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Since 2004, collapse sinkholes occur on the sports field of Münsterdorf, a village north of Hamburg in Germany. The sinkholes, around 2–5 m in diameter and 3–5 m deep, develop in peri-glacial sand, which at around 20 m depth is underlain by Cretaceous chalk. The chalk has been pushed up close to the surface by a salt diapir. The sinkhole formation initiated suddenly and occurs with a frequency of one every 2 years. We use a variety of geophysical results (e.g. gravity, electrical resistivity imaging, ground-penetrating radar) from previous fieldwork campaigns together with a new data set from direct-push-based methods to infer mechanical and hydrological properties of the material beneath the sports field (peri-glacial sand, glacial marl, Cretaceous chalk). Based on the derived material properties, we develop a mechanical model for the sinkhole collapse, starting from simple analytical considerations and then moving towards a three-dimensional distinct-element model explaining the sudden onset of collapse sinkholes for the sports field. The mechanical model supports our hypothesis that the sudden onset of sinkholes is triggered by changes in groundwater level. [ABSTRACT FROM AUTHOR]

Since 2004, collapse sinkholes occur on the sports field of Münsterdorf, a village north of Hamburg in Germany. The sinkholes, around 2-5 m in diameter and 3-5 m deep, develop in peri-glacial sand, which in around 20 m depth is underlain by cretaceous chalk. The chalk has been pushed up close to the surface by a salt diapir. The sinkhole formation initiated suddenly and occurs with a frequency of one every two years. We use a variety of geophysical results (e.g. gravity, electrical resistivity imaging, ground-penetrating radar) from previous field-work campaigns together with a new data set from direct-push based methods to infer mechanical and hydrological properties of the material beneath the sports field (peri-glacial sand, glacial marl, cretaceous chalk). Based on the derived material properties, we develop a mechanical model for the sinkhole collapse, starting from simple analytical considerations and then moving towards a three-dimensional distinct-element model explaining the sudden onset of collapse sinkholes for the sports field. The mechanical model supports our hypothesis that the sudden onset of sinkholes is triggered by changes in groundwater level. [ABSTRACT FROM AUTHOR]

Submarine sinkholes are found on carbonate platforms around the world. They are thought to form and grow when groundwater interactions generate conditions corrosive to carbonate minerals. Because their morphology can restrict mixing and water exchange, the effects of biogeochemical processes can accumulate such that the sinkhole water properties considerably diverge from the surrounding ocean. Studies of sinkhole waters can therefore reveal new insights into marine biogeochemical cycles, thus sinkholes can be considered as 'natural laboratories' where the response of marine ecosystems to environmental variations can be investigated. We conducted the first measurements in recently discovered sinkholes on Luymes Bank, part of Saba Bank in the Caribbean Netherlands. Our measurements revealed a plume of gas bubbles rising from the seafloor in one of the sinkholes, which contained a constrained body of dense, low-oxygen ([O2] = 60.2 ± 2.6 µmol⋅kg-1), acidic (pHT = 6.24 ± 0.01) seawater that we term the 'acid lake'. Here, we investigate the physical and biogeochemical processes that gave rise to and sustain the acid lake, the chemistry of which is dominated by the bubble plume. We determine the provenance and fate of the acid lake's waters, which we deduce must be continuously flowing through. We show that the acid lake is actively dissolving the carbonate platform, so the bubble plume may provide a novel mechanism for submarine sinkhole formation and growth. It is likely that the bubble plume is ephemeral and that other currently non-acidic sinkholes on Luymes Bank have previously experienced 'acid lake' phases. Conditions within the acid lake are too extreme to represent coming environmental change on human timescales but in some respects reflect the bulk ocean billions of years ago. Other Luymes Bank sinkholes host conditions analogous to projections for the end of the 21st century and could provide a venue for studies on the impacts of anthropogenic CO[sub 2] uptake by the ocean. [ABSTRACT FROM AUTHOR]

Rock salt has remarkable mechanical properties and high economic importance; however, the strength of salt compared to other rocks makes it a rather vulnerable material. Human activities could lead to acceleration of the dissolution of soluble rock salt and collapse of subsurface caverns. Although sinkhole development can be considered a local geological disaster regarding the characteristic size of surface depressions, the deformations can result in catastrophic events. In this study we report the spatiotemporal evolution of surface deformation in the Solotvyno salt mine area in Ukraine based on Sentinel-1 interferometric synthetic aperture radar measurements. Although the mining operations were finished in 2010, several sinkholes have been opened up since then. Our results show that despite the enormous risk management efforts, the sinkholes continue to expand with a maximum line-of-sight deformation rate of 5 cm/yr. The deformation time series show a rather linear feature, and unfortunately no slowdown of the processes can be recognized based on the investigated 4.5-year-long data set. We utilized both ascending and descending satellite passes to discriminate the horizontal and vertical deformations, and our results revealed that vertical deformation is much more pronounced in the area. Analytical source modeling confirmed that the complex deformation pattern observed by Sentinel-1 radar interferometry has a direct connection to the former mining activity and is confined to the mining territory. With the 6 d repetition time of Sentinel-1 observations, the evolution of surface changes can be detected in quasi real time, which can facilitate disaster response and recovery. [ABSTRACT FROM AUTHOR]

In several countries, the rising occurrence of sinkholes has led to severe social and economic damage. Based on the mechanism of sinkhole development, researchers have investigated the correlation between rainfall intensity and sinkholes caused by damaged sewer pipes. In this study, the effect of rainfall intensity on the formation of eroded zones, as well as the occurrence of sinkholes caused by soil erosion due to groundwater infiltration through pipe defects, has been analyzed through model tests. The ground materials in Seoul were represented by weathered granite soil, which is generally used for backfill sewer pipes, and groundwater levels corresponding to three different rainfall intensity conditions were considered. The ground level changes and ground displacements were measured continuously, and the particle image velocimetry (PIV) algorithm was applied to measure the displacement at each position of the model ground. The results indicate that impeding the excessive rise in groundwater levels by securing sufficient sewage treatment facilities can effectively prevent the development of sinkholes caused by pipe defects. [ABSTRACT FROM AUTHOR]

Sinkhole risks are becoming particularly severe in urban areas that lack careful planning and where karst depressions are frequently filled and developed. Sinkholes frequently have a higher probability of occurrence and a bigger genetic diversity in evaporite rocks than in carbonate rocks. This is because evaporites rocks (halite, gypsum, etc) have a higher solubility. Subsidence damage resulting from this dissolution generates considerable losses at the world. To contract with these risks, is needed the identification, investigation, prediction, and mitigation of sinkholes. Corrective measures might be applied to reduce the subsidence processes. A more practical solution for safe development is to reduce the vulnerability of the structures by using subsidence-proof designs. Therefore, this case study is located in the town of Perdiguera (Zaragoza, Spain), within the Ebro Basin. This town is affected by subsidence problems, which are associated with the dissolution of gypsiferous silts that generate sinking. These sinkholes are affecting the buildings threatening its structural integrity. [ABSTRACT FROM AUTHOR]

