Dedication: Dr. Scott W. Nixon (1943–2012)

The availability of suspended sediments will be a dominant factor influencing the stability of tidal wetlands as sea levels rise. Watershed-derived sediments are a critical source of material supporting accretion in many tidal wetlands, and recent declines in wetland extent in several large river delta systems have been attributed in part to declines in sediment delivery. Little attention has been given, however, to changes in sediment supply outside of large river deltas. In this study, significant declines in suspended sediment concentrations (SSCs) over time were observed for 25 of 61 rivers examined that drain to the East and Gulf Coasts of the USA. Declines in fluvial SSC were significantly correlated with increasing water retention behind dams, indicating that human activities play a role in declining sediment delivery. There was a regional pattern to changes in fluvial sediment, and declines in SSCs were also significantly related to rates of relative sea level rise (RSLR) along the coast, such that wetlands experiencing greater RSLR also tend to be receiving less fluvial sediment. Tidal wetlands in the Mid-Atlantic, Mississippi River Delta, and Texas Gulf especially may become increasingly vulnerable due to rapid RSLR and reductions in sediment. These results also indicate that past rates of marsh accretion may not be indicative of potential future accretion due to changes in sediment availability. Declining watershed sediment delivery to the coastal zone will limit the ability of tidal marshes to keep pace with rising sea levels in some coastal systems.     

Flood hazard and damage assessment in the Ebro Delta (NW Mediterranean) to relative sea level rise

The impact of relative sea-level rise (RSLR), damage to and possible responses in the Ebro Delta (NW Mediterranean) has been analyzed. Impact was determined by delineating delta areas prone to flooding under different RSLR scenarios. The surface areas of the different habitats were then quantified for flooding impact and affected ecosystems were assessed. The obtained results enabled us to characterize the Ebro Delta as a coastal environment that is highly sensitive to changes in sea level, with affected flooded areas likely to range between about 45 and 60 % for different RSLR scenarios, from which about 26 % would be inundated by subsidence only. In absolute terms, the habitat most likely to be affected by flooding was cropland. In relative terms, the most affected habitats were those typical of the lowest areas: saltwater wetlands, riparian buffer and areas of saline vegetation. Under present deltaic evolution with no sediment supply, adaptation is considered a plausible option for managing the Ebro delta under a RSLR scenario. This implies permitting surface area losses or land use changes in the lower parts of the delta, where natural values will be reinforced, and concentrating agriculture in the higher parts of the deltaic plain.     

Response scenarios for the deltaic plain of the Rhône in the face of an acceleration in the rate of sea-level rise with special attention to<Emphasis Type="Italic">Salicornia</Emphasis>-type environments

One of the most critical problems facing many deltaic wetlands is a high rate of relative sea-level rise due to a combination of eustatic sea-level rise and local subsidence. Within the Rhône delta, the main source of mineral input to soil formation is from the river, due to the low tidal range and the presence of a continuous sea wall. We carried out field and modeling studies to assess the present environmental status and future conditions of the more stressed sites, i.e.,Salicornia-type marshes with a shallow, hypersaline groundwater. The impacts of management practices are considered by comparing impounded areas with riverine areas connected to the Rhône River. Analysis of vegetation transects showed differences between mean soil elevation ofArthrocnemum fruticosum (+31.2 cm),Arthrocnemum glaucum (+26.5 cm), bare soil (+16.2 cm), and permanently flooded soil (?12.4 cm). Aboveground and belowground production showed that root:shoot ratio forA. fruticosum andA. glaucum was 2.9 and 1.1, respectively, indicating more stressful environmental conditions forA. glaucum with a higher soil salinity and lack of soil drainage. The annual leaf litter production rate of the two species is 30 times higher than annual stem litter production, but with a higher long-term decomposition rate associated with leaves. We developed a wetland elevation model designed to predict the effect of increasing rates of sea-level rise on wetland elevation andSalicornia production. The model takes into account feedback mechanisms between soil elevation and river mineral input, and primary production. In marshes still connected to the river, mineral input decreased quickly when elevation was over 21 cm. Under current sea-level rise conditions, the annual amount of riverine mineral input needed to maintain the elevation of the study marshes is between 3,000 and 5,000 g m^?2 yr^?1. Simulations showed that under the Intergovernmental Panel on Climate Change best estimate sea-level rise scenario, a mineral input of 6,040 g m^?2 yr^?1 is needed to maintain marsh elevation. The medium term response capacity of the Rhône deltaic plain with rising sea level depends mainly on the possibility of supplying sediment from the river to the delta, even though the Rhône Delta front is wave dominated. Within coastal impounded marshes, isolated from the river, the sediment supply is very low (10 to 50 g m^?2 yr^?1), and an increase of sea-level rise would increase the flooding duration and dramatically reduce vegetation biomass. New wetland management options involving river input are discussed for a long-term sustainability of low coastal Mediterranean wetlands.     

