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1.
This study presents a detailed reconstruction of the sedimentary effects of Holocene sea‐level rise on a modern coastal barrier system. Increasing concern over the evolution of coastal barrier systems due to future accelerated rates of sea‐level rise calls for a better understanding of coastal barrier response to sea‐level changes. The complex evolution and sequence stratigraphic framework of the investigated coastal barrier system is reconstructed using facies analysis, high‐resolution optically stimulated luminescence and radiocarbon dating. During the formation of the coastal barrier system starting 8 to 7 ka rapid relative sea‐level rise outpaced sediment accumulation. Not before rates of relative sea‐level rise had decreased to ca 2 mm yr?1 did sediment accumulation outpace sea‐level rise. From ca 5·5 ka, rates of regionally averaged sediment accumulation increased to 4·3 mm yr?1 and the back‐barrier basin was filled in. This increase in sediment accumulation resulted from retreat of the barrier island and probably also due to formation of a tidal inlet close to the study area. Continued transgression and shoreface retreat created a distinct hiatus and wave ravinement surface in the seaward part of the coastal barrier system before the barrier shoreline stabilized between 5·0 ka and 4·5 ka. Back‐barrier shoreline erosion due to sediment starvation in the back‐barrier basin was pronounced from 4·5 to 2·5 ka but, in the last 2·5 kyr, barrier sedimentation has kept up with and outpaced sea‐level. In the last 0·4 kyr the coastal barrier system has been prograding episodically. Sediment accumulation shows considerable variation, with periods of rapid sediment deposition and periods of non‐deposition or erosion resulting in a highly punctuated sediment record. The study demonstrates how core‐based facies interpretations supported by a high‐resolution chronology and a well‐documented sea‐level history allow identification of depositional environments, erosion surfaces and hiatuses within a very homogeneous stratigraphy, and allow a detailed temporal reconstruction of a coastal barrier system in relation to sea‐level rise and sediment supply.  相似文献   

2.
The extent of multi‐year sea ice impacts climate processes worldwide, such as ocean–atmosphere carbon dioxide exchange and deep ocean current formation. Reconstructing these processes in the past, and assessing the distribution of ecologically and climatically significant features, such as polynas, requires recognition of sediments deposited under multi‐year sea ice, but little is known about their characteristics. Textural analysis of subaerial and sea floor sediment in Explorers Cove, McMurdo Sound, at the mouth of Taylor Valley, Antarctica, augmented with observations of sedimentary structures and faunal components, elucidates how sediment is transported to the sea floor and allows characterization of the deposits. Comparison of grain‐size characteristics of subaerial (moraine, delta and sea‐ice surface) sediment and sea floor sediment from short cores taken at depths of 7 to 25 m indicates that the likely source of the moderately to poorly sorted sea floor sand is deltaic sediment; small glacial meltwater streams have built deltas since Taylor Valley became ice‐free ca 7000 years ago. Windblown sediment accumulating on the multi‐year sea ice close to the coast typically is coarser grained than sediment on the sea floor; this suggests that the transport of sediment through the ice to the sea floor is not the predominant mode of sediment transfer. However, supra‐sea‐ice sediment does move to the sea floor through local fractures. The rate of sedimentation under multi‐year sea ice is low because of limited stream flow and biogenic sedimentation; the ice cover inhibits primary productivity and dampens waves, precluding physical re‐suspension. The upper centimetres of sea floor sediment are churned by epifaunal scallops and brittle stars that leave no telltale biogenic structures and whose calcite ossicles and shells may be poorly preserved. The resulting deposits under multi‐year sea ice are poorly sorted, massive sand that provides little evidence of the bioturbators that have masked the indicators of the original physical depositional processes.  相似文献   

