Oil contamination of sediments by freeze-drying versus air-drying for organic geochemical analysis.

Freeze-drying is widely used in geochemical laboratories for preparing wet solid environmental samples such as sediments and soils before being analyzed for their contents and states of various metal elements and labile organic components that may be temperature- and/or redox-sensitive. Screening bulk geochemical analysis of two Artic lake sediment samples prepared by freeze-drying displayed unexpectedly high contents of labile organic matter (OM) represented by the Rock-Eval S1 peaks (e.g., 8.12 and 4.84 mg HC/g sediment). The amount of labile OM was reduced greatly for the freeze-dried sediment samples after a thorough cleaning of the freeze-drier sample chamber (e.g., 2.75 and 1.46 mg HC/g sediment), but was still significantly higher than that of the equivalent air-dried samples (e.g., 0.76 and 0.23 mg HC/g sediment). Compositional analysis of the labile OM fractions by gas chromatography (GC) of both freeze-dried and air-dried aliquots of the same sediments indicates the presence of unresolved complex mixture (UCM) humps of C10-C23 hydrocarbons in the freeze-dried samples. In contrast, air-dried samples, either real sediments or blank laboratory materials represented by clean sand and thermally spent shale, do not show the C10-C23 hydrocarbon UCM humps on their GC traces. The hydrocarbon UCM humps persist in the freeze-dried samples even they further went through air-drying at ambient conditions. Both bulk and compositional analytical results in this work appear to indicate the potential risk of introduction of external hydrocarbons to the prepared materials during freeze-drying process, especially if an aged freeze-drier was used without being thoroughly cleaned and if pump oil and cooling fluids were components of the device.

Postdiagenetic Changes in Kerogen Properties and Type by Bacterial Oxidation and Dehydrogenation.

A significant part of organic carbon found on the earth is deposited as fossil organic matter in the lithosphere. The most important reservoir of carbon is shale rocks enriched with organic matter in the form of kerogen created during diagenesis. The purpose of this study was to analyze whether the bacterial communities currently inhabiting the shale rocks have had any impact on the properties and type of kerogen. We used the shale rock located on the Fore-Sudetic Monocline, which is characterized by oil-prone kerogen type II. We were able to show that shale rock inhabited by bacterial communities are characterized by oxidized and dehydrated kerogen type III (gas-prone) and type IV (nonproductive, residual, and hydrogen-free). Bacterial communities inhabiting shale rock were dominated by heterotrophs of the Proteobacteria, Firmicutes, and Actinobacteria phyla. Additionally, we detected a number of protein sequences in the metaproteomes of bacterial communities matched with enzymes involved in the oxidative metabolism of aliphatic and aromatic hydrocarbons, which may potentially contribute to the postdiagenetic oxidation and dehydrogenation of kerogen. The kerogen transformation contributes to the mobilization of fossil carbon in the form of extractable bitumen dominated by oxidized organic compounds.

Geochemical Characteristics and Significance of Organic Matter in Hydrate-Bearing Sediments from Shenhu Area, South China Sea.

Rock-Eval pyrolysis and the biomarker composition of organic matter were systematically studied in hydrate-bearing sediments from the Shenhu area, South China Sea. The n-alkane distribution patterns revealed that the organic matter in the sediments appeared to originate from mixed sources of marine autochthonous input, terrestrial higher plants, and ancient reworked organic matter. The low total organic carbon contents (average < 0.5%) and the low hydrogen index (HI, <80 mg HC/g TOC) suggested the poor hydrocarbon-generation potential of the deposited organic matter at a surrounding temperature of <20 °C in unconsolidated sediments. The abnormally high production index and the fossil-originated unresolved complex mixture (UCM) accompanied by sterane and hopane of high maturity indicated the contribution of deep hydrocarbon reservoirs. Preliminary oil-to-source correlation for the extracts implied that the allochthonous hydrocarbons in the W01B and W02B sediments might have originated from the terrestrial source rocks of mature Enping and Wenchang formations, while those of W03B seem to be derived from more reduced and immature marine source rocks such as the Zhuhai formation. The results of the organic extracts supported the previous identification of source rocks based on the isotopic composition of C2+ hydrate-bound gases. The biomarker of methanogens, squalane, was recognized in the sediments of this study, possibly suggesting the generation of secondary microbial gases which are coupled with the biodegradation of the deep allochthonous hydrocarbons.

