Global monitoring data shows grain size controls turbidity current structure

The first detailed measurements from active turbidity currents have been made in the last few years, at multiple sites worldwide. These data allow us to investigate the factors that control the structure of these flows. By analyzing the temporal evolution of the maximum velocity of turbidity currents at different sites, we aim to understand whether there are distinct types of flow, or if a continuum exists between end-members; and to investigate the physical controls on the different types of observed flow. Our results show that the evolution of the maximum velocity of turbidity currents falls between two end-members. Either the events show a rapid peak in velocity followed by an exponential decay or, flows continue at a plateau-like, near constant velocity. Our analysis suggests that rather than triggers or system input type, flow structure is primarily governed by the grain size of the sediment available for incorporation into the flow

Determining the controls on flow behaviour, bedform development and stratigraphic architecture from detailed surveys and monitoring of active submarine channels

Seafloor-hugging flows, known as turbidity currents, transport sediment from shallow to deep water via submarine channels. These flows carry globally important volumes of sediment, and transport organic carbon, oxygenated waters, nutrients and contaminants that accumulate within submarine channels and at their downslope terminal lobes or submarine fans. The often-powerful nature of turbidity currents poses a significant hazard to critical seafloor infrastructure. Previous studies have largely relied upon the study of ancient deposits or scaled-down measurements of laboratory-scale flows to understand turbidity currents. Several conceptual models exist, but it remains unclear as to whether turbidity currents show a distinct behaviour at different scales, or if a continuum of behaviour exists from small to large events. Recent technological advances allow us to investigate these issues. The advent of Autonomous Underwater Vehicles enables mapping of the seafloor at unprecedented detail, repeat surveys record previously-unseen seascape changes, while Acoustic Doppler Current Profilers record the range of internal structures observed in field-scale turbidity currents for the first time. In this thesis, I use high-resolution data acquired in several modern offshore systems to analyse turbidity current behaviour across various spatial and temporal scales. First, a global analysis of direct velocity measurements of turbidity currents reveals two end-member modes of turbidity current behaviour that range from: i) a sudden peak in velocity that decays exponentially, lasting minutes to hours; to ii) sustained flow that lasts for days. I show that a continuum exists between these flow modes; likely controlled by the proportion of sand or mud within the flow. Second, an extensive (65 x 50 km) and detailed (5 m bin size) seafloor survey offshore East Africa, reveals a variety of bedforms within two deep-sea canyons. Morphometric analysis reveals a continuum from small-scale (10s m wavelength) crescentic bedforms to large-scale (kms wavelength) sediment waves. This continuum is in contrast with a previous global study, but that study did include such high-resolution deep-water data. Previous studies may have missed an intermediate scale of bedform due to the decreasing resolution with increasing water depth-related. Small to medium-scale bedforms may be more common in the deep sea than currently thought. Third, I analyse repeat mapping of an active submarine delta to reveal how turbidity currents build stratigraphy. As a result of the reworking caused by repeated flows, the completeness of the stratigraphy record over three months is found to be 10% on average. The stratigraphic record is dominated by large events. Large slope failures are more likely to be preserved than smaller bedforms, while erosion is dominated by rare, but powerful turbidity currents that can obscure the record of smaller flows.I conclude that a continuum in turbidity current behaviour exists across various scales of flow, from small fjord channel systems to the largest submarine channels on the planet. The mode of flow is dominantly controlled by the grain size of the sediment available in the system. The continuum in bedform scales reflects both the downstream evolution of turbidity currents, as they expand due to mixing with ambient seawater and entrainment of seafloor sediment, and modifications caused by seafloor morphology. What becomes recorded in stratigraphy does not show a gradual continuum, however, and instead appears to be strongly biased by larger but infrequent events. These findings from modern systems provide new insights to inform the understanding of ancient depositional records and have implications for assessing seafloor hazards and understanding deep-sea sediment transport in general

Quantifying the three‐dimensional stratigraphic expression of cyclic steps by integrating seafloor and deep‐water outcrop observations

Deep‐water deposits are important archives of Earth’s history including the occurrence of powerful flow events and the transfer of large volumes of terrestrial detritus into the world’s oceans. However the interpretation of depositional processes and palaeoflow conditions from the deep‐water sedimentary record has been limited due to a lack of direct observations from modern depositional systems. Recent seafloor studies have resulted in novel findings, including the presence of upslope‐migrating bedforms such as cyclic steps formed by supercritical turbidity currents that produce distinct depositional signatures. This study builds on process to product relationships for cyclic steps using modern and ancient datasets by providing sedimentological and quantitative, three‐dimensional architectural analyses of their deposits, which are required for recognition and palaeoflow interpretations of sedimentary structures in the rock record. Repeat‐bathymetric surveys from two modern environments (Squamish prodelta, Canada, and Monterey Canyon, USA) were used to examine the stratigraphic evolution connected with relatively small‐scale (average 40 to 55 m wavelengths and 1.5 to 3.0 m wave heights) upslope‐migrating bedforms interpreted to be cyclic steps within submarine channels and lobes. These results are integrated to interpret a succession of Late Cretaceous Nanaimo Group deep‐water slope deposits exposed on Gabriola Island, Canada. Similar deposit dimensions, facies and architecture are observed in all datasets, which span different turbidite‐dominated settings (prodelta, upper submarine canyon and deep‐water slope) and timescales (days, years or thousands of years). Bedform deposits are typically tens of metres long/wide, <1 m thick and make up successions of low‐angle, backstepping trough‐shaped lenses composed of massive sands/sandstones. These results support process‐based relationships for these deposits, associated with similar cyclic step bedforms formed by turbidity currents with dense basal layers under low‐aggradation conditions. Modern to ancient comparisons reveal the stratigraphic expression of globally prevalent, small‐scale, sandy upslope‐migrating bedforms on the seafloor, which can be applied to enhance palaeoenvironmental interpretations and understand long‐term preservation from ancient deep‐water deposits

