Geomorphological and managerial implications of fish trapping in the Kosi Bay Estuary,KwaZulu-Natal,South Africa

The tidal basin of the Kosi Bay lagoonal-lacustrine complex is dominated by fish traps that line the intertidal and subtidal environments. These have been considered significant sediment sinks, responsible for transformation of parts of the system from subtidal and intertidal sand-flat settings to mangrove habitats. Mangrove progradation has been attributed to sediment retention within the trap structures. Fish-trap counts since 1942 indicate no significant increase in trap number prior to 1980, but thereafter trap numbers have increased exponentially. Historical GIS analysis indicates a linear growth rate of mangrove areas since 1942, a trend not mirrored in the numbers of fish traps. Modelling of mangrove progradation indicates lateral extension rates per annum much lower than global figures for choked systems elsewhere. Modelling of choking thresholds for the Kosi Bay system yields a minimum date of ～2500 AD. By including sea-level rise within the model, newly formed mangrove communities may either be drowned or remain in step with predicted sea-level rises within the next 150–200 years. The role of fish traps in the conversion of subtidal environments to sedimentary environments dominated by mangroves is negligible, and this shallowing would not be of concern in light of the short-term nature of managerial timescales.     

Holocene reef growth in Torres Strait

The platform and fringing reefs of Torres Strait are morphologically similar to reefs of the northern Great Barrier Reef to the south, except that several are elongated in the direction of the strong tidal currents between the Coral Sea and the Gulf of Carpentaria. Surface and subsurface investigations and radiocarbon dating on Yam, Warraber and Hammond Islands reveal that the initiation and mode of Holocene reef growth reflect antecedent topography and sea-level history. On the granitic Yam Island, fringing reefs have established in some places over a Pleistocene limestone at about 6 m depth around 7000 years BP. Emergent Holocene microatolls of Porites sp. indicate that the reefs have prograded seawards while sea level has fallen gradually from at least 0.8 m above present about 5800 years BP. On the Warraber Island reef platform drilling near the centre indicated a Pleistocene limestone foundation at a depth of about 6 m over which reefs established around 6700 years BP. Reef growth lagged behind that on Yam Island. Microatolls on the mature reef flat indicate that the reef reached sea level around 5300 years BP when the sea was around 0.8–1.0 m above present. On the reef flat on the western side of Hammond Island bedrock was encountered at 7–8 m depth, overlain by terrigenous mud. A progradational reef sequence of only 1–2 m thickness has built seaward over these muds, as sea level has fallen over the past 5800 years. Reef-flat progradation on these reefs is interpreted to have occurred by a series of stepwise buildouts marked by lines of microatolls parallel to the reef crest, marking individual coalescing coral heads. Detrital infill has occurred between these. This pattern of reef progradation is consistent with the radiocarbon dating results from these reefs, and with seismic investigations on the Torres Reefs.     

Vast amount of accommodation space controlled evolution of a continuous Pliocene–Pleistocene mixed cool-water carbonate-siliciclastic prograding wedge in the Bay of Oran (Western Mediterranean)

The Bay of Oran is part of the northern Algerian continental margin, located in the Western Mediterranean Sea between Europe and northern Africa. A regional terrace in ca. 320 m water depth described in earlier studies and a second deeper located one (∼1200 m water depth) provide an unusually vast amount of accommodation space for an observed prograding wedge. Seismo-stratigraphic interpretation of high-resolution reflection seismic data show different phases of mixed cool-water carbonate-siliciclastic deposition: (Ia) Initial aggradation with low dipping foreset deposition during early-Pliocene relative sea-level highstand. (Ib) Deposition transitions to progradation when aggradation reaches the base level. (IIa) Once progradation reaches the shelf break, terrace deposition is reduced to coarse fraction foreset deposits until it ceases entirely. (IIb) Finer sediments are bypassed and start to aggrade on the lower slope terrace until deposits reach the shelf terrace depth. (III) Due to accommodation space prolongation progradation recommences. Phase IIa and phase III deposits are separated by a hiatus. A drop in mean sea-level during the mid-Pleistocene will have caused the base level to fall below the upper strata, hence causing some reworking and redeposition. However, sea-level variations are not considered to be a main controlling factor of the depositional sequences. The evolution of this continuous Pliocene–Pleistocene mixed cool-water carbonate-siliciclastic prograding wedge is instead attributed to the controlling factor of this unusually vast amount of accommodation space. In closest proximity to the sea-floor, sparse recent sedimentation in form of 5–10 m thick sediment lobes can be observed in subbottom profiler data only. From a tectonic point of view, a prolongation of the Yusuf Fault into the survey area though expected by other authors could not be supported with the available dataset.     

