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1.
Rare earth elements in the sedimentary cycle: A summary   总被引：2，自引：0，他引：2  
The relative and absolute concentrations of rare earth elements (REE) in authigenic and biogenic phases of deep-sea sediments are quite different. Competition between these phases for REE has resulted in fractionation from the parent material, the latter consisting predominantly of terrigenous material, but with a contribution from marine volcanism. The strongest feature of this fractionation is a depletion of Ce, relative to La, in CaCO3, opalline silica, phillipsite, phosphorite, barite, and montmorillonitic clays; and a Ce enrichment in Fe/Mn nodules. The distribution of REE in different masses of seawater strongly reflects their fractionation in sediments. Whereas the relative concentration of REE in rivers resembles that of shale, their removal from seawater by authigenic and biogenic phases results in: (1) a decrease of their total concentration; (2) a depletion of Ce; and (3) an enrichment of heavy REE relative to light REE. The order of fractionation for water masses in the Atlantic Ocean is:Antarctic intermediate water > North Atlantic deep water > Antarctic bottom water> shelf water > river water ～ shale.The shale-normalized pattern for the sum of REE in the authigenic and biogenic phases of pelagic sediment and in seawater resembles that of an admixture of shale and basalt corresponding presumably to the realtive inputs from continents and marine volcanism respectively. The estimated rate of accumulation of each REE in the sediment, however, is approximately 12 times the estimated rate of input of REE from these two sources.  相似文献   

2.
Organic carbon (OC) and total nitrogen (TN) concentrations and stable isotope ratios (δ13C, δ15N) of fine (<50 μm) size fractions of deep-sea sediments from the central North Atlantic were employed to identify changes in sources of organic matter over the past 50 ka BP. Ambient glacial sediments are characterised by values that reflect mixtures of marine and terrestrial inputs (averages ± 1σ: OC/TN = 7.6 ± 0.8; δ13C = −22.8 ± 1.0‰; δ15N = 5.5 ± 0.6‰). δ13C, OC, and TN concentrations shift to higher values during the Holocene, indicating a gradual decrease of fine terrigenous supply to the North Atlantic. The unchanged δ15N record between last glacial and Holocene stages indicates that the central North Atlantic region remained oligotrophic at least during the past 50 ka BP, but additional studies are required to support this result in terms of nitrogen oceanic budget. During the phases of enhanced ice-rafted detrital supply corresponding to prominent Heinrich events (HL1, HL2, HL4, and HL5), fine-sized sedimentary organic matter has lower OC and TN concentrations, contrasting sharply with those of ambient glacial sediments. Lower δ13C (down to −28‰) and δ15N (down to 1.6‰) values and high OC:TN ratios (up to 14.7 ± 1.1) are found for HL1, HL2, and with lesser extent for HL4. These values reflect enhanced detrital supply originating from poorly differentiated soil horizons that characterise periglacial climate conditions and from organic matter-bearing rock sources of the underlying geological basement. During HL5, only the δ13C offset records the input of fine size ice-rafted organic matter. Gradually changing soil development conditions during the time interval covering HL5 to HL1 (marine isotope stages 5 to 2), as well as varying erosion levels, have been hypothesized on the basis of constant δ13C, increasing OC/TN and decreasing δ15N values.  相似文献   

3.
 Sediments in the rivers and basins around Washington, DC, have high concentrations of phosphorus, which, based on geographic distributions, is largely derived from urban runoff and municipal sewage. Dissolved-particulate phosphate exchange reactions and biological uptake of dissolved phosphorus from the water column may be an added source of phosphorus to the sediments. Concentrations of total sedimentary phosphorus ranged from 24 to 56 μm P/g-dw, and were highest in areas near combined sewer outfalls. As a part of this study, sedimentary phosphorus was fractionated into Fe-P, Ca-P, Al-P, and organic phases using a selective-sequential leaching procedure. The distribution of the phases in all sediments analyzed follow the order , Fe-P>Ca-P>Al-P. Spatial variations in the amounts of phosphorus in the different phases is related to the sources of phosphorus to the area. The proportions of occluded Al-P and organic P are 10–20% of the total P, respectively. This suggests that phosphorus from natural sources is small compared to anthropogenic inputs in this area. The high leachable Fe-P and Ca-P in these sediments might contribute a substantial amount of P to the water column under conditions of remobilization. Received: 20 February 1996 · Accepted: 2 April 1996  相似文献   