Integrating Successional Ecology and the Delta Lobe Cycle in Wetland Research and Restoration

Inactive deltas are more extensive than active deltas in most deltaic landscapes; thus, the subsurface generally is dominated by mineral sediments that rapidly accreted at different times, whereas the landscape at any one time generally is dominated by ephemeral emergent wetlands that are slowly accreting via vegetative growth. Subsidence is slow enough in most deltas that emergent wetlands, although ephemeral, can persist for millennia but accelerating global sea level rise probably will slow wetland creation in active deltas and accelerate the loss of existing wetlands in inactive deltas this century worldwide. A recent publication created confusion regarding the effects of river management on coastal Louisiana, where spatially variable subsidence is great enough in some areas to mimic extremely rapid sea level rise. I show how integrating Successional Ecology with the Delta Lobe Cycle, and correcting some omissions and errors in recent publications, clarifies the effects of river management in coastal Louisiana and provides a framework for predicting deltaic landscape dynamics worldwide. Successional Ecology provides a framework for understanding changes in natural and managed environments worldwide, whereas the Delta Lobe Cycle provides a framework for understanding river-dominated deltas worldwide. Sediment diversions are a form of river management that removes artificial barriers to river flow and are designed to mimic hydrologic conditions during the active delta stage of the Delta Lobe Cycle by focusing rapid mineral sedimentation in open water and thus creating new emergent wetlands. Freshwater diversions are another form of river management that also removes artificial barriers to river flow but are designed to mimic hydrologic conditions during the inactive stages of the Delta Lobe Cycle by reducing salinity stress over large areas of emergent wetlands and thus promoting marsh vertical accretion via vegetative growth. The Delta Lobe Cycle and both types of river diversions also create salinity gradients that simultaneously increase the sensitivity of emergent wetlands to disturbance while increasing the ability of emergent wetlands to recover from disturbance. Freshwater diversions only slow the loss of existing wetlands because the natural Delta Lobe Cycle, artificial channels that increase salinity stress, artificial ridges that increase flooding stress, and repeated disturbances eventually will cause vertical accretion via vegetative growth to become inadequate. Formally integrating these concepts might advance research and restoration in deltaic landscapes worldwide especially in the majority of deltas where inactive deltas are more extensive than active deltas.     

A Modeling Study of the Impacts of Mississippi River Diversion and Sea-Level Rise on Water Quality of a Deltaic Estuary

Freshwater and sediment management in estuaries affects water quality, particularly in deltaic estuaries. Furthermore, climate change-induced sea-level rise (SLR) and land subsidence also affect estuarine water quality by changing salinity, circulation, stratification, sedimentation, erosion, residence time, and other physical and ecological processes. However, little is known about how the magnitudes and spatial and temporal patterns in estuarine water quality variables will change in response to freshwater and sediment management in the context of future SLR. In this study, we applied the Delft3D model that couples hydrodynamics and water quality processes to examine the spatial and temporal variations of salinity, total suspended solids, and chlorophyll-α concentration in response to small (142 m³ s^?1) and large (7080 m³ s^?1) Mississippi River (MR) diversions under low (0.38 m) and high (1.44 m) relative SLR (RSLR = eustatic SLR + subsidence) scenarios in the Breton Sound Estuary, Louisiana, USA. The hydrodynamics and water quality model were calibrated and validated via field observations at multiple stations across the estuary. Model results indicate that the large MR diversion would significantly affect the magnitude and spatial and temporal patterns of the studied water quality variables across the entire estuary, whereas the small diversion tends to influence water quality only in small areas near the diversion. RSLR would also play a significant role on the spatial heterogeneity in estuary water quality by acting as an opposite force to river diversions; however, RSLR plays a greater role than the small-scale diversion on the magnitude and spatial pattern of the water quality parameters in this deltaic estuary.     