Acoustic characters and distribution pattern of modern fine-grained deposits in a tide-dominated coastal bay: Jinhae Bay,Southeast Korea

Jinhae Bay, a semi-enclosed, tide-dominated coastal embayment on the southeastern coast of Korea, receives large amounts of sediment derived from the Nakdong River. The irregular surface of the acoustic basement is overlain by a modern sedimentary sequence up to 25 m thick, characterized by an acoustically semitransparent subbottom. Sediments, consisting mainly of terrigenous and bioturbated mud, accumulate at a rate of 2–5 mm/yr. About 21% of the suspended sediments discharged from the Nakdong River, that is approximately 1.0 × 10⁶ tons per year, accumulate in Jinhae Bay. Modern sedimentation began probably at about 5000 yr BP, when sea level approached its present level.     

Holocene sedimentation of a wave-dominated barrierisland shoreline: Cape Lookout, North Carolina

The sedimentary record of 130 km of microtidal (0.9 m tidal range) high wave energy (1.5 m average wave height) barrier island shoreline of the Cape Lookout cuspate foreland has been evaluated through examination of 3136 m of subsurface samples from closely spaced drill holes. Holocene sedimentation and coastal evolution has been a function of five major depositional processes: (1) eustatic sea-level rise and barrier-shoreline transgression; (2) lateral tidal inlet migration and reworking of barrier island deposits; (3) shoreface sedimentation and local barrier progradation; (4) storm washover deposition with infilling of shallow lagoons; and (5) flood-tidal delta sedimentation in back-barrier environments.

Twenty-five radiocarbon dates of subsurface peat and shell material from the Cape Lookout area are the basis for a late Holocene sea-level curve. From 9000 to 4000 B.P. eustatic sea level rose rapidly, resulting in landward migration of both barrier limbs of the cuspate foreland. A decline in the rate of sea-level rise since 4000 B.P. resulted in relative shoreline stabilization and deposition of contrasting coastal sedimentary sequences. The higher energy, storm-dominated northeast barrier limb (Core and Portsmouth Banks) has migrated landward producing a transgressive sequence of coarse-grained, horizontally bedded washover sands overlying burrowed to laminated back-barrier and lagoonal silty sands. Locally, ephemeral tidal inlets have reworked the transgressive barrier sequence depositing fining-upward spit platform and channel-fill sequences of cross-bedded, pebble gravel to fine sand and shell. Shoreface sedimentation along a portion of the lower energy, northwest barrier limb (Bogue Banks) has resulted in shoreline progradation and deposition of a coarsening-up sequence of burrowed to cross-bedded and laminated, fine-grained shoreface and foreshore sands. In contrast, the adjacent barrier island (Shackleford Banks) consists almost totally of inlet-fill sediments deposited by lateral tidal inlet migration. Holocene sediments in the shallow lagoons behind the barriers are 5–8 m thick fining-up sequences of interbedded burrowed, rooted and laminated flood-tidal delta, salt marsh, and washover sands, silts and clays.

While barrier island sequences are generally 10 m in thickness, inlet-fill sequences may be as much as 25 m thick and comprise an average of 35% of the Holocene sedimentary deposits. Tidal inlet-fill, back-barrier (including flood-tidal delta) and shoreface deposits are the most highly preservable facies in the wave-dominated barrier-shoreline setting. In the Cape Lookout cuspate foreland, these three facies account for over 80% of the sedimentary deposits preserved beneath the barriers. Foreshore, spit platform and overwash facies account for the remaining 20%.     

