A rapid estimation of organic carbon in silty lake sediments

A rapid and convenient method is described for the estimation of active organic carbon in silty lake sediments collected in regional geochemical surveys. The method utilizes measurement of optical density at 500 nm, of a 4 M nitric acid ? 0.1 M hydrochloric acid leach of a sediment sample and has a precision of ± 26% as determined from separate replicate analyses. An attractive feature of the method lies in its use of the same leach solution on which atomic absorption analyses of trace metals may be made.     

Classification of torbanite and cannel coal: II. Insights from pyrolysis-GC/MS and multivariate statistical analysis

Petrographic and megascopic criteria have traditionally been used as the basis for the classification of torbanite and cannel coal. For this study, it was hypothesized that modern analytical organic geochemical and multivariate statistical techniques could provide an alternative approach. Towards this end, the demineralized residues of 14 torbanite (rich in Botryococcus-related alginite) and cannel (essentially, rich in organic groundmass and/or sporinite) coal samples were analyzed by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Cluster analysis performed on the Py-GC/MS data clearly distinguished the torbanite from the cannel coal, demonstrating a consistency between the chemical properties and the petrographic composition. All the torbanite samples group into one cluster, their pyrolyzates having an overwhelming predominance of straight chain hydrocarbons, a characteristic typical of Botryococcus. The presence of the C₉–C₂₆ n-α,ω-alkadiene series is the key feature distinguishing the torbanites from the other samples. The cannel coals exhibit more chemical diversity, reflecting their greater variability in petrographic composition. The Breckinridge cannel, dominated by a highly aliphatic lamalginitic groundmass, chemically fits the torbanite category. The bituminitic groundmass-dominated cannel coals fall into a cannel sub-cluster, their pyrolyzates having a characteristic predominance of n-alk-1-enes and n-alkanes (particularly the long-chain homologues), with no detectable alkadienes. The vitrinitic groundmass-dominated Ohio Linton cannel and the sporinite-rich Canadian Melville Island cannel are readily distinguishable from the other cannels by the relatively abundant aromatic and phenolic compounds in their pyrolyzates. The internal distribution patterns of alkylaromatic and alkylphenolic isomers are shown to be less significant in the classification of this sample set. Multivariate statistical analysis of the pyrolysis data not only successfully discriminated torbanites from cannel coals, but recognized subtler differences between the examples of these two coal types, in substantial agreement with the petrographic characterization. As such, these methods can substitute for or supplement the traditional microscope-based approach.     

A detailed pyrolysis-GC/MS analysis of a black carbon-rich acidic colluvial soil (Atlantic ranker) from NW Spain

Despite the potentially large contribution of black carbon (BC) to the recalcitrant soil organic matter pool, the molecular-level composition of aged BC has hardly been investigated. Pyrolysis-GC/MS, which provides structural information on complex mixtures of organic matter, was applied to the NaOH-extractable organic matter of an acidic colluvial soil (Atlantic ranker) sampled with high resolution (5 cm) that harbours a fire record of at least 8.5 ka. Additionally, 5 charcoal samples from selected soil layers were characterised using pyrolysis-GC/MS for comparison. Pyrolysis-GC/MS allowed distinguishing between BC and non-charred organic matter. It is argued that a large proportion of the polycyclic aromatic hydrocarbons (PAHs), benzenes and benzonitrile in the pyrolysates of the extractable organic matter, together accounting for 21–54% of total identified peak area, derived from BC. In charcoal samples, these compounds accounted for 60–98% of the pyrolysis products. The large quantity of BC in almost all samples suggested a key role of fire in Holocene soil evolution. The high C content of the soil (up to 136 g C kg⁻¹ soil) may be attributed to the presence of recalcitrant organic C as BC, in addition to the sorptive preservation processes traditionally held responsible for long-term C storage in acid soils. Interactions between reactive Al hydroxides and BC could explain the longevity of BC in the soil. This work is the first thorough pyrolysis-GC/MS based study on ancient fire-affected organic matter.     

