Influence of relative sea level changes on the construction of the Mississippi Fan

A conceptual sea-level-driven depositional model for individual fanlobes (channel-overbank systems) of the Mississippi Fan does not permit direct application of the sequence stratigraphic principles of Vail and colleagues. Deep Sea Drilling Project Leg 96 results suggest that, during initial relative lowering of sea level, the canyon and upper fan channel were formed; excavated fine-grained slope sediments may have formed a debris flow deposit base for the fanlobe. Continued lowering produced constructional channel-levee-overbank deposits. Rising relative sea level inhibited input of coarse clastics, and channel depressions filled with muds. A blanket of (hemi)pelagics represents relative high sea level stand.     

Mississippi Fan: Internal structure and depositional processes

The Mississippi Fan is a Quarternary accumulation composed of more than seven elongated fan lobes. Isopach and structure maps show frequent shifting of these lobes. The Mississippi Canyon, formed by retrogressive slumping, connects to the youngest fan lobe. The upper fan-lobe is characterized by a large, incised, partially infilled, leveed channel. The middle fan-lobe is aggradational, convex in cross section, with a channel-levee complex on its apex. The lower fan-lobe contains a recently active small channel and several abandoned ones. Depositional patterns can be explained by several processes: “fluvial,” debris flows, and turbidity currents. Margin setting represents fan and/or source area     

Mississippi Fan: Internal structure and depositional processes

The Mississippi Fan is a Quarternary accumulation composed of more than seven elongated fan lobes. Isopach and structure maps show frequent shifting of these lobes. The Mississippi Canyon, formed by retrogressive slumping, connects to the youngest fan lobe. The upper fan-lobe is characterized by a large, incised, partially infilled, leveed channel. The middle fan-lobe is aggradational, convex in cross section, with a channel-levee complex on its apex. The lower fan-lobe contains a recently active small channel and several abandoned ones. Depositional patterns can be explained by several processes: “fluvial,” debris flows, and turbidity currents.     

Channel piracy on the lower Bengal Fan

Differences in morphology between the most recently active valley and older valleys suggest that channel piracy occurred on the lower Bengal Fan. Cutoff of two older valleys took place in the late Wisconsinan fan lobe between Sri Lanka and the 85°E Ridge at about 5°N and 83°E.     

国际业务化海洋学系统建设进展

介绍了国外业务化海洋学系统建设的概况和特点;重点介绍了法国业务化海洋学系统的实施规划、系统组成和各组成部分的主要内容。     

Miocene Blanca Fan, Northern Channel Islands, California: Small fans reflecting tectonism and volcanism

Blanca fan is a submarine fan composed of Miocene volcaniclastic strata. Parts of the fan system are exposed on Santa Cruz and Santa Rosa Islands, and possibly correlative strata crop out on San Miguel and Santa Catalina Islands. The Blanca fan and underlying breccia reflect regional transcurrent faulting in the California Continental Borderland and development of a system of rapidly subsiding basins and uplifted linear ridges during early and middle Miocene time. Erosion of uplifted crystalline basement rocks followed by the onset of silicic volcanism created linear sediment sources for the alluvial and submarine fans, respectively. Margin setting represents fan and/or source area     

Miocene Blanca Fan,Northern Channel Islands,California: Small fans reflecting tectonism and volcanism

Blanca fan is a submarine fan composed of Miocene volcaniclastic strata. Parts of the fan system are exposed on Santa Cruz and Santa Rosa Islands, and possibly correlative strata crop out on San Miguel and Santa Catalina Islands. The Blanca fan and underlying breccia reflect regional transcurrent faulting in the California Continental Borderland and development of a system of rapidly subsiding basins and uplifted linear ridges during early and middle Miocene time. Erosion of uplifted crystalline basement rocks followed by the onset of silicic volcanism created linear sediment sources for the alluvial and submarine fans, respectively.     

A high sinuosity,laterally migrating submarine fan channel-levee-overbank: results from DSDP Leg 96 on the Mississippi Fan,Gulf of Mexico

