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1.
Geoacoustic and physical properties of carbonate sediments of the Lower Florida Keys 总被引:4,自引:0,他引:4
Near-surface sediment geoacoustic and physical properties were measured from a variety of unconsolidated carbonate sediments
in the Lower Florida Keys. Surficial values of compressional and shear speed correlate with sediment physical properties and
near-surface acoustic reflectivity. Highest speeds (shear 125–150 m s-1; compressional 1670–1725 m s-1) are from sandy sediments near Rebecca Shoal and lowest speeds (shear 40–65 m s-1; compressional 1520–1570 m s-1) are found in soft, silty sediments which collect in sediment ponds in the Southeast Channel of the Dry Tortugas. High compressional
wave attenuation is attributed to scattering of acoustic waves from heterogeneity caused by accumulation of abundant shell
material and other impedance discontinuities rather than high intrinsic attenuation. Compared to siliciclastic sediments,
carbonate sediment shear wave speed is high for comparable values of sediment physical properties. Sediment fabric, rather
than changes due to the effects of biogeochemical processes, is responsible for these differences. 相似文献
2.
We study the distribution of Fe, Mn, Co, Ni, Cu, Zn, Cd, Hg, Ag, and Au in present-day and ancient bottom sediments in the
shelf, slope, and deep-trench areas of the northeast part of the Black Sea. The samples were taken in 1990 in the Crimean,
Kerch-Taman, and Caucasian regions. The revealed character of the fields of concentration of metals does not allow one to
establish exact regularities in the distribution of Mn, Co, and Cu with Fe. It is shown that the values of the ratios of concentrations
of metals increase in the following direction: Au/Cu → Zn/Cu → Co/Fe → Ni/Fe → Mn/Fe → Hg/Cu → Ag/Cu → Cu/Fe. The data on
Ag are presented for the first time. 相似文献
3.
Box cores were collected close to river mouths along the eastern Brazilian shelf at water depths of 10–30 m. One core was
taken from more than 1000 m depth at the shelf slope. 210Pb and 226Ra activities were measured to establish sediment accumulation rates. Seven of the 10 cores exhibited an exponential decrease
with depth of excess 210Pb activities. The sediments from the sheltered Sudeste Channel off Caravelas revealed the highest sediment accumulation rate
of 0.81 cm yr−1. The sediments at the shelf slope seaward of the Rio Doce revealed the lowest accumulation rate of 0.13 cm yr−1. Sediment accumulation rates increased towards the Caravelas Bank. Current patterns and the morphology of the seabed favor
sediment deposition in this area. 相似文献
4.
Sedimentary processes on an estuarine marsh island within the turbidity maximum zone of the Yangtze River mouth 总被引:5,自引:0,他引:5
Sedimentary characteristics and processes on an estuarine marsh island in the Yangtze Estuary (East China Sea) show that
(1) the mean grain size of the surficial sediments varies between 0.003 and 0.16 mm, the finest sediment being more enriched
in the marsh center particularly under calm weather conditions during the flood season, and the coarsest sediment being more
enriched on the low seaward tidal flats particularly after storms; (2) the sediment organic carbon content is generally less
than 1.4 wt%; (3) the annual vertical marsh accretion is in the range of 20 cm, with a maximum value of 40 cm in the middle
of the marsh; and (4) the seasonal cycle in plant growth strongly influences sediment grain size and erosion-accretion events.
Received: 6 January 1999 / Revision accepted: 11 April 2000 相似文献
5.
Holocene accumulation of organic carbon at the Laptev Sea continental margin (Arctic Ocean): sources, pathways, and sinks 总被引:1,自引:0,他引:1
Composition and accumulation rates of organic carbon in Holocene sediments provided data to calculate an organic carbon budget
for the Laptev Sea continental margin. Mean Holocene accumulation rates in the inner Laptev Sea vary between 0.14 and 2.7 g C cm−2 ky−1; maximum values occur close to the Lena River delta. Seawards, the mean accumulation rates decrease from 0.43 to 0.02 g C cm−2 ky−1. The organic matter is predominantly of terrigenous origin. About 0.9 × 106 t year−1 of organic carbon are buried in the Laptev Sea, and 0.25 × 106 t year−1 on the continental slope. Between about 8.5 and 9 ka, major changes in supply of terrigenous and marine organic carbon occur,
related to changes in coastal erosion, Siberian river discharge, and/or Atlantic water inflow along the Eurasian continental
margin.
