Landward Shift Research Articles

Variable-density solute transport in unconfined coastal aquifers with a subsurface dam

Recently, the influence of subsurface dams on solute transport in coastal aquifers has become a hot research topic. Although many studies have been conducted, the combined effects of subsurface dam and tides on variable-density solute transport remain unclear, and this study aimed to fill this research gap. In the current study, a numerical model representing a 2-D cross-shore coastal aquifer was established. The model simulated cases with and without a subsurface dam, and sensitivity analysis cases with different height and location of the subsurface dam and solute concentration. The results show that a subsurface dam blocks a portion of the solute plume, which can only be discharged by dilution at the edges, thereby altering its discharge pattern and reducing their discharge rate. The addition of a subsurface dam may either prolong or shorten the residence time of solute,depending on the location rather than the height of the subsurface dam. In particular, a more landward subsurface dam would significantly increase the residence time; sensitivity analysis demonstrates that both the landward shift and the height increase of the subsurface dam contribute to a heightened ratio of dynamic mass distribution for the solute plume within the freshwater and saltwater zones of the aquifer, with maximum changes in mass distribution ratios of 87.22% and 300%, respectively. Also, these factors cause the solute to migrate both seaward and landward, respectively, across the primary outflow regions of the aquifer-ocean interface. Results from this study may provide theoretical guidance for the optimal design and environmental impact assessment of subsurface dams.

Geohazard features of the Aeolian Island slopes and the North-Eastern Sicily offshore

ABSTRACT The MaGIC project (Marine Geohazard along the Italian Coasts) had the aim of mapping the geohazard in the Italian seas and resulted in the production of numerous maps covering parts of the Italian Seas. In this paper, we present the maps: ‘The submerged portions of the Aeolian volcanic islands and the north-eastern Sicilian margin’, located in the south-eastern Tyrrhenian Sea. Both areas are affected by active geological processes, which represent important geohazards elements. Inthe submarine parts of the Stromboli volcanoremobilization of volcaniclastic deposits occur along the Sciara del Fuoco, where small-scale instabilities may represent a source of geohazard. Hydrothermal activity occurs on Enarete and Enaretino conical seamounts. The north-eastern Sicilian margin has a narrow continental shelf. Numerous canyon heads indent the shelf and, sometimes, reach close to the coast. Canyons have often a retrogradational trend and further eventual landward shift through sliding can iendangeri coastal or offshore infrastructures. Many of the canyons connect with leveed channels with widespread sediment instability. In the Gioia Basin, some of the channels connect to form the Stromboli slope Valley. Volcanic unrest or local and regional earthquakes are proven to have caused submarine landslides and tsunamis.

Projections of changes in the global distribution of shallow water ecosystems through 2100 due to climate change

The global area and distribution of shallow water ecosystems (SWEs), and their projected responses to climate change, are fundamental for evaluating future changes in their ecosystem functions, including biodiversity and climate change mitigation and adaptation. Although previous studies have focused on a few SWEs, we modelled the global distribution of all major SWEs (seagrass meadows, macroalgal beds, tidal marshes, mangroves, and coral habitats) from current conditions (1986–2005) to 2100 under the representative concentration pathway (RCP) 2.6 and 8.5 emission scenarios. Our projections show that global coral habitat shrank by as much as 75% by 2100 with warmer ocean temperatures, but macroalgal beds, tidal marshes, and mangroves remained about the same because photosynthetic active radiation (PAR) depth did not vary greatly (macroalgal beds) and the shrinkage caused by sea-level rise was offset by other areas of expansion (tidal marshes and mangroves). Seagrass meadows were projected to increase by up to 11% by 2100 because of the increased PAR depth. If the landward shift of tidal marshes and mangroves relative to sea-level rise was restricted by assuming coastal development and land use, the SWEs shrank by 91.9% (tidal marshes) and 74.3% (mangroves) by 2100. Countermeasures may be necessary for coastal defense in the future; these include considering the best mix of SWEs and coastal hard infrastructure because the significant shrinkage in coral habitat could not decrease wave energy. However, if appropriate coastal management is achieved, the other four SWEs, which have relatively high CO2 absorption rates, can help mitigate the climate change influences.

