The Cosmonaut Sea Wedge

A set of multi-channel seismic profiles (∼15,000 km) is used to study the depositional evolution of the Cosmonaut Sea margin of East Antarctica. We recognize a regional sediment wedge, below the upper parts of the continental rise, herein termed the Cosmonaut Sea Wedge. The wedge is situated stratigraphically below the inferred glaciomarine section and extends for at least 1,200 km along the continental margin with a width that ranges from 80 to about 250 km. The morphology of the wedge and its associated depositional features indicate a complex depositional history, where the deep marine depositional sites were influenced by both downslope and alongslope processes. This interaction resulted in the formation of several proximal depocentres, which at their distal northern end are flanked by elongated mounded drifts and contourite sheets. The internal stratification of the mounded drift deposits indicates that westward flowing bottom currents reworked the marginal deposits. The action of these currents together with sea-level changes is considered to have controlled the growth of the wedge. We interpret the Cosmonaut Sea Wedge as a composite feature comprising several bottom current reworked fan systems. The wide spectrum of depositional geometries in the stratigraphic column reflects dramatic variations in sediment supply from the continental margin as well as varying interaction between downslope and alongslope processes.     

Along-slope oceanographic processes and sedimentary products around the Iberian margin

This contribution to this special volume represents the first attempt to comprehensively describe regional contourite (along-slope) processes and their sedimentary impacts around the Iberian margin, combining numerically simulated bottom currents with existing knowledge of contourite depositional and erosional features. The circulation of water masses is correlated with major contourite depositional systems (CDSs), and potential areas where new CDSs could be found are identified. Water-mass circulation leads to the development of along-slope currents which, in turn, generate contourite features comprising individual contourite drifts and erosional elements forming extensive, complex CDSs of considerable thickness in various geological settings. The regionally simulated bottom-current velocities reveal the strong impact of these water masses on the seafloor, especially in two principal areas: (1) the continental slopes of the Alboran Sea and the Atlantic Iberian margins, and (2) the abyssal plains in the Western Mediterranean and eastern Atlantic. Contourite processes at this scale are associated mainly with the Western Mediterranean Deep Water and the Levantine Intermediate Water in the Alboran Sea, and with both the Mediterranean Outflow Water and the Lower Deep Water in the Atlantic. Deep gateways are essential in controlling water-mass exchange between the abyssal plains, and thereby bottom-current velocities and pathways. Seamounts represent important obstacles for water-mass circulation, and high bottom-current velocities are predicted around their flanks, too. Based on these findings and those of a selected literature review, including less easily accessible ??grey literature?? such as theses and internal reports, it is clear that the role of bottom currents in shaping continental margins and abyssal plains has to date been generally underestimated, and that many may harbour contourite systems which still remain unexplored today. CDSs incorporate valuable sedimentary records of Iberian margin geological evolution, and further study seems promising in terms of not only stratigraphic, sedimentological, palaeoceanographic and palaeoclimatological research but also possible deep marine geohabitats and/or mineral and energy resources.     

Depositional characteristics and spatial distribution of deep-water sedimentary systems on the northwestern middle-lower slope of the Northwest Sub-Basin, South China Sea

Based upon 2D seismic data, this study confirms the presence of a complex deep-water sedimentary system within the Pliocene-Quaternary strata on the northwestern lower slope of the Northwest Sub-Basin, South China Sea. It consists of submarine canyons, mass-wasting deposits, contourite channels and sheeted drifts. Alongslope aligned erosive features are observed on the eastern upper gentle slopes (<1.2° above 1,500 m), where a V-shaped downslope canyon presents an apparent ENE migration, indicating a related bottom current within the eastward South China Sea Intermediate Water Circulation. Contourite sheeted drifts are also generated on the eastern gentle slopes (~1.5° in average), below 2,100 m water depth though, referring to a wide unfocused bottom current, which might be related to the South China Sea Deep Water Circulation. Mass wasting deposits (predominantly slides and slumps) and submarine canyons developed on steeper slopes (>2°), where weaker alongslope currents are probably dominated by downslope depositional processes on these unstable slopes. The NNW–SSE oriented slope morphology changes from a three-stepped terraced outline (I–II–III) east of the investigated area, into a two-stepped terraced (I–II) outline in the middle, and into a unitary steep slope (II) in the west, which is consistent with the slope steepening towards the west. Such morphological changes may have possibly led to a westward simplification of composite deep-water sedimentary systems, from a depositional complex of contourite depositional systems, mass-wasting deposits and canyons, on the one hand, to only sliding and canyon deposits on the other hand.     

