Deltas Research Papers - Academia.edu (original) (raw)

Four drowned shelf-edge delta complexes, two drowned shelf deltas, three drowned barrier islands, large areas of "hardground", and fields of bedforms on the mid and outer continental shelf and uppermost slope north of the head of De Soto Canyon, NE Gulf of Mexico were mapped with high-resolution multibeam echosounder. Deltas formed not during the last eustatic low stand, but during one or more interstadials when eustatic sea levels were only 60 to 80 m below present sea level. The barrier islands and deltas must have been cemented prior to rapid falls of eustatic sea level that occurred during global glaciations. Cementation is necessary to have preserved the barrier islands from erosion and subsequent destruction by the rapid sea-level rise during the last deglaciation. The preservation of the relict bathymetry is so good that features that superficially resemble trough blowouts are found in association with one of the relict barrier islands. Asymmetric bedforms on the midshelf in water depths of 50 to 60 m indicate transport directions to the SW but asymmetric bedforms in water depths of the upper slope between 100 and 120 m on the S and SE flanks of the drowned shelf-edge deltas indicate a different current direction, a separate flow that is a continuation of a SW-flowing current that was previously found on the upper slope off NW Florida.

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- Geology, Geomorphology, Sea Level, Depth

Among the large rivers rising on the Tibetan Plateau and adjacent high mountains, the discharge and suspended sediment load of the Ayeyarwady (Irrawaddy) River are the least well known. Data collected between 1969 and 1996 at Pyay (Prome) are analysed to provide the best available modern estimate of discharge (379 š 47 ð 10 9 m 3 /year) and suspended sediment load (325 š 57 ð 10 6 t/year) for the river upstream of the delta head. A statistical comparison with data collected in the nineteenth century (1871 to 1879) shows discharge has significantly decreased in the last ¾100 years. Regression and correlation analyses between discharge in the modern period and indices of El Niño-Southern Oscillation (ENSO) show a relationship.

- by Robert Wasson
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- Environmental Engineering, Civil Engineering, Far East, Discharge

Chikurachki volcano (Northern Kurile Islands Chain, Paramushir Island 50-20VN, 155-27VE; elevation 1816 m, stratovolcano) has been in a state of unrest for over twenty years. Its most recent eruption that began in April 2003 was preceded by an eruption between January and May 2002. Thermal infrared images from the Japanese -United States' Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER-5 TIR channels, 8 -12 Am, 90 m/pixel, Noise Equivalent Delta Temperature [NEDT]¨0.1 -0.3 K; onboard the U.S. National Aeronautics and Space Administration's Terra polar-orbiting spacecraft) of a snow-covered Chikurachki Volcano taken in January 2003 show muted evidence of thermal activity. ASTER thermal infrared (TIR) images from February 2003, however, indicate warmer areas within the summit crater. Average whole-pixel temperatures of adjacent summit slopes are in the range of 250 -252 K, while many summit-crater whole pixel temperatures are¨2 -6 K greater. A two component Planck solution for the warmer pixels yields a solution of 275 -277 K for 10 -15% of the Chikurachki summit crater and 285 -295 K for 25 -35% of a prominent ''hotspot'' on its flank. An interpretation of this enhanced preeruption heat flow is the presence of surface melt water. The detection in ASTER data of such subtle precursory heat-flow enhancement, even retrospectively, raises important issues for remote monitoring of ''dormant'' snow-capped volcanoes, especially those that threaten nearby populations, like Mt. Rainier. D

- by David Pieri
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- Interpretation, Noise, Volcanoes, Temperature

Deltas in the world are densely populated and have rich bio-diversities. Distributary rivers have a significant contribution in the delta building process. The abandonment and development of distributaries over time are common phenomena in the deltas. Human interventions may disrupt the natural process and cause rapid deterioration of these distributaries. Time series satellite images and hydrological data would be analyzed to reveal the causes of deterioration and probable measures to restore the abandoned distributaries. The Gorai River is a right bank distributary of the Ganges River. The morphology of the river off-take experienced large changes at the end of the 1980s. This article reports the morphological development of the Gorai off-take and outlines probable measures for the restoration of the Gorai flow. The high rate of right bank erosion upstream of Talbaria, and the angle between the Ganges main channel and the Gorai, have a significant influence on the sediment distribution at the off-take. Shortening of the flood recession period of the Ganges has contributed to the reduction of both dry and monsoon flow in th Gorai. Study results show that the study of the river processes in the longer time-scale in the active deltas has made it possible to design effective and efficient interventions.

- by Sudipta Kumar Hore and +1
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- Hydrology, Deltas, Satellite Images

The Upper Triassic -Lower Jurassic Åre Formation comprising the deeper reservoir in the Heidrun Field offshore mid-Norway consists of fluvial channel sandstones (FCH), floodplain fines (FF), and sandy and muddy bay-fill sediments (SBF, MBF) deposited in an overall transgressive fluvial to lower delta plain regime. The formation has been investigated to examine possible sedimentary facies controls on the distribution of cementation and compaction based on petrography and SEM/micro probe analyses of core samples related to facies associations and key stratigraphic surfaces. The most significant authigenic minerals are kaolinite, calcite and siderite. Kaolinite and secondary porosity from dissolution of feldspar and biotite are in particular abundant in the fluvial sandstones. The carbonate minerals show complex compositional and micro-structural variation of pure siderite (Sid I), Mg-siderite (Sid II), Fe-dolomite, ankerite and calcite, displaying decreasing Fe from early to late diagenetic carbonate cements. An early diagenetic origin for siderite and kaolinite is inferred from micro-structural relations, whereas pore filling calcite and ankerite formed during later diagenesis. The Fe-dolomite probably related to mixingzone dolomitization from increasing marine influences, and a regional correlatable calcite cemented layer has been related to a flooding event. Porosity values in non-cemented sandstone samples are generally high in both FCH and SBF facies associations averaging 27%. Differential compaction between sandstone and mudstone has a ratio of up to 1:2 and with lower values for MBF. We emphasize the role of eogenetic siderite cementation in reducing compactability in the fine-grained, coal-bearing sediments most prominent in MBF facies. This has implications for modeling of differential compaction between sandstone and mudstones deposited in fluvial-deltaic environments.

