Wide Shelf Research Articles

Narrow is normal: Exploring the extent and significance of flooded marine shelves in icehouse, transitional, and greenhouse climate settings

Abstract Marine shelves are a ubiquitous feature of modern Earth, developed across a wide range of scales in many sedimentary basins and representing the flooded portion of basin-margin clinoform topsets. Analysis of 80 clinoforms from 10 basins spanning Cenozoic and Mesozoic icehouse, transitional, and greenhouse climate settings indicates that normalized mean greenhouse marine shelf width is 33% of normalized mean total measured clinoform topset length. The equivalent value for transitional settings is 43%, and 72% for icehouse marine shelves. These values demonstrate that greenhouse marine shelves were substantially narrower than icehouse equivalents, suggesting that narrower shelves with persistent shelf-edge deltas were a consequence of lower rates of accommodation change in greenhouse climate intervals that lacked the large ice sheets required to drive high-amplitude high-frequency glacio-eustasy. Because greenhouse climates have been the dominant mode through Earth history, narrow shelves have probably been the dominant form, and conceptual models based on modern relatively wide shelves may be poor predictors of paleogeography, sediment routing, and sediment partitioning throughout much of Earth history.

Controls on submarine canyon morphology along a convergent tectonic margin. The Southern Caribbean of Colombia

Submarine canyons are major conduits for sediment transfer from continental shelf to deep marine environments. Mass failures and faults play a key role in the initiation and evolution of submarine canyons along convergent tectonic margins, in addition to continental sediment supply. Here we analyze high-resolution bathymetry and seismic data from the Colombian southern Caribbean to investigate the controls on submarine canyon morphology along a convergent tectonic margin. We propose three end-member types of canyons: Type I canyons have a direct connection to a major river associated with high sediment supply. These canyons are highly erosive and cut topographic barriers created by tectonic deformation. Type II canyons are associated with faults at the canyon's head, where the location, orientation, and sinuosity are controlled by fault strike. Also, the presence of faults is related to an increase in the occurrence of mass failures from the steep canyon borders, increasing the depth and width asymmetry. Finally, Type III canyons evolve toward the continental shelf edge through retrogressive mass failures. These mass failures produce erosional scars that merge downslope to create submarine canyons in early stages of development, which later evolve to wider but shallow canyons due to the progressive basinward merging. We argue that the morphology of each canyon type is controlled by the interaction between continental sediment supply, mass failures, shelf width, and fault occurrence. We also demonstrate that seafloor topography influences the size of submarine fan deposits at the mouths of canyons. We identified large fan deposits (>650 km2) on the continental rise, whereas in piggyback sub-basins we found smaller fans (<60 km2). Our analysis of downdip canyon morphology and associated deposits can be used to predict the development of submarine canyons and associated fans in convergent tectonic margins worldwide.

High-resolution sequence stratigraphy in continental lacustrine basin: A case of Eocene Shahejie formation in the Dongying Depression, Bohai Bay Basin

Approximate 500 m deltaic-lacustrine successions were formed within 2 Ma in the Liangjialou area of the Dongying Depression in the Bohai Bay Basin (eastern China). However, few studies on sequence stratigraphy were previously conducted in the east of Liangjialou. In the present study, we integrate high-resolution three-dimensional (3D) seismic data with well-logging data to study the sequence stratigraphy and the depositional system.Our study shows that: (1) Four systems tracts have been identified in the study interval, they are HST, FSST, LST and TST. The HST has an ascending shelf-edge trajectory and aggradation to progradation stacking pattern. FSST has a descending shelf-edge trajectory and progradation to degradation stacking pattern. LST has a low-angle ascending shelf-edge trajectory and progradation to aggradation stacking pattern. TST has a backstepping shelf-edge trajectory and retrogradation stacking pattern. (2) Five high-resolution sequences (equal to 4th-order sequence), HRS1∼HRS5, were further subdivided within the 3rd-order LST. In addition, the depositional systems of HRS1∼HRS5 are significantly variable in 3D space. The delta and deep-water deposits in the north are larger than those in the south. (3) The formation of the sequence is controlled by both tectonics and dry-wet climatic cycles. The tectonic subsidence and high sediment supply in overall wet climate play a major role in the formation of the 3rd-order sequence. The higher frequency dry-wet climatic cycles dominate the formation of internal systems tracts. We further divide the climate of study interval into three stages that are excellent correlated to the specific systems tracts: Stage 1 (evaporation < precipitation) corresponds to HST; Stage 2 (evaporation > precipitation) corresponds to FSST; Stage 3 (evaporation < precipitation) corresponds to LST. (4) The basin morphology and source location mainly control the lateral variability of the depositional system of LST. Low paleotopography, narrow shelf width and steep slope dip in the north cause a large accommodation, and promote the development of the deltaic and deep-water deposits. In contrast, a high relief, wide shelf width and gentle slope cause only minor accommodation in the south, and limit the development of the deltaic and deep-water deposits.