Received: 26 October 1998 / Revision accepted: 15 June 1999 相似文献
6.
Relationships between sedimentary features and the distribution and structure of Posidonia oceanica meadows were investigated in the Gulf of Oristano (Sardinia) in a study site stretching from the mouth of the Tirso River
in a more sheltered, inner sector of the gulf to an outer, more exposed sector near the rocky Cape San Marco. The results
demonstrate that gradients in sediment texture and composition can be related to meadow distribution and areal coverage. We
suggest that this involves the mechanical trapping of fine particles by the plants, enhanced mud deposition due to dampening
of wave action in the meadows, and the admixture of bioclastic sediments linked to increased production by epiphytes and invertebrates.
Independent evidence of seagrass erosion was recorded in an unvegetated breach in the meadow. The mass of fine sediment which
would be resuspended in the wake of meadow destruction was estimated at 30–90 × 103 tonnes km−2, an amount which could substantially influence water quality and ecosystem stability in the gulf.
Received: 27 May 1999 / Revision accepted: 2 February 2000 相似文献
7.
Correlation of side-scan backscatter intensity with grain-size distribution of shelf sediments, New Jersey margin 总被引:2,自引:0,他引:2
The dependence of acoustic backscatter variations on sediment grain-size distribution is examined using a combined analysis
of 95 kHz side-scan data and approx. 300 grab samples from the New Jersey margin. We show that, under well-controlled circumstances,
an unprecedented degree of correlation can be obtained between backscatter intensity and mean grain size. However, backscatter
is disproportionately affected by larger grain sizes. Addition of just a few extra weight percentage of the larger grain sizes
(>4 mm), which in our study is typically represented by more abundant shell hash among the older sediments at greater water
depths, can completely degrade this correlation.
Received: 29 September 1999 / Revision accepted: 14 February 2000 相似文献
8.
Akira Ijiri Urumu Tsunogai Toshitaka Gamo Fumiko Nakagawa Tatsuhiko Sakamoto Saneatsu Saito 《Geo-Marine Letters》2009,29(5):301-308
Substantial amounts of adsorbed methane were detected in authigenic carbonate concretions recovered from sedimentary layers
from depths between 245 and 1,108 m below seafloor during Ocean Drilling Program Leg 186 to ODP sites 1150 and 1151 on the
deep-sea terrace of the Japan Trench. Methane contents were almost two orders of magnitude higher in the concretions (291–4,528
nmol/g wet wt) than in the surrounding bulk sediments (5–93 nmol/g wet wt), whereas methane/ethane ratios and stable carbon
isotopic compositions were very similar. Carbonate content of surrounding bulk sediments (0.02–3.2 wet wt%) and methane content
of the surrounding bulk sediments correlated positively. Extrapolation of the carbonate contents of bulk sediments suggests
that 100 wt% carbonate would correspond to 1,886±732 nmol methane per g bulk sediment, which is similar to the average value
observed in the carbonate concretions (1,321±1,067 nmol/g wet wt, n = 13). These data support the hypothesis that, in sediments, adsorbed hydrocarbon gases are strongly associated with authigenic
carbonates. 相似文献
9.