In several countries, the rising occurrence of sinkholes has led to severe social and economic damage. Based on the mechanism of sinkhole development, researchers have investigated the correlation between rainfall intensity and sinkholes caused by damaged sewer pipes. In this study, the effect of rainfall intensity on the formation of eroded zones, as well as the occurrence of sinkholes caused by soil erosion due to groundwater infiltration through pipe defects, has been analyzed through model tests. The ground in Seoul was adopted using weathered granite soil, which is generally used for backfill sewer pipes, and groundwater levels corresponding to three different rainfall intensity conditions were considered. The ground level changes and ground displacements were measured continuously, and the particle image velocimetry (PIV) algorithm was applied to measure the displacement at each position of the model ground. The results indicate that impeding the excessive rise of groundwater levels by securing sufficient sewage treatment facilities can effectively prevent the development of sinkholes caused by pipe defects. [ABSTRACT FROM AUTHOR]

Rocksalt has remarkable mechanical properties and a high economic importance, however, this strength of salt compared to other rocks makes it a rather vulnerable material. Human activities could lead to acceleration of the dissolution of soluble rocksalt and collapse of subsurface caverns. Although sinkhole development can be considered local geological disaster regarding the characteristic size of surface depressions the deformations can result in catastrophic events. In this study we report the spatiotemporal evolution of surface deformation in Solotvyno salt mine area in Ukraine based on Sentinel-1 interferometric synthetic aperture radar measurements. Although the mining operations were finished in 2010 several sinkholes have been opened up since then. Our results show that even though the enormous risk managing efforts the sinkholes continue to expand with a maximum line-of-sight deformation rate of 5 cm/yr. The deformation time series show a rather linear feature and unfortunately no slowdown of the processes can be recognized based on the investigated 4.5 year-long data set. We utilized both ascending and descending satellite passes to discriminate the horizontal and vertical deformations and our results revealed that vertical deformation is much more dominant in the area. With the 6-day repetition time of Sentinel-1 observations the evolution of surface changes can be detected in quasi real-time which can facilitate disaster response and recovery. [ABSTRACT FROM AUTHOR]

Cover-collapse sinkholes can present significant hazards to human habitation and communal facilities in soil-covered karst regions. Therefore, for human security and land-use planning in sinkhole-prone areas, appropriate approaches are required prior to construction in order to understand the cover-collapse sinkhole genesis and its likely evolution. The study seeks to contribute to performing an integrated analysis of karst hazards in mantle karst regions where karst evidence can be masked, with the ultimate goal of developing a methodological framework utilizing different techniques and approaches. A small area located in Guangzhou City of southeastern China’s Guangdong Province was analyzed. The detailed typology, morphometry, and chronology inventory of 49 cover-collapse sinkholes in the study area were analyzed using various surface investigation methods, such as field surveys, aerial photography, and photogrammetry. The Quaternary deposits and indicators of the active underground karst features in the aforementioned mantle karst region were geotechnically characterized using drilling and geophysical techniques. These techniques included ground penetrating radar (GPR), electrical resistivity imaging (ERI), natural source audio frequency magnetotellurics (NSAMT), and micro-tremors. During this study’s investigations, three karst fissure zones covered by Quaternary soil were observed using multiple techniques. In addition, it was found that the groundwater dynamic monitoring data confirmed that the sinkholes in the study area were closely related to changes in groundwater levels. Therefore, the efforts which have been made to investigate and monitor the sinkhole development will be required to continue into the immediate future. [ABSTRACT FROM AUTHOR]

In the Yucatán Peninsula (YP), southern Mexico, cities and towns are settled on a platform of calcareous sedimentary sequence, where karst processes have formed numerous sinkholes, underground water conduits, and caverns. Anthropogenic activities there threaten the only source of freshwater supply, which is in a regional unconfined aquifer; there are no lakes or rivers on the surface. For the sustainable management of this resource in the YP, mathematical tools are needed in order to model groundwater. To determine the geometry of the aquifer, for example the positions of caves, sinkholes, and underground principal conduits, we modified a software to invert three-dimensional electromagnetic low-induction number (3-D EM-LIN) data for a set of profiles at arbitrary angles. In this study we used the EM-LIN geophysical method to explore the Chac-Mool sinkhole system in the state of Quintana Roo, Mexico. We performed inverse modeling in 3-D using the EM-34 instrument for vertical and horizontal magnetic dipoles. The 3-D inversion process yields models that enable us to correlate the path of the underground principal conduits with the subsurface electrical resistivity. In this work we show that inverse modeling of EM-LIN data can give us information about how close to surface the underground water conduits and the location of the boundary between fresh and salty water are. [ABSTRACT FROM AUTHOR]

Seismic reflection S-wave technique is very effective and has demonstrated nice results in previous investigations of various authors. However, the salt layer was not detected in the Ghor Al-Haditha area (Jordan) because of some reasons. The main reason is that about ~ 80 % of reflection lines were carried outside the salt area delineated by Ezersky et al. (2013b) based on results of El-Isa et al. (1995). Other possible factor is too strong filtering of seismic data obtained from the upper part of the section (up to 50 m deep). Our and Polom (2018) assessment of the work of other authors diverges. We affirm that the salt layer of 7-10 m thickness is located at ~ 40 m depth in the Ghor Al-Haditha area. [ABSTRACT FROM AUTHOR]

Enclosed topographic depressions are characteristic of karst landscapes on Earth. The developmental relationship between depression types, such as sinkholes (dolines) and uvalas, has been the subject of debate, mainly because the long developmental timescales in classical limestone karst settings impede direct observation. Here we characterize the morphometric properties and spatio-temporal development of ∼1150 sinkholes and five uvalas formed from ∼1980 to 2017 in an evaporite karst setting along the eastern coast of the hypersaline Dead Sea (at Ghor Al-Haditha, Jordan). The development of sinkhole populations and individual uvalas is intertwined in terms of onset, evolution and cessation. The sinkholes commonly develop in clusters, within which they may coalesce to form compound or nested sinkholes. In general, however, the uvalas are not defined by coalescence of sinkholes. Although each uvala usually encloses several clusters of sinkholes, it develops as a larger-scale, gentler and structurally distinct depression. The location of new sinkholes and uvalas shows a marked shoreline-parallel migration with time, followed by a marked shoreline-perpendicular (i.e. seaward) growth with time. These observations are consistent with theoretical predictions of karstification controlled by a laterally migrating interface between saturated and undersaturated groundwater, as induced by the 35 m fall in the Dead Sea water level since 1967. More generally, our observations indicate that uvalas and the sinkhole populations within them, although morphometrically distinct, can develop near-synchronously by subsidence in response to subsurface erosion. [ABSTRACT FROM AUTHOR]

The 2-D distinct element method (DEM) code (PFC2D_V5) is used here to simulate the evolution of subsidence-related karst landforms, such as single and clustered sinkholes, and associated larger-scale depressions. Subsurface material in the DEM model is removed progressively to produce an array of cavities; this simulates a network of subsurface groundwater conduits growing by chemical/mechanical erosion. The growth of the cavity array is coupled mechanically to the gravitationally loaded surroundings, such that cavities can grow also in part by material failure at their margins, which in the limit can produce individual collapse sinkholes. Two end-member growth scenarios of the cavity array and their impact on surface subsidence were examined in the models: (1) cavity growth at the same depth level and growth rate; (2) cavity growth at progressively deepening levels with varying growth rates. These growth scenarios are characterised by differing stress patterns across the cavity array and its overburden, which are in turn an important factor for the formation of sinkholes and uvala-like depressions. For growth scenario (1), a stable compression arch is established around the entire cavity array, hindering sinkhole collapse into individual cavities and favouring block-wise, relatively even subsidence across the whole cavity array. In contrast, for growth scenario (2), the stress system is more heterogeneous, such that local stress concentrations exist around individual cavities, leading to stress interactions and local wall/overburden fractures. Consequently, sinkhole collapses occur in individual cavities, which results in uneven, differential subsidence within a larger-scale depression. Depending on material properties of the cavity-hosting material and the overburden, the larger-scale depression forms either by sinkhole coalescence or by widespread subsidence linked geometrically to the entire cavity array. The results from models with growth scenario (2) are in close agreement with surface morphological and subsurface geophysical observations from an evaporite karst area on the eastern shore of the Dead Sea. [ABSTRACT FROM AUTHOR]

