Search

The NW Iberian continental margin is characterised by a complex morphology and by a sedimentary system which was highly dynamic over glacial to interglacial times. The sedimentary history of the continental slope was strongly influenced by the interaction of bottom currents with topographic highs of structural origin leading to the accumulation of several sediment drifts. A combined analysis of gravity cores from different water depth with hydroacoustic data reveals the vertical behaviour of the upper Mediterranean Outflow Water (MOW) core after the Last Glacial Maximum (LGM). A coarser grained interval during Deglacial and early Holocene times (17.2 to 9.9 cal ka BP) points to an increase in bottom current strength. This increase in velocity was probably related to oceanic density fronts, which migrated through the 300 m thick transition zone between the underlying Labrador Sea Water and the overlying MOW. Radiocarbon dates timed the current strengthening to 17.2 cal ka BP, and a following weakening of the bottom current to 13.3 cal ka BP at 1965 m water depth and to 9.9 cal ka BP at 1885 m water depth. The depth-dependent current weakening suggests an upward shifting of the transition zone by 80 m that was related either to an overall shallowing of MOW or a vertical contraction of this water mass. The upward movement happened over a time interval of approximately 3.4 thousand years. In addition sediment core analysis reveals significant lateral heterogeneities within cm to dm thick sediment layers in the contourite drift. These heterogeneities suggest a need of a detailed core coverage across current-influenced deposits for palaeoceanographic studies to minimize misinterpretations.

The Río de la Plata estuary (RdlP) and adjacent continental shelf exhibit complex hydrological processes as a result of a highly variable fluvial discharge and associated terrigenous supply, which are primarily controlled by regional climatic forcing in interaction with the oceanographic system. Previous hydrological studies indicate that Little Ice Age and Medieval Climate Anomaly resulted in increased/decreased river runoff due to an enhanced/weakened precipitation regime. This study aims to analyze the cyclicity in the runoff-related continental supply and the associated climatic oscillation behind it over the past 1,000 cal yr BP. To achieve this, we used a 10-m-long sediment core retrieved from the RdlP mud depocenter, which exhibits an extremely high and fairly constant linear sedimentation rate of 1.1 cm yr-1. We performed continuous 1-cm XRF element intensity scans and performed time series analysis on Fe/K, Ti/Al, Fe/Ca, Ti/Ca and Si/Al ratios, considered as regional proxies for inferring fluvial supply. The most significant cyclicities occur at 215, 192, 115, 100, 49, 47, 37, 35, and 2.5-8 yrs recurrence times. The long-term cyclicity is persistent throughout the whole record for all ratios, and it is probably related to solar forcing, i.e., the 200 yr Vries/Suess cycle, which determines the activity of the South American Monsoon System activity. The shorter climate-related cyclicities (˂100 yr) are probably related to the Atlantic Multidecadal Oscillation, the Pacific Decadal Oscillation and the El Niño Southern Oscillation climatic modes, and were registered only for Ti/Al, Fe/K and Si/Al ratios. We further identified an intensification of such shorter climate-related cyclicities over the past 500 yr BP. This study provides new evidence that both tropical Pacific and Atlantic SSTs and solar forcing are the main drivers of changes in rainfall over Southeastern South America on interannual to decadal, multidecadal and centennial scales, and thus modulates the RdlP river discharge. ResumoO estuário do Río de la Plata (RdlP) e a plataforma continental adjacente exibem processos hidrológicos complexos, controlados principalmente pela forçante climática regional em interação com o sistema oceanográfico, o que