Optically stimulated luminescence age controls on late Pleistocene and Holocene coastal lithosomes, North Carolina, USA

Luminescence ages from a variety of coastal features on the North Carolina Coastal Plain provide age control for shoreline formation and relative sea-level position during the late Pleistocene. A series of paleoshoreline ridges, dating to Marine Isotope Stage (MIS) 5a and MIS 3 have been defined. The Kitty Hawk beach ridges, on the modern Outer Banks, yield ages of 3 to 2 ka. Oxygen-isotope data are used to place these deposits in the context of global climate and sea-level change. The occurrence of MIS 5a and MIS 3 shorelines suggests that glacio-isostatic adjustment (GIA) of the study area is large (ca. 22 to 26 m), as suggested and modeled by other workers, and/or MIS 3 sea level was briefly higher than suggested by some coral reef studies. Correcting the shoreline elevations for GIA brings their elevation in line with other sea-level indicators. The age of the Kitty Hawk beach ridges places the Holocene shoreline well west of its present location at ca. 3 to 2 ka. The age of shoreline progradation is consistent with the ages of other beach ridge complexes in the southeast USA, suggesting some regionally contemporaneous forcing mechanism.     

Impact of hydro-isostatic holocene sea-level change on the geologic context of Island archaeological sites,Northern Ha'apai group,Kingdom of Tonga

Archaeological sites in the northern Ha'apai Group of central Tonga occur on small islands within the uplifted forearc belt of the Tonga-Kermadec arc-trench system. The present inland positions of occupation sites that probably once occupied coastal settings imply significant expansion of some island shorelines during late Holocene time (ca. 3250 B.P. to present). Geologic processes leading potentially to enlargement of the islands include continuing forearc uplift, eustatic or glacio-hydro-isostatic fall in sea level following a mid-Holocene highstand, and progressive accretion of beach ridges to island coasts, with or without changes in relative sea level. Radiometric dates for uplifted coral terraces in Tonga indicate that forearc uplift has been negligible during Holocene time. By contrast, theoretical considerations, regional analysis of shoreline indicators throughout the South Pacific, and limited empirical data from Tonga itself all imply that regional sea level has declined locally by 1–2 m since a mid-Holocene highstand (ca. 6000-3000 B.P.), which was a hydro-isostatic response to transfer of water mass from Pleistocene ice caps to the ocean basins. Emergence of originally coastal sites is thus expected since initial settlement of the islands by Lapita peoples. Accretionary coastal flats composed of multiple beach ridges are 250–500 m wide on favorable leeward shores and the flanks of sand cays, but some presently unknown proportion of this incremental island growth may have occurred prior to the post-mid-Holocene decline in relative sea level. Ash falls from tephra eruptions at Tongan volcanoes also modified island environments through Quaternary time. Evidence for significant change in the configuration and morphology of islands in Ha'apai during the period of human settlement highlights the need for systematic interdisciplinary archaeological and geological research in the study of Pacific prehistory. © 1994 John Wiley & Sons, Inc.     

Coarse‐sand beach ridges at Cowley Beach,north‐eastern Australia: Their formative processes and potential as records of tropical cyclone history

Storm surges generated by tropical cyclones have been considered a primary process for building coarse‐sand beach ridges along the north‐eastern Queensland coast, Australia. This interpretation has led to the development of palaeotempestology based on the beach ridges. To better identify the sedimentary processes responsible for these ridges, a high‐resolution chronostratigraphic analysis of a series of ridges was carried out at Cowley Beach, Queensland, a meso‐tidal beach system with a >3 m tide range. Optically stimulated luminescence ages indicate that 10 ridges accreted seaward over the last 2500 to 2700 years. The ridge crests sit +3·5 to 5·1 m above Australian Height Datum (ca mean sea‐level). A ground‐penetrating radar profile shows two distinct radar facies, both of which are dissected by truncation surfaces. Hummocky structures in the upper facies indicate that the nucleus of the beach ridge forms as a berm at +2·5 m Australian Height Datum, equivalent to the fair‐weather swash limit during high tide. The lower facies comprises a sequence of seaward‐dipping reflections. Beach progradation thus occurs via fair‐weather‐wave accretion of sand, with erosion by storm waves resulting in a sporadic sedimentary record. The ridge deposits above the fair‐weather swash limit are primarily composed of coarse and medium sands with pumice gravels and are largely emplaced during surge events. Inundation of the ridges is more likely to occur in relation to a cyclone passing during high tide. The ridges may also include an aeolian component as cyclonic winds can transport beach sand inland, especially during low tide, and some layers above +2·5 m Australian Height Datum are finer than aeolian ripples found on the backshore. Coarse‐sand ridges at Cowley Beach are thus products of fair‐weather swash and cyclone inundation modulated by tides. Knowledge of this composite depositional process can better inform the development of robust palaeoenvironmental reconstructions from the ridges.     