- by Erik Hammer and +1
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- Geology, Geophysics, Diagenesis, Clay Minerals

Shelf-edge deltas (SEDs) forming during periods of relative sea level fall and lowstand are generally efficient in transferring sediments to the slope and basins, and their identification in subsurface data is often considered a good indication of coeval development of slope and basin-floor turbidite reservoirs. This study investigates the seismic stratigraphic evolution of a forced-regressive and normal regressive shelf-edge delta (Bonaparte SED) that accumulated on the edge of the NW Australian margin during the late Quaternary. High resolution 2D and 3D reflection seismic data allow reconstruction of the main episodes of delta progradation and understanding of the extrinsic and intrinsic controls on their deposition. The lack of a significant turbidite system forming off the shelf-edge delta throughout the Quaternary is a striking feature of the Bonaparte SED. Instead, slope sedimentation is dominated by the accumulation of plume-derived mud belts and their reworking through mass-transport processes. Seismic geomorphology permits interpretation of the process regime of the youngest shelf-edge depocentre by applying a new process-based shallow-marine classification scheme to the 3D seismic attribute data. Results suggest either a tide or wave dominated delta with fluvial processes being of tertiary significance. A tide or wave-dominated, fluvial-affected shelf-edge delta classification is consistent with the paleogeographical reconstruction of the margin during the last glacial maximum (ca. 25 ka BP). The comparison of this mixed-process shelf-edge delta and starved slope system with a fluvial-dominated counterpart with significant sandy slope deposits emphasizes the potential of assessing the process regime of shelf-edge deltas as a rapid, first approach for predicting the presence or absence of coeval slope and basin-floor reservoirs.

Most present day estuaries formed within incised fluvial valleys, created during the last glacial, that drowned during post-glacial sea-level rise. The sedimentary archive of the associated river-mouth areas contains important information on estuarine evolution under different rates of sea-level rise. This thesis presents a study on the development of the mouth of the Rhine-Meuse system in the Rotterdam area, western Netherlands, between 12000-6000 BP. During the study tens of thousands of core descriptions and cone penetration test results, as well as seismic data, pollen and diatom analyses, tens of OSL-dates and hundreds of radiocarbon dates were used. The objectives were to explain: 1) the early-mid Holocene sedimentary succession of the Rhine-Meuse river-mouth area; 2) the development of the river-mouth area in the early-mid Holocene in response to rapid sea-level rise (SLR) and 3) the interaction of the fluvial and coastal systems during the early-mid Holocene transgression.
Between 10.5-8 ka BP, the effects of sea-level rise started to influence the study area: groundwater rise resulted in the formation of extensive wetlands, fluvial flood basins became more frequently flooded, sediment-aggradation rates increased and finally the river valley changed into an estuary with adjacent tidal basins. Sea level reached rates of 1 m/100 yr before 8 ka BP with a period of 2 m/100 yr between 8.5-8.3 ka BP as a result of sea-level jumping. During the latter period sea level rose 4.06 ± 0.5 m: 1.95 ± 0.74 m background sea-level rise and 2.11 ± 0.89 m sea-level jump. This jump is linked to the drainage of Lakes Agassiz and Ojibway in the Hudson Bay area and linked to the 8.2 event.
After 8 ka BP the rate of SLR slowed down to 0.6 m/100 yr. North and south of the estuary, retrogradation of the coastline occurred at a faster rate than near the mouth at Hoek van Holland, leading to the formation of a promontory. In the upper estuary a bay-head delta was formed. Around 7.3 ka BP the main branch of the Rhine connected to a tidal inlet 20 km north of the promontory that was strongly eroded. The eroded sediment was partly used to fill in the abandoned estuary, but also to fill in tidal basins north of the promontory. The closure of the several tidal inlets resulted in poor drainage conditions and the wetland area increased rapidly after 6.5 ka BP and around 6 ka BP the coastline reached its most eastward position.
The sedimentary succession was also described using sequence-stratigraphic concepts. The transition from lowstand to transgressive systems tract (TST) occurred when widespread basal-peat formation halted and gyttja accumulation and clastic deposition started. In large parts of the study area this occurred near-instantaneously during the sea-level jump. The highstand systems tract in the study area started after 6.5 ka BP with widespread peat formation in the closing tidal basins. The relatively late start of the TST is most likely typical for river-mouth areas along wide, low-gradient continental shelves where base-level changes are dominant for a relatively short period.

- by Marc Hijma
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- Holocene sea level change, Estuaries, Transgression, Deltas

Coastal areas which are very sensitive with ecologicaly are highly influenced by naturaly and human
activities and faced huge chabges by years. Delta areas that one of the most important geomorphological units of
coastal areas higly humen-used areas and changes rapidly by human pressures last decades. Wetlands of Sakarya
River Delta’s Coasts are one of the most affected places by this changes. In this study, Landsat Satellite images that
belongs to Sakarya Delta area dated 1984 and 2014 are examined by analysis methods of Remote Sensing (RS)
and Geographical Information Sciences (GIS) and land use change of coastal area is idendified. Major reason
for this change in Sakarya River Delta is shrinking of agricultural areas caused by expansion of residential and
industry areas. Effects of land use changes and precautions will be listed with this study.
Keywords: Delta change, Geographic Information Systems, Geographical analysis, Remote Sensing, Sakarya
River Delta, Use of Coastal Areas.

- by Sümeyra Kurt
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- Coastal Processes, Ecology, Rivers, Land Use Change
- by Usman Qamar
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- Engineering, Climate Change, Groundwater, Construction

The integration of core sedimentology, seismic stratigraphy and seismic geomorphology has enabled interpretation of delta-scale (i.e. tens of metres high) subaqueous clinoforms in the upper Jurassic Sognefjord Formation of the Troll Field. Mud-prone subaqueous deltas characterized by a compound clinoform morphology and sandy delta-scale subaqueous clinoforms are common in recent tide-influenced, wave-influenced and current-influenced settings, but ancient examples are virtually unknown. The data presented help to fully comprehend the criteria for the recognition of other ancient deltascale subaqueous clinoforms, as well as refining the depositional model of the reservoir in the super-giant Troll hydrocarbon field. Two 10 to 60 m thick, overall coarsening-upward packages are distinguished in the lower Sognefjord Formation. Progressively higher energy, wave-dominated or current-dominated facies occur from the base to the top of each package. Each package corresponds to a set of seismically resolved, westerly dipping clinoforms, the bounding surfaces of which form the seismic 'envelope' of a clinoform set and the major marine flooding surfaces recognized in cores. The packages thicken westwards, until they reach a maximum where the clinoform 'envelope' rolls over to define a topset-foreset-toeset geometry. All clinoforms are consistently oriented sub-parallel to the edge of the Horda Platform (N005-N030). In the eastern half of the field, individual foresets are relatively gently dipping (1°to 6°) and bound thin (10 to 30 m) clinothems. Core data indicate that these proximal clinothems are dominated by fine-grained, hummocky crossstratified sandstones. Towards the west, clinoforms gradually become steeper (5°to 14°) and bound thicker (15 to 60 m) clinothems that comprise mediumgrained, cross-bedded sandstones. Topsets are consistently well-developed, except in the westernmost area. No seismic or sedimentological evidence of subaerial exposure is observed. Deposition created fully subaqueous, near-linear clinoforms that prograded westwards across the Horda Platform. Subaqueous clinoforms were probably fed by a river outlet in the north-east and sculpted by the action of currents sub-parallel to the clinoform strike.