Morphological controls on delta‐canyon‐fan systems: Insights from stratigraphic forward models

AbstractMorphology is one of the principal driving forces governing the sedimentology and evolution of delta‐canyon‐fan systems. However, quantified and systematic studies of morphological controls are still very limited. This study applied hydraulic‐based stratigraphic forward modelling to investigate the impacts of morphological parameters on sediment budget partitioning and channel network of delta‐canyon‐fan systems on passive margins. A total of six sets of stratigraphic forward models are built using 47 initial bathymetries with six varying morphological parameters: shelf gradient; shelf width; slope gradient; canyon sinuosity; canyon depth; and basin gradient. The quantified relationships between morphological parameters and sedimentological parameters (dimensions, deposition/erosion volume/area, number of tributaries and slope channels) are investigated. The causes behind the relationships are explored by analyzing the response of qualified sedimentary features to morphological controls, such as grain‐size distribution, channel migration and sequence stratigraphic frame. The results suggest that low shelf gradient, low shelf width, high canyon gradient and low canyon sinuosity are beneficial for sediment budget partitioning into deep basins and turbidity currents to flow inside canyons. Low canyon depth also promotes sediment delivery but results in more channels on canyon flanks. For the delta, channel lateral migration increases then decreases with increasing shelf gradient due to the gradient threshold in determining channel sinuosity; delta size increases then stops increasing with increasing shelf width due to the shelf width threshold in determining the balance between accommodation and sediment supply. For the canyon, low canyon gradient, high canyon sinuosity and low canyon depth have similar effects on canyons, all resulting in significant channel translation and canyon widening but less canyon head retrogradation. This study improves the knowledge of controlling factors and sediment transport regime of a delta–canyon–fan system. Moreover, the observed relationships could provide semi‐quantitative guidelines to predict the dimensions of delta–canyon–fan systems and the distribution of hydrocarbon reservoirs.

Tectonic and eustatic control on channel formation, erosion, and deposition along a strike-slip margin, San Diego, California, USA

Active margins are characterized by small mountainous streams, which contribute significantly to global sediment transport from land to sea. The response of active margin streams to base-level change due to eustatic sea-level cycles is less well understood than passive margin streams, which form the basis of most sequence stratigraphic models. The southern California margin offers an opportunity to investigate the evolution of small mountainous streams along a strike-slip margin over a sea-level cycle. High-resolution Chirp subbottom data were examined from the Silver Strand littoral cell in San Diego, California, which is located in a region of localized transtension associated with a step in the Newport Inglewood-Rose Canyon fault zone. Chirp data reveal active faulting on multiple fault strands, a subaerial surface likely formed from MIS 4 to MIS 2, fluvial incision associated with MIS 2, and the transgressive surface of erosion formed during sea-level rise from MIS 2 to MIS 1. Interpreted acoustic units observed in the data include the Cretaceous outcropping Rosario Group, Miocene to Pliocene prograding clinoforms, subaerial deposits associated with MIS 4 to MIS 3, channel fill and transgressive lag deposits formed during the most recent sea-level transgression, and Holocene marine sediment. The Chirp data examined in this study build upon findings from other seismic studies, sediment cores, trenching, and outcrops that have been examined on- and offshore San Diego. Combined, these data suggest that the fault-controlled pull-apart basin formed in this region influences sediment transport and deposition throughout the last glacial cycle. During highstand, the fault-controlled wide shelf, subsiding basin, and circulation patterns result in trapping of sediment in San Diego Bay and nearshore, cutting off sediment delivery to the deep-sea. During lowstand, the fault-controlled morphology creates a drainage divide that deflects streams to the south which lengthens and reduces slope of the drainage pathways. These findings are in contrast to observations in other fault controlled littoral cells along the southern California margin where sediment delivery to the deep-sea remains active during highstands, highlighting the variability of stream response to base-level change along active strike-slip dominated margins.