DSDP Leg 96 drilled four sites in a channel-levee-overbank system on the Mississippi Fan, Gulf of Mexico, approximately 300 km from the present Mississippi River Delta in water depths of about 2500 m (Sites 617, 620, 621 and 622). Apart from the uppermost 20–25 cm of Holocene marly foraminiferal ooze in most of the drilled sites, the entire cored intervals are in the Pleistocene Ericson Y Zone. Eight sedimentary facies are recognized: (1) biogenic oozes and muddy oozes; (2) calcareous muds; (3) clays and muds; (4) silty muds and muddy silts; (5) silt-laminated muds; (6) silts and sands; (7) muddy gravels and pebbly muds, and (8) gravels. Sediment accumulation rates on this part of the fan during the Wisconsin glaciation were as much as 11 m/1000 yrs, although most of the sediments probably accumulated from discrete, geologically instantaneous events. Site 621 and Site 622 are located within a prominent channel, Site 617 on an adjacent levee, and Site 620 in overbank deposits approximately 18 km northeast from the channel sites. In this part of the fan, there is one prominent high sinuosity channel, asymmetric in cross section and flanked by levees with probable ridge-and-swale topography. Near these drill sites, the channel width is 3–4 km and its bathymetric relief ranges from 25–45 m. Downfan, the dimensions of the channel decrease. Site 617 (to 191.2 m sub-bottom) and Site 620 (to 422.7 m sub-bottom) mainly comprise fine-grained, thin-bedded turbidites, with Site 617 tending to be slightly coarser grained and showing considerably more evidence of wet-sediment deformation. Site 621 (to 214.8 m sub-bottom), in the channel axis near the deepest part of a meander, contains mainly muds with a downhole increase in the silt content above 195 m sub-bottom, where pebbly muds overlie clean gravel that was obviously washed during core-retrieval and probably was a sandy gravel or gravelly sand. Site 622 (to 208 m sub-bottom) shows similar lithologies to Site 621 although the sediments generally contain more silt, and towards the base of the hole become thoroughly laminated silts and sandy silts: pebbles within muds and silts occur at 199 m sub-bottom. Based on overall grain size trends over tens of metres, the channel sites show ill-defined fining-upward sequences, whereas the levee and overbank sites show coarsening-upward sequences, although the upper part of Site 617 is a fining-upward sequence. Biogenic components of sediments at the channel sites are dominated by shallow-water benthic foraminifera derived from the continental shelf, with the coarser grained clastic intervals containing reworked late Cretaceous planktonic foraminifera and radiolaria from the Upper Mississippi River Valley. The levee and overbank sites have a larger percentage of Quaternary radiolaria, pelagic algal cysts, and more planktonic foraminifera than the channel sites. Seismic reflection profiles across this most recent fan channel show high-amplitude reflectors in the lower part of the channel fill, thought to correspond to the coarsest grained channel lag deposits. Isopach maps show that the lag deposits are up to 6.5 km wide, slightly more than 200 m thick, and that the northernmost meander belt has migrated about 2 km laterally, 1.2 km downfan, and has climbed 175 m stratigraphically (Kastens and Shor, 1985; Sterling et al., 1985). Evolution of the meander belt shows features common to point-bar migration in high sinuosity fluviatile systems. While the location of Sites 617, 620, 621 and 622 have been drilled within a middle fan environment, the width/depth ratios and the fact that this channel is a single conduit in this part of the fan, perhaps suggest a more appropriate comparison with many inner or upper fan environments that have been described in the literature.     

Controls on turbidite sedimentation: Insights from a quantitative approach of submarine channel and lobe architecture (Late Quaternary Congo Fan)

The role of internal and external forcing of sedimentation in turbidite systems remains a subject of debate. Here we propose new insights from the quantitative analysis of architectural parameters of the Congo Axial Fan.Fifty-two channel-levee-lobe systems, spanning the last ca. 200 ka, are visible on the seafloor, most of them having slightly elongated lobe complexes at their termination. Volumes of lobe complexes (usually 3–196 km³) are highly variable in time and space. The cumulative volume of the lobe complexes represents approximately 30% of the volume of the Axial Fan.The Axial Fan is sequentially divided into periods of increasing/decreasing channel lengths and basinward/landward migrations of avulsion points, representing successive prograding/retrograding architectural patterns called architectural cycles. These cycles are either symmetrical saw toothed and bell-shaped with progressive progradation and retrogradation phases, or asymmetrical, with long-lasting progradation phases and abrupt retrogradation phases that correspond to channel avulsions occurring high up on the fan.Our study points to the interplay between internal and external factors controlling the architecture of the Congo Axial Fan. The local topographic constraint is a major factor in the fan's stacking pattern. However, cyclic evolution of the architecture reveals major shifts in the deposition site that are linked to very upfan avulsion events. These events are interpreted to be driven by external factors (e.g. climate and/or eustatic sea-level change) that were able to drastically increase and/or coarsen the sediment supply to the fan.     