Seamount and Ridge Subduction at the Java Margin, Indonesia: Effects on Structural Geology and Seismogenesis

AbstractThe Java ‐ Lesser Sunda margin, which features multi‐scale subducting oceanic basement relief, is classified as neutral (Lombok and Sumbawa) to erosional (Central Java to Bali) in comparison to its accretionary counterpart offshore Sumatra. However, a comprehensive analysis of how plate boundary and upper plate structure across the neutral to erosional transition are modulated by the subduction of oceanic basement relief is lacking to date. To shed light on the tectonic parameters that push the margin into the neutral or erosional domain, we combine multi‐channel reflection seismic images derived through a grid‐based P‐wave velocity inversion, and multibeam bathymetric maps. The data document how different scales of subducting topography modify seafloor morphology, upper plate structure, and décollement position. Large‐scale subducting features cause a landward shift of the deformation front, shortening of the accretionary wedge, and seafloor steepening at the relief's trailing edge. Small‐scale subducting ridges primarily impact the frontal prism resulting in over‐steepening at the trench and localized slope failure. Ahead of subducting relief, deformation of the accretionary wedge encompasses enhanced compression and a reduction in seafloor slope but appears independent of the size of the relief. Ridge and seamount subduction induce frontal erosion and basal erosion offshore Lombok and Bali, respectively. Our P‐wave velocity models indicate that the rigidity of the upper plate's base along the eastern Sunda margin is significantly lower than the worldwide trend. We conclude that this favors the genesis of tsunami earthquakes that have occurred on the Java margin.

Resolving Uncertainties in Foraminifera-Based Relative Sea-Level Reconstruction: a Case Study from Southern New Zealand

Abstract Since the pioneering work of David Scott and others in the 1970s and 1980s, foraminifera have been used to develop precise sea-level reconstructions from salt marshes around the world. In New Zealand, reconstructions feature rapid rates of sea-level rise during the early to mid-20th century. Here, we test whether infaunality, taphonomy, and sediment compaction influence these reconstructions. We find that surface (0–1 cm) and subsurface (3–4 cm) foraminiferal assemblages show a high degree of similarity. A landward shift in assemblage zones is consistent with recent sea-level rise and transgression. Changes associated with infaunality and taphonomy do not affect transfer function-based sea-level reconstructions. Applying a geotechnical modelling approach to the core from which sea-level changes were reconstructed, we demonstrate compaction is also negligible, resulting in maximum post-depositional lowering of 2.5 mm. We conclude that salt-marsh foraminifera are indeed highly accurate and precise indicators of past sea levels.

Modelling the impact of anthropogenic measures on saltwater intrusion in the Weser estuary

Abstract. The Weser estuary has been subject to profound changes in topography in the past 100 years through natural variations and river engineering measures, leading to strong changes in hydrodynamics. These changes are also expected to have affected the dynamics of saltwater intrusion. Using numerical modelling, we examined saltwater intrusion in the Weser estuary in four different system states (1966, 1972, 1981, 2012). Models of each system state were set up with the respective topography and boundary values. We calibrated and validated each model individually to account for differences in sediments, bedforms, and the resolution of underlying bathymetric data between historical and recent system states. In simulations of 1 hydrological year, each with realistic forcing (hindcasting study), the influence of topography is overshadowed by the effects of other factors, particularly river discharge. At times of identical discharge, results indicate a landward shift of the salinity front between 1966 and 2012. Subsequent simulations with different topographies but identical boundary conditions (scenario study) confirm that topography changes in the Weser estuary affected saltwater intrusion. Solely through the topography changes, at a discharge of 300 m3 s−1, the position of the tidally averaged and depth-averaged salinity front shifted landwards by about 2.5 km between 1972 and 1981 and by another 1 km between 1981 and 2012. These changes are significant but comparatively small, since due to seasonal variations in run-off, the tidally averaged saltwater intrusion can vary by more than 20 km. An analysis of the salt flux through a characteristic cross section showed that saltwater intrusion in the Weser estuary is primarily driven by tidal pumping and only to a lesser degree due to estuarine circulation. However, results indicate that the contribution of individual processes has changed in response to anthropogenic measures.