3.
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.  相似文献   

4.
This study analyses the three‐dimensional geometry of sedimentary features recorded on the modern sea floor and in the shallow subsurface of a shelf to upper slope region offshore Australia that is characterized by a pronounced internal wave regime. The data interpreted comprise an extensive, >12 500 km2 industrial three‐dimensional seismic‐reflection survey that images the northern part of the Browse Basin, Australian North West Shelf. The most prominent seismic–morphological features on the modern sea floor are submarine terrace escarpments, fault‐scarps and incised channels, as well as restricted areas of seismic distortion interpreted as mass wasting deposits. Besides these kilometre‐scale sea floor irregularities, smaller bedforms were discovered also, including a multitude of sediment waves with a lateral extent of several kilometres and heights up to 10 m. These sedimentological features generally occur in extensive fields in water depths below 250 m mostly at the foot of submerged terraces, along the scarps of modern faults and along the shelf break between the outer shelf and the upper continental rise. Additional bedforms that characterize the more planar regions of the outer shelf are elongate, north‐west/south‐east oriented furrows and ridges. The formation of both sediment waves and furrow‐ridge systems requires flow velocities between 0·3 m sec?1 and 1·5 m sec?1, which could be generated by oceanic currents, gravity currents or internal waves. In the studied setting, these velocities can be best explained as being generated by bottom currents induced by internal waves, an interpretation that is discussed against oceanographic background data and modelling results. In addition to the documentation of three‐dimensional seismic–geomorphological features of the modern sea floor, it was also possible to map kilometre‐scale buried sediment wave fields in the seismic volume down to ca 500 ms two‐way‐time below the present sea floor, indicating the general potential for the preservation of such bedforms in the sedimentary record.  相似文献   

5.
The universally known subsidence theory of Darwin, based on Bora Bora as a model, was developed without information from the subsurface. To evaluate the influence of environmental factors on reef development, two traverses with three cores, each on the barrier and the fringing reefs of Bora Bora, were drilled and 34 uranium‐series dates obtained and subsequently analysed. Sea‐level rise and, to a lesser degree, subsidence were crucial for Holocene reef development in that they have created accommodation space and controlled reef architecture. Antecedent topography played a role as well, because the Holocene barrier reef is located on a Pleistocene barrier reef forming a topographic high. The pedestal of the fringing reef was Pleistocene soil and basalt. Barrier and fringing reefs developed contemporaneously during the Holocene. The occurrence of five coralgal assemblages indicates an upcore increase in wave energy. Age–depth plots suggest that barrier and fringing reefs have prograded during the Holocene. The Holocene fringing reef is up to 20 m thick and comprises coralgal and microbial reef sections and abundant unconsolidated sediment. Fringing reef growth started 8780 ± 50 yr bp ; accretion rates average 5·65 m kyr?1. The barrier reef consists of >30 m thick Holocene coralgal and microbial successions. Holocene barrier‐reef growth began 10 030 ± 50 yr bp and accretion rates average 6·15 m kyr?1. The underlying Pleistocene reef formed 116 900 ± 1100 yr bp , i.e. during marine isotope stage 5e. Based on Pleistocene age, depth and coralgal palaeobathymetry, the subsidence rate of Bora Bora was estimated to be 0·05 to 0·14 m kyr?1. In addition to subsidence, reef development on shorter timescales like in the late Pleistocene and Holocene has been driven by glacioeustatic sea‐level changes causing alternations of periods of flooding and subaerial exposure. Comparisons with other oceanic barrier‐reef systems in Tahiti and Mayotte exhibit more differences than similarities.  相似文献   

6.
Small turbidite systems offshore from southern California provide an opportunity to track sediment from river source through the turbidity‐current initiation process to ultimate deposition, and to evaluate the impact of changing sea level and tectonics. The Santa Monica Basin is almost a closed system for terrigenous sediment input, and is supplied principally from the Santa Clara River. The Hueneme fan is supplied directly by the river, whereas the smaller Mugu and Dume fans are nourished by southward longshore drift. This study of the Late Quaternary turbidite fill of the Santa Monica Basin uses a dense grid of high‐resolution seismic‐reflection profiles tied to new radiocarbon ages for Ocean Drilling Program (ODP) Site 1015 back to 32 ka. Over the last glacial cycle, sedimentation rates in the distal part of Santa Monica Basin averaged 2–3 mm yr?1, with increases at times of extreme relative sea‐level lowstand. Coarser‐grained mid‐fan lobes prograded into the basin from the Hueneme, Mugu and Dume fans at times of rapid sea‐level fall. These pulses of coarse‐grained sediment resulted from river channel incision and delta cannibalization. During the extreme lowstand of the last glacial maximum, sediment delivery was concentrated on the Hueneme Fan, with mean depositional rates of up to 13 mm yr?1 on the mid‐ and upper fan. During the marine isotope stage (MIS) 2 transgression, enhanced rates of sedimentation of > 4 mm yr?1 occurred on the Mugu and Dume fans, as a result of distributary switching and southward littoral drift providing nourishment to these fan systems. Longer‐term sediment delivery to Santa Monica Basin was controlled by tectonics. Prior to MIS 10, the Anacapa ridge blocked the southward discharge of the Santa Clara River into the Santa Monica Basin. The pattern and distribution of turbidite sedimentation was strongly controlled by sea level through the rate of supply of coarse sediment and the style of initiation of turbidity currents. These two factors appear to have been more important than the absolute position of sea level.  相似文献   