Assessment of anthropogenic pollution in Guanabara Bay (SE Brazil) through biogeochemical data and stable isotope mixing models.

This work intends to identify pollution sources along the margins of Guanabara Bay (GB; SE Brazil) through a multiproxy approach and Bayesian stable isotopic mixture model (BSIMM). For this purpose, 33 surface sediment samples were collected and analyzed for granulometry, geochemistry (heavy metals, total organic carbon-TOC, stable isotopes of carbon and nitrogen-δ13C and δ15N, Rock-Eval pyrolysis parameters-REPP), and physicochemical parameters. Metal concentrations (E) dissolved in water (EW), adsorbed by organic matter (EOM) and by Mn hydroxides (EMn), and total extracted concentrations (ET) were analyzed. Sampling was conducted in 2018 after an oil spill from Reduc Oil Refinery. Potential Ecological risk index (PERI), based on metals, classified 85% of the analyzed stations as having moderate to considerable ecological risk. The metals with the potential to cause the highest ecological risk were CdW, CdOM, PbOM, and HgOM. The combination of BSIMM and REPP data was an effective proxy for oil spill detection by indicating the presence of polycyclic aromatic hydrocarbons (PAHs). Relatively high TOC contents suggested that the analyzed stations are eutrophicated environments. BSIMM discriminated three groups of stations with different sources of organic matter (OM), endorsing the result previously shown by the cluster analysis: (A) Niterói region, Botafogo marina, Glória marina, Fiscal and Fundão islands with diffuse sources of OM, including marine phytoplankton and material of continental origin from highly polluted rivers and domestic sewage; (B) region near Fundão and Governador islands and Mangue Channel outlet with OM (≃70%) supplied by highly polluted streams and a small contribution of PAHs; (C) Duque de Caxias and Botafogo-Urca inlet with significant contributions of PAHs, materials from C-3 plants and rivers polluted by urban sewage. Results of linear regressions in conjunction with BSIMM indicate that HgMn and PbOM mainly affect Group A's stations. Although the eastern margin of GB (Niterói; Group A) showed greater oceanic interaction than the other groups, it presented substantial concentrations of metals, potentially harmful (i.e., Hg and Pb) to marine biota and human health.

Characterisation of former manufactured gas plant soils using parent and alkylated polycyclic aromatic hydrocarbons and Rock-Eval(6) pyrolysis.

Soils sampled from 10 former manufactured gas plants (MGP) in the UK were investigated using gas chromatography mass spectrometry (GC-MS/MS) and Rock-Eval (6) Pyrolysis (RE). RE is a screening tool used to characterise bulk organic matter in soils via the release of carbon compounds during pyrolysis and oxidation. Both the distributions and concentrations of 30 parent and 21 alkylated polycyclic aromatic hydrocarbons (PAHs) and the parameters of RE were analysed to establish relationships between soils and the MGP processes history. Principal component analysis (PCA) using the PAHs distributions and RE parameters can assist with differentiating between MGP processes. MGP processes utilizing oil provided the clearest results, attributed to petrogenic signatures with high proportions of low molecular weight PAHs. Processes using lower temperature processes were distinguished by higher proportions of high molecular weight PAHs. RE parameters alone were unable to distinguish MGP processes but showed potential in estimating the lability and thus the amount of PAH that could be released from soils. This research provides new insights that may be useful in understanding and characterising the risks posed to human health from PAHs in soils.

Polymer quantification using the Rock-Eval® device for identification of plastics in sediments.

In this work, artificial thermal degradation experiments using the Rock-Eval® device were performed on selected polymer microsphere samples (PE, PP, PE100, PA6, PA11, PFA and PET). The main idea of this work is first to create a database of different polymer standard responses using the specific Rock-Eval® FID/IR peak signals. Several specific Rock-Eval® parameters are now defined to characterize each polymer family. For instance, each polymer is characterized by specific quantified parameters like Total HCpolymer, Total COpolymer, TotalCO2polymer, Tpeakpolymer, among others. This study attempts to demonstrate if this quick thermal degradation method can be also used to characterize the plastic contents (detection, type, and quantity) in sedimentary samples. Results indicate that each investigated polymer shows specific Rock-Eval® parameters that can be considered as useful characteristics of polymer families (mainly Tpeakpolymer, TOCpolymer, PCpolymer, RCpolymer, total HCpolymer, total COpolymer and total CO2polymer parameters). Samples containing different mineral matrices (e.g. sand, shale, marl and carbonate) were also mixed with polymers at different concentrations varying between 0.2 and 4.2 wt%. These composite samples were also analyzed in order to evaluate their thermal degradation comparing their specific Rock-Eval® FID/IR signatures. For example, most composite samples show an excellent linear correlation between TOC, PC, RC, total HC, CO and CO2 parameters versus the amount polymer at different concentrations. Although more work is still needed, a methodology is here proposed to distinguish and quantify the presence of plastics in the environment applying the proposed polymer Rock-Eval® database.