环渤海海平面上升与三角洲湿地保护

环渤海地区三角洲是我国滨海重要的湿地发育区,在淤泥质滩地型湿地上发育着众多的动、植物群落,成为若干珍稀水禽的栖息地。从地面垂直形变与潮位资料等分析,黄河三角洲和辽河三角洲的地面下降速率为3～4mm/a和3.5～4.5mm/a,而相对海平面上升速率为45～5.5mm/a和5～6mm/a,预计至2050年总体的相对海平面上升量可达40～55cm。海平面上升对三角洲湿地的影响首先是直接淹没大片农田、油井和市区,其次是加剧海岸线的侵蚀与后退,还有风暴潮与洪涝灾害的加剧。针对三角洲湿地生态系统所面临的生态风险与人为活动干扰,有必要采取更加科学合理的保护与开发模式。本文介绍了生境更新与湿地调整的管理策略,以及淤长型滨海湿地的滚动开发模式。交替采用“渐进”与“跃进”的滚动开发,可保持湿地总量的动态平衡,有利于三角洲的可持续发展。     

Sea-level fluctuations and the geometric variability of tide-dominated sandbodies

This paper presents examples of various large tidal sandbodies from the Eocene Roda Sandstone in the southern Pyrenees and the Late Pleistocene and Early Holocene in the East China Sea. An attempt is made to summarize the geometric variability of these large tidal sandbodies in relation to the sediment supply and tidal discharge of the depositional system. Transverse sand bars were developed in low-sinuosity, high-gradient channels with high influxes of coarse sediments and water from fluvial systems. Tidal point bars were formed in meandering low-gradient estuarine channel where tidal influence was stronger and sediment was finer than those of the transverse sand bar. A tidal delta complex was built up at the estuary mouth with an abundant sediment supply and an increased tidal discharge. Tidal sand ridges were formed when relict fluvial or deltaic sands were eroded and reworked by strong tidal currents during subsequent sea-level rise.

Since the sediment supply and the tidal discharge of the depositional system were closely related to the eustatic sea-level change and basin subsidence, i.e. the relative sea-level change, special attention will be given to the relationship between geometric variability of tidal sandbodies and the sequence stratigraphic framework in which various sandbodies occurred. Three orders of eustatic sea-level fluctuations can be recognized. The third-order eustatic sea-level cycle, together with basin subsidence, controlled the development of systems tracts and the occurrence of different tidal sandbodies, such as estuary and tidal flat facies during the late stage of a LSW systems tract (type 1 sequence) or a SM systems tract (type 2 sequence); tidal point bar facies, tidal delta facies or tidal sand-ridge facies during a TR systems tract; estuary facies during an early HS systems tract; and fluvial sand bar facies in a late HS systems tract and the early stage of a SM or LSW systems tract. There are also the fourth-order and fifth-order eustatic fluctuations, which are superimposed on the third-order eustatic changes and have important control on the build-up, abandonment and preservation of composite and single tidal sandbodies, respectively.

Since the deposition of tidal sandbodies is very sensitive to eustatic sea-level changes, recognition of various tidal sandbodies is important in sequence stratigraphy analyses of sedimentary basins and in the facies prediction of clastic sediments in basin modelling.     