resulta em grande variabilidade da descarga fluvial e do fornecimento de materiais terrígenos ao sistema oceânico. Estudos hidrológicos anteriores indicam que a Pequena Idade do Gelo e a Anomalia Climática Medieval resultaram no aumento / diminuição do escoamento do rio devido a um regime de precipitação aumentado/ enfraquecido. Este estudo tem como objetivo analisar a ciclicidade no fornecimento continental condicionada por oscilações climáticas nos últimos 1.000 anos e com mudanças no caudal do RdlP. Com este objetivo, foi estudado um testemunho de sedimentos, com 10 m de comprimento, obtido no depósito lodoso do RdlP. O testemunho apresenta uma taxa de acumulação extremamente alta e razoavelmente constante de 1,1 cm ano-1. Foram efetuadas análises elementares de XRF, com resolução de 1 cm, das razões Fe/K, Ti/Al, Fe/Ca, Ti/Ca e Si/Al, consideradas como proxies regionais para inferência de suprimento fluvial. As ciclicidades de recorrência mais significativas ocorreram há 215, 192, 115, 100, 49, 47, 37, 35 e 2,5-8 anos. A ciclicidade mais longas, persentes ao longo de todo o registro, estão provavelmente relacionadas com o forçamento solar, ou seja, com o ciclo de 200 anos de Vries/Suess, que determina a atividade do Sistema de Monção Sul-Americana. As ciclidades mais curtas (˂100 anos) estão provavelmente relacionadas com a Oscilação Multidecadal Atlântica, Oscilação Decadal do Pacífico e fenômeno El Niño-Oscilação Sul, tendo sido registrados por mudanças nos valores das razões Ti/Al, Fe/K e Si/Al. Além disso, identificámos uma intensificação de tais ciclicidades, relacionadas com o clima, nos últimos 500 anos BP. Este estudo fornece novas evidências de que tanto as temperaturas superficiais do oceano Pacífico e Atlântico tropicais, como o forçamento solar, são as causas principais responsáveis pelas mudanças na precipitação no sudeste da América do Sul, á escala interanual a decadal, multidecadal e centenária, e modulam a descarga do Río de la Plata.

In order to strengthen and update knowledge on climatic, environmental and oceanographic changes on the inner Uruguayan continental shelf, w e studied pollen, dinocyst and other non-pollen palynomorph assemblages on the sediment core GeoB13813-4. This core was taken from the mud depocenter and its remarkable high sedimentation rate for the last ca. 1000 yr cal BP reflects the high terrigenous supply from the Rio de la Plata (RdlP). Most pollen and spores are originated from the RdlP grasslands, the vegetation that covers the lower reaches of the La Plata Drainage Basin (LPDB). They mainly represent the regionally dominant grasslands, but also the riparian forests, Butia yatay palm populations, and the herbaceous-bushy marshes around the mouth of the estuary. Pollen from salt marshes, Atlantic rainforest, and Araucaria forests located in southern Brazil reached the study site, probably transported by coastal ocean currents, whereas pollen from Andean regions would represent a long-distance transport by wind. Changes in both proportion and concentration of freshwater and marine palynomorphs indicate variability in freshwater input to the inner Uruguayan shelf. From ca. 1000 to 230 yr cal BP, significant marine influence of Subtropical Shelf Waters (STSW) was inferred, which was diluted by the freshwater supply from the Uruguayan mainland. This time interval was interrupted between ca. 690 to 575 yr cal BP by an increased freshwater contribution to the study area under the influence of Subantarctic Shelf Waters (SASW). From ca. 230 to 25 yr cal BP, a strong influence of RdlP waters was detected, only followed by another phase of dominant STSW during the past century. Such changes were related to regional climatic variability, i.e., Medieval Climate Anomaly, Little Ice Age and Current Warm Period. After ca. 1960 AD, the anthropogenic impact within the LPDB was clearly evidenced both by eutrophication and the first occurrence of Pinus pollen. Resumo A fim