Unique bell-shaped underwater speleothems were recently reported from the deep (∼ 55 m) meromictic El Zapote sinkhole (cenote) on the Yucatán Peninsula, Mexico. The local diving community has termed these speleothems as Hells Bells because of their shape and appearance in a dark environment in ∼ 28–38 m water depth above a sulfidic halocline. It was also suggested that Hells Bells form under water, yet the mystery of their formation remained unresolved. Therefore, we conducted detailed hydrogeochemical and geochemical analyses of the water column and Hells Bells speleothems including stable carbon isotopes. Based on the comprehensive results presented in this study we deduce that both biogeochemical processes in the pelagic redoxcline and a dynamic halocline elevation of El Zapote cenote are essential for Hells Bells formation. Hells Bells most likely form in the redoxcline, a narrow 1–2 m thick water layer immediately above the halocline where a pelagic chemolithoautotrophic microbial community thrives from the upward diffusion of reduced carbon, nitrogen and sulfur species released from organic matter degradation in organic-rich debris. We hypothesize that chemolithoautotrophy, in particular proton-consuming nitrate-driven anaerobic sulfide oxidation, favors calcite precipitation in the redoxcline and hence Hells Bells formation. A dynamic elevation of the halocline as a hydraulic response to droughts, annual tidal variability and recharge events is further discussed, which might explain the shape of Hells Bells as well as their occurrence over a range of 10 m water depth. Finally, we infer that highly stagnant conditions, i.e., a thick halocline, a low-light environment and sufficient input of organic material into a deep meromictic cenote are apparent prerequisites for Hells Bells formation. This might explain their exclusivity to only a few cenotes in a restricted area of the northeastern Yucatán Peninsula. [ABSTRACT FROM AUTHOR]

Hypoxia is an increasing problem in marine ecosystems around the world, and recent projections indicate that anoxic dead zones will be spreading in the forthcoming decades. While major advances have been made in our understanding of the drivers of hypoxia, it fundamentally hinges on patterns of water circulation that can be difficult to resolve in coastal regions. The complexity of many coastal areas and lack of detailed in situ data has hindered the development of models describing oxygen dynamics at a sufficient resolution for efficient management actions to take place. We hypothesized that the enclosed nature of seafloors facilitates hypoxia formation. We developed simple proxies of seafloor heterogeneity and modelled oxygen deficiency in complex coastal areas in the northern Baltic Sea. We discovered that topographically sheltered seafloors and sinkholes with stagnant water are prone to the development of hypoxia. Approximately half of the monitoring sites in Stockholm Archipelago and one third of sites in southern Finland experienced severe hypoxia (O2<2mgL-1). Based only on topography, area potentially affected by hypoxia is smaller than anticipated. Developed models could boost the performance of numerical models, aid nutrient abatement measures, and pinpoint areas where management actions are most urgently needed. [ABSTRACT FROM AUTHOR]

The stability of man-made underground cavities in soft rocks interacting with overlying structures and infrastructures represents a challenging problem to be faced. Based upon the results of a large number of parametric two-dimensional (2D) finite-element analyses of ideal cases of underground cavities, accounting for the variability of cave geometrical features and rock mechanical properties, specific charts have been recently proposed in the literature to assess at a preliminary stage the stability of the cavities. The purpose of the present paper is to validate the efficacy of the stability charts by means of the application to several case studies of underground cavities, either subjected to collapse in the past or still stable. The stability charts proposed result to be performing to catch the stability conditions and, eventually, the conditions that lead to failure occurrence. For sinkholes already occurred, they show the importance of structural elements as pillars and internal walls in the stability of the whole quarry system, whereas, for cavities that have not reached failure, they can provide useful indications about the eventual proneness of the underground cavity to local or general instability phenomena. [ABSTRACT FROM AUTHOR]

Mechanical and/or chemical removal of material from the subsurface may generate large subsurface cavities, the destabilisation of which can lead to ground collapse and the formation of sinkholes. Numerical simulation of the interaction of cavity growth, host material deformation and overburden collapse is desirable to better understand the sinkhole hazard but is a challenging task due to the involved high strains and material discontinuities. Here, we present 2-D distinct element method numerical simulations of cavity growth and sinkhole development. Firstly, we simulate cavity formation by quasi-static, stepwise removal of material in a single growing zone of an arbitrary geometry and depth. We benchmark this approach against analytical and boundary element method models of a deep void space in a linear elastic material. Secondly, we explore the effects of properties of different uniform materials on cavity stability and sinkhole development. We perform simulated biaxial tests to calibrate macroscopic geotechnical parameters of three model materials representative of those in which sinkholes develop at the Dead Sea shoreline: mud, alluvium and salt. We show that weak materials do not support large cavities, leading to gradual sagging or suffusion-style subsidence. Strong materials support quasi-stable to stable cavities, the overburdens of which may fail suddenly in a caprock or bedrock collapse style. Thirdly, we examine the consequences of layered arrangements of weak and strong materials. We find that these are more susceptible to sinkhole collapse than uniform materials not only due to a lower integrated strength of the overburden but also due to an inhibition of stabilising stress arching. Finally, we compare our model sinkhole geometries to observations at the Ghor Al-Haditha sinkhole site in Jordan. Sinkhole depth / diameter ratios of 0.15 in mud, 0.37 in alluvium and 0.33 in salt are reproduced successfully in the calibrated model materials. The model results suggest that the observed distribution of sinkhole depth / diameter values in each material type may partly reflect sinkhole growth trends. [ABSTRACT FROM AUTHOR]

Near-surface geophysical imaging of alluvial fan settings is a challenging task but crucial for understating geological processes in such settings. The alluvial fan of Ghor Al-Haditha at the southeast shore of the Dead Sea is strongly affected by localized subsidence and destructive sinkhole collapses, with a significantly increasing sinkhole formation rate since ca. 1983. A similar increase is observed also on the western shore of the Dead Sea, in correlation with an ongoing decline in the Dead Sea level. Since different structural models of the upper 50m of the alluvial fan and varying hypothetical sinkhole processes have been suggested for the Ghor Al-Haditha area in the past, this study aimed to clarify the subsurface characteristics responsible for sinkhole development. For this purpose, high-frequency shear wave reflection vibratory seismic surveys were carried out in the Ghor Al- Haditha area along several crossing and parallel profiles with a total length of 1.8 and 2.1 km in 2013 and 2014, respectively. The sedimentary architecture of the alluvial fan at Ghor Al-Haditha is resolved down to a depth of nearly 200m at a high resolution and is calibrated with the stratigraphic profiles of two boreholes located inside the survey area. The most surprising result of the survey is the absence of evidence of a thick (> 2-10 m) compacted salt layer formerly suggested to lie at ca. 35-40m depth. Instead, seismic reflection amplitudes and velocities image with good continuity a complex interlocking of alluvial fan deposits and lacustrine sediments of the Dead Sea between 0 and 200m depth. Furthermore, the underground section of areas affected by sinkholes is characterized by highly scattering wave fields and reduced seismic interval velocities. We propose that the Dead Sea mud layers, which comprise distributed inclusions or lenses of evaporitic chloride, sulfate, and carbonate minerals as well as clay silicates, become increasingly exposed to unsaturated water as the sea level declines and are consequently destabilized and mobilized by both dissolution and physical erosion in the subsurface. This new interpretation of the underlying cause of sinkhole development is supported by surface observations in nearby channel systems. Overall, this study shows that shear wave seismic reflection technique is a promising method for enhanced near-surface imaging in such challenging alluvial fan settings. [ABSTRACT FROM AUTHOR]

