Facies, internal structures and sequences of modern Gironde-derived muds on the Aquitaine inner shelf, France

Seventy cores from the Aquitaine continental shelf were examined using radiographic and grain-size techniques in order to describe the sedimentary structures of the muddy deposits, and to evaluate their depositional processes. Four lithofacies are identified in this fine-grained deposit: (a) homogeneous silty sand, (b) interbedded homogeneous mud and sand, (c) silty-clayey mud, and (d) mottled mud. They show a logical pattern in relationship to the water depth and the distance from the coast.

Primary structures are present particularly in the landward and central portion of the mud fields, where the sediment is organized into sequences with a sharp-based erosional contact, overlain by a fining-up succession (centimetre to decimetre scale). The beginning of each of these is characteristic of a high-energy storm event, which is common on this shelf. The settling of suspended fine sediment corresponds to the flood estuarine discharge during quiet periods. Primary sedimentary structures decrease in the distal area where the muddy sediment is frequently reworked by infauna. Finally, primary structures and their preservation depend on the relative magnitudes of surface waves, storms, infaunal mixing and fluvial sediment deposition rates (i.e. floods).     

冲绳海槽陆源碎屑峡谷通道搬运与海底扇沉积

应用“向阳红16号”1992年地质调查和“向阳红9号”1995年地球物理调查的实际资料，并参照80年代以来有关研究成果，对冲绳海槽沉积物类型、陆源组分的堆积形式、沉积速率、物质通量以及沉积环境状况等进行了研究，结果表明，冲强海槽陆源碎屑主要集中在海底峡谷口外，形成海底扇沉积，海底扇以其与峡谷伴生而地势和缘、陆源组分含量高、沉积通量大、沉积物楔入体复合叠置为标志，揭示出海底峡谷在陆源碎屑向海槽输送过程中的通道作用；提出陆架潮流与海底峡谷内波、内潮汐的联合作用是陆源碎屑经峡谷通道向海槽持续搬运的主要动力因素，而黑潮摆动及其涡旋分支对峡谷上游沉积物的供给具有积极作用。     

Past and present sedimentary activity in the Capbreton Canyon, southern Bay of Biscay

Located in the south-eastern part of the Bay of Biscay, the Capbreton Canyon incises the continental shelf up to the 30 m isobath contour, and acts as a natural conduit for continental and shelf-derived sediments. EM1000 multibeam bathymetry shows two main features characterising the canyon — a deeply entrenched meandering channel, bordered by fluvial-like terraces constituting large sediment traps. A dataset of cores and seismic profiles together with a multibeam bathymetry map has enabled the characterisation of recent sedimentary activity in the axial channel and on the terraces. Data analysis evidenced the major role of the canyon head in recent sediment dynamics. This part of the canyon is a temporary reservoir for sediments, accumulated by coastal hydrodynamic processes. Exceptional climatic, tectonic or hydrodynamic events can mobilise the sediments and generate gravity-driven flows. Under the present-day sea-level highstand conditions, these flows are not powerful enough to bring their bedload to the deep sea, and are confined mainly to the upper part of the canyon. Turbidity currents model the axial channel pathway and are at the origin of terrace formation. Terraces in the Capbreton Canyon are not typical but rather are reduced to confined levees. Three factors control the vertical growth of a terrace: (1) the amount of overspilled sediments brought by turbidity currents, (2) hemipelagic sedimentation and (3) terrace height. The amount of sediment spilling over a terrace decreases with increased terrace elevation. Concurrently, the proportion of hemipelagic fallout depositing on a terrace increases. Terraces are considered to be fossil when the height of the terrace prevents further deposition by overspilling. The terraces studied in this paper are interpreted as having formed during the Holocene, implying that the sediment dynamics of the Capbreton Canyon is continuous through time. Highstand periods differ from lowstand periods because they show a decrease in the energy of erosive processes. Temporal variations in erosive and depositional processes in the canyon are controlled by the Adour River, which delivers large amounts of sediment to the system.     