Internal architecture of a raised beach ridge system (Anholt,Denmark) resolved by ground-penetrating radar investigations

The internal architecture of raised beach ridge and associated swale deposits on Anholt records an ancient sea level. The Holocene beach ridges form part of a progradational beach ridge plain, which has been interpreted to have formed during an isostatic uplift and a relative fall in the sea level over the past 7700 years. The ridges are covered by pebbles and cobbles and commonly show evidence of deflation. Material presumably removed from the beach ridges and adjacent swales form the present dune forms on Anholt. Ground-penetrating radar (GPR) reflection lines have been collected with 250 MHz shielded antennae across the fossil ridge and swale structures. The signals penetrate the subsurface to a maximum depth of ～ 10 m below the fossil features. The GPR data resolve the internal architecture of the beach ridges and swales with a vertical resolution of about 0.1 m. GPR mapping indicates that the Holocene beach ridges are composed of seaward-dipping beachface deposits as well as minor amounts of inland dipping deposits of wash-over origin. The beachface deposits downlap on underlying shoreface deposits, and we use these surfaces as markers of a relative palaeo-sea level. The new data indicate that the highest relative sea level at about 8.5 m was reached 6500 years ago; 700 years later the relative sea level had dropped 0.7 m indicating a change in the relative sea level around 1 mm/year. This fall in the relative sea level most likely records the influence of an isostatic rebound causing younger beach ridge deposits to indicate lower sea levels.     

Coastal lagoons and beach ridges as complementary sedimentary archives for the reconstruction of Holocene relative sea‐level changes

Coastal lagoons and beach ridges are genetically independent, though non‐continuous, sedimentary archives. We here combine the results from two recently published studies in order to produce an 8000‐year‐long record of Holocene relative sea‐level changes on the island of Samsø, southern Kattegat, Denmark. The reconstruction of the initial mid‐Holocene sea‐level rise is based on the sedimentary infill from topography‐confined coastal lagoons (Sander et al., Boreas, 2015b). Sea‐level index points over the mid‐ to late Holocene period of sea‐level stability and fall are retrieved from the internal structures of a wide beach‐ridge system (Hede et al., The Holocene, 2015). Data from sediment coring, georadar and absolute dating are thus combined in an inter‐disciplinary approach that is highly reproducible in micro‐tidal environments characterised by high sediment supply. We show here that the commonly proximate occurrence of coastal lagoons and beach ridges allows us to produce seamless time series of relative sea‐level changes from field sites in SW Scandinavia and in similar coastal environments.     

Holocene coastal evolution and uplift mechanisms of the northeastern Raukumara Peninsula,North Island,New Zealand

The coastal geomorphology of the northeastern Raukumara Peninsula, New Zealand, is examined with the aim of determining the mechanisms of Holocene coastal uplift. Elevation and coverbed stratigraphic data from previously interpreted coseismic marine terraces at Horoera and Waipapa indicate that, despite the surface morphology, there is no evidence that these terraces are of marine or coseismic origin. Early Holocene transgressive marine deposits at Hicks Bay indicate significant differences between the thickness of preserved intertidal infill sediments and the amount of space created by eustatic sea level rise, therefore uplift did occur during early Holocene evolution of the estuary. The palaeoecology and stratigraphy of the sequence shows no evidence of sudden land elevation changes. Beach ridge sequences at Te Araroa slope gradually toward the present day coast with no evidence of coseismic steps. The evolution of the beach ridges was probably controlled by sediment supply in the context of a background continuous uplift rate. No individual dataset uniquely resolves the uplift mechanism. However, from the integration of all evidence we conclude that Holocene coastal uplift of this region has been driven by a gradual, aseismic mechanism. An important implication of this is that tectonic uplift mechanisms do vary along the East Coast of the North Island. This contrasts with conclusions of previous studies, which have inferred Holocene coastal uplift along the length of the margin was achieved by coseismic events. This is the first global example of aseismic processes accommodating uplift at rates of >1 mm yr⁻¹ adjacent to a subduction zone and it provides a valuable comparison to subduction zones dominated by great earthquakes.     