Depositional styles of deltaic systems from the inner shelf to shelf edge and their controlling processes: A case study from the Upper Oligocene to Lower Miocene in the Pearl River Mouth Basin

Late Oligocene to Early Miocene deltaic systems (the Zhuhai Formation) comprise the most important oil and gas reservoirs in the Pearl River Mouth Basin in the northern South China Sea and their changes in depositional architecture reflect the interplay of basin kinetic energy, sea level change and sediment supply. Based on the analysis of well log, seismic and core data, depositional styles of deltaic systems, sequence stratigraphy and their controlling factors are systematically documented. Three kinds of deltaic systems (braided river, meandering river and incised river deltas) are distinguished from the inner shelf, middle–outer shelf to shelf edge within a dynamic sequence framework. The braided river and braided river deltaic deposits along the inner shelf are mainly composed of vertically-stacked, thick-bedded channel sandstones and broad-lobate shaped delta front sandy deposits. The meandering river deltaic systems distributed in the middle–outer shelf are characterized by relatively thick delta front sandstones, interdistributary bay fills and prodelta mudstones with turbidite sheets in places. Incised river deltaic systems were generally developed at the shelf margin, with clinoform thickness ranging from 200 to 1200 m, and were commonly associated with large-scale slump and slope fan deposits. The temporal–spatial distribution of deltaic systems within the sequence framework shows a distinct pattern: the braided river delta system mainly developed in the early stage of highstand systems tracts; meandering river delta systems occurred in the middle–late stage of highstand systems tracts; and incised river delta systems generally formed in the falling stage systems tract. Depositional architectures of deltaic systems reveal that the basin kinetic condition varied from the inner shelf to shelf edge. Inner shelf braided river deltaic systems were dominated by fluvial processes, whereas middle–outer shelf meandering river delta and shelf edge incised river delta systems were reworked by waves and alongshore currents. The migration of deltaic systems across the entire wide shelf within the transgression–regression cycle is regarded as the result of high-amplitude sea level fluctuations under coolhouse or icehouse climatic condition. High progradation rates of shelf edge deltaic systems reflect enormous sediment supply resulting from the continuous uplift of the drainage area during the early post-rift period and the intense erosion triggered by glacial climate.

Coastal trapped waves propagation along the Southwestern Atlantic Continental Shelf

This study investigates the propagation of coastal trapped waves (CTWs) along the Brazilian continental shelf between 34°S and 11°S using in situ data combined with the outputs from a high-resolution ocean simulation with HYCOM. The CTWs generation area covers a wide region ranging from the Patagonian shelf to the southern Brazilian shelf. The spectral analysis of coastal sea level series between 54°S and 10.5°S shows three bands of high energy associated with periods from 5 to 12 days, 15–22 days, and 25–40 days. The energy of the CTWs decreases along their propagation for all frequency bands, showing a drastic reduction north of 22°S, due to abrupt variations in the width and depth of the continental shelf between Tubarão Bight and Abrolhos Bank. Their phase speed propagation varies along the coast, being faster (>25 m/s) in the southernmost region (between 42°S and 41°S), reaching ~11 m/s north of 41°S, and reducing to ~3 m/s further north (equatorward of 24°S). The free Continental Shelf Wave theory supports the notion that the intense deceleration north of 24°S can be explained by the narrowing of the continental shelf. The stratification parameter indicates that the Brazilian continental shelf has a barotropic response to wind-generated disturbances.