Sinkhole collapse is a major hazard causing substantial social and economic losses. However, the surface deformations and sinkhole evolution are rarely recorded, as these sites are known mainly after a collapse, making the assessment of sinkhole-related hazard challenging. Furthermore, more than 40% of the sinkholes of Italy are in seismically hazardous zones; it remains unclear whether seismicity may trigger sinkhole collapse. Here we use a multidisciplinary data set of InSAR, surface mapping and historical records of sinkhole activity to show that the Prà di Lama lake is a long-lived sinkhole that was formed in an active fault zone and grew through several events of unrest characterized by episodic subsidence and lake-level changes. Moreover, InSAR shows that continuous aseismic subsidence at rates of up to 7.1 mm yr-1 occurred during 2003-2008, between events of unrest. Earthquakes on the major faults near the sinkhole do not trigger sinkhole activity but low-magnitude earthquakes at 4-12 km depth occurred during sinkhole unrest in 1996 and 2016. We interpret our observations as evidence of seismic creep at depth causing fracturing and ultimately leading to the formation and growth of the Prà di Lama sinkhole. [ABSTRACT FROM AUTHOR]

We present results of a sophisticated, high-precision time-lapse gravity survey that was conducted over four years in Bad Frankenhausen (Germany). To our knowledge, this is the first successful attempt to monitor subrosion-induced mass changes in urban areas with repeated gravimetry. The method provides an approach to estimate the mass of dissolved rocks in the subsurface. Subrosion, i.e. leaching and transfer of soluble rocks, occurs worldwide. Especially in urban areas, any resulting ground subsidence can cause severe damage, especially if catastrophic events, i.e. collapse sinkholes occur. Monitoring strategies typically make use of established geodetic methods, such as levelling, and therefore, focus on the associated deformation processes. In this study, we combine levelling and highly precise time-lapse gravity surveys. Our investigation area is the urban area of Bad Frankenhausen in Central Germany, which is prone to subrosion, as many subsidence and sinkhole features on the surface reveal. The city and the surrounding areas are underlain by soluble Permian deposits, which are continuously dissolved by meteoric water and groundwater in a strongly fractured environment. Between 2014 and 2018, a total of 17 high-precision time-lapse gravity and 18 levelling campaigns were carried out in quarter-yearly intervals within a local monitoring network. This network covers historical sinkhole areas, but also areas that are considered to be stable. Our results reveal ongoing subsidence of locally up to 30.4mma-1, with distinct spatio-temporal variations. Furthermore, we observe significant time-variable gravity changes in the order of 8μGal over four years at several measurement points. In the processing workflow, after the application of all required corrections and least squares adjustment to our gravity observations, a significant effect of varying soil water content on the adjusted gravity differences was figured out. Therefore, we place special focus on the correlation of these observations and the correction of the adjusted gravity differences for soil water variations using the global soil water model GLDAS Noah to separate these effects from subrosion-induced gravity changes. Our investigations demonstrate the feasibility of high-precision time-lapse gravity in urban areas for sinkhole investigations. Although the observed rates of gravity changes of 1-2μGala-1 are small, we suggest that it is significantly associated with subterranean mass loss due to subrosion processes. We discuss limitations and implications of our approach, as well as give a first quantitative estimation of mass transfer at different depths and for different densities of dissolved rocks. [ABSTRACT FROM AUTHOR]

The fall of hydrological base-level is long established as a driver of geomorphological change in both fluvial and karst systems, but few natural occurrences occur on timescales suitable for direct observation. Here we document the spatiotemporal development of fluvial and karstic landforms along the eastern coast of the hypersaline Dead Sea (at Ghor al-Haditha, Jordan) during a 50-year period of regional base-level decline from 1967 to 2017. Combining remote sensing data with close-range photogrammetric surveys, we show that the 35 m base-level fall has caused shoreline retreat of up to 2.5 km, and resulted in: (1) incision of new meandering or straight/braided stream channels and (2) formation of > 1100 sinkholes and several salt-karst uvalas. Both alluvial incision and karst-related subsidence represent significant hazards to local infrastructure. The development of groundwater-fed meandering stream channels is in places interlinked with that of the sinkholes and uvalas. Moreover, active areas of channel incision and sinkhole development both migrate seaward in time, broadly in tandem with shoreline retreat. Regarding theoretical effects of base-level fall, our observations show some deviations from those predicted for channel geometry, but are remarkably consistent with those for groundwater-related salt karstification. Our results present, for the first time in the Dead Sea region, the dual response of surface and subsurface hydrological systems to base level drop as indicated by fluvial and karst geomorphological analysis. [ABSTRACT FROM AUTHOR]

In southern Mexico at the Yucatan Peninsula (YP), cities and towns are settled on a platform of calcareous sedimentary sequence which has originated a wide formation of sinkholes, underground rivers and caverns due of karst process. The anthropogenic activities threat the only source of fresh water supply which is located in a regional unconfined aquifer; there are not lakes and rivers. For sustainable use of these resources at the YP, it is required to develop mathematical tools to help the groundwater modeling. In order to determine the geometry of the aquifer as the positions of caves, sinkholes and underground rivers we have developed software to invert three-dimensional electromagnetic low-induction numbers (3D EM-LIN) data for a set of profiles at arbitrary angle. In this work we have explored with the aid of EM-LIN geophysical method, the Chac-Mool sinkhole system at the state of Quintana Roo, Mexico. We have performed inverse modeling in 3D using the EM-34 instrument for vertical and horizontal magnetic dipoles. The 3D inversion process gives us models that allow us to correlate the path of the underground rivers with the subsurface electrical resistivity. In this work we have shown that inverse modeling of EM-LIN data is necessary to explore and understand coastal karst systems. [ABSTRACT FROM AUTHOR]

The occurrence frequency and intensity of many natural geohazards, such as landslides, debris flows and earthquakes, have increased in response to global warming. However, the effects of such on development and spread of sinkholes has been largely overlooked. Most research shows that water pumping and related drawdown is the most important factor in sinkhole development, but in this paper evidence is presented which highlights the role played by global warming in causing sinkholes. The state of Florida, USA is used as a case study in which the role of global warming is evident, based on correlation analysis between sinkhole collapse and peak drought periods. Three distinct drought and sinkhole collapse phases are evident between 1965 and 2006, along with eight peak periods of sinkhole collapse that lag slightly behind eight peak drought periods. A prediction equation is derived according to curve fitting and a correlation coefficient of 0.999 is determined. The results of this study confirm that global warming related to climate change has led to an increase in sinkhole collapse events in Florida over the past 50 years, which is of significance for studying the occurrence and prediction of other sinkhole collapse events and global warming on an international scale. [ABSTRACT FROM AUTHOR]