Emergent coasts of Akimiski Island,James Bay,Northwestern Territories,Canada: geology,geomorphology, and vegetation

Akimiski Island contains good examples of emergent coastal landscapes of a cold mesotidal inland sea. The Paleozoic reefal trend of the Attawapiskat Formation dictated the overall shape and the main structure of the island. Differential erosion and deposition by Pleistocene glaciers have fluted the island in a north-south direction. That surface was later modified by emergent landforms developed in the last 3500–4000 yrs.The modern steeper southern area reaches an elevation of 60 m approximately 3 km inland from the southern shoreline, and contains well-developed sandy and gravelly longitudinal beach ridges and spits, now inactive and covered by a lichen-rich taiga (boreal forest). The flatter, northern part of the island shows a wide transition between the primarily erosional, sand-starved, coastal marshland and the inland organic-rich fens. Partially paludified longitudinal and transversal beach ridges subdivide those northern flat wetlands forcing a straight course to the north-flowing streams.The vegetation zonations of the marshes are as varied as the coasts, facing different oceanographic conditions. Longshore and tidal currents affect the western and southern coasts greatly. Tides, waves and sea ice affect the others more. The marshes resemble those of the mainland in having well-defined Puccinellia phryganodes lower marsh, Carex subspathacea upper marsh, and peat-forming coastal fens. Some components of the marshes of the island, such as isolated mounds with vegetation minisequences, incipient permafrost features generated by seasonal frozen ground conditions, and intense grazing by large populations of geese, are typical of cold settings, more commonly found along mainland coasts farther to the north in James Bay and Hudson Bay.     

Evidence for a transgressive barrier within a regressive strandplain system: Implications for complex coastal response to environmental change

Clastic, depositional strandplain systems have the potential to record changes in the primary drivers of coastal evolution: climate, sea‐level, and the frequency of major meteorological and oceanographic events. This study seeks to use one such record from a southern Brazilian strandplain to highlight the potentially‐complex nature of coastal sedimentological response to small changes in these drivers. Following a 2 to 4 m highstand at ca 5·8 ka in southern Brazil, falling sea‐level reworked shelf sediment onshore, forcing coastal progradation, smoothing the irregular coastline and forming the 5 km wide Pinheira Strandplain, composed of ca 500 successive beach and dune ridges. Sediment cores, grab samples and >11 km of ground‐penetrating radar profiles reveal that the strandplain sequence is composed of well‐sorted, fine to very‐fine quartz sand. Since the mid‐Holocene highstand, the shoreline prograded at a rate of ca 1 to 2 m yr^?1 through the deposition of a 4 to 6 m thick shoreface unit; a 1 to 3 m thick foreshore unit containing ubiquitous ridge and runnel facies; and an uppermost beach and foredune unit. However, the discovery of a linear, 100 m wide barrier ridge with associated washover units, a 3 to 4 m deep lagoon and 250 m wide tidal inlet within the strandplain sequence reveals a period of shoreline transgression at 3·3 to 2·8 ka during the otherwise regressive developmental history of the plain. The protected nature of Pinheira largely buffered it from changes in precipitation patterns, wave energy and fluvial sediment supply during the time of its formation. However, multiple lines of evidence indicate that a change in the rate of relative sea‐level fall, probably due to either steric or ice‐volume effects, may have affected this coastline. Thus, whereas these other potential drivers cannot be fully discounted, this study provides insights into the complexity of decadal‐scale to millennial‐scale coastal response to likely variability in sea‐level change rates.     

