1,720,981 research outputs found
Submarine channel flow processes and deposits: a process-product perspective
No full textProcess-product studies have been central to the development of process sedimentology over the past few decades, with the ability to first measure flows, and then examine the resulting deposits, removing much of the ambiguity associated with previous interpretations. However, perhaps uniquely for large geomorphic systems on Earth, there are no field-scale process-product studies of submarine channels. In fact, there are remarkably few direct measurements even of the flow dynamics as a result of the difficulties of measuring these powerful, infrequent, and often inaccessible flows. Over the past decade, physical experimentation has provided the first process-product studies for model submarine channel systems, enabling us to link flow behaviour and sedimentation patterns. This has been supplemented by numerical simulations, particularly of submarine channel flow dynamics. Here for the first time, we synthesise these observations, in the context of our direct knowledge of submarine channels, to derive an overview of submarine channel flow dynamics, and process-orientated intra-channel architecture models for low and high latitude systems. In addition, we propose new models for the development and evolution of point bars and for inner bend sedimentary accumulations that can comprise point bars overlain by finer-grained oblique accretion deposits. The work reveals a rich range of flow behaviour and associated sedimentation patterns in submarine channels that are far more complex than in fluvial systems
Swept away by a turbidity current in Mendocino submarine canyon, California
No full textWe present unique observations and measurements of a dilute turbidity current made with a remotely operated vehicle in 400?m water depth near the head of Mendocino Canyon, California. The flow had a two-layer structure with a thin (0.5 to 30?m), relatively dense (<0.04?vol %) and fast (up to ~1.7?m/s) wedge-shaped lower layer overlain by a thicker (up to 89?m) more dilute and slower current. The fast moving lower layer lagged the slow moving, dilute flow front by 14?min, which we infer resulted from the interaction of two initial pulses. The two layers were strongly coupled, and the sharp interface between the layers was characterized by a wave-like instability. This is the first field-scale data from a turbidity current to show (i) the complex dynamics of the head of a turbidity current and (ii) the presence of multiple layers within the same event
Can turbidites be used to reconstruct a paleoearthquake record for the central Sumatran margin: Reply
Full text linkOn how thin submarine flows transported large volumes of sand for hundreds of kilometres across a flat basin plain without eroding the seafloor
Full text linkSubmarine gravity currents, especially long run-out flows that reach the deep ocean, are exceptionally difficult to monitor in action, hence there is a need to reconstruct how these flows behave from their deposits. This study mapped five individual flow deposits (beds) across the Agadir Basin, offshore north-west Africa. This is the only dataset where bed shape, internal distribution of lithofacies, changes in grain size and seafloor gradient, bed volumes, flow thickness and depth of erosion into underlying hemipelagic mud are known for individual beds. Some flows were 30 to 120 m thick. However, flows with the highest fraction of sand were less than 5 to 14 m thick. Sand was most likely to be carried in the lower 5 to 7 m of these flows. Despite being relatively thin, one flow was capable of transporting very large volumes of sediment (ca 200 km3) for large distances across very flat seafloor. These observations show that these relatively thin flows could travel quickly enough on very low gradients (0.02º to 0.05º) to suspend sand several metres to tens of metres above the sea floor, and maintain those speeds for up to 250 km across the basin. Near uniform hemipelagic mud interval thickness between beds, and coccolith assemblages in the mud caps of beds, suggest that the flows did not erode significantly into the underlying seafloor mud. Simple calculations imply that some flows, especially in the proximal part of the basin, were powerful enough to have eroded hemipelagic mud if it was exposed to the flow. This suggests that the flows were depositional from the moment they arrived at a basin plain location, and deposition shielded the underlying hemipelagic mud from erosion. Reproducing the field observations outlined in this exceptionally detailed field dataset is a challenge for future experimental and numerical models
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The structure of the deposit produced by sedimentation of polydisperse suspensions
No full textTo interpret the deposits from particle-laden flows it is necessary to understand particle settling at their base. In this paper a quantitative model is developed that not only captures how particles settle out of suspension but also the composition of the final deposit in terms of its vertical distribution of grain sizes. The theoretical model is validated by comparison to published experimental data that has been used to interpret the field deposits of submarine sediment-laden flows (Amy et al., 2006). The model explains two intriguing features of the experimental deposits that are also observed in natural deposits. First, deposits commonly have an ungraded, or poorly normally graded, region overlain by a strongly normally graded region. Second, the normalized thickness of the ungraded region increases as the initial concentration of the suspension is increased. In the theoretical model, the poorly normally graded region results from a constant mass flux into the bed that persists until the largest grain size present within the flow has been completely deposited. The effect of increasing the concentration of the initial suspension is to increase the thickness of the poorly graded part of the deposit and to decrease its average grain size. This work suggests that deposits with relatively thick, poorly graded bases can form from relatively high-concentration polydisperse suspensions, when the initial volume fraction of sediment is greater than approximately 20% and indicates that it is important to include these hindered settling effects in models of depositing flows
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Cretaceous–Paleogene boundary exposed: Campeche Escarpment, Gulf of Mexico
No full textWe present the first multibeam bathymetric maps of the Campeche Escarpment, a Mesozoic carbonate platform in the Gulf of Mexico, which represents the closest Cretaceous–Paleogene (K-Pg) boundary outcrops to the Chicxulub impact structure. The impact of an extraterrestrial-body ~ 65 million years ago on top of this platform is implicated in the end of the Cretaceous mass extinction and caused the largest debris flow yet described on Earth, which is found across the floor of the Gulf of Mexico and the Caribbean Sea. The location of the K-Pg boundary has been identified in the escarpment face by combining the new multibeam data with existing information from boreholes. The boundary is represented by an abrupt change in gradient on the escarpment face. The morphology of the escarpment combined with seismic data reveals that a significant amount of material is missing from the face, which failed catastrophically due to seismic shaking produced by the impact. The escarpment face is inferred to be an important source for the extensive debris flows triggered by the impact, whose deposits are found throughout much of the Gulf of Mexico
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