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1.
Biologically dominated lower Chesapeake Bay and the physically dominated York River subestuary are contrasted in terms of the dynamics of sediment mixing, strata formation and sea-bed particle residence times. Two lower bay sites were examined; both are located within the bay stem plains and are characterized by muddy sand and an abundance of large, deep-dwelling organisms. X-radiographs indicate extensive biological reworking of sediments, with no long-term preservation of physical stratification.210Pb profiles reveal low sediment accumulation rates at both lower bay sites (<0·1 cm year−1), but significant differences in biological mixing depths (25vs40 cm) and biodiffusivity (>80vs6–30 cm2year−1). In contrast, the York River site, located within a partially-filled palaeochannel, is predominantly mud with a depauperate benthic community dominated by small, short-lived, shallow-dwelling organisms. Although210Pb accumulation rates at the York River site (<0·2 cm year−1) are similar to those measured in the lower bay, there is little bioturbation. In addition, transient bed forms at the York River site form laterally persistent, linear ridges and furrows sub-parallel to the channel, spaced 10–20 m apart. These observations, coupled with evidence of episodic erosion and deposition from radioisotope and porosity profiles, and X-radiographs, suggest that the upper 60–120 cm of the sea-bed are dominated by physical mixing. Deep mixing and low accumulation rates result in long residence times of particles in the mixed upper portion of the sea-bed (102year) at both locations, despite different mixing controls [i.e. biological (diffusive)vsphysical (advective)].  相似文献   

2.
《Marine Chemistry》2001,73(2):125-152
We report here bioturbation and sediment accumulation rates determined from replicate sediment cores at four different sampling sites on the Palos Verdes shelf, Southern California, using bomb fallout and natural radionuclides (137Cs, 239,240Pu, 210Pb, 234Th, and 14C), along with supporting measurements of organic carbon (OC), porosity and granulometry. Present-day particle reworking, on time scales of several months, is restricted to the upper 3 cm, with rates ranging from 13 to 200 cm2/year, as deduced from 234Thxs profiles. There is little evidence that particle reworking reached depths significantly greater than 5 cm. Post-1963 (or post-1971) sediment accumulation rates ranged from 0.7 to 1.4 g/cm2/year (equivalent to 1.1–1.8 cm/year for surficial sediments), as calculated from Pu and Cs isotope profiles, with little change over time or distance from the outfall. Lateral transport of older sediment and multiple sediment sources on the Palos Verdes shelf is suggested from radiocarbon measurements on foraminifera and bulk sedimentary organic matter at two sampling sites, which showed variable, old and refractory sources of OC. Pre-1953 sediments accumulated at rates that were at least 0.4 g/cm2/year (≥0.3 cm/year), based on 210Pbxs dating. Given the abundance of sediment sources to the Palos Verdes shelf, the high sedimentation rates, and shallow particle mixed layers, contaminant-enriched layers should continue to move deeper into the sediments.  相似文献   

3.
《Marine Geology》2006,225(1-4):103-127
This paper examines the spatial and temporal variability in the volumetric sediment balance of Allen Creek marsh, a macro-tidal salt marsh in the Bay of Fundy. The volumetric balance was determined as the balance of inputs of sediments and organic matter via accretion on the marsh surface and outputs of sedimentary material primarily due to erosion of the marsh margin. Changes in marsh surface elevation were measured at 20 buried plates and 3 modified sediment elevation tables from 1996–2002, and detailed margin surveys were conducted in 1997, 1999 and 2001 using a differential global positioning system. Changes in surface area were calculated using GIS overlay analysis and used in conjunction with accretion and erosion data to derive volumetric estimates of gains and losses of sedimentary material in the marsh system.Currently the volumetric sediment balance at Allen Creek marsh is positive. However the processes of erosion and accretion demonstrate seasonal, annual and spatial variability. Inputs to the system include deposition on the marsh surface from sediment laden waters and from ice rafting of sediments. Sediment is deposited onto the marsh surface year round, even during the winter when vegetation cover is sparse, and the amount of deposition in general is not significantly correlated with the frequency of tidal inundations. Based on the data from 1996 to 2002, the mid and high marsh zones experience mean accretion rates of approximately 1.4 cm year− 1 whereas accretion rates in the low marsh region are statistically significantly lower (0.8 cm year− 1). The absolute amount of accretion varies between seasons and from year to year. The main loss to the marsh is through erosion of the marsh margin cliffs which can remove a comparatively large volume of sedimentary material in one mass wasting event and which also decreases the vegetated surface area available for deposition from sediment laden waters. The volume of material removed from the marsh margin almost tripled between 1997 (169 m3) and 2001 (502 m3) following breaching of the side of a tidal creek channel, altering the patterns of margin erosion and deposition in the marsh system. During this time, however, other sheltered areas of the marsh system, such as along the tidal creek banks, showed evidence of new vegetation growth, increasing the amount of vegetated surface area available for deposition.The processes of erosion and deposition on the marsh surface exhibit considerable spatial variability, with different regions of the marsh being more or less sensitive to seasonal variability in the dominant controls influencing sediment deposition and erosion in this system, namely wave activity, vegetation, ice and water depths. A key factor in predicting how a marsh will evolve and respond to a number of different controls, e.g. sea-level rise or reduced sediment supply, is to quantify both accretion of the marsh surface and erosion of the marsh margin, evaluating the marsh system as a volumetric whole. This study demonstrates that a marsh system should be assessed in three dimensions rather than simply as a surface of accumulation. This is particularly important for open coastal marshes exposed to the erosive action of waves.  相似文献   