Morphology and sedimentary architecture of a beach‐ridge system (Anholt,the Kattegat sea): a record of punctuated coastal progradation and sea‐level change over the past ∼1000 years

Flakket on the island of Anholt in Denmark is a cuspate foreland facing the microtidal Kattegat sea. It is composed of a number of beach ridges typically covered by dune sand and separated by swales and wetlands. OSL dating indicates that the evolution of Flakket began c. AD 1000. Foreland growth was punctuated by a major episode of coastal reorganization leading to coastal retreat c. AD 1800. Coastal retreat led to the formation of an erosion surface that separates older and higher‐lying beach‐ridge and swale deposits from younger and lower‐lying deposits. The palaeo‐sea level is deduced from the architecture of the deposits, and interpretation of ground‐penetrating radar data and geomophological observations indicates that relative sea level was about 1.90±0.25 m above present sea level c. AD 1000, but about 0.00±0.25 m relative to present sea level c. AD 1830 and c. AD 1870. Anholt is situated at the margin of the uplifted Fennoscandian area; assuming uplift to be about 1.2 mm a^?1 it follows that absolute sea level was about +0.70±0.25 m at AD 1000, but around ?0.22±0.25 m at AD 1830 and around ?0.17±0.25 m at AD 1870. Within the uncertainties of the age control, the sea‐level indicators mapped by ground‐penetrating radar reflections and the variability of estimates of uplift found in the literature, the result obtained for AD 1000 is consistent with findings from the Stockholm area in Sweden and with a recently published global sea‐level curve.     

Impact of environmental parameters on coral reef development and drowning: Forward modelling of the last deglacial reefs from Tahiti (French Polynesia; IODP Expedition #310)

The sedimentological and chronological analysis of the last deglacial reef sequences of Tahiti (French Polynesia), drilled during the Integrated Ocean Drilling Program Expedition 310, provide a high‐resolution data set allowing a well‐constrained forward modelling study. This study represents the first attempt to model in three dimensions the coral reef development of Tahiti during the last deglacial sea‐level rise (23 000 to 6000 cal yr bp ) using the software dionisos developed by IFP Energies nouvelles. It allows the testing of the reconstructed last deglacial sea‐level curve and the different environmental parameters (for example, wave energy and sediment fluxes) that could have influenced the reef development. These last deglacial reef sequences form two prominent ridges occurring seaward of the living barrier reef that consist of successive submerged reefs. These reefs have been prone to drowning because the window of maximum carbonate production rate is inhibited by high water turbidity (sediment supply from a nearby river), shallow depth of wave action and substrate availability. These factors, combined with rapid sea‐level rise, have driven the growth of retrograding reef pinnacles. Local factors (substratum nature, sediment supply and wave energy) were the main processes that induced the drowning of the inner ridge, whereas interplay of local and global factors (acceleration of the sea‐level rise) was responsible for the drowning of the outer ridge. This particular acceleration of sea‐level rise of 16 m between 14·6 ka and 14 ka bp corresponds to meltwater pulse 1A.     

Mid-to-late Holocene sea level influence on coastal wetland development in Trinidad

Understanding how Holocene sea levels influenced coastal wetland development in the Caribbean will aid wetland management in the context of predicted sea level rise. Nine radiocarbon dates from the Maracas and Nariva Swamps on wave-dominated coasts from Trinidad, show sea level was –9 m approximately 7000 yr BP, and rose gradually to –2 m by 2000 yr BP. Since then there may have been isostatic readjustment. Wetlands developed with a transgression of dry upland habitats by rising seas and the facultative halophyte Rhizophora colonized the new brackish water environment. A freshwater plant community gradually replaced the Rhizophora as the marine influence decreased. At Maracas, higher sea levels caused wetland retreat as beach and lagoon habitats migrated inland. Sand ridges in Nariva Swamp indicate that, as in Maracas Swamp, sea level rise created beaches and lagoons, but that these landforms prograded as additional nearshore sediments were deposited. Basins were also filled with sediment delivered by streams that drain the watershed, and by mangrove peat accumulation.     

