Sediment accumulation rates along the inner eastern Brazilian continental shelf

Box cores were collected close to river mouths along the eastern Brazilian shelf at water depths of 10–30 m. One core was taken from more than 1000 m depth at the shelf slope. ²¹⁰Pb and ²²⁶Ra activities were measured to establish sediment accumulation rates. Seven of the 10 cores exhibited an exponential decrease with depth of excess ²¹⁰Pb activities. The sediments from the sheltered Sudeste Channel off Caravelas revealed the highest sediment accumulation rate of 0.81 cm yr⁻¹. The sediments at the shelf slope seaward of the Rio Doce revealed the lowest accumulation rate of 0.13 cm yr⁻¹. Sediment accumulation rates increased towards the Caravelas Bank. Current patterns and the morphology of the seabed favor sediment deposition in this area.     

210Pb sedimentation rates from the Northwestern Mediterranean margin

A total of 83 cores were collected in the Gulf of Lions continental margins and analysed for ²¹⁰Pb_xs (excess ²¹⁰Pb) in order to understand sedimentation patterns. Apparent sedimentation rates (ASR) range from 0.65 cm year⁻¹ in the vicinity of the Rhône River mouth to 0.01 cm year⁻¹ in the deep basin. Except for the prodelta area, rates decrease with depth linearly with the water depth. On the slope, ASR do not differ between canyons and open slope, except for the western area where the rates are slightly higher in the Lacaze–Duthiers canyon compared to its adjacent, open slope. In the canyon and open slope areas, mass accumulation rates determined from ²¹⁰Pb_xs profiles (0.10 and 0.08 g cm⁻² year⁻¹, respectively) are in good agreement with particulate fluxes calculated from 5 years of near-bottom sediment trap data, even when the trap particle fluxes and the apparent accumulation rates are overestimated in response to resuspension and bioturbation effects.However, differences in sediment trap data, between west and east portion of the slope, are not observed in the sedimentation rates calculated with ²¹⁰Pb_xs. The outer shelf area may have an important role in trapping sediment but it is not sufficiently documented. Sediment surface mixed layer depths decrease with water depth, with a mean value for the whole margin of 8±6 cm.²¹⁰Pb_xs inventories in the sediment are systematically higher than the net ²¹⁰Pb export flux estimated from the above water column. Over the margin, the ratio between accumulated ²¹⁰Pb and available ²¹⁰Pb is about 3, suggesting boundary scavenging effects and advective transport.     

Cyclical variability of suspended sediment concentration over a low-energy tidal flat in Jiaozhou Bay,China: effect of shoaling on wave impact

During the spring-neap period of 17–24 August 2004, turbidity, horizontal and vertical current velocities and echo intensity were measured using OBS-3A and ADP-XR instruments over an intertidal flat within the semi-enclosed Jiaozhou Bay, China, to examine patterns in suspended sediment concentration (SSC) and possible control factors. SSC was found to be lower than 30 mg l⁻¹ in most of the water column and for most of the tidal cycle. This is attributed mainly to the low hydrodynamic energy, in particular weak currents (near-bottom maximum 1- and 8-min-interval velocities were only 26.1 and 14.2 cm s⁻¹, respectively), and limited fine-grained sediment supply by rivers. However, high SSC values ranging from 100 to >1,000 mg l⁻¹ occurred over short periods at the beginning and the end of inundation. This phenomenon is attributed to the shoaling effect of frequent wind-generated waves, as a result of which near-bottom SSC fluctuations display a U-shaped trend during each tidal cycle.     

A high-resolution geophysical investigation of sediment distribution controlled by catchment size and tides in a multi-basin turbid outwash fjord: Simpson Bay,Prince William Sound,Alaska