4.
Profiles of 210Pb and 239 + 240Pu from sediment cores collected throughout Massachusetts Bay (water depths of 36–192 m) are interpreted with the aid of a numerical sediment-mixing model to infer bioturbation depths, rates and processes. The nuclide data suggest extensive bioturbation to depths of 25–35 cm. Roughly half the cores have 210Pb and 239 + 240Pu profiles that decrease monotonically from the surface and are consistent with biodiffusive mixing. Bioturbation rates are reasonably well constrained by these profiles and vary from 0.7 to 40 cm2 yr−1. As a result of this extensive reworking, however, sediment ages cannot be accurately determined from these radionuclides and only upper limits on sedimentation rates (of 0.3 cm yr−1) can be inferred. The other half of the radionuclide profiles are characterized by subsurface maxima in each nuclide, which cannot be reproduced by biodiffusive mixing models. A numerical model is used to demonstrate that mixing caused by organisms that feed at the sediment surface and defecate below the surface can cause the subsurface maxima, as suggested by previous work. The deep penetration depths of excess 210Pb and 239 + 240Pu suggest either that the organisms release material over a range of >15 cm depth or that biodiffusive mixing mediated by other organisms is occurring at depth. Additional constraints from surficial sediment 234Th data suggest that in this half of the cores, the vast majority of the present-day flux of recent, nuclide-bearing material to these core sites is transported over a timescale of a month or more to a depth of a few centimeters below the sediment surface. As a consequence of the complex mixing processes, surface sediments include material spanning a range of ages and will not accurately record recent changes in contaminant deposition.  相似文献   

5.
The role of the Setúbal–Lisbon canyon in accumulation and transport of labile organic matter from the coastal sea and ocean surface water towards the deep sea was assessed by investigating the distribution of organic matter of different quality in sedimentary aggregates and surface sediments of the canyon and adjacent slopes. Total hydrolysable amino acids (THAA) and organic carbon (Corg) were measured from aggregates, and contents of Corg, chlorophyll a (chl a), phaeopigments (phaeo), chloroplastic pigment equivalents (CPE) from sediments. As indices of organic matter (OM) quality THAA:Corg, degradation index (DI), chl a:phaeo, chl a:Corg and C:N ratio were determined. Sediment profiles of chl a and the isotope 210 of lead (210Pb) were used as tracers in a transport model to estimate deposition rates and background levels of the tracers, and sediment mixing rates (Db). Whereas bulk Corg contents of canyon and slope sediments were practically similar at all depths, higher contents of THAA, chl a and CPE, as well as higher THAA:Corg, DI and chl a:Corg, in aggregates and sediments from the upper reaches of the canyon indicate that labile organic matter accumulates in the upper canyon. This is confirmed by higher chl a and 210Pb deposition and Db calculated from the model. Hence, the Setúbal–Lisbon canyon, specially the upper region, acts as a natural trap of organic matter that is transported to the region via lateral transport and vertical settling from primary productivity. Organic matter might be further transported in downward canyon direction via rebound processes. The chl a and 210Pb profiles reveal active sediment mixing by physical processes and/or animal reworking.  相似文献   