Internal architecture and evolution of bioclastic beach ridges in a megatidal chenier plain: Field data and wave flume experiment

Beach ridges in macrotidal environments experience strong multi‐annual to multi‐decennial fluctuations of tidal inundation. The duration of tide flooding directly controls the duration of sediment reworking by waves, and thus the ridge dynamics. Flume modelling was used to investigate the impact of low‐frequency tidal cycles on beach ridge evolution and internal architecture. The experiment was performed using natural bioclastic sediment, constant wave parameters and low‐frequency variations of the mean water level. The morphological response of the beach ridge to water level fluctuations and the preservation of sedimentary structures were monitored by using side‐view and plan‐view photographs. Results were compared with the internal architecture of modern bioclastic beach ridges in a macrotidal chenier plain (Mont St. Michel Bay, France) surveyed with ground‐penetrating radar. The experimentally obtained morphologies and internal structures matched those observed in the field, and the three ridge development stages identified in ground‐penetrating radar profiles (early transgressive, late transgressive and progradational) were modelled successfully. Flume experiments indicate that flat bioclastic shapes play a key role in sediment sorting in the breaker zone, and in sediment layering in the beach and washover fans. Water level controls washover geometry, beach ridge evolution and internal structure. Low water levels allow beach ridge stabilization and sediment accumulation lower on tidal flats. During subsequent water level rise, accumulated sediment becomes available for deposition of new washover units and for bayward extension of the beach ridges. In the field, low‐frequency water level fluctuations are related to the 4·4 year and 18·6 year tidal cycles. Experimental results suggest that these cycles may represent the underlying factor in the evolution of the macrotidal chenier coast at the multi‐decadal to centennial time scale.     

Late Holocene Tectonic Uplift and the Silting Up of Lechaion,the Western Harbor of Ancient Corinth,Greece

Lechaion's ancient harbor is now a coastal swamp filled with sediments. Two natural factors explain the harbor's abandonment: (1) tectonic uplift during historical times and (2) the location of the harbor basin in a serpentine depression protected from the sea. Although it undoubtedly functioned as a very efficient sediment trap, only modest sedimentation rates (<1 mm/yr) have been measured in the basin. This paradox suggests that the basin was dredged and that the extracted sediments were dumped, forming a number of mounds around the harbor edges. The transition from marine organics to silt is dated to 750–400 cal. B.C. and precedes the 1.2 m uplift of the harbor at around 340 B.C., which underscores the minimal impact of tectonic forcing factors. The presence of fine‐grained sediments is consistent with an increasingly protected environment. The macrofauna indicate a low‐energy environment enriched with organic matter and brackish conditions. All data suggest that this environment became isolated from the sea. Although a seismic uplift around 340 B.C. played a partial role in the evolution of the harbor, it is not the sole natural forcing agent involved in the silting up of the basin. © 2012 Wiley Periodicals, Inc.     

Coastal uplift rate at Matanzas (Cuba) inferred from MIS5e phreatic overgrowths on speleothems

Many morphological elements in Cuba's landscape (e.g. marine terraces, tidal notches) demonstrate that coastal uplift has taken place, but the rate at which this occurs is not known. Carbonate phreatic overgrowths on speleothems have been found in a cave in Central North Cuba, ~1 km from the present coastline at 16 m asl. They form exceptional and unique mushroom‐shaped speleothems and balconies decorating the walls of the rooms. These phreatic overgrowths on speleothems (POS) formed at the oscillating air–water interface in sea‐level controlled anchialine lakes. U/Th dating of these overgrowths suggests ages that are compatible with the Marine Isotope Stage 5e (i.e. 130–115 ka). These POS have fixed this sea‐level highstand and demonstrate that this part of Cuba has been subjected to a much lower uplift rate than previously reported, that is, less than 0.1 mm/year since the last interglacial.     

Allerød- Younger Dryas sea level changes in southwestern Sweden and their relation to the Baltic Ice Lake development

Mt. Kroppefjall is situated just south of the Middle Swedish (Younger Dryas) ice-marginal zone. Its abundance of lake basins makes it very suitable for detailed shore displacement studies close to the Younger Dryas ice margin. Altogether 12 lakes at altitudes between 157 and 78 m were studied and all but one situated above the marine limit contained marine sediments. The dating of their isolation from the sea resulted in a shore displacement curve from c. 11,200 to c. 98M)BP. The relative uplift almost ceased between 10,900 and 10,300 BP, which is mainly related to an ice readvance in the Lake Vanern basin. This period of balance between uplift and sea level rise was preceded by a relative uplift rate of 5 m/lW yr and followed by as high rates as 7–8 m/100 yr, possibly caused by a delayed uplift effect and perhaps also a local fall in sea level caused by the rapidly receding ice margin. The time difference between the formation of two delta surfaces at Odskolts Moar is estimated at 60&800 years. Shoreline diagrams along the Swedish west and east coasts, mainly based on a number of shore displacement curves, reveal large anomalies that are believed to have been caused by dammings and drainages of the Baltic basin. The southwards extrapolated shorelines indicate that the bedrock threshold in the Oresund Strait, between Denmark and Sweden, functioned as the outlet threshold for the Baltic Ice Lake during its dammed stages, while the erosion of the Store Balt and Darss Sill straits began at the culmination of the Ancylus transgression and continued during the rapid IS20 m Ancylus regression.     

