Fluorescence spectroscopy of U(VI)-silicates and U(VI)-contaminated Hanford sediment

Time-resolved U(VI) laser fluorescence spectra (TRLFS) were recorded for a series of natural uranium-silicate minerals including boltwoodite, uranophane, soddyite, kasolite, sklodowskite, cuprosklodowskite, haiweeite, and weeksite, a synthetic boltwoodite, and four U(VI)-contaminated Hanford vadose zone sediments. Lowering the sample temperature from RT to ∼ 5.5 K significantly enhanced the fluorescence intensity and spectral resolution of both the minerals and sediments, offering improved possibilities for identifying uranyl species in environmental samples. At 5.5 K, all of the uranyl silicates showed unique, well-resolved fluorescence spectra. The symmetric O = U = O stretching frequency, as determined from the peak spacing of the vibronic bands in the emission spectra, were between 705 to 823 cm⁻¹ for the uranyl silicates. These were lower than those reported for uranyl phosphate, carbonate, or oxy-hydroxides. The fluorescence emission spectra of all four sediment samples were similar to each other. Their spectra shifted minimally at different time delays or upon contact with basic Na/Ca-carbonate electrolyte solutions that dissolved up to 60% of the precipitated U(VI) pool. The well-resolved vibronic peaks in the fluorescence spectra of the sediments indicated that the major fluorescence species was a crystalline uranyl mineral phase, while the peak spacing of the vibronic bands pointed to the likely presence of uranyl silicate. Although an exact match was not found between the U(VI) fluorescence spectra of the sediments with that of any individual uranyl silicates, the major spectral characteristics indicated that the sediment U(VI) was a uranophane-type solid (uranophane, boltwoodite) or soddyite, as was concluded from microprobe, EXAFS, and solubility analyses.     

Distributions of pesticides and organic contaminants between water and suspended sediment,San Francisco Bay,California

Suspended-sediment and water samples were collected from San Francisco Bay in 1991 during low river discharge and after spring rains. All samples were analyzed for organophosphate, carbamate, and organochlorine pesticides; petroleum hydrocarbons; biomarkers; and polynuclear aromatic hydrocarbons. The objectives were to determine the concentrations of these contaminants in water and suspended sediment during two different hydrologic conditions and to determine partition coefficients of the contaminants between water and sediment. Concentrations of hydrophobic contaminants, such as polynuclear aromatic hydrocarbons, varied with location of sample collection, riverine discharge, and tidal cycle. Concentrations of hydrophobic contaminants in suspended sediments were highest during low river discharge but became diluted as agricultural soils entered the bay after spring rains. Polynuclear aromatic hydrocarbons defined as dissolved in the water column were not detected. The concentrations sorbed on suspended sediments were variable and were dependent on sediment transport patterns in the bay. In contrast, the relatively hydrophilic organophosphate pesticides, such as chlorpyrifos and diazinon, has a more uniform concentration in suspended sediment. These pesticides were detected only after spring rains. Most of the measured diazinon, at least 98% for all samples, was in the dissolved phase. Measured partition coefficients for diazinon generally were uniform, which suggests that suspended-sediment concentrations were close to equilibrium with dissolved concentrations. The concentration of diazinon sorbed to suspended sediments, at any given sampling site, was driven primarily by the more abundant solution concentration. The concentrations of diazinon sorbed to suspended sediments, therefore, were independent of the patterns of sediment movement.     