Vertically stacked Gilbert-type deltas of Ventimiglia (NW Italy): The Pliocene record of an overfilled Messinian incised valley

Overfilled incised valleys develop when the rate of sediment supply outpaces the rate of accommodation. An overfilled incised valley presents simple or compound valley-fill architecture, depending on the depth of the valley incision, compared with the height reached by the following sea-level rise.The Ventimiglia incised valley, exposed on the Ligurian coast, north-western Mediterranean margin, presents a spectacular example of compound incised-valley fill, developed in perennial “overfill” conditions. The valley was subaerially incised during the Messinian Salinity Crisis and rapidly flooded by the sea at the beginning of Pliocene, then filled by eleven coarse-grained Gilbert-type deltas during Early–Middle Pliocene time.The basal Messinian unconformity is locally paved with subaerial scree breccias and bioclastic shallow-marine sandstones, and blanketed by bathyal marls. These deposits record the lowstand, transgressive and early-highstand systems tracts of the first valley-fill sequence. The subsequent progradation of Gilbert-type deltas occurred in four stages, or depositional sequences, separated by transgressive marine-marl intervals. Within each depositional sequence, the deltaic bodies display offlapping architecture, recording falling shoreline trajectory, downward shifts in facies, and overall forced regression. The water depth and accommodation in the inundated coastal valley was gradually decreasing with time. The reduced accommodation allowed the youngest deltas to prograde out to the shelf edge, triggering mass collapses and subsequent filling into the newly created slump scars. Some of the deltas probably acted as “canyon-perched deltas” and supplied sediment to the deep-water slope and floor of the Ligurian Basin.The vertical stacking of Gilbert-type deltas is usually attributed, in tectonically active basins, to fault-related subsidence pulses. In Ventimiglia, the accommodation was created by high-frequency eustatic sea-level rises that, probably accompanied by climate controlled reductions in sediment supply, temporarily outpaced uplift, leading to the development of multiple cycles of infill.     

Sediment density flow distribution on wave-influenced deltas

Deltas are at the transition between fluvial and marine sedimentary environments where sediment density flows are often triggered during high river discharge events, forming submarine channels and sediment waves. On wave-influenced deltas, longshore currents are particularly efficient at transporting sediment alongshore, reducing the likelihood of sediment density flows from occurring at river mouths. This study describes four deltaic sedimentary systems at different stages of their evolution on a formerly glaciated continental inner shelf of eastern Canada in order to better understand the distribution of sediment density flows on wave-influenced deltas. Three types of settings are recognized as being prone to sediment density flows: (i) in the early stages of wave-influence and on large deltas, converging longshore currents can lead to offshelf sediment transport; (ii) on wave-influenced to wave-dominated deltas, a sandy spit can re-route the river mouth and sediment density flows form where the spit intersects the delta lip; (iii) in advanced stages of wave-dominated deltas and during their demise, rocky headlands are exposed and can intersect the slope, where off-shelf sediment transport occurs. These types of sediment density flows were all characterized by debris flows or surge-type turbidity currents which have limited offshore run-out. More rarely, hyperpycnal flows form at the river mouths, especially where the river incises glaciomarine clays prone to landsliding in the river, which increases fine-grained fluvial suspended sediment concentration. Overall, these results highlight the predominance of fluvial-dominated deltas during a phase of relative sea-level fall combined with high sediment supply. However, as soon as sediment supply diminishes, wave action remobilizes sediment alongshore modifying the distribution and types of sediment density flows occurring on wave-influenced deltas.     