de melhorar e atualizar o conhecimento sobre mudancas climaticas, ambientais e oceanograficas na Plataforma Continental Uruguaia, foi estudado o polen, os dinocistos e outras associacoes de palinomorfos nao polinicos no testemunho de sedimentos GeoB13813-4. Este testemunho foi recolhido no deposito lodoso e a sua taxa de sedimentacao durante os ultimos ≈1000 anos antes do presente (BP; before present ) reflete o elevado contributo terrigeno do Rio de la Plata (RdlP) a area de estudo. Os resultados obtidos revelam que a maioria dos polenes e esporos e originaria das pastagens e da vegetacao que cobre o curso inferior da Bacia de Drenagem do Rio de la Plata (LPDB). Representam principalmente as pastagens, as quais sao regionalmente dominantes, mas tambem matas ciliares, populacoes da palmeira Butia yatay e sapais herbaceos ao redor da foz do estuario. Polen de sapais, Mata Atlântica e florestas de Araucaria localizadas no sul do Brasil chegaram ao local de estudo, provavelmente transportados por correntes oceânicas costeiras. O polen das regioes andinas podera representar um transporte de longa distância pelo vento. Mudancas na proporcao e concentracao de palinomorfos de agua doce e marinhos indicam variabilidade na entrada de agua doce na plataforma continental interna uruguaia. Entre ≈1000-230 anos BP, os dados indicam a ocorrencia de uma significativa influencia marinha das Aguas Subtropicais da Plataforma (STSW), misturadas com agua doce do continente uruguaio. Este intervalo foi marcado, entre ≈690-575 anos BP, com uma maior contribuicao de agua doce para a area de estudo sob a influencia de Aguas Subantarticas (SASW). Entre ≈230-25 anos BP, foi detectada uma forte influencia das aguas do RdlP. Esta fase foi seguida, durante o seculo passado, por outra em que as STSW foram dominantes. Tais mudancas poderao ter sido causadas pela variabilidade climatica regional, ou seja, Anomalia Climatica Medieval, Pequena Idade do Gelo e Periodo Quente Atual. Desde ≈1960, o impacto antropogenico no LPDB esta claramente evidenciado tanto pela eutrofizacao quanto pela primeira ocorrencia de polen de Pinus .

High-resolution sediment echosounder data combined with radiocarbon-dated sediment cores allowed us to reconstruct the Late Quaternary stratigraphic architecture of the Kveithola Trough and surrounding Spitsbergenbanken. The deposits display the successive deglacial retreat of the Svalbard-Barents Sea Ice Sheet. Basal subglacial till indicates that the grounded ice sheet covered both bank and trough during the Late Weichselian. A glaciomarine blanket inside the trough coinciding with laminated plumites on the bank formed during the initial ice-melting phase from at least 16.1 to 13.5 cal ka BP in close proximity to the ice margin. After the establishment of open-marine conditions at around 13.5 cal ka BP, a sediment drift developed in the confined setting of the Kveithola Trough, contemporary with crudely laminated mud, an overlying lag deposit, and modern bioclastic-rich sand on Spitsbergenbanken. The Kveithola Drift shows a remarkable grain-size coarsening from the moat towards the southern flank of the trough. This trend contradicts the concept of a separated drift (which would imply coarser grain sizes in proximity of the moat) and indicates that the southern bank is the main sediment source for the coarse material building up the Kveithola Drift. This depocenter represents, therefore, a yet undescribed combination of off-bank wedge and confined drift. Although the deposits inside Kveithola Trough and on Spitsbergenbanken display different depocenter geometries, time-equivalent grain-size changes imply a region-wide sediment-dynamic connection. We thus relate a phase of coarsest sediment supply (8.8–6.3 cal ka BP) to an increase in bottom current strength, which might be related to a stronger Atlantic Water inflow from the Southeast across the bank leading to winnowing and off-bank export of sandy sediments.

