Refractory organic matter in sediments from the North–West African upwelling system: abundance,chemical structure and origin

Refractory (insoluble and nonhydrolyzable) organic matter (ROM) was studied in sediments underlying the north–west African upwelling system. Two core samples: beginning of glacial isotope stage 4 (ca. 70,000 yr BP; depth bsf 360 cm) and interglacial isotope substage 5d (ca. 90,000 yr BP; depth bsf 480 cm) were analyzed, along with a surface sample. ROM, which accounts for a substantial part of the total organic matter (OM) in these sediments (ca. 20% for the two core samples and 10% for the surface sample), was isolated and examined for its morphological and chemical features using a combination of transmission and scanning electron microscopy, spectroscopy (FTIR, solid state ¹³C NMR) and analytical pyrolysis (Curie point flash pyrolysis–GC–MS). These studies allowed the chemical structure of the three ROMs (chiefly based on melanoidin-type macromolecules) and their mechanism of formation (degradation–recondensation of products mainly derived from proteinaceous material) to be established. Such a formation probably began in the water column and continued within the sediment upper layers. Important differences, concerned both with OM chemical structure and preservation mechanism, were noted upon comparison with recently studied sediments underlying the Peru upwelling system. These differences between the two systems, in spite of similarities in primary production, must reflect the combined influence of factors such as upwelling intensity (primary productivity), water depth and iron supply.     

Elucidation of different forms of organic carbon in marine sediments from the Atlantic coast of Spain using thermal analysis coupled to isotope ratio and quadrupole mass spectrometry

Analysis of river, estuary and marine sediments from the Atlantic coast of Spain using thermogravimetry–differential scanning calorimetry–quadrupole mass spectrometry–isotope ratio mass spectrometry (TG–DSC–QMS–IRMS) was used to (a) distinguish bulk chemical hosts for C within a sediment and humic acid fraction, (b) track C pools with differing natural C isotope ratios and (c) observe variation with distance from the coast. This is the first application of such a novel method to the characterisation of organic matter from marine sediments and their corresponding humic acid fractions. Using thermal analysis, a labile, a recalcitrant and a refractory carbon pool can be distinguished. Extracted humic fractions are mainly of recalcitrant nature. The proportion of refractory carbon is greatest in marine sediments and humic acid fractions. Quadrupole mass spectrometry confirmed that the greatest proportion of m/z 44 (CO₂) and m/z 18 (H₂O) were detected at temperatures associated with recalcitrant carbon (510–540 °C). Isotope analysis detected progressive enrichment in δ¹³C for the sediment samples with an increase in marine influence. Isotopic heterogeneity in the refractory organic matter in marine sediments could be due to products of anthropogenic origin or natural combustion products. Isotope homogeneity of humic acids confirms the presence of terrigenous C in marine sediments, allowing the terrestrial input to be characterised.     

Oxidation and Origin of Organic Matter in Surficial Eastern Mediterranean Hemipelagic Sediments

Aerobic mineralisation of C_org in surface sedimentsof the deep (>2000 m water depth) eastern Mediterranean Sea has been quantified by analysis of detailedbox core C_org concentration versus depth profiles and the modelling environment for early diageneticproblems MEDIA. The reactive fraction comprises 60–80% of the total C_org reachingthe sediments and is largely oxidised within the surficial 10 cm. A non-reactive C _orgfraction (G_NR) dominates at depths >10 cm, and makes up20–40% of the total C _org flux to the sediments. First-order rateconstants for decomposition of the reactive fraction calculated from theC _org profiles range from 5.4 × 10^-3 to8.0 × 10^-3 y^-1 to 8.0 × 10^-3 y^-1. Total mineralization rates in thesurface sediment are between 1.7 and 2.6 mol C cm^-2 y^-1 and thus are typical for oligotrophic, deep-seaenvironments. The low fluxes and rapid remineralisation of C _org are accompanied by²¹⁰Pb_excess surface mixed layers which are only 2 cm deep, among the thinnest reported for oxygenated marine sediments.Model results indicate a mismatch between the C _org profiles and O₂ microprofileswhich were measured onboard ship. This can be attributed to a combination of decompression artefactsaffecting onboard measurement of the O₂ profiles or the leakage ofoxygen into the core during handling on deck. Furthermore, the used D_b values, based on ²¹⁰Pb, may not befully appropriate; calculations with higher D_b values improve the O₂ fits. The surficial sediment¹³C _org values of -22 become less negative with increasing depth and decreasing C _orgconcentrations. The major ¹³C change occurs in the top 3 to 4 cm and coincides with the interval weremost of the organic carbon oxidation takes place. This indicates that the reactive fractionof organic matter, commonly assumed to be marine, has a more negative ¹³C _orgthan the refractory fraction, usually held to be terrestrial. Palaeoproductivity estimates calculated from thesediment data by means of literature algorithms yield low surface productivities(12–88 gC m^-2 y^-1), which are in good agreement with field measurements of primary productivity in otherstudies. Such values are, however, significantly lower than those indicated by recent productivitymaps of the area derived from satellite imagery (>100 gC m^-2 y^-1).     