- by Stefano Patruno
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- Geology, Sedimentology, Upper Jurassic, Deltas

Kıyılar, karmaşık ve çok yönlü doğal ilişkilerin bütünleştiği ekosislerin başında gelmektedir. Deltalar başta olmak üzere alçak rakımlı kıyı alanları, dünya üzerinde en hızlı değişime uğrayan birimleri oluşturmaktadırlar. Türkiye kıyılarında da hem sosyo-ekonomik, hem de ekolojik açıdan değeri yüksek deltaların bulunması, bu kouda yapılması gereken çalışmaların önemini göstermektedir. Son yıllarda uydu görüntüleri ve hava fotoğrafları, kıyı alanlarında meydana gelen zamansal değişimlerin belirlenmesinde yararlanabilecek önemli veri kaynakları haline gelmiştir. Bu çalışmanın amacı da uydu teknolojilerini kullanarak Göksu deltası sahilinde kıyı çizgisinin zamansal değişimini ortaya koymak ve CBS ortamında değişim haritalarını üretmektir. Sonuçlar, deltanın doğu kıyılarında meydana geln bir gerilemenin, güncel ağız bölgesinde ise ilerlemenin gerçekleştiğini göstermiştir. Paradeniz lagünü kıyı kordonunda şiddetli bir kıyı gerilemesinin, İncekum burnunun batı ve güneydoğusunda ise kumul ilerlemesinin oluştuğu tespit edilmiştir. Altınkum mevkiinde ise
şiddetli bir kıyı gerilemesi mevcuttur.

- by Ahmet Karakoç
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- Remote Sensing, Deltas, CBS, Geographic Information Systems (GIS)
- by William Athens
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- History, Geomorphology, Wetlands, Culture

Fire Island, New York, is a 50-kilometer-long barrier island that has remained positionally stable without any formation of breach inlets for nearly 200 years. Some researchers have attributed its stability to a major supply of sand moving onshore from relatively deep water (i.e., >10 m depths). Others have demonstrated via sediment budgets that the principal sand sources at decadal to century time scales are littoral sediments derived from eroding beaches, bluffs, and cannibalization of inlet shoals in shallower depths (i.e., <10 m). Published sediment budgets indicate that the quantity in question is of the order 10 5 m 3 /yr. The possibility that this deep-water source of sand is significant, active, and persistent at decadal to century time scales has led to reluctance to mine deep-water shoals for beach nourishment of Fire Island. Herein, the authors review five factors related to the potential for a significant deep-water sand source in this setting: (1) spatial and temporal frames of reference necessary for this flux of sand; (2) studies of scour and sediment transport over offshore features; (3) sediment size distribution across the foreshore; (4) depth of closure (DOC); and (5) contribution of abandoned inlet shoals. The authors conclude that evidence for an onshore flux of sediment (i.e., order of 10 5 m 3 /yr) is lacking and suggest that reluctance to mine the offshore for beach nourishment is unfounded.

- by Julie Rosati
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- Engineering, Earth Sciences, Coastal Erosion, Coastal

Clinoforms are ubiquitous deltaic, shallow-marine and continental-margin depositional morphologies, occurring over a range of spatial scales (1-10 4 m in height). Up to four types of progressively larger-scale clinoforms may prograde synchronously along shoreline-to-abyssal plain transects, albeit at very different rates. Paired subaerial and subaqueous delta clinoforms (or 'delta-scale compound clinoforms'), in particular, constitute a hitherto overlooked depositional model for ancient shallow-marine sandbodies. The topset-to-foreset rollovers of subaqueous deltas are developed at up to 60 m water depths, such that ancient delta-scale clinoforms should not be assumed to record the position of ancient shorelines, even if they are sandstone-rich. This study analyses a large dataset of modern and ancient delta-scale, shelf-prism-and continental-margin-scale clinoforms, in order to characterise diagnostic features of different clinoform systems, and particularly of deltascale subaqueous clinoforms. Such diagnostic criteria allow different clinoform types and their dominant grain-size characteristics to be interpreted in seismic reflection and/or sedimentological data, and prove that all clinoforms are subject to similar physical laws. The examined dataset demonstrates that progressively larger scale clinoforms are deposited in increasingly deeper waters, over progressively larger time spans. Consequently, depositional flux, sedimentation and progradation rates of continental-margin clinoforms are up to 4-6 orders of magnitude lower than those of deltas. For all clinoform types, due to strong statistical correlations between these parameters, it is now possible to calculate clinoform paleobathymetries once clinoform heights, age spans or progradation rates have been constrained. Muddy and sandy delta-scale subaqueous clinoforms show many different features, but all share four characteristics.

- by Stefano Patruno
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- Earth Sciences, River Deltas, Deltas, Holocene Evolution of Deltas

C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 332 (2001) 37-44  2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS. Tous droits réservés S1251-8050(00)01487-7/FLA Abstract -North-Pyrenean foreland basin evolution during Palaeogene times (Adour basin) : stratigraphic constraints. The study of 50 wells correlated according to the principles of High Resolution Sequence Stratigraphy, shows that the Adour basin is filled during two main steps. (1) The Palaeocene is characterised by aggradational carbonate platforms, passing southward to turbiditic sedimentation. During this initiation stage, the carbonate production balances accommodation space creation. The Ypresian-Priabonian is characterised by large progradational deltaic systems, migrating westward. During this stage, siliciclastic supply was higher than accommodation space creation. This basin is interpreted during Palaeocene to Middle Eocene as a compressional basin due to lithospheric buckling. The foreland history starts during the Upper Eocene to Oligocene.  2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS

- by Eric Leroy
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- Earth Sciences, Sequence Stratigraphy, Carbonate Platform, Deltas

A consistent database was established to characterize key environmental factors known to control delta morphology. The database includes the location, basin morphology, fluvial and sediment discharge to the deltas, delta morphology, ocean energy, and shelf depth reached by the sub-aqueous delta. Fifty-one deltas were selected to cover the global parameter range of rivers entering all major oceans and coastal seas. Seasonal satellite images of the deltas were processed (IKONOS, SPOT, LANDSAT, and MODIS). Predictive statistical relationships were obtained, suitable for hypothesis testing or to constrain/verify numerical models used to simulate the evolution of coastal systems. The area of a delta is best predicted from average discharge, the total sediment load feeding the delta, and the offshore accommodation space. The gradient of a delta plain, measured from the apex of the delta to the coast along the main channel, is best predicted with a ratio of sediment supply to sediment retention, sediment concentration used as a proxy of delta plain sedimentation, and mean water discharge. Widths of distributary channels form a lognormal distribution, with the cumulative width of the river mouths directly related to the maximum discharge, tidal and wave energy. The grain size of topset deposits scales with the river length. Hundreds of millions of people occupy deltas and human engineering is now a major influence on the growth and evolution of many deltas, through control of the flow path of distributary channels, and mitigation of the seasonal flood wave with concomitant change in the delivery of sediment load. More and more deltas are moving away from their pre-Anthropocene morphology, as influenced by pristine sediment supply and sediment dispersal.