A first estimation of tsunami hazard of the Pacific coast of Costa Rica from local and distant seismogenic sources

Costa Rica has been affected by several local and distant tsunamis in the past, but the historical information is scarce and incomplete. Its Pacific coast stretches for over a thousand kilometers, and tsunami hazard has never been evaluated for its full extent. Numerical modeling of tsunami propagation and inundation is a useful tool to assess tsunami hazard, particularly in cases with limited historical information available. Here, we perform a first estimation of tsunami hazard for the Pacific coast of Costa Rica from seismogenic sources, by numerical propagation of 57 local and distant tsunamis to a depth of 20 m. The results of our study identified tsunami sources that are particularly threatening for Costa Rica and determined locations with higher tsunami hazard. For the analysis, the Pacific coast of Costa Rica was divided into segments and subsegments based on differences in continental slope morphology. Subsegments with higher tsunami heights were Southwest Nicoya Peninsula and West Osa Peninsula, and in a lesser extent North Guanacaste, North Nicoya Peninsula, and Central Pacific. Regions with long and gentle slopes and narrow continental shelf were affected by higher tsunami waves, due to more efficient tsunami energy transmission to the shelf and reduced energy loss while traveling through a narrow shelf. On the opposite, steeper continental slopes reflected most of the tsunami energy, causing smaller tsunami heights nearshore, regardless of the shelf width. Nevertheless, other effects played a major role, like curved coastlines that focused tsunami energy, wave refraction, interference, and trapped edge waves. Distant tsunamis dominated the threat, with tsunamis coming from the Tonga-Kermadec and the Colombia-Ecuador Trenches causing the greatest heights due to directivity, and arrival times of about 15 h and 75 min, respectively. Local tsunamis had short arrival times but a localized impact, mainly at the shoreline in front of the generation region but were also affected by tsunami focusing, wave refraction, and edge waves. Outer rise and Osa sources caused the lowest impact within local sources. These results provide a guide for emergency planners to prioritize coastal locations and tsunami sources for tsunami preparedness actions and warning protocols.

High-Resolution 3D Seismic Imaging of Fault Interaction and Deformation Offshore San Onofre, California

The Inner California Borderland (ICB) records a middle Oligocene transition from subduction to microplate capture along the southern California and Baja coast. The closest nearshore fault system, the Newport-Inglewood/Rose Canyon (NIRC) fault complex is a dextral strike-slip system that extends primarily offshore approximately 120 km from San Diego to Newport Beach, California. Holocene slip rates along the NIRC are 1.5–2.0 mm/year in the south and 0.5 mm/year along its northern extent based on trenching and well data. High-resolution 3D seismic surveys of the NIRC fault system offshore of San Onofre were acquired to define fault interaction across a prominent strike-slip step-over. The step-over deformation results in transpression that structurally controls the width of the continental shelf in this region. Shallow coring on the shelf yields a range of sedimentation rates from 0.27–0.28 mm/year. Additionally, a series of smaller anticlines and synclines record subtle changes in fault trends along with small step-overs and secondary splay faults. Finally, sedimentary units onlapping and dammed by the anticline, place constraints on the onset of deformation of this section of the NIRC fault system. Thickness estimates and radiocarbon dating yield ages of 560,000 to 575,000 years before present for the onset of deformation.