Geochemical assessment of arsenic contamination in well water and sediments from several communities in the Nawalparasi District of Nepal

Arsenic contamination of well water is a serious issue in the Nawalparasi District of the Terai region in Nepal. A local investigation was carried out on 137 tube wells in 24 communities of the district in December 2011. The investigation revealed that the average arsenic concentration in the tube wells was 350 μg/L, and that nearly 98 % of the wells exceeded the WHO guideline arsenic level limit of 10 μg/L. Highly contaminated well water, with more than 400 μg/L of arsenic, was found within the limited depth ranges of 18–22 and 50–80 m. High arsenic levels exceeding 500 μg/L were detected in shallower wells at Patkhauli, Mahuawa, Thulokunwar, and Goini located between 27.517° and 27.543°N and between 83.648° and 83.748°E. Boring sampling at five communities of Kashiya, Goini, Sanokunwar, Thulokunwar, and Mahuawa revealed two aquifers located at the two depths around 14–22 and 41–50 m in each community. Dark gray or black-colored peaty clay layers rich in organic matter were distributed at depths of 18–21 m beside the upper aquifers with high arsenic concentration in each community. Positive correlations were shown between iron and arsenic in the sediments from the five communities. It can be inferred that these results were caused by dissolution of iron-oxyhydroxide molecules with arsenic from solid phases. Microbial metabolisms have a great potential to induce the dissolution and release arsenic attached on the solid phases into aqueous phases depending on the level of redox potential and pH.     

Composition of waters and sediments in the Antwerp harbor

Industrial expansion in the harbor areas of Antwerp has altered environmental conditions to a great extent. This study examines the relation between the water economy of the port of Antwerp docks and the composition of dock waters, sediments, and interstitial waters, with emphasis on trace metals and chlorides. Some conclusions are reached as to the possible use of the dredged deposits in agriculture.     

An assessment of selected heavy metal contamination in the surface sediments from the South China Sea before 1998

Rapid economic development in East Asian countries has inevitably resulted in environmental degradation in the surrounding seas, and concern for both the environment and protection against pollutants is increasing. Identification of sources of contaminants and evaluation of current environmental status are essential to environmental pollution management, but relatively little has been done in the South China Sea (SCS). In order to investigate the metal pollution status and source within the SCS, a total of 52 surface sediment samples were collected in 1998 from the SCS for the selected heavy metal measurements such as Pb, Zn, Cu, V, Cr, Cd and Sc. The total concentrations (in mg kg^− 1 dry weight) in sediments ranged and averaged (mean ± S.D.): Pb, 4.18 to 58.7 (23.6 ± 8.93); Zn, 10.7 to 346 (87.4 ± 47.7); Cu, 5.29 to 122 (38.1 ± 24.6); V, 0.03 to 148 (78.0 ± 37.0); Cr, 4.48 to 589 (105 ± 86); Cd, 0.08 to 2.14 (0.40 ± 0.40) and Sc, 0.33 to 20.6 (10.6 ± 4.4), respectively. Enrichment factor (EF) values and geoaccumulation indexes (I_geo) suggest that Cu, Pb, Zn and V contamination exists only in few localized areas, but Cr and Cd contamination can be found in large-scale area of the SCS before 1998. Further studies are needed to reconstruct deposition history and for trend analysis.     