Caroline Schmidt Claudia Hanfland Pierre Regnier Philippe Van Cappellen Michael Schl��ter Ulrich Knauthe Ingrid Stimac Walter Geibert 《Geo-Marine Letters》2011,31(4):259-269
Activities of the naturally occurring radium nuclides 228Ra, 226Ra, 224Ra and 223Ra were determined in waters of the open German Bight and adjacent nearshore areas in the North Sea, in order to explore the
potential use of radium isotopes as natural tracers of land–ocean interaction in an environment characterised by extensive
tidal flats, as well as riverine and groundwater influx. Data collected at various tidal phases from the Weser Estuary (228Ra: 46.3 ± 4.6; 226Ra: 17.1 ± 1.1; 224Ra: 26.1 ± 8.2 to 36.5 ± 6.1; 223Ra: 1.8 ± 0.1 to 4.0 ± 0.4), tidal flats near Sahlenburg (228Ra: 39.3 ± 3.8 to 46.0 ± 4.5; 226Ra: 15.5 ± 1.5 to 16.5 ± 1.7; 224Ra: 34.3 ± 2.2 to 85.3 ± 6.3; 223Ra: 3.6 ± 0.5 to 8.0 ± 1.2), freshwater seeps on tidal flats near Sahlenburg (228Ra: 42.1 ± 4.1; 226Ra: 21.3 ± 2.2; 224Ra: 5.1 ± 0.9; 223Ra: 2.6 ± 1.3) and also in permanently inundated parts of the North Sea (228Ra: 23.0 ± 2.3 to 28.2 ± 2.8; 226Ra: 8.2 ± 0.8 to 11.8 ± 1.2; 224Ra: 3.1 ± 1.0 to 10.1 ± 0.9; 223Ra: 0.1 ± 0.02 to 0.9 ± 0.05; units: disintegrations per minute per 100 kg water sample) reveal that, except for the fresh
groundwater, the potential end-members of nearshore water mass mixing have quite similar radium signatures, excluding a simple
discrimination between the sources. However, the decreasing activities of the short-lived 224Ra and 223Ra isotopes recorded towards the island of Helgoland in the central German Bight show a potential to constrain fluxes of land-derived
material to the open North Sea. The largest source for all radium isotopes is generally found on the vast tidal flats and
in the Weser Estuary. Future work could meaningfully combine this so-called radium quartet approach with investigations of
radon activity. Indeed, preliminary data from a tidal flat site with fresh groundwater seepage reveal a 222Rn signal that is clearly lower in seawater. 相似文献
10.
Thomas Leipe Franz Tauber Henry Vallius Joonas Virtasalo Szymon U?cinowicz Nicole Kowalski Sven Hille Susanna Lindgren Tero Myllyvirta 《Geo-Marine Letters》2011,31(3):175-188
In this study, particulate organic carbon (POC) contents and their distribution pattern in surficial sediments of the Baltic
Sea are presented for 1,471 sampling stations. POC contents range from approx. 0.1% in shallow sandy areas up to 16% in deep
muddy basins (e.g. Gotland Basin). Some novel relationships were identified between sediment mass physical properties (dry
bulk density (DBD), grain size) and POC levels. Notably, the highest POC concentrations (about 10–17 mg cm–3) occur in sandy mud to mud (60–100% mud content) with intermediate POC contents of about 3–7% and DBDs of 0.1–0.4 g cm–3. Areas with this range in values seem to represent the optimum conditions for POC accumulation in the Baltic Sea. The maximum
POC contents (8–16%) are found in fluid mud of the central Baltic Sea characterized by extremely low DBDs (<0.1 g cm–3) and moderate POC concentrations (4–7 mg cm–3). Furthermore, sediment mass accumulation rates (MAR), based on 210Pb and 137Cs measurements and available for 303 sites of the Baltic Sea, were used for assessing the spatial distribution of POC burial
rates. Overall, these vary between 14 and 35 g m–2 year–1 in the mud depositional areas and, in total, at least 3.5 (±2.9) Mt POC are buried annually. Distribution patterns of POC
contents and burial rates are not identical for the central Baltic Sea because of the low MAR in this area. The presented
data characterize Baltic Sea sediments as an important sink for organic carbon. Regional differences in organic carbon deposition
can be explained by the origin and transport pathways of POC, as well as the environmental conditions prevailing at the seafloor
(morphology, currents, redox conditions). These findings can serve to improve budget calculations and modelling of the carbon
cycle in this large brackish-water marginal sea. 相似文献
11.