6.
Sedimentary records of naturally occurring and fallout-derived radionuclides are widely used as tools for estimating both the ages of recent sediments and rates of sedimentation and bioturbation. Developing these records to the point of data interpretation requires careful sample collection, processing, analysis and data modeling. In this work, we document a number of potential pitfalls that can impact sediment core records and their interpretation. This paper is not intended as an exhaustive treatment of these potential problems. Rather, the emphasis is on potential problems that are not well documented in the literature, as follows: (1) the mere sampling of sediment cores at a resolution that is too coarse can result in an apparent diffusive mixing of the sedimentary record at rates comparable to diffusive bioturbation rates observed in many locations; (2) 210Pb profiles in slowly accumulating sediments can easily be misinterpreted to be driven by sedimentation, when in fact bioturbation is the dominant control. Multiple isotopes of different half lives and/or origin may help to distinguish between these two possible interpretations; (3) apparent mixing can occur due simply to numerical artifacts inherent in the finite difference approximations of the advection diffusion equation used to model sedimentation and bioturbation. Model users need to be aware of this potential problem. Solutions to each of these potential pitfalls are offered to ensure the best possible sediment age estimates and/or sedimentation and bioturbation rates can be obtained.  相似文献   

7.
 Sedimentological studies were undertaken in the Dry Tortugas to examine environmental influences on the formation of sedimentary fabric. 234Th, 210Pb, grain size, porosity, and fabric analyses reveal the presence of a soft, fine-grained, well mixed surface layer underlain by compact, shelly, intensely bioturbated carbonate muds. Vertical zonation of biogenic structures indicates that deep bioturbation is advective and results in transport of fine material to the seabed surface. The impact of physical oceanographic processes appears to be restricted to the surface layer; however, potential for preservation of this layer in the sediment record is low due to intense mixing in deeper sediments. Thus, preserved sediments retain an incomplete record of the dominant benthic processes at the study area.  相似文献   

8.
The behavior of radon in the sea-floor region provides a useful test of theories which describe mass transport in sediments. We have made measurements of Rn-222 and Ra-226 in near-bottom waters and near-surface sediments at the same location. The distribution of radon in sediments can be described by a simplified diagenetic equation when advection, adsorption, and bioturbation are ignored. Sediment measurements show a radon deficit relative to radium emanation. A reasonable balance is found between integrated radon deficit in sediment and radon surplus in the overlying water.In most cores radium increased with depth in the top 10 cm of sediment. This implies that bioturbation and other mixing processes do not homogenize the radium concentration in the zone of diffusive radon loss, and that radium is diffusing out of the sediments.Radon leakage is less than that predicted by previous authors. Radon leakage depends upon the physical distribution of radium in marine sediments. We present a model that predicts leakage of 30–40% for normal deep-sea sediments, in agreement with measured values.Radon surplus in near-bottom waters depends upon the radium distribution, radon leakage, and effective diffusion coefficients. These in turn depend on the properties of the sediment, such as composition, accumulation rate, and porosity. As we learn how these factors interact we may be able to infer sedimentary features from measurements of radon in overlying waters.  相似文献   

9.
We investigated the impact of sediment reworking fauna and hydrodynamics on mobilization and transport of organic matter and fine particles in marine sediments. Experiments were conducted in an annular flume using lugworms (Arenicola marina) as model organisms. The impact of lugworms on sediment characteristics and particle transport was followed through time in sediments experimentally enriched with fine particles (< 63 μm) and organic matter. Parallel experiments were run at low and high water current velocity (11 and 25 cm s− 1) to evaluate the importance of sediment erosion at the sediment–water interface. There was no impact of fauna on sediment composition and particle transport at current velocity below the sediment erosion threshold. At current velocity above the erosion threshold, sediment reworking by lugworms resulted in dramatic particle transport (12 kg dry matter m− 2) to an adjacent particle trap within 56 days. The transported matter was enriched 6–8 times in fine particles and organic matter when compared to the initial sediment. This study suggests that sediment reworking fauna is an important controlling factor for the particle composition of marine sediments. A. marina mediated sediment reworking greatly increases the sediment volume exposed to hydrodynamic forcing at the sediment–water interface, and through sediment resuspension control the content of fine particles and organic matter in the entire reworked sediment layer (> 20 cm depth).  相似文献   