Surficial sediment distribution within Simpson Bay is a function of antecedent bedrock and recently deposited glacial geology, as well as active physical processes both within Simpson Bay and Prince William Sound (PWS). Simpson Bay is a turbid, outwash fjord located in northeastern PWS, Alaska. Freshwater from heavy precipitation, and the melting of high alpine glaciers enter the bay through bay head rivers and small shoreline creeks. The catchment has a high watershed/basin surface area ratio (∼8:1), and easily erodible bedrock that contribute to high sediment loads. The system can be divided into three discrete basins, each with specific morphologic and circulatory characters. Side scan sonar, swath bathymetry, and seismic profiles reveal that bathymetric highs are areas of outcropping glacial surfaces. High backscatter coupled with surface grab samples reveal these surfaces to be composed of coarse sediment and bedrock outcrops. Bathymetric lows are areas of low backscatter, and grab samples reveal these areas to be ponded deposits of organic-rich estuarine muds. The data provide evidence of terminal morainal bank systems, and glacial grounding line deposits at the mouth of the bay and rocky outcrops were identified as subsurface extensions of aerial rocky promontories. Radioisotope analyses of short cores reveal that the bay has an average accumulation rate of approx. 0.5 cm year⁻¹, but that this varies in function of the watershed/basin surface area ratios of the different basins. The interaction of tidal currents and sediment source drives sediment distribution in Simpson Bay. Hydrographic data reveal high spatial variability in surface and bottom currents throughout the bay. Subsurface currents are tide dominated, but generally weak (5–20 cm s⁻¹), while faster currents are found along shorelines, outcrops, and bathymetric highs. Bathymetric data reveal steep slopes with little to no modern sediment throughout the bay, suggesting lack of deposition due to tidal currents.     

Decadal-scale sediment dynamics and environmental change in the Albemarle Estuarine System, North Carolina

During the summer of 2001, several short cores (<50 cm) were collected in the Albemarle Estuarine System (AES). Down-core measurements for radiochemical tracers (²¹⁰Pb, ¹³⁷Cs) and organic matter signatures (δ¹³C, δ¹⁵N, C:N ratio, and LOI) have been used to elucidate potential temporal changes in fluxes and cycles of organic matter in the AES. Pb-210 geochronology indicates temporal and spatial variations in sediment deposition rates (0.08–0.57 cm y⁻¹) with highest rates near the AES western limit relative to the rest of the estuary. Low accumulation rates, deficient excess ²¹⁰Pb inventories, and near linear ¹³⁷Cs profiles throughout the AES suggest that sediments are resuspended by wind-generated waves and currents and flushed from the system by river discharge and wind-tides, probably to Pamlico Sound to the south. Sediments in the AES are accumulating at rates less than the current rate of relative sea-level rise for this region except in protected portions of the estuary. Thus sediment accumulation in the AES is controlled in the short term by storm wave-base and in the long term by the creation of accommodation space by basin subsidence and sea-level rise. The geochemical and sedimentological data characterize the evolution of the Albemarle Sound and associated tributaries over the past 200–300 years. The majority of cores collected throughout the system show a significant decrease in ¹³C and increase in ¹⁵N isotopic signatures up-core. Thus, the estuarine system of eastern North Carolina has changed from a marine-influenced, high brackish environment to the modern-day system, which is a highly variable, terrestrially influenced, low brackish environment.     

Sedimentary environment and glacial history during the last 40 ka of the Vøring continental margin,mid-Norway

This study is based on detailed investigation of sediment cores and high resolution seismics. We identified and describe five lithofacies on the Vøring Plateau and eight on the mid-Norwegian continental slope. The various lithofacies are mainly related to the fluctuations of the Fennoscandian Ice Sheet and the varying intensity of bottom currents and inflow of Atlantic water masses. Ocean circulation was highly variable between 40 and 22 ¹⁴C ka BP, being vigorous during interstadials and sluggish during stadials. Between ca 22 and 15 ¹⁴C ka BP the sedimentary environment was significantly influenced by fluctuations of the Fennoscandian Ice Sheet, repeatedly reaching the outermost shelf. These fluctuations are reflected in the sedimentary record as ice-rafted debris (IRD) accumulation peaks, deposition of stratified diamicton, and glacigenic debris flows on the continental slope. During this period the sediment accumulation rate increased, bottom currents influenced the sedimentary pattern, and surface waters were seasonally ice-free, indicating inflow of Atlantic waters. Subsequent to ca 15 ¹⁴C ka BP the glacier influence decreased as the margin of the Fennoscandian Ice Sheet retreated to reach the coast before 12.5 ¹⁴C ka BP. The modern sedimentary environment is characterised by relatively strong bottom current action, causing winnowing or non-deposition down to approximately 1000 m water depth.     