Effects of swell waves on the dynamics of the estuarine turbidity maximum in an idealized convergent partially mixed estuary

The dynamics of estuarine sediments plays a key role in the morphological evolution, water quality, and ecosystem health of estuaries. Estuarine turbidity maxima (ETMs) are ubiquitous in estuaries worldwide and have drawn enormous attention from researchers. Here, we use a coupled ocean–atmosphere–wave–sediment transport modeling system to examine the effects of swell waves on the location, extent, and suspended sediment concentration (SSC) of ETMs in a convergent partially mixed estuary. Our results reveal that swell waves influence the residual circulation, mixing, and bottom stress in the estuary. The wave–current interaction increases the bottom stress in the estuary but reduces mixing in the lower to middle reaches of the estuary, owing to the enhanced landward salt transport, and the increased stratification. The reduced mixing enhances the estuarine circulation in the middle reach of the estuary. These changes cause a landward shift of the primary ETM and an increase in the SSC in the lower estuary. Moreover, the Coriolis force changes the ETM distribution and influences the wave effects. Overall, the findings of this study further elucidate ETM dynamics in an estuary. • Swell wave advects more saline water in the estuary and increases stratification. • Wave-current interaction increases the bottom stress in the lower estuary. • Swell wave causes a landward shift of the primary estuarine turbidity maxima (ETM).

Coastal barrier morphogenic categories in Mississippi Delta plain development

Coastal barriers are defined as shore-parallel, aggraded-prograded supratidal beach ridge sets and individual single ridge-landforms, composed of sand and/or gravel. Island, spit, and mainland barriers represent the three major topographic subdivisions. In terms of bathymetric and hydrographic characteristics, bottom sediment types and associated habitats, barriers separate the marine offshore from inshore facies. This definition is not uniformly accepted and applied in the literature. Barriers impose reduced salinities inshore, diminish storm impact and moderate inland inundation, protect critical habitats and may mark ancient longshore transport directions, shore and sea-level positions. A wide range of dimensions, topography, and other factors characterize mainland strandplains. Shoal aggradation appears to be the prime island-forming mechanism, followed by accretion around semi-consolidated, elevated cores. Smaller islands may also form by barrier spit fragmentation. Detachment of dune ridges from the mainland shore is an often cited but at best unlikely mode of island genesis. Review of the literature and remote-sensed images created new insights, utilized barrier definitions and genetic theories, as applied in the Mississippi Delta Complex. A three-stage concept, involving dune ridge or barrier spit-detachment by marine encroachment and landward shift of fragmented transgressive islands forms the basis of a recent invited Marine Geology research article. That model assumes spit-growth from delta headlands, followed by splintering into landward-migrating islands. Submerged, they become permanent shoals. For several reasons, this “island-cycle” hypothesis is questionable and a reassessment of the geomorphic and stratigraphic context of this delta coast is required.Contradicting the geomorphic setting of assumed Stage 1, key component of the cycle-model, evidence is lacking for subrecent delta lobes and erosional headland in spawning barrier islands. In addition, the delta headland-flanking islands reveal only progradational (regressive) development and in situ disintegration,. Transgressive landward migrations is known only from the Chandeleur chain's prehistory. Sand from decaying Terrebonne-Lafourche 6 Subdelta headland was the dominant source for the Caminada Strandplain and for the shoal /or island predecessor of Grand Isle. Wave refraction around an inferred Lafourche delta lobe (dated provisionally between ∼2900 - ∼ 2300 yrs. BP) and its decayed erosional headland remnant resulted in diametric reversal of littoral sand transport. An earlier proferred ∼750–300 yrs. BP Caminada Strandplain age range is now replaced by a loosely defined ∼2200- to- ∼800 yrs. BP value, based on ages of Grand Isle and reworked delta hinterland sediments incorporated into the Strandplain.