Stratigraphy, sedimentology and engineering aspects of Holocene organo-calcareous silts, Suva Harbour, Fiji

A deposit of post-glacial organo-calcareous silt 25–40 m thick blankets the floor of Suva Harbour and its marginal inlets, including Draunibota Bay, on the southeast coast of Viti Levu, Fiji. This deposit is notable for its involvement in a large number of foundation failures in the area. The organo-calcareous silt is the latest sedimentary infill over a late Pleistocene erosional surface which is dissected characteristically down to around 50 m below current mean sea level at the harbour margins, and to at least 90 m at the seaward end of the study area. The gross distribution of Pleistocene and Holocene sedimentary units is strongly influenced by Pliocene graben formation in the Mio-Pliocene bedrock. The silt deposit forms part of a transgressive and highstand systems tract related to the latest glacio-eustatic sea-level rise, and includes a series of lobate delta deposits currently prograding at the mouths of streams entering the harbour. The silt was deposited in a lagoonal/estuarine environment of restricted circulation in a marginal/back-barrier reef situation and received a variety of marine carbonate and terrigenous sediment contributions, as well as organic substances from both autochthonous and allochthonous sources. Undisturbed sampling indicates the silt to be homogeneous, non-layered and thoroughly bioturbated, but a distinct layering with a spacing in the order of 1–5 m, probably related to periodic influxes of terrigenous material, is obvious on continuous seismic profiles. Acoustic turbidity due to biogenic gas in thick accumulations over palaeo-channels may well represent zones of high pore fluid pressure, and indicate lower effective soil strength. There is also evidence that the passage of the overpressured gas front results in hydraulic fracturing and consequent degassing of the silt. The silt deposit displays a slight upward coarsening in response to shoaling conditions, accompanied by an upward increase in both organic carbon and calcium carbonate proportions, and a decreasing terrigenous contribution. The increase in organic carbon is closely mirrored by increased diagenetic development of bacterially-deposited agglomerates of iron sulfide crystals in the pore spaces. The resulting pore-space microstructure appears to exert a direct control on the in-situ void ratio, and hence directly affects the consolidation and strength characteristics of the sediment. Radiocarbon dates from the silt deposit indicate a consistent rate of deposition in the Draunibota palaeo-channel of 4.3 m/1000 yr since the marine transgressive front passed the site some 9600 yr B.P. The dates extend the reliable local sea-level data for southeast Viti Levu back some further 4000 years, and support an earlier contention that sea level reached its current position here at around 4000–4500 yr B.P., about 2000 years later than on the coast of southeast Australia. Water depth at the Cement Works Bridge site reached a maximum of 20 m some 5000 yr B.P. and, subsequently, has gradually shoaled to its present position just below mean sea level.     

Stratigraphy of Quaternary inner-shelf sediments in Tai O Bay, Hong Kong, based on ground-truthed seismic profiles

High-resolution boomer profiles from Tai O Bay, Hong Kong SAR, were ground-truthed using ten discontinuously sampled boreholes penetrating bedrock with a maximum length of 82.1 m. The relationship between depth below seabed and seismic profiles was established through the measurement of two borehole compressional-wave velocity profiles. In departure from previous interpretations, nine Quaternary seismic units were identified, which can be divided into eight systems tracts formed by cycles of fourth-order sea-level fluctuations dating back at least to marine isotope stage (MIS) 7 (ca. 190–245 ka). These consist of two lowstand systems tracts, two transgressive systems tracts, and four highstand systems tracts. Secondary unconformities within the highstand deposits are interpreted to document fifth-order sea-level fluctuations. Lowstand deposits are less common because, as soon as the sea level drops by a few metres, Tai O Bay becomes sub-aerially exposed, leading to widespread non-deposition or erosion. At the same time, extensive fluvial erosion and channel incision take place. Filling of the fluvial channels occurs during rising sea level. Lowstand sediments (if present) are generally landslide deposits laid down on a basal alluvial plain. Uncorrected accelerator mass spectrometry (AMS) radiocarbon dates of mollusc shells show that the depositional environment was marine since 6.2 ka, becoming gradually more brackish as a result of progradation of the Pearl River delta. The computed average sedimentation rate for the period 6.2–4.1 ka is 4.4 m/1,000 year, and approximately 1 m/1,000 year since 4.1 ka.     