Holocene Environmental Change and River-Mouth Sedimentation in the Baie des Anges, French Riviera

River mouths on the steep, high-relief coast of the French Riviera exhibit thick sequences of Holocene marine, estuarine, deltaic, and river channel-floodplain sediments that overlie basal fluvial Pleistocene gravel. Gravel is uncommon in most of the early to middle Holocene aggradational-progradational marine, estuarine, deltaic sediments, despite an ample supply from rock units in the steep adjoining uplands. River-mouth gravel is common only in late Holocene river channels and in barrier beaches perched on finer-grained nearshore sediments. Neither downslope grain-size fining on alluvial fans nor sediment stacking patterns during sea-level (base-level) rise readily account for the lack of early to middle Holocene gravel in the river-mouth sediment wedges. Holocene sea-level rise led to the storage of fine-grained sediments in shallow marine, estuarine, and deltaic environments in the present coastal zone. We infer that humid temperate conditions, a dense forest cover, landscape stabilization, and a regular quiescent river flow regime associated with the Atlantic climatic optimum limited gravel supply in the adjoining catchments and gravel entrainment downstream during the early Holocene. Sea-level stabilization in the middle and late Holocene coincided with a marked change in bioclimatic conditions toward the present Mediterranean-type regime, which is characterized by a less dense forest cover, soil erosion, and episodic catastrophic floods. The late Holocene was thus a time of downstream bedload channel aggradation, fine-grained floodplain and paludal sedimentation, and seaward flushing of clasts leading to the formation and consolidation of the gravel barrier beaches that bound the rivermouths and embayments.     

Late Quaternary coastal and inner shelf stratigraphy, Apalachicola Delta region, Florida

Since the beginning of the Tertiary the sedimentology of the Gulf of Mexico Basin has been dominated by the depositional activity of the Mississippi River. The sedimentologic influence of the Mississippi diminishes with distance east or west of the Louisiana shelf, however. The Texas and northwest Florida shelf margins, for example, are characterized by a series of smaller deltas. In the inner and mid-shelf areas of these regions the near-surface sedimentary units include infilled stream channels and small deltas. Such features are commonly observed in sub-bottom seismic records from the middle and inner shelf of the northeastern Gulf, along the Apalachicola River coast of northwest Florida.

The Apalachicola River is the principal source of clastic sediment to the northeastern Gulf of Mexico. During the late Holocene virtually all of the river's sediment load has been deposited in the modern Apalachicola Delta and in the river's estuary, Apalachicola Bay, which has been filling rapidly. During late Quaternary lowstands, prior to the development of the modern estuary, the river traversed the present-day inner and mid-shelf, incising a network of channels. Based on seismic records, many of these buried shelf channels were considerably larger than their modern counterparts.

During lowstands the Apalachicola River also deposited coarse sediment on the shelf as deltaic and associated river-mouth sediments. These deposits comprise the modern near-surface sediments of the inner and middle shelf. An investigation of subsurface sedimentary features observed in seismic profiles provides details on the late Quaternary development of the northeastern Gulf of Mexico shelf. Seismic reflection profiles obtained on the inner and mid-shelf regions of northwest Florida reveal an approximately 50 m thickness of late Quaternary sediments, comprised of two and sometimes three discrete clastic sequences. Two lower fluvial sequences total as much as 40–50 m in thickness. A transgressive marine sand deposit overlies the older features in some places, varying in thickness from 0 to 5 m. Identification of seismic facies, combined with stratigraphic data from a suite of coastal boreholes, enables correlation of offshore seismic stratigraphic units with late Tertiary and Quaternary coastal stratigraphy.     

Sustainability of Mediterranean Deltaic and Lagoon Wetlands with Sea-Level Rise: The Importance of River Input

We report on a decadal trend of accretionary dynamics in the wetlands of several northwestern Mediterranean deltas and a lagoon system, all of them with high rates of wetland loss. Wetland vertical accretion and surface elevation change were measured at 55 riverine, marine, and impounded sites in four coastal systems: the Ebro delta, Spain; the Rhône delta, France; and the Po delta and Venice Lagoon, Italy. Vertical accretion and elevation change ranged between 0 and 25 mm year^?1 and were strongly correlated. The highest rates of elevation gain occurred at riverine sites where vertical accretion was highest. We conclude that areas with high sediment input, mainly riverine, are the only ones likely to survive accelerated sea-level rise, especially if recent higher estimates of 1 m or more in the twenty-first century prove to be accurate. This is the first study where the importance of river input on wetland survival has been demonstrated at a decadal time scale over a broad geographical area.     