Present-day trends of vertical ground motion along the coast lines

Vertical ground motion (VGM) rates stand as crucial information, either for predicting the impact of the actual sea level rise along low-lying coasts or refining geodynamic problems. Because present day VGM rates have a magnitude smaller than 10 mm/yr, they remain challenging to quantify and often elusive. We focus on the quantification of global-scale VGM rates in order to identify global or regional trends. We computed VGM rates by combining tide gauges records and local satellite altimetry, which yield a new dataset of 634 VGM rates. We further compare this database to previous studies that use geodetic techniques and tide gauges records in order to evaluate the consistency of both our results and previous ones. The magnitudes differ by less than 5 mm/yr, and similar subsidence and uplift general tendencies appear. Even if the asset of our database stands in the greater number of sites, the combination of all studies, each with different pros and cons, yields a hybrid dataset that makes our attempt to extract VGM trends more robust than any other, independent study. Fennoscandia, the West coast of North America, and the eastern coast of Australia are uplifting, while the eastern coast of North America, the British Isles and Western Europe, the eastern Mediterranean Sea, Japan, and the western coast of Australia are subsiding. Glacial Isostatic Adjustment (GIA) is expected to provide a major contribution to the present-day signal. Aside from Fennoscandia, observed VGM often depart from the GIA model predictions of Peltier (2004). This either results from an underestimate of the model predictions or from the influence of other processes: indeed, the influence of the geodynamic setting appears in particular along the coasts of western North America or Japan, where the alternation of transform faults and subduction zones makes it possible to assign contrasted behaviours to the local geodynamic context. Local mechanisms like anthropogenic processes or sediment compaction, also contribute to VGM. This remains true for the critical cases of Venice, the Gulf of Mexico, the Ganges delta, and the Maldives, which are particularly exposed to the current sea level rise.     

The Early Lake Ontario barrier beach: evidence for sea level about 12.8–12.5 cal. ka BP beneath western Lake Ontario in eastern North America

An overstepped, concave‐eastward, barrier beach beneath Holocene mud in western Lake Ontario has been delineated by acoustic and seismic reflection profiles and piston cores, and related to Early Lake Ontario (ELO). The average ELO barrier depth below present mean lake level is 77.4 to 80.6 m, or about ?6 to ?2.8 m above present sea level. Trend surface analysis of Champlain Sea (Atlantic Ocean) marine limits defined the contemporaneous marine water surface, and projections of this surface pass ~25 m above the outlet sill of the Lake Ontario basin and extend to the ELO palaeo‐barrier, a unique sand and gravel deposit beneath western Lake Ontario. ELO was connected to the Champlain Sea above the isostatically rising outlet sill for up to three centuries after about 12.8 cal. ka BP, while the glacio‐isostatically depressed St. Lawrence River Valley was inundated by the Atlantic Ocean. During the period of this connection, ELO level was confluent with slowly rising sea level, and the lake constructed a transgressive beach deposit with washover surfaces. ELO remained fresh due to a high flux of meltwater inflow. The marine water level connection stabilized water level in ELO relative to its shore and facilitated shore erosion, sediment supply and barrier construction. Glacio‐isostatic uplift of the outlet sill, faster than sea‐level rise, lifted ELO above the Champlain Sea about 12.5 cal. ka. Shortly after, a hydrological deficit due mainly to a combination of diverted meltwater inflow and dry climate, well known from regional pollen studies, forced the lake into a lowstand. The lowstand stranded the barrier, which remains as evidence of sea level, the farthest inland in eastern North America north of the Gulf of Mexico at the time. The highest palaeo‐washover surface provides a sea‐level index point.     

