519 research outputs found
Studies of deep-sea sedimentary microtopography in the North Atlantic Ocean
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the
Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January, 1978Many of the small-scale topographic features (dimensions
of centimeters to kilometers) found on the Blake-Bahama Outer
Ridge (western North Atiantic, water depth greater than 4000 m)
and in the Rockall Trough (northeastern North Atlantic, water
depth greater than 2000 m) have been formed as bed forms of
deep currents. These bed forms, all developed in cohesive sediments,
include current ripples (spacings of tens of centimeters,
formed transverse to the flow), longitudinal triangular ripples
(spacings of meters, formed in sandy muds and parallel to the
flow), furrows (spacings of tens to 100's of meters, formed
parallel to the flow and presently either erosional or depositional),
and regular sediment waves (spacings of a few kilometers,
now found oblique to the flow and migrating either
upstream or downstream). The local distribution of any given
bed form is influenced by the presence of larger features. Bed
forms are often found in zones which strike parallel to the
regional contours.
Debris flows, affecting areas of 1000's to 10,000's of
square kilometers, are also present in these areas. A debris
flow studied in the Rockall Trough is erosional at its shallowest
depths and depositional at greater depths. Gravitational flows strike perpendicular to the contours. Pockmarks (tens
of meters in diameter, marking fluid seeps) are also found on
the Blake-Bahama Outer Ridge.
The larger topographic features (greater than several
meters) with steep slopes (greater than about 20°) can be
observed on surface echo-sounding profiles either as fields of
regular hyperbolic echoes (e.g., echoes from regularly spaced
furrows), fields of irregularly spaced, dissimilar hyperbolae
(e.g., echoes from blocks, ridges, and folds in debris flows),
or as regular features whose structure is often obscured by
side echoes (e.g., echoes from sediment waves). Although near-bottom investigations are required to describe the features,
the nature of the sea floor can often be inferred from the
character of the echo-sounding profile. Similar echo-sounding
records in different areas of the ocean indicate the presence
of similar sea-floor features.
The morphology of the bed forms studied and the current
and temperature structure of the overlying water column lead
to conclusions about bed form origin and present-day interactions
with deep currents.
Furrows form as erosional bed forms during high-velocity
(>20? cm/sec) current events by large, helical secondary
circulations in the bottom boundary layer. Once formed, furrows
may develop into depositional features, or they may continue
as erosional ones, depending on the local currents and the
sediment supply.
Large, regular sediment waves may be formed at current
speeds of 5 to 10 cm/sec by lee waves generated by topographic
irregularities on the sea floor, such as submarine canyons,
or by instabilities in the flow of deep, contour-following
currents. Sediment waves develop where there is an abundant
supply of sediment and steady mean currents. Waves appear to
migrate upstream where tidal current fluctuations are smaller
than the mean velocity, and downstream where they are larger.
Near-bottom currents appear to be faster on the downstream side
of upstream-migrating sediment waves than on their upstream
side. The resulting variations in bed shear stress lead to
higher sedimentation rates on the upstream side and bed form
migration in that direction.This research was made possible by National Science Foundation grants DES 73-06657 and OCE 76-22152, and Office
of Naval Research contract N00014-74-C-0262; NR083-004 to Woods Hole Oceanographic Institution, NSF grant OCE 74-01671
to Lamont-Doherty Geological Observatory, and numerous NSF
grants and ONR contracts to Scripps Institution of Oceanography
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Contourite or turbidite?: magnetic fabric of fine-grained Quaternary sediments, Nova Scotia continental rise
Samples of three piston cores and one gravity core from the Nova Scotia continental rise (depths 4210-4925 m) have been examined to differentiate parallel-to-slope and downslope depositional processes in Quaternary deposits from a region presently influenced by a strong contour current. Measurement of anisotropy of magnetic susceptibility of samples of a red-brown, silt-laminated lutite 'contourite' facies shows grain alignments which are consistent with both parallel-to-slope (contour current) flow and downslope (turbidity current) flow. We believe that these results provide support for the hypothesis that 'geologically significant' contour currents have influenced continental rise deposition during the Pleistocene. However, our observation that both alongslope and downslope alignments are present in lithologically similar units clearly demonstrates the need for studies on the relationship between lithofacies and process in this geological setting
Haverstraw Bay Benthic Habitat Characterization
High-resolution backscatter and bathymetric maps created by multibeam and sidescan sonar surveys were used to identify five different seafloor bottom types within Haverstraw Bay. Grab samples were collected within these areas to characterize sediment properties and macrofauna. Selected sampling locations were revisited and seafloor images were obtained with an HD underwater camera. Multivariate analysis was used to identify the most important factors explaining variations in community structure. Results indicated that categorical variables defining bottom types, grain size, and water depth can explain about 42% of community structure variation. In addition, shell length data collected for Rangia cuneata, an introduced species, indicated that successful spawning and recruitment occurred for this species during 2011, 2012, and 2013. An attempt to relate 2012-2014 hydrophone location data for Atlantic and Shortnose sturgeon to identified bottom types did not produce clear bottom preferences