Rebesco, Michele ... et al.-- Special Issue: PAST Gateways (Palaeo-Arctic Spatial and Temporal Gateways).-- 16 pages, 12 figures, 2 tables, supplementary data http://dx.doi.org/10.1016/j.quascirev.2016.02.007, Kveithola is a glacially-carved, E-W trending trough located in the NW Barents Sea, an epicontinental shelf sea of the Arctic Ocean located off northern Norway and Russia. A set of confined sediment drifts (the “Kveithola Drift”) is located in the inner part of the trough. In general, drift deposits are commonly characterized by high lateral continuity, restricted occurrence of hiatuses and relatively high accumulation rates, and thus represent excellent repositories of paleo-environmental information. We provide for the first time a detailed morphological and seismostratigraphic insight into this sediment drift, which is further supported by some preliminary lithological and sedimentological analyses. The complex morphology of the drift, imaged by combining all available multibeam data, includes a main and a minor drift body, two drift lenses in the outer part of the trough, more or less connected drift patches in the innermost part and small perched sediment patches in a structurally-controlled channel to the north. The seismic (PARASOUND) data show that the main and minor drift bodies are mainly well-stratified, characterized by sub-parallel reflections of moderate to high amplitude and good lateral continuity. The reflectors show an abrupt pinch-out on the northern edge where a distinct moat is present, and a gradual tapering to the south. Internally we identify the base of the drift and four internal horizons, which we correlate throughout the drift. Two units display high amplitude reflectors, marked lensoidal character and restricted lateral extent, suggesting the occurrence of more energetic sedimentary conditions. Facies typical for contourite deposition are found in the sediment cores, with strongly bioturbated sediments and abundant silty/sandy mottles that contain shell fragments. These characteristics, along with the morphological and seismic information, suggest a strong control by a bottom current flowing along the moat on the northern edge of the drift. Though both Atlantic and Arctic waters are known to enter the trough, from the west and the north respectively, brine-enriched shelf water (BSW) produced during winter and flowing westward in the moat, is suggested to be responsible for the genesis of the Kveithola Drift. The formation of BSW is inferred to have started around 13 cal ka BP, the onset of drift deposition, suggesting that conditions leading to atmospheric cooling of the surface waters and/or the presence of coastal polynyas and wind or floating ice shelves have persisted on the western Barents Shelf since that time. The units inferred to have been deposited under more energetic sedimentary conditions (tentatively dated to the Younger Dryas and to 8.9–8.2 cal ka BP) are suggestive of stronger BSW formation. In general, we infer that variations in the bottom current regime were mainly related to BSW formation due to atmospheric changes. They could also have been a response to successive episodes of grounded and sea ice retreat that allowed for a first limited, later open shelf current, which progressively established on the western Barents Sea shelf, The research cruise MSM30 CORIBAR and this study were partly funded through the MARUM DFG-Research Center/Cluster of Excellence “The Ocean in the Earth System” as part of MARUM project SD-2. This study contributes to the IPY initiative 367 NICESTREAM (Neogene Ice Streams and Sedimentary Processes on High- Latitude Continental Margins). The work was funded by the Italian projects OGS-EGLACOM, PNRA-CORIBAR-IT (PdR 2013/C2.01), ARCA (grant n. 25_11_2013_973) and PNRA-VALFLU, by the Council of Norway through its Centres of Excellence funding scheme (project number 223259), by the Spanish projects DEGLABAR (CTM2010-17386) and CORIBAR-ES (CTM2011-14807-E) funded by the “Ministerio de Economia y Competitividad”. The “Generalitat de Catalunya” is acknowledged for support through an excellence research group grant (2014SGR940). J.L. was funded by an FPI grant BES-2011-043614

Mud accumulates on continental shelves under a variety of environmental conditions and results in a diverse formation of mud depocenters (MDCs). Their three-dimensional architectures have been in the focus of several recent studies. Due to some terminological confusion concerning MDCs, the present study sets out to define eight individual MDC types in terms of surface sediment distribution and internal geometry. Under conditions of substantial sediment supply, prodeltas (distal zones off river deltas; triangular sheets), subaqueous deltas (disconnected from deltas by strong normal-to-shore currents; wedge-like clinoforms), and mud patches (scattered distribution) and mud blankets (widespread covers) are formed. Forced by hydrodynamic conditions, mud belts in the strict sense (detached from source; elongated bodies), and shallow-water contourite drifts (detached from source; growing normal to prevailing current direction; triangular clinoforms) develop. Controlled by local morphology, mud entrapments (in depressions, behind morphological steps) and mud wedges (triangular clinoforms growing in flow direction) are deposited. Shelf mud deposition took place (1) during early outer-shelf drowning (~14 ka), (2) after inner-shelf inundation to maximum flooding (9.5–6.5 ka), and (3) in sub-recent times (

