valleys were examined, the more western Yengweni dominated by clastic sediment at the surface, and the more eastern Totweni with peat. The Mkuze floodplain is characterised by silt with a low organic content. Surface sediments fine downstream and with distance from the main channel. Tributary sediment south of the lakes (adjacent to the floodplain) contains little organic material at the surface, but increases with depth. North (upstream) of Yengweni lake, the tributary valley contains peat up to 1.5 m thick, with organic contents up to 30% (generally 10 to 20%). In contrast, north (upstream) of Mpanza lake, peat up to 7 m thick is extensive with high organic contents (typically >60% at the surface but decreasing with depth). The thickness and width of the peat deposits increase longitudinally from the head of the tributary valley toward Mpanza lake. The distribution of clastic and organic sediments illustrates that as aggradation of the Mkuze floodplain progresses, tributary valleys initially fill with sediment from the local tributary catchment, lakes form, there is a phase of peat formation and finally, peat is buried by sediment from the Mkuze floodplain. We hypothesise that peat formation in subtropical and tropical settings through these processes is likely to be an important long-term sink for carbon.
|
|
| 6. |
- Eriksson, Bert, et al.
(författare)
-
Surface destabilisation by the invasive burrowing engineer Mus musculus on a sub-Antarctic island
- 2014
-
Ingår i: Geomorphology. - : Elsevier BV. - 0169-555X .- 1872-695X. ; 223, s. 61-66
-
Tidskriftsartikel (refereegranskat)abstract
- Invasive species are known to have substantial trophic effects on ecosystems and ecosystem processes. The invasion of the house mouse (Mus musculus) onto sub-Antarctic islands has had a devastating effect on plants, invertebrates, and birds with substantial changes in ecosystem functions. Less well understood, however, are the nontrophic, geomorphic effects of mice resulting from their burrowing activities. We examined the extent of burrow construction by M. musculus across an area of about 20 ha on Marion Island and the effects of burrows on water flow and sediment movement. We recorded a density of 0.59 +/- 0.48 (mean +/- SD) burrows m(-2), with more burrows at lower altitudes and shallower slopes, and twice the density in the solifluction risers (0.86 +/- 0.54 m(-2)) than the intervening terraces or treads (0.40 +/- 0.51 m(-2)). Most burrows were dug horizontally into the slope and tended to extend about 20 cm deep before turning. A very conservative estimate of sediment removed from burrows from this depth is 2.4 t ha(-1). However, taking into account more detailed data on burrow morphology based on excavations, actual amounts may be closer to 8.4 t ha(-1). Average soil displacement rate for a single burrow, measured over 5 days, was 0.18 kg burrow(-1) day(-1). Burrows acted as conduits for water and warmer air. Stones at burrow entrances were moved eight times farther by water (10.4 cm) than those not associated with burrows. Similarly, temperatures adjacent to burrow entrances were 4.1 degrees C higher than sites 10 cm away. Together our data indicate that mice are having substantial deleterious and geomorphic effects on sub-Antarctic ecosystems through their burrowing. With lower rates of mouse mortality resulting from warmer climates predicted under global climate models, we can expect an increase in damage resulting from mouse activity.
|
|
| 7. |
- Fu, Ping, 1982-, et al.
(författare)
-
Glacial geomorphology and paleoglaciation patterns in Shaluli Shan, the southeastern Tibetan Plateau — Evidence for polythermal ice cap glaciation
- 2013
-
Ingår i: Geomorphology. - : Elsevier BV. - 0169-555X .- 1872-695X. ; 182, s. 66-78
-
Tidskriftsartikel (refereegranskat)abstract
- Glacial geomorphological mapping from satellite imagery and field investigations provide the basis for a reconstructionof the extent and style of glaciation of the Shaluli Shan, a mountainous area on the southeastern TibetanPlateau. Our studies provide evidence for multiple glaciations, including the formation of regional ice caps andvalley glaciers. The low-relief topographywithin the Shaluli Shan, the Haizishan Plateau, and Xinlong Plateau displayzonal distributions of glacial landforms that is similar to those imprinted by Northern Hemisphere ice sheetsduring the last glacial cycle, indicating the presence of regional, polythermal ice caps. Abundant alpine glaciallandforms occur on high mountain ranges. The pattern of glaciated valleys centered on high mountain rangesand ice-scoured low relief granite plateaus with distinctive patterns of glacial lineations indicate a strong topographiccontrol on erosional and depositional patterns by glaciers and ice caps. In contrast to the Shaluli Shan,areas farther north and west on the Tibetan Plateau have not yielded similar landform evidence for regionalice capswith complex thermal basal conditions. Such spatial differences across the Tibetan Plateau are the resultof variations in climate and topography that control the extent and style of glaciations and that reinforce the importanceof detailed geomorphological mapping for understanding paleoclimate variations and characteristics offormer glaciations.
|
|
| 8. |
- Jónsson, Sverrir Aðalsteinn, et al.