Mediterranean shelf-edge muddy contourites: examples from the Gela and South Adriatic basins

We present new evidence of shallow-water muddy contourite drifts at two distinct locations in the central Mediterranean characterized by a relatively deep shelf edge (between 170 and 300 m below sea level): the south-eastern Adriatic margin and the north-western Sicily Channel. The growth of these shelf-edge contourite drifts is ascribed to the long-term impact of the Mediterranean themohaline circulation. The Levantine Intermediate Water flows continuously, with annual or inter-annual variations, and affects the shelf edge and the upper slope in both study areas. In addition, the SW Adriatic margin is impinged by the seasonally modulated off-shelf cascading of North Adriatic Dense Water. This water mass has formed ever since the large Adriatic continental shelf was drowned by the post-glacial sea-level rise. It energetically sweeps the entire slope from the shelf edge to the deep basin. These bottom currents flow parallel or oblique to the depth contours, and are laterally constricted along markedly erosional moats aligned parallel to the shelf edge where they increase in flow velocity. The internal geometry and growth patterns of the shelf-edge contourites reflect changes in oceanographic setting affecting the whole Mediterranean Sea. In particular, seismic correlation with published sediment cores documents that these deposits are actively growing and migrating during the present interglacial, implying an enhancement in bottom-water formation during intervals of relative sea-level rise and highstand. Regardless of the specific mechanisms of formation, sediment drifts in both study areas have been affected by widespread thin-skinned mass-wasting events during post-glacial times. Repeated mass-transport processes have affected in particular the downslope flank of the shelf-edge contourite drifts, indicating that these muddy deposits are prone to failure during, or soon after, their deposition.     

Contourites in the Gulf of Cadiz: a cautionary note on potentially ambiguous indicators of bottom current velocity

Facies associations in cores collected in the deep part of the Gulf of Cadiz, which is under the influence of the lower branch of the Mediterranean Outflow Water, are investigated in terms of the classical contourite model using grain-size analyses and thin sections of indurated sediment. Cores include both low-energy (contourite drift) and high-energy (channel) environments. The thin sections and grain-size distributions show that clayey fine silts and sandy coarse silts are the most common facies associations in the studied contourite sequences, while coarse-grained, gravelly contourites are less common. Grain-size distributions are unimodal in the fine-grained and bi- or trimodal in the coarser-grained contourites. This change in grain-size composition is related both to the partial removal of the fine-grained fraction and to the replenishment of the coarser-grained one. In addition, most of the contacts between individual facies are sharp rather than transitional. This suggests that the contourite sequence is only in part related to changes in bottom current velocity and flow competency, but may also be related to the supply of a coarser terrigeneous particle stock, provided by either increased erosion of indurated mud along the flanks of confined contourite channels (mud clasts), or by increased sediment supply by rivers (quartz grains) and downslope mass transport on the continental shelf and upper slope. The classical contourite facies association may therefore not be solely controlled by current velocity, but may be the product of a variety of depositional histories. The classical contourite depositional sequence should therefore be interpreted with greater care and in the light of the regional sedimentological background. In addition, the wisdom of exclusively using mean or modal particle size for the interpretation of depositional contourite processes is questioned. Instead, it is proposed that the vertical evolution of grain-size populations in the facies successions forming contourite sequences be assessed.     

Stratigraphy and facies distribution of the Utsira formation and the Pliocene sequences in the northern North Sea

The Upper Cenozoic deposits of the northern North Sea have been analysed in order to establish a regional and detailed stratigraphy. The Utsira Formation is subdivided into four log-units and mapped, and two main depocentres are outlined. The lower part of the Utsira Formation consists of thick marine, mounded sand bodies, interpreted as overall stacked lowstand fan deposits, while the upper part of the formation consists of more clayey-silty intervals, indicating increased relative sea level. The succeeding progradational Pliocene deposits are subdivided into 13 high-frequency depositional sequences and are mapped. The sequences are grouped into four composite sequences. Each of the four Pliocene composite sequences is composed of one or two rather locally distributed, prograding sequences (lowstand sequence set), succeeded by one or two more widely distributed aggrading-prograding sequences (transgressive-highstand sequence set). Boundaries between the composite sequences are recorded as marked changes in distribution of depocentres and sequence architecture. The regional uplift of Scandinavia is believed to be the main control on sediment input, feeding the succeeding general prograding Pliocene sequences. Oscillations of the eustatic sea level punctuated the tectonically controlled progradation and affected variations in the accommodation space, and thus created the high-frequency sequences.     