10.
A sediment budget for the Choptank River, one of the three largest estuaries on the eastern shore of Chesapeake Bay, was developed from measurements of sediment carried in upland runoff, shore erosion, sedimentation, and levels of suspended sediments in estuarine waters. Shore erosion was the major source of sediment (340 × 106 kg y?1), contributing seven times more sediment than upland runoff. Low relief, the rural character of the Coastal Plain drainage basin, and the susceptibility of poorly consolidated shoreline materials to erosion contributed to the dominance of shore erosion over runoff as a sediment source. Box modelling indicated a net annual flux (14–44 × 106 kg y?1) of sediment from the Choptank River to Chesapeake Bay. A mass balance estimate of sedimentation, calculated as the difference between total inputs and loss at the mouth of the estuary, (350 × 106 kg y?1) agreed well with an estimate based on 210Pb profiles (340 × 106 kg y?1) measured along the longitudinal axis of the estuary. Lead-210 sedimentation rates correspond to accumulation rates of 1·5–7·9 mm y?1.  相似文献   

11.
Radiocarbon dating has been carried out on three cores from areas of muddy sediments in the N. Irish Sea to estimate rates of sediment accumulation. 14C age profiles of the two eastern basin cores revealed a near-constant age from the sediment surface to the base of the core (12 500±1000 years bp). The 14C age profile of the western basin core revealed a zone of apparent mixing to a depth of 55 cm, underlain by a zone of constant sedimentation rate (0·018 cm y−1) to 160 cm. These data are discussed in relation both to previously reported sedimentological studies of the area and to the authorised discharges of low-level radioactive waste from the Sellafield nuclear fuel reprocessing plant.  相似文献   

12.
The distribution of trace metals in sediments and their exchange between sediments and overlying water is governed by multiple processes including molecular diffusion, bioturbation (porewater advection, porewater mixing, and particle mixing), chemical reactions and adsorption–desorption. To understand these processes and their relative contributions, a one-dimensional model was built, which includes bioturbation and adsorption–desorption processes, to describe the transport of 224Ra. Because 224Ra is adsorbed on MnO2, 224Ra may serve as a proxy for trace metal transport. Three sites were sampled and both dissolved and adsorbed 224Ra were analyzed and modeled to understand the transport and exchange processes. It was found that particle transport of adsorbed 224Ra followed by desorption at the sediment/water interface typically represents the dominant flux. We have further been able to define conditions where the porewater transport for adsorption reactive metals like 224Ra (and other metals) may be out of the sediments whereas the active scavenging of 224Ra from the water column at the sediment water interface via adsorption reactions can result in a flux of 224Ra into the sediment. These processes are both predicted by the model and observed in sediment samples.  相似文献   

13.
《Marine Chemistry》2001,74(4):227-243
The distribution of trace metals in sediments and their exchange between sediments and overlying water is governed by multiple processes including molecular diffusion, bioturbation (porewater advection, porewater mixing, and particle mixing), chemical reactions and adsorption–desorption. To understand these processes and their relative contributions, a one-dimensional model was built, which includes bioturbation and adsorption–desorption processes, to describe the transport of 224Ra. Because 224Ra is adsorbed on MnO2, 224Ra may serve as a proxy for trace metal transport. Three sites were sampled and both dissolved and adsorbed 224Ra were analyzed and modeled to understand the transport and exchange processes. It was found that particle transport of adsorbed 224Ra followed by desorption at the sediment/water interface typically represents the dominant flux. We have further been able to define conditions where the porewater transport for adsorption reactive metals like 224Ra (and other metals) may be out of the sediments whereas the active scavenging of 224Ra from the water column at the sediment water interface via adsorption reactions can result in a flux of 224Ra into the sediment. These processes are both predicted by the model and observed in sediment samples.  相似文献   

14.
Analyses of DSRV “Alvin” core samples on the Cape Hatteras margin indicate major textural and compositional changes at depths of about 1000 and well below 2500 m. The distribution patterns of petrologic parameters correlate well with water mass flow and suspended-sediment plumes measured on this margin by other workers. Our study also shows: (a) vigorous erosion and sediment transport at depths of less than 400 m resulting from the NE-trending Gulf Stream flow; (b) deposition, largely planktonic-rich sediment released from the Gulf Stream, on the upper- to mid-slope, to depths of about 800–1200 m; (c) winnowing, resuspension and deposition induced by periodically intensified slope currents on the mid-slope to uppermost rise, between about 1000 and 2500 m; and (d) prevailing deposition on the upper rise proper (below 2500 m), from transport by the SW-trending Western Boundary Undercurrent. Sediments moved by bottom currents have altered the composition and distribution patterns of material transported downslope by offshelf spillover; this mixing of gravity-emplaced and bottom-current-transported sediment obscures depositional boundaries. Moreover, reworking of the seafloor by benthic organisms alters physical properties and changes erodability of surficial sediments by bottom currents. Measurement of current flow above the seafloor and direct observation of the bottom are insufficient to delineate surficial sediment boundaries. Detailed petrologic analyses are needed to recognize the long-term signature of processes and define depositional provinces.  相似文献   