4.
Deep sea sediment cores taken between 50° and 75°N in the North Atlantic, in water depths varying between 1340 and 3850 m, were examined to provide an astronomically calibrated late Quaternary time-scale based on physical property records. Magnetic susceptibility and gamma ray attenuation porosity evaluator (GRAPE) density changes of these cores revealed significant responses to orbital forcing in the eccentricity (100 kyr), obliquity (41 kyr) and precessional (23, 19 kyr) bands. At 75°N (Greenland Sea), a response to obliquity forcing was weak despite the fact that it should become more pronounced in sediments at high latitudes. Application of bandpass filtering at the obliquity period (41 kyr), however, showed that variance at this period did exist in the magnetic susceptibility record, but at a very low power. At 50°N stacked curves of magnetic susceptibility correlated strongly with the SPECMAP curve for the past 500 ka. Since about 65 ka, dropstone layers are recorded in both magnetic susceptibility and GRAPE data of Rockall Plateau sediments. Although Rockall Plateau sediments show peaks in physical properties that correlate with Heinrich events (H1, H2, H4, H5, H6), such a relationship was not readily observed in Norwegian-Greenland Sea records. Heinrich events at Rockall Plateau sites indicate a northward flow of icebergs in the eastern North Atlantic. This flow pattern and the presence of Heinrich events during the past 65 ka raise the questions of whether similar events occurred before this time period, and to what kind of ice sheet dynamics and climatic-oceanographic conditions favoured major iceberg surges from the Laurentide ice sheet to the North Atlantic at 50°N.  相似文献   

5.
Remobilization of authigenic uranium in marine sediments by bioturbation   总被引：1，自引：0，他引：1  
Uranium behaves as a nearly conservative element in oxygenated seawater, but it is precipitated under chemically reducing conditions that occur in sediments underlying low-oxygen bottom water or in sediments receiving high fluxes of particulate organic carbon. Sites characterized by a range of bottom-water oxygen (BWO) and organic carbon flux (OCF) were studied to better understand the conditions that determine formation and preservation of authigenic U in marine sediments. Our study areas are located in the mid latitudes of the northeast Pacific and the northwest Atlantic Oceans, and all sites receive moderate (0.5 g/cm2 kyr) to high (2.8 g/cm2 kyr) OCF to the sediments. BWO concentrations vary substantially among the sites, ranging from <3 to ∼270 μM. A mass balance approach was used to evaluate authigenic U remobilization at each site. Within each region studied, the supply of particulate nonlithogenic U associated with sinking particles was evaluated by means of sediment traps. The diffusive flux of U into sediments was calculated from pore-water U concentration profiles. These combined sources were compared with the burial rate of authigenic U to assess the efficiency of its preservation. A large fraction (one-third to two-thirds) of the authigenic U precipitated in these sediments via diffusion supply is later regenerated, even under very low BWO concentrations (∼15 μM). Bioturbating organisms periodically mix authigenic U-containing sediment upward toward the sediment-water interface, where more oxidizing conditions lead to the remobilization of authigenic U and its loss to bottom waters.  相似文献   

6.
The sedimentary rocks of the Metlaoui Formation in the Gafsa basin (southern Tunisia), which may be grouped in three units: the basal (Thèlja), middle (Chouabine) and upper unit (Kef Eddour), provide a record of preserved sedimentary, authigenic and biological processes. This paper presents the findings of sedimentological investigations of the biosiliceous deposits of the middle unit. This unit contains either well-preserved (Opal-A) or diagenetically altered (Opal-CT, clinoptilolite, quartz and even clays) diatom frustules. Such diagenetic changes are commonly described in marine and lacustrine biosiliceous deposits. The fossil content of theses diatomaceous layers implies shallow-marine conditions.The opal-rich sediments, and the associated facies record the transgressive transitions associated with high organic productivity, probably enhanced by seasonal input of nutrients, and high sea level stands, and a close association with stratified water column conditions. The formation of bedded diatomaceous sediments is known to require either high organic productivity or anoxic conditions in bottom/intermediate water, and eventually both processes. The initial organic content of the biogenic deposits was impoverished in early stages of sedimentation and diagenesis. A large part of the organic matter could have been destroyed during early diagenetic processes and from further oxidation in outcrops.  相似文献   