In November 2010, a large sinkhole opened up in the urban area of Schmalkalden, Germany. To determine the key factors which benefited the development of this collapse structure and therefore the dissolution, we carried out several shear-wave reflection-seismic profiles around the sinkhole. In the seismic sections we see evidence of the Mesozoic tectonic movement in the form of a NW-SE striking, dextral strike-slip fault, known as the Heßleser Fault, which faulted and fractured the subsurface below the town. The strike-slip faulting created a zone of small blocks (<100m in size), around which steep-dipping normal faults, reverse faults and a dense fracture network serve as fluid pathways for the artesian-confined groundwater. The faults also acted as barriers for horizontal groundwater flow perpendicular to the fault planes. Instead groundwater flows along the faults which serve as conduits and forms cavities in the Permian deposits below ca. 60m depth. Mass movements and the resulting cavities lead to the formation of sinkholes and dissolutioninduced depressions. Since the processes are still ongoing, the occurrence of a new sinkhole cannot be ruled out. This case study demonstrates how S-wave seismics can characterize a sinkhole and, together with geological information, can be used to study the processes that result in sinkhole formation, such as a near-surface fault zone located in soluble rocks. The more complex the fault geometry and interaction between faults, the more prone an area is to sinkhole occurrence. [ABSTRACT FROM AUTHOR]

Sinkholes are being a natural hazard which threads economic and human life. Sudden occurrence characteristic of sinkholes make it unable to escape. There are a lot of factor that activate sinkholes such as geology, irrigation, land use and human related factors. In Karapınar, Konya, there are over 200 sinkholes and this count is getting increased in recent years. Especially active agricultural lands, decreasing ground water level, extreme irrigation by 55267 water wells increase the risk factor of Karapınar. Nowadays, considering the economic contribution of Karapınar to Turkey economy in the field of agriculture, solar energy fields and thermal reactor which will be planned in next few years, prediction of sinkholes and searching for preventation ways are being more important issue. In this study, sinkhole susceptibility map via AHP was carried out for Karapınar in Konya. Slope, land use, elevation, geology, water wells, distance to roads and settlements criteria are included to determine susceptibility. The weights are calculated with AHP for each criterion and generated susceptibility map is overlapped with existing sinkholes. Suggestions and results are shared for this study. [ABSTRACT FROM AUTHOR]

Sinkhole is not classified as new phenomenon in this country, especially surround Klang Valley. Since 1968, the increasing numbers of sinkhole incident have been reported in Kuala Lumpur and the vicinity areas. As the results, it poses a serious threat for human lives, assets and structure especially in the capital city of Malaysia. Therefore, a Sinkhole Hazard Model (SHM) was generated with integration of GIS framework by applying Analytical Hierarchical Process (AHP) technique in order to produced sinkhole susceptibility hazard map for the particular area. Five consecutive parameters for main criteria each categorized by five sub classes were selected for this research which is Lithology (LT), Groundwater Level Decline (WLD), Soil Type (ST), Land Use (LU) and Proximity to Groundwater Wells (PG). A set of relative weights were assigned to each inducing factor and computed through pairwise comparison matrix derived from expert judgment. Lithology and Groundwater Level Decline has been identified gives the highest impact to the sinkhole development. A sinkhole susceptibility hazard zones was classified into five prone areas namely very low, low, moderate, high and very high hazard. The results obtained were validated with thirty three (33) previous sinkhole inventory data. This evaluation shows that the model indicates 64% and 21% of the sinkhole events fall within high and very high hazard zones respectively. Based on this outcome, it clearly represents that AHP approach is useful to predict natural disaster such as sinkhole hazard. [ABSTRACT FROM AUTHOR]

In November 2010, a large sinkhole opened up in the urban area of Schmalkalden, Germany. To determine the key factors which benefited the development of this collapse structure and therefore the subrosion, we carried out several shear wave reflection seismic profiles around the sinkhole. In the seismic sections we see evidence of the Mesozoic tectonic movement, in the form of a NW-SE striking, dextral strike-slip fault, known as the Heßleser Fault, which faulted and fractured the subsurface below the town. The strike-slip faulting created a zone of small blocks (< 100 m in size), around which steeply-dipping normal faults, reverse faults, and a dense fracture network serve as fluid pathways for the artesian-confined groundwater. The faults also acted as barriers for horizontal groundwater flow perpendicular to the fault planes. Instead the groundwater is flowing along the faults which serve as conduits. A near-surface fault zone located in soluble rocks can enhance subrosion in two ways: (1) tectonic movements accompanied by strain variations lead to the formation of small fault blocks and a fracture network that increases the rock permeability and creates fluid pathways, and (2) the faults can serve as groundwater conduits. Also note, the more complex the fault geometry and the more interaction between faults, the more fractures are generated and the more prone to sinkholes occurrence is the area. [ABSTRACT FROM AUTHOR]

In karst environments, heavy rainfall is known to cause multiple geo-hydrological hazards, including inundations, flash floods, landslides, and sinkholes. We studied a period of intense rainfall from 1 to 6 September 2014 in the Gargano Promontory, a karst landscape in Puglia, southern Italy. In the period, a sequence of torrential rainfall events caused severe damage, and claimed two fatalities, triggering different types of geo-hydrological hazards. The amount and accuracy of the geographical and the temporal information varied for the different hazards. The temporal information was most accurate for the inundation caused by a major river, less accurate for flash floods caused by minor torrents, and even less accurate for landslides. For sinkholes, only generic information on the period of occurrence of the failures was available. Our analysis revealed that in the Promontory, rainfall-driven geo-hydrological hazards occurred in response to extreme meteorological conditions, and that the karst landscape responded to the torrential rainfall with a threshold behaviour. We exploited the rainfall and the landslide information to design the new Ensemble - Non Exceedance Probability, E-NEP algorithm for the quantitative evaluation of the possible occurrence of rainfall-induced landslides, and of related geo-hydrological hazards. The ensemble of the metrics produced by the E-NEP algorithm provided better diagnostics than the single metrics often used for landslide forecasting, including rainfall duration, cumulated rainfall and rainfall intensity. We expect that the E-NEP algorithm will be useful for landslide early warning in karst areas and in other similar environments. We acknowledge that further tests are needed to evaluate the algorithm in different meteorological, geological, and physiographical settings. [ABSTRACT FROM AUTHOR]