Wetland loss in the Northern Gulf of Mexico: Multiple working hypotheses

I examined four hypotheses about causes for the dramatically high coastal wetland losses (0.86% yr^?1) in the northern Gulf of Mexico: an extensive dredged canal and spoil bank network, a decline in sediments in the Mississippi River during the 1950s, Mississippi River navigation and flood protection levees, and salinity changes. Natural factors contributing to these habitat changes include eustatic sea-level rise and geological compaction, which appear to have remained relatively constant this century, although variation does occur. These four hypotheses were tested using data on land-to-water changes in 15-min quadrangle maps inventoried for four intervals between the 1930s and 1990. Land loss rates were directly proportional to changes in wefland hydrology in time and space. A linear regression of the direct losses due to dredging versus the losses due to all other factors (indirect losses) had a zero intercept and a slope that increased with time. The ratio indirect:direct land loss was highest nearest the estuarine entrance. The coastwide patterns of land loss do not appear to be affected by riverine sediment reductions over the last 60 yr. The effects of changes in wetland hydrology from dredging human-made channels and forming dredged spoil banks appear to be the most efficacious hypothesis explaning these dramatic losses. The effects of extensive human-induced changes on this coast have apparently overwhelmed the causal linkages identified in the historical re-constructionist view of deltaic gain and loss that emphasizes the role of mineral sediments. A paradigm shift is therefore proposed that emphasizes a broad ecological view as contrasted to a mostly physical view emphasizing the role of sediment supply in wetland maintenance. In this view, plants are not an ancillary consequence of strictly geological dynamics such as sediment supply but are dominant agents controlling factors relevant to coastal restoration and management efforts.     

Initiation of the Mekong River delta at 8 ka: evidence from the sedimentary succession in the Cambodian lowland

Modern deltas are understood to have initiated around 7.5–9 ka in response to the deceleration of sea-level rise. This episode of delta initiation is closely related to the last deglacial meltwater events and eustatic sea-level rises. The initial stage of the Mekong River delta, one of the world's largest deltas, is well recorded in Cambodian lowland sediments. This paper integrates analyses of sedimentary facies, diatom assemblages, and radiocarbon dates for three drill cores from the lowland to demonstrate Holocene sedimentary evolution in relation to sea-level changes. The cores are characterized by a tripartite succession: (1) aggrading flood plain to natural levee and tidal–fluvial channel during the postglacial sea-level rise (10–8.4 ka); (2) aggrading to prograding tidal flats and mangrove forests around and after the maximum flooding of the sea (8.4–6.3 ka); and (3) a prograding fluvial system on the delta plain (6.3 ka to the present). The maximum flooding of the sea occurred at 8.0 ± 0.1 ka, 2000 years before the mid-Holocene sea-level highstand, and tidal flats penetrated up to 20–50 km southeast of Phnom Penh after a period of abrupt ～5 m sea-level rise at 8.5–8.4 ka. The delta progradation then initiated as a result of the sea-level stillstand at around 8–7.5 ka. Another rapid sea-level rise at 7.5–7 ka allowed thick mangrove peat to be widely deposited in the Cambodian lowland, and the peat accumulation endured until 6.3 ka. Since 6.3 ka, a fluvial system has characterized the delta plain, and the fluvial sediment discharge has contributed to rapid delta progradation. The uppermost part of the sedimentary succession, composed of flood plain to natural-levee sediments, reveals a sudden increase in sediment accumulation over the past 600–1000 years. This increase might reflect an increase in the sediment yield due to human activities in the upper to middle reaches of the Mekong, as with other Asian rivers.     