7.
Sediment accumulation rate studies utilizing excess 210Pb and 137Cs were conducted as part of recent investigations of biogeochemical cycling at a single site in Cape Lookout Bight, a rapidly changing coastal basin on the Outer Banks of North Carolina (U.S.A.). Cores three meters in length reveal a depositional history for the bight interior characterized by a gradual transition in texture from coarse-grained to fine-grained material over the period 1946–1979. This transition is controlled by progressive enclosure of the bight by an active northerly migrating recurved spit. The textural gradation is periodically interrupted by layers of well-sorted sand associated with major storm events. Lead-210 data indicate that the upper meter of the sediment has accumulated at a rate of 3.35 to 4.71 g · cm?2 · yr?1 or approximately 8.4 to 11.8 cm · yr?1 (at ø = 0.84). Below 120 cm depth, dilution of clay and silt by low activity sand necessitates correction of the 210Pb profile in order to establish a geochronology. Grain size 210Pb distribution measurements at three depths reveal that the specific activity (dpm · g?1) of clay is 3.2 times that of silt and 24.7 times that of sand. Corrections of bulk sediment excess 210Pb activities based on these measurements lead to dates for textural changes which are consistent with charted changes in basin morphology and major storm events.Sixteen 137Cs measurements between 33–241 cm depth reveal a peak activity at 105–115 cm and indicate a minimum sedimentation rate of approximately 2.7 g · cm?2 · yr?1.  相似文献   

8.
Thick bay‐fill sequences that often culminate in strandplain development serve as important sedimentary archives of land–ocean interaction, although distinguishing between internal and external forcings is an ongoing challenge. This study employs sediment cores, ground‐penetrating radar surveys, radiocarbon dates, palaeogeographic reconstructions and hydrodynamic modelling to explore the role of autogenic processes – notably a reduction in wave energy in response to coastal embayment infilling – in coastal evolution and shoreline morphodynamics. Following a regional 2 to 4 m highstand at ca 5·8 ka, the 75 km2 Tijucas Strandplain in southern Brazil built from fluvial sediments deposited into a semi‐enclosed bay. Holocene regressive deposits are underlain by fluvial sands and a Pleistocene transgressive–regressive sequence, and backed by a highstand barrier‐island. The strandplain is immediately underlain by 5 to 16 m of seaward‐thickening, fluvially derived, Holocene‐age, basin‐fill mud. Several trends are observed from the landward (oldest) to the seaward (youngest) sections of the strandplain: (i) the upper shoreface and foreshore become finer and thinner and shift from sand‐dominated to mud‐dominated; (ii) beachface slopes decrease from >11° to ca 7°; and (iii) progradation rates increase from 0·4 to 1·8 m yr?1. Hydrodynamic modelling demonstrates a correlation between progressive shoaling of Tijucas Bay driven by sea‐level fall and sediment infilling and a decrease in onshore wave‐energy transport from 18 to 4 kW m?1. The combination of allogenic (sediment supply, falling relative sea‐level and geology) and autogenic (decrease in wave energy due to bay shoaling) processes drove the development of a regressive system with characteristics that are rare, if not unique, in the Holocene and rock records. These findings demonstrate the complexities in architecture styles of highstand and regressive systems tracts. Furthermore, this article highlights the diverse internal and external processes and feedbacks responsible for the development of these intricate marginal marine sedimentary systems.  相似文献   