深沪湾末次冰期以来的地层结构及古环境演变

通过深沪湾高分辨率浅地层剖面声学地层和地质钻孔沉积地层的对比,并结合沉积物的粒度、微体古生物以及AMS14C测年的综合分析,揭示了研究区晚更新世末次冰期以来的地层层序,探讨了深沪湾的古环境演变。深沪湾高分辨率浅地层剖面自上而下划分的5个声学地层单元与钻孔岩芯划分的5个沉积地层单元具有较好的对应关系。8.2 ka BP左右,全新世海侵使得海水进入深沪湾海域,海平面低于现今海平面10~12 m,气候凉爽;7 ka BP左右海水到达现今海平面位置,并于6 ka BP左右到达最高,约比现今海平面高2~3 m,气候温暖湿润,这一时期,近岸大量裸子植物被海水淹没并被沉积物快速掩埋;5 850~5 642 a BP研究区温度降低,该降温活动持续到2 ka BP左右,气候凉爽干燥;2 ka BP以来温度逐渐上升,600 a BP左右有一个相对冷期,之后温度又逐渐上升至现今水平。     

杭州湾北岸岸滩变化对海平面上升的响应——以龙泉—南竹港岸段为例

相对海平面上升引发的海岸潜在侵蚀是海岸带资源利用与规划的重点关注内容.基于杭州湾北岸龙泉—南竹港岸段实测断面资料,利用历史岸线后退和淹没法则计算法分析了该地区的海岸变化对海平面上升的响应.结果表明:近10 a来岸滩呈侵蚀后退趋势,年侵蚀速率为3.7～5.7 m/a,相对海平面上升对岸滩迁移后退的贡献为2％～6％;未来1...     

Mid-Holocene evolution and paleoenvironments of the shoreface–offshore transition, north-eastern Argentina: New evidence based on benthic microfauna

A sedimentary record spanning 5792–5511 cal yr BP and 3188–2854 cal yr BP was recovered at 36° 45′ 43″ S–56 ° 37′ 13″ W, south-west South Atlantic. The sedimentological features and micropaleontological (benthic foraminifera and ostracoda) content were analyzed in order to reconstruct paleoenvironmental conditions. Considerable environmental fluctuations are indicated by all these proxies. Five different stages were distinguished: Stage 1 (ca. 5800–5000 cal yr BP) consists of muddy sand with abundant microfossils. In this interval, species typical for inner marine shelf environments maintained a high abundance. Stage 2 consists of plastic light greenish grey clays barren of microfossils, and probably represents fluvial input from the de la Plata River to the shelf contemporaneous of a lowering of sea level. Stage 3 is composed of brownish yellow sandy silts, and represents increasing marine conditions in the area as reflected by higher faunal diversity and typical foraminifera of inner shelf environments. Stage 4 is made of homogeneous mud, barren of microfossil, which represents a new pulse of fluvial input to the shelf in consequence of a new fall in sea level. The final part of the core (Stage 5) is a coarsening upward sequence, grading from greeny brown clayey sandy silts to coarse shelly sands and represents the modern sedimentation in the area. This interpretation strengthens the stepped model of late-Holocene sea-level fall between 5511–5792 cal yr BP and 2854–3188 cal yr BP in Buenos Aires coast, and agrees with the relative sea-level history previously proposed by some authors from western South Atlantic coasts.     