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The magnetic fabric of surficial deep-sea sediments in the HEBBLE area (Nova Scotian continental rise)
The anisotropy of magnetic susceptibility (AMS) of surficial sediments in the HEBBLE area (depth 4820 m, Nova Scotian continental rise) has been measured to study the fabric of these sediments and to relate the sediment fabric to depositional conditions. Previous investigators have established that strong but variable bottom currents are important agents in redistributing bottom sediments in the area. We observe that the upper 1.5 cm of the sediments has a primary fabric (dominantly foliar with horizontal bedding) created by bottom-current activity while sediments deeper than 2.5 cm have a predominantly secondary fabric due to bioturbation. The magnitude of the AMS signal in these sediments is generally low making it difficult to measure some of the fabric elements, especially the direction of the lineated fabric. In the upper 1.5 cm magnetic grains are predominantly oriented parallel to the regional bathymetric contours, but there is also a significant grouping of alignments perpendicular to the contours. Primary fabrics are observed in a few of the samples from the bioturbated sediments with the magnetic grains oriented parallel to the contours. Some of the variability observed between box cores in the magnetic fabric may reflect the variability of depositional conditions within the region. An analysis of the magnitude of the AMS fabric suggests that the parameter Fs, often used to denote bottom-current fabrics, is strongly dependent on the total anisotropy and only weakly dependent on the lineated nature of the fabric. Both primary and secondary fabrics can have relatively large Fs values. Thus Fs does not appear sufficient for delineating bottom current activity in the deep sea
The impact of Hurricane Sandy on the shoreface and inner shelf of Fire Island, New York : large bedform migration but limited erosion
© The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution.. The definitive version was published in Continental Shelf Research 98 (2015): 13-25, doi:10.1016/j.csr.2015.03.001.We investigate the impact of superstorm Sandy on the lower shoreface and inner shelf
offshore the barrier island system of Fire Island, NY using before-and-after surveys involving
swath bathymetry, backscatter and CHIRP acoustic reflection data. As sea level rises over the
long term, the shoreface and inner shelf are eroded as barrier islands migrate landward; large
storms like Sandy are thought to be a primary driver of this largely evolutionary process. The
“before” data were collected in 2011 by the U.S. Geological Survey as part of a long-term
investigation of the Fire Island barrier system. The “after” data were collected in January, 2013,
~two months after the storm. Surprisingly, no widespread erosional event was observed. Rather,
the primary impact of Sandy on the shoreface and inner shelf was to force migration of major
bedforms (sand ridges and sorted bedforms) 10’s of meters WSW alongshore, decreasing in
migration distance with increasing water depth. Although greater in rate, this migratory behavior
is no different than observations made over the 15-year span prior to the 2011 survey.
Stratigraphic observations of buried, offshore-thinning fluvial channels indicate that long-term
erosion of older sediments is focused in water depths ranging from the base of the shoreface
(~13-16 m) to ~21 m on the inner shelf, which is coincident with the range of depth over which
sand ridges and sorted bedforms migrated in response to Sandy. We hypothesize that bedform
migration regulates erosion over these water depths and controls the formation of a widely
observed transgressive ravinement; focusing erosion of older material occurs at the base of the
stoss (upcurrent) flank of the bedforms. Secondary storm impacts include the formation of
ephemeral hummocky bedforms and the deposition of a mud event layer.This work was funded primarily by a rapid response grant from the Jackson
School of Geosciences, The University of Texas/Austi
IODP workshop: developing scientific drilling proposals for the Argentina Passive Volcanic Continental Margin (APVCM) – basin evolution, deep biosphere, hydrates, sediment dynamics and ocean evolution
The Argentine margin contains important sedimentological, paleontological and chemical records of regional and local tectonic evolution, sea level, climate evolution and ocean circulation since the opening of the South Atlantic in the Late Jurassic–Early Cretaceous as well as the present-day results of post-depositional chemical and biological alteration. Despite its important location, which underlies the exchange of southern- and northern-sourced water masses, the Argentine margin has not been investigated in detail using scientific drilling techniques, perhaps because the margin has the reputation of being erosional. However, a number of papers published since 2009 have reported new high-resolution and/or multichannel seismic surveys, often combined with multi-beam bathymetric data, which show the common occurrence of layered sediments and prominent sediment drifts on the Argentine and adjacent Uruguayan margins. There has also been significant progress in studying the climatic records in surficial and near-surface sediments recovered in sediment cores from the Argentine margin.
