Spread of denudational processes in seepage channels in a postglacial area (north-western Poland)

<p>Erosion by emerging groundwater (i.e., seepage erosion or groundwater sapping) is the primary mechanism initiating stream channels (so-called seepage channels) and headward growth in lowland areas with a high infiltration capacity of sediment where the surface runoff is relatively rarely observed. Around groundwater outflows, as a result of the impact of denudational processes, develops an alcove, which is an amphitheatrical depression, often of steep slopes, separated from the slopes of the initial depression with clear edges. A spring-formed alcove is the upper boundary of a concentrated flow of water and sediment transport between well-marked channel margins. The influence of groundwater remains one of the least understood factors in the landform evolution in the postglacial zone of Western Pomerania (north-western Poland).</p><p>Morphometric and lithological surveying of about 80 spring-formed alcoves were studied in the southern part of the Parsęta catchment (NW Poland) made it possible to identify morphological effects of seepage erosion which are combined with surface wash and mass movement processes. The co-occurrence of various denudational processes in the headwater zones produces variations in the accumulation conditions, and as a result, a diversity of deposits. The mineral series includes erosional pavements, colluvium, and alluvial deposits. Changes in hydrodynamic conditions are favourable to organic accumulation (peats and organic-mineral muds) as well as  chemical and biochemical deposition (calcareous tufa and precipitation of Fe-oxides). Seepage channels grow when they attract enough groundwater to remove clastic material from the heads. Depending on the discharge volume of the outflow from the ten observed spring-formed alcoves (1-73 L/s), products of mechanical denudation (4-54 mg/L) are transported from the slope system to the fluvial system.</p><p>The morphometry of the spring-formed alcoves as well as deposits found in them reflect stages of their development. Changes in the development of the channel heads occur as a result of variations in the groundwater table that are due to changes in climatic conditions or land use. The determination of the place and formation of the beginning of a river channel initiated by groundwater outflows is of key importance for the modelling of the development of a stream network.</p>

Groundwater seepage landscapes from local or distal sources in experiments and on Mars

Theater-headed valleys can form due to groundwater sapping, but these valleys could also be the result of knick-point (waterfall) erosion generated by overland flow. This morphological ambiguity hampers the interpretation of such valleys on Mars, especially due to insufficient knowledge of material properties, but the climate implications are quite different. Instead of single-valley morphology, metrics of the entire landscape may provide diagnostic insight in the formative hydrological conditions. However, flow patterns and the resulting landscapes are different for different sources of groundwater and poorly understood. We aim to increase our understanding of the formation of the entire landscapes by sapping from different sources of groundwater and to provide a framework of landscape metrics of such systems to aid interpretation of such landscapes. We study sapping from local and distal sources of groundwater in sandbox experiments and combine our results with previous experiments. Key results are that groundwater piracy acts on distally-fed valleys, which results in a sparsely dissected landscape of many small and a few large valleys while locally-fed valleys result in a densely dissected landscape. In addition, distally-fed valleys grow into the direction of the groundwater source while locally-fed channels grow in a broad range of directions and have a strong tendency to bifurcate, particularly on flat horizontal surfaces. As an example, we apply these results to two Martian cases. The valleys of Louros Valles show properties of sapping by a local source and Nirgal Vallis shows evidence of a distal source, which is likely groundwater from Tharsis.

Groundwater sapping as the cause of irreversible desertification of Hunshandake Sandy Lands, Inner Mongolia, northern China

In the middle-to-late Holocene, Earth’s monsoonal regions experienced catastrophic precipitation decreases that produced green to desert state shifts. Resulting hydrologic regime change negatively impacted water availability and Neolithic cultures. Whereas mid-Holocene drying is commonly attributed to slow insolation reduction and subsequent nonlinear vegetation–atmosphere feedbacks that produce threshold conditions, evidence of trigger events initiating state switching has remained elusive. Here we document a threshold event ca. 4,200 years ago in the Hunshandake Sandy Lands of Inner Mongolia, northern China, associated with groundwater capture by the Xilamulun River. This process initiated a sudden and irreversible region-wide hydrologic event that exacerbated the desertification of the Hunshandake, resulting in post-Humid Period mass migration of northern China’s Neolithic cultures. The Hunshandake remains arid and is unlikely, even with massive rehabilitation efforts, to revert back to green conditions.

Origin of collapsed pits and branched valleys surrounding the Ius chasma on Mars

Chasma is a deep, elongated and steep sided depression on planetary surfaces. Several hypothesis have been proposed regarding the origin of chasma. In this study, we analysed morphological features in north and south of Ius chasma. Collapsed pits and branched valleys alongwith craters are prominent morphological features surrounding Ius Chasma, which forms the western part of the well known Valles Marineris chasma system on Martian surface. Analysis of images from the High Resolution Stereo Camera (HRSC) in ESA’s Mars Express (MEX) with a spatial resolution of 10 m shows linear arrangement of pits north of the Ius chasma. These pits were initially developed along existing narrow linear valleys parallel to Valles Merineris and are conical in shape unlike flat floored impact craters found adjacent to them. The width of conical pits ranges 1–10 km and depth ranges 1–2 km. With more subsidence, size of individual pits increased gradually and finally coalesced together to create a large depression forming a prominent linear valley. Arrangement of pits in this particular fashion can be attributed to collapse of the surface due to l arge hollows created in the subsurface because of the withdrawal of either magma or dry ice. Branched valleys which are prominent morphologic features south of the Ius chasma could have been formed due to groundwater sapping mechanism as proposed by previous researchers. Episodic release of groundwater in large quantity to the surface could have resulted in surface runoff creating V-shaped valleys, which were later modified into U-shaped valleys due to mass wasting and lack of continued surface runoff.