48ka以来日本海Ulleung海盆南部的海洋沉积环境演化

晚第四纪以来伴随底层水含氧量的剧烈变化,浅色和深色沉积层的交替出现是日本海半远洋沉积物的主要特征。沉积特征分析表明,日本海Ulleung海盆南部KCES1孔的沉积物具有四种不同的沉积构造:均质、纹层、纹层状和混杂构造。深色沉积层一般具有纹层和纹层状构造,并且与我国内陆的千年尺度东亚夏季风强弱变化记录有很好的对应关系,表明纹层沉积物也具有千年尺度的变化规律,从而进一步说明了冰川性海平面变化和东亚夏季风波动应该是Ulleung海盆南部底层水溶解氧含量变化的主要原因。在暖期,在东亚夏季风降水相对增强的影响下,低温、低盐的东海沿岸水对日本海表层水体的贡献要大于对马暖流的贡献,日本海水体间的交换减弱,最终造成缺氧的海底沉积环境。在冷期,夏季风强度的减弱(冬季风增强)加快了日本海西北部深层水的生成,Ulleung海盆南部的底层水含氧量高,相应地沉积了具均质构造的浅色沉积物;在末次盛冰期最低海平面时,日本海成为一个封闭的海盆,降雨量高于蒸发量,水体出现分层,底层水处于停滞缺氧状态。自距今17.5 ka(日历年,下同)以来底层水含氧量较高,对马暖流逐渐成为影响日本海海洋沉积环境的主要因素。Ulleung海盆南部底层水的含氧量在YD期间有一定程度的降低,东海沿岸水的短暂强盛制约了深层水的流通。自距今10.5 ka以来对马暖流强盛,日本海海底处于富氧的沉积环境。     

等深流沉积特征

根据国内外等深流沉积的研究成果,从产状、成分、结构、沉积构造、垂向层序和与海平面变化的关系等方面总结了等深流沉积特征以及与其他类型深水沉积的区别。等深流沉积是深水牵引流沉积中的一种重要类型,蕴含有重要的古海洋学和古气候信息,对其进行解析是近年国际海洋学、气候学研究的热点。等深流沉积最明显的特征包括对称递变层序以及强烈的生物扰动,其储集性能优于浊流沉积,具有非常重要的含油气潜能。南海是中国具有深海环境的唯一边缘海盆,在南海北部深水陆坡环境已确认等深流沉积普遍存在,是等深流研究的理想场所,但南海的等深流沉积类型、形态特征以及控制其发育的主要因素等问题有待进一步研究。     

Contrasting facies patterns in subtropical and temperate continental slope sediments: inferences from east Australian late Quaternary records

Studies of latest Quaternary continental slope sediments at two localities on the east Australian margin have revealed markedly different responses to late Quaternary sea level fluctuations. Offshore of Noosa, in the sub-tropics, the sediment is predominantly a mixture of fine metastable carbonate, siliciclastic material, and pelagic carbonate. Important features of the stratigraphy include a siliciclastic-dominated facies deposited relatively slowly during the last glacial lowstand (sedimentation rate ≤8 cm/ka), and a calcareous facies, rich in metastable carbonate, deposited more rapidly during the late post-glacial transgression (sedimentation rates 15–24 cm/ka). Highstand and transgressive sedimentation rates are greater than lowstand rates by a factor of 2.5–6 due to increased shelf carbonate productivity after flooding of the mid-shelf. Off Sydney, in temperate latitudes, continental slope sediment is largely a mixture of fine siliciclastic material and pelagic carbonate. Mean sedimentation rates range from 2 to 5 cm/ka over the last four oxygen isotope stages, with mean glacial/interstadial rates higher than Holocene rates by a factor of ∼1.36. This largely reflects the transfer of siliciclastic mud from the shelf to the slope during sea level regression. In both localities, facies changes on the slope are not related to specific sea level states (e.g. lowstand facies, transgressive facies, etc.), but reflect instead the interaction of changing sea level with shelf morphology.     