The biogeochemistry of sorbed methane in marine sediments

Sorption of volatile hydrocarbon gases (VHCs) to marine sediments is a recognized phenomenon that has been investigated in the context of petroleum exploration. However, little is known about the biogeochemistry of sorbed methane and higher VHCs in environments that are not influenced by thermogenic processes. This study evaluated two different extraction protocols for sorbed VHCs, used high pressure equipment to investigate the sorption of methane to pure clay mineral phases, and conducted a geochemical and mineralogical survey of sediment samples from different oceanographic settings and geochemical regimes that are not significantly influenced by thermogenic gas. Extraction of sediments under alkaline conditions yielded higher concentrations of sorbed methane than the established protocol for acidic extraction. Application of alkaline extraction in the environmental survey revealed the presence of substantial amounts of sorbed methane in 374 out of 411 samples (91%). Particularly high amounts, up to 2.1 mmol kg⁻¹ dry sediment, were recovered from methanogenic sediments. Carbon isotopic compositions of sorbed methane suggested substantial contributions from biogenic sources, both in sulfate-depleted and sulfate-reducing sediments. Carbon isotopic relationships between sorbed and dissolved methane indicate a coupling of the two pools. While our sorption experiments and extraction conditions point to an important role for clay minerals as sorbents, mineralogical analyses of marine sediments suggest that variations in mineral composition are not controlling variations in quantities of sorbed methane. We conclude that the distribution of sorbed methane in sediments is strongly influenced by in situ production.     

Approaches to surface complexation modeling of Uranium(VI) adsorption on aquifer sediments

Uranium(VI) adsorption onto aquifer sediments was studied in batch experiments as a function of pH and U(VI) and dissolved carbonate concentrations in artificial groundwater solutions. The sediments were collected from an alluvial aquifer at a location upgradient of contamination from a former uranium mill operation at Naturita, Colorado (USA). The ranges of aqueous chemical conditions used in the U(VI) adsorption experiments (pH 6.9 to 7.9; U(VI) concentration 2.5 · 10⁻⁸ to 1 · 10⁻⁵ M; partial pressure of carbon dioxide gas 0.05 to 6.8%) were based on the spatial variation in chemical conditions observed in 1999-2000 in the Naturita alluvial aquifer. The major minerals in the sediments were quartz, feldspars, and calcite, with minor amounts of magnetite and clay minerals. Quartz grains commonly exhibited coatings that were greater than 10 nm in thickness and composed of an illite-smectite clay with occluded ferrihydrite and goethite nanoparticles. Chemical extractions of quartz grains removed from the sediments were used to estimate the masses of iron and aluminum present in the coatings. Various surface complexation modeling approaches were compared in terms of the ability to describe the U(VI) experimental data and the data requirements for model application to the sediments. Published models for U(VI) adsorption on reference minerals were applied to predict U(VI) adsorption based on assumptions about the sediment surface composition and physical properties (e.g., surface area and electrical double layer). Predictions from these models were highly variable, with results overpredicting or underpredicting the experimental data, depending on the assumptions used to apply the model. Although the models for reference minerals are supported by detailed experimental studies (and in ideal cases, surface spectroscopy), the results suggest that errors are caused in applying the models directly to the sediments by uncertain knowledge of: 1) the proportion and types of surface functional groups available for adsorption in the surface coatings; 2) the electric field at the mineral-water interface; and 3) surface reactions of major ions in the aqueous phase, such as Ca²⁺, Mg²⁺, HCO₃⁻, SO₄²⁻, H₄SiO₄, and organic acids. In contrast, a semi-empirical surface complexation modeling approach can be used to describe the U(VI) experimental data more precisely as a function of aqueous chemical conditions. This approach is useful as a tool to describe the variation in U(VI) retardation as a function of chemical conditions in field-scale reactive transport simulations, and the approach can be used at other field sites. However, the semi-empirical approach is limited by the site-specific nature of the model parameters.     

Uranium(VI) sorption on iron oxides in Hanford Site sediment: Application of a surface complexation model