Topography and morphodynamic study of intertidal mudflats along the eastern coast of the Gulf of Khambhat, India using remote sensing techniques

The intertidal mudflats that are exposed subaerially between the high and low tides have a complex morphology comprising ebb and flood tidal deltas, spits, shoals, and tidal sandbanks intervened with a network of meandering and branching tidal channels and inlets. The intertidal zones are highly transient and understanding their morphodynamics is critical to the ecology and economy of the region. Monitoring intertidal mudflat dynamics is imperative for sustainable development activities along coastal regions. The topography of intertidal mudflat is estimated for the eastern coast of the Gulf of Khambhat (north-western coast of India) using the waterline detection method from a series of satellite images. We have used Modified Normalized Difference Water Index and image threshold techniques to delineate waterlines from satellite images of high to low tidal conditions at different tidal heights. The waterlines are considered as elevation contours corresponding to tidal height during satellite observation, and a series of elevation contours delineated from satellite images of various tidal heights are then used to calculate the mudflat topography. The estimated topography is validated with the Differential Global Positioning System (DGPS), the Sound and navigation ranging (Sonar), and ICESat-2 ATL03 dataset observations, which results in R2 values of 0.97, 0.89, and 0.9, respectively, as well as RMSE values of 0.39 m, 0.33 m, and 0.49 m, respectively. The mudflat dynamics are studied by comparing the topographical changes estimated between the satellite images of 1998-99 and 2019. The major morphological changes are the landward shift of the low waterline, lateral migration of tidal bars, narrowing of the river mouth, as well as anthropogenic influences such as mudflat reclamation and impedance of long-shore sediment transport. The region with deposition of sediment above Mean Sea Level (MSL) has flattened the slope, making the mudflat stable and promoting mangroves expansion, while erosion has steepened the mudflats with loss of mangrove cover. These mudflats exhibit highly complex morphological changes mainly induced by sediment transport, two-way tidal currents, and fluvial fluxes. The estimated topography shall find its application in modeling coastal processes, identifying vulnerable coastal stretches, and site suitability for mangrove plantations.

Shoreline change assessment of Indus delta using GIS-DSAS and satellite data

Indus River Delta, Pakistan, is under continual threat of coastal erosion due to natural and anthropogenic activities. The variations in the shoreline position have significantly degraded agricultural lands, increased tidal floodplain areas, and subsequently affected the livelihood of the coastal communities under changing climatic scenarios. The present study was conducted to quantify the Indus delta shoreline variations using satellite data and the Digital Shoreline Analysis System (DSAS 4.3) integrated with ArcGIS software. The statistical parameters End Point Rate (EPR) and Linear regression rate (LRR) were used for estimation of the shoreline change rate, while Net Shoreline Movement (NSM) was used to estimate the net change in the shoreline from 1972 to 2017. The analysis of results revealed that the erosion of the shoreline was dominant over the accretion, which ranged between 1 to 160 m/year with a net erosion rate of 18.87 ± 2.22 m/year. The net shoreline erosion rate was higher, i.e., 30.12 ± 3.13 m/year on the left bank side of the river Indus, compared to that of on the right bank side, i.e., 6.34 ± 2.85 m/year. The net landward shift in the shoreline of the delta from 1972 to 2017 was 860 ± 92 m. The DSAS software integrated with ArcGIS could successfully be used for shoreline change analysis for similar river deltaic areas of the World. The study recommends the employment of mitigation measures for the protection of the shoreline of the Indus delta.