- by Yoshiki Saito
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- Earth Sciences, Wave Energy, Seasonality, Grain size

Reçu le 12 novembre 2003 ; accepté après révision le 10 mai 2005 Disponible sur Internet le 13 septembre 2005 Présenté par Michel Durand-Delga Résumé Dans le Haut Atlas de Marrakech, la sédimentation s'est effectuée dans deux bassins différents durant l'Arénig-Llanvirn. Dans le bassin du Haut Atlas occidental s'est développé un delta influencé par vagues et tempêtes, alimenté par une source située à l'emplacement actuel du couloir d'Argana et soumis à des fluctuations eustatiques et à la subsidence. Dans le Haut Atlas central se sont développés des deltas influencés par la marée et les tempêtes épisodiques et des estuaires qui étaient alimentés par des sources situées à l'emplacement actuel des massifs du Siroua et de l'Ouzellagh et soumis à des fluctuations eustatiques et une tectonique verticale. Pour citer cet article : C. Chacrone, N. Hamoumi, C. R. Geoscience 337 (2005).  2005 Académie des sciences. Publié par Elsevier SAS. Tous droits réservés.

- by naima hamoumi
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- Palaeogeography, Geology, Tectonics, Subsidence
- by Robin Davidson-arnott
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- Geology, Geomorphology, Computer Science, Climate Change

The Mekong River Delta in Southern Vietnam is a tide-dominated delta and has been formed by the Mekong River system. The sediments are dominantly ®ne grained and were deposited in the receiving basin with slight inclination of pre-existing deposits in the South China Sea and Gulf of Thailand. Around 6000 to 5000 yr BP maximum Holocene transgression occurred and seawaves surrounded uplands of the Late Pleistocene terrace, basement rock and weathered land in the northern parts of the delta. Double notches were undercut by seawaves on the limestone hill in Hatien at about 4.5 and 2.5 m above present sea level. Marine deposits prevailed throughout the delta. During the highstand and regressions of relative sea level over the last 4550 yr BP, delta progradation has produced a great¯at-plain of 62,520 km 2. This extremely fast progradation could be due to: (a) very high sediment supply depositing in the slight inclination of the receiving basin; (b) neotectonic movements and relative sea-level changes; (c) widespread mangrove forests playing an important role to enhance sediment accumulation. Former mangrove marshy deposits were signi®cantly developed in the Plain of Reeds and Longxuyen Quadrangle. Mangrove marshes have also been developing well in the Camau Peninsula and present coastlines. Moreover, regressions over the last 4550 yr BP have produced many distinct sets of relict beach ridges which are useful in distinguishing the former coastlines and evolutionary history of the delta.

- by Masaaki TATEISHI
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- Geology, Geochemistry, Geomorphology, Geophysics

This study examines the accuracy of the predictions of gas hydrate saturations made based on five-step analysis of 3D seismic data prior to 2005 drilling, logging, conventional coring, and pressure core sampling through the gas hydrate stability zone at two focus sites in the northern Gulf of Mexico. These predictions are detailed in Part I (2008). Here we conduct a detailed analysis of the gas hydrate saturation using both resistivity and P-wave velocity log data and analyze the pre-drilling predictions, which were made almost exclusively on the basis of seismic data, with no local logging control. Well log measurements, core data analysis, and pressure core-degas experiments all indicated general agreement with the pre-cruise analysis regarding the location and approximate concentration of gas hydrates in the sediments. We find that seismic predictions are generally consistent with log-based estimates after upscaling to seismic frequencies. We recalibrated the pre-drill model based on the new field data so that a refined version of the model could be used for future work.

- by Nader Dutta
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- Geology, Geophysics, Data Analysis, Prediction

1] Lightning return-stroke models are needed for specifying the source in studying the production of transient optical emission (elves) in the lower ionosphere, the energetic radiation from lightning, and characterization of the Earth's electromagnetic environment, as well as studying lightning interaction with various objects and systems. Reviewed here are models based on Maxwell's equations and referred to as electromagnetic models. These models are relatively new and most rigorous of all models suitable for computing lightning electromagnetic fields. Maxwell's equations are numerically solved to yield the distribution of current along the lightning channel. Different numerical techniques, including the method of moments (MoM) and the finite difference time domain (FDTD) method, are employed. In order to achieve a desirable current-wave propagation speed (lower than the speed of light in air), the channel-representing wire is embedded in a dielectric (other than air) or loaded by additional distributed series inductance. Capacitive loading has been also suggested. The artificial dielectric medium is used only for finding the distribution of current along the lightning channel, after which the channel is allowed to radiate in air. Resistive loading is used to control current attenuation with height. In contrast with distributed circuit and so-called engineering models, electromagnetic return-stroke models allow a self-consistent full-wave solution for both lightning-current distribution and resultant electromagnetic fields. In this review, we discuss advantages and disadvantages of four return-stroke channel representations: a perfectly conducting/resistive wire in air, a wire embedded in a dielectric (other than air), a wire in air loaded by additional distributed series inductance, and a wire in air having additional distributed shunt capacitance. Further, we describe and compare different methods of excitation used in electromagnetic return-stroke models: closing a charged vertical wire at its bottom with a specified grounded circuit, a delta-gap electric field source, and a lumped current source. Finally, we review and compare representative numerical techniques used in electromagnetic modeling of the lightning return stroke: MoMs in the time and frequency domains and the FDTD method. We additionally consider the so-called hybrid model of the lightning return stroke that employs a combination of electromagnetic and circuit theories and compare this model to electromagnetic models. Citation: Baba, Y., and V. A. Rakov (2007), Electromagnetic models of the lightning return stroke,

- by Vladimir Rakov
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- Radiation, Earth, Multidisciplinary, Ionosphere

C. Ibáñez). are severely impacted by humans. These impacts range from rapid hydromorphological and ecological disruption due to alterations in river basins (i.e., flow regulation, sediment retention, etc.) and delta plains (i.e., dikes, altered hydrology, fluid withdrawals, etc.), to consequences of increased relative sea level rise (RSLR), such as increasing flooding risks . Many deltas are heavily populated, especially in Asia , which exacerbates the consequences of natural and anthropogenic impacts. Moreover, deltas undergo rapid changes at geological, ecological and human scales with strong feed-backs among these three levels, so their management and restoration must be based on a comprehensive 0925-8574/$ -see front matter

- by Carles Ibáñez
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- Engineering, Ecological Engineering, Earth Sciences, Resilience