Sorption of U(VI) on Hanford fine sand (HFS) with varying Fe-oxide (especially ferrihydrite) contents showed that U(VI) sorption increased with the incremental addition of synthetic ferrihydrite into HFS, consistent with ferrihydrite being one of the most reactive U(VI) sorbents present in natural sediments. Surface complexation model (SCM) calculations for U(VI) sorption, using only U(VI) surface-reaction constants obtained from U(VI) sorption data on freshly synthesized ferrihydrite at different pHs, were similar to the measured U(VI) sorption results on pure synthetic ferrihydrite and on HFS with high contents of ferrihydrite (5 wt%) added. However, the SCM prediction using only U(VI) sorption reactions and constants for synthetic ferrihydrite overestimated U(VI) sorption on the natural HFS or HFS with addition of low amounts of added ferrihydrite (1 wt% added). Over-predicted U(VI) sorption was attributed to reduced reactivity of natural ferrihydrite present in Hanford Site sediments, compared to freshly prepared synthetic ferrihydrite. Even though the SCM general composite (GC) approach is considered to be a semi-quantitative estimation technique for contaminant sorption, which requires systematic experimental data on the sorbent–sorbate system being studied to obtain credible SCM parameters, the general composite SCM model was still found to be a useful technique for describing U(VI) sorption on natural sediments. Based on U(VI) batch sorption results, two simple U(VI) monodentate surface species, SO_UO₂HCO₃ and SO_UO₂OH on ferrihydrite and phyllosillicate in HFS, respectively, can be successfully used to describe U(VI) sorption onto Hanford Site sediment contacting varying geochemical solutions.     

An investigation into the effects of diffusion on salinity distribution beneath the Fraser River Delta, Canada

Electrical conductivity (EC) and geochemical data were interpreted to determine the nature, origin and distribution of salinity in pore waters of sediments in a deltaic environment. The role of diffusion as a mechanism for transporting saline water within lower permeability prodelta and delta slope sediments is specifically investigated. Characteristic vertical salinity profiles at several different regions of the Fraser River Delta, British Columbia (Canada) are identified, including relatively shallow salinity zones in areas currently and historically near main river channels, and deeper salinity zones reaching up to 300 m depth in delta-front and inland areas. Comparison of salinity profiles with the results of a simple salt transport model suggests that diffusion may be a significant mechanism controlling the observed distribution of salinity in current or former estuarine areas of the delta.Density-effects were found not to be significant given the low permeability of the silt through which the salt is diffusing; however, in similar environments with higher permeability sediments, density effects may be significant. In inland and delta front areas, salinity extends to a considerable depth in the silts, beyond what would appear to be possible by diffusion alone, and points to a connate origin.     

Seasonal transport pattern of chromium(III and VI) in a stream receiving wastewater from tanneries

Water, suspended matter and sediment samples from a system heavily impacted by wastewater from numerous small tanneries (the upper Dunajec River, southern Poland) were collected to establish the annual cycle of Cr. To study possible oxidation processes the speciation of Cr and Mn concentrations were also investigated. This study showed that Cr(III and VI) temporal and spatial distributions are regulated by coupled anthropogenic (source location and emissions) and hydrologic factors (water flow and particle transport). Chromium(III) concentrations in all compartments varied seasonally as a function of the hydrological regime, production cycle in tanneries and distance from the discharge location. Cr(III) was largely associated with the particulate phase and rapidly deposited in river bed sediments. Contaminated river sediments were however flushed during flood periods to the reservoir located downstream from tanneries. During the tanning season (November to March), Chromium(III) concentrations in the water column and total Cr concentration in sediments exceeded relevant ecotoxicological guidelines only upstream from the reservoir, which trapped about 70% of the annual Cr(III) load transported by the Dunajec river. A correlation between Cr(VI)/Cr(III) ratios and Mn concentration in sediments downstream from the reservoir suggests the possibility of Cr(III) oxidation in natural conditions.     

Behavior of Uranium in the Yellow River Plume (Yellow River Estuary)

The Yellow River (Huanghe) is the second largest river in China and is known for its high turbidity. It also has remarkably high levels of dissolved uranium (U) concentrations (up to 38 nmol 1^-1). To examine the mixing behavior of dissolved U between river water and seawater, surface water samples were collected along a salinity gradient from the Yellow River plume during September 2004 and were measured for dissolved U concentration,²³⁴U:²³⁸U activity ratio, phosphate (PO₄ ^3–), and suspended particulate matter. Laboratory experiments were also conducted to simulate the mixing process in the Yellow River plume using unfiltered Yellow River water and filtered seawater. The results showed a nonconservative behavior for dissolved U at salinities < 20 with an addition of U to the plume waters estimated at about 1.4 X 10⁵ mol yr^–1. A similarity between variations in dissolved U and PO₄ ^3– with salinity was also found. There are two major mechanisms, desorption from suspended sediments and diffusion from interstitial waters of bottom sediments, that may cause the elevated concentrations of dissolved U and PO₄ ^3– in mid-salinity waters. Mixing experiments indicate that desorption seems more responsible for the elevated dissolved U concentrations, whereas diffusion influences more the enrichment of PO₄ ^3–.     