Mantle control on magmatic flare-ups in the southern Coast Mountains batholith, British Columbia

AbstractThe southern Coast Mountain batholith was episodically active from Jurassic to Eocene time and experienced four distinct high magmatic flux events during that period. Similar episodicity has been recognized in arcs worldwide, yet the mechanism(s) driving such punctuated magmatic behavior are debated. This study uses zircon Hf and O isotopes, with whole-rock and mineral geochemistry, to track spatiotemporal changes in southern Coast Mountains batholith melt sources and to evaluate models of flare-up behavior and crust formation in Cordilleran arc systems. Zircon Hf isotope analysis yielded consistently primitive values, with all zircon grains recording initial εHf between +6 and +16. The majority (97%) of zircons analyzed yielded δ18O values between 4.2‰ and 6.5‰, and only five grains recorded values of up to 8.3‰. These isotopic results are interpreted to reflect magmatism dominated by mantle melting during all time periods and across all areas of the southern batholith, which argues against the periodic input of more melt-fertile crustal materials as the driver of episodic arc magmatism. They also indicate that limited crustal recycling is needed to produce the large volumes of continental crust generated in the batholith. Although the isotopic character of intrusions is relatively invariant through time, magmas emplaced during flare-ups record higher Sr/Y and La/Yb(N) and lower zircon Ti and Yb concentrations, which is consistent with melting in thickened crust with garnet present as a fractionating phase. Flare-ups are also temporally associated with periods when the southern Coast Mountains batholith both widens and advances inboard. We suggest that the landward shift of the arc into more fertile lithospheric mantle domains triggers voluminous magmatism and is accompanied by magmatic and/or tectonic thickening. Overall, these results demonstrate that the magmatic growth of Cordilleran arcs can be spatially and temporally complex without requiring variability in the contributions of crust and/or mantle to the batholith.

Brazilian Semi-Arid Mangroves-Associated Microbiome as Pools of Richness and Complexity in a Changing World.

Mangrove microbiomes play an essential role in the fate of mangroves in our changing planet, but the factors regulating the biogeographical distribution of mangrove microbial communities remain essentially vague. This paper contributes to our understanding of mangrove microbiomes distributed along three biogeographical provinces and ecoregions, covering the exuberant mangroves of Amazonia ecoregion (North Brazil Shelf) as well as mangroves located in the southern limit of distribution (Southeastern ecoregion, Warm Temperate Southwestern Atlantic) and mangroves localized on the drier semi-arid coast (Northeastern ecoregion, Tropical Southwestern Atlantic), two important ecotones where poleward and landward shifts, respectively, are expected to occur related to climate change. This study compared the microbiomes associated with the conspicuous red mangrove (Rhizophora mangle) root soils encompassing soil properties, latitudinal factors, and amplicon sequence variants of 105 samples. We demonstrated that, although the northern and southern sites are over 4,000 km apart, and despite R. mangle genetic divergences between north and south populations, their microbiomes resemble each other more than the northern and northeastern neighbors. In addition, the northeastern semi-arid microbiomes were more diverse and displayed a higher level of complexity than the northern and southern ones. This finding may reflect the endurance of the northeast microbial communities tailored to deal with the stressful conditions of semi-aridity and may play a role in the resistance and growing landward expansion observed in such mangroves. Minimum temperature, precipitation, organic carbon, and potential evapotranspiration were the main microbiota variation drivers and should be considered in mangrove conservation and recovery strategies in the Anthropocene. In the face of changes in climate, land cover, biodiversity, and chemical composition, the richness and complexity harbored by semi-arid mangrove microbiomes may hold the key to mangrove adaptability in our changing planet.

Quaternary Evolution of Coastal Plain in Response to Sea-Level Changes: Example from South-East Sicily (Southern Italy)