Facies and architecture of an estuarine tidal bar (the Trompeloup bar, Gironde Estuary, SW France)

This study describes the sedimentary facies and internal architecture of a modern estuarine tidal bar present in the bay-head delta of a wave- and tide-dominated estuary, the Gironde Estuary, in southwest France. Based on 51 cores (2 to 5 m long), this work demonstrates that such bars are composite sedimentary bodies made up of individual sigmoidal sand units isolated from one another by thick muddy layers. The bar appeared in the study area around 1874. Since then, its evolution has been characterized by phases of rapid downstream bar migration alternating with periods of bar abandonment. During the periods of active bar progradation, which commonly last between 10 and 20 years, the bar progrades up to 7 km seaward and forms a narrow (200–800 m wide) sand ridge up to 6 m thick. During abandonment phases which also last around 10–20 years, the bar is starved of sand and a mud layer (1 to 3 m thick) is deposited. The vertical succession formed by the seaward progradation of the tidal bar records these alternating phases of growth and abandonment. A typical vertical succession in the bar system is 8 m thick and comprises several superimposed, upward-coarsening units of cross-stratified sand, deposited during phases of seaward progradation, separated by a thick layer of thinner-bedded mud and sand laminae deposited during phases of bar abandonment. When a tidal bar builds up to the supratidal zone, marshes accumulate and the bar becomes a permanent island in the estuary. The large-scale architecture of the bar is thus made of an alternation of potential sandy reservoir units and muddy internal seals. The reservoir units are characterized by a sigmoidal shape. The muddy layer, which is deposited between the two sigmoidal units, is a thick, continuous potential internal seal. This complex internal architecture is due to the fact that there is a considerable amount of mud present in the bay-head delta of the Gironde Estuary. It is thought to be representative of the architecture of tidal bars in the inner part of wave- and tide-dominated estuaries.     

海南岛1605年历史地震的海岸沉积记录

海南岛铺前港湾地区1605年发生了华南历史记录的最强烈的地震,地震强度为M7.5,地震造成陆地沉陷成海。根据野外海岸地貌调查和海岸钻孔沉积物岩性特征与粒度、元素比值分析结果,对历史地震的海岸沉积记录进行了初步的研究,4个钻孔位于铺前港湾西岸博度村附近的小型古海湾,昌德、演丰附近的野菠萝岛和道学附近的红树林区潮间带,钻孔沉积记录显示铺前港湾内湾沉陷幅度为2 m,存在明显的差异性,东寨港道学和野菠萝岛附近的钻孔岩心下部为泥状红土,属于成土的风化壳顶部,根据曲口半岛下山村附近的海岸调查,强海洋动力事件形成了高出现代高潮位3.0 m的高位海岸混杂沉积体,可能与1605年地震下沉引发的类似海啸的沉积事件相关。     

Sequence stratigraphic modelling and reservoir architecture of the shallow marine successions of Baram field,West Baram Delta,offshore Sarawak,East Malaysia