Distribution of copper, zinc, lead and cadmium concentrations in stream sediments from the Mapocho River in Santiago, Chile

The Mapocho river, which crosses downtown Santiago, is one of the most important rivers in contact with a population of about six million inhabitants. Anthropogenic activities, industrialization, farming activities, transport, urbanization, animal and human excretions, domestic wastes and copper mining have affected the river, contaminating it and its sediments with heavy metals. Concentration and distribution of Cu, Zn, Pb and Cd were studied with the purpose of determining their bioavailability and their relation with the characteristics of the sediments. Freshly deposited seasonal sediments were collected from 0–8 cm depths from 6 locations (S₁ to S₆) along the 30-km long channel length, in the four seasons of year on the following dates: May 2001 (D₁, autumn); August 2001 (D₂, winter); October 2001 (D₃, spring) and January 2002 (D₄, summer). The dried samples were sifted to obtain the < 63-μm sediment fraction, since it has been shown that large amounts of heavy metals are bound in the fine-grained fraction of the sediment. Cu and Zn were analyzed by atomic absorption spectrophotometry and Pb and Cd by square wave anodic stripping voltammetry. The highest concentrations of Cu (2850 μg g^− 1) were found in the northern part of the river (S₁, average D₁–D₄), near the mountains and a copper mine, and then decreased downstream to 209 μg g^− 1 (S₆). Total Zn showed an irregular variation, with higher values at S₁ (1290 μg g^− 1) and high values in some winter sampling (1384 μg g^− 1 S₄, S₅–D₂). Pb showed different trends, increasing from S₁ to S₆ (17 to 61 μg g^− 1), with the highest values in the summer samples (83 μg g^− 1, S₄–S₆, D₄), and total Cd increased slightly from mean values of 0.2 and 0.5 μg g^− 1. Partition into five fractions was made using Tessier's analytical sequential extraction technique; the residue was treated with aqua regia for recovery studies, although this step is not part of the Tessier procedure. The results show that Cu, Zn and Pb in the sediments were dependent on the sampling places along the river, and variation in two years was low (D₁–D₄). The highest values of total organic matter, carbonate and conductivity were found in S₆, which has the smallest size particles, while at S₁ the sediments were predominantly sand and contain larger amounts of silica. Cu associated with carbonate decreased gradually from 58% (1771 μg g^− 1, S₁) to 16% (32 μg g^− 1, S₆); Cu bonded to reducible fraction was almost constant (33% to 37%), and Cu associated with oxidizable fraction increased from 7% (S₁) to 34% (S₆), but copper content was lower (214 to 68 μg g^− 1). Zn had a similar fractionation profile. However, Pb bound to oxidizable fraction did not show significant percent variation along the river (20% to 19%), but the amount bounded was 4 to 12 μg g^− 1. The residual fraction increased from 24% to 41% (5 to 25 μg g^− 1, S₁ to S₆). The distribution of Cd in the sediment was almost independent of the sampling stations and was bound to carbonate, reducible and residual fraction in similar proportion. Cu and Zn at S₁ were mainly bound to carbonates and reducible phases with 91% and 73% (2779 and 965 μg g^− 1, respectively), and with a change in the pH and/or the redox potential of the sediment–water system, these contaminants could easily enter the food chain. In S₆ the amount of Cu and Zn in these phases was 50% and 53% (100 to 313 μg g^− 1, respectively).     