Hydraulic control of continental shelf waves

Abstract

Resource-saving steel hopper for loading of railway cars

The article is devoted to the study of geometric characteristics of corrugated sheets of railway grain hopper. In particular, the problem of finding the optimal parameters of a corrugated sheet of a given length, such as the height and width of the corrugation shelves, is solved. The optimality condition in this problem is determined by the minimum mass of the corrugated sheet. Because of analytical calculations, it was found that, regardless of the angle of tip of the corrugation wall, there will always be such a value of optimum height at which the corrugation area will be the smallest. For the analysis of this parameter, a series of curves of dependence of the corrugation cross-section area on the height for different angles of tip were constructed. The study also provided concise and engineering-friendly equations to determine the optimum height and width of corrugation shelves for wall angles up to 60º. These expressions can be applied to the case of corrugated sheet with a limited number of erection joints. For other cases of jointing of the corrugated sheets with each other (complete or partial overlapping of the lower part), additional studies should be performed similar to the procedure presented.

Seasonal Variability of Temperature Fronts in the Black Sea from Satellite Data

The structure and seasonal variability of temperature frontal zones and fronts in the Black Sea are studied on the basis of a modern high-resolution satellite data set. The distributions of the total temperature gradient allow us to distinguish the following frontal zones: the western frontal zone; the Turkish coastal frontal zone; the Anatolian upwelling frontal zone; the southeastern coastal frontal zone; northeastern, Crimea, and Kerch-Feodosia frontal zones; and the frontal zone of the deep part of the Black Sea. Large-scale fronts are identified within most frontal zones based on an analysis of distributions of meridional and zonal temperature gradients. It is shown that the combined effect of large-scale processes (seasonal warming and cooling of waters and their advection by the Rim Current) and regional factors (river discharge, coastline configuration, shelf width, development of upwelling, and formation of coastal eddies) produces significant spatiotemporal variability of the temperature field and, hence, frontal zones and fronts. It is ascertained that most fronts exist for several months. In the northern part of the sea, the fronts intensify mainly in winter due to an increase in the contrasts between warm Rim Current waters and colder coastal waters. The frontal zones near the southern coast of the sea intensify mainly in late spring–summer during the spring flood, the development of Anatolian upwelling, and active warming of waters in the eastern part of the sea.

Dinosaur Footprints in Terrigenous Sediments on the Early Jurassic Shelf of the Central North Caucasus: Sedimentological and Paleoecological Settings

Dinosaur footprints were found in Pliensbachian terrigenous rocks in the central North Caucasus. Tracks of different sizes (12‒35 cm) were identified at three levels of the Mizur Formation section. The sediments accumulated on the wide (15‒20 km) shelf of a paleobasin. The coastal land was covered with abundant vegetation. The warm and humid climate favored the development of weathering crusts, lake–bog systems, and peat accumulation. The seawater temperature ranged from 21 to 25°C. The accumulation of sediments on the shallow shelf and their distribution over the area were influenced by wave activity, currents, and tidal phenomena. At low tide, small dinosaurs visited the shelf zone in order to search for food in the form of benthic fauna, which was abundant in clayey–silty sediments. The simultaneous presence of significantly different–size animal tracks may indicate that large individuals preyed on smaller ones. The presence of dinosaur tracks at different levels of the upper Pliensbachian strata indicates that representatives of the terrestrial fauna periodically visited the paleoshelf during the Domerian.

Regional controls in the distribution and morphometry of deep-water gravitational deposits along a convergent tectonic margin. Southern Caribbean of Colombia