Depositional environment in the southern Ulleung Basin, East Sea (Sea of Japan), during the last 48 000 years

The present study is based on the sedimentological data from a piston core KCES1 off the southern Ulleung Basin margin, the East Sea (Sea of Japan). The data include sediment color (L^*), X-ray radiographs, grain size distribution and AMS¹⁴C date. Four kinds of sediments (homogeneous, laminated, crudely laminated and hybrid sediments) are identified according to the characters of the sedimentary structures that were considered to reflect changes in bottom-water oxygenation. Alternations of dark laminated/crudely laminated sediments and light homogeneous sediments represent millennial-scale variations that are possibly associated with the high-resolution changes in the East Asian monsoon (EAM). The relative contributions of the East China Sea Coastal Water (ECSCW) and the Tsushima Warm Current (TWC) were likely the main reasons for the repetition of the anoxic and oxic depositional conditions in the East Sea since the last 48 ka BP. During the interstadial, the strengthen summer EAM was attributed to the expansion of the ECSCW because of more humid climate in central Asia, and then more strongly low-salinity, nutrient-enriched water was introduced into the East Sea. The ventilation of deep water was restricted and therefore the dark laminated layer deposited under the anoxic bottom water condition. During the lowest stand of sea level in the last glacial maximum (LGM), the isolated East Sea dominated by stratiˉed water masses and the euxinic depositional environment formed. The homogenous sediments have been predominating since 17.5 ka BP indicating that the TWC has intruded into the East Sea gradually with the stepwise rise of sea level and the bottom water oxygen level was high. During the late Younger Dryas (YD) period, the last dark laminated layer deposited because the ventilation of bottom water was restricted by stronger summer EAM. The TWC strengthened and the bottom water became oxic again from 10.5 ka BP.     

Seismic expression of contourites and related deposits: a preface

Contourites are widespread throughout the deep sea, ranging from those that build up individually distinct bodies (mounded drifts) to those that occur closely interbedded with other deep-water facies. Although seismic data should not be used to make a firm identification of contourites without supporting evidence, much progress has been made in determining the combination of seismic criteria that best represent contourite deposits. With some modifications and additions, these criteria are in broad agreement with those recently proposed by Faugères and colleagues (1999). The papers in this special issue are mainly drawn from those presented at an IGCP workshop held in Trieste, Italy (October 2000), together with selected additions. Some of the new and important topics covered include: discussion of bottom-current controlled deposits on continental shelves and in large lakes (shallow-water and lacustrine `contourites'); the unique characteristics of high-latitude contourite systems that show interaction with turbidity current, debris flow and glacigenic processes; and the importance of local slope topography, developed as a result of synsedimentary tectonics and downslope mass movement, in focussing and shaping drift deposits (infill and fault-related drifts). An introduction to the powerful use of 3D seismic data in understanding contourite systems points the way towards important future research.     

A record of fluctuating bottom currents on the slopes west of the Porcupine Bank,offshore Ireland — implications for Late Quaternary climate forcing

Both local and regional controls on slope sedimentation west of Porcupine Bank are assessed using an array of 25 gravity cores, integrated with shallow seismic, TOBI side-scan and high-resolution bathymetry data. The cores were retrieved from an area of smooth, distally steepened slope (between 52° and 53°N) in water depths of 950 to 2750 m. The slope here is unmodified by gravity failures and is swept by bottom currents that flow from S to N along the margin. The cores reveal a coherent shallow stratigraphy that can be traced along and between transects at upper-, mid- and lower-slope levels. AMS ¹⁴C dating, oxygen-isotopes and carbonate profiles suggest the cored record could extend as far back as 500 ka in the longest cores, with most cores providing details of the slope response to the last interglacial, last glacial and Holocene forcing. The facies indicate deposition was dominated by a combination of bottom currents, ice-rafting and hemipelagic settling, with carbonate-prone deposits during interglacials, and siliciclastic deposits during glacials. Inferred contourites imply that strong currents operated during interglacials, with weaker current reworking during glacial conditions. A pair of erosion surfaces record significant mid- and upper-slope scouring during Marine Isotope Stage (MIS 3) and in the Early Holocene. The lateral facies distribution implies stronger currents at shallower levels on the slope, although there is evidence that the core of the current migrated up and down the slope, and that sand might locally have spilt down-slope. The bathymetry influenced both the wider geometry of the condensed contourite sheet and the local thickness and facies variation across the slope. A significant result of the study is the identification of a pair of thin sand–mud contourite couplets that record enhanced bottom-current reworking corresponding to periods of interstadial warming during MIS 3. The couplets can be correlated to the terrestrial records onshore Ireland and imply that the NE Atlantic margin oceanographic and onshore climate records are strongly coupled at interstadial level.     