Characterization of the reactivity of riverine heterogeneous sediments using a facies-based approach; the Rhine–Meuse delta (The Netherlands)

Large groundwater resources are found in densely populated lowland areas, which consist often of young unconsolidated and reduced sediments. When anthropogenic activities lead to oxygenation of the aquifer, breakdown of the main reduced fractions, i.e. sedimentary organic matter (SOM) and pyrite, could lead to severe groundwater deterioration such as acidification, heavy metal mobilization, and increased hardness. The characterization of the reactive properties of these sediments is important in predicting groundwater deterioration, but is often complicated by the high degree of heterogeneity of these sediments. In this study, the potential reduction capacity (PRC, based on SOM and pyrite content), the potential buffer capacity (PBC, based on carbonate content), potential acidification capacity (PAC, based on the potential acid production by sulfide oxidation), and the measured reduction capacity (MRC) of five facies, which are typical of the riverine sediments in the Rhine–Meuse delta (The Netherlands) were determined. A universal facies-classification model was used to classify the deposits into more homogeneous sub-units based on lithologic and geogenic properties, with a further sub-division into oxic or anoxic redox environment based upon groundwater data and field observations. The bulk chemical data show strong variation across facies for the median values of PRC (186–9093 mmol O₂ kg⁻¹), PBC (17–132 mmol O₂ kg⁻¹), and PAC (36–1530 mmol H⁺ kg⁻¹). The MRC was measured as reactivity to molecular O₂ exposure and was 0.5–567.3 mmol O₂ kg⁻¹. Steady-state oxidation rates were in the wide range of 0.001–10.355 mmol O₂ kg⁻¹ day⁻¹ but were typically about 3–8 times faster in fine facies than in coarse facies. Both the PRC and MRC depend strongly on grain size, but also on the syn/post-depositional environment and redox conditions. The main part of the PRC consists of SOM, but pyrite reactivity is higher than SOM reactivity as shown by the relative depletion of pyrite in oxic subfacies and the preferential oxidation during the oxidation experiments. Some facies are very prone to acidification because the PAC is higher than the PBC, but the oxidation experiments also show that acidification could already start before the PRC is fully exhausted. This study, is one of the few that combines bulk chemical data, groundwater data, and reactivity measurements and shows that a facies-based approach is a practical tool in characterizing the reactivity of heterogeneous deposits.     

Sulphide-bearing Waters in Northern Apennines, Italy: General Features and Water-rock Interaction

Sulphide-bearing Ca-carbonate, Na-carbonate, Na-hydroxide, Na-chloride and Ca-sulphate waters from Northern Apennines were investigated in order to determine their main chemical and isotopic composition and draw inferences on water-rock interaction. ²H and ¹⁸O values suggest an origin mostly meteoric for the analysed waters but a well drilled in Miocenic sediments. The Na-carbonate and the Ca-sulphate waters are the most interesting geochemically. Na-carbonate type, which sometimes reaches extreme composition (Na/Ca up to 228, equivalent ratio), may have been derived through prolonged interaction of Ca-carbonate waters with rocks containing feldspar, montmorillonite and illite under calcite saturation/oversaturation; the high F and pH and the very low P_{CO 2} agree with prograde dissolution of silicates and lasting water-rock interaction. However, Ca–Na ion exchange, involving clays of marine origin, cannot be excluded in addition. The Ca-sulphate waters, occurring in Messinian gypsum-bearing sediments, are saturated in gypsum and calcite and exhibit very high total H₂S (up to 219 mg dm^-3) and P_{CO 2} (up to 0.32 bar). Mass balance of sulphate sulphur, sulphide sulphur and delta³⁴S suggests sulphate – derived from gypsum – as source for H₂S; CH₄ and organic matter generate the reducing conditions and sulphate reduction is mediated by bacteria. One Na-chloride water from a well in Miocenic sediments has unusual composition, containing about 700 mgdm^-3 of potential CaCl₂ and having ²H and ¹⁸O (-47.5 and -4.9 respectively) which plot far from the meteoric water lines; probably it is derived by mixing of meteoric and formation waters. The Na-hydroxide water, with very high pH (11.2), is generated through protracted interaction of meteoric waters with ultramafites.     