Deep-water fold and thrust belts often develop in convergent tectonic margins, creating irregular slope profiles that control the distribution of deep-water gravity deposits. However, in areas with high sediment supply, the erosion and sedimentation can minimize structural relief and smooth the slope. Using multibeam bathymetry with 3D seismic data, we analyze the distribution of deep-water gravity-driven deposits along the convergent margin of the southern Caribbean of Colombia, comparing areas with different continental sediment supply, slope profile, and shelf width. We identify three geomorphological zones: The Northern, Central and Southern Zones. The Northern Zone is characterized by a gentle slope topography, high sediment supply, and large (>100 km length) channel-levee systems traversing the slope and basin floor. In this zone, shelf-attached mass-transport deposits erode and smooth sea-floor topography. The Central Zone is characterized by low sediment supply and steep and irregular slope topography. Here, short-runout mass-transport deposits sourced from the crests and steep flanks of emergent anticlines are common. The irregular relief created by tectonic deformation forms barriers for sediment transport, leading to tortuous sediment-flow pathways. Submarine canyons incise the thrust-cored anticlines, transporting sediment through interconnected, adjacent piggyback sub-basins. Finally, the Southern Zone is characterized by steep slope and moderate sediment supply. Here, tectonic deformation has been smoothed by numerous shelf-attached mass-transport deposits. The erosional scours carved by mass flows merge downslope and evolve into submarine canyons that can deliver mass-transport deposits more than 80 km into the basin. We analyze the impact of slope profile, sediment input and shelf width on the distribution and morphology of deep-water deposits along the southern Colombian Caribbean margin, and present a predictive model for the depositional patterns more likely to develop in other continental margins affected by deep-water fold and thrust belts.

Rigid diaphragm wall with a relief shelf to mitigate the deformations of soil and shallow foundations subjected to normal faulting

Previous studies developed several types of weak and soft walls to deviate reverse fault ruptures and to absorb the compressional soil deformation. However, different strategies are required to address the extensional soil deformation caused by normal faulting. This study proposes a rigid diaphragm wall with a relief shelf (RDWRS) to mitigate soil and shallow foundation deformations in normal fault. The relief shelf is applied to keep the stability of the diaphragm wall and to protect the shallow foundation on the ground surface. Numerical modeling is performed to investigate the diaphragm wall-normal fault interaction mechanisms and the effects of RDWRS on deformation mitigation of shallow foundations. The RDWRS shows promising effects concerning the mitigation of soil and shallow foundation deformations. The protection effects depend on soil depth, foundation width, foundation location, surcharge pressure, wall depth relative to the rupture, the width of the relief shelf, and the distance between the wall and the foundation. As this paper presents the results of numerical modeling, experimental studies are suggested for future studies. Besides, the economy of the RDWRS should be comprehensively considered when applying this method.

Tidal amplification and along‐strike process variability in a mixed‐energy paralic system prograding onto a low accommodation shelf, Edgeøya, Svalbard

AbstractThe study describes the depositional development and sediment partitioning in a prograding paralic Triassic succession. The deposits are associated with the advance of large prism‐scale clinoforms across a shallower platform area. Approaching the platform, the limited accommodation and associated relative higher rates of deposition generated straighter clinoforms with lower foreset angles. The vertical restriction across the platform is interpreted to have amplified the tidal signature. Sediment was redistributed from the coast into increasingly sandy delta‐front deposits, compared to offshore equivalents. The deposits comprise extensive compound dune fields of amalgamated and increasingly clean sandbodies up‐section. Rapid deposition of significant amounts of sand led to differential subsidence and growth‐faulting in the delta front, with downthrown fault blocks further amplifying the tidal energy through funnelling. A mixed‐energy environment created along‐strike variability along the delta front with sedimentation governing process‐regime. Areas of lower sedimentation were reworked by wave and storm‐action, whereas high sedimentation rates preserved fluvially dominated mouth bars. A major transgression, however, favoured tidally dominated deposits also in these areas, attributed to increasing rugosity of the coastline. Formation of an extensive subaqueous platform between the coast and delta front dampened incoming wave energy, and tidally dominated deposits dominate the near‐shore successions. Meanwhile formation of wave‐built sand‐bars atop the platform attest to continued wave influence. The strong tidal regime led to the development of a heterolithic near‐shore tidally dominated channel system, and sandier fluvial channels up‐river. The highly meandering tidal channels incising the subaqueous platform form kilometre wide successions of inclined heterolithic stratification. The fluvially dominated channels which govern deposition on the delta plain are narrower and slightly less deep, straighter, generally symmetric and filled with cleaner sands. This study provides important insight into tidal amplification and sand redistribution during shallowing on a wide shelf, along with along‐strike process‐regime variability resulting from variations in sediment influx.