Rykalová, Gabriela (2009): Entwicklung in der Tagespresse : dargestellt an journalistischen Textsorten der deutschsprachigen Zeitungen

Submarine channels have been important throughout geologic time for feeding globally significant volumes of sediment from land to the deep sea. Modern observations show that submarine channels can be sculpted by supercritical turbidity currents (seafloor sediment flows) that can generate upstream-migrating bedforms with a crescentic planform. In order to accurately interpret supercritical flows and depositional environments in the geologic record, it is important to be able to recognize the depositional signature of crescentic bedforms. Field geologists commonly link scour fills containing massive sands to crescentic bedforms, whereas models of turbidity currents produce deposits dominated by back-stepping beds. Here we reconcile this apparent contradiction by presenting the most detailed study yet that combines direct flow observations, time-lapse seabed mapping, and sediment cores, thus providing the link from flow process to depositional product. These data were collected within the proximal part of a submarine channel on the Squamish Delta, Canada. We demonstrate that bedform migration initially produces back-stepping beds of sand. However, these back-stepping beds are partially eroded by further bedform migration during subsequent flows, resulting in scour fills containing massive sand. As a result, our observations better match the depositional architecture of upstream-migrating bedforms produced by fluvial models, despite the fact that they formed beneath turbidity currents

First source-to-sink monitoring shows dense head controls sediment flux and runout in turbidity currents

Until recently, despite being one of the most important sediment transport phenomena on Earth, few direct measurements of turbidity currents existed. Consequently, their structure and evolution were poorly understood, particularly whether they are dense or dilute. Here, we analyze the largest number of turbidity currents monitored to date from source to sink. We show sediment transport and internal flow characteristic evolution as they runout. Observed frontal regions (heads) are fast (>1.5 m/s), thin (<10 m), dense (depth averaged concentrations up to 38%vol), strongly stratified, and dominated by grain-to-grain interactions, or slower (<1 m/s), dilute (<0.01%vol), and well mixed with turbulence supporting sediment. Between these end-members, a transitional flow head exists. Flow bodies are typically thick, slow, dilute, and well mixed. Flows with dense heads stretch and bulk up with dense heads transporting up to 1000 times more sediment than the dilute body. Dense heads can therefore control turbidity current sediment transport and runout into the deep sea

First source-to-sink monitoring shows dense head controls sediment flux and runout in turbidity currents

Until recently, despite being one of the most important sediment transport phenomena on Earth, few direct measurements of turbidity currents existed. Consequently, their structure and evolution were poorly understood, particularly whether they are dense or dilute. Here, we analyze the largest number of turbidity currents monitored to date from source to sink. We show sediment transport and internal flow characteristic evolution as they runout. Observed frontal regions (heads) are fast (>1.5 m/s), thin (<10 m), dense (depth averaged concentrations up to 38% vol), strongly stratified, and dominated by grain-to-grain interactions, or slower (<1 m/s), dilute (<0.01% vol), and well mixed with turbulence supporting sediment. Between these end-members, a transitional flow head exists. Flow bodies are typically thick, slow, dilute, and well mixed. Flows with dense heads stretch and bulk up with dense heads transporting up to 1000 times more sediment than the dilute body. Dense heads can therefore control turbidity current sediment transport and runout into the deep sea

Survey of neonatal respiratory care and surfactant administration in very preterm infants in the Italian neonatal network

Introduction: Variation of respiratory care is described between centers around the world.The Italian Neonatal Network (INN), as a national group of the Vermont-Oxford Network (VON) allows to perform a wide analysis of respiratory care in very low birth weight infants. Methods:We analyzed the dataset of infants enrolled in the INN in 2009 and 2010 and, for surfactant administration only, from 2006 to 2010 from 83 participating centers. All definitions are those of the (VON). A questionnaire analysis was also performed with a questionnaire on centers practices. Results: We report data for 8297 infants. Data on ventilator practices and outcomes are outlined. Variation for both practices and outcome is found. Trend in surfactant administration is also analyzed. Conclusions. The great variation across hospitals in all the surveyed techniques points to the possibility of implementing potentially better practices with the aim of reducing unwanted variation. These data also show the power of large neonatal networks in identifying areas for potential improvement. \ua9 Mattioli 1885

Survey of neonatal respiratory care and surfactant administration in very preterm infants in the Italian neonatal network

Introduction: Variation of respiratory care is described between centers around the world.The Italian Neonatal Network (INN), as a national group of the Vermont-Oxford Network (VON) allows to perform a wide analysis of respiratory care in very low birth weight infants. Methods:We analyzed the dataset of infants enrolled in the INN in 2009 and 2010 and, for surfactant administration only, from 2006 to 2010 from 83 participating centers. All definitions are those of the (VON). A questionnaire analysis was also performed with a questionnaire on centers practices. Results: We report data for 8297 infants. Data on ventilator practices and outcomes are outlined. Variation for both practices and outcome is found. Trend in surfactant administration is also analyzed. Conclusions. The great variation across hospitals in all the surveyed techniques points to the possibility of implementing potentially better practices with the aim of reducing unwanted variation. These data also show the power of large neonatal networks in identifying areas for potential improvement. © Mattioli 1885