Loading and fate of particulate organic carbon from the Himalaya to the Ganga-Brahmaputra delta

We use the evolution of river sediment characteristics and sedimentary C_org from the Himalayan range to the delta to study the transport of C_org in the Ganga-Brahmaputra system and especially its fate during floodplain transit.A detailed characterisation of both mineral and organic particles for a sampling set of river sediments allows taking into account the sediment heterogeneity characteristic of such large rivers. We study the relationships between sediment characteristics (mineralogy, grain size, specific area) and C_org content in order to evaluate the controls on C_org loading. Contributions of C3 and C4 plants are estimated from C_org stable isotopic composition (δ¹³C_org). We use the evolution of δ¹³C_org values from the Himalayan range to the delta in order to study the fate of C_org during floodplain transit.Ganga and Brahmaputra sediments define two distinct linear relations with specific area. In spite of 4-5 times higher specific area, Ganga sediments have similar C_org content, grain size and mineralogy as Brahmaputra sediments, indicating that specific area does not exert a primary control on C_org loading. The general correlation between the total C_org content and Al/Si ratio indicates that C_org loading is mainly related to: (1) segregation of organic particles under hydrodynamic forces in the river, and (2) the ability of mineral particles to form organo-mineral aggregates.Bed and suspended sediments have distinct δ¹³C_org values. In bed sediments, δ¹³C_org values are compatible with a dominant proportion of fossil C_org derived from Himalayan rocks erosion. Suspended sediments from Himalayan tributaries at the outflow of the range have low δ¹³C_org values (−24.8‰ average) indicating a dominant proportion of C3 plant inputs. In the Brahmaputra basin, δ¹³C_org values of suspended sediments are constant along the river course in the plain. On the contrary, suspended sediments of the Ganga in Bangladesh have higher δ¹³C_org values (−22.4‰ to −20.0‰), consistent with a significant contribution of C4 plant derived from the floodplain. Our data indicate that, during the plain transit, more than 50% of the recent biogenic C_org coming from the Himalaya is oxidised and replaced by floodplain C_org. This renewal process likely occurs during successive deposition-erosion cycles and river course avulsions in the plain.     

Stable Carbon and Nitrogen Isotopic

The perennially ice-covered lakes in the McMurdo Dry Valleys, Antarctica, are part of the coldest and driest ecosystem on earth. To understand lacustrine carbon and nitrogen cycling in this end-member ecosystem, and to define paleolimnological proxies for ice-covered lakes, we measured the stable carbon and nitrogen isotopic composition of particulate organic matter (POM) and benthic organic matter (BOM) within the lakes of Taylor Valley. The ¹³C compositions of seasonally ice-free edges of the lakes (moats) are enriched relative to under-ice organic matter. Thus, the organic carbon isotopic composition of buried sediments may be a proxy for sample position within the lake. In the moats, ¹³C values are governed by limited CO₂diffusion across benthic cyanobacterial cell membranes. During a high glacial melt (2001–2002) season, both ¹³C_POM and ¹³C_BOM in the moats were more depleted than during previous low melt years. We propose that this occurred in response to higher [CO₂]_(aq) and/or reduced growth rates resulting from turbidity-induced light limitation. Though moats and under-ice environments are usually poorly connected, during the 2001–2002 season, the enrichment of the ¹³C_POM values at 6 m depth in the stream-proximal sites relative to deep-profile sites implies enhanced connectivity between these environments. The ¹³C compositions of BOM and POM profiles in Lake Hoare and Lake Fryxell indicate that these lakes are dominated by benthic productivity. In contrast, in Lake Bonney, the similarity of the ¹³C values of BOM and POM indicates the pelagic component dominance in the carbon cycle.     

The role of supergene sulphuric acid during weathering in small river catchments in low mountain ranges of Central Europe: Implications for calculating the atmospheric CO2 budget

The geochemistry of dissolved and suspended loads in river catchments of two low mountain ranges in Central Europe allows comparison of pertinent chemical weathering rates. Distinct differences in lithology, i.e. granites prevailing in the Black Forest compared to Palaeozoic sediments in the Rhenish Massif, provide the possibility to examine the influence of lithology on weathering. Here we determine the origin of river water using the stable isotope ratio δ¹⁸O_H2O and we quantify the geogenic proportions of sulphate from stable isotope ratios δ³⁴S_SO4 and δ¹⁸O_SO4. Particularly in catchments with abundant pyrite, determination of the geogenic amount of sulphate is important, since oxidation of pyrite leads to acidity, which increases weathering. Our results show that spatially averaged silicate weathering rates are higher for the river catchments Acher and Gutach in the Black Forest (10–12 t/km²/yr) compared to the river catchments of the Möhne dam and the Aabach dam in the Rhenish Massif (2–6 t/km²/yr). Correspondingly, the CO₂ consumption by silicate weathering in the Black Forest (334–395 × 10³ mol/km²/yr) is more than twice as high as in the Rhenish Massif (28–151 × 10³ mol/km²/yr). These higher rates for watersheds of the Black Forest are likely due to steeper slopes leading to higher mechanical erosion with respective higher amounts of fresh unweathered rock particulates and due to the fact that the sediments in the Rhenish Massif have already passed through at least one erosion cycle. Carbonate weathering rates vary between 12 and 38 t/km²/yr in the catchments of the Rhenish Massif. The contribution of sulphuric acid to the silicate weathering is higher in the catchments of the Rhenish Massif (9–16%) than in the catchments of the Black Forest (5–7%) due to abundant pyrite in the sediments of the Rhenish Massif. Three times higher long-term erosion rates derived from cosmogenic nuclides compared to short-term erosion rates derived from river loads in Central Europe point to three times higher CO₂ consumption during the past 10³ to 10⁴ years.     