Comparison of Modern and Pleistocene (MIS 5e) Coastal Boulder Deposits from Santa Maria Island (Azores Archipelago, NE Atlantic Ocean)

Modern and palaeo-shores from Pleistocene Marine Isotope Substage 5e (MIS 5e) featuring prominent cobble/boulder deposits from three locations, on the southern and eastern coast of Santa Maria Island in the Azores Archipelago, were compared, in order to test the idea of higher storminess during the Last Interglacial. A total of 175 basalt clasts from seven transects were measured manually in three dimensions perpendicular to one another. Boulders that exceeded the minimum definitional diameter of 25 cm contributed to 45% of the clasts, with the remainder falling into the category of large cobbles. These were sorted for variations in shape, size, and weight pertinent to the application of two mathematical formulas to estimate wave heights necessary for traction. Both equations were based on the “Nott-Approach”, one of them being sensitive to the longest axis, the other to the shortest axis. The preponderance of data derived from the Pleistocene deposits, which included an intertidal invertebrate fauna for accurate dating. The island’s east coast at Ponta do Cedro lacked a modern boulder beach due to steep rocky shores, whereas raised Pleistocene palaeo-shores along the same coast reflect surged from an average wave height of 5.6 m and 6.5 m. Direct comparison between modern and Pleistocene deposits at Ponta do Castelo to the southeast and Prainha on the island’s south shore produced contrasting results, with higher wave heights during MIS 5e at Ponta do Castelo and higher wave heights for the modern boulder beach at Prainha. Thus, our results did not yield a clear conclusion about higher storminess during the Last Interglacial compared to the present day. Historical meteorological records pit the seasonal activity of winter storms arriving from the WNW-NW against the scant record of hurricanes arriving from the ESE-SE. The disparity in the width of the marine shelf around Santa Maria Island with broad shelves to the north and narrow shelves to the south and east suggested that periodic winter storms had a more regular role in coastal erosion, whereas the rare episodic recurrence of hurricanes had a greater impact on southern and southeastern rocky shores, where the studied coastal boulder deposits were located.

Palynomorphs in surface sediments of the North-Western Black Sea as indicators of environmental conditions

Most previous Black Sea palynology studies have used pollen-spore assemblages as proxies for climate and landscape changes, and dinocyst assemblages as proxies for surface water salinity. However, there are few data on within-region variations in these assemblages using large sets of surface samples and a full suite of palynomorphs, including terrigenous pollen + spores, freshwater algal spores, and organic remains of marine microplankton, micro- and meiobenthos. Here we fill this knowledge gap, using results from a palynological study of 43 surface samples from water depths of 71–905 mbsl on the Ukrainian Shelf and adjoining continental slope, NW Black Sea. The palynology samples were collected in conjunction with water and sediment chemistry data, grain-size, micropaleontological and meiobenthos data from the EU-FP6 project HERMES “Hotspot Ecosystems Research on the Margins of European Seas”. This dataset uniquely covers shelf and upper slope areas east and west of the Odessa-Sinop Fault Zone (OSFZ) that delimits areas of lower and higher hydrocarbon gas contents, respectively.The new data show that on the wide shelf with a shallow (ca. 80 m) oxycline, pollen assemblages differ from classical models for marine pollen transport. On the Ukrainian Shelf, pollen concentrations are extraordinarily high far offshore and the grains are often pyrite-infilled. These features apparently reflect high pollen preservation potential in low oxygen sediments, probably enhanced by cross-shelf transport of land-derived particulate organic matter. The new data have important consequences for accurate modelling of carbon burial and storage in epicontinental seas, and they provide a baseline for evaluating marine pollen concentrations expected to reflect the early Neolithic farming on the Ukrainian shelf. Furthermore, although large concentrations of dinocysts comprise more than half of the total Non-Pollen Palynomorphs (NPP), the high pollen concentrations skew the P:D index (pollen:dinocyst ratio) commonly used to evaluate changes in paleo-shoreline (proxy sea-level) positions. Despite the discharge of several large rivers to the Ukrainian Shelf, there are negligible traces of freshwater algae in the HERMES samples except at Sta. 15 where Pediastrum is abundant. Overall, palynomorphs of terrestrial origin make up about half (52%) of the acid-resistant organic-walled particles on the outer NW Black Sea Shelf and adjacent slope.The abundances of the major micro- and meiobenthic palynomorph groups show the same trends east and west of the OSFZ as the micropaleontological data reported for the HERMES samples taken at the same sites. The combined microzooplankton and zoobenthos surface assemblage data provide an important new baseline for evaluating historical and past changes in biodiversity and aquatic foodweb structure in the now polluted NW Black Sea. Comparison can also be made with foodweb reconstructions for Permian black shales.