During a cycle of sea-level variation, coastal environments develop in different position of the continental shelf following seaward and landward shift of the coastline. They vary widely in character, reflecting the wide range of process-regimes that are brought about during the different stages of sea-level variations. Within this scenario, the morphology of continental shelves, mainly resulting from the combined effect of tectonic activity and eustatism, plays an important role in controlling the features and the preservation of coastal environments. Coastal deposits formed along continental shelves in the past, during different stages of sea-level changes, consist of discontinuous and thin depositional bodies, thus their reconstruction can be best carried out through the interpretation of high-resolution seismic data. Such a research approach is adopted in the present study to investigate a portion of the continental shelf of the southernmost sector of SE Sicily, in the offshore of Marzamemi village (Syracuse). The interpretation of high-resolution “Sparker” profiles allowed us to reconstruct the evolution of alluvial and lagoonal environments, established on a substratum of Pliocene or more ancient marine deposits, with the detection of several seismic units and unconformity surfaces, which have been related to alternating sedimentation and erosional processes, depicting the sea-level change framework of glacial-interglacial phases, from the late Pleistocene onward.

Modeling Delta Smelt Distribution for Hypothesized Swimming Behaviors

Delta Smelt, Hypomesus transpacificus, is an endangered pelagic fish native to the San Francisco Estuary. The distribution of Delta Smelt in the estuary shifts landward from low-salinity habitat to freshwater habitat before spawning. This spawning migration often coincides with the first substantial freshwater inflow to the estuary during winter. To accomplish this landward shift in distribution, Delta Smelt are believed to use the tides by swimming to faster-moving currents during flood tides and then repositioning themselves to slower-moving currents to reduce seaward movement on ebb tides. Studies have hypothesized that the swimming behavior of Delta Smelt during this period is influenced by environmental conditions such as salinity and turbidity. The details of these swimming behaviors—including the extent to which flows, salinity, and turbidity affect behaviors and distributions—are uncertain. The spawning migration is of management interest because an increase in observed counts of Delta Smelt at the South Delta water-export facilities has coincided roughly with the spawning migration in many years. In this study, we investigated a range of hypothesized swimming behaviors using a three-dimensional particle-tracking model for water year 2002 during the spawning migration, and compared the predicted distributions of Delta Smelt to distributions inferred from catch data. Our goal was to improve understanding of the influence of Delta Smelt swimming on distribution, and, ultimately, to develop a modeling tool to help management agencies identify conditions associated with entrainment losses. Predictions of Delta Smelt distributions and entrainment varied greatly among behaviors. Without swimming, Delta Smelt would be rapidly transported seaward of Suisun Bay, while continuous tidal migration would move them deep into the interior Delta. These behaviors and a simple turbidity-driven behavior model predicted distributions inconsistent with observations, while more complex behavior rules allowed improved predictions.

Modeling Delta Smelt Distribution for Hypothesized Swimming Behaviors

Delta Smelt, Hypomesus transpacificus, is an endangered pelagic fish native to the San Francisco Estuary. The distribution of Delta Smelt in the estuary shifts landward from low-salinity habitat to freshwater habitat before spawning. This spawning migration often coincides with the first substantial freshwater inflow to the estuary during winter. To accomplish this landward shift in distribution, Delta Smelt are believed to use the tides by swimming to faster-moving currents during flood tides and then repositioning themselves to slower-moving currents to reduce seaward movement on ebb tides. Studies have hypothesized that the swimming behavior of Delta Smelt during this period is influenced by environmental conditions such as salinity and turbidity. The details of these swimming behaviors—including the extent to which flows, salinity, and turbidity affect behaviors and distributions—are uncertain. The spawning migration is of management interest because an increase in observed counts of Delta Smelt at the South Delta water-export facilities has coincided roughly with the spawning migration in many years. In this study, we investigated a range of hypothesized swimming behaviors using a three-dimensional particle-tracking model for water year 2002 during the spawning migration, and compared the predicted distributions of Delta Smelt to distributions inferred from catch data. Our goal was to improve understanding of the influence of Delta Smelt swimming on distribution, and, ultimately, to develop a modeling tool to help management agencies identify conditions associated with entrainment losses. Predictions of Delta Smelt distributions and entrainment varied greatly among behaviors. Without swimming, Delta Smelt would be rapidly transported seaward of Suisun Bay, while continuous tidal migration would move them deep into the interior Delta. These behaviors and a simple turbidity-driven behavior model predicted distributions inconsistent with observations, while more complex behavior rules allowed improved predictions.