Early diagenetic alteration of chlorophyll-a and bacteriochlorophyll-a in a contemporaneous marl ecosystem; Florida Bay

This report covers the analyses of tetrapyrrole pigments in sediments and numerous source biota from northern central Florida Bay. Sediment cores were all carbonate marls and ‘bedrock’ (Pleistocene limestones) was typically at 0.6–1.2 m bsf. (below sea floor). Extraction of the sediments was performed using tetrahydrofuran, shown to be 3–7 times as effective as more common solvents. Surficial sediments were found to contain a pigment distribution indicating a diatomaceous-cyanobacterial biofilm/mat underlain with purple sulfur bacteria as the microphytobenthos. Downhole trends in pigment diagenesis revealed differences due to paleoenvironment. That is, when bacteriochlorophyll-a (BCHLa) plus derivatives were elevated, indicating strong syn-depositional anoxia, the pheophytins-a (PTINsa) were the major chlorophyll-a (CHLa) derivatives. Conversely, when BCHL-a plus derivatives were low, indicating oxic to dys-oxic syn-depositional conditions, then pyropheophorbide-a (pPBIDa) and 13², 17²-cyclopheophorbide-a (CYCLO) were strongly dominant. The generation of cyclopheophorbide in carbonate marls with weakly anoxic to dysoxic syn-depositional conditions may yield an oil-source paleoenvironmental marker for bitumen containing compounds such as bicycloalkanoporphyrins (BiCAPs). Downhole conversions were observed for the following diagenetic steps; CHLa to pheophytin-a (PTINa), PTINa to pyropheophytin-a (pPTINa), pPBIDa to CYCLO, BCHLa to bacteriopheophytin-a (BPTINa), and BPTINa to bacteriopyropheophytin-a (BpPTINa).     

Geochemical studies of pore fluid in surface sediment on the Daini Atsumi Knoll

We collected sediment samples and pore water samples from the surface sediment on the Daini Atsumi Knoll, and analyzed the sediments for CH₄, C₂H₆, and δ¹³C_CH4, and the pore fluids for CH₄, C₂H₆, δ¹³C_CH4, Cl⁻, SO₄²⁻, δ¹⁸O_H2O, and δD_H2O, respectively. A comparison of the measured concentration and isotopic composition of methane in pore water samples with those in sediment samples revealed that methane was present in the sediment samples at a higher concentration and was isotopically heavier than those in the pore water samples. It suggests that the effect of the release of a sorbed gas bound to organic particles when heated prior to analysis of hydrocarbons was larger than that of the degassing process. A large amount of a sorbed gas would be a significant source of natural gas. Two striking features are the chemical and isotopic composition of the pore water samples taken from the different sites around the Daini Atsumi Knoll. In the KL09, KL10, and KP07 samples, Cl⁻ concentrations in the pore water samples showed depletion to a minimum of 460 mmol/kg, correspond to  17% dilution of seawater, however the latter was not enriched in CH₄. The isotopic compositions of pore water samples suggested the low-Cl⁻ fluids in the pore water were not derived from dissociation of methane hydrate, but were derived from input of meteoric water. In contrast, in the KP05 samples from the north flank of the Daini Atsumi Knoll, pore water were characterized by CH₄ enrichment more than 370 μmol/kg, but not depleted in Cl⁻ concentrations. The observed methane concentration in the KP05 samples is not sufficient for methane hydrate to form in situ, indicating that the existence of methane hydrate in the surface sediment is negligible, as supported by Cl⁻ concentration. Based on the stable carbon isotope ratio of methane in the pore fluid from the KP05 site (δ¹³C_CH4 < − 50‰PDB), methane is thought to be of microbial origin. The pore waters in the surface sediments in the north flank of the Daini Atsumi Knoll were not directly influenced by upward fluid bearing methane of thermogenic origin from a deeper part of the sedimentary layer. However, extremely high methane concentration in the north flank site as compared with the concentration of pore water taken from the normal seafloor suggests that the north flank site is not the normal seafloor. We hypothesize that upward migration of chemically-reduced fluids from a deeper zone of the sedimentary layer reduces chemically-oxidized solutes in the surface sediment. As a consequence methane production replaced sulfate reduction as the microbial metabolism in the reduced environment of the surface sediment.     