粤东及闽南近岸上升流对局地风场变化的响应

本文利用2010年6-7月的实测温盐、水位、海流等资料,结合风场数据,讨论了在台风影响较小的情况下,粤东及闽南近岸上升流对局地风场变化的响应特征,主要结论如下:（1）谱分析结果显示,沿岸风、水位、海流、近底层水温均具有3.5～4.0 d、5.0～5.5 d、8.3～9.0 d的波动周期,沿岸风的变化引起上升流强度在3～9 d周期上的波动;（2）上升流对局地风场变化的响应过程如下:利于上升流产生的局地风场发生变化时,沿岸风作用下产生的Ekman输运促使的上升流区水位的下降幅度发生改变,随即向岸方向的压强梯度力也发生变化,进而导致沿岸流及近底层向岸流的增强或减弱,而近底层向岸流强度的改变又会引起近底层水温的变化;（3）相关分析及交叉谱分析的结果表明,沿岸风的变化将在3 d以内影响上升流区近底层水温。以34 m向岸流代表近底层向岸流,则“沿岸风-水位-近底层向岸流-近底层水温”这一过程的响应时间依次为24 h、7 h、27 h左右。     

The Cenozoic western Svalbard margin: sediment geometry and sedimentary processes in an area of ultraslow oceanic spreading

The northeastern high-latitude North Atlantic is characterised by the Bellsund and Isfjorden fans on the continental slope off west Svalbard, the asymmetrical ultraslow Knipovich spreading ridge and a 1,000 m deep rift valley. Recently collected multichannel seismic profiles and bathymetric records now provide a more complete picture of sedimentary processes and depositional environments within this region. Both downslope and alongslope sedimentary processes are identified in the study area. Turbidity currents and deposition of glacigenic debris flows are the dominating downslope processes, whereas mass failures, which are a common process on glaciated margins, appear to have been less significant. The slide debrite observed on the Bellsund Fan is most likely related to a 2.5–1.7 Ma old failure on the northwestern Barents Sea margin. The seismic records further reveal that alongslope current processes played a major role in shaping the sediment packages in the study area. Within the Knipovich rift valley and at the western rift flank accumulations as thick as 950–1,000 m are deposited. We note that oceanic basement is locally exposed within the rift valley, and that seismostratigraphic relationships indicate that fault activity along the eastern rift flank lasted until at least as recently as 1.5 Ma. A purely hemipelagic origin of the sediments in the rift valley and on the western rift flank is unlikely. We suggest that these sediments, partly, have been sourced from the western Svalbard—northwestern Barents Sea margin and into the Knipovich Ridge rift valley before continuous spreading and tectonic activity caused the sediments to be transported out of the valley and westward.     

Spreading of Antarctic Bottom Water and its effects on the floor of the Indian Ocean inferred from bottom-water potential temperature,turbidity, and sea-floor photography