Trace metal concentrations in sediments of the Eastern Mississippi Bight

Sediments collected from 3 onshore-offshore transects (12 stations total) in the Eastern Mississippi Bight at 5 different times during 1987–1988 have been analyzed for total Ag, As, Ba, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sb, Se, Sn, Th and Zn. The data show considerable variation both spatially and temporally, largely as a result of natural variability in grain size and mineralogy. Clay-rich samples from deep water were always more metal-rich than sandy samples from shallow areas. Many samples from near the Mississippi River Delta appeared to be enriched in Ba by about a factor of two, probably as a result of contamination from oil well drilling mud, but there were few other indications of pollution influences. Many samples from throughout the area gave Mn/Fe and Cd/Fe ratios a factor of two or more lower than their Mississippi River source material. This shows that biochemical activity in the sediment is capable of solubilizing reducible and adsorbed metals.     

Analysis of the coastal Mississippi storm surge hazard

Following the extreme flooding caused by Hurricane Katrina, the Federal Emergency Management Agency (FEMA) commissioned a study to update the Mississippi coastal flood hazard maps. The project included development and application of new methods incorporating the most recent advances in numerical modeling of storms and coastal hydrodynamics, analysis of the storm climatology, and flood hazard evaluation. This paper discusses the methods that were used and how they were applied to the coast of the State of Mississippi.     

Importance of organic Fe complexing ligands in the Mississippi River plume

As part of a larger program focused on understanding the biogeochemistry of large river plumes, we participated in two expeditions during 2000 to sample the Mississippi River plume. Surface water samples were collected using a trace metal clean towed fish and analyzed for total dissolved Fe, organic Fe complexing ligands and their associated conditional stability constants. The ligands in the river plume have conditional stability constants (log K′_FeL between 10.5 and 12.3 with an average of 11.2 and standard deviation of 0.6) very similar to ligands found in the open ocean. Comparison of high flow and low flow regimes indicates that variability in flow may be the main cause of the variability in Fe concentrations in the plume. The organic Fe complexing ligands are in greatest excess during a time of higher flow. These ligands are responsible for maintaining very high (5 nM) Fe concentrations throughout the plume. Due to complexation with these organic ligands, the concentration of Fe remains above the Fe-hydroxide solubility level until a salinity above 35 is reached where there appears to be a sink for Fe in the less productive waters. Therefore, Fe is transported a great distance from the river source and is available for biological utilization in the coastal zone.     

Morpho-acoustic and sedimentologic characteristics of the Indus Fan

The upper Indus Fan is characterized by an average 1∶500 gradient, chanels with 100 m high levees, several continuous subbottom reflectors on 3.5-kHz records, and generally fine-grained sediments. Multichannel seismics show the levee complexes typified by overlapping wedge-shaped reflection sets and channel axis by high-amplitude discontinuous reflections. The middle fan has 1∶500–1∶1000 gradients and channels with ≈20 m high levees. The lower fan has gradients less than 1∶1000, channels with 8–20 m high levees, few or no subbottom reflectors on 3.5-kHz records, and high sand content. Besides the dominant unchannelized turbidity currents, channelized and overbank flows also played a significant role in the sedimentation of the lower fan.     

Morpho-acoustic and sedimentologic characteristics of the Indus Fan

The upper Indus Fan is characterized by an average 1∶500 gradient, chanels with 100 m high levees, several continuous subbottom reflectors on 3.5-kHz records, and generally fine-grained sediments. Multichannel seismics show the levee complexes typified by overlapping wedge-shaped reflection sets and channel axis by high-amplitude discontinuous reflections. The middle fan has 1∶500–1∶1000 gradients and channels with ≈20 m high levees. The lower fan has gradients less than 1∶1000, channels with 8–20 m high levees, few or no subbottom reflectors on 3.5-kHz records, and high sand content. Besides the dominant unchannelized turbidity currents, channelized and overbank flows also played a significant role in the sedimentation of the lower fan. Margin setting represents fan and/or source area     

Particulate and dissolved amino acids in the lower Mississippi and Pearl Rivers (USA)