9.
Hydrodynamics and sediment dynamics of The Wash embayment, eastern England   总被引：6，自引：0，他引：6  
Water and sediment movement in The Wash embayment has been determined from an extensive set of data, consisting of tidal current readings and suspended sediment concentration measurements. Instantaneous and residual currents in the embayment show a distinct lateral inhomogeneity, whereas vertically the water column is almost homogenous. The central deep water area (30–40 m) of the embayment is dominated by a residual landward water movement, whilst on the margins, the residual movement is seaward. Sediment is supplied predominantly in suspension from the north, through the northern extremity of Boston Deep. Suspended sediment pathways are coincident with the spring tide water movements and the subtidal channels act as the main conduits. Approximately 6·8 × 106 tonnes yr?1 of suspended sediments are supplied to the embayment from offshore areas. Bedload sediment supply is of lesser importance, ～ 1·4 × 104 tonnes yr?1. Whereas suspended sediment movement appears to be the dominant mode of transport throughout the embayment, bedload transport is important in reforming the sea bed into a variety of bedforms which are particularly well developed on the margins of channels and shoals.  相似文献   

10.
Loessial colluvial sediments and aeolian aprons are common deposits in the Negev Desert Highlands. In an attempt to monitor the amounts and distributional pattern of loess, monthly dust measurements were carried out during 2004 to 2006 in 10 cm diameter traps located at 18 stations along four slopes, north‐facing, south‐facing, east‐facing and west‐facing in a second‐order drainage basin near Sede Boqer, Negev Desert Highlands, Israel. Annual total dust depositions ranged between 110 g and 178 g m?2 with an average of 151·1 g m?2. The average annual dust deposition in the catchment was 23·5% higher than the average amount recorded at the hilltops (122·4 g m?2) and may be a consequence of sheltering opportunities in the hilly topography. When analysed according to season and aspect, significantly higher monthly amounts were received during the wet rainy season of December to March (17·0 g m?2), in comparison with the rest of the year (8·1 g m?2). As for the aspect, while no significant differences characterized north‐facing and south‐facing slopes, east‐facing slopes received significantly higher amounts (by 43·3%) than west‐facing slopes, pointing to preferential dust deposition at the leeward slope. Concurring with the classical model that anticipates higher dust deposition at the leeside slope, but in disagreement with some reports published in the literature, the findings of this study were also supported by a field survey that showed preferential loess accumulation at the eastern and north‐eastern aspects. These findings may shed light on distributional patterns of colluvial sediments and aeolian aprons in the Negev, on soil‐forming processes and on past cycles of dust deposition.  相似文献   

11.
Cold‐water coral ecosystems present common carbonate factories along the Atlantic continental margins, where they can form large reef structures. There is increasing knowledge on their ecology, molecular genetics, environmental controls and threats available. However, information on their carbo‐nate production and accumulation is still very limited, even though this information is essential for their evaluation as carbonate sinks. The aim of this study is to provide high‐resolution reef aggradation and carbonate accumulation rates for Norwegian cold‐water coral reefs from various settings (sunds, inner shelf and shelf margin). Furthermore, it introduces a new approach for the evaluation of the cold‐water coral preservation within cold‐water coral deposits by computed tomography analysis. This approach allows the differentiation of various kinds of cold‐water coral deposits by their macrofossil clast size and orientation signature. The obtained results suggest that preservation of cold‐water coral frameworks in living position is favoured by high reef aggradation rates, while preservation of coral rubble prevails by moderate aggradation rates. A high degree of macrofossil fragmentation indicates condensed intervals or unconformities. The observed aggradation rates with up to 1500 cm kyr?1 exhibit the highest rates from cold‐water coral reefs so far. Reef aggradation within the studied cores was restricted to the Early and Late Holocene. Available datings of Norwegian cold‐water corals support this age pattern for other fjords while, on the shelf, cold‐water coral ages are reported additionally from the early Middle Holocene. The obtained mean carbonate accumulation rates of up to 103 g cm?2 kyr?1 exceed previous estimates of cold‐water coral reefs by a factor of two to three and by almost one order of magnitude to adjacent sedimentary environments (shelf, slope and deep sea). Only fjord basins locally exhibit carbonate accumulation rates in the range of the cold‐water coral reefs. Furthermore, cold‐water coral reef carbonate accumulation rates are in the range of tropical reef carbonate accumulation rates. These results clearly suggest the importance of cold‐water coral reefs as local, maybe regional to global, carbonate sinks.  相似文献   