Phosphorus Speciation in Stream Bed Sediments from an Agricultural Watershed: Solid-Phase Associations and Sorption Behavior

The sorption behavior and solid-phase associations of phosphorus (P) in fine-grained sediments (<63 μm) from two upstream tributaries and one downstream main stem site of the Spoon River in west-central Illinois were characterized to better understand phosphorus bioavailability in this agriculturally dominated watershed. The P sorption affinities, as indicated by linear distribution coefficients (K _d), of all sediments were 330–5,150 L/kg, and negatively correlated with equilibrium phosphorus concentration (EPC_o) values, which ranged between 0.2 and 2.2 μM. pH values measured at the conclusion of the sorption experiments varied only slightly (7.45–8.10) but were nonetheless strongly positively correlated to EPC_o values, and negatively correlated to K _d values, suggesting the importance of pH to the observed sorption behavior. K _d values were generally lower and EPC_o values higher at the main stem site than at the upstream tributary sites, suggesting dissolved reactive P (DRP) bioavailability (specifically orthophosphate) increased downstream. The solid phase associations of P were operationally assessed with the streamlined SEDEX (sedimentary extraction) procedure, and most sediment P (≥50%) was released during the step designed to determine iron oxide–associated P. On average, 70–90% of the total sediment P pool was potentially bioavailable, as estimated by the sum of the iron oxide-, authigenic carbonate-, and organic-associated P fractions. Considerable calcium was also extracted from some sediments during the step designed to specifically remove iron oxide–associated P. It is hypothesized that the severe drought conditions that persisted between April and October, 2005 allowed authigenic carbonates (perhaps partly amorphous) to accumulate, and that these carbonates dissolved during the iron oxide extraction step. The extensive benthic algal populations also present may have aided carbonate precipitation, which under more normal hydrologic conditions would be periodically flushed downstream and replaced by fresh sediment. This suggests antecedent hydrologic conditions played a dominant role in the P sorption and solid phase associations identified.     

Uranium(VI) adsorption and surface complexation modeling onto vadose sediments from the Savannah River Site

Uranium U(VI) adsorption was measured as function of pH (3–10) on goethite, kaolinite, quartz, two binary mixtures of goethite and kaolinite, and a vadose zone sediment collected on The Department of Energy’s Savannah River Site (SRS), the clay mineral fraction of which is composed largely of kaolinite and goethite. Diffuse-layer surface complexation models were parameterized using the code PEST together with PHREEQC to fit U(VI) sorption data for the pure goethite, kaolinite, and quartz. U(VI) adsorption on kaolinite and goethite was modeled as the formation of two bidentate U(VI) complexes at mineral edge sites on a variable charge site. U(VI) adsorption on quartz was described using a one-site diffuse-layer with the formation of bidentate complex on a variable charge site. These models were used to predict U(VI) adsorption on the binary sorbent mixtures and the SRS sediment using a simple component-additivity approach. In general, the predicted adsorption edges were in good agreement with measured data, with statistically similar goodness of fit compared to that obtained for the pure mineral systems.     