Relationship between Se/S and sulfur isotope ratios of hydrothermal sulfide minerals

The pH and f_{O 2} dependences of the [Se^2–]/[S^2–] ratio in chloride solutions at 100°, 200° and 300°C are predicted thermodynamically. Under the high f_{O 2} conditions where sulfate species are dominant in solution, the [Se^2–]/[S^2–] ratio always increases with increasing pH and/or f_{O 2}. Under the low f_{O 2} conditions where sulfide species are dominant in solution, the pH and f_{O 2} dependences of the [Se^2–]/[S^2–] ratio are seriously affected by the presence of native selenium. With native selenium present, the [Se^2–]/[S^2–] ratio decreases with increasing f_{O 2}, but almost independent of pH in geologically important pH regions. When native selenium is absent, the [Se^2–]/[S^2–] ratio is solely a function of pH and independent of f_{O 2}. Combining the above with the pH and f_{O 2} dependences of ³⁴S value of aqueous sulfur species, we discuss the possible influences of the pH and f_{O 2} of ore-forming solutions on the relationship between the Se/S ratio and ³⁴S value of hydrothermal sulfide minerals. The results are applied to some Japanese sulfide ore deposits.     

Algal pigments in sediments as a measure of eutrophication in the Baltic environment

A method of quantitative comparison of eutrophication of an area is proposed for the Baltic Sea, based on pigment content in sediments. The pigments concerned were chlorins a (i.e. chlorophylls a, b and selected chlorophyll a derivatives) and chlorophylls c. The analyses were performed on 300 samples from different layers of recent (0–10 cm) sediments, collected from about 50 stations, at different sites of the southern Baltic, including the estuaries of the two largest Polish rivers, in different seasons between 1992 and 2001, before and after the great flood of July 1997. The results are related to sampling site, sediment layer and hydrological conditions and also to organic carbon and E_h in sediments, oxygen and salinity in near-bottom waters. Depending on different chlorin a content in 0–1 and 0–10 cm layers, the sampling sites are classified into one of three groups: 1. Szczecin Lagoon and the Deep of Gda sk stations (permanently eutrophic, chl a in 0–1 cm >40 nmol/g, Σchlns a in 0–1 in 0–10 cm layer, Σchlns a_A in Σchlns a=55–65%), 2. Open sea stations (mesotrophic/oligotrophic, chl a in 0–1 cm <10 nmol/g, Σchlns a in 0–1 in 0–10 cm layer, Σchlns a_A in Σchlns a 50%; and 3. Coastal stations (periodically eutrophic, chl a in 0–1 cm 10–40 nmol/g, Σchlns a_A in Σchlns a 40%). The correlation coefficient between chlorophyll a and chlorophylls b and c indicates the classes of algae, which could be the main source of organic matter in the sediments. A high correlation with chlorophylls c is a marker of diatoms; a high correlation with chlorophyll b is a marker of green algae; and low correlation both with chlorophylls b and c—indicates a high blue–green algae input.     

Sedimentary record of black carbon in the Pearl River estuary and adjacent northern South China Sea

The concentrations of black carbon (BC) and δ¹³C_BC were determined in sediments of three dated cores from the Pearl River estuary (core PR-3) and adjacent northern South China Sea (cores SS-30, E2). For comparison, the total organic C (TOC) contents and δ¹³C_TOC in the sediments were also measured. Relatively higher concentrations and fluxes of BC were found in sedimentary core PR-3, taken in the Pearl River estuary. The BC concentration profiles or fluxes correlated well with fossil-fuel usage in the Pearl River Delta. Maximum BC fluxes occurred in the late 1970s to early 1980s as recorded in core PR-3, and in the 1950s (core SS-30), reflecting the maximum BC emission in the Pearl River Delta and Hong Kong region, respectively. After the 1980s, a rapid decrease of BC fluxes and a light δ¹³C_BC excursion were presumably due to improvements in combustion and pollution-control technologies and a shift of energy structure from biomass and coal to a mixture of coal, gas, oil and biomass. The fossil BC that contributed to total BC in core PR-3 increased from 20–30% to 70–80% during the last five decades. The study also shows that BC correlates well with terrestrial organic matter and that the ratio of BC to TOC is a good pollution indicator in relation to anthropogenic activities.     