Ammonia Nitrogen at the Water–Sediment Interface in Puck Bay (Baltic Sea)

The magnitude of the exchange flux at the water–sediment interface was determined on the basis of the ammonia concentration gradient at the near-bottom water–interstitial interface and Fick's first law. It was established that in Puck Bay, ammonia almost always passes from the sediment to water. Ammonia flux varied from 5 to 1434 μmol NH₄-N m⁻² day⁻¹. In total,c. 138·2 tonneammonia year⁻¹pass from sediments of Internal Puck Bay to near-bottom water, the equivalent value for External Puck Bay being 686·9 tonne year⁻¹. In total, about 825 tonne ammonia year⁻¹passes from the sediment to near-bottom water of Puck Bay. In interstitial waters, ammonia occurred in concentrations varying over a wide range (3–1084 μmol NH₄-N dm⁻³).The basic factors affecting the magnitude of ammonia concentration in interstitial waters included: oxidation of organic matter, type of sediment, and inflow of fresh underground waters to the region examined.This paper involves preliminary studies only and constitutes a continuation of the studies on ionic macrocomponents and phosphorus in interstitial waters of Puck Bay undertaken previously.     

Cross-slope zonation of erosion and deposition in the Faeroe-Shetland Channel,North Atlantic Ocean

Boundary currents and internal waves determine cross-slope zonation of erosion and deposition in the Faeroe-Shetland Channel. Currents were measured at 8 and 34–50 m above the bottom at three mooring sites (502, 595 and 708 m depth) for 14 days. The structure of the water column was evaluated from CTD sections, and included nepheloid layers and particulate matter concentrations. Indicators for recent deposition in the sediment (organic carbon, phytopigments, ²¹⁰Pb) were measured at eight stations across the slope. Strong near-bottom currents at the upper slope sustain down-slope particle transport in a benthic nepheloid layer, which is eroded under the influence of critically reflecting M₂ internal tidal waves at 350–550 m, where the major pycnocline meets the sloping bottom. Beam attenuation profiles confirmed the presence of intermediate nepheloid layers intruding into the Channel along the major pycnocline, and elevated concentrations of particulate matter and chlorophyll-a were measured at this depth. Near-bottom currents decreased with depth, thus allowing particle deposition down the slope. Inventories of excess ²¹⁰Pb activity in the sediment deeper than 600 m were higher than what was expected on the basis of atmospheric input of ²¹⁰Pb and production in the water column, thus indicating additional lateral inputs. Simple calculations showed that off-slope input of particles from areas shallower than 600 m may be responsible for the enhanced deposition at greater depths.     

Chlorophyll a in Arctic sediments implies long persistence of algal pigments

Sediment cores were collected from the shelf, slope, and basin of the Bering, Chukchi, and Beaufort Seas during May–June (under ice cover) and July–August (largely ice-free) 2004. Measurements of chlorophyll a (chl a), total organic carbon (TOC), and C/N ratios were made in surface and some subsurface core increments. Surface sediment chl a decreased with increasing water depth. Significant positive correlations were found between chl a and TOC and chl a and C/N ratios in the basin (>2000 m), but there were significant negative correlations between chl a and C/N ratios on the shelf (⩽200 m). Chl a values generally declined in down-core profiles, but in some deeper slope and basin cores, measurable inventories of subsurface chl a were present at depth. In some cases, these subsurface chlorophyll inventories coincident with peak activities of the anthropogenic radionuclide ¹³⁷Cs were detected, which had maximal deposition following the atmospheric nuclear weapons testing era in the 1960s. A sedimentation rate independently determined for one of these cores using ²¹⁰Pb was consistent with the depths of subsurface ¹³⁷Cs peaks in slope sediments reflecting steady, relatively undisturbed deposition over a several-decade period. The depth of penetration of ¹³⁷Cs in some continental slope sediments, together with detectable chl a, suggests that chl a can be buried in some of these deeper-water sediments under cold conditions for decadal periods in the absence of deposit feeders. Because organic deposition from the water column is episodic at high latitudes and concentrated following the spring bloom, these buried sources of organic materials, whether on the shelf or in deeper basin sediments, may ultimately be important for benthic invertebrates that could utilize this food source during times of the year when primary production flux from the overlying water column is reduced.     