15.
Fluids discharged from subaerial springs along faults on a sediment diapir near the mouth of the Mississippi River are derived from buried marine pore waters which have been extensively altered chemically by processes of bacterial respiration, mineral precipitation and, possibly, by fractionation due to the presence of clays of high exchange capacity. Vertical mass transport of dissolved components in many shallow marine sediments is controlled by long-term compaction, diffusion and bioturbation. In areas of rapid sediment deposition, however, these processes can be overwhelmed by catastrophic episodes of sediment failure, vertical mixing and upward discharge of water, gas and dissolved species.  相似文献   

16.
The Little Ice Age (∼1600–1900 AD) and 20th century sediment accumulation rates in Billefjorden, a subpolar fjord on Svalbard, were reconstructed by applying 210Pb, 137Cs and AMS 14C datings. The modern sediment accumulation rate decreases from more than 0.39 cm y−1 at the fjord head to 0.08 cm y−1 close to the fjord mouth. However, during the Little Ice Age the sediments accumulated at a much lower rate of 0.02 cm y−1 in the central fjord basin. This difference is most likely related to the rapid retreat of glaciers during the 20th century, when most of them withdrew up to 2 km. The post-Little Ice Age increases in temperature and a negative glacier mass balance resulted in a larger meltwater discharge transferring substantial amounts of sediments released from the glaciers, as well as those eroded from previously stored unconsolidated glacial sediments. A comparison of data from the subpolar fjords of Svalbard suggests that the increase in the sediment accumulation rate is a common trend, and further increases might be expected if climate warming continues. The properties of the fjord sediments (grain size, IRD, coarse-fraction composition, clay mineralogy) from the Little Ice Age and the 20th century showed no distinct differences. The change in the accumulation rate may be the most evident sedimentary record of this climatic change.  相似文献   

17.
The influence of bioturbation on certain aspects of the biogeochemistry of sulfur and iron was examined in shallow-water sediments of Great Bay Estuary, New Hampshire. A bioturbated (JEL) and non-bioturbated (SQUAM) site were compared. Annual sulfate reduction measured with 35S, was 4·5 times more rapid at JEL. A significant portion of this difference was attributed to rapid rates which occurred throughout the upper 12 cm of sediment at JEL due to infaunal reworking activities. Sulfate reduction decreased rapidly with depth at SQUAM. FeS in the upper 2 cm at JEL increased in concentration from 3 to 45 μmol ml−1 from early May to late July while only increasing from 3 to 8 μmol ml−1 at SQUAM. Infaunal irrigation and reworking activities caused rapid and continous subsurface cycling of iron and sulfur at JEL. This maintained dissolved iron concentrations at 160–170 μM throughout the summer despite rapid sulfide production. Therefore, dissolved sulfide never accumulated in JEL pore waters. Although dissolved organic carbon (DOC) was generated during sulfate reduction, bioturbation during summer caused a net removal of DOC from JEL pore waters. Sulfate reduction rates, decomposition stoichiometry and nutrient concentrations were used to calculate turnover times of nutrients in pore waters. Nutrient turnover varied temporally and increased three-to five-fold during bioturbation. A secondary maximum in the abundance of recoverable sulfate-reducing bacteria occurred at 10 cm in JEL sediments only during periods of active bioturbation, demonstrating the influence of macrofaunal activities on bacterial distributions.  相似文献   