Simultaneous determination of sulfide, polysulfides and thiosulfate as an aid to ore exploration

The interaction of water and sulfide minerals yields dissolved species which can be utilized to trace back the presence of sulfide minerals and associated minerals. Computer modeling and laboratory and field results show that the most characteristic dissolved species are hydrogen sulfide (H₂S, HS⁻), polysulfide ions (S_n²⁻) and thiosulfate (S₂O₃²⁻), derived from the hydrolysis of sulfide minerals. Typical concentration ranges are: 10⁻⁵ – 10⁻⁷ mole/l for hydrogen sulfide, 10⁻⁶ – 10⁻⁹ mole/l for polysulfides and 10⁻⁵ – 10⁻⁸ mole/l for thiosulfate. The chemical reactivity of these species at contact with air makes them difficult to assess unless determined immediately after sampling.These sulfur species can be determined rapidly and accurately in field conditions by simultaneous titration with mercuric chloride employing an Ag/Ag₂S electrode for the determination of the end points.The application to ore exploration is exemplified by the results of the research on roll-type uranium deposits in the southwest of France.     

Detection of petroleum contamination in river sediments from Quebec City region using GC–IRMS

Isotopic analysis by compound specific gas chromatography–isotope ratio mass spectrometry (GC–IRMS) is used to detect and characterize petroleum pollution in surficial sediments along the St Lawrence River, near Quebec City. Unusually mature n-alkane distributions have been found in some recent intertidal sediments in the region. GC–IRMS results suggest that the n-alkanes are not derived from indigenous organic sources because they carry δ¹³C values between −30.0 and −27.0‰, as well as very small isotopic differences between odd and even numbered n-alkanes, which are both typically associated with petroleum products. Comparison of these sediments with bunker fuel, an oil used in the shipping industry, has shown a close isotopic correlation in some sites, which is further supported by biomarkers. Overall, the contamination has been dispersed along the river but is generally localized around the industrial region where hydrocarbon transfer from shore storage to ships takes place. This study illustrates how GC–IRMS can be used effectively in the detection and characterization of petroleum pollutants in sediments.     

Origin of groundwater salinity and hydrogeochemical processes in a confined coastal aquifer: Case of the Rhône delta (Southern France)

The Rhône delta, South of France (Camargue, 750 km²) is a coastal saline wetland located along the Mediterranean Sea. The confined aquifer of this delta shows high values of electrical conductivity rising from the north (4 mS/cm) to the shoreline (58 mS/cm). This work aims to identify the origin of groundwater salinity and the geochemical processes occurring in this coastal confined aquifer according to the degree of salinity. A natural tracing approach is considered using monthly sampling in 8 piezometers for chemical and isotopic analyses (¹⁸O, ²H, ¹³C_TDIC). Ionic and isotopic ratios demonstrate that strong salinities are due to a simple mixing between Mediterranean seawater and freshwater; seawater contribution reaches up to 98% at 8 km from the shoreline. Seawater intrusion induces a particular groundwater chemistry which varies with the degree of seawater contribution: (1) In the less saline part of the aquifer (seawater contribution <20%), the intrusion induces an increase of Na⁺ in groundwater leading to Ca²⁺/Na⁺ exchange processes. The δ¹³C_TDIC analyses show that matrix exchange processes most likely occur for the less saline samples. (2) In the saline part of the aquifer (seawater contribution >20%), the intrusion induces SO₄ reduction which is confirmed by highly depleted δ¹³C_TDIC values (up to −19‰). The δ¹³C_TDIC also reveals that methanogenesis processes may occur in the most reductive part of the aquifer. Due to SO₄ reduction, the intrusion induces a shift in carbonate equilibrium leading to supersaturation with respect to dolomite and/or magnesian calcite. Thus carbonate precipitation may occur in the area strongly influenced by seawater.     