Deltaic and estuarine environments and their Late Quaternary dynamics on the Sunda and Sahul shelves

Deltaic and estuarine environments have been, and continue to be, some of the most rapidly changing environments. Those associated with the Sunda shelf generally receive large volumes of sediment and were characterised by a diverse and productive vegetation before much of it was cleared and converted for agriculture, silviculture or urban development. By contrast estuaries in northern Australia receive far less sediment supply, and record a much less modified pattern of landform change during the Holocene. Three periods of change are discussed: first, the long-term geological development and response of deltaic–estuarine plains to eustatic cycles of sea-level change, particularly postglacial sea-level rise to present; second, Holocene development of deltaic–estuarine environments, dominated by patterns of coastal progradation and distributary migration, under relatively stable sea level; and third, the impact of human modifications. These observations provide a framework within which response of the deltaic–estuarine environments to future, anticipated environmental change can be assessed.     

Last Interglacial marine environments in the White Sea region, northwestern Russia

Marine sediments from river sections in the Mezen River drainage, northwest Russia, have been analysed for dinoflagellate cysts, foraminifers and molluscs. The sediments were dated by pollen analysis and by reference to the local sea-level history, and are Late Saalian to late Eemian (c. 133 to 119.5 kyr in age). The Late Saalian deglaciation was characterized by Arctic conditions, but a few centuries into the Eemian the Gulf Stream system carried warm Atlantic water into the region. At 129.8 kyr BP there was a marked increase in the influx of Atlantic water, and the advection of warm Atlantic water was stronger and probably penetrated further eastwards than at present. The molluscs, dinoflagellate cysts and foraminifers reflect conditions warmer than present and that the optimum temperature occurred at the time of the early Eemian global sea-level rise. Around 128 kyr BP, the eustatic sea-level rise was curbed by isostatic rebound and accompanying regression and constriction of marine passages to the White Sea. Local, low-saline, stratified basins developed and characterized the next five to six millennia.     

Alexandria-Nile Delta coast,Egypt: update and future projection of relative sea-level rise

Previous studies have indicated that the Nile River deltaic plain is vulnerable to a number of aspects, including beach erosion, inundation, and relatively high rates of land subsidence. This issue motivates an update and analysis of new tide-gauge records, from which relative sea-level changes can be obtained. Estimated rates from five tide gauges are variable in terms of magnitude and temporal trend of rising sea level. Analysis of historical records obtained from tide gauges at Alexandria, Rosetta, Burullus, Damietta, and Port Said show a continuous rise in mean sea level fluctuating between 1.8 and 4.9 mm/year; the smaller rate occurs at the Alexandria harbor, while the higher one at the Rosetta promontory. These uneven spatial and temporal trends of the estimated relative sea-level rise (RSLR) are interpreted with reference to local geological factors. In particular, Holocene sediment thickness, subsidence rate and tectonism are correlated with the estimated rates of relative sea-level change. From the relatively weak correlation between them, we presume that tectonic setting and earthquakes, both recent and historical ones, contribute more to accelerated RSLR than that of dewatering and compression/dewatering of Holocene mud underlying the Nile Delta plain. As a result, large areas of the coastal plain have been subsided, but some sectors have been uplifted in response to tectonic activities of thick underlying older strata. Projection of averaged sea-level rise trend reveals that not all the coastal plain of the Nile Delta and Alexandria is vulnerable to accelerated sea-level rise at the same level due to wide variability of the land topography, that includes low-lying areas, high-elevated coastal ridges and sand dunes, accretionary beaches, and artificially protective structures. Interaction of all aspects (tectonic regime, topography, geomorphology, erosion rate, and RSLR rate) permitted to define risk areas much vulnerable to impacts of sea incursion due to accelerated sea-level rise.     

Variations of continental discharge pattern in space and time: implications from the Laptev Sea continental margin, Arctic Siberia