Quantitative links between fluvial sediment discharge,trapped terrigenous flux and sediment accumulation,and implications for temporal and spatial distributions of sediment fluxes

The southwestern-most Okinawa Trough (SOT), characterized by high sedimentation rates (>0.1 cm/yr), has the potential for recording high-resolution episodic events, such as storm floods and seismic activities, at least on a regional scale. To retrieve data on past climate change from nearby sediment cores and quantitatively reconstruct it, particularly with respect to precipitation (or typhoon-induced flood events), a linkage between fluvial sediment discharge and terrigenous sediment flux is warranted. Apparent sediment fluxes, observed with four arrays of sediment traps deployed in the SOT, were found to vary with fluvial sediment discharges. Empirical equations for individual arrays of sediment traps are site-dependent and related to the scenario of initial supply, transport and final deposition of terrigenous sediments (i.e. land–sea interaction). Using these equations and hydrological data from 1950 to 2000, the long-term temporal and spatial variations of settling sediment fluxes were simulated. Simulation results agree well with sediment mass accumulation rates derived from literature data on ²¹⁰Pb and ¹³⁷Cs chronology. The simulated spatial patterns of sediment fluxes along a slope–trough section illustrate that sediment plumes can disperse concurrently in two manners, namely near-bottom and mid-depth plumes, and the flood-driven plumes can travel very long distances, approaching 125°E or beyond. The sediment burial budget in the SOT was estimated to be approximately 5.2 Mt/yr, representing about 80% of riverine exports from the Lanyang Hsi, Taiwan. This is the first study dealing qualitatively and quantitatively with two parameters, namely terrigenous sediment flux and fluvial sediment discharge.     

Deep-sea terrigenous organic carbon transfer and accumulation: Impact of sea-level variations and sedimentation processes off the Ogooue River (Gabon)

Sedimentary, isotopic and bulk geochemical proxies measured in sediment samples of five gravity cores collected in the distal part of the Ogooue turbidite system (around 4000 m-depth) were used to develop a conceptual model to describe the accumulation of terrigenous organic matter (OM) during the last 200,000 yrs BP in the eastern part of the Gulf of Guinea. This model takes into account the influence of the different depositional processes (turbiditic vs hemipelagic sedimentation), geomorphological features and sea-level variations.Total organic carbon (TOC) and the stable organic carbon isotopes of the OM (δ¹³C) variability follow the highstand/lowstand (interglacial/glacial) cyclicity with a very low accumulation rate of terrigenous OM during periods of high sea-level and higher accumulation rate during period of low sea-level. A sea-level of 80–120 m below present day seems to favor the transfer of terrigenous sediments to the deep offshore environment through the turbidite system and thanks to the connection of the canyons heads with the river system presently located at the shelf edge at −120 m water depth.In this system, terrigenous OM matter delivered by the river accumulate in the sediments via two main processes. Indeed, a part of the terrigenous OM settles in combination with the finest particles forming hemipelagites, while another part, formed of very well preserved land plant debris, is transported and deposited far offshore with turbidity currents. The proportion of terrigenous OM accumulated due to turbidity currents is important as it can represent more than 70% of the carbon accumulated during sea-level lowstand. Moreover, terrigenous OM seems to preferentially accumulate in the levees and the lobes of the system notably due to the higher frequency of organic-rich turbidites.This study demonstrates that gravity flows, influenced by the sea-level variations, can significantly affect the terrigenous OM budget of the deep offshore Atlantic margins and that channel-levee complexes as well as turbidite lobes can be regarded as good sink for terrestrial organic carbon. These processes should be taken into consideration in the context of source rocks exploration but also for the estimation of the general carbon accumulation in ocean sediment.     