Processes controlling very low sedimentation rates on the continental slope of the Gonone-Orosei canyon system,NE Sardinia—terrestrial and oceanic significance

The narrow shelf and upper slope immediately above the Gonone canyon head off NE Sardinia represent areas of very low sedimentation rates. Along the sides of the canyon head (1,600 m water depth), the sediment deposits are homogeneous but show alternating light-grey intervals rich in carbonate and dark-grey ones rich in organic matter, possibly related to distal turbidite processes. Deposits older than 50,000 years are already encountered at core depths of 2.50 m, the sedimentation rates varying from 6–21 cm/10³ years in the lower parts of two cores and from 1.5–3 cm/10³ years in the upper parts. At about 35,000 years BP, both cores show a simultaneous drop in sedimentation rate by a factor of 3, probably in response to local mechanisms of channel avulsion. Lithological, mineralogical and geochemical properties reveal the environmental factors which are responsible for the extremely slow sediment accumulation. The southernmost sector of the coast, and partly also of the shelf, consists of Jurassic limestones which supply only small amounts of fine-grained material transported in suspension. During the last sea-level highstand, the accumulation of the Cedrino River pro-delta remained restricted to the coast, the low siliciclastic sediment yields resulting in poor shelf sediment trapping. The present morphology of the canyon head prevented the occurrence of gravity processes in the deeper part of the canyon system, including the coring sites. Accordingly, deposition was mainly fed by hemipelagic material of planktonic origin, together with only moderate terrigenous inputs. On a wider late Pleistocene timescale, seismic data indicate the occurrence of a coarse-grained, layered turbidite facies, implying a very different architecture of the canyon drainage system probably prior to 60,000 years BP.     

南海北部陆架柱状沉积物记录的残留沉积

对南海北部外陆架珠江口西南的E602孔进行了粒度分析和AMS14C年龄测定,通过沉积学分析,认为该孔沉积物形成于近滨环境,时间为全新世初期。根据岩性的变化将该孔分为上下2段,下段岩性较均匀,粒度参数变化较小,主要是风暴成因的递变悬浮沉积序列,形成于近滨上部环境;上段为均匀悬浮,为近滨下部环境。该孔下段的沉积作用主要是对晚更新世低海平面时沉积物的改造和再沉积,基本没有新的陆源物质加入,属于残留沉积或准残留沉积,只是在晚期的上段沉积中才有少量以细粒物质为主的新物源加入。     

Fine-grained sediment budget on the continental margin of the Bay of Biscay

Under present-day conditions, rivers are the main source of fine sediments dispersed to the Bay of Biscay. They deliver about 2.5×10⁶ t yr⁻¹ of continental fine sediments, 60% of which is derived from the Gironde estuary. Of this flux, 65% is believed stored on the shelf. Two kinds of mud fields can be found in the Bay of Biscay: coastal mud and shelf mud belts. The total mass of fine sediments stored during the past 2000 years is 3.2×10⁹ t. Consequently, about 0.9×10⁶ t yr⁻¹ could reach the shelf edge and eventually the open sea. From this amount of displaced material and the deposition surface areas, an evaluation of sediment fluxes across the margin during the late Holocene period is discussed. This evaluation is compared with results obtained from ECOsystéme du canyon du cap-FERret (ECOFER) data from sediment traps and surficial box cores.     

Evidence of bioturbation in the Cap-Ferret Canyon in the deep northeastern Atlantic

In the sedimentary column, a combined quantification of burrows and macrobenthic community provides evidence of bioturbation features in the submarine canyon of Cap-Ferret between 2000 and 3000 m depth. An image-processing technique allows accurate quantification of burrow volumes with depth in the sedimentary column. The major bioturbation mode seems to be different in the channel compared to the interfluve. Macrobenthic activity is more inclined to mix the sediment in the channel in response to increased organic matter supplies. Sediment mixing leads to burrow destruction in the upper mixed layer of sediment in the canyon. Burrows are better preserved on the interfluve where mixing is slower. Under the mixed zone, the volume of recorded burrows is higher when sedimentation rate increases, as in the upper canyon. In this transition layer, the burrow volume is estimated to be between 3 and 64% of the total sediment volume depending on the sediment depth. The fill-down of numerous burrows with surface sediment by bioregeneration suggests that anaerobic degradation of fresh organic matter is dominant in this canyon. In the sedimentary column, the negative relationship between carbonate content and macrobenthic abundance confirms that carbonate dissolution is largely influenced by bioturbation.     