12.
Entrainment of planktonic foraminifera: effect of bulk density   总被引：1，自引：0，他引：1  
Depositional hydrodynamics have been studied using settling rate distributions of Norwegian deep sea sediments (between Jan Mayen Island and the Vøring Plateau), together with Shields’ critical shear stress velocities. Planktonic foraminifera are the dominant sand sized component of these sediments. The bulk density of the foraminifera was calculated from their settling velocity, sieve size and shape. Density decreases from 2·39 g cm?3 at 0·05 mm diameter to 1·37 g cm?3 at 0·35 mm diameter. These density and size data were used to construct a threshold sediment movement curve. From the similarity in their Shield's critical shear-stress velocities and the observed correlation of foraminifera size with decreasing percentage of fine fraction, it is concluded that the two components, the sand size foraminifera and the quartz and carbonate silt, are transport-equivalent.  相似文献   

13.
Subaqueous sand dunes are common bedforms on continental shelves dominated by tidal and geostrophic currents. However, much less is known about sand dunes in deep‐marine settings that are affected by strong bottom currents. In this study, dune fields were identified on drowned isolated carbonate platforms in the Mozambique Channel (south‐west Indian Ocean). The acquired data include multibeam bathymetry, multi‐channel high‐resolution seismic reflection data, sea floor imagery, a sediment sample and current measurements from a moored current meter and hull‐mounted acoustic Doppler current profiler. The dunes are located at water depths ranging from 200 to 600 m on the slope terraces of a modern atoll (Bassas da India Atoll) and within small depressions formed during tectonic deformation of drowned carbonate platforms (Sakalaves Seamount and Jaguar Bank). Dunes are composed of bioclastic medium size sand, and are large to very large, with wavelengths of 40 to 350 m and heights of 0·9 to 9·0 m. Dune migration seems to be unidirectional in each dune field, suggesting a continuous import and export of bioclastic sand, with little sand being recycled. Oceanic currents are very intense in the Mozambique Channel and may be able to erode submerged carbonates, generating carbonate sand at great depths. A mooring located at 463 m water depth on the Hall Bank (30 km west of the Jaguar Bank) showed vigorous bottom currents, with mean speeds of 14 cm sec?1 and maximum speeds of 57 cm sec?1, compatible with sand dune formation. The intensity of currents is highly variable and is related to tidal processes (high‐frequency variability) and to anticyclonic eddies near the seamounts (low‐frequency variability). This study contributes to a better understanding of the formation of dunes in deep‐marine settings and provides valuable information about carbonate preservation after drowning, and the impact of bottom currents on sediment distribution and sea floor morphology.  相似文献   

14.
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.  相似文献   

15.
A natural smoky quartz crystal from Shandong province, China, was characterised by laser ablation ICP‐MS, electron probe microanalysis (EPMA) and solution ICP‐MS to determine the concentration of twenty‐four trace and ultra trace elements. Our main focus was on Ti quantification because of the increased use of this element for titanium‐in‐quartz (TitaniQ) thermobarometry. Pieces of a uniform growth zone of 9 mm thickness within the quartz crystal were analysed in four different LA‐ICP‐MS laboratories, three EPMA laboratories and one solution‐ICP‐MS laboratory. The results reveal reproducible concentrations of Ti (57 ± 4 μg g?1), Al (154 ± 15 μg g?1), Li (30 ± 2 μg g?1), Fe (2.2 ± 0.3 μg g?1), Mn (0.34 ± 0.04 μg g?1), Ge (1.7 ± 0.2 μg g?1) and Ga (0.020 ± 0.002 μg g?1) and detectable, but less reproducible, concentrations of Be, B, Na, Cu, Zr, Sn and Pb. Concentrations of K, Ca, Sr, Mo, Ag, Sb, Ba and Au were below the limits of detection of all three techniques. The uncertainties on the average concentration determinations by multiple techniques and laboratories for Ti, Al, Li, Fe, Mn, Ga and Ge are low; hence, this quartz can serve as a reference material or a secondary reference material for microanalytical applications involving the quantification of trace elements in quartz.  相似文献   