Coastal erosion risk assessment in the coastal area of Muara Asam Asam, Tanah Laut Regency

Coastal erosion is the process of land erosion in coastal areas due to waves and ocean currents which reduce land and can adversely affect socio-economic activities in coastal areas. Coastal erosion risk assessment in the estuary is a development of previous research, because the erosion and accretion processes do not only occur along the shoreline, but also on the side facing river water bodies, due to the confluence of fluvial and marine processes. The landward shift of the shoreline in the Muara Asam Asam coastal area has reached 90 meters in the last 28 years, and has been detrimental because the area is a densely populated area with major economic activities in the fisheries and agriculture sectors. This study aims to determine the coastal erosion risk level zones based on the relationship among vulnerability and consequence parameters, through distance decay weighted based method. The results indicate that Muara Asam Asam has high risk zones of coastal erosion, especially in densely populated residential areas and dry land agriculture on the west side of the estuary, due to the lack of implementation of preventive measures through hard coastal structure and coastal zoning policy to protect socio-economic activities and coastal ecological environments.

Palaeogeographic reconstruction of a fluvio-marine transitional system in Narmada rift basin, India \u2014 Implications on Late Cretaceous global sea-level rise

Rising sea-levels in tectonically active epicontinental basins often lead to varied depositional settings and palaeogeography, mostly influenced by the net accommodation resulting from mutual interference of the extent and nature of landward encroachment by the sea and the net sedimentation. The Cenomanian Nimar Sandstone Formation, Bagh Group, Narmada rift basin, uniquely portrays the effect of sea-level rise within an intra-cratonic setting and attributes to the corresponding palaeogeographic changes in west-central India. An integrated sedimentological–sequence-stratigraphic study of the broadly fining-upward Nimar Sandstone Formation (thickness ~ 20–30 m) depicts the actual nature of changeover from a fluvial to a marine-dominated transitional depositional setting. Detailed sedimentological study reveals total seventeen facies, grouped in five facies associations, viz., the channel-fill facies association (FA-1), the overbank facies association (FA-2), the fluvial-dominated fluvio-tidal facies association (FA-3), the tide-dominated fluvio-tidal facies association (FA-4), and the shoreface facies association (FA-5). Overall facies architecture indicates a west-to-eastward marine encroachment, resulting in stacking of three distinct palaeo-depositional conditions: (i) an initial fluvial system with channel and overbank, changing into a tide-influenced fluvial bay-head delta in the inner estuary, followed by (ii) marine encroachment leading to a tide-dominated central estuary with inter- to sub-tidal settings, and finally, (iii) with further intense marine encroachments, a wave-reworked open shore condition in the outer estuary zone. The overall fining-up succession with a systematic change from fluvial to marine-dominated depositional systems points to a landward shift of the shoreline, signifying a major transgressive event correlated to the Cenomanian global sea-level rise. Characteristic stratal stacking patterns point to four coarsening- and fining-up hemicycles, embedded within the major transgressive succession. These high-frequency cycles attest to the varied interplay of sedimentation, tectonics and sea-level changes, and the resultant net accommodations. A palaeogeographic model is proposed based on the high-frequency transgressive–regressive hemicycles, which envisages the evolution of the depositional environments in relation to the Cenomanian eustatic rise in the intra-cratonic riftogenic fluvio-marine transitional basinal setup.