Nina Stark Hendrik Hanff Christian Svenson Verner B. Ernstsen Alice Lefebvre Christian Winter Achim Kopf 《Geo-Marine Letters》2011,31(4):249-258
In-situ geotechnical measurements of surface sediments were carried out along large subaqueous dunes in the Knudedyb tidal
inlet channel in the Danish Wadden Sea using a small free-falling penetrometer. Vertical profiles showed a typical stratification
pattern with a resolution of ∼1 cm depicting a thin surface layer of low sediment strength and a stiffer substratum below
(quasi-static bearing capacity equivalent: 1–3 kPa in the top layer, 20–140 kPa in the underlying sediment; thickness of the
top layer ca. 5–8 cm). Observed variations in the thickness and strength of the surface layer during a tidal cycle were compared
to mean current velocities (measured using an acoustic Doppler current profiler, ADCP), high-resolution bathymetry (based
on multibeam echo sounding, MBES) and qualitative estimates of suspended sediment distributions in the water column (estimated
from ADCP backscatter intensity). The results revealed an ebb dominance in sediment remobilization, and a general accretion
of the bed towards low water. A loose top layer occurred throughout the tidal cycle, likely influenced by bedload transport
and small events of suspended sediment resettlement (thickness: 6 ± 2 cm). Furthermore, this layer showed a significant increase
in thickness (e.g. from 8 cm to 16 cm) related to periods of overall deposition. These findings imply that dynamic penetrometers
can conveniently serve to (1) quantify potentially mobile sediments by determining the thickness of a loose sediment surface
layer, (2) unravel sediment strength development in potentially mobile sediments and (3) identify sediment accumulation. Such
data are an important complement and add a new geotechnical perspective during investigations of sediment remobilization processes
in highly dynamic coastal environments. 相似文献
12.
O. S. Chauhan B. S. Sukhija A. R. Gujar P. Nagabhushanam A. L. Paropkari 《Geo-Marine Letters》2000,20(2):118-122
Down-core variations in illite, chlorite, smectite and kaolinite (the major clays) in two 14C-dated cores collected along the SW continental margin of India show that illite and chlorite have enhanced abundance during
20–17, 12.5, 11–9.5, and 5–4.8 ka b.p., whereas smectite accumulation is higher between 17 and 12.5, and after 9 ka b.p. The climate may have been predominantly arid at 17 (20–17), 12.5, 10.5 (11–9.5), and 4.8 ka b.p. The first three dates correspond to the last glacial maximum, Bolling-Allerod, and Younger Dryas events, respectively. The
SW monsoon was variable between 17 and 15 ka b.p., and it was more stable and intense after the Younger Dryas until about 6 ka b.p.
Received: 2 December 1999 / Revision accepted: 11 April 2000 相似文献
13.
Seventy-five surface and subsurface sediment samples collected on the Amazon continental shelf were analyzed by X-ray diffraction
(XRD) in the grain-size fractions smaller than 2 μm. The groups of clay minerals thus identified were smectite, kaolinite,
illite, and mixed-layer illite/smectite. Calculation of relative abundance shows a predominance of smectite (37%) in the whole
area, followed by kaolinite (27%), illite (23%), and a small percentage of mixed-layer illite/smectite (12%). The distal,
northwest region of the continental shelf is characterized by a high abundance of smectite (41%), high salinity values, and
high sediment accumulation rates (10 cm year−1), whereas the proximal region is characterized by a comparatively low abundance of smectite (30%), low salinity values, and
low sediment accumulation rates (<1 cm year−1). This trend of increasing abundance northwestwards documents a preferential settling of smectite in the distal and more
saline regions of this estuarine system.
Received: 17 September 1999 / Revision accepted: 17 January 2000 相似文献
14.
Sediment transport and morphodynamics of the Douro River estuary 总被引:1,自引:1,他引:0
A combination of dredging data, hydrographic surveys and numerical modelling has been used to assess morphological change
and sediment transport in the Douro River estuary. The system is dominated by sand- and gravel-sized sediments and confined
by resistant rock types. The evolution of the bed in the last 20 years has been strongly influenced by the opening of a navigation
channel. According to the data available to date, the average maintenance dredging volume has been of the order of 0.4 × 106 m3 year−1. Comparisons of hydrographic surveys reveal a rate of volume loss of the same magnitude. Apparently, maintenance dredging
mainly involves local material, transported into the channel from shallower areas of the estuary. The results of numerical
modelling indicate that the sediment transport capacity due to tidal currents is very limited. River flood events increase
the transport capacity by several orders of magnitude, thus playing a critical role in sediment redistribution and supply
to the coast. The average sediment transport capacity is estimated to be of the order of 0.1 × 106 m3 year−1 in most of the estuary and 0.5 × 106 m3 year−1 at the inlet, with a large uncertainty. It is concluded that, if morphological stability is set as an environmental objective,
the dredged material should not be removed from the system but rather be used to nourish the estuarine beaches and the barrier
spit. 相似文献
15.