Late Quaternary sedimentation within a submarine channel–levee system offshore Cap Timiris, Mauritania

Aeolian and fluvial sediment transport to the Atlantic Ocean offshore Mauritania were reconstructed based on grain-size distributions of the carbonate-free silt fraction of three marine sediment records of Cap Timiris Canyon to monitor the climatic evolution of present-day arid north-western Africa. During the late Pleistocene, predominantly coarse-grained particles, which are interpreted as windborne dust, characterise glacial dry climate conditions with a low sea level and extended sand seas that reach onto the exposed continental shelf off Mauritania. Subsequent particle fining and the abrupt decrease in terrigenous supply are attributed to humid climate conditions and dune stabilisation on the adjacent African continent with the onset of the Holocene humid period. Indications for an ancient drainage system, which was discharging fluvial mud offshore via Cap Timiris Canyon, are provided by the finest end member for early to mid Holocene times. However, in comparison to the Senegal and Niger River further south, the river system connecting Cap Timiris Canyon with the Mauritanian hinterland was starved during the late Holocene and is non-discharging under present-day arid climate conditions.     

Core-based reconstruction of paleoenvironmental conditions in the southern Drake Passage (West Antarctica) over the last 150 ka

A deep-sea sediment core (GC98-06) from the southernmost Drake Passage, West Antarctica, shows late Quaternary depositional environments distinctly different from sedimentary drifts commonly found along the southwestern Pacific margin of the Drake Passage. The chronology of the core has been inferred using geochemical tracers of paleoproductivity and diatom biostratigraphy, and represents the paleoceanographic conditions in a continental rise setting during the last 150,000 years. Three dominant sediment types associated with distinct sedimentary processes have been identified using textural/compositional analyses: (1) hemipelagic mud (interglacial sediments) deposited from pelagic settling of bioclasts, meltwater plumes, and ice-rafted detritus; (2) terrigenous mud (glacial sediments) delivered by turbid meltwater plumes; and (3) massive muds marking the boundaries from interglacial to glacial periods. The succession of the sedimentary facies in core GC98-06 is interpreted to reflect temporal changes in environmental conditions prevailing on the continental rise of the southern Drake Passage in the course of successive climatic stages over the last 150 ka: from the bottom upward, these are glacial, interglacial, glaciation, glacial, and interglacial episodes. Variability in sediment flux and diatom abundance seem to have been related to changes in glacial advance, sea-ice extent, and specific sedimentary environments, collectively influenced by mid- to late Quaternary climatic changes.     

南海西沙海域永乐海底峡谷形成演化过程分析

远海孤立碳酸盐台地周缘发育了碳酸盐岩峡谷,对其开展研究有助于深刻理解碳酸盐碎屑沉积物的“源-汇”体系及深水油气成藏等方面。文章利用多波束测深、高分辨率二维多道地震等数据,精细刻画南海西沙海域永乐海底峡谷的地貌形态及内部充填特征,揭示该峡谷沉积演化过程,分析峡谷成因控制因素及稳定性。永乐海底峡谷形成演化可分为萌芽、汇聚和拓展3个阶段,随着演化过程的发展,峡谷规模及对沉积物输运作用增加。永乐海底峡谷形成及演化主要受古地貌隆起形成的负地形和沉积物重力流侵蚀作用影响。峡谷在第四纪以后仍有较明显的活动迹象。分析显示永乐海底峡谷是西北次海盆的重要物质输送通道,其沉积演化过程及稳定性对研究碳酸盐台地沉积物输运等深水沉积过程及岛礁工程建设具有一定参考意义。     