The so-called El Trébol is a critical road interchange in Quito connecting the north and south regions of the city. In addition, it connects Quito with the highly populated Los Chillos Valley, one of the most traveled zones in the Ecuadorian capital. El Trébol was constructed in the late 1960s in order to resolve the traffic jams of the capital city and for that purpose the Machángara River was rerouted through an underground concrete box tunnel. In March 2008, the tunnel contained a high amount of discarded furniture that had been impacting the top portion of the tunnel, compromising the structural integrity. On 31 March 2008 after a heavy rainfall a sinkhole of great proportions formed in the Trébol traffic hub. In the first few minutes, the sinkhole reached an initial diameter of 30 m. The collapse continued to grow in the following days until the final dimensions of 120m in diameter and some 40m of depth, revealing the Machángara River at the base of the sinkhole. A state of emergency was declared. The cause of the sinkhole was a result of the lack of monitoring of the older subterranean infrastructure where trash had accumulated and damaged the concrete tunnel that channelized the Machángara River until it was worn away for a length of some 20 m, leaving behind the sinkhole and the fear of recurrence in populated areas. With the intent to understand the causes and consequences of this sinkhole event, rainfall data are shown together with hydrogeological characteristics and a view back to the recent history of sinkhole lineation or arrangement of the city of Quito. The economic impact is also emphasized, where the direct costs of the damage and the reconstruction are presented and compared to indirect costs associated with this socio-natural disaster. These analyses suggest that the costs of indirect financial damage, like time loss or delay, and subsequent higher expenses for different types of vehicles, are equivalent to many times the costs of the reconstruction of El Trébol. [ABSTRACT FROM AUTHOR]

Airborne laser scans present an optimal tool to describe geomorphological features in natural environments. However, a challenge arises in the detection of such phenomena, as they are embedded in the topography, tend to blend into their surroundings and leave only a subtle signature within the data. Most object-recognition studies address mainly urban environments and follow a general pipeline where the data are partitioned into segments with uniform properties. These approaches are restricted to man-made domain and are capable to handle limited features that answer a well-defined geometric form. As natural environments present a more complex set of features, the common interpretation of the data is still manual at large. In this paper, we propose a data-aware detection scheme, unbound to specific domains or shapes. We define the recognition question as an energy optimization problem, solved by variational means. Our approach, based on the level-set method, characterizes geometrically local surfaces within the data, and uses these characteristics as potential field for minimization. The main advantage here is that it allows topological changes of the evolving curves, such as merging and breaking. We demonstrate the proposed methodology on the detection of collapse sinkholes. [ABSTRACT FROM AUTHOR]

Erosion and sedimentation pose ubiquitous problems for land and watershed managers, requiring delineation of sediment sources and sinks across landscapes. However, the technical complexity of many spatially explicit erosion models precludes their use by practitioners. To address this critical gap, we demonstrate a contemporary use of applied geomorphometry through a straightforward GIS analysis of sediment sources in the San Francisco Bay Area in California, USA, designed to support erosion reduction strategies. Using 2 m LiDAR DEMs, we delineated the entire river network in the Arroyo Mocho watershed (573 km²) at the scale of ~30 m segments and identified incised landforms using a combination of hillslope gradient and planform curvature. Chronic erosion to the channel network was estimated based on these topographic attributes and the density and size of vegetation, and calibrated to sediment gage data, providing a spatially explicit estimate of sediment yield from incised channels across the basin. Rates of erosion were summarized downstream through the channel network, revealing patterns of sediment supply at the reach scale. Erosion and sediment supply were also aggregated to subbasins, allowing comparative analyses at the scale of tributaries. The erosion patterns delineated using this approach provide land use planners with a robust framework to design erosion reduction strategies. More broadly, the study demonstrates a modern analysis of important geomorphic processes affected by land use that is easily applied by agencies to solve common problems in watersheds, improving the integration between science and environmental management. [ABSTRACT FROM AUTHOR]

Subsidence was measured for the first time on railway tracks in the central sector of Ebro Valley (NE Spain) using Differential Synthetic Aperture Radar Interferometry (DInSAR) techniques. This area is affected by evaporite karst and the analysed railway corridors traverse active sinkholes that produce deformations in these infrastructures. One of the railway tracks affected by slight settlements is the Madrid-Barcelona high-speed line, a form of transport infrastructure highly vulnerable to ground deformation processes. Our analysis based on DInSAR measurements and geomorphological surveys indicates that this line shows dissolution-induced subsidence and compaction of anthropogenic deposits (infills and embankments). Significant sinkhole-related subsidence was also measured by DInSAR techniques on the Castejón-Zaragoza conventional railway line. This study demonstrates that DInSAR velocity maps, coupled with detailed geomorphological surveys, may help in the identification of the railway track sections that are affected by active subsidence. [ABSTRACT FROM AUTHOR]

The traditional approach for defining sinkholes characteristics is based on topographic maps and air photographs with derived digital terrain models. This method is sometimes not accurate, requiring costly, time consuming and potentially dangerous fieldwork. Investigations have shown that airborne scanning laser data (lidar) is useful in detection of karst depressions due to the high density of ground points that can be obtained. This is especially important under dense forest canopy, where classical photogrammetric methods do not allow ground points to be measured. The objective of this work was to map and determine geomorphometric characteristics of a large number of sinkholes located in a diverse karst terrain under a dense forest tree-canopy using lidar data. We tested an algorithm described in previous literature which uses only information from the DTM. It is based on water flow simulations on a surface (DTM) and incorporates four phases: (i) watershed delineation, (ii) confining of sinkholes, (iii) confining of higher rank sinkholes and (iv) extraction of non-karstic sinkholes. Sinkholes were confined by effluent level with cells below the effluent level designated as part of the sinkhole. In the third step sinkholes were ranked according to their location and size - first rank sinkholes are the smallest and are located within a larger sinkhole. Results are that the sinkhole fraction of 1st, 2nd, 3rd, 4th and 5th rank in the study area was 3.25%, 4.26%, 5.68%, 3.65% and 3.14%, respectively. Sinkhole distribution shows a peculiar directionality in their spatial distribution, which seems to be significantly towards a northwest - southeast direction. It was not possible to compare results with ground-truth data due to very low accessibility, nevertheless a statistical and visual assessment of the results shows that lidar is a very effective technique to model sinkholes under dense canopy. [ABSTRACT FROM AUTHOR]

In this case study, an approach for finding regions endangered by extreme rain is presented. The approach is based on the assumption that sinks in the surface are more endangered than their surroundings. The surface data, which are the source for the classification, are generated using a Cartosat stereo scene. The classification is performed by using an algorithm for retrieving the terrain positioning index. Different classification schemes are possible, therefore a set of input parameters is iteratively computed. The classification results are then evaluated. For validating the classification stock data of an insurance are used. We compare the position of the reported damages caused by extreme rain with our classification. By doing so we got the confirmation of the assumption. [ABSTRACT FROM AUTHOR]

The analysis of mining-induced sinkholes occurrence is a very important issue as far as the spatial development optimization is concerned. Research conducted for this paper was focussed on examining the applicability of GIS and the associated AHP method (analytic hierarchy process) for estimating the risk of discontinuous deformation occurrence on the surface. Qualitative factors were accounted for in the sinkhole risk assessment, thus creating bases for the research. These elements play an important role in the process of sinkholes formation; however, they were not used in prediction models. Another assumption lay in minimizing the number of variables in the model. Accordingly, the most important qualitative and quantitative risk factors were finally selected on the basis of whether the risk of cave-ins occurrence on the surface could be calculated. The results of the estimation of potential sinkhole zones were verified. The locations of actual and high-risk potential discontinuous deformation were compared. The congruence between predicted values and the actual observations of sinkholes was very high. The results of research presented prove the necessity to evaluate sinkhole hazards in view of qualitative factors. [ABSTRACT FROM AUTHOR]