Laboratory experiments to simulate landscapes and stratigraphy often suffer from scale effects, because reducing length- and time scales leads to different behaviour of water and sediment. Classically, scaling proceeded from dimensional analysis of the equations of motion and sediment transport, and minor concessions, such as vertical length scale distortion, led to acceptable results. In the past decade many experiments were done that seriously violated these scaling rules, but nevertheless produced significant and insightful results that resemble the real world in quantifiable ways. Here we focus on self-formed fluvial channels and channel patterns in experiments. The objectives of this paper are 1) to identify what aspects of scaling considerations are most important for experiments that simulate morphodynamics and stratigraphy of rivers and deltas, 2) to establish a design strategy for experiments based on a combination of relaxed classical scale rules, theory of bars and meanders, and small-scale experiments focussed at specific processes. We present a number of small laboratory setups and protocols that we use to rapidly quantify erosive and sedimentary types of forms and dynamics that develop in the landscape experiments as a function of detailed properties such as effective material strength and to assess potential scale effects. Most importantly, the width-to-depth ratio of channels determines the bar pattern and meandering tendency. The strength of floodplain material determines these channel dimensions, and theory predicts that laboratory rivers should have 1.5 times larger width-to-depth ratios for the same bar pattern. We show how floodplain formation can be controlled by adding silt-sized silica flour, bentonite, Medicago sativa (alfalfa) or Partially Hydrolyzed PolyAcrylamide (a synthetic polymer) to poorly sorted sediment. The experiments demonstrate that there is a narrow range of conditions between no mobility of bed or banks, and too much mobility. The density of vegetation and the volume proportion of silt allow well-controllable channel dimensions whereas the polymer proved difficult to control. The theory, detailed methods of quantification and experimental setups presented here show that the rivers and deltas created in the laboratory seem to behave as natural rivers when the experimental conditions adhere to the relaxed scaling rules identified herein, and that required types of fluvio-deltaic morphodynamics can be provoked by conditions and sediments selected on the basis of a series of small-scale experiments.

- by Wout van Dijk and +2
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- Morphology, Rivers, Deltas, Flume Experiments

This paper considers recent studies of global environmental change and their impact on the deltas, social and ecological patches that epitomize an Anthropocene environmental dynamism. Looking into these delta studies, we explore an emerging imagination about human-planet relations. Specifically, we indicate that the relationship between the changing Earth and human activities depicted in these studies is comparable to the kind of affective relations to which the anthropology of science has recently brought attention. While affective relations as anthropologically described depend on the capacity of the body to be affected by other entities, global change research on deltas asks the public to imagine collective life, including infrastructure, land use, resource consumption, and companion species, as composite bodies affected by the changing planet. This imagination is made possible by analogies developed in the vicinity of the notion of resilience, a term that originated in mathematical ecology and complex adaptive systems in computer science. In exploring this interdisciplinary traffic of ideas and models, we elucidate an analogical imagination that crosses the border of machines and organisms.

- by Atsuro Morita
- •
- Anthropology, Water, Resilience, Sustainable Development

The deeper groundwater (depending on definition) of the Bengal basin (Ganges–Brahmaputra delta) has long been considered as an alternate, safe drinking-water source in areas with As-enrichment in near-surface groundwater. The present study provides the first collective discussion on extent and controls of elevated As in deeper groundwater of a regional study area in the western part of the Bengal basin. Deeper groundwater is defined here as non-brackish, potable (Cl− ⩽ 250 mg/L) groundwater available at the maximum accessed depth (∼80–300 m). The extent of elevated As in deeper groundwater in the study area seems to be largely controlled by the aquifer–aquitard framework. Arsenic-enriched deeper groundwater is mostly encountered north of 22.75°N latitude, where an unconfined to semi-confined aquifer consisting of Holocene- to early Neogene-age gray sand dominates the hydrostratigraphy to 300 m depth below land surface. Aquifer sediments are not abnormally enriched in As at any depth, but sediment and water chemistry are conducive to As mobilization in both shallow and deeper parts of the aquifer(s). The biogeochemical triggers are influenced by complex redox disequilibria. Results of numerical modeling and profiles of environmental tracers at a local-scale study site suggest that deeper groundwater abstraction can draw As-enriched water to 150 m depth within a few decades, synchronous with the advent of wide-scale irrigational pumping in West Bengal (India).► First detailed study of deeper groundwater arsenic (As) in western Bengal basin. ► The study shows presence of elevated As in contrast to existing hypothesis. ► Regionally, hydrostratigraphy has a strong control on As distribution. ► As mobilization conducive sediment-groundwater chemistry is present in >200 m depth. ► Locally, prolific deep pumping can attract As-enriched shallow water to deeper depth.

- by Prosun Bhattacharya
- •
- Geochemistry, Applied Geochemistry, Quaternary, Depth

The low lying Nal region in western India, linking the Gulf of Kachchh with the Gulf of Khambhat through the Little Rann and Nal Sarovar is barely 15 m above msl and lacks surface exposures. The evolutionary history of the Nal region using remote sensing data and sub-surface lithological correlation indicated that late Quaternary sedimentation in the Nal region was governed by changes in sea level and by tectonism in the region of Cambay Graben. The geomorphic evidence for changes in sea level was found in the form of inland palaeo-deltas and old mud flats. Abrupt changes in lithological data in the vicinity of Nal region pointed to the role of tectonism. Contrary to the earlier view, a shallow sea linked the Gulf of Kachchh to the Gulf of Khambhat only in a time period around Marine Isotope Stage 5. Our studies also suggest that the Nal Ž . region itself may not have witnessed any major uplift beyond ; 10 m during late Quaternary. q 1998 Elsevier Science B.V. All rights reserved.

- by sushma prasad
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- Geology, Geomorphology, Sea Level, Quaternary
- by Jorrie smith
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- Geomorphology, Interpretation, Multidisciplinary, Sediment transport

The Zambezi River is the fourth largest river in Africa (after the Congo, Nile, and Niger), and it is the largest African river flowing into the Indian Ocean. The lower Zambezi in Mozambique is influenced by the presence of two very large reservoirs (Kariba dam and Cahora Bassa dam) that have modified the natural seasonal flows, as well as the sediment balance and morphology of the river. In particular, downstream of the Cahora Bassa reservoir down to the delta, no negligible effects are reported to take place, such as local scour, bank collapse, and shoreline progressive erosion . In order to quantify and possibly mitigate these effects, a simplified numerical model of the sediment transport and erosion/sedimentation phenomena along the lower Zambezi has been developed, capable to deal with the scanty and uncertain data available. Indeed, besides the systematic flow records at the dam sites and few occasional measurements of turbidity and bottom granulometry, only the Digital Elevation Model (DEM) of the river will be used for the model. The objective of this paper is to predict the present and future effects of the presence of the Kariba and Cahora Bassa dams on the downstream morphology, integrating the few coarse and nonsimultaneous data, somehow improving their overall quality. The model reproduces the time and space propagation of the disturbances, that confirm, with more generality, the qualitative response of the river to the constructions of dams, anticipated by various geomorphologists. In fact, the reduction of waterflow seems to have an immediate effect downstream by initially fostering the sediment deposition. Subsequently, the total interception of sediment by the dam slowly takes over and inverts this tendency. A slightly smaller aggradation (or slightly larger degradation) rate with respect to the natural conditions (no dams) seems to represent the dominant effect of damming in the long term evolution of the lower Zambezi.