Echo character of the western equatorial Atlantic floor and its relationship to the dispersal and distribution of terrigenous sediments

The floor of the western equatorial Atlantic Ocean can be divided into several distinct provinces based on detailed characteristics of the bottom echos recorded with short-ping (< msec.) 3.5 and 12 kHz sound sources. Two major types of echos are recorded: (I) distinct echos; and (II) indistinct echos.Indistinct echos can be further sub-divided into (A) continuous prolonged echos; and (B) hyperbolic echos. Each class of echos contains two or more unique echo types. The regional distributions of the various echo types recorded from the continental rise, Amazon Cone, and abyssal plains reveal much information about sedimentary processes.In the western equatorial Atlantic, hyperbolic echos are recorded only from small, isolated portions of the continental rise. This contrasts with the continental rise of the western North Atlantic where previous investigators have shown that hyperbolic echos parallel bathymetric contours along the entire rise and thus reflect shaping of the rise by geostrophic contour currents (Heezen et al., 1966; Hollister, 1967). The fact that regions of hyperbolic echos show little or no relationship to bathymetric contours of the continental rise of the western equatorial Atlantic suggests that contour currents have been unimportant in shaping the rise in this region.The three most widespread echo types recorded from the continental rise, Amazon Cone, and abyssal plains reveal much information about terrigenous sediment dispersal and deposition in the western equatorial Atlantic. Comparison of the thicknesses and frequencies of coarse (silt- to gravel-size), bedded, terrigenous sediment in piston cores with the echo type recorded at each coring site shows a correlation between echo type and the relative amount of coarse, bedded sediment within the upper few meters of the sea floor. The regional distributions of these three echo types indicate that dispersal of coarse terrigenous sediment has been downslope across the continental rise and Amazon Cone to the abyssal plains via gravity-controlled sediment flows. The Amazon River is the major sediment source and most coarse sediment is deposited on the lower Amazon Cone and proximal portions of the Demerara abyssal plain.     

Morphotectonics and evolutionary controls on the Pearl River Canyon system, South China Sea

The Pearl River Canyon system is a typical canyon system on the northern continental slope of the South China Sea, which has significant implications for hydrocarbon exploration. Through swath bathymetry in the canyon area combined with different types of seismic data, we have studied the morphotectonics and controlling factors of the canyon by analyzing its morphology and sedimentary structure, as well as the main features of the continental slope around the canyon. Results show that the Pearl River Canyon can be separated into three segments with different orientations. The upper reach is NW-oriented with a shallowly incised course, whereas the middle and lower reaches, that are located mainly in the Baiyun Sag, have a broad U-shape and have experienced consistent deposition. Seventeen deeply-cut canyons have developed in the slope north of the Baiyun Sag, playing an important role in the sedimentary processes of the middle and lower reaches of the Pearl River Canyon. These canyons display both asymmetrical V- and U-shapes along their lengths. Numerous buried channels can be identified below the modern canyons with unidirectionally migrating stacking patterns, suggesting that the canyons have experienced a cyclic evolution with several cut and fill phases of varying magnitude. These long established canyons, rather than the upper reach of the Pearl River Canyon, are the main conduits for the transport of terrigenous materials to the lower slope and abyssal basin during lowstand stage, and have contributed to the formation of vertically stacked deep-water fans in the middle reach. Canyon morphology is interpreted as a result of erosive sediment flows. The Pearl River Canyon and the 17 canyons in the slope area north of the Baiyun Sag probably have developed since the Miocene. Cenozoic tectonics, sea level change and sediment supply jointly control the morphology and sedimentary structure. The middle and lower reaches of the Pearl River Canyon developed on the paleo-terrain of the Baiyun Sag, which has been a persistently rapid depositional environment, receiving most of the materials transported via the canyons.     