The Mar del Plata (MdP) Canyon at the Argentine continental margin is incorporated into a major contourite depositional system, built by the incursion of southern-sourced water masses affecting the seafloor at different waters depths. The new sedimentological, morphological and hydro acoustic data provide novel insights into contour and turbidity current interactions in mid-slope (blind) canyons, which do not have a connection to the shelf or an onshore river system. Such canyons are capable to record climate-related ocean stratification changes, current variability, and slope stability. Three sediment cores were obtained along the MdP Canyon thalweg covering the last 20,000 years and compiled with two cores from the adjacent Ewing Terrace. Turbidity-current activity within the MdP Canyon was limited to the time interval from Last Glacial Maximum (LGM) to the late deglacial. During the LGM and early deglacial, turbidites reached both the proximal sector and the distal northern flank of the canyon. During the late deglacial only the proximal sector was characterized by turbidite deposition. Similarities in mineralogy and grain-size data indicate that the material transported by the turbidity currents originated from the mid-slope Ewing Terrace. Glacial turbidity-current activity was most probably favored by increased sediment supply along the Ewing Terrace from a shallowed and/or enhanced glacial Antarctic Intermediate Water (AAIW) nepheloid layer. These sediments were trapped by the MdP Canyon, in particular at the head area. During the late deglacial, a displacement or limited AAIW nepheloid layer resulted in less sediment transfer along the Ewing Terrace and immediate accumulation in the MdP Canyon head restricting turbidite deposition to the proximal sector of the canyon. In general, contourite-turbidite interactions provide valuable information on variations in thermohaline circulation such as AAIW distribution and current strength.

Continental shelves represent areas of highest economical and ecological importance. Nevertheless, these sedimentary systems remain poorly understood due to a complex interplay of various factors and processes which results in highly individual construction schemes. Previous studies of sedimentary shelf systems have mainly focused on a limited number of cores, retrieved from Holocene fine-grained depocentres. As such, the relation between shelf architecture and sedimentary history remains largely obscure. Here, we present new data from the NW Iberian shelf comprising shallow-seismic profiles, a large number of sediment cores, and an extended set of radiocarbon dates to reveal the Late Quaternary evolution of a low-accumulation shelf system in detail. On the NW Iberian shelf, three main seismic units are identified. These overly a prominent erosional unconformity on top of the basement. The lowermost Unit 1 is composed of maximal 75-m thick, Late Tertiary to Pleistocene deposits. The youngest sediments of this unit are related to the last glacial sea-level fall. Unit 2 was controlled by the deglacial sea-level rise and shows a maximum thickness of 15 m. Finally, Unit 3 comprises deposits related to the late stage of sea-level rise and the modern sea-level highstand with a thickness of 4 m in mid-shelf position. Two pronounced seismic reflectors separate these main units from each other. Their origin is related to (1) exposure and ravinement processes during lower sea level, and (2) to reworking and re-deposition of coarse sediments during subsequent sea-level rise. According to the sediment core ground-truthing, sediments of the Late Tertiary to Pleistocene unit predominantly display homogenous fine sands with exceptional occurrences of palaeosols that indicate an ancient exposure surface. Fine sands which were deposited in the run of the last sea-level rise show a time-transgressive retrogradational development. The seismic reflectors, bounding the individual units, appear in the cores as 0.1 to 1-m thick deposits consisting either of shell gravels or siliceous coarse sands with gravels. The modern sea-level highstand stage is characterised by zonal deposition of mud forming a mud belt in mid-shelf position, and sediment starvation on outer shelf zones. Radiocarbon ages indicate that this mud belt was the main depocentre for river-supplied fine material on the NW Iberian shelf at least over the past 5.32 ka BP. The initial onset of this depocentre is proposed to be related to a shift in the balance between rate of sea-level rise and amount of terrigenous sediment supply. Various other stratigraphical shelf reconstructions reveal analogies in architecture which indicate that timing and shaping of the individual units on low-accumulation shelves is fundamentally controlled by eustatic sea-level changes. Other factors of local importance such as differential elevation of the basement and the presence of morphological barriers formed by rocky outcrops on the seafloor have additionally modifying influence on the sedimentary processes.