Artificial tracers are powerful tools for investigating karst systems. Tracers are commonly injected into sinking streams or dolines, while springs serve as monitoring sites. The obtained flow and transport parameters represent mixed information from the vadose, epiphreatic and phreatic zones (that is, the aquifer remains a black box). Accessible active caves constitute valuable but underexploited natural laboratories to gain detailed insights into the hydrologic functioning of the aquifer. Two multi-tracer tests in the catchment of a major karst spring (Blautopf, Germany) with injections and monitoring in two associated water caves aimed at obtaining spatially and temporally resolved information on groundwater flow in different compartments of the system. Two tracers were injected into the caves to characterize the hydraulic connections between them and with the spring. Two injections at the land surface, far from the spring, aimed at resolving the aquifer's internal drainage structure. Tracer breakthrough curves were monitored by field fluorimeters in caves and at the spring. Results demonstrate the dendritic drainage structure of the aquifer. It was possible to obtain relevant flow and transport parameters for different sections of this system. The highest mean flow velocities (275mh–1) were observed in the near-spring epiphreatic section (openchannel flow), while velocities in the phreatic zone (pressurized flow) were one order of magnitude lower. Determined conduit water volumes confirm results of water balances and hydrograph analyses. In conclusion, experiments and monitoring in caves can deliver spatially resolved information on karst aquifer heterogeneity and dynamics that cannot be obtained by traditional investigative methods. [ABSTRACT FROM AUTHOR]

Sedimentation in the ocean is fed by large aggregates produced in the surface mixed layer that sink rapidly through the water column. These particles sampled by sediment traps have often been proposed to interact by disaggregation and scavenging with a pool of fine suspended matter with very slow sinking velocities and thus a long residence time. We investigated the amino acid composition and stable nitrogen isotopic ratios of suspended matter sampled during the late SW monsoon season in the Arabian Sea and compared them to those of sinking particles to investigate organic matter degradation/modification during passage through the water column. We found that amino acid (AA) composition of mixed layer suspended matter corresponds more to fresh plankton and their aggregates, whereas AA composition of suspended matter in the sub-thermocline water column deviated progressively from mixed layer composition. We conclude that suspended matter in deep waters and in the mixed layers of oligotrophic stations is dominated by fine material that has a long residence time and organic matter that is resistant to degradation. Whereas SPM in areas of high primary productivity is essentially derived from fresh plankton and thus has a strong imprint of the subsurface nitrate source, SPM at oligotrophic stations and at subthermocline depths appears to exchange amino acids with the DOC pool influencing also the δ15N values. [ABSTRACT FROM AUTHOR]

In this study, we establish a nested atmospheric inversion system with a focus on China using the Bayesian method. The global surface is separated into 43 regions based on the 22 TransCom large regions, with 13 small regions in China. Monthly CO2 concentrations from 130 GlobalView sites and 3 additional China sites are used in this system. The core component of this system is an atmospheric transport matrix, which is created using the TM5 model with a horizontal resolution of 3° x 2°. The net carbon fluxes over the 43 global land and ocean regions are inverted for the period from 2002 to 2008. The inverted global terrestrial carbon sinks mainly occur in boreal Asia, South and Southeast Asia, eastern America and southern South America. Most China areas appear to be carbon sinks, with strongest carbon sinks located in Northeast China. From 2002 to 2008, the global terrestrial carbon sink has an increasing trend, with the lowest carbon sink in 2002. The inter-annual variation (IAV) of the land sinks shows remarkable correlation with the El Nino Southern Oscillation (ENSO). The terrestrial carbon sinks in China also show an increasing trend. However, the IAV in China is not the same as that of the globe. There is relatively stronger land sink in 2002, lowest sink in 2006, and strongest sink in 2007 in China. This IAV could be reasonably explained with the IAVs of temperature and precipitation in China. The mean global and China terrestrial carbon sinks over the period 2002-2008 are -3.20 ± 0.63 and -0.28 ± 0.18PgCyr-1, respectively. Considering the carbon emissions in the form of reactive biogenic volatile organic compounds (B VOCs) and from the import of wood and food, we further estimate that China's land sink is about -0.3 lPgCyr-1. [ABSTRACT FROM AUTHOR]

It is estimated that tropical forest soils contribute 6.2 Tg yr-1 (28 %) to global methane (CH4) uptake, which is large enough to alter CH4 accumulation in the atmosphere if significant changes would occur to this sink. Elevated deposition of inorganic nitrogen (N) to temperate forest ecosystems has been shown to reduce CH4 uptake in forest soils, but almost no information exists from tropical forest soils even though projections show that N deposition will increase substantially in tropical regions. Here we report the results from two long-term, ecosystem-scale experiments in which we assessed the impact of chronic N addition on soil CH4 fluxes from two old-growth forests in Panama: (1) a lowland, moist (2.7 my r-1 rainfall) forest on clayey Cambisol and Nitisol soils with controls and N-addition plots for 9-12 yr, and (2) a montane, wet (5.5 my r-1 rainfall) forest on a sandy loam Andosol soil with controls and N-addition plots for 1-4 yr. We measured soil CH4 fluxes for 4 yr (2006-2009) in four replicate plots (40 m x 40 m each) per treatment using vented static chambers (four chambers per plot). CH4 fluxes from the lowland control plots and the montane control plots did not differ from their respective N-addition plots. In the lowland forest, chronic N addition did not lead to inhibition of CH4 uptake; instead, a negative correlation of CH4 fluxes with nitrate (NO3-) concentrations in the mineral soil suggests that increased NO3- levels in N-addition plots had stimulated CH4 consumption and/or reduced CH3 production. In the montane forest, chronic N addition also showed negative correlation of CH3 fluxes with ammonium concentrations in the organic layer, which suggests that CH3 consumption was N limited. We propose the following reasons why such N-stimulated CH4 consumption did not lead to statistically significant CH4 uptake: (1) for the lowland forest, this was caused by limitation of CH4 diffusion from the atmosphere into the clayey soils, particularly during the wet season, as indicated by the strong positive correlations between CH4 fluxes and water-filled pore space (WFPS); (2) for the montane forest, this was caused by the high WFPS in the mineral soil throughout the year, which may not only limit CH4 diffusion from the atmosphere into the soil but also favour CH4 production; and (3) both forest soils showed large spatial and temporal variations of CH4 fluxes. We conclude that in these extremely different tropical forest ecosystems there were indications of N limitation on CH4 uptake. Based on these findings, it is unlikely that elevated N deposition on tropical forest soils will lead to a rapid reduction of CH4 uptake. [ABSTRACT FROM AUTHOR]

We present results from a study of canopy-atmosphere fluxes of carbon dioxide from 2007 to 2009 above a site in Wytham Woods, an ancient temperate broadleaved deciduous forest in southern England. Gap-filled net ecosystem exchange (NEE) data were partitioned into gross primary productivity (GPP) and ecosystem respiration (Re) and analysed on daily, monthly and annual timescales. Over the continuous 24 month study period annual GPP was estimated to be 21.1 Mg C ha-1 yr-1 and Re to be 19.8 Mg C ha-1 yr-1; net ecosystem productivity (NEP) was 1.2 Mg C ha-1 yr-1. These estimates were compared with independent bottom-up estimates derived from net primary productivity (NPP) and flux chamber measurements recorded at a plot within the flux footprint in 2008 (GPP = 26.5 ± 6.8 Mg C ha-1 yr-1, Re = 24.8 ± 6.8 Mg C ha-1 yr-1, biomass increment = ~1.7 Mg C ha-1 yr-1). Over the two years the difference in seasonal NEP was predominantly caused by changes in ecosystem respiration, whereas GPP remained similar for equivalent months in different years. Although solar radiation was the largest influence on daily values of CO2 fluxes (R² = 0.53 for the summer months for a linear regression), variation in R e appeared to be driven by temperature. Our findings suggest that this ancient woodland site is currently a substantial sink for carbon, resulting from continued growth that is probably a legacy of past management practices abandoned over 40 years ago. Our GPP and Re values are generally higher than other broadleaved temperate deciduous woodlands and may represent the influence of the UK's maritime climate, or the particular species composition of this site. The carbon sink value of Wytham Woods supports the protection and management of temperate deciduous woodlands (including those managed for conservation rather than silvicultural objectives) as a strategy to mitigate atmospheric carbon dioxide increases. [ABSTRACT FROM AUTHOR]