16.
Spencer Gulf is a large (ca 22 000 km2), shallow (<60 m water depth) embayment with active heterozoan carbonate sedimentation. Gulf waters are metahaline (salinities 39 to 47‰) and warm‐temperate (ca 12 to ?28°C) with inverse estuarine circulation. The integrated approach of facies analysis paired with high‐resolution, monthly oceanographic data sets is used to pinpoint controls on sedimentation patterns with more confidence than heretofore possible for temperate systems. Biofragments – mainly bivalves, benthic foraminifera, bryozoans, coralline algae and echinoids – accumulate in five benthic environments: luxuriant seagrass meadows, patchy seagrass sand flats, rhodolith pavements, open gravel/sand plains and muddy seafloors. The biotic diversity of Spencer Gulf is remarkably high, considering the elevated seawater salinities. Echinoids and coralline algae (traditionally considered stenohaline organisms) are ubiquitous. Euphotic zone depth is interpreted as the primary control on environmental distribution, whereas seawater salinity, temperature, hydrodynamics and nutrient availability are viewed as secondary controls. Luxuriant seagrass meadows with carbonate muddy sands dominate brightly lit seafloors where waters have relatively low nutrient concentrations (ca 0 to 1 mg Chl‐a m?3). Low‐diversity bivalve‐dominated deposits occur in meadows with highest seawater salinities and temperatures (43 to 47‰, up to 28°C). Patchy seagrass sand flats cover less‐illuminated seafloors. Open gravel/sand plains contain coarse bivalve–bryozoan sediments, interpreted as subphotic deposits, in waters with near normal marine salinities and moderate trophic resources (0·5 to 1·6 mg Chl‐a m?3) to support diverse suspension feeders. Rhodolith pavements (coralline algal gravels) form where seagrass growth is arrested, either because of decreased water clarity due to elevated nutrients and associated phytoplankton growth (0·6 to 2 mg Chl‐a m?3), or bottom waters that are too energetic for seagrasses (currents up to 2 m sec?1). Muddy seafloors occur in low‐energy areas below the euphotic zone. The relationships between oceanographic influences and depositional patterns outlined in Spencer Gulf are valuable for environmental interpretations of other recent and ancient (particularly Neogene) high‐salinity and temperate carbonate systems worldwide.  相似文献   

17.
The oceanographic Polar Front separates the East Greenland and Iceland margins. Surface water temperatures across Denmark Strait vary by 8–12 °C and represent one of the steepest oceanographic gradients on earth. The East Greenland margin is a polar environment, with extensive sea‐ice cover and calving glacier margins; in contrast, the Iceland shelf is much more temperate, and freshwater run‐off is a key component in land–ocean sediment transfers. Average sediment properties from these two contrasting climate and oceanographic continental shelf environments are compared in the spatial domain at 13 sites; the data represent the last 10 000 radiocarbon years of `normal' marine sedimentation for the two regions. The two regions have similar average rates of sediment accumulation (around 43·5 cm kyr?1), so that this key variable is factored out in explaining any differences in sediment properties. Dry sediment density, moisture content, hygroscopic moisture, total organic carbon and carbonate contents, mass magnetic susceptibility and the percentages of sand and silt are compared focusing on: (1) median values for sediment properties; and (2) downcore variability, measured by the coefficient of variation (CV). There are significant differences in all but one (hygroscopic moisture) of the sediment properties between Iceland and East Greenland; in four cases, the sense of the differences was not as predicted. In terms of downcore variation (CV), no difference was found between the two regions, nor between the 13 sites, whereas there are some significant differences between the variables. Carbonate and mass magnetic susceptibility have the largest spreads, and moisture content and dry sediment density are the least variable. Protocols are developed to identify the `type core' in a regional series of sites. The results indicate a need to develop a regional perspective on sediment properties, both as inputs to models of sedimentary processes in different polar/arctic environments, and as an indication of which sediment properties might be best suited for palaeoenvironmental downcore time series.  相似文献   

18.
A laboratory flume experiment was carried out in which the hydrodynamic and sedimentary behaviour of a turbidity current was measured as it passed through an array of vertical rigid cylinders. The cylinders were intended primarily to simulate aquatic vegetation canopies, but could equally be taken to represent other arrays of obstacles, for example forests or offshore wind turbines. The turbidity currents were generated by mixing naturally sourced, poly‐disperse sediment into a reservoir of water at concentrations from 1·0 to 10·0 g l?1, which was then released into the experimental section of the flume by removing a lock gate. For each initial sediment concentration, runs with obstacle arrays with solid plant fractions of 1·0% and 2·5%, and control cases with no obstacles, were carried out. The progress of the current along the flume was characterized by the array drag term, CDaxc (where CD is the array drag coefficient, at the frontal area of cylinders per unit volume, and xc is the position of the leading edge of the current along the flume). The downward depositional flux of sediment out of the current as it proceeded was measured at 13 traps along the flume. Analysis of these deposits divided them into fine (2·2 to 6·2 μm) and coarse (6·2 to 104 μm) fractions. At the beginning of their development, the gravity currents proceeded in an inertia‐dominated regime until CDaxc = 5. For CDaxc > 5, the current transitioned into a drag‐dominated regime. For both fine and coarse sediment fractions, the rate of sediment deposition tended to decrease gradually with distance from the source in the inertial regime, remained approximately constant at the early drag‐dominated regime, and then rose and peaked at the end of the drag‐dominated stage. This implies that, when passing through arrays of obstacles, the turbidity currents were able to retain sufficient sediment in suspension to maintain their flow until they became significantly influenced by the drag exerted by the obstacles.  相似文献   