Behavior and budget of dissolved uranium in the lower reaches of the Yellow (Huanghe) River: Impact of Water–Sediment Regulation Scheme

The Water–Sediment Regulation Scheme (WSRS) is an important water conservancy project in the Yellow River basin, which is usually operated annually from June to July to control water and sediment release from the Xiaolangdi Reservoir in the middle reaches. As a greatly concentrated period of delivering terrigenous materials from the Yellow River to the sea, the WSRS can serve as a natural laboratory to examine the geochemical behavior of elements during their transport along the river. Uranium isotopes (²³⁴U and ²³⁸U) were measured in Yellow River waters at stations Xiaolangdi (located in the middle reaches of the Yellow River) and Lijin (the last hydrologic station near the Yellow River estuary) during the WSRS 2012. Compared with station Xiaolangdi, dissolved uranium concentration at station Lijin was markedly higher, showing a significant impact from the WSRS. Budget calculation for dissolved uranium during the WSRS indicated that two major sources of new added dissolved uranium in the section of the Yellow River between Xiaolangdi and Lijin: suspended particles (46%) and porewater of bottom sediment (45%). The flux of dissolved uranium from the Yellow River to the sea was estimated to be 2.40 × 10⁷ g during the WSRS 2012.     

Distribution of AVS-SEM, transformation mechanism and risk assessment of heavy metals in the Nanhai Lake in China

Distribution of AVS (acid volatile sulfide)-SEM (simultaneously extracted metals), transformation mechanism and risk assessment of heavy metals in the Nanhai Lake in Baotou City were discussed in this work. The results showed that the content of heavy metals in sediments increased due to the water pumped from the Yellow River, domestic sewage, municipal runoff and yacht waste release. Increasing water depth, domestic sewage influx and hydrophyte booming made the AVS level higher in downstream than upstream. The vertical distribution of AVS is characterized as multiple-peak in the sediment cores from the studied lake. Comparatively, the control abilities of the carbonate and sulfate to the heavy metals were five orders of magnitude lower than the sulfide phase. Therefore, AVS was the key factor controlling the precipitation of heavy metals in the Nanhai Lake. The ratio of SEM/AVS in the sediments, the acute sediment quality criteria and the chronic sediment quality criteria indicated that no acute toxicity for benthic organisms can be expected, and the AVS plays an important role in controlling the bioavailability and toxicity of heavy metals in the Nanhai Lake.     

Spatial variability of soils and stream sediments and the remediation effects in a Portuguese uranium mine area

The old Senhora das Fontes uranium mine, located in central Portugal, was closed down in 1971. The treatment of ores from this mine and other mines by heap-leach ended in 1982. Seven dumps partially covered by vegetation were left in the area. Soil and stream sediment samples were collected in December 2009. The remediation was carried out from May 2010 to January 2011. Stream sediment samples were collected again in October 2013. Before the remediation, soils from inside the mine influence area have higher Al, As, Co, Cr, Cu, Fe, Ni, Sr, Th, U and Zn concentrations than soils from outside this area, due to radionuclides, metals and metalloid released from the mine dumps. The principal component analysis (PCA) shows a distinction between soils from inside and outside the mine influence area. The U(VI), As(V) and metals from soils can be adsorbed to Fe-oxyhydroxides and the humic acid can increase the U uptake. Soils must not be used for public or private green and residential areas, because they are contaminated in U, As, Co, Cd and Ni. Before the remediation, downstream sediments have higher Al, As, Cu, Mn, Ni, Pb, U and Zn than upstream sediments, due to erosion and percolation of water through the mine dumps. The PCA shows a distinction between downstream and upstream sediments. The U(VI), Th and As(V) can be adsorbed to Fe-oxyhydroxides. The stream sediments are contaminated in As, Mn, Th and U. Downstream sediments are the most contaminated in U and As. After the remediation, upstream and downstream sediments have generally higher Al, Fe, As, Cr, Ni, Th, U and Zn concentrations than before the remediation, attributed to the relocation of dumps. Radionuclides, metals and metalloids were transported by surface water. Consequently downstream sediments have higher Al, As, Cu, Mn, Ni, Th, U and Zn concentrations than upstream sediments. The U(VI), Th and As(V) can be adsorbed to Fe-oxyhydroxides. Stream sediments became more contaminated in U, Th and As than before the remediation, but more intensively downstream.     