18.
Bioturbation is one of the major processes influencing ecosystem functioning. Population parameters such as species density, burrow density and species-specific life modes, determine the impact of bioturbation on the ecosystem. A laboratory experiment was developed, using microcosms mimicking a marine intertidal sediment–water interface which allowed for quantification of different population parameters. The vertical redistribution, bioturbation rate and maximum penetration depth of two sizes (41 and 129 μm) of luminophores were measured in five treatments (control, low density of burrows with and without Corophium (1989 ind./m2), and high density of burrows with and without Corophium (14,921 ind./m2)) after 1, 7 and 14 days. Results indicate that the behavioural activities of Corophium are of the utmost importance in sediment reworking, since they contributed to a five-fold increase in bioturbation rate compared to the passive transport induced by the static structure of the burrows. Furthermore, density is an important parameter because only high densities play a prominent role in particle transport and hence in organic matter processing, while the role of low Corophium densities is limited in sediment reworking. No evidence for differentiation in sediment size fractions was observed. Finally, bioturbation rates in this study were low compared to other studies, and these results suggest an influence of the tidal rhythmicity in the behavioural activity of Corophium on the bioturbation rate.  相似文献   

19.
To quantify recent sediment accumulation, carbon fluxes and cycling, three N.W. European Continental Margin transects on Goban Spur and Meriadzek Terrace were extensively studied by repeated box- and multicore sampling of bottom sediments. The recent sediment distribution and characteristics appear directly related to the near-bed hydrodynamic regime on the margin, which at the upper slope break on the Goban Spur results in along-slope and periodic off-slope directed transport of particles, possibly by entrainment of particles in a detached bottom or intermediate nepheloid layer. From the shelf to the abyssal plain the surface sediments on the Goban Spur change from terrigenous sandy shelf sediments into clayey silts. 210Pb activity decreases exponentially down core, reaching a stable background value at 10 cm (shallower stations) to 5 cm (deeper stations) sediment depth. 210Pb profiles of repeatedly sampled stations indicate negligible annual variability of mixing and flux. The 210Pbxs flux to the sediment shows a decreasing trend with increasing water depth. Below about 2000 m the average 210Pbxs flux is about 0.3 dpm cm−2 y−1, a third of the fluxes measured on the shelf and upper slope stations. Sediment mixing rates (Db) correlate with macro- and meiofaunal density changes and are within the normal oceanic ranges. Lower mixing rates on the lower slope likely reflect lower organic carbon fluxes there. Mass accumulation rates on Meriadzek Terrace are at maximum 80 g m−2 y−1, almost twice as high as at Goban Spur stations of comparable depth. A minimum accumulation rate of 16.6 g m−2 y−1 is found at the Goban Spur upper slope break. Organic carbon burial rates are low compared to other margins and range from a lowest value of 0.05 g m−2 y−1 at the upper slope break to 0.11 g m−2 y−1 downslope. A maximum organic carbon burial rate of 0.41 g m−2 y−1 is found on Meriadzek Terrace. Carbonate burial rates increase along the northern transect from the shelf (13 g m−2 y−1) via a low (9.3 g m−2 y−1) on the upper slope break to the deep sea (30.7 g m−2 y−1). Carbonate burial is highest on Meriadzek Terrace (44.5 g m−2 y−1). The N.W. European Margin at Goban Spur and Meriadzek Terrace cannot be considered a major carbon depocenter.  相似文献   

20.
Even though the continental slope off Cape Hatteras has sediment accumulation rates on the order of 1 cm/year, large areas of soft sediment are intensively reworked by infaunal organisms. Primary sedimentary structures have been completely replaced with biogenic structures. Surface sedimentary structures are dominated by the bioturbational activities of a deep burrowing infauna (to at least 30 cm). The layer actively mixed by the benthos, as evidenced by sediment profile and X-ray images, is estimated to range from 5 to 20 cm, with the residence time for particles within the surface mixed layer ranging from about 4.5 to 18 years. The biological mixing parameter (G) ranges from 0.4 to 5.5, which indicates moderate to strong biological mixing relative to accumulation and strata formation. Bioturbation contributes to the dynamic forces affecting the surface sediments by decreasing compaction of sediment layers and dilating sediment fabrics by sediment mixing, and introducing large water-filled burrows and voids to subsurface sediments. The sediment profile images captured numerous subsurface feeding voids, and worms in the process of making deep burrows, many of which extended below the 5 to 10 cm average depth of the apparent color redoxpotential discontinuity layer. High rates of accumulation of organic-rich sediment lead to high standing stocks of benthos and intensive feeding/burrowing activity that result in organic rich stratagraphic sequences that are thoroughly mixed. Cape Hatteras is a apparent focusing point for the transport of shallow water sediments to the deep sea. Sediments across other areas of the continental slope just 100's of kilometers south of Cape Hatteras are not as thoroughly mixed or biologically active.  相似文献   

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