- by Michael Nones
- •
- Geology, Geomorphology, Morphology, Seasonality

The first millennium AD encompasses the Roman period (12 BC to AD 450) and the Early Middle Ages (AD 450 to 1050). In the Netherlands, this millennium saw population growth, steep decline and subsequent revival. In addition, many changes occurred in the physical landscape, marking a transition from a mainly natural prehistorical lowland landscape to an increasingly human-affected landscape. This paper synthesises the main landscape changes and human-landscape interactions in the Netherlands during this dynamic period. The degree of landscape change is compared between the coastal plain, the delta and the Pleistocene sand area. Human activities caused major often unintended geomorphological changes in all studied landscapes. Landscape sensitivity to human impact, however, strongly varied as a function of preceding landscape state. The most prominent changes took place in many parts of the coastal plain, where back-barrier peatlands transformed into open tidal basins. Presence of compaction-prone peat and intensified artificial drainage led to subsidence. This precondition and slow anthropogenic forcing combined, made the area more sensitive to stochastically occurring storms, which could serve as tipping points for large-scale drowning. Eventually, major peatlands turned into tidal areas that for many centuries would remain unsuitable for habitation. Human-induced peatland subsidence also led to the formation of the new Hollandse IJssel and Lek river branches. This marked a major reorganisation of the river network in the lower Rhine-Meuse delta. In the middle and upstream parts of the delta, the landscape was more stable. Yet, settlements on the natural levees show adaption to increasing flooding frequency from the Late Roman period onwards. The settlements shifted towards higher positions, while route networks between them largely remained intact. Smaller-scale landscape changes were found in the Pleistocene sand area. Here, local sand drifting occurred, most frequently occurring close to human movement corridors. Drift sand intensity became larger as population density increased after c.AD 900.

- by Harm Jan Pierik
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- Geomorphology, Geoarchaeology, Landscape Archaeology, Coastal Geomorphology

1] MODIS (Moderate Resolution Imaging Spectroradiometer) derived aerosol optical depths (AODs) were compared against the ground based observations from Microtops sunphotometer over Ahmedabad (72.5°E, 23.03°N) in Western India. The region is semiarid and poses challenge for the satellite remote sensing of aerosols. Besides comparing the ground truth with the Collection Version 4 of MODIS aerosol product, the paper reports the first ever validation of the updated Collection Version 5 of the MODIS aerosol product over India. The AOD data from Aqua platform is averaged over 0.5°Â 0.5°c entered at Ahmedabad and compared with the sunphotometer observation taken within half an hour to the satellite overpass time. The Version 4 data comparison showed a large scatter. Further, the comparison for 470 nm and 660 nm behave differently over different years. Overall, the comparison shows considerable improvement in the Collection Version 5 aerosol product. Among seasons, Pre-Monsoon (April to May) has the best correlation and Dry season (December to March) the least. The updated product has scope for further improvement as the correlations are less than unity, and the extent of underestimation for 470 nm is more during Dry and Post-Monsoon seasons whereas that for 660 nm is more during Pre-Monsoon and Monsoon seasons which are dominated by fine and coarse particles respectively. The results show a better surface reflectance parameterization by the MODIS Collection Version 5 algorithm as compared to Version 4 but the aerosol model used in the retrieval algorithm is still not adequate.

- by Marina Aloysius
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- Geophysics, Algorithms, Water, Modeling

Export-oriented shrimp aquaculture appeared in the coastal landscape of Peru in the late 1970s. Its rapid development has entailed both positive and negative environmental and socioeconomic impacts across the newly emerging shrimp farming territory of the Tumbes river delta. Here we combine ground surveys and interviews with remote sensing analysis of the transformed environment in order to characterize and quantify the nature, origin and scale of the environmental and social impacts caused by the intrusion of aquaculture. Results show that shrimp farming has encroached on several land cover categories and converted 17% of the Peruvian mangrove. We provide an overview of the history and nature of those impacts. Minimum full-time equivalent employment, which occurred during a white-spot epidemic, was attained in 2001 with 439 full-time jobs; employment peaked in 2006, with 2660 fulltime jobs. However, considering indirect benefits, remittances and the prevalence of part-time jobs, the population reliant on aquaculture probably exceeds 10,000. A geographical analysis of social networks reveals how shrimp farming draws a sustained influx of population from the Andes and how it modifies the interaction between population and the natural coastal environment. Recent sectorial improvements towards achieving a more sustainable management of the coastal resources are analysed, and additional recommendations predicated on a more integrated approach are provided.

- by C. Mering
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- Management, Earth Sciences, Remote Sensing, Migration

Accelerated rise in sea-level, and associated geological adjustment, may result in a high impact on shorelines. On the basis of hourly sea-level observations over a 20 year period, sea-level change along the Turkish coast has been investigated. In addition to relative sea-level rise, the high level of human activities in coastal areas puts a significant amount of stress on Turkish coastal zones, especially on coastal lowland plains. Coastal erosion and ground water salinization are the most important impacts and two case studies are discussed in this paper: the Kizilirmak river delta on the Black Sea; and the tourism center of Erdek on the Marmara Sea. Coastal retreat on the Kizilirmak river delta is mostly attributed to decreasing sediment supply to the coast. The shore barriers and the enclosed lagoons are likely to respond readily to future sea-level rise. At Erdek, the most important threat is intrusion of salt water into freshwater systems caused by a combination of accelerated sea-level rise, land reclamation and overexploitation of underground aquifers.

- by B. Alpar
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- Geology, Geomorphology, Case Studies, Subsidence

- by Nader Dutta
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- Geology, Geophysics, Data Analysis, Prediction

Detailed facies and palaeogeographical analysis of Lower and Middle Jurassic sequences in the Turpan-Hami oil producing basin of northwest China led to the reconstruction of a basin-wide depositional model. Twenty distinct lithofacies have been identified which have been further grouped into sedimentary facies and five major facies associations; braided fluvial, meandering fluvial, braid delta, (meandering channel) delta, and lacustrine associations. Coal-forming swamps occur in each facies association but the preferred sites of accumulation are interdelta bay and lower delta plain environments. A series of age specific, basin-wide, palaeogeogeographical maps have been constructed leading to a depositional model for the basin. Results indicate that the basin experienced cyclic swamp developing -- lake flooding sequences, and that these characterize the deposition of the Early and early Middle Jurassic coal measures. Analyses indicate that the Bogda-Harlik Mountains were uplifted prior to the Early Jurassic and the two mountain regions were separated by a lake during the Early and early Middle Jurassic. From these results it is predicted that oil bearing coal sequences are to be found in other parts of the basin and as such, the full economic potential of the basin is not yet fully realised.