Distribution,fractionation, and contamination assessment of heavy metals in offshore surface sediments from western Xiamen Bay,China

Surface sediment samples were collected at 21 offshore sites in western Xiamen Bay, Southeast China. Total concentrations of Li, V, Cr, Co, Ni, Cu, Zn, Sr, Mn, Pb, Ba, Fe, and Ti were determined by inductively coupled plasma-optical emission spectrometry; Hg was determined by atomic fluorescence spectrometry. A modified BCR sequential extraction procedure was used to extract fractions of the above elements. Concentrations of Pb, Cr, and Hg at most sites met the primary standard criteria of Marine Sediment Quality except site S12 for Pb and S7 for Cr, while concentrations of Zn at 17 sites and Cu at seven sites exceeded the criteria. The mean concentration of Hg was three times higher than the background, with a possible source being the Jiulong River. Fe, Ti, Ba, Co, V, and Li dominated the residual phase, mainly from terrestrial input. Ni, Cr, Pb, and Hg in the non-residual phase varied largely between sites. Sr, Mn, Cu, and Zn were mainly in the non-residual fraction. Most sites showed considerable ecological risk; exceptions were site S7 (very high) and sites S10, S11, and S14 (moderate). Cu showed moderate-to-high pollution and Pb exhibited no-to-low pollution, while other metals had a non-pollution status according to their ratios of secondary phase to primary phase (RSP). Results of two assessment methods showed moderate pollution and a very high ecological risk for Cu, Zn, Ni, and Cr at site S7, which might be due to the local sewage treatment plant.     

A rapid field method for extracting the magnetic fraction from stream sediments

Magnetic fraction concentrates for geochemical analysis can be obtained with an automagnet directly from wet stream sediments, without prior concentration by panning.     

Geochemistry of surface sediments and heavy metal contamination assessment: Messolonghi lagoon complex,Greece

The Messolonghi lagoon complex in Western Greece receives agricultural and domestic effluents both from point and diffused sources. Surface sediments were analyzed for grain size, organic carbon, total nitrogen, total sulfur, major and minor elements, aiming at the identification of geochemical relationships between all variables. Enrichment factors and the modified degree of contamination methods were applied to assess potential heavy metal enrichment related to human activities. Sediment texture was highly variable, with muddy sediments prevailing. In the central sector of the Messolonghi lagoon, organic carbon contents were high. Principal factor analysis revealed the following main groups of variables with common geochemical behavior: (1) terrigenous aluminosilicates (2) organic matter, (3) biogenic carbonates, (4) mineral quartz-aluminosilicates, and (5) Mn-oxides. Enrichment factors estimated for V, Cr, Mn, Co, Ni, Cu, Zn, and Pb using local pre-industrial sediment showed that all metals exhibit almost natural background levels, except for Pb, which was found to be slightly elevated (legacy of leaded fuel). Estimation of contamination factors concluded in similar results, whereas the overall modified degree of contamination was at the lowest level, therefore suggesting that this transitional water body has not been affected by anthropogenic activities. The data set may be considered as a baseline for future monitoring projects according to EU policy.     

The diagenetic geochemistry and contamination assessment of iron,cadmium, and lead in the sediments from the Shuangtaizi estuary,China

Sediment and pore water samples have been collected from the coastal tidal flat in the Shuangtaizi estuary, China, in order to investigate the geochemical behavior of iron, cadmium, and lead during diagenesis and to assess the degree of contamination. The calculated enrichment factors and geoaccumulation indices for separate elements show that anthropogenic activities have had no significant influence on the distribution of Fe and Pb in the study area, whereas the distribution of Cd has been closely influenced in this way. The high percentage of exchangeable Cd (average of 56.34%) suggests that Cd represents a potential hazard to benthic organisms in the estuary. The calculated diffusive fluxes of metals show that the most mobilized metal is Fe (9.22 mg m^?2 a^?1), followed by Cd (0.54 mg m^?2 a^?1) and Pb (0.42 mg m^?2 a^?1). Low Fe²⁺ contents in surface pore water, alongside high chromium-reducible sulfur contents, and low acid-volatile sulfur, and elemental sulfur contents at 0–25 cm depth in sediments show that Fe²⁺ is formed by the reduction of Fe oxides and is transformed first to a solid phase of iron monosulfides (FeS) and eventually to pyrite (FeS₂). The release of adsorbed Pb due to reductive dissolution of Fe/Mn oxides during early diagenesis could be a source of Pb²⁺ in pore water. From the relatively low total organic carbon contents measured in sediments (0.46–1.28%, with an average of 0.94%) and the vertical variation of Cd²⁺ in pore water, sulfide or Fe/Mn oxides (instead of organic matter) are presumed to exert a significant influence on carrying or releasing Cd by the sediments.     