The Baram Delta Province is located in the northern part of Sarawak (West Baram Delta) and extends north-eastward into Brunei and further into the south-western part of Sabah (East Baram Delta). The delta is a Neogene basin which developed over an accretionary wedge implying Cretaceous to Eocene sediments during the Late Eocene to Late Miocene times (Tongkul, 1991; Hutchison et al., 2000; Morley et al., 2003; Sapin et al., 2011).Facies and well log analyses were carried out on core and well data for the Late Miocene successions of Baram field, a medium-sized oilfield located in the north-eastern flank of the Baram Delta Oil Province, offshore Sarawak. A numerical model of sea-level fluctuations and progradational basin-fill was generated using the Clastic Modeling Program (Hardy and Waltham, 1992a and 1992b; Waltham, 1992) software to evaluate the possible controls of sea-level changes in the development of the siliciclastic successions and their bounding surfaces. This model was based on four lines of evidence, namely core data, fieldwide wireline logs correlation, seismic sections and average thickness variations across the field.Cored intervals of the Upper Cycle V (Late Miocene) display reservoir successions dominated by thick swaley cross-stratified (SCS) sandstones, thin hummocky cross-stratified sandstones and other shallow marine, wave and storm-dominated facies, interbedded with laminated shelfal mudstones. The vertical facies organisation suggests deposition during shoreface progradation associated with a fall of relative sea level.Analysis and correlation of well logs reveal stacking patterns comprising three scales of depositional cyclicity: the parasequences (∼10–∼30 m thick), the parasequence sets (∼45–∼130 m thick) and the major cycles (∼600–800 m thick).Field-wide, dip-oriented seismic sections show very well-developed horizontal to slightly upward convex layers traceable over great distances, which suggests a ramp-type margin, in which the basin floor dipped gradually seaward and lacked a distinct shelf-slope margin.The evidences gathered demonstrate that the deposition and build-up stratigraphy of the Late Miocene sedimentary successions could have been strongly controlled by superimposed short-term, medium-term and long-term sea-level changes.The simulated sea level and sedimentary basin-fill model, generated by the Clastic Modelling Program, match to the well log correlation. This model illustrates that high frequency sea-level fluctuations enable sands to be distributed over large areas within a shallow, low gradient shelf. Our study shows that integrated studies incorporating cores, well logs, seismic sections and simulated models can be employed as important tools for correlation and reservoir modelling.     

东亚和南亚典型大河三角洲晚第四纪地层结构及成因对比

通过整理东亚、南亚典型河口三角洲末次冰消期以来演化历史的研究进展,对比各河口晚第四纪地层结构、沉积体系演替和三角洲开始建造的时间,分析其沉积历史的主要控制因素。结果显示,末次冰消期以来各河口具相似的地层结构和演变过程,即早全新世下切古河谷充填和河口湾发育,中—晚全新世三角洲建造,该过程主要受海平面变化的控制。但是各河口地层结构和沉积历史也存在差异,其中以三角洲开始建造的时间差别最为明显。对比发现这种差异与流域地貌、基岩以及河口沉积盆地的差别有关。流程短、流域基岩易侵蚀的河流,入海泥沙量大,其三角洲开始建造的时间显著早于其他河流,其中以恒河三角洲最为典型。另外,以珠江三角洲为典型,其半封闭、基底浅的河口沉积盆地特征,也有助于中全新世湾顶三角洲的建造。     

Modeling erosion of sedimentary coasts in the western Russian Arctic

Morphodynamic modeling is employed in the present work to predict the long-term evolution (over the next 100 years) of typical sedimentary coasts in the western Russian Arctic. The studied objects are the coasts of Varandey (the Barents Sea), Baydaratskaya Bay and Harasavey (the Kara Sea). The model developed takes into account both the short-term processes (storm events) and long-term factors (for example, changes in sea level, inter-annual variations in gross sediment flux, lack or excess of sediment supply). Predicted and observed morphological changes in coastal profiles are shown to agree well for time scales ranging from weeks to decades. It is revealed that under given environmental conditions, the morphological evolution is strongly influenced by storm surges and associated wind-driven circulation. The water level gradient created by a surge generates a seaward flow at the bed. This outflow is shown to be an important destructive mechanism contributing to the erosion and recession of Arctic coasts. The rate of change is found to depend on both the exposure of the coast (relative to the direction of dominant winds) and its height above the sea. The open coast of Varandey is expected to retreat as much as 300–500 m over 100 years, while recession of the less exposed coasts of Baydaratskaya Bay would not exceed about 100 m/century. If long-term sediment losses are insignificant, the rate of erosion decays with time and the morphodynamic system may tend toward equilibrium. It is concluded that the expected relative sea-level rise (up to 1 m over the nearest 100 years) is non-crucial to the future coastal evolution if an erosion activity is already high enough.     