- by Jason Hilton
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- Geology, Depression, Carbon, China

are also sincerely thanked, particularly Cornel for many, many long discussions which sharpened the arguments presented here. This dissertation benefited greatly from a strong foundation of experimental study conducted with Tetsuji Muto at Nagasaki University, Japan. Tetsuji's insights, encouragement, and enthusiasm have been greatly appreciated. The experimental work presented in Chapter Two was financially supported in part by a Japanese Grant-in-Aid for Scientific Research (15340171) to Tetsuji Muto. Funding for fieldwork in Utah from BHP Billiton, the William Dow Hamm AAPG Grant-in-Aid, and the RioMAR Consortium is gratefully acknowledged and appreciated. Of course, I have to thank my fellow graduate students for stimulating discussions in the office or over beers which were critical to pushing forward scientific thought, particularly Cristian Carvajal (who led the way towards solving the sediment-flux problem),

- by william fisher
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- Geology, Geomorphology, Stratigraphy, Geometric model

Surface water-groundwater interactions play a crucial role in the hydrology and ecology of the Okavango Delta. The hydrology of the Delta is dominated by the annual arrival of a flood wave which is distributed over an number of branches. Subsequently, the flood water feeds the phreatic aquifers underlying the Delta islands. In order to evaluate the seasonal and long-term dynamics of the surface water-groundwater interactions between the floodplains and the islands, a network of piezometers located in various locations of the Delta was monitored. Groundwater table fluctuations observed for up to 6 years were analysed and modelled using groundwater flow models. The floodplain-island groundwater flow is in general very dynamic and driven by island evaporation and transpiration. A typical small to medium sized island (width !500 m), appear not to be influenced by long-term antecedent conditions. Only on large islands (width O500 m) and at the perimeter of the flooded area is the influence of long-term antecedent conditions apparent. The knowledge gained during this study will be used for the improvement of the hydrological and hydro-ecological model of the Delta, and can be useful for the description of floodplain dynamics in semi-arid regions in general. q

- by Piotr Wolski
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- Hydrology, Ecology, Evaporation, Multidisciplinary

A comprehensive understanding of the groundwater dynamics of a transboundary aquifer system is highly needed for any successful transboundary cooperation policy. Moreover, an analysis of the NWSAS can be of particular interest for policy makers and researchers. This paper aims to reveals and to assess the renewability of North-Western Sahara Aquifer System (NWSAS) as one of the major transboundary multi-layered aquifer system, in North Africa, shared by Algeria, Tunisia, and Libya and is often referred to as the Système Aquifère du Sahara Septentrional (SASS). The paper is primarily intended for exploring whether it receives a considerable fraction of modern water as recharge or it is at risk of being depleted and excessively pumped, where the main challenge for NWSAS, is that it should be abstracted rationally for equitable use.

- by Samir Al-Gamal
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- Hydrology, Water resources, Stable Isotopes, Multidisciplinary

Valid representations of geological heterogeneity are fundamental inputs for quantitative models used in managing subsurface activities. Consequently, the simulation of realistic facies distributions is a significant aim. Realistic facies distributions are typically obtained by pixel-based, object-based or process-based methods. This work presents a pixel-based geostatistical algorithm suitable for reproducing lateral gradual facies transitions (LGFT) between two adjacent sedimentary bodies. Lateral contact (i.e. interfingering) between distinct depositional facies is a widespread geometric relationship that occurs at different scales in any depositional system. The algorithm is based on the truncation of the sum of a linear expectation trend and a random Gaussian field, and can be conditioned to well data. The implementation introduced herein also includes subroutines to clean and geometrically characterize the obtained LGFT. The cleaned sedimentary body transition provides a more appropriate and realistic facies distribution for some depositional settings. The geometric measures of the LGFT yield an intuitive measure of the morphology of the sedimentary body boundary, which can be compared to analogue data. An example of a LGFT obtained by the algorithm presented herein is also flow simulated, quantitatively demonstrating the importance of realistically reproducing them in subsurface models, if further flow-related accurate predictions are to be made.

- by Pau Arbues and +1
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- Engineering, Earth Sciences, Algorithms, Prediction
- by maran gabriel
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- Deltas

The lithologic composition and grain size distribution of sediments are primary determinants of their inherent reflectance properties. However, moisture content is also known to have a strong influence on reflectances of soils and sediments. If the effects of sediment composition, grain size and moisture content could be distinguished spectrally, it might be possible to map these properties at synoptic scales using hyperspectral, or perhaps even broadband, remote sensing. Mapping the spatiotemporal distribution of sediment composition and moisture content could provide unique constraints on both the processes by which the sediments are deposited as well as the constraints they may impose on subsequent water flow and sediment transport. The Ganges-Brahmaputra delta (GBD) is formed by the convergence of these two great rivers and is superlative in both size and geologic activity. Sediment redistribution and channel migration associated with the annual floods disrupt the lives of hundreds of thousands of people living on the GBD but is also critical for maintaining the delta area fertile and above sea level. The 30+ year archive of Landsat imagery could provide a basis for spatiotemporal analysis of these fluvial dynamics if sediment properties could be inferred or measured from reflectance spectra. However, before confronting the challenge of broadband detection we must understand the spectral properties of the sediments under more controlled laboratory conditions. Bidirectional reflectance spectroscopy of 109 sediment samples from the GBD yields a spectral mixing space that appears to be structured by variations in moisture content, grain size and possibly lithology. Although the individual Empirical Orthogonal Functions of the Principal Components do not correspond to unique absorption features, clustering within the mixing space is clearly influenced by moisture content and grain size. Laboratory spectra of sediment reflectance measured under varying moisture content yield distinct trajectories through the spectral mixing space for different grain size distributions of sieved sediments. These variations in moisture content account for N98% of spectral variance observed in these samples. Drying trajectories of coarse, fine and mixed sediments are distinct and suggest that moisture and grain size might be spectrally distinguishable. These results are consistent with Angstrom's hypothesis of moisture-driven spectral absorption but more controlled experiments are necessary to test the hypothesis rigorously.