Seismic expressions of deep-shelf depositional and erosional morphologies,Miocene Utsira formation,North Sea Basin

The Middle–Late Miocene Utsira Formation of the North Sea Basin contains a fully preserved, regional marine sand deposit that records a stable paleogeographic setting of sand transport and accumulation within a deep, epeiric seaway which persisted for >8 Ma. The sediment dispersal system was defined by (1) input through a marginal prograding strandplain platform, coast-to-basin bypass, transport along a narrow strait, and accumulation in strait-mouth shoal complexes within a shelf sea; (2) a high-energy marine regime; (3) very low time-averaged rates of sediment supply and accumulation; and (4) consequent high sediment reworking ratio. Sand distribution and stratal architectures reflect regional along-strike sediment transport and local to sub-regional landward sediment transport. Plume-shaped, south-building, submarine sand shoals that formed along the recurved arc of the strandplain margin nourished the shoal system. Very low-angle sigmoid clinoforms and down-stepping, aggradational top sets are distinctive architectures of these strike-fed sand bodies. The combination of strong marine currents and slow but long-lived sand supply from the Shetland strandplain created regional, sandy shelf shoal depositional systems that individually covered 3,500 to 6,000 km² of the basin floor. Defining attributes of the shelf shoal systems include their location within the basin axis, abundance of autochthonous sediment, and sandy marine facies composition. Diagnostic depositional architectures include the along-strike-dipping sigmoidal clinoforms, poly-directional low-angle accretionary bedding at both regional and local scales, and mounded depositional topography. Erosional features include regional hummocky, low-relief shelf deflation surfaces, broad, elongate scours and sub-circular scour pits.     

Modern sediments and sediment accumulation rates on the narrow shelf off central Vietnam,South China Sea

The narrow shelf along the coast of central Vietnam is seasonally supplied by large amounts of sediment from the adjacent mountainous hinterland following monsoonal precipitation. This study examines the fate of these sediments, and their accumulation rates along two transects across the shelf, based on analyses of radionuclides (²¹⁰Pb, ¹³⁷Cs), sediment texture and structure, as well as carbonate content. The inner shelf is covered by sands, and probably serves as bypass zone for fine sediments transported offshore. Sediment characteristics suggest that the transport to the mid and outer shelf is related to flood events. Averaged over the last century, the ²¹⁰Pb-based mud mass accumulation rates on the mid and outer shelf vary between 0.25 g cm ⁻² and 0.56 g cm ⁻² year ⁻¹ (corresponding to linear sediment accumulation rates of 0.20–0.47 cm year ⁻¹). Along with high excess ²¹⁰Pb inventories, these high accumulation rates suggest a significant sediment depocentre on the mid shelf. The ²¹⁰Pb-derived sediment accumulation rates were found to be several times higher than ¹⁴C-derived rates previously reported for the Holocene, at the same location on the outer shelf. This is probably due to the incompleteness of the Holocene record, and an overestimation of the modern rate. Another explanation would be increased erosion within the rivers’ drainage basins, due to 20th century deforestation. This hypothesis is supported by the difference between recent (less sand, more lithic grains in the sand fraction) and older sediments. In terms of modern sedimentation processes and rates, the central Vietnam shelf, although being a part of a narrow passive continental margin, is similar to active flood-dominated continental margins.     

近百年来黄河改道及输沙量变化对山东半岛泥质楔沉积物粒度特征的影响

对山东半岛泥质楔柱状样进行沉积物粒度分析,并在210Pb测年基础上结合黄河年输沙量、悬浮泥沙粒径、河口位置及沿岸流强弱替代指标等数据研究近百年来黄河改道及输沙量变化对远端沉积区沉积特征的影响。研究结果显示,研究区沉积物粒度的变化是黄河入海泥沙特征和沿岸流等水动力作用共同影响的结果。1884–1939年及1947–1999年,研究区沉积物粒度特征的变化主要受到沿岸流强弱的影响;1939–1947年及1999–2012年,受1938–1947年花园口决堤以及20世纪90年代末以来黄河年输沙量持续偏低致使源区泥沙不足的影响,沉积物粒度相对粗化,与沿岸流强度的相关性减弱。与黄河入海泥沙近端沉积区不同,研究区粒度特征对黄河尾闾改道事件不敏感,且对花园口决堤及年输沙量低于临界值的响应有一定程度的延迟,这与该区沉积物经历了再悬浮过程及中间复杂的物质混合有关。黄河改道及输沙量变化仍然是远端沉积区沉积特征演化不可忽视的因素。     