杭州湾近期环境演变与沉积速率

本文追述了杭州湾4万年来的沉积环境演变,论述了近期湾内沉积环境的差异,以及不同时间尺度的沉积速率。从钻孔样品的综合分析得知,4万年来杭州湾和东海大陆架一样亦二经沧桑。陆海变迁不仅在岩性组合、微体古生物诸方面留下确凿证据,而且有确切地貌部位样品~(14)C测年印证。杭州湾3～5m厚的沉积结构特征,显示出其各沉积区环境的明显差异和数百年来的继承性。从全新世沉积层的厚度与年代推算,8000～9000a来其沉积速率一般平均为0.11～0.25cm/a;其口门段中北部细粒沉积物区(锋面活动区),近百年来的沉积速率(~(210)Pb)为1.0～1.2cm/a;有机农药以及风暴层比对的沉积速率与其吻合。     

Yangtze offshore,China: highly laminated sediments from the transition zone between subaqueous delta and the continental shelf

This study examines the deposition of highly laminated muddy sediment in vibrocores recovered from a depth of 40–55 m of water and located 120 km east of the Yangtze River mouth. X-radiographs show numerous sharp-based sedimentary rhythmics with nearly parallel and undulated laminations, interbedded with silty lenses and interbeds. The laminated sediment varies from clayey silt to silty clay. AMS radiocarbon dates on well-preserved bivalves are primarily younger than 5500 y BP, indicating the formation of the laminated sediment under the present sea-level conditions. Macro- and microfossils in the sediments are consistent with the modern offshore sedimentary setting. The laminated sediment originated from the Yangtze estuary, but the sedimentation processes were not deltaic. The highly laminated sediment that lies below the normal wave base of 10 m was deposited on the seaward margin of the Yangtze subaqueous delta, closely associated with submarine tidal and storm-generated currents.     

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.     

Global sea-level control on local parasequence architecture from the Holocene record of the Po Plain,Italy

Holocene deposits exhibit distinct, predictable and chronologically constrained facies patterns that are quite useful as appropriate modern analogs for interpreting the ancient record. In this study, we examined the sedimentary response of the Po Plain coastal system to short-term (millennial-scale) relative fluctuations of sea level through high-resolution sequence-stratigraphic analysis of the Holocene succession.Meters-thick parasequences form the building blocks of stratigraphic architecture. Above the Younger Dryas paleosol, a prominent stratigraphic marker that demarcates the transgressive surface, Early Holocene parasequences (#s 1–3) record alternating periods of rapid flooding and gradual shoaling, and are stacked in a retrogradational pattern that mostly reflects stepped, post-glacial eustatic rise. Conversely, Middle to Late Holocene parasequences (#s 4–8) record a complex, pattern of coastal progradation and delta upbuilding that took place following sea-level stabilization at highstand, starting at about 7 cal ky BP. The prominent transgressive surface at the base of parasequence 1 correlates with the period of rapid, global sea-level rise at the onset of the Holocene (MWP-1B), whereas flooding surfaces associated with parasequences 2 and 3 apparently reflect minor Early Holocene eustatic jumps reported in the literature. Changes in shoreline trajectory, parasequence architecture and lithofacies distribution during the following eustatic highstand had, instead, an overwhelming autogenic component, mostly driven by river avulsions, delta lobe switching, local subsidence and sediment compaction. We document a ∼1000-year delayed response of the coastal depositional system to marine incursion, farther inland from the maximum landward position of the shoreline. A dramatic reduction in sediment flux due to fluvial avulsion resulted in marine inundation in back-barrier position, whereas coastal progradation was simultaneously taking place basinwards.We demonstrate that the landward equivalents of marine flooding surfaces (parasequence boundaries) may be defined by brackish and freshwater fossil assemblages, and traced for tens of kilometers into the non-marine realm. This makes millennial-scale parasequences, whether auto- or allogenic in origin, much more powerful than systems tracts for mapping detailed extents and volumes of sediment bodies.The Holocene parasequences of the Po coastal plain, with strong age control and a detailed understanding of sea-level variation, may provide insight into the driving mechanisms and predictability of successions characterized by similar depositional styles, but with poor age constraint, resulting in more robust interpretations of the ancient record.