Evidence linking calcium to increased organo-mineral association in soils.

Geochemical indicators are emerging as important predictors of soil organic carbon (SOC) dynamics, but evidence concerning the role of calcium (Ca) is scarce. This study investigates the role of Ca prevalence in SOC accumulation by comparing otherwise similar sites with (CaCO3-bearing) or without carbonates (CaCO3-free). We measured the SOC content and indicators of organic matter quality (C stable isotope composition, expressed as δ 13C values, and thermal stability) in bulk soil samples. We then used sequential sonication and density fractionation (DF) to separate two occluded pools from free and mineral-associated SOC. The SOC content, mass, and δ 13C values were determined in all the fractions. X-ray photoelectron spectroscopy was used to investigate the surface chemistry of selected fractions. Our hypothesis was that occlusion would be more prevalent at the CaCO3-bearing site due to the influence of Ca on aggregation, inhibiting oxidative transformation, and preserving lower δ 13C values. Bulk SOC content was twice as high in the CaCO3-bearing profiles, which also had lower bulk δ 13C values, and more occluded SOC. Yet, contrary to our hypothesis, occlusion only accounted for a small proportion of total SOC (< 10%). Instead, it was the heavy fraction (HF), containing mineral-associated organic C, which accounted for the majority of total SOC and for the lower bulk δ 13C values. Overall, an increased Ca prevalence was associated with a near-doubling of mineral-associated SOC content. Future investigations should now aim to isolate Ca-mediated complexation processes that increase organo-mineral association and preserve organic matter with lower δ 13C values. SUPPLEMENTARY INFORMATION: The online version of this article (10.1007/s10533-021-00779-7) contains supplementary material, which is available to authorized users.

Neutral monosaccharides and their relationship to metal contamination in mangrove sediments.

Mangrove sediments act as an important natural sink and a secondary source for trace metals. The main objective of this study was to investigate metal contamination and its relationship to mangrove-derived carbohydrates in mangrove sediments. Sixteen metals (Be, V, Cr, Co, Ni, Cu, Zn, Ga, As, Sr, Cd, Sn, Sb, Ba, Tl, and Pb）were analyzed in the surface sediments from four sites at different latitudes on the southeast coastline of China. The sedimentary organic matter was characterized by Rock-Eval pyrolysis, and the neutral sugars were examined by gas chromatograph mass spectrometry. Our results from the enrichment factors indicated that the mangrove sediments were no enriched by Ga, Sr, and Ba, minor enriched by Be, V, Cr, Co, Ni, Cu, Zn, As, Sn, Sb, Tl, and Pb, and moderate enriched by Cd. Litterfall was a major source of organic matter in the mangrove sediments, and the neutral sugars were mainly derived from this litterfall. Significant correlations were detected between the total organic carbon, pyrolytic parameters, neutral sugars, and enrichment factors of V, Cr, Co, Ni, Zn, and Cd, suggesting the input of neutral carbohydrates played an important role in enhancing the metal accumulation in the mangrove sediments. The mangrove litterfall itself was a major source of metals for the sediments, and the mangrove-derived organic matter enhanced the sediment's metal accumulation.

Spatial variability of organic matter properties determines methane fluxes in a tropical forested peatland.

Tropical peatland ecosystems are a significant component of the global carbon cycle and feature a range of distinct vegetation types, but the extent of links between contrasting plant species, peat biogeochemistry and greenhouse gas fluxes remains unclear. Here we assessed how vegetation affects small scale variation of tropical peatland carbon dynamics by quantifying in situ greenhouse gas emissions over 1 month using the closed chamber technique, and peat organic matter properties using Rock-Eval 6 pyrolysis within the rooting zones of canopy palms and broadleaved evergreen trees. Mean methane fluxes ranged from 0.56 to