Data on bottom-water potential temperature, turbidity and current indications show that in the Southern Ocean west of the Kerguelen Plateau, Antarctic Bottom Water (AABW) of Weddell Sea origin spreads northwards from the Atlantic—Indian Basin in two directions: (1) AABW enters the Agulhas Basin through relatively deep areas in the Mid-Indian Ridge at 20–25°E and possibly at 35°E, and flows northwards into the Mozambique Basin as far as its northern limits; (2) a more easterly spreading path extends from the Atlantic—Indian Basin through the Crozet into the Madagascar, Mascarene, Somali and Arabian Basins. The passage in the western branch of the Indian Ridge for the AABW spreading from the Crozet into the Madagascar Basin appears to be at 29-26°S and 60–64°E.East of the Kerguelen Plateau in the South Indian Basin, the bottom water formed mainly along the Adélie Coast and Ross Sea travels west towards the Kerguelen Plateau and then parallel to it. This water finally flows eastwards hugging the Southeast Indian Ridge. Significant deviations from this general circulation pattern occur due to local topographic effects. Some AABW in the South Indian Basin exits through a passage at 120–125°E in the region of the Australian—Antarctic discordance in the Southeast Indian Ridge and enters the South Australian Basin and subsequently the Wharton Basin. This passage is clearly indicated by the northward extension of a cold, bottom-water tongue as shown by the temperature distribution in the region; the bottom-water effects in the passage are reflected in the high turbidity and current lineations on the sea floor.In the Southern Ocean basins, bottom-water turbidity is generally high, reflecting in part the strong bottom-water activity. The effects of AABW circulation on the sea floor—in the form of well-developed small- or large-scale current ripples and erosional/depositional features, manganese-nodule formations, and unconformities and reworking of sediments observed in cores — are also marked in these basins. Even though the AABW in the Wharton Basin is cold, its spreading effects on the sea floor are minimal in this basin in contrast to the basins west of the Mid-Indian Ridge at comparable latitudes.     

Deep-water Circulation: Processes & Products (16�C18 June 2010, Baiona): introduction and future challenges

Deep-water circulation is a critical part of the global conveyor belt that regulates Earth??s climate. The bottom (contour)-current component of this circulation is of key significance in shaping the deep seafloor through erosion, transport, and deposition. As a result, there exists a high variety of large-scale erosional and depositional features (drifts) that together form more complex contourite depositional systems on continental slopes and rises as well as in ocean basins, generated by different water masses flowing at different depths and at different speeds either in the same or in opposite directions. Yet, the nature of these deep-water processes and the deposited contourites is still poorly understood in detail. Their ultimate decoding will undoubtedly yield information of fundamental importance to the earth and ocean sciences. The international congress Deep-water Circulation: Processes & Products was held from 16?C18 June 2010 in Baiona, Spain, hosted by the University of Vigo. Volume 31(5/6) of Geo-Marine Letters is a special double issue containing 17 selected contributions from the congress, guest edited by F.J. Hernández-Molina, D.A.V. Stow, E. Llave, M. Rebesco, G. Ercilla, D. Van Rooij, A. Mena, J.-T. Vázquez and A.H.L. Voelker. The papers and discussions at the congress and the articles in this special issue provide a truly multidisciplinary perspective of interest to both academic and industrial participants, contributing to the advancement of knowledge on deep-water bottom circulation and related processes, as well as contourite sedimentation. The multidisciplinary contributions (including geomorphology, tectonics, stratigraphy, sedimentology, paleoceanography, physical oceanography, and deep-water ecology) have demonstrated that advances in paleoceanographic reconstructions and our understanding of the ocean??s role in the global climate system depend largely on the feedbacks among disciplines. New insights into the link between the biota of deep-water ecosystems and bottom currents confirm the need for this field to be investigated and mapped in detail. Likewise, it is confirmed that deep-water contourites are not only of academic interest but also potential resources of economic value. Cumulatively, both the congress and the present volume serve to demonstrate that the role of bottom currents in shaping the seafloor has to date been generally underestimated, and that our understanding of such systems is still in its infancy. Future research on contourites, using new and more advanced techniques, should focus on a more detailed visualization of water-mass circulation and its variability, in order to decipher the physical processes involved and the associations between drifts and other common bedforms. Moreover, contourite facies models should be better established, including their associations with other deep-water sedimentary environments both in modern and ancient submarine domains. The rapid increase in deep-water exploration and the new deep-water technologies available to the oil industry and academic institutions will undoubtedly lead to spectacular advances in contourite research in terms of processes, morphology, sediment stacking patterns, facies, and their relationships with other deep-marine depositional systems.     

Peculiarities of oceanographic processes in the north-western Black Sea

The paper examines how various factors affect the intensity of photosynthesis and oxidation reaction in the north-western part of the Black Sea. In the course of the evolution of the basin, natural aspects were widely changing; therefore their role is evaluated by the content of organic matter and diatoms in the bottom sediments. The influence of anthropogenic factors is determined by the current tendencies in the variation of physico-chemical and biological characteristics of the basin. The main factor upsetting steady functioning of the north-western Black Sea ecosystem is the large biogenic influx to the sea. The possible changing of the ecosystem during the nearest few decades is assessed. Translated by Vladimir A. Puchkin.     