The glacimarine sedimentary environment of Expedition Fiord, Canadian High Arctic

Expedition Fiord is a small, shallow inlet on the west coast of Axel Heiberg Island near 80°N latitude. It receives runoff and sediment at its head from a 1079 km² drainage basin, 72% of which is glacier-covered. Subbottom acoustic survey and cores from the fiord floor were used to assess the sedimentary environment. Most of the sediment is deposited within 3 km of the inflow from suspension in the overflowing cap and by gravity flows on the foreset beds of the delta. Occasionally, weak turbidity currents reach the mid fiord where they deposit fine-grained sediments. Icebergs from a large calving glacier in an adjacent fiord raft additional sediment, especially to the outer part of the fiord. They also scour the seafloor, although the persistent ice cover and slow currents in the fiord restrict this process. Except near the inflow, the total sediment accumulation since deglaciation is less than 20 m, and the rates of 0.5–1 mm/yr have not varied significantly to the present. A thicker deposit in the outer fiord is probably related to an early Holocene glacier margin near that location.     

Organic carbon accumulation and sulfate reduction rates in slope and basin sediments of the Ulleung Basin,East/Japan Sea

This study investigated the organic carbon accumulation rates (OCARs) and sulfate reduction rates (SRRs) in slope and basin sediments of the Ulleung Basin, East/Japan Sea. These sediments have high organic contents at depths greater than 2,000 m; this is rare for deep-sea sediments, except for those of the Black Sea and Chilean upwelling regions. The mean organic carbon to total nitrogen molar ratio was estimated to be 6.98 in the Ulleung Basin sediments, indicating that the organic matter is predominantly of marine origin. Strong organic carbon enrichment in the Ulleung Basin appears to result from high export production, and low dilution by inputs of terrestrial materials and calcium carbonate. Apparent sedimentation rates, calculated primarily from excess ²¹⁰Pb distribution below the zone of sediment mixing, varied from 0.033 to 0.116 cm year⁻¹, agreeing well with previous results for the basin. OCARs fluctuated strongly in the range of 2.06–12.5 g C m⁻² year⁻¹, these rates being four times higher at the slope sites than at the basin sites. Within the top 15 cm of the sediment, the integrated SRRs ranged from 0.72 to 1.89 mmol m⁻² day⁻¹, with rates approximately twice as high in the slope areas as in the basin areas. SRR values were consistently higher in areas of high sedimentation and of high organic carbon accumulation, correlating well with apparent sedimentation rates and OCARs. The sulfate reduction rates recorded in the basin and slope sediments of the Ulleung Basin are higher than those reported for other parts of the world, with the exception of the Peruvian and Chilean upwelling regions. This is consistent with the high organic carbon contents of surface sediments of the Ulleung Basin, suggesting enhanced organic matter fluxes.     

Sedimentation rates and heavy metal pollution of sediments in the Seto Inland Sea

In the Suo-Nada area of the Seto Inland Sea, Japan, sedimentation rates and the sedimentary record of anthropogenic metal loads were determined by combining the Pb-210 dating technique with heavy metal analysis of the sediments. The sedimentation rates vary from 0.11 to 0.27 g cm^–2 yr^–1. Lower sedimentation rates were observed in the eastern part of the basin which is characterized by a bottom with sand and gravel, and fast tidal currents.Anthropogenic and natural loads of copper and zinc into the sediments are 34 and 326, and 65 and 375 ton yr^–1, respectively. The anthropogenic loads are fairly low compared with those of the other main areas of sediment accumulation in the Seto Inland Sea. The highest level of zinc and copper pollution was observed in the western part of the basin because of waste discharge from an old and big ironworks outside basin since the early 1900's.     