Land-derived organic matter in surface sediments from the Gulf of Mexico

Lignin oxidation products and ¹³C/¹²C ratios were compared as indicators of land-derived organic matter in surface sediments from the western Gulf of Mexico. Whole sediments were reacted with cupric oxide to yield phenolic oxidation products that indicated the types and relative amounts of the lignins that were present.Measurements of lignin concentration and carbon isotope abundances both indicated a sharp offshore decrease of land-derived organic matter in most areas of the western Gulf. This decrease results primarily from mixing of terrestrial and marine organic matter. The terrestrially derived material in these sediments has a lignin content similar to that of grasses and tree leaves. Flowering plants contribute most of the sedimented lignin compounds. These lignins apparently occur in the form of well-mixed plant fragments that are transported to sea by rivers and deposited primarily on the inner continental shelf.     

Dissolved organic matter in pore water of carbonate sediments from Bermuda

Pore water samples from seven nearshore areas in Bermuda were obtained under in situ conditions and analyzed for dissolved organic carbon, dissolved carbohydrates, dissolved free amino acids and dissolved humic substances. The concentration of dissolved organic carbon is higher than in the overlying nearshore waters indicating significant diagenetic remobilization of carbon in these recently deposited carbonate sediments. Dissolved carbohydrates decrease with depth due to microbial utilization.     

Early diagenesis of organic matter in recent Black Sea sediments: characterization and source assessment

The organic matter in 9 recent (not more than 250 years old) and ‘organic-rich’ sediments from the southern Black Sea shelf and upper slope have been characterized semi-quantitatively by Pyrolysis/Gas Chromatography/Mass Spectrometry (PY/GC/MS) and¹³C Cross Polarization Magic Angle Spinning Nuclear Magnetic Resonance (CPMAS-NMR) spectrometry. The organic matter of 7 of the studied sediments was found to be ligno-carbohydrate with a proteinaceous component, one sediment appeared to contain oxidized coal dust and one contained thiophenes in association with pyrite. The ligno component is derived from grasses and soft wood lignin. Material entrapped in an anoxic environment contained the highest proportions of carbohydrate and protein. All the samples had suffered diagenesis as is generally shown by the attachment of carboxyl groups and the removal of methoxyl groups. The evidence suggests that diagenesis occurred whilst the particles traversed the oxic water column.     

Evolution of organic matter in lignite-containing sediments revealed by analytical pyrolysis (Py–GC–MS)

Newly vegetated sites provide opportunities to enlighten organic matter (OM) transformation mechanisms in soils and sediments at very early stages of development which, in turn, is relevant to better understand general ecosystem functioning. Mine acid soils and sediments in the Lusatian open cast lignite mining district (Germany) contains a high concentration of fossil carbon (lignite) in ad mixture with recent OM from the local vegetation, both contributing to the humified OM pool. In this study, analytical pyrolysis (Py–GC–MS) was used to monitor the different C sources (lignite or plant derived) in developing mine tailing soils and sediments and their degree of degradation in contrasting environments. Representative vegetation and the organic carbon (OC) rich soil/sediment fraction (humus fraction) were sampled at two depths (0–5 and 5–10 cm) in three plots along a transect covering an upland forest soil, a partially submerged sediment at the land–water interface and a constantly submerged sediment. The analysis of plant (lipds, isoprenoids, methoxyphenols and carbohydrates) and possible lignite (alkyl napththalenes, alkyl benzenes and PAHs) biomarkers released after pyrolysis supports previous findings in the area using other proxies. It was possible to discern OM sources in soil/sediment humus fractions, both from the substrate (lignite) as well as from the prevailing vegetation of the area. Environmental conditions in the submerged sediment seem to favour OM protection and the accumulation of decomposing plant material, whereas more intense OM degradation seems to prevail in the land–water interface areas characterized by fluctuating water level. In addition, a well resolved series of organic sulfur compounds (OSCS) found in the submerged sediments of rehabilitated acid lakes, indicates the possible occurrence of particular mechanisms of C preservation in this extreme anoxic S rich environment, i.e. via sulphur “quenching” with plant derived lipids during early diagenesis.     