Molecular and stable carbon isotopic compositions of saturated fatty acids within one sedimentary profile in the Shenhu,northern South China Sea: Source implications

This study examined the distributions and stable carbon isotopic compositions of saturated fatty acids (SaFAs) in one 300 cm long sedimentary profile, which was named as Site4B in Shenhu, northern South China Sea. The concentrations of total SaFAs in sediments ranged from 1.80 to 10.16 μg/g (μg FA/g dry sediment) and showed an even-over-odd predominance in the carbon chain of C₁₂ to C₃₂, mostly with n-C₁₆ and n-C₁₈ being the two major components. The short-chain fatty acids (ScFAs; n-C₁₂ to n-C₁₈) mainly from marine microorganisms had average δ¹³C values of −26.7‰ to −28.2‰, whereas some terrigenous-sourced long-chain fatty acids (LcFAs; n-C₂₁ to n-C₃₂) had average δ¹³C values of −29.6‰ to −34.1‰. The other LcFAs (n-C₂₄ & n-C₂₆ ∼ n-C₂₈; average δ¹³C values are −26.1‰ to −28.0‰) as well as n-C₁₉ and n-C₂₀ SaFAs (average δ¹³C values are −29.1‰ and −29.3‰, respectively) showed a mixed signal of carbon isotope compositions.The relative bioproductivity calculation (marine vs. terrigenous) demonstrated that most of organic carbon accumulation throughout the sedimentary profile was contributed by marine organism. The high marine productivity in Shenhu, South China Sea may be related to the hydrocarbon seepage which evidenced by diapiric structures. Interestingly, there is a sever fluctuation of terrigenous inputs around the depth of 97 cm below the seafloor (bsf), probably resulting from the influence of the Dansgaard–Oeschger events and the Younger Dryas event as revealed by ¹⁴C age measurements.     

Invasion of anthropogenic CO<Subscript>2</Subscript> recorded in planktonic foraminifera from the northern Gulf of Aqaba

The stable carbon isotopic composition of the planktonic foraminifera Globigerinoides sacculifer and G. ruber (white) and sedimentary organic matter from the northern Gulf of Aqaba have been investigated to estimate changes in ¹³C_DIC in surface waters during the last 1,000 years. The high sedimentation rates at the core sites (about 54 cm/Kyear) provide high temporal resolution (~10 years). Recent sediments at the top of the cores reflect conditions younger than 1950. The ¹³C records of the planktonic foraminifera from three multicores display similar trends, showing a uniform and consistent pattern before the 1750s, and a gradual decrease of approximately 0.63 over the last two centuries. This decrease seems to track the decrease of ¹³C_DIC in surface waters, which is mainly caused by the increase of anthropogenic input of ¹³C-depleted CO₂ into the atmosphere. Similarly, a trend towards lighter values of the carbon isotopic composition of sedimentary organic matter (¹³C_org) during the last 200 years supports the interpretation obtained from the planktonic foraminiferal ¹³C. Furthermore, direct measurements of seawater show that ¹³C of the dissolved inorganic carbon (DIC) in the northern Gulf of Aqaba has decreased by about 0.44 during the period 1979–2000. The average annual decrease is 0.021, which is similar to that observed globally. The ¹³C values of planktonic foraminifera combined with organic matter ¹³C from marine sediments are good indicators for reconstructing past changes in atmospheric CO₂ concentrations from the northern Gulf of Aqaba.     

Genesis of copper mineralisation,Myall Creek prospect,South Australia

The Myall Creek copper prospect is in unmetamorphosed carbonaceous dolosiltstone and sandstone at the base of the late Proterozoic (Adelaidean) Tapley Hill Formation. It contains disseminated, fine-grained chalcopyrite, zincian tennanite, bornite, chalcocite, pyrite, sphalerite and galena, and irregular to straight chalcopyrite-rich veinlets. Some ore minerals rim and/or partially replace pyrite or clastic grains. There is no evidence of hydrothermal activity. The ³⁴S_CDT values of pyrite and the other sulfides fall in the wide range –3.6 to +44.2. Dolomite in both mineralised and unmineralised samples has ¹³C_PDB values concentrated around –3, and ¹⁸O_SMOW values around +25. It is concluded that the mineralising fluids were near-neutral brines which leached metals from the basement and early Adelaidean rocks. They entered the Tapley Hill sediments at moderately low temperatures via permeable strata and faults. The metals were precipitated by biogenic H₂S, and also fixed by reaction with iron sulfides and, possibly, organic matter. Continuing ascent of brines into the mineralised strata caused breakdown of detrital feldspars and Fe-Ti oxides, and some solution-remobilisation of early-formed sulfides.