Contourite drift construction influenced by capture of Mediterranean Outflow Water deep-sea current by the Portimão submarine canyon (Gulf of Cadiz, South Portugal)

The margin of the Gulf of Cadiz is swept by the deep current formed by the Mediterranean Outflow Water (MOW) flowing from the Mediterranean to the Atlantic. On the northern margin of the Gulf (Algarve Margin, South Portugal), the MOW intensity is low and fine-grained contourite drifts are built up with an alongslope development. From new sedimentological data, this study emphasizes the presence of two types of contourite drifts separated only by a deep submarine canyon incising the slope with a north-south orientation (Portimão Canyon). High-resolution seismic and bathymetry interpretation shows that on the eastern side of the canyon, the MOW forms a thick and large detached drift (Albufeira Drift) prograding toward both north and west, as shown in seismic profiles, with a high sedimentation rate. On this side of the canyon, the MOW intensity is high enough to erode the slope forming a moat channel (Alvarez Cabral). On the western side of Portimão Canyon, the MOW energy is lower, preventing moat channel erosion. Only flat and thin drift develops (Portimão and Lagos Drifts) with slow aggradation and a low sedimentation rate. This difference in drift development is due to the presence of the canyon which generates an important change in hydrodynamic of the MOW, confirmed by temperature-density measurements showing that MOW flows down Portimão Canyon. The canyon is responsible for the deviation of the direction of the MOW as it partly catches the deep-sea current flowing westward (i.e. capture phenomenon). It creates, thus, a decrease of the flow energy, competency and capacity between the east and west sides of the canyon. Through this phenomenon of MOW deep-sea current capture, the canyon constitutes a morphologic feature generating an important change in the contourite deposition pattern.In addition to already known climatic and oceanographic influences, our results show the role of canyons on contourite drift building. This study provides new elements on autocyclic factors influencing the contourite sedimentation, which are important to consider in future sedimentary paleo-reconstruction interpretations.     

Late Pleistocene-Holocene sediments on the Spanish continental shelves: Model for very high resolution sequence stratigraphy

Sequence stratigraphy analysis of high resolution seismic profiles (Geopulse, Uniboom and 3.5 kHz) of late Pleistocene-Holocene sediments has been carried out on five sectors of the Spanish continental margin. Four types of depositional settings are distinguished in these sectors: (1) low subsident ramps (Alborán Margin-Cádiz Gulf); (2) high subsident (2m/kyr) ramps (Alicante-Valencia); (3) “Ria”-type morphology on the Atlantic passive margin (Ria de Muros); and (4) fault-scarp morphology systems with subsidence (Balearic Margin). A Type 1 sequence is interpreted in all these sectors, being composed of lowstand systems tract, transgressive systems tract and highstand systems tract. This conforms to the basic concepts of sequence stratigraphy and each systems tract correlates with a particular part of the last eustatic hemicycle. Characteristic shelf features such as terraces, terraces with beach deposits and progradational sediment wedges evidence a complex stacking of lesser sub-sequences in all the systems tracts, which must be related to very-short period sea-level stillstands and fall. We propose a very high resolution sequence stratigraphy model in which the last sea-level hemicycle is punctuated by: “P” cycles (4500 years), which give rise to the neo-glacial events; “h” cycles (2200-950 years), and “c” cycles (500-50 years). These cycles interact with each other, thus establishing the placing of the high and low sea levels.

This attention to detail: (1) explains sedimentary evolution on both the shelf and upper slope during late Pleistocene-Holocene time; (2) illustrates some departures from the classical sequence stratigraphy model; and (3) also demonstrates that the late Pleistocene-Holocene eustatic curve is not one simple transgression but is modulated by three differing-period cycle groups below the Milankovitch band. Our model is delimited by fluctuating sea level during Pleistocene-Holocene times. Such features should be identifiable on any continental margin. However, localized features occur due to subsidence and continental shelf morphology which determ ine the location and depth of sedimentary bodies generated in each eustatic cycle.