Encouraged by these recent results, our 3.5-day IODP (International Ocean Discovery Program) workshop in Buenos Aires (8–11 September 2015) focused on opportunities for scientific drilling on the Atlantic margin of Argentina, which lies beneath a key portion of the global ocean conveyor belt of thermohaline circulation. Significant opportunities exist to study the tectonic evolution, paleoceanography and stratigraphy, sedimentology, and biosphere and geochemistry of this margin.Facultad de Ciencias Naturales y Muse
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Integrative Acoustic Mapping Reveals Hudson River Sediment Processes and Habitats
Rivers and estuaries around the world are the focus of human settlements and activities. Needs for clean water, ecosystem preservation, commercial navigation, industrial development, and recreational access compete for the use of estuaries, and management of these resources requires a detailed understanding of estuarine morphology and sediment dynamics. This article presents an overview of the first estuary-wide study of a heavily used estuary, the Hudson River, based on high-resolution acoustic mapping of the river bottom. The integration of three high-resolution acoustic methods with extensive sampling reveals an unexpected complexity of bottom features and allows detailed classification of the benthic environment in terms of riverbed morphology, sediment type, and sedimentary processes
IODP workshop: developing scientific drilling proposals for the Argentina Passive Volcanic Continental Margin (APVCM) – basin evolution, deep biosphere, hydrates, sediment dynamics and ocean evolution
The Argentine margin contains important sedimentological, paleontological and chemical records of regional and local tectonic evolution, sea level, climate evolution and ocean circulation since the opening of the South Atlantic in the Late Jurassic–Early Cretaceous as well as the present-day results of post-depositional chemical and biological alteration. Despite its important location, which underlies the exchange of southern- and northern-sourced water masses, the Argentine margin has not been investigated in detail using scientific drilling techniques, perhaps because the margin has the reputation of being erosional. However, a number of papers published since 2009 have reported new high-resolution and/or multichannel seismic surveys, often combined with multi-beam bathymetric data, which show the common occurrence of layered sediments and prominent sediment drifts on the Argentine and adjacent Uruguayan margins. There has also been significant progress in studying the climatic records in surficial and near-surface sediments recovered in sediment cores from the Argentine margin.
Encouraged by these recent results, our 3.5-day IODP (International Ocean Discovery Program) workshop in Buenos Aires (8–11 September 2015) focused on opportunities for scientific drilling on the Atlantic margin of Argentina, which lies beneath a key portion of the global ocean conveyor belt of thermohaline circulation. Significant opportunities exist to study the tectonic evolution, paleoceanography and stratigraphy, sedimentology, and biosphere and geochemistry of this margin.Facultad de Ciencias Naturales y Muse
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Integrated Reconnaissance of the Physical and Biogeochemical Characteristics of Jamaica Bay
Researchers at The Earth Institute of Columbia University have carried out an integrated, coordinated pilot reconnaissance of the physical, chemical, geological, and biological systems within Jamaica Bay, entitled "Integrated Reconnaissance of the Physical and Biogeochemical Characteristics of Jamaica Bay." We believe that such an integrated approach is necessary to fully understand the complex inter-relationship of the wetland ecosystem. The program focused on obtaining a synergistic view of the varied elements of Jamaica Bay by carrying out coordinated research in four areas: submarine sediment morphology, sediment and soil sampling, circulation and mixing, and chemical analysis of the Bay waters
A reference time scale for Site U1385 (Shackleton Site) on the SW Iberian Margin
We produced a composite depth scale and chronology for Site U1385 on the SW Iberian Margin. Using log(Ca/Ti) measured by core scanning XRF at 1-cm resolution in all holes, a composite section was constructed to 166.5 meters composite depth (mcd) that corrects for stretching and squeezing in each core. Oxygen isotopes of benthic foraminifera were correlated to a stacked δ^18O reference signal (LR04) to produce an oxygen isotope stratigraphy and age model.
Variations in sediment color contain very strong precession signals at Site U1385, and the amplitude modulation of these cycles provides a powerful tool for developing an orbitally-tuned age model. We tuned the U1385 record by correlating peaks in L* to the local summer insolation maxima at 37^oN. The benthic δ^18O record of Site U1385, when placed on the tuned age model, generally agrees with other time scales within their respective chronologic uncertainties.
The age model is transferred to down-core data to produce a continuous time series of log(Ca/Ti) that reflect relative changes of biogenic carbonate and detrital sediment. Biogenic carbonate increases during interglacial and interstadial climate states and decreases during glacial and stadial periods. Much of the variance in the log(Ca/Ti) is explained by a linear combination of orbital frequencies (precession, tilt and eccentricity), whereas the residual signal reflects suborbital climate variability. The strong correlation between suborbital log(Ca/Ti) variability and Greenland temperature over the last glacial cycle at Site U1385 suggests that this signal can be used as a proxy for millennial-scale climate variability over the past 1.5 Ma.
Millennial climate variability, as expressed by log(Ca/Ti) at Site U1385, was a persistent feature of glacial climates over the past 1.5 Ma, including glacial periods of the early Pleistocene (‘41-kyr world’) when boundary conditions differed significantly from those of the late Pleistocene (‘100-kyr world’). Suborbital variability was suppressed during interglacial stages and enhanced during glacial periods, especially when benthic δ^18O surpassed ~ 3.3-3.5‰. Each glacial inception was marked by appearance of strong millennial variability and each deglaciation was preceded by a terminal stadial event. Suborbital variability may be a symptomatic feature of glacial climate or, alternatively, may play a more active role in the inception and/or termination of glacial cycles.This research was supported by the Natural Environmental Research Council Grant NE/K005804/1 to DH and LS and NE/J017922/1 to DH.This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.gloplacha.2015.07.00
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