19.
A 4·7 km2 field of sediment waves occurs in front of the Slims River delta in Kluane Lake, the largest lake in the Yukon Territory. Slims River heads in the Kaskawulsh Glacier, part of the St Elias Ice Field and discharges up to 400 m3 s?1 of water with suspended sediment concentrations of up to 7 g l?1. The 19 km long sandur of Slims River was created in the past 400 years since Kaskawulsh Glacier advanced and dammed the lake and the sandur has advanced into Kluane Lake at an average rate of 48 m a?1. However, this rate is decreasing as flow is diverted from Slims River because of the retreat of the Kaskawulsh Glacier. The sandur and a road constructed on the delta remove coarse‐grained sediment, so the river delivers dominantly mud to the lake. Inflow during summer generates quasi‐continuous turbidity currents with velocities up to 0·6 m s?1. The front of the delta consists of a plane surface sloping lakeward at 0·0188 (1·08°). A field of sediment waves averaging 130 m in length and 2·3 m in amplitude has developed on this surface. Slopes on the waves vary from ?0·067 (?3·83°, i.e. sloping in the opposite direction to the regional slope) to 0·135 (7·69°). The internal structure of the sediment waves, as documented by seismic profiling, shows that sedimentation on the stoss portion of the wave averages 2·7 times that on the lee portion. Rates of sediment accumulation in the wave field are about 0·3 m a?1, so these lacustrine waves have formed in a much shorter period of time (less than 200 years) and are advancing upslope towards the delta much more quickly (1 to 2 m a?1) than typical marine sediment waves. These waves formed on the flat surface of the lake floor, apparently in the absence of pre‐existing forms, and they are altered and destroyed as the wave field advances and the characteristics of the turbidity currents change.  相似文献   

20.
Thick sequences of sediment surround the Whitsunday Islands on the middle shelf of the Great Barrier Reef (GBR) Platform. Much of this sediment is siliciclastic material deposited since the sea‐level highstand at around 6·5 ka. This raises a mass balance dilemma because modern terrigenous discharge to the GBR Platform is restricted to the inner shelf. Shallow seismic profiles and sediment samples were collected over 450 km2 around the Whitsunday Islands to quantify the mass of siliciclastic sediment for a dynamic model of the shelf. The sea floor and pre‐Holocene surfaces were mapped using 4584 stations along the seismic profiles and a graphical computer program. The total volume of sediment between these two surfaces is 3·67 ± 0·45 × 109 m3. This volume is composed of buried reefs (0·13 ± 0·01 × 109 m3), medium‐ (0·70 ± 0·30 × 109 m3) and fine‐grained shoals (2·84 ± 0·35 × 109 m3). The volume estimates combined with measurements of carbonate concentration, density and porosity indicate that 1850 ± 380 Mt of Holocene siliciclastic sediment surround the Whitsunday Islands in medium‐ (510 ± 225 Mt) and fine‐grained shoals (1340 ± 155 Mt). The total mass of siliciclastic material is 1·7–2·6 times that stored in Cleveland Bay, a similar sized repository on the inner shelf. A simple numerical model has been constructed to explain this large quantity of Holocene siliciclastic sediment. The model results in the appropriate siliciclastic mass next to the Whitsunday Islands by integrating regional shelf processes over time. Unlike the present day, rivers discharged sediment to the middle shelf during the early Holocene. This material was subsequently focused by northward transport into the vicinity of the islands, a geomorphologically complex region that serves as a sediment trap. Although direct riverine inputs to the middle shelf have stopped during the present sea‐level highstand, previously deposited siliciclastic sediment is continually being winnowed from the middle shelf and redeposited next to the Whitsunday Islands. The transport and distribution of siliciclastic sediment on the GBR Platform is thus influenced significantly by storage around islands on the middle shelf.  相似文献   

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