In this study, we present seasonal changes (monthly samples from September 2001 to August 2003) in the abundance and composition of dissolved and particulate amino acids, at one station in the lower Mississippi and Pearl Rivers (LA, MS: USA). Spatial changes over a 4-day transmit from river km 390 to river mouth (Head of Passes, LA) in the Mississippi River, and a two-day downstream sampling from Jackson (MS) to Stennis Space Center (MS) were also determined. Temporal data in the lower Mississippi River showed significantly lower concentrations of dissolved combined amino acids (DCAA, 0.8 to 2.2 μM) and dissolved amino acids in high molecular weight fraction (HMW DAA, 0.2–0.4 μM) than in the Pearl River (DCAA, 1.4–4.3 μM; HMW DAA, 0.4–1.4 μM). Dissolved free amino acids (DFAA) were significantly lower than DCAA in both rivers, and displayed minimal seasonal variability. DCAA, HMW DAA, and particulate amino acids (PAA) were generally higher during high-flow periods, which may have suggested dominance in terrestrial sources. Carbon-normalized yield of PAA (%C-PAA) was generally higher during low-flow conditions and positively correlated with chlorophyll-a (chl-a), reflective of in situ sources. Downstream variability in the lower Mississippi River showed stable DCAA concentrations, a decline in PAA (from 1.06 to 0.43 μM), and a gradual increase in mole percent of non-protein amino acids (%NPAA). This likely reflected bacterial degradation of phytoplankton biomass during falling discharge. Nitrogen-normalized yield of PAA (%N-PAA) was inversely correlated with PAA (R = − 0.7, n = 48), indicative of short-term sedimentation and resuspension events. Conversely, downstream decreases in DCAA and middle-reach peaks of PAA and %N-PAA in the Pearl River, likely resulted from photochemical degradation of DOM as well as algal production during base-flow conditions. The comparisons in abundance and composition of DAA and PAA in these different river systems provides important information on in situ nitrogen and carbon cycling as related to riverine inputs of organic matter to coastal ocean.     

Fluid Mud Measurement and Siltation Analysis in A Trial Excavated Channel in the Approach Channel of the Xiangshan Port

In order to clarify the distribution and variation of silt and fluid mud in the Waiganmen shallow section of the 50000-ton intake channel of the Xiangshan Port, and to understand the influence of the channel excavation on the surrounding flow conditions and the strength of the backsilting, especially the impact of typhoon on the sudden silting of the channel, so as to demonstrate the feasibility and stability of the channel excavation. The fluid mud,hydraulic, sediment and topographic measurements were carried out in the study area, and the thickness of the fluid mud layers, tidal current, sediment and topographic data were obtained. Dual-frequency sounder, gamma-ray densitometer and SILAS navigational fluid mud measurement system were used to monitor the fluid mud, and the results were compared and verified. The adaptability and accuracy of the three methods were analyzed. The SILAS navigational continuous density measurement system and gamma-ray fixed-point fluid mud measurement are used to detect the density, thickness and variation of the fluid mud accurately. Based on the hydrological observation data,the process of erosion and deposition in excavation channel and its influence mechanism are analyzed, and the distribution characteristics and evolution law of siltation in engineering area are given in the form of empirical formula. The research shows that the super typhoon can produce large siltation, which results in sudden siltation of the channel. The tidal current is the main dynamic factor of the change of erosion and siltation of the excavation trench. Under the influence of reciprocating tidal current and excavation topography, the trial excavation trench is silted on the whole. There is fluid mud in the monitoring area of the trench, and the distribution of fluid mud is different in space. The thickness of the fluid mud at the bottom of the trench is generally larger than that outside the trench and the slope of the trench, and the siltation of the trench tends to be slow. The research results can provide scientific evaluation for channel excavation and maintenance, and support for the implementation of the project.     

Temporal variability in the composition and abundance of terrestrially-derived dissolved organic matter in the lower Mississippi and Pearl Rivers

Here we report on temporal changes in the concentration and composition of lignin phenols in high molecular weight (< 0.2 μm, > 1 kDa) dissolved organic matter (HMW DOM) collected from the lower Mississippi and Pearl Rivers (MR and PR) (USA). Monthly water samples were collected at a station in the lower reach in each river from August 2001 to August 2003. Significantly higher concentrations of lignin and Λ₈ values (mg lignin phenols in 100 mg organic carbon) in the Pearl River than in the Mississippi River, reflected sporadic inputs of terrestrial DOM during rainstorm events from wetlands and forest soils. Larger seasonal variations in lignin concentration and composition in the Pearl River, compared to the Mississippi River, were attributed to shifts in organic matter sources from topsoil inputs during rainstorm events to groundwater inputs and in situ production during base flow in this small river. Conversely, lower Λ₈ and vanillic acid to vanillin ratios [(Ad/Al)v] in the HMW DOM of the lower Mississippi River may be a result of a lower export rate of lignin from agricultural soils due to lower carbon storage in the expansive agricultural systems of the Mississippi River watershed, as well as dilution of phytoplankton DOM inputs. Large seasonal changes in lignin concentration and Λ₈ (linked at times with river discharge), and minimal variability in the composition of lignin phenols, likely represented an integrated signal of soil-derived vascular inputs from the upstream drainage basin. If we are to better understand the controls of organic matter delivery to the coastal zone from both small and large rivers, sampling strategies need to be adjusted to account for the different scales of hydrologic response time and in situ processing associated with different residence times.     