Holocene climate in the Persian Gulf as deduced from grain-size and pteropod distribution

Detailed coarse-fraction analyses have been made of twelve sediment cores from the Persian Gulf (40–100 m water depth) in order to detect Holocene climatic changes.The only indication of such a change was provided by the terrigenous material (i.e., the < 63 μ fraction) carried into the Gulf by rivers. All cores show characteristic variations in the quantity of terrigenous material and alternating fine- and coarse-grained layers.There was no other parameter which could point to variations in climate. This might be due to the fact that the sediments are sorted as a result of reworking by tidal currents (velocities 25–50 cm/sec): fine and light particles are removed. Any climatically-produced variations in amount or size of benthonic and planktonic organisms have probably been obliterated by tidal currents. The importance of this factor varied with time: during increased terrigenous supply, tidal currents modified the sediment surface for only short periods and a fine-grained layer was, therefore, deposited. A lesser supply of terrigenous material, on the other hand, resulted in the deposition of a coarse-grained layer.Contemporaneous sedimentation of layers with much terrigenous material (fine-grained layer) and with little terrigenous material (coarse-grained layer) is suggested. Observed petrographic variations are presumably due to large-scale changes of climate and not to local sedimentary variations (including shifts of river-mouth processes).Accordingly, the cores were correlated by examining the percentage of coarse fraction and the median diameter of the sand fraction. The assumption is that the higher the percentage of coarsegrained material, the lower the amount of terrigenous material transported into the Persian Gulf.In support of this correlation, another parameter - varying independently from grain size within one core - was examined: the species composition of pteropod shells. These quantitative changes could also be correlated, because there were different core sections with different species compositions.Correlation of the cores by grain size and by pteropod species provides the same results.Four different climatic zones could be distinguished between the lower and the upper part of the Holocene: a relatively arid period at about 9000 years B.P. was succeeded by a more humid period; this then was followed by a period of less rainfall and finally a period during the Late Holocene when rainfall increased again. This sequence is comparable to the established European climate chronology.     

Geoacoustic character,sedimentology and chronology of a cross-shelf Holocene sediment deposit off Cabo Frio,Brazil (southwest Atlantic Ocean)

The Cabo Frio region in the state of Rio de Janeiro, southeast coast of Brazil, is characterized by a local coastal upwelling system and converging littoral sediment transport systems that are deflected offshore at Cabo Frio, as a consequence of which a thick cross-shelf sediment deposit has developed over time. To investigate the evolution of this muddy deposit, geophysical, sedimentological and geochemical data from four sediment cores (3.8–4.1 m in length) recovered in water depths between 88 and 141 m were analyzed. The high-resolution seismic data show variable sediment thicknesses ranging from 1 to 20 m, comprising two sedimentary units separated by a high-impedance layer at a depth of about 10 m below the seafloor at the coring sites. According to the available age datings, the upper sedimentary unit is late Pleistocene to Holocene in age, whereas the lower unit (not dated) must, by implication, be entirely Pleistocene in age. The boomer-seismic reflection signal can be divided into three echo-types, namely transparent (inner shelf), stratified (middle shelf) and reflective (outer shelf), each type seemingly related to the local sediment composition. The upper 4 m of the upper sedimentary unit is dominated by silty sediment on the middle shelf, and by upward-fining sediments (silty sand to sandy silt) on the inner and outer shelf. The downcore trends of P-wave velocity, gamma-ray density and acoustic impedance are largely similar, but generally reversed to those of water and organic carbon contents. Total organic carbon contents increase with decreasing mean grain size, periodic fluctuations suggesting temporal changes in the regional hydrodynamics and primary productivity fuelled by the local upwelling system. The reconstruction of sedimentation rates in the course of the Holocene is based on 35 AMS age datings of organic material recovered from variable downcore depths. These range from a maximum of 13.3 cm/decade near the base of the inner shelf core (7.73–7.70 ka BP) to generally very low values (<0.11 cm/century) over the last thousand years in all cores. Over the last 6 ka there appear to have been three distinct sedimentation peaks, one between 6 and 5 ka BP, another between 4 and 3 ka PB, and one around 1 ka BP. Due to different time intervals between dates, not every peak is equally well resolved in all four cores. Based on the similar sedimentology of the inner and outer shelf cores, an essentially identical sedimentation model is proposed to have been active in both cases, albeit at different times. Thus, already during the last glacial maximum, alongshore sediment transport was deflected offshore by a change in shoreline orientation caused by the Cabo Frio structural high. The source of terrigenous material was probably a barrier-island complex that was subsequently displaced landward in the course of sea-level rise until it stabilized some 6.5 ka BP along the modern coast.     

Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen Gulf region,Canadian Arctic

To establish the relative importance of terrigenous and marine organic matter in the southern Beaufort Sea, we measured the concentrations and the stable isotopic compositions of organic carbon and total nitrogen in sediments and in settling particles intercepted by sediment traps. The organic carbon content of surface sediment in the Chukchi and southern Beaufort Seas ranged from 0.6 to 1.6% dry wt., without a clear geographical pattern. The C_ORG:N_TOT ratio ranged from 7.0 to 10.4 and did not vary significantly downcore at any one station. Values of δ¹³C_ORG and δ¹⁵N_TOT in the sediment samples were strongly correlated, with the highest values, indicative of a more marine contribution, in the Amundsen Gulf. In contrast, the organic matter content, elemental (C_ORG:N_TOT ratio) and isotopic (δ¹³C_ORG and δ¹⁵N_TOT) composition of the settling particles was different from and much more variable than in the bottom sediments. The isotopic signature of organic matter in the Beaufort Sea is well constrained by three distinct end-members: a labile marine component produced in situ by planktonic organisms, a refractory marine component, the end product of respiration and diagenesis, and a refractory terrigenous component. A three-component mixing model explains the scatter observed in the stable isotope signatures of the sediment trap samples and accommodates an apparent two-component mixing model of the organic matter in sediments. The suspended matter in the water column contains organic matter varying from essentially labile and marine to mostly refractory and terrigenous. As it settles through the water column, the labile marine organic matter is degraded, and its original stable isotope signature changes towards the signature of the marine refractory component. This process continues in the bottom sediment with the result that the sedimentary organic matter becomes dominated by the refractory terrigenous and marine components.     

Sedimentological processes in the southern margin of the Cretan Sea (NE Mediterranean)

The Cretan Basin can be characterized as a back-arc basin of the Hellenic Trench System, that is related to the subduction zone of the African Plate under the Eurasia Plate. The study area includes the narrow and relatively steep (gradient 1.5°) continental shelf of the island of Crete followed by the steep slope (2°–4°) and the rather flat deeper part of the Cretan basin (water depths >1700 m).Surficial sediments of the coastal zone are coarser and of terrigenous origin, while in deeper waters finer sediments, of biogenic origin, are more abundant. Sand-sized calcareous sediment accumulations, identified in middle-lower slope, may be attributed to the aggregation of seabed biogenic material related to the near bed current activity.High resolution profiles (3.5 kHz) taken from the inner shelf shows a typical sigmoid-oblique progradational configuration, implying prodelta sediment accumulation during the Holocene. In the upper-middle slope, sub-bottom reflectors indicate continuous sedimentation of alternating fine and/or coarse grained material. Small-scale gravity induced synsedimentary faults appeared, locally. In contrast, a series of gravity induced faults, identified in the lower slope, are associated with sediment instabilities due to seismotectonic activity. Sediment cores taken from the shelf-break consists of calcareous muddy sand with small amounts of terrigenous silt and fine sand, while the cores recovered from the middle slope has revealed a more homogeneous fine sediment texture of hemipelagic deposition.The prevailing accumulation processes in the southern margin of the Cretan basin are: (i) prodelta deposition in the inner-middle shelf; (ii) settling from bottom nepheloid layers in the shelf and upper slope; (iii) calcareous sediment formation due to settling from suspension and post accumulation aggregation (middle-lower slope); (iv) long-term episodic sediment gravity processes in the lower slope; and (v) to a lesser extent, redeposition from resuspension due to gravity processes and bottom currents.     