The article discusses the purpose of karst, a unique landform developed by soluble rock, in environmental geosciences and engineering. It states that karst offers important water resources, and relates to the earth system and environmental protection. Also, karst can create a potential natural hazard such as land subsidence in urban area and sinkhole flooding. Moreover, karst's relationship with hydrogeology is important for studying water pollution.

Grasslands represent canopies with a complex structure where sources and sinks of ammonia (NH3) may coexist at the plant level. Moreover, management practices such as mowing, hay production and grazing may change the composition of the sward and hence the source-sink relationship at the canopy level as well as the interaction with the atmosphere. There is therefore a need to understand the exchange of ammonia between grasslands and the atmosphere better, especially regarding the location and magnitude of sources and sinks. Fluxes of atmospheric NH3 within a grassland canopy were assessed in the field and under controlled conditions using a dynamic chamber technique (cuvette). These cuvette measurements were combined with extraction techniques to estimate the ammonium (NH4+) concentration and the pH of a given part of the plant or soil, leading to an estimated ammonia compensation point (Cp). The combination of the cuvette and the extraction techniques was used to identify the potential sources and sinks of NH3 within the different compartments of the grassland: the soil, the litter or senescent "litter leaves", and the functioning "green leaves". A set of six field experiments and six laboratory experiments were performed in which the different compartments were either added or removed from the cuvettes. The results show that the cuvette measurements agree with the extraction technique in ranking the strength of compartment sources. It suggests that in the studied grassland the green leaves were mostly a sink for NH3 with a compensation point around 0.1-0.4 μg m-3 and an NH3 flux of 6 to 7 ng m-2 s-1. Cutting of the grass did not increase the NH3 fluxes of the green leaves. The litter was found to be the largest source of NH3 in the canopy, with a Cp of up to 1000 μg m-3 NH3 and an NH3 flux up to 90 ng m-2 s-1. The litter was found to be a much smaller NH3 source when dried (Cp = 160 μg m-3 and FNH3=35 ng m-2 s-1 NH3). Moreover emissions from the litter were found to vary with the relative humidity of the air. The soil was a strong source of NH3 in the period immediately after cutting (Cp = 320 μg m-3 and FNH3 = 60 ng m-2 s-1), which was nevertheless always smaller than the litter source. The soil NH3 emissions lasted, however, for less than one day, and were not observed with sieved soil. They could not be solely explained by xylem sap flow extruding NH4+. These results indicate that future research on grassland-ammonia relationships should focus on the post-mowing period and the role of litter in interaction with meteorological conditions. [ABSTRACT FROM AUTHOR]

Thousands of sinkholes have appeared in the Dead Sea (DS) coastal area in Israel and Jordan during two last decades. The sinkhole development is recently associated with the buried evaporation karst at the depth of 25-50m from earth's surface caused by the drop of the DS level at the rate of 0.8-1.0 m/yr. Drop in the Dead Sea level has changed hydrogeological conditions in the subsurface and caused surface to collapse. The pre-existing cavern was detected using microgravity mapping in the Nahal Hever South site where seven sinkholes of 1-2m diameter had been opened. About 5000 gravity stations were observed in the area of 200x200m² by the use of Scintrex CG-3M AutoGrav gravimeter. Besides the conventional set of corrections applied in microgravity investigations, a correction for a strong gravity horizontal gradient (DS Transform Zone negative gravity anomaly influence) was inserted. As a result, residual gravity anomaly of -(0.08/0.14) mGal was revealed. The gravity field analysis was supported by resistivity measurements. We applied the Emigma 7.8 gravity software to create the 3-D physical-geological models of the sinkholes development area. The modeling was con- firmed by application of the GSFC program developed especially for 3-D combined gravity-magnetic modeling in complicated environments. Computed numerous gravity models verified an effective applicability of the microgravity technology for detection of karst cavities and estimation of their physical-geological parameters. A volume of the karst was approximately estimated as 35 000m³. The visual analysis of large sinkhole clusters have been forming at the microgravity anomaly site, confirmed the results of microgravity mapping and 3-D modeling. [ABSTRACT FROM AUTHOR]

Sinkhole development occurs in many areas of the world where soluble rocks crop out. Sinkholes are generally the surface expression of the presence of caves and other groundwater flow conduits in carbonate rocks, which are solutionally enlarged secondary permeability features. Their formation may be either natural or caused by man's activities. In both cases, heavy consequences have to be registered on the anthropogenic environment and related infrastructures. Knowledge of the mechanism of formation of this subtle geohazard is therefore necessary to planners and decision makers for performing the most appropriate and suitable programs of land use and development. The Apulia region of southern Italy is characterized for most of its extension by carbonate rocks, which makes it one of the most remarkable example of karst in the Mediterranean Basin. Based on analysis of literature and in situ surveys, including caving explorations, we have identified in Apulia three main types of possible mechanisms for sinkhole formation: 1) collapse of a chamber in a natural cave or in man-made cavities; 2) slow and gradual enlargement of doline through dissolution; 3) settlement and internal erosion of filling deposits of pre-existing dolines. Since sinkhole formation very often affects directly the human settlements in Apulia, and have recently produced severe damage, some considerations are eventually presented as regards the interrelationships between sinkholes and the anthropogenic environment. [ABSTRACT FROM AUTHOR]

Detailed simulation of hydrodynamics in karst areas is extremely tough because of strong heterogeneity of geology and geomorphology, e.g. rock fractures and sinkholes, and a dual flow system of surface stream and underground channel. In this study, the distributed hydrology-soil-vegetation model (DHSVM) is improved to simulate both surface stream flow and underground channel flow and to quantify sinkhole effects on collecting overland flow and subsurface flow. The model has be successfully applied in the karst watershed of Houzhai, a typical karst watershed located in Guizhou province of southwest China, with detailed observation data of surface stream and underground channel flow, hydraulic properties of soil and fractures and karst topography. The model parameter sensitivity and simulation uncertainty regarding surface stream and underground channel flow are executed using Monte Carlo simulation. The results indicate that the surface streams located in relatively thick soil in the north of the watershed primarily receives overland storm flow, while underground channels in the south obtain the incepted overland flow, and subsurface flow from soil, epikarst and deep aquifer flow. It is found that sinkholes in the karst watershed play an important role in transferring overland flow into underground flow. The model simulation results indicate that the underground channel flow peaks are noticeably reduced due to the large proportion of overland storm flow intercepted by sinkholes in the eastern mountainous area of watershed. Meanwhile, water amount recharging into the underground channel via sinkholes is proportional to the rainfall amount. [ABSTRACT FROM AUTHOR]