High-resolution measurements of chromophoric dissolved organic matter in the Mississippi and Atchafalaya River plume regions

Chromophoric dissolved organic matter (CDOM) was measured in the spring and summer in the northern Gulf of Mexico with the ECOShuttle, a towed, instrumented, undulating vehicle. A submersible pump mounted on the vehicle supplied continuously flowing, uncontaminated seawater to online instruments in the shipboard laboratory and allowed discrete samples to be taken for further analysis. CDOM in the northern Gulf of Mexico was dominated by freshwater inputs from the Mississippi River through the Birdfoot region and to the west by discharge from the Atchafalaya River. CDOM was more extensively dispersed in the high-flow period in the spring but in both time periods was limited by stratification to the upper 12 m or so. Thin, subsurface CDOM maxima were observed below the plume during the highly stratified summer period but were absent in the spring. However, there was evidence of significant in situ biological production of CDOM in both seasons.The Mississippi River freshwater end member was similar in spring and summer, while the Atchafalaya end member was significantly higher in the spring. In both time periods, the Atchafalaya was significantly higher in CDOM and dissolved organic carbon (DOC) than the Mississippi presumably due to local production and exchange within the coastal wetlands along the lower Atchafalaya which are absent along the lower Mississippi. Nearshore waters may also have higher CDOM due to outwelling from coastal wetlands. High-resolution measurements allow the differentiation of various water masses and are indicative of rapidly varying (days to weeks) source waters. Highly dynamic but conservative mixing between various freshwater and marine end members apparently dominates CDOM distributions in the area with significant in situ biological inputs (bacterial degradation of phytoplankton detritus), evidence of flocculation, and minor photobleaching effects also observed. It is clear that high-resolution measurements and adaptive sampling strategies allow a more detailed examination of the processes that control CDOM distributions in river-dominated systems.     

Ambient and dynamic pore pressures in fine-grained submarine sediments: Mississippi Delta

A multisensor piezometer probe has measured pore pressures in fine-grained submarine sediments of the Mississippi Delta over a period of approximately eight months. Data presented here cover the initial 2650 hours of the experiment. Dynamic and ambient pore pressures were recorded. Analogue data collected from the time of probe insertion include decay characteristics, steady-state (ambient) excess pore pressures, and the response of pore pressures to surface wave activity. The probe was installed in 43–44 ft of water near an offshore platform in the East Bay area of the Delta. Sensors were located at 21, 41 and 51 ft below the mudline. Ambient excess pore pressures were determined to be 0.7, 2.8 and 6.6 psi (lb/in²) at the respective depths. The relatively high excess pressures and the measured laboratory wet unit weights of the soil result in a significantly low effective stress. Pressure fluctuations due to tidal and surface wave activity were observed to produce significant pore pressure response in these soils. Preliminary data obtained using high-airentry and corundum stones indicate that considerably more research is necessary in order to fully understand the peculiarities observed in the data and to assess the role of dissolved and free gas on the pore pressures in submarine sediments.     

A Deep-sea Channel in the Northwestern Mediterranean Sea: Morphology and seismic structure of the Valencia Channel and its surroundings

The 400 km long Valencia Channel occupies the axis of the Valencia Trough in the Northwestern Mediterranean. Four different types of seismic reflection profiles were used to analyze the morphology and structure of the Valencia Channel with regard to the role played by both margins, Balearic and Iberian, of the Valencia Trough. From a detailed morphoseismic analysis of the Valencia Channel, its upper, middle, and lower courses can be characterized as follows: (1) in the upper course, tributaries are short and only slightly incised, with recent mass-transport deposits occurring on the adjacent continental slopes; (2) in the middle course, the channel deepens, and tributary valleys merge into it; and (3) the lower course begins after a sudden change in the direction of the channel, has a meandering path, is flanked by levees, and is fed by some valleys.During the Pliocene and Quaternary, at least four erosional and filling phases are observed in seismic profiles of the lower course of the Valencia Channel. The varying intensity of mass-transport processes and associated retrogressive slumping, which are related with fluctuations in sediment supply and relative sea-level changes, have played a major role in the formation, maintenance and deepening of the Valencia Channel. In addition to these sedimentary processes, a basement tectonic control and some morphostructural features affect the direction of the Valencia Channel locally.