7.
We measured U in sediments (both pore waters and solid phase) from three locations on the middle Atlantic Bight (MAB) from the eastern margin of the United States: a northern location on the continental shelf off Massachusetts (OC426, 75 m water depth), and two southern locations off North Carolina (EN433-1, 647 m water depth and EN433-2, 2648 m water depth). These sediments underlie high oxygen bottom waters (250-270 μM), but become reducing below the sediment-water interface due to the relatively high organic carbon oxidation rates in sediments (EN433-1: 212 μmol C/cm2/y; OC426: 120 ± 10 μmol C/cm2/y; EN433-2: 33 μmol C/cm2/y). Pore water oxygen goes to zero by 1.4-1.5 cm at EN433-1 and OC426 and slightly deeper oxygen penetration depths were measured at EN433-2 (∼4 cm).All of the pore water profiles show removal of U from pore waters. Calculated pore water fluxes are greatest at EN433-1 (0.66 ± 0.08 nmol/cm2/y) and less at EN433-2 and OC426 (0.24 ± 0.05 and 0.13 ± 0.05 nmol/cm2/y, respectively). Solid phase profiles show authigenic U enrichment in sediments from all three locations. The average authigenic U concentrations are greater at EN433-1 and OC426 (5.8 ± 0.7 nmol/g and 5.4 ± 0.2 nmol/g, respectively) relative to EN433-2 (4.1 ± 0.8 nmol/g). This progression is consistent with their relative ordering of ‘reduction intensity’, with greatest reducing conditions in sediments from EN433-1, less at OC426 and least at EN433-2. The authigenic U accumulation rate is largest at EN433-1 (0.47 ± 0.05 nmol/cm2/y), but the average among the three sites on the MAB is ∼0.2 nmol/cm2/y. Pore water profiles suggest diffusive fluxes across the sediment-water interface that are 1.4-1.7 times greater than authigenic accumulation rates at EN433-1 and EN433-2. These differences are consistent with oxidation and loss of U from the solid phase via irrigation and/or bioturbation, which may compromise the sequestration of U in continental margin sediments that underlie bottom waters with high oxygen concentrations.Previous literature compilations that include data exclusively from locations where [O2]bw < 150 μM suggest compelling correlations between authigenic U accumulation and organic carbon flux to sediments or organic carbon burial rate. Sediments that underlie waters with high [O2]bw have lower authigenic U accumulation rates than would be predicted from relationships developed from results that include locations where [O2]bw < 150 μM.  相似文献   

8.
Utilizing a sequential extraction technique this study provides the first quantitative analysis on the abundance of sedimentary phosphorus and its partitioning between chemically distinguishable phases in sediments of the Bering Sea, the Chukchi Sea and the Mackenzie River Delta in the western Arctic Ocean. Total sedimentary phosphorus (TSP) was fractionated into five operationally defined phases: (1) adsorbed inorganic and exchangeable organic phosphorus, (2) Fe-bound inorganic phosphorus, (3) authigenic carbonate fluorapatite, biogenic apatite and calcium carbonate-bound inorganic and organic phosphorus, (4) detrital apatite, and (5) refractory organic phosphorus. TSP concentrations in surface sediments increased from the Chukchi Sea (18 μmol g−1 of dried sediments) to the Bering Sea (22 μmol g−1) and to the Mackenzie River Delta (29 μmol g−1). Among the five pools, detrital apatite phosphorus of igneous or metamorphic origin represents the largest fraction (~43%) of TSP. The second largest pool is the authigenic carbonate fluorapatite, biogenic apatite as well as CaCO3 associated phosphorus (~24% of TSP), followed by the Fe-bound inorganic phosphorus, representing ~20% of TSP. The refractory organic P accounts for ~10% of TSP and the readily exchangeable adsorbed P accounts for only 3.5% of TSP. Inorganic phosphorus dominates all of phosphorus pools, accounting for an average of 87% of the TSP. Relatively high sedimentary organic carbon and total nitrogen contents and low δ13C values in the Mackenzie River Delta together with the dominance of detrital apatite in the TSP demonstrate the importance of riverine inputs in governing the abundance and speciation of sedimentary phosphorus in the Arctic coastal sediments.  相似文献   