Exchange of polychlorinated biphenyls between sediment and water in the Hudson River estuary

Sediment-bound polychlorinated biphenyls (PCBs) measured at several sites in the lower Hudson River Estuary are above equilibrium with the overlying water, providing a thermodynamic driving force for exchange from sediment to water. The fluxes of PCB congeners are estimated for a number potential processes: diffusive release of dissolved and colloidal PCBs from the bed, resuspension and subsequent desorption from resuspended particles, and sediment accumulation and burial. All processes are potentially significant, at least for some congeners. The sediment exchange fluxes of PCB solutes for five sites between Governors Island and Haverstraw Bay are estimated to be 20–860 μm m^?2 d^?1, which is between 2 and 100 times more than the flux of dissolved PCBs coming down the river at Haverstraw Bay. Much of the exchange from sediment to water may be balanced by burial of sorbed PCBs by sediment accumulation. Advection down the river and sediment exchange dominate other potential sources of PCBs to the lower estuary under current loading conditions. The magnitude of the calculated fluxes is consistent with the nonconservative behavior of PCBs in the same region but is higher than earlier modeling estimates that employed different assumptions.     

南海表层沉积物中的碳酸钙含量分布及其影响因素

通过测定南海213个表层沉积物样品中的碳酸钙含量, 综合分析整个南海海域碳酸钙含量分布特征及其控制因素.结果表明, 不同的区域海洋环境, 控制表层沉积物中碳酸钙含量变化的因素也不尽相同: 大陆架区, 碳酸钙含量主要受陆源非碳酸盐物质的稀释作用而较低; 大陆坡区, 碳酸钙因丰富的物源量、低的陆源物质输入量和弱的碳酸钙溶解作用等因素而呈较高含量; 深海盆区, 碳酸钙含量因强烈的溶解作用而较低.根据碳酸钙含量在南海整个表层沉积物中的分布趋势, 推测南海纬度14°N以北的海域碳酸钙补偿深度(CCD)为3700m左右, 纬度14°N以南的海域CCD为4000m左右.Pearson相关分析表明, 南海表层沉积物中钙质超微化石对碳酸钙的含量分布具有较高的贡献率.      

Effect of organic matter on DOM sorption on lake sediments

The effect of organic matter on the sorption of dissolved organic matter (DOM) on lake sediments is critical to understanding the fate and transport of contaminants at the sediment–water interface in lake ecosystems. Results indicate that DOM sorption on sediment is largely due to ligand exchange between the DOM and hydroxyl groups, and the amount of DOC sorbed is a linear function of added DOC. With increasing organic matter content the sediment has lower binding strength, higher releasing ability for DOM, and the higher amount of DOM sorbed by sediment naturally. There was no clear difference before and after the sediment was treated with H₂O₂, but the constant b implied that after the sediments were treated DOC release was promoted. Organic matter in the sediment tends to impede the sorption of DOC and results in a remarkable decrease in DOC sorption rates.     

南沙海区晚第四纪的碳酸盐旋回

根据水深200～2 800m范围内的17个沉积柱状样分析结果,讨论南沙海区晚第四纪沉积中碳酸钙相对含量的垂向和横向变化。南沙海区的碳酸盐旋回普遍属于"大西洋型",冰期时含量低、间冰期含量高,反映出陆源物输入量的控制作用。在平面上,南沙海区的碳酸钙含量与纬度几乎呈线性关系,由北向南递减,冰期时趋势不变而梯度加大。南海南部陆坡由于集水盆地处于热带湿热条件而且有大河注入海区,使碳酸钙含量相对北部陆坡为低。     

The effect of sediment-water exchange on manganese deposition and nodule growth in Jervis Inlet,British Columbia

The processes controlling Mn deposition in the vicinity of a nodule site in Jervis Inlet, a fjord in the British Columbia mainland coast, were studied. The results indicate that Mn, due to reductive remobilization in the subsurface sediments, undergoes intensive exchange across the sediment-water interface and, because of this exchange and the effects of water circulation processes within the inlet, tends to be stripped from shallow water sediments and to be redeposited as an oxide precipitate in the surface sediment of the deepest areas. There it undergoes diagenetic reactions in the subsurface sediments that, as demonstrated by saturation of the pore waters, finally result in its burial as a carbonate phase. The distribution of properties in the vicinity of the Mn nodule site indicates that concretion growth occurs in an area of strong currents and low sedimentation rate where rocks resting on the bottom act as centers to which Mn-oxide particles suspended in the bottom water are apparently being cemented.     