G. H. Hong S. H. Kim C. S. Chung D.-J. Kang D.-H. Shin H. J. Lee S.-J. Han 《Geo-Marine Letters》1997,17(2):126-132
Recent sediment accumulation rates are 18–230 mg cm-2 yr-1 (0.02–0.2 cm yr-1) based on excess 210Pb activity profiles in the southwestern part of the East Sea (Sea of Japan). Assuming no mixing beneath surface mixed layers,
210Pb-derived sediment accumulation rates are 18–32 mg cm-2 yr-1 in the northern part of the Yamato Ridge and the Ulleung Basin, 29–136 mg cm-2 yr-1 in the Korea Plateau, and 230 mg cm-2 yr-1 in the southern shelf. These values generally agree with long-term sedimentation rates estimated from dated ash layers.
Received: 6 October 1995 / Revision received: 31 May 1996 相似文献
16.
Franck Lartaud Laurent Emmanuel Marc de Rafelis Stephane Pouvreau Maurice Renard 《Geo-Marine Letters》2010,30(1):23-34
Compared to oxygen isotopes, the carbon isotope composition of biogenic carbonates is less commonly used as proxy for palaeoenvironmental
reconstructions because shell δ13C is derived from both dissolved inorganic (seawater) and organic carbon sources (food), and interactions between these two
pools make it difficult to unambiguously identify any independent effect of either. The main purpose of this study was to
demonstrate any direct impact of variable food supply on bivalve shell δ13C signatures, using low/high rations of a 13C-light mixed algal diet fed to 14-month-old (adult) cultured Japanese Crassostrea gigas under otherwise essentially identical in vitro conditions during 3 summer months (May, June and July 2003, seawater temperature
means at 16, 18 and 20 °C respectively) in experimental tanks at the Argenton laboratory along the Brittany Atlantic coast
of France. At a daily ration of 12% (versus 4%) oyster dry weight, the newly grown part of the shells (hinge region) showed
significantly lower δ13C values, by 3.5‰ (high ration: mean of −5.8 ± 1.1‰, n = 10; low ration: mean of −2.3 ± 0.7‰, n = 6; ANOVA Scheffe’s test, p < 0.0001). This can be explained by an enhanced metabolic activity at higher food supply, raising 13C-depleted respiratory CO2 in the extrapallial cavity. Based on these δ13C values and data extracted from the literature, and assuming no carbon isotope fractionation between food and shell, the
proportion of shell metabolic carbon would be 26 ± 7 and 5 ± 5% for the high- and low-ration C. gigas shells respectively; with carbon isotope fractionation (arguably more realistic), the corresponding values would be 69 ± 14
and 24 ± 9%. Both groups of cultured shells exhibited lower δ13C values than did wild oysters from Marennes-Ol éron Bay in the study region, which is not inconsistent with an independent
influence of diet type. Although there was no significant difference between the two food regimes in terms of δ18O shell values (means of 0.1 ± 0.3 and 0.4 ± 0.2‰ at high and low rations respectively, non-significant Scheffe’s test),
a positive δ13C vs. δ18O relationship recorded at high rations supports the interpretation of a progressive temperature-mediated rise in metabolic
activity fuelled by higher food supply (in this case reflecting increased energy investment in reproduction), in terms not
only of δ13C (metabolic signal) but also of δ18O (seawater temperature signal). Overall, whole-shell δ18O trends faithfully recorded summer/winter variations in seawater temperature experienced by the 17-month-old cultured oysters. 相似文献
17.