Variations in sediment input and distribution to the Laptev Sea continental margin during the Holocene and Termination I could be identified based on radiocarbon dated magnetic susceptibility logs and sediment thickness in high-resolution seismic profiles. Magnetic susceptibility of surface samples reveals an increased input of magnetic grains to the Laptev Sea deriving from the Anabar and Khatanga river catchments. Exposed magnetite schists and volcanic rocks of the Anabar shield and Putoran Plateau, respectively, function as major source of magnetic material. The distribution of magnetic susceptibility in association with the thickness of the Holocene sediments indicates bottom-current induced sediment transport guided by major submarine valleys on the Laptev Sea shelf. The sites of filled paleoriver channels identified in the seismic profiles suggest that during the Late Weichselian sea-level lowstand river runoff continued through four of the major valleys on the exposed Laptev Sea shelf. The sediments at the top of the lowstand deposits in front of the Anabar-Khatanga valley, represented in the seismic profiles by prograding deltas, are characterized by outstandingly high magnetic susceptibility values. Radiocarbon datings approximate the deposition of these high magnetic sediments between 10 and 13.4 ka. It is suggested that this increased input of magnetic material is related to the deglaciation of the Anabar shield and the Putoran Plateau and thus support their glaciation during marine isotope stage (MIS) 2.     

Late Quaternary climatic events and sea-level changes recorded by turbidite activity, Dakar Canyon, NW Africa

The relationship of sea-level changes and short-term climatic changes with turbidite deposition is poorly documented, although the mechanisms of gravity-driven sediment transport in submarine canyons during sea-level changes have been reported from many regions. This study focuses on the activity of the Dakar Canyon off southern Senegal in response to major glacial/interglacial sea-level shifts and variability in the NW-African continental climate. The sedimentary record from the canyon allows us to determine the timing of turbidite events and, on the basis of XRF-scanning element data, we have identified the climate signal at a sub-millennial time scale from the surrounding hemipelagic sediments. Over the late Quaternary the highest frequency in turbidite activity in the Dakar Canyon is confined to major climatic terminations when remobilisation of sediments from the shelf was triggered by the eustatic sea-level rise. However, episodic turbidite events coincide with the timing of Heinrich events in the North Atlantic. During these times continental climate has changed rapidly, with evidence for higher dust supply over NW Africa which has fed turbidity currents. Increased aridity and enhanced wind strength in the southern Saharan-Sahelian zone may have provided a source for this dust.     

Depositional bias and environmental change—important factors in sequence stratigraphy

Differences among depositional systems, here called depositional bias, strongly influence sequence patterns. Siliciclastics and shallow-water carbonates, for instance, shed most of their sediment during opposite phases of a sea-level cycle (lowstand shedding and highstand shedding, respectively). Furthermore, the two systems generate their own, system-specific relief on the sea floor, disperse their sediment load along different avenues and differ in the way they are deactivated: reefs and carbonate platforms can be drowned, whereas siliciclastic deposition can be shut off and renewed at any depth. As a consequence of these differences, pronounced unconformities (drowning unconformities) develop where carbonate platforms are terminated and buried by siliciclastics (the siliciclastic-to-carbonate transition tends to be more gradual). Drowned platforms and drowning unconformities appeared world-wide in great abundance in the Miocene, Cretaceous (Valanginian-Turonian), Jurassic (Toarcian) and Devonian (Frasnian/Famennian). Examples of drowning unconformities interpreted as sequence boundaries, include those of the Early Cretaceous platforms off New Jersey and off Morocco, the mid-Cretaceous unconformity in the Gulf of Mexico and Miocene unconformities on top of reefs in the Far East.

The eustatic cycles postulated from sequence stratigraphy are a very unlikely cause for the mass drownings of reefs and platforms: either the rates of rise are an order of magnitude lower than the growth potential of platforms, or cycle amplitudes are too small, or the cycles are too short and subsidence too slow to remove platforms permanently from the photic zone. The critical element in the mass drowning seems to be environmental stress that reduces the growth potential of carbonate systems. Thus, drowning unconformities demonstrate the importance of environmental change as a control on sequence development. Other examples of dominantly environment-controlled sequences include slope deposits shaped by shifting currents (e.g. Florida and Blake Plateau) and basin fills that were controlled by changes in sediment input related to tectonic alteration of the drainage in the hinterland (e.g. Gulf of Mexico).

Environmental change must be considered as a third, independent factor that competes with eustatic and tectonically driven regional changes of sea-level for control of sequences. The definition of sequences and sequence boundaries should be broad enough to include the possibility of non-sea-level controls.