Late Quaternary siliciclastic/carbonate sedimentation model for the Capricorn Channel,southern Great Barrier Reef province,Australia

A model is presented for hemipelagic siliciclastic and carbonate sedimentation during the last glacial–interglacial cycle in the Capricorn Channel, southern Great Barrier Reef (GBR). Stable isotope ratios, grainsize, carbonate content and mineralogy were analysed for seven cores in a depth transect from 166 to 2892 m below sea level (mbsl). Results show variations in the flux of terrigenous, neritic and pelagic sediments to the continental slope over the last sea level cycle.During the glacial lowstand terrigenous sediment influenced all the cores down to 2000 mbsl. The percentages of quartz and feldspar in the cores decreased with water depth, while the percentage of clay increased. X-ray diffraction analysis of the glacial lowstand clay mineralogy suggests that the siliciclastic sediment was primarily sourced from the Fitzroy River, which debouched directly into the northwest sector of the Capricorn Channel at this time. The cores also show a decrease in pelagic calcite and an increase in aragonite and high magnesium calcite (HMC) during the glacial. The influx of HMC and aragonite is most likely from reworking of coral reefs exposed on the continental shelf during the glacial, and also from HMC ooids precipitated at the head of the Capricorn Channel at this time. Mass accumulation rates (MARs) are high (13.5 g cm^? 3 kyr^? 1) during the glacial and peak at ~ 20 g cm^? 3 kyr^? 1 in the early transgression (16–14 ka BP). MARs then decline with further sea level rise as the Fitzroy River mouth retreats from the edge of the continental shelf after 13.5 ka BP. MARs remain low (4 cm^? 3 kyr^? 1) throughout the Holocene highstand.Data for the Holocene highstand indicate there is a reduction in siliciclastic influx to the Capricorn Channel with little quartz and feldspar below 350 mbsl. However, fine-grained fluvial sediments, presumably from the Fitzroy River, were still accumulating on the mid slope down to 2000 mbsl. The proportion of pelagic calcite in the core tops increases with water depth, while HMC decreases, and is present only in trace amounts in cores below 1500 mbsl. The difference in the percentage of HMC in the deeper cores between the glacial and Holocene may reflect differences in supply or deepening of the HMC lysocline during the glacial.Sediment accumulation rates also vary between cores in the Capricorn Channel and do not show the expected exponential decrease with depth. This may be due to intermediate or deep water currents reworking the sediments. It is also possible that present bathymetry data are too sparse to detect the potential role that submarine channels may play in the distribution and accumulation of sediments.Comparison of the Capricorn Channel MARs with those for other mixed carbonate/siliciclastic provinces from the northeast margin of Australia indicates that peak MARs in the early transgression in the Capricorn Channel precede those from the central GBR and south of Fraser Island. The difference in the timing of the carbonate and siliciclastic MAR peaks along the northeast margin is primarily related to differences in the physiography and climate of the provinces. The only common trend in the MARs from the northeast margin of Australia is the near synchronicity of the carbonate and siliciclastic MAR peaks in individual sediment cores, which supports a coeval sedimentation model.     

Sedimentation in a tropical fjord: Golfo Dulce, Costa Rica

Sedimentation processes in Golfo Dulce, a periodically anoxic fjord-like embayment on the Pacific coast of Costa Rica, have been investigated by analyses of surface sediments and 3.5-m-long sediment cores. The large proportion of non-biogenic material (>90%) reflects the dominance of terrigenous sediment input to Golfo Dulce. Biogenic components such as organic carbon and carbonate are also supplied from terrigenous sources. The two components, however, originate from different parts of the coast surrounding the gulf. The sediment cores did not show any laminated sections. The sediments can be divided into turbiditic basin deposits and almost undisturbed, hemipelagic slope deposits.     

Grain-size distribution patterns of suspended sediment in response to hydrodynamics on the Dafeng intertidal flat, Jiangsu, China

1Introduction The sediment dynamics of intertidal flats in-volves erosion, transport, and settling processes. A-mong these, the mechanisms of sediment transportare a key question. Progress has been made in awide range of associated areas, including sedime…