U. von Rad U. Berner G. Delisle H. Doose-Rolinski N. Fechner P. Linke A. Lückge H. A. Roeser R. Schmaljohann M. Wiedicke SONNE / Scientific Parties M. Block V. Damm J. Erbacher J. Fritsch B. Harazim J. Poggenburg G. Scheeder B. Schreckenberger N. von Mirbach M. Drews S. Walter A. Ali Khan A. Inam M. Tahir A. R. Tabrez A. H. Cheema M. Pervaz M. Ashraf 《Geo-Marine Letters》2000,20(1):10-19
The Makran accretionary complex shows a distinct bottom-simulating reflector, indicating a thick gas-hydrate-bearing horizon
between the deformational front and about 1350 m water depth which seals off the upward flow of gas-charged fluids. A field
of presently inactive mud diapirs with elevations up to 65 m was discovered in the abyssal plain seawards of the deformation
front, suggesting that in the past conditions were favorable for periodic but localized vigorous mud diapirism. Regional destabilization
of the gas hydrate leading to focused flow was observed where deep-penetrating, active faults reach the base of the gas-hydrate
layer, as in a deeply incised submarine canyon (2100–2500 m water depth). At this location we discovered seeps of methane
and H2S-rich fluids associated with chemoautotrophic vent faunas (e.g., Calyptogena sp.). Driven by the accretionary wedge dynamics, the landward part of the gas-hydrate layer below the Makran margin is being
progressively uplifted. Due to reduced hydrostatic pressure and rising ocean bottom-water temperatures, gas hydrates are progressively
destabilized and dissociated into hydrate water, methane and H2S. Sediment temperatures lie outside the methane stability field wherever water depth is less than 800 m. Above this depth,
upward migration of fluids to the seafloor is unimpeded, thus explaining the abundance of randomly distributed gas seeps observed
at water depths of 350 to 800 m.
Received: 14 June 1999 / Revision accepted: 6 February 2000 相似文献
18.
We analyze the space-and-time variability of the meridional heat transport in the North Atlantic. The contribution of various
mechanisms to the integral meridional heat transport (MHT) is estimated. The key role played by the drift transport of the
Tropical Atlantic in the formation of the meridional oceanic heat transport is confirmed. On the basis of the general analysis
of estimations obtained by various authors according to the data accumulated for 1870–2008 and the results of numerical analyses
based on the data of NCEP/NCAR reanalysis, we show that the long-term average meridional drift heat (mass) transport attains its maximum values equal to (1.6 ± 0.1)
PW [(17.4 ± 1.5) Sv] in the vicinity of 12.5°N in the Tropical Atlantic. The contribution of the heat transport caused by
the horizontal Sverdrup circulation to the integral meridional heat transport is maximum in the vicinity of 30° N. On the
average, it is equal to ∼ 40%. In the Subtropical Atlantic, the meridional heat transport varies with a period of ∼ 50–70 yr.
The minimum value of the integral meridional heat transport was attained in the mid-1960s and its maximum value was at attained
at the beginning of the 1990s. The location of the center of Azores pressure maximum makes it possible to conclude that the
intensification of the total meridional heat transport in the Subtropical Atlantic on these time scales is accompanied by
the displacement of the center of the North Subtropical anticyclonic gyre in the southwest direction. 相似文献
19.
A calibration procedure is presented for generating bulk density profiles of intact marine sediment cores using X-ray computed
tomography (CT). Developed using both artificial and natural marine sediment samples, the correlation is strong with an R
2 value of 0.98. Similar experiments with carbonate sediments were also strongly linear, but offset from the terrigenous curve,
illustrating the influence of sediment chemistry on scanner response. Comparison between newly computed densities for both
sediment types with those from individually calibrated cores during our previous CT studies reveals good correspondence between
the data sets.
Received: 22 November 1998 / Revision received: 4 March 1999 相似文献
20.
P. R. Vogt J. Gardner K. Crane E. Sundvor F. Bowles G. Cherkashev 《Geo-Marine Letters》1999,19(1-2):97-110
Numerous small (50- to 300-m-diameter) strong-backscatter objects were imaged on the 1200- to 1350-m deep crest of Vestnesa
Ridge (Fram Strait) and along the 900- to 1000-m deep northeast margin of the Storegga slide valley. Ground-truthing identified
most of these objects as 2- to 10-m-deep pockmarks, developed within soft, acoustically stratified silty clays (typical wet
bulk density: 1400–1600 kg m-3; sound speed: 1480– 1505 m s-1; porosity, 65–75%; shear strength: 5–10 kPa; water content: 80–120%; and thermal conductivity: 0.8–0.9 W m-1 deg C-1 in the top 3 m). Gas wipeouts, enhanced reflectors, and reflector discontinuities indicate recent or ongoing activity, but
the absence of local heat flow anomalies suggests that any upward fluid flows are modest and/or local. 相似文献