Separation of total nitrogen from sediments into organic and inorganic forms for isotopic analysis

Nitrogen isotopic (δ¹⁵N) analysis is widely applied in palaeoceanography, palaeolimnology and soil science. Most measurements are performed on untreated bulk samples containing both organic and inorganic nitrogen (ON and IN), which could sometimes confound isotopic signals. Here we propose a sample pretreatment method to physically and quantitatively separate the total nitrogen into ON and IN for subsequent concentration and isotopic measurements. In the method, the sediment is first demineralized with HCl and HCl-HF, adsorbed and bound IN being liberated into the acid solutions during the demineralization. Then, organic matter (OM) dissolved in the acid solutions is recovered by a sorbent of styrene divinyl benzene polymer (Bond Elut PPL, Varian) and is then combined with the acid-resistant OM to obtain the total OM. Finally, IN in the acid solutions passing through the PPL sorbent is recovered by way of steam distillation. Total nitrogen recovery with the method is 92.4% and isotopic mass balance calculations show that the δ¹⁵N value of total nitrogen can be quantitatively partitioned into the organic and inorganic forms. The δ¹³C values of the separated OM show a small positive excursion of 0.2‰ relative to those of bulk OM, ruling out significant loss of ON during the pretreatment. By comparison, the KOBr method, widely used to remove ON from bulk samples for isotope measurements, gives lower IN concentration and higher δ¹⁵N_IN values, attributed to the partial loss of IN and escape of some oxidation-resistant ON.     

Triterpenols in mangrove sediments as a proxy for organic matter derived from the red mangrove (Rhizophora mangle)

Mangroves are the dominant type of vegetation along many tropical coasts. Organic matter (OM) derived from mangrove leaf litter and root material is stored in sediments and is a major contributor to the amount and chemical composition of sedimentary OM. A set of organic biomarkers in sediments was applied as a palaeo-indicator for the Holocene dynamics of a mangrove Estuary (Rio Caeté, Pará, Brazil). Six sediment cores were collected perpendicular to the present coast line and analysed for triterpenols and sitosterol. The influence of microbial biomarker degradation was implemented from a previous study. Biomarker profiles were validated with pollen data and multivariate statistics to test whether these compounds were suitable indicators for the palaeo-vegetation. Sediments deposited up to 2 Ma BP showed biomarker assemblages similar to those of recent surface sediment. In two cores, the biomarker composition revealed a transition from marsh to mangrove vegetation. Taraxerol, germanicol and β-amyrin provided the most significant chemotaxonomical information and, especially in combination, served as reliable proxies for OM from Rhizophora mangle in northern Brazil. The maximum age of the mangrove system ranged between 1000 and 5100 yr depending on the topographic elevation of the drilling location.     

Sulfur isotopic composition of individual organic compounds from Cariaco Basin sediments