(författare)
-
The drumlin field and the geomorphology of the Múlajökull surge-type glacier, central Iceland
- 2014
-
Ingår i: Geomorphology. - : Elsevier BV. - 0169-555X .- 1872-695X. ; 207, s. 213-220
-
Tidskriftsartikel (refereegranskat)abstract
- Here we present a new geomorphological map of the active drumlin field and the forefield of Múlajökull, a surge-type outlet glacier, Iceland. The map is based on aerial photographs taken in 1995 and LiDAR data recorded in 2008. Mapping was done using ArcGIS 10 software on orthorectified imagery, LiDAR data and digital elevation models. The mapped landforms were initially identified on the aerial imagery and LiDAR and then ground-checked in the field. We mapped subglacial, supraglacial, ice-marginal, periglacial, and glaciofluvial landforms. The geomorphology of the Múlajökull forefield is similar to that of the forefields of other surge-type glaciers in Iceland: with a highly streamlined forefield, crevasse-fill ridges, and series of glaciotectonic end moraines. However, the large number (i.e., 110) of drumlins forming the drumlin field is unique for modern Icelandic surge-type glaciers and, as yet, unique for contemporary glaciers in general. Also apparent is that the drumlins are wider and shorter in the distal part of the drumlin field and narrower and longer in the proximal part. Hence, the mapping reveals a development of the drumlins toward a more streamlined shape of the proximal landforms that have experienced more surges. The drumlins in the drumlin field are active, i.e., they form during the modern surges of Múlajökull.
|
|
| 9. |
- Kolstrup, Else, et al.
(författare)
-
Stone heave field experiment in clayey silt
- 2010
-
Ingår i: Geomorphology. - : Elsevier BV. - 0169-555X .- 1872-695X. ; 117:1-2, s. 90-105
-
Tidskriftsartikel (refereegranskat)abstract
- We recorded movements of 24 cm large globes and cuboids made of granite, concrete and wood buried at three different depths in silty soil, in a cool temperate environment during four and a half years. We also monitored 4-hourly soil moisture and air and ground temperature. All objects heaved during freezing and fell back during thawing, and some sank during the summers. Freeze thaw in a very shallow surface layer may result in heave of objects due partly to sediment infill in unfrozen soils beneath the objects. The net result was sinking of some of the granite and concrete objects and rise of wooden ones. The recorded object movements, in relation to the thermal properties and specific gravity of the objects and the embedding soil, show that an (almost) isodiametric object tends to sink or remain in place if its thermal conductivity and density exceed those of the surrounding soil, and to rise if the reverse is the case. (C) 2009 Elsevier B.V. All rights reserved.
|
|
| 10. |
- Kolstrup, Else, et al.
(författare)
-
Stone heave field experiment in sand
- 2011
-
Ingår i: Geomorphology. - : Elsevier BV. - 0169-555X .- 1872-695X. ; 129:3-4, s. 361-375
-
Tidskriftsartikel (refereegranskat)abstract
- Movements of 24 cm large globes and cuboids of granite, concrete and wood were recorded during four and a half years. The objects were buried at three different depths in well-sorted sand in a natural cool temperate environment. All objects moved during the experiment and at the end most wood objects, which had a relatively low thermal conductivity had moved up, and the granite and concrete objects with higher thermal conductivities had sunk a little or remained almost in place. Also the soil surface moved and its final height was up to 1 cm above that at the start. The movements of the objects started within a few hours after temperature shifts around 0 degrees C with the granite reacting more readily than the wood. It is hypothesized that the movements are related to the thermal properties of the objects and the soil. The experiment was done in parallel with an experiment in clayey silt and the net results in sand show clear parallels to the movements of similar objects in the silt. It is concluded that the material and shape of the object are more important to movements than whether they are embedded in sand or silty clay. (C) 2011 Elsevier B.V. All rights reserved.
|
|