The sedimentation on the Mauritanian shelf off the western Saharan desert is strongly influenced by dust supply and is investigated in this study by combining surface mapping, shallow-acoustic profiling and the analysis of sediment cores. Coarse-grained carbonates dominate wide areas of the open shelf at places where the shelf is exposed to the Atlantic Ocean. The sheltered Golfe d'Arguin, in contrast, is characterized by muddy shoals in its inner area, and locally restricted and wedge-shaped muddy depocenters which extend contour-parallel in mid-shelf settings. In both cases, cross-shelf currents control sediment distribution and accumulation, not location or amount of primary eolian input. A major discontinuity on the outer shelf is located in 85–110 m water plus sediment depth and is refilled by a sedimentary facies succession of shoreface, tidal and open-shelf deposits. This succession has formed during temporary lower sea levels along the last overall regression (MIS-4 and MIS-3), the Last Glacial Maximum, and during the past deglacial sea-level rise. Significant sediment deposition and preservation is limited to very restricted areas. Concerning the overall shelf architecture, sea level was the main factor enabling or suppressing deposition in terms of 1) creating local erosional swales which have favored subsequent trapping and preservation of sediments; 2) determining the type of sedimentary facies refilling these depressions under a variety of nearshore conditions; 3) interacting with the seafloor morphology and, by this, controlling timing of initiation and growth geometry of mud wedges. These mud wedges have formed since ca. 9 cal. ka under a continuous sedimentation regime. These bodies must have formed in hydraulic equilibrium but they are detached from a defined sediment point source which is in contrast to common mudbelt formation on shelves. In general, sediment storage on shelves under hyperarid conditions seems to be very limited due to comparably low sediment input, strong ravinement activity during changing sea levels, significant bypassing and winnowing processes, and a low preservation potential of the non-cohesive materials. These conditions lead to the development of almost sediment-starved systems with low subsidence rates over longer geological timescales.

The continental margin off the La Plata Estuary (SE South America) is characterized by high fluvial sediment supply and strong ocean currents. High-resolution sediment-acoustic data combined with sedimentary facies analysis, AMS-14C ages, and neodymium isotopic data allowed us to reconstruct late Quaternary sedimentary dynamics in relation to the two major sediment sources, the La Plata Estuary and the Argentine margin. Sediments from these two provinces show completely different dispersal patterns. We show that the northward-trending La Plata paleo-valley was the sole transit path for the huge volumes of fluvial material during lower sea levels. In contrast, material from the Argentine margin sector was transported northwards by the strong current system. Despite the large sediment volumes supplied by both sources, wide parts of the shelf were characterized by either persistent non-deposition or local short-term depocenter formation. The location and formation history of these depocenters were primarily controlled by the interplay of sea level with current strength and local morphology. The high sediment supply was of secondary importance to the stratigraphic construction, though locally resulting in high sedimentation rates. Thus, the shelf system off the La Plata Estuary can be considered as a hydrodynamic-controlled end-member.