Pollen and diatom evidence of sea water intrusion, east flood canal (Banjir Kanal Timur), Semarang, Indonesia

Semarang City, Indonesia is plagued by a combination of widespread flooding during the wet season and a lack of fresh (potable) water during the dry season, the latter related to saltwater intrusion. The East Flood Canal (Banjir Kanal Timur = BKT) was built to address these problems by improving runoff (drainage) and increasing groundwater recharge. This research was conducted to reconstruct the history of environmental change, including seawater intrusion, along the coast of Semarang using pollen and diatom assemblages from a 190-cm-long, 210Pb-dated sediment core extracted from the estuary at the mouth of the BKT. The age of the sediments within the core ranged over 160 years (the surface dated to 2014). The sediments could be subdivided into three zones on the basis of pollen assemblages and 2 zones on the basis of diatom assemblages. Lower zone assemblages, dominated by Sonneratia alba (mangrove), Acrostichum aureum (Pteridophyte), and Cocconeis pediculus (an epiphytic diatom), indicate that historically, the estuarine ecosystem was relatively open to the Java Sea and characterized by periods of marine and freshwater conditions. Thereafter, an upward increase in saline mangrove species of pollen and diatoms, combined with the loss of freshwater species (e.g., Avicennia and Rhizophora), indicates an increasing prevalence of marine conditions at the mouth of the BKT. Increasing salinity was associated with a landward shift in the mangrove ecosystem, a trend indicated by the progressive increase in mangrove plant diversity as statically manifested by the three pollen zones. This landward shift was associated with a “closing off” of marine flow in response to a significant (order of magnitude) increase in the estuarine sedimentation rates at about 1981. Increased sedimentation was presumably related to land-use changes that enhanced sediment delivery to the coast via the BKT. Inherent in these sediments were allochthonous fragments of Asterionella formosa, a freshwater species found in the upper sediment layers. The results of this study are consistent with the increased saltwater intrusion along the coast and a decline in groundwater quality.

Sedimentary zonation shift of tidal flats in a meso-tidal estuary

Understanding the spatial-temporal pattern of sedimentary dynamics on tidal flats is important for examining their ecological function and evolutionary trend. In this study, the dynamic state of sediments across an open-coast tidal flat of the meso-tidal Changjiang Estuary, China, on a monthly scale from December 2014 to June 2016 was examined, based on a high-spatial-resolution sedimentological-topographic survey and a collection of related meteorological-hydrological data. The results revealed a continuously distinct zonation of sediments, which transformed seaward from sand-dominance to silt dominance over a short distance. A zoning, to divide the tidal flat into a sand zone, a mixture zone and a silt zone, was delimited based on the cluster analysis of grain size frequency distributions. The zonation was highly dynamic, with the mixture zone migrating upward during summer and experiencing an uplift of 1 m in its lower boundary over the time period examined. Within each zone, the medium size and sorting coefficient of sediments tended to be smaller in winter than those in summer. Alterations of the mean tidal level likely controlled the zonation shift and sediment variations. A higher mean tidal level indicated a landward shift of the location exhibiting shear stress maxima and the mixture zone had to migrate upward to maintain an appropriate stress for the development of mixed sediments. The impacts of waves were masked under normal conditions, but during storms strong waves dominated an embedded downward displacement of the mixture zone. Characteristics of sediments covering a large region of the tidal flat changed along with zonation shifts, resulting from altered proportions among sandy, mixed and silty sediments. The study highlights that sedimentary zonation shift, under the impacts of seasonal variations in tidal level, can be a major manifestation of tidal flat sedimentary dynamics.

Recent salinity intrusion in the Bengal delta: Observations and possible causes

Salinization stands among the most prominent environmental hazards of the largest delta on Earth, the Bengal delta. It has significant impacts on the local societies and the economy. Using an unprecedented collection of in situ river salinity records over the Bengal delta, extending from the Hooghly estuary in the west to the Meghna estuary in the east, we report a sudden salinization of the central part of the delta that occurred in 2006–2007. This results in a sudden landward shift of the seasonal march of the salinity front by about 20 km, taking place in the pre-monsoon season. Such a regime shift was never reported before. We investigate the various drivers of this sudden change and identify three possible forcing factors: the decrease in Ganges freshwater discharge, the rise of sea level and the depletion of the groundwater level. These factors may act independently, or in concert. Given the threat of the ongoing climate change and its cohort of adverse effects expected in the course of the 21st century in the Bengal delta, our study contributes to set the observational basis for the development of the next generation of salinization modeling platforms.