Grain size and geochemistry of surface sediments in northwestern continental shelf of the South China Sea

Eighty-six surface sediments collected from the northwestern continental shelf of the South China Sea (SCS) were analyzed for grain size distribution, calcium carbonate, organic carbon, and major and trace element compositions to investigate sediment provenance and factors controlling their geochemical composition. Sediments from the eastern continental shelf of Hainan Island have higher sand and lower clay content, while the samples from the nearshore Hainan Island have higher contents of gravel and clay. Calcium carbonate contents in samples show a positive correlation with water depth in northwestern shelf of SCS, suggesting that it is related to biological factors. However, the nearshore sediments have higher contents of organic carbon compared to those of the outer shelf, possibly suggesting that the terrigenous organic matter usually deposited in nearshore environments such as bays and estuaries. Compared with the upper continental crust, the samples have relatively lower contents of SiO₂ and Al₂O₃, higher than those of the Pearl and Red river sediments. The low contents of K₂O and Na₂O in sediments from the northwestern continental shelf are consistent with intense chemical weathering in the river basin due to the seasonally hot and humid climate regime. The sediments mainly consist of three components, including the gravel fraction composed of calcareous debris, the sand fraction composed of quartz, and the silt and clay fractions mainly composed of clay minerals. The content of each component depends on grain size, sediment source, biogenesis, and hydrodynamic conditions, which finally controls the chemical composition of the sediments. The distributions of Co/Al₂O₃, Cr/Al₂O₃, and Zr/Sc ratios for sediments in the northwestern continental shelf suggest that source rocks are mainly composed of felsic rocks rather than mafic rocks. There is a distinct difference in sediment source between eastern and western shelf sediments; the eastern shelf sediments are characterized by high Zr/Sc ratios mainly derived from the Pearl River, while the western shelf sediments have relatively low values of Zr/Sc indicating a main contribution possibly sourced from the Red River Basin. Terrigenous materials from Hainan Island usually influence the geochemistry of sediments deposited in the nearshore area.     

Interaction of organic compounds with calcium carbonate—III. Amino acid composition of sorbed layers

Nitrogenous organic compounds in sorbed surface layers and in calcified organic matter associated with calcium carbonate sediment particles consist of 40–50% amino acids, 2% amino sugars and 25% ammonia. In grain size classes > 20 μm these compounds are mainly contained in the calcified protein of carbonate secreting organisms but with smaller grain sizes—and consequently increased specific surface area—they are contained in sorbed layers at the mineral surface. The composition of the sorbed layer is characterized by a predominance of neutral amino acids, a relative enrichment of basic and weakly polar amino acids, and a deficiency of acidic amino acids in comparison with the proteinaceous matter of calcifying organisms. The respective abundances for sorbed and calcified matter are: 505 and 380 Res./ of neutral amino acids, 262 and 450 Res./1000 of acidic amino acids, 92 and 51 Res./l000 of basic amino acids, and 141 and 129 Res./1000 of weakly polar amino acids.The composition of the sorbed layer appears to be the result of sorption of proteinaceous matter from solution since it reflects the free and peptide-bound amino acid composition of seawater. The characteristic amino acid assemblage could also be the result of preferential decomposition of protein and subsequent enrichment of neutral and basic amino acids; however, sorption from solution appears more likely since the total amount of amino acids sorbed to calcium carbonate (0.58 mg m ^?2) corresponds closely to the amount of protein known to cover one m² of aqueous substrate in monolayer arrangement. Sorption from solution is further supported by the low arginine/ornithine ratios in both the sorbed layer and the natural dissolved organic matter. This process might lead to a characteristic amino acid spectrum in fine grained calcareous sediments that reflects the composition of the dissolved organic matter in seawater rather than that of the carbonate secreting proteinaceous matter.