Reactions between reduced inorganic sulfur and organic compounds are thought to be important for the preservation of organic matter (OM) in sediments, but the sulfurization process is poorly understood. Sulfur isotopes are potentially useful tracers of sulfurization reactions, which often occur in the presence of a strong porewater isotopic gradient driven by microbial sulfate reduction. Prior studies of bulk sedimentary OM indicate that sulfurized products are ³⁴S-enriched relative to coexisting sulfide, and experiments have produced ³⁴S-enriched organosulfur compounds. However, analytical limitations have prevented the relationship from being tested at the molecular level in natural environments. Here we apply a new method, coupled gas chromatography – inductively coupled plasma mass spectrometry, to measure the compound-specific sulfur isotopic compositions of volatile organosulfur compounds over a 6 m core of anoxic Cariaco Basin sediments. In contrast to current conceptual models, nearly all extractable organosulfur compounds were substantially depleted in ³⁴S relative to coexisting kerogen and porewater sulfide. We hypothesize that this ³⁴S depletion is due to a normal kinetic isotope effect during the initial formation of a carbon–sulfur bond and that the source of sulfur in this relatively irreversible reaction is most likely the bisulfide anion in sedimentary porewater. The ³⁴S-depleted products of irreversible bisulfide addition alone cannot explain the isotopic composition of total extractable or residual OM. Therefore, at least two different sulfurization pathways must operate in the Cariaco Basin, generating isotopically distinct products. Compound-specific sulfur isotope analysis thus provides new insights into the timescales and mechanisms of OM sulfurization.     

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.     

Role of prior exposure on anaerobic degradation of naphthalene and phenanthrene in marine harbor sediments

The anaerobic degradation of the polycyclic aromatic hydrocarbons (PAHs) naphthalene and phenanthrene was investigated in several marine harbor sediments. In sediments from Boston Harbor that were heavily contaminated with petroleum, [¹⁴C]-naphthalene and [¹⁴C]-phenanthrene were oxidized to ¹⁴CO₂ without a lag, suggesting that the microbial community was adapted for anaerobic PAH oxidation in situ. The addition of molybdate, a specific inhibitor of sulfate-reducing microorganisms, inhibited PAH mineralization which suggested that sulfate reducers were involved in the anaerobic oxidation of the PAHs. PAHs were also anaerobically oxidized at another site in Boston Harbor that was less heavily contaminated, but at a slower rate than in the most heavily contaminated sediments. Sediments not contaminated with petroleum did not significantly oxidize the PAHs. A similar correspondence between rates of anaerobic PAH oxidation and the degree of PAH contamination was observed in sediments from Tampa Bay and San Diego Bay. When relatively pristine sediments from San Diego Bay that did not have a significant capacity for anaerobic PAH oxidation were exposed to high concentrations of naphthalene, they developed a potential for naphthalene degradation that was comparable to that in sediments that had a history of PAH contamination. The increase in potential for naphthalene degradation in the sediments exposed to naphthalene was associated with an increase in naphthalene-degrading microorganisms. These results suggest that many marine harbor sediments contain microorganisms capable of anaerobically oxidizing PAHs under sulfate-reducing conditions and that these microorganisms will respond with an increase in their activity when PAHs are introduced into the sediments. Thus, if PAH inputs into harbor sediments from petroleum can be reduced there may be a widespread potential for microorganisms to remove this PAH contamination from the sediments, despite anaerobic conditions.     

Volatile C1-C7 organic compounds in surface sediments from Walvis Bay

C₁-C₇ volatile organic compounds were analyzed in three gravity cores taken from Walvis Bay shelf. The compounds detected included alkanes (methane, ethane, propane, i- and n-butane, and i- and n-pentane, and heptane), alkenes (2-methyl-2-butene, dimethylcyclopentenes, cyclohexene), oxygen containing compounds (2- and 3-methylfuran, 2,5-dimethylfuran, 2- and 3-methylbutanal and 3-pentanone), sulfur compounds (dimethylsulfide, thiophene, 2- and 3-methylthiophene) and aromatic compounds (benzene and toluene). In situ biological and low temperature chemical (less than 15°C) formation processes are proposed, possibly from marine terpene precursors. Subsequent to this work, these compounds were found to be widely distributed in surface gravity cores from other areas. Many of these compounds do not survive deeper burial. Furans, ketocompounds, and alkenes are generally not found in more than trace quantities in deeper (?10m subbottom) DSDP cores we have examined from other areas.