- by Michael S Steckler
- •
- Cartography, Remote Sensing, Principal Component Analysis, Spectroscopy

Estuaries are among the most invaded ecosystems on the planet. Such invasions have led in part, to the formation of a massive $1 billion restoration effort in California's Sacramento-San Joaquin River Delta. However, invasions of weeds into riparian, floodplain, and aquatic habitats threaten the success of restoration efforts within the watershed and jeopardize economic activities. The doctrine of early detection and rapid response to invasions has been adopted by land and water resource managers, and remote sensing is the logical tool of choice for identification and detection. However meteorological, physical, and biological heterogeneity in this large system present unique challenges to successfully detecting invasive weeds. We present three hyperspectral case studies which illustrate the challenges, and potential solutions, to mapping invasive weeds in wetland systems: 1) Perennial pepperweed was mapped over one portion of the Delta using a logistic regression model to predict weed occurrence. 2) Water hyacinth and 3) submerged aquatic vegetation (SAV), primarily composed of Brazilian waterweed, were mapped over the entire Delta using a binary decision tree that incorporated spectral mixture analysis (SMA), spectral angle mapping (SAM), band indexes, and continuum removal products. Perennial pepperweed detection was moderately successful; phenological stage influenced detection rates. Water hyacinth was mapped with modest accuracies, and SAV was mapped with high accuracies. Perennial pepperweed and water hyacinth both exhibited significant spectral variation related to plant phenology. Such variation must be accounted for in order to optimally map these species, and this was done for the water hyacinth case study. Submerged aquatic vegetation was not mapped to the species level due to complex non-linear mixing problems between the water column and its constituents, which was beyond the scope of the current study. We discuss our study in the context of providing guidelines for future remote sensing studies of aquatic systems.

The delta of the Colorado River in Mexico supports a rich mix of estuarine, wetland and riparian ecosystems that provide habitat for over 350 species of birds as well as fish, marine mammals, and other wildlife. An important part of the delta ecosystem is the riparian corridor, which is supported by agricultural return flows and waste spills of water originating in the U.S. and Mexico. These flows may be curtailed in the future due to climate change and changing land use practices (out-of-basin water transfers, increased agricultural efficiency, and more optimal management of dams) in the U.S. and Mexico, and resource managers need to monitor the effects of their water management practices on these ecosystems. We developed ground-validated, remote sensing methods to monitor the vegetation status, habitat value, and water use of wetland and riparian ecosystems using multi-temporal, multi-resolution images. The integrated methodology allowed us to project species composition, leaf area index, fractional cover, habitat value, and evapotranspiration over seasons and years throughout the delta, in response to variable water flows from the U.S. to Mexico. Waste spills of water from the U.S. have regenerated native cottonwood and willow trees in the riparian corridor and created backwater and marsh areas that support birds and other wildlife. However, the main source of water supporting the riparian vegetation is the regional aquifer recharged by underflow from U.S. and Mexico irrigation districts. Native trees have a short half-life in the riparian zone due to human-set fires and harvesting for timber. Active management, monitoring, and restoration programs are needed to maintain the habitat value of this ecosystem for the future.

- by Osvel Hinojosa-Huerta
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- Remote Sensing, Climate Change, Wetlands, Agriculture

- by Stefano Patruno
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- Earth Sciences, River Deltas, Deltas, Holocene Evolution of Deltas

Ecological sub-regions are a way of viewing coastal zone (CZ) regions that have been developed to enhance the capability of NGOs, governmental organizations, and academics to assess conditions and trends of the major ecosystems in the Gulf of Mexico (Gulf), mainly as a management tool for defining priority actions towards sustainable development. Major ''geographical regions'' at scale-1-level (1:40) are: (a) the warm-temperate Gulf, (b) the tropical ARTICLE IN PRESS www.elsevier.com/locate/ocecoaman 0964-5691/$ -see front matter r

- by Alejandro Yanez
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- Management, Earth Sciences, Wetlands, Human Factors

Elevated As is well known to be present in aquifers utilised for drinking water and irrigation in West Bengal and Bangladesh. This problem has also more recently been discovered in other parts of Asia, including Vietnam, Cambodia, Inner Mongolia and the Middle Ganges Plain. Analysis of groundwaters in Kandal Province of Cambodia found waters with comparable geochemistry to the As-rich groundwaters of the West Bengali Delta. Similarities included high but heterogeneous As distributions, predominantly in the form As(III), high Fe, moderate to high HCO À 3 , circumneutral pH, low SO 2À 4 and geochemical components indicative of reducing conditions. Good positive correlations between As, Fe, HCO À 3 and NH þ 4 , and dissolved organic C is consistent with As release predominantly via microbially mediated reductive dissolution of As bearing Fe(III) oxides. Further evidence for such a process is found from correlations between As, Fe and organic matter from analysis of aquifer sediments, by the presence of goethite in the finer fractions and from the association of As with amorphous, poorly crystalline and well crystallised hydrous Fe oxides. The presence of several high As, but low Fe, wells implies that microbes could have a more direct role in mediating As release via the direct utilisation of Fe(III) or As(V) as electron acceptors. The presence of elevated As in waters with short aquifer residence times (as indicated by their geochemical signature) highlights the possible vulnerability of these aquifers to the influx of surface derived waters, providing an additional source of labile organic C that could exacerbate As release by stimulating microbial activity.

- by Helen Rowland
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- Geochemistry, Irrigation, Arsenic, Vietnam

This paper examines the differences between tide-dominated and tide-influenced deltas, as well as tide-dominated deltas and tide-dominated estuaries. The deltaic deposits of the Middle Devonian Kernave and Aruküla formations were documented in cores and outcrops in the Baltic Basin and interpreted as tide-dominated delta deposits. These tide-dominated deposits consist of three vertically stacked progradational to aggradational packages, 20-40 m thick. Each package consists of two stratigraphic intervals. The lower upward-coarsening interval contains seaward-accreting prodelta to distal tidal-bar and proximal tidal-bar deposits. The upper upward-fining interval consists of tidal-flat deposits and minor tidal gully, distributarychannel, supratidal muds, and paleosol deposits. The overall character of these delta deposits indicates a subaqueous delta with no river-dominated delta-plain. Comparison of these successions with modern and ancient tide-dominated and tide-influenced deltas suggests that tide-dominated deltaic deposits tend to form in conditions of relative sea-level rise succeeding transgressions, when tidal currents are strong enough to redeposit most river-derived sediments. Tide-dominated deltas form subaqueous deltas, where the bulk of the deposits are tidally reworked. In contrast, tide-influenced deltas contain tidal indicators in delta-front and lower-delta-plain deposits, whereas the upper delta plain is river-dominated. Our data suggest that tide-dominated deltas may change into tide-influenced deltas during delta evolution when they prograde to the mouth of the restricted or funnel-shaped bay, given the rate of fluvial sediment supply exceeds the rate of accommodation increase.

- by Piret Plink-Bjorklund
- •
- Geology, Stratigraphy, Tides, Estuaries
- by michele pipan
- •
- History, Geology, Geoarchaeology, Environmental Archaeology