Variability in Particle Mixing Rates in Sediments with Polymetallic Nodules in the Equatorial Eastern Pacific as Determined from Measurements of Excess <Superscript>210</Superscript>Pb

Radionuclide activities of ²¹⁰Pb and ²²⁶Ra were measured to determine bioturbation coefficients (D_b) in seven sediment cores from the Korean licensed block for polymetallic nodules in the Clarion–Clipperton Fracture Zone. Variability in D_b is considered in the context of the sedimentological, geochemical, and geotechnical properties of the sediments. D_b values in the studied cores were estimated using a steady-state diffusion model and varied over a wide range from 1.1 to 293 cm²/yr with corresponding mixing depths (L) of 26 to 144 cm. When excepting for spurious results obtained from cores where diffusive mixing does not apply, D_b values range from 1.1 to 9.0 cm²/yr with corresponding mixing depths (L) of 26 to 63 cm. Such wide variability in D_b and L values is exceptional in sites with water depths of ～5000 m and is attributed in this study to an uneven distribution of sediment layers with different shear strengths and total organic carbon (TOC) contents, caused by erosion events. The studied cores can be grouped into two categories based on lithologic associations: layers with high maximum shear strength (MSS) and low TOC content, showing a narrow range of D_b values (1.1–9.0 cm²/yr); and layers with low MSS and high TOC content, yielding much higher D_b values of over 30 cm²/yr. The distribution of different lithologies, and the resultant spatial variability in MSS and labile organic matter content, controls the presence and maximum burrowing depth of infauna by affecting their mobility and the availability of food. This study provides a unique case showing that shear strength, which relates to the degree of sediment consolidation, might be an important factor in controlling rates of bioturbation and sediment mixing depths.     

Particulate organic carbon (POC) in surface sediments of the Baltic Sea

In this study, particulate organic carbon (POC) contents and their distribution pattern in surficial sediments of the Baltic Sea are presented for 1,471 sampling stations. POC contents range from approx. 0.1% in shallow sandy areas up to 16% in deep muddy basins (e.g. Gotland Basin). Some novel relationships were identified between sediment mass physical properties (dry bulk density (DBD), grain size) and POC levels. Notably, the highest POC concentrations (about 10–17 mg cm^–3) occur in sandy mud to mud (60–100% mud content) with intermediate POC contents of about 3–7% and DBDs of 0.1–0.4 g cm^–3. Areas with this range in values seem to represent the optimum conditions for POC accumulation in the Baltic Sea. The maximum POC contents (8–16%) are found in fluid mud of the central Baltic Sea characterized by extremely low DBDs (<0.1 g cm^–3) and moderate POC concentrations (4–7 mg cm^–3). Furthermore, sediment mass accumulation rates (MAR), based on ²¹⁰Pb and ¹³⁷Cs measurements and available for 303 sites of the Baltic Sea, were used for assessing the spatial distribution of POC burial rates. Overall, these vary between 14 and 35 g m^–2 year^–1 in the mud depositional areas and, in total, at least 3.5 (±2.9) Mt POC are buried annually. Distribution patterns of POC contents and burial rates are not identical for the central Baltic Sea because of the low MAR in this area. The presented data characterize Baltic Sea sediments as an important sink for organic carbon. Regional differences in organic carbon deposition can be explained by the origin and transport pathways of POC, as well as the environmental conditions prevailing at the seafloor (morphology, currents, redox conditions). These findings can serve to improve budget calculations and modelling of the carbon cycle in this large brackish-water marginal sea.     

鸭绿江河口地区沉积物的粒度变化及影响因素

通过对比鸭绿江河口不同地区柱状样的粒度参数及敏感组分变化规律,分析了河口不同地区沉积动力环境和沉积特征的差异;结合代表性柱状样的210Pb测年,分析了柱状样在不同时期自然和人类活动影响下的沉积特征变化。结果表明,西水道主要受潮流作用而含有泥沙互层,粒度参数变化小;西岸潮滩受潮流和波浪作用含有贝壳碎屑,并在顶部出现粗化层。近几十年来,鸭绿江河口地区沉积特征的变化主要由于人类活动对水沙入海通量的影响:1941年以前柱状样的波动沉积特征反映了河流山溪性特征;1941—1970(1980)年随水沙入海通量减少而呈现砂含量降低、粉砂增加的特征;1970(1980)—1990年由于水沙入海通量变化较小,而呈现沉积物组成和敏感组分保持稳定的特征;但1990年后随水沙入海通量的进一步降低,邻近河口区域的沉积物出现明显的粗化。