Dependence of mid-ocean ridge morphology on spreading rate in numerical 3-D models

The morphology of natural mid-ocean ridges changes significantly with the rate of extension. Full spreading rate on Earth varies over more than one order of magnitude, ranging from less than 10 mm/yr at the Gakkel Ridge in the Arctic Ocean to 170 mm/yr at the East Pacific Rise. The goal of this study is to reproduce and investigate the spreading patterns as they vary with extension rate using 3-D thermomechanical numerical models. The applied finite difference marker-in-cell code incorporates visco-plastic rheology of the lithosphere and a crustal growth algorithm. The evolution of mid-ocean ridges from nucleation to a steady-state is modelled for a wide range of spreading rates. With increasing spreading rate, four different regimes are obtained: (a) stable alternating magmatic and amagmatic sections (≈ 10 mm/yr), (b) transient features in asymmetrically spreading systems (≈ 20 mm/yr), (c) stable orthogonal ridge-transform fault patterns (≈ 40 mm/yr) and (d) stable curved ridges (≥ 60 mm/yr). Modelled ultraslow and slow mid-ocean ridges share key features with natural systems. Abyssal hills and oceanic core complexes are the dominant features on the flanks of natural slow-spreading ridges. Numerically, very similar features are produced, both generated by localised asymmetric plate growth controlled by a spontaneous development of large-offset normal faults (detachment faults). Asymmetric accretion in our models implies a lateral migration of the ridge segment, which might help explaining the very large offsets observed at certain transform faults in nature.     

Beach erosion under rising sea‐level modulated by coastal geomorphology and sediment availability on carbonate reef‐fringed island coasts

This study addresses gaps in understanding the relative roles of sea‐level change, coastal geomorphology and sediment availability in driving beach erosion at the scale of individual beaches. Patterns of historical shoreline change are examined for spatial relationships to geomorphology and for temporal relationships to late‐Holocene and modern sea‐level change. The study area shoreline on the north‐east coast of Oahu, Hawaii, is characterized by a series of kilometre‐long beaches with repeated headland‐embayed morphology fronted by a carbonate fringing reef. The beaches are the seaward edge of a carbonate sand‐rich coastal strand plain, a common morphological setting in tectonically stable tropical island coasts. Multiple lines of geological evidence indicate that the strand plain prograded atop a fringing reef platform during a period of late‐Holocene sea‐level fall. Analysis of historical shoreline changes indicates an overall trend of erosion (shoreline recession) along headland sections of beach and an overall trend of stable to accreting beaches along adjoining embayed sections. Eighty‐eight per cent of headland beaches eroded over the past century at an average rate of ?0·12 ± 0·03 m yr^?1. In contrast, 56% of embayed beaches accreted at an average rate of 0·04 ± 0·03 m yr^?1. Given over a century of global (and local) sea‐level rise, the data indicate that embayed beaches are showing remarkable resiliency. The pattern of headland beach erosion and stable to accreting embayments suggests a shift from accretion to erosion particular to the headland beaches with the initiation of modern sea‐level rise. These results emphasize the need to account for localized variations in beach erosion related to geomorphology and alongshore sediment transport in attempting to forecast future shoreline change under increasing sea‐level rise.     

全球变化中的浅海沉积作用与物理环境演化——以渤、黄、东海区域为例

以渤、黄、东海陆架区为例 ,综合评述入海沉积物通量、浅海沉积作用和人类活动影响下的物理环境变化。黄河、长江沉积物入海后主要堆积于河口附近 ,沉积速率为 10～ 10 0mm/a ,部分物质输运至陆架区 ,形成泥质沉积体 ,其沉积速率为 1mm/a量级。但目前河流入海沉积物通量呈减小趋势 ,今后河口附近的堆积将会减缓 ,并在总体上转化为侵蚀状态。在过去的 7ka里 ,来自黄河、长江等河流的沉积物形成了河口三角洲、滨海平原、潮流脊和潮汐汊道沉积体系 ,其演化方式是双重的 ,既有常态边界条件和外力作用下的演化 ,又有边界条件和外力作用发生变异时的演化。边界条件和外力作用变异的原因包括入海通量变化、全球气候变化、海面上升和人类活动等。建议从中国海岸带城市化发展的现状和需求出发 ,对此进行深入研究。