Deep-water deposition in response to sea-level fluctuations in the past 30 kyr on the northern margin of the South China Sea

Sea-level fluctuations exert a major influence on deep-water depositional processes through changing sediment supply and oceanographic processes. By utilizing the bathymetric map and six piston cores (P1 and P5–P9) from the two major canyon systems (i.e., Pearl River Canyon (PRC) and Shenhu Submarine Canyon Group (SSCG)) within the Baiyun Sag, northern South China Sea (SCS), this study investigated the deep-water depositional responses to the sea-level fluctuations in the past 30 kyr, during which the last glacial maximum (LGM) shelf edge remained farther (>60 km) seawards of the contemporary shoreline and had water depths greater than 120 m. In the past 30 kyr, the Baiyun Sag was dominated by mixed sedimentary processes involving in both turbidity and bottom currents. Coarse-grained turbidites were mostly deposited at the upper-middle segments of the slope channel within the PRC and the canyon heads of the SSCG from the LGM to the middle deglacial stage (ca. 11.5 ka); while the fine-grained contourites were continuously and widely deposited within the Baiyun Sag. It is noted that the contourites show fining- and coarsening-upwards trends before and after 6.7 ka, respectively.The turbidity currents in the past 30 kyr were mainly triggered by oceanographic processes (i.e., contour currents and internal waves) within the Baiyun Sag. Internal waves exerted greater effects on the occurrences of turbidity currents flowing within the canyons of the SSCG. The near-seafloor intensities of contour currents and internal waves around the shelf edge weakened with sea level rising. Internal waves were responsible for triggering high-frequency turbidity currents to deposit the multiple thin-bedded turbidites at the slope canyon heads of the SSCG. The fining-upwards trend of the contourites during the period of 29.5–6.7 ka was mainly attributed to the decreasing sediment supply from the Pearl River with sea level rising, whereas the coarsening-upwards trend in the past 6.7 kyr was resulted from the circulation intensification, in response to the reopening of all straits around the SCS. This study provides a new insight into the significance of varying oceanographic processes for deep-water deposition in response to sea-level fluctuations on sediment-starved continental margin with wide shelf.

Investigation of active earth pressures and failure mechanism of retaining walls with a relief shelf

Various studies of retaining walls with relief shelves tend to rely on the proposition of reducing soil pressures behind the wall, according to the location and the width of relief shelf. The previous numerical studies did not predict the optimum reduction rate of earth pressure and the shape of failure mechanism. The finite-difference code fast Lagrangian analysis of continua is used to evaluate the active earth pressure behind a retaining wall with one relief shelf placed at different levels with a variable width. The soil behind the wall is modelled by an elastoplastic constitutive model with a Mohr–Coulomb yield criterion. The effects of soil non-associativity as well as the interface of soil–wall and relief shelf–soil are investigated. The reduction of earth pressure is estimated by the rate R, defined as the ratio of the reduced active thrust to the total active thrust of a wall without a relief shelf. The rate R is evaluated numerically and by using Klein’s solution. The obtained results enable determination of the appropriate position and width of the relief shelf to evaluate the optimum values of R.