9.
Solid phase P speciation has been determined in sediments from a transect across the central section of the continental shelf and slope of the Great Barrier Reef (GBR) lagoon. This region is characterized by a gradient of riverine aluminosilicate clay and silt nearshore, seawards of which biogenic carbonate sediment predominates. Phosphorus speciation results show large variations along this transect. Organic P and authigenic (apatite) P are the major chemical forms of phosphorus in the central GBR continental shelf sediments. Post-depositional reorganization of P was also observed, converting organic P and iron bound P (Fe-P) to authigenic (apatite) P. Phosphorus burial rate was estimated from measurements of total P concentration and excess 210Pb sediment mass accumulation rates. Burial efficiency varies significantly over the shelf. Inshore areas showed significant P remobilization from sediments to the water column (up to ∼50%). The mid and the outer shelf showed little evidence for remobilization (except for coral reef platform sediments), with more of the sediment P being in the less reactive authigenic apatite phases. An appreciable fraction of this non-labile authigenic apatite phase was identified as fish bone. P sources and sinks over the central part of the GBR shelf were quantified using a mass balance approach. This showed that Coral Sea shelf edge upwelling events are essential to satisfy the large P nutrient demand of the whole GBR lagoon. P inputs due to upwelling events were greater than those contributed by local rivers over an average year.  相似文献   

10.
We measured the vertical distributions of Mo, Fe, Mn, sulfide, sulfate, organic carbon, major ions, and pH in sediment porewater from one perennially oxic and three seasonally anoxic lacustrine basins in Eastern Canada, as well as those of Mo, acid volatile sulfide, Fe, Mn, Al, organic C, 210Pb and 137Cs in sediment cores from the same sites. The only input of anthropogenic Mo to these lakes comes from atmospheric deposition.The relatively monotonous distribution of Mo in the porewater of three seasonally anoxic basins suggests that Mo is not redistributed in the sedimentary column during periods of anoxia. In contrast, porewater Mo profiles obtained at three sampling dates in a perennially oxic basin display sharp Mo peaks below the sediment-water interface, indicating redistribution subsequent to deposition. Modeling of these latter porewater Mo profiles with a diagenetic reaction-transport equation coupled to comparisons among the various porewater and solid phase profiles reveal that Mo is released at 1-2 cm depth as a consequence of the reductive dissolution of Fe oxyhydroxides and scavenged both at the vicinity of the sediment-water interface, by re-adsorption onto authigenic Fe oxyhydroxides, and deeper in the sediments where dissolved sulfide concentrations are higher. The estimated rate constant for the adsorption of Mo onto Fe oxyhydroxides is 36 ± 45 cm3 mol−1 s−1.Diagenetic modeling indicates that authigenic Mo in sediments of the perennially oxic basin represents about one-third of the total solid phase Mo in the first cm below the sediment-water interface and only one tenth below this horizon. If we assume that no authigenic Mo is accumulated in the seasonally anoxic lake sediments we conclude that the sediment Mo concentrations, which are up to 3-16 times higher than the average lithogenic composition, depending on the lake, are mainly due to atmospheric deposition of anthropogenic Mo and not to the formation of authigenic Mo phases. Reconstructed historical records of the atmospheric Mo deposition indicate maximum values in the 1970s and 1980s and significant decreases since then. Emissions to the atmosphere associated with the smelting of non-ferrous ores and coal combustion appear to be the most important sources of anthropogenic Mo.  相似文献   

11.
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12.
Many (bio)geochemical processes that bring about changes in sediment chemistry normally begin at the sediment-water interface, continue at depth within the sediment column and may persist throughout the lifetime of sediments. Because of the differential reactivity of sedimentary phosphate phases in response to diagenesis, dissolution/precipitation and biological cycling, the oxygen isotope ratios of phosphate (δ18OP) can carry a distinct signature of these processes, as well as inform on the origin of specific P phases. Here, we present results of sequential sediment extraction (SEDEX) analyses combined with δ18OP measurements, aimed at characterizing authigenic and detrital phosphate phases in continental margin sediments from three sites (Sites 1227, 1228 and 1229) along the Peru Margin collected during ODP Leg 201. Our results show that the amount of P in different reservoirs varies significantly in the upper 50 m of the sediment column, but with a consistent pattern, for example, detrital P is highest in siliciclastic-rich layers. The δ18OP values of authigenic phosphate vary between 20.2‰ and 24.8‰ and can be classified into at least two major groups: authigenic phosphate precipitated at/near the sediment-water interface in equilibrium with paleo-water oxygen isotope ratios (δ18Ow) and temperature, and phosphate derived from hydrolysis of organic matter (Porg) with subsequent incomplete to complete re-equlibration and precipitated deeper in the sediments column. The δ18OP values of detrital phosphate, which vary from 7.7-15.4‰, suggest two possible terrigenous sources and their mixtures in different proportions: phosphate from igneous/metamorphic rocks and phosphate precipitated in source regions in equilibrium with δ18Ow of meteoric water. More importantly, original isotopic compositions of at least one phase of authigenic phosphates and all detrital phosphates are not altered by diagenesis and other biogeochemical changes within the sediment column. These findings help to understand the origin and provenance of P phases and paleoenvironmental conditions at/near the sediment-water interface, and to infer post-depositional activities within the sediment column.  相似文献   

13.
J. B. KEENE 《Sedimentology》1983,30(3):449-454
ABSTRACT Chalcedony is the most abundant form of quartz in silicified pelagic sediments from the North Pacific. Varieties of chalcedonic quartz present in chert of deep-sea origin include chalcedony (length-fast and zebraic), quartzine (length-slow), and lutecite.
These occurrences of quartzine in known pelagic sediments emphasize the dangers of using quartzine as an indicator of former evaporitic environments. Quartzine is a diagenetic mineral and does indicate pore fluids rich in sulphate and magnesium. In pelagic sediments, it is always associated with authigenic barite and in many cases with authigenic dolomite. Quartzine should not be used, by itself, as an indicator of any particular environment of deposition.  相似文献   

14.
Studies of the δ13C of pore water dissolved inorganic carbon (δ13C-DIC) were carried out in shallow water carbonate sediments of the Great Bahamas Bank (GBB) to further examine sediment-seagrass relationships and to more quantitatively describe the couplings between organic matter remineralization and sediment carbonate diagenesis. At all sites studied δ13C-DIC provided evidence for the dissolution of sediment carbonate mediated by metabolic CO2 (i.e., CO2 produced during sediment organic matter remineralization); these observations are also consistent with pore water profiles of alkalinity, total DIC and Ca2+ at these sites. In bare oolitic sands, isotope mass balance further indicates that the sediment organic matter undergoing remineralization is a mixture of water column detritus and seagrass material; in sediments with intermediate seagrass densities, seagrass derived material appears to be the predominant source of organic matter undergoing remineralization. However, in sediments with high seagrass densities, the pore water δ13C-DIC data cannot be simply explained by dissolution of sediment carbonate mediated by metabolic CO2, regardless of the organic matter type. Rather, these results suggest that dissolution of metastable carbonate phases occurs in conjunction with reprecipitation of more stable carbonate phases. Simple closed system calculations support this suggestion, and are broadly consistent with results from more eutrophic Florida Bay sediments, where evidence of this type of carbonate dissolution/reprecipitation has also been observed. In conjunction with our previous work in the Bahamas, these observations provide further evidence for the important role that seagrasses play in mediating early diagenetic processes in tropical shallow water carbonate sediments. At the same time, when these results are compared with results from other terrigenous coastal sediments, as well as supralysoclinal carbonate-rich deep-sea sediments, they suggest that carbonate dissolution/reprecipitation may be more important than previously thought, in general, in the early diagenesis of marine sediments.  相似文献   

15.
The behavior of phosphorus is considered at major stages of the sedimentary cycle beginning with supply sources for its delivery into the ocean to precipitation and its sedimentation, localization and subsequent diagenetic redistribution in bottom sediments. River runoff represents the main phosphorus source in the ocean. It delivers annually about 1.5 Mt of dissolved phosphorus and more than 20 Mt of suspended phosphorus into the ocean. Up to 80% of the suspended phosphorus incorporated in the lithogenic material precipitates within submarine continental margins. Phosphorus dissolved in seawater repeatedly participates in biogeochemical processes owing to its assimilation by phytoplankton that annually consumes from 1.5 to 2.5 Gt of phosphorus. Dissolved phosphorus is incorporated in organic remains and precipitates from seawater by a biogenic mechanism, too. Only a part of phosphorus settled onto the bottom is buried in sediments. Due to reducing diagenetic processes, up to 30–40% of the primarily precipitated phosphorus diffuses from the upper layer of sediments into bottom water. Diffusion flux into the ocean significantly exceeds the supply of dissolved phosphorus from river runoff. The absolute mass phosphorus dispersed in sediments is several orders of magnitude greater than the mass concentrated in phosphorite deposits. However, the majority of phosphorite formation epochs coincide with the intensification of total phosphorus accumulation in marine sediments in conditions of humid climate, intense chemical weathering of rocks on continents, and considerable expansion of the oceanic shelf area.  相似文献   

16.
Pore water and solid phase data for redox-sensitive metals (Mn, Fe, V, Mo and U) were collected on a transect across the Peru upwelling area (11°S) at water depths between 78 and 2025 m and bottom water oxygen concentrations ranging from ∼0 to 93 μM. By comparing authigenic mass accumulation rates and diffusive benthic fluxes, we evaluate the respective mechanisms of trace metal accumulation, retention and remobilization across the oxygen minimum zone (OMZ) and with respect to oxygen fluctuations in the water column related to the El Niño Southern Oscillation (ENSO).Sediments within the permanent OMZ are characterized by diffusive uptake and authigenic fixation of U, V and Mo as well as diffusive loss of Mn and Fe across the benthic boundary. Some of the dissolved Mn and Fe in the water column re-precipitate at the oxycline and shuttle particle-reactive trace metals to the sediment surface at the lower and upper boundary of the OMZ. At the lower boundary, pore waters are not sufficiently sulfidic as to enable an efficient authigenic V and Mo fixation. As a consequence, sediments below the OMZ are preferentially enriched in U which is delivered via both in situ precipitation and lateral supply of U-rich phosphorites from further upslope. Trace metal cycling on the Peruvian shelf is strongly affected by ENSO-related oxygen fluctuations in bottom water. During periods of shelf oxygenation, surface sediments receive particulate V and Mo with metal (oxyhydr)oxides that derive from both terrigenous sources and precipitation at the retreating oxycline. After the recurrence of anoxic conditions, metal (oxyhydr)oxides are reductively dissolved and the hereby liberated V and Mo are authigenically removed. This alternation between supply of particle-reactive trace metals during oxic periods and fixation during anoxic periods leads to a preferential accumulation of V and Mo compared to U on the Peruvian shelf. The decoupling of V, Mo and U accumulation is further accentuated by the varying susceptibility to re-oxidation of the different authigenic metal phases. While authigenic U and V are readily re-oxidized and recycled during periods of shelf oxygenation, the sequestration of Mo by authigenic pyrite is favored by the transient occurrence of oxidizing conditions.Our findings reveal that redox-sensitive trace metals respond in specific manner to short-term oxygen fluctuations in the water column. The relative enrichment patterns identified might be useful for the reconstruction of past OMZ extension and large-scale redox oscillations in the geological record.  相似文献   

17.
Organic matter was isolated from the water columns and sediments of two pond systems in the south-eastern United States. Water column material was ultrafiltered to provide three fractions, i.e. <0.45 μm, but > 50,000 daltons; <50,000 daltons, but > 5000 daltons; and <5000 daltons. Sedimentary organic matter was separated into humic acid and fulvic acid fractions based on solubility criteria and the humic acid fraction was ultrafiltered to provide the same fractions as the water column isolates. All fractions were analysed for organic carbon, Al, Ca, Cu, Fe, Mg and Mn. Infra-red spectra were also measured for the sedimentary organic fractions. Organic matter isolated from the water column of the two ponds had similar organic carbon and elemental distributions, as did the organic matter isolated from the two sediments. However, significant differences in the organic carbon and elemental distributions were observed for water column and sedimentary organic matter isolated from the same pond. These studies have relevance to diagenetic alterations of organic matter and geochemical cycles of elements within lakes.  相似文献   

18.
Marine sediments typically exhibit steep porosity gradients in their uppermost centimeters. Although the decrease in porosity with depth below the sediment-water interface is primarily due to compression arising from the accumulation of overlying sediment, early diagenetic mineral dissolution and precipitation reactions may potentially also affect the porosity gradient. Here, we present a steady state compaction model, based on the mass and momentum conservation of total fluid and solid phases, in order to quantify the relative contributions of mineral reactions and physical compaction on porosity changes. The compaction model is applied to estimate hydraulic conductivity and compressive response coefficients of deep-sea sediments from measured porosity depth profiles. The results suggest an inverse relation between the compressive response coefficient and the lithogenic content of marine sediments. For deep-sea sediments exhibiting high rates of dissolution of siliceous shell fragments, the compaction model ignoring mineral reactions overestimates the hydraulic conductivity and compressive response coefficients. In contrast to non-compacting porous media, mineral dissolution in surficial sediments can lead to lower porosity. However, as illustrated for a deep-sea sediment in the equatorial Atlantic characterized by extensive dissolution of calcareous shell fragments, the effect of mineral dissolution and precipitation reactions on porosity gradients is, in most cases, negligible.  相似文献   

19.
Despite recent advances, diagenetic processes in fine-grained sediments are still relatively poorly understood. Key questions still to be resolved include the types of diagenetic minerals present in mudstones and the extent of element mobility in these low permeability systems. This study utilizes data from the Haynesville-Bossier Shale, USA, to analyze lithologies, discriminate authigenic phases and identify mobile elements during diagenesis. It has implications for understanding how authigenic minerals develop and the sources of those authigenic minerals in fine-grained sediments. On the basis of grain-size and mineralogy five lithologies are designated: (i) silica-rich argillaceous mudstones; (ii) argillaceous siliceous mudstones; (iii) mixed siliceous mudstones; (iv) mixed mudstones; and (v) authigenically-dominated mudstones. The diagenetic development of the Haynesville-Bossier Shale can be divided into early and late diagenesis. Ferroan and non-ferroan dolomite, framboidal pyrite and bioclast pore-filling kaolinite and calcite grain replacements and cements all formed during early diagenesis. Late diagenetic mineral phases include illite formed by the illitization of smectite, replacive and displacive chlorite, calcite-replacive albite, quartz-replacive calcite and replacive and/or displacive quartz. The presence of extensive late diagenetic mineral precipitates indicates that there was a degree of element mobility on at least the local scale. Aluminium present in albite is most likely to have resulted from the illitization of smectite. Quartz overgrowths probably resulted from illitization and the pressure dissolution at quartz silt grain boundaries. Externally, hydrothermal fluids resulting from regional-scale igneous activity appear to have played a role in the formation of chlorite and possibly albite. The work indicates that extensive mineral development and element mobility occurred during late diagenesis.  相似文献   

20.
This study characterizes organic carbon (Corganic) and phosphorus (P) geochemistry in surface sediments of the Sacramento-San Joaquin Delta, California. Sediment cores were collected from five sites on a sample transect from the edge of the San Francisco Bay eastward to the freshwater Consumnes River. The top 8 cm of each core were analyzed (in 1-cm intervals) for Corganic, four P fractions, and redox-sensitive trace metals (uranium and manganese). Sedimentary Corganic concentrations and Corganic: P ratios, decreased, while reactive P concentrations increased moving inland in the Delta. The fraction of total P represented by organic P increased inland, while that of authigenic P was higher bayward than inland reflecting increased diagenetic alteration of organic matter toward the bayward end of the transect. The redox indicator metals are consistent with decreasing sedimentary suboxia inland. The distribution of P fractions and C:P ratios, reflect the presence of relatively labile organic matter in upstream surface sediments. Sediment C and P geochemistry is influenced by site-specific particulate organic matter sources, the sorptive power of the sedimentary material present, physical forcing, and early diagenetic transformations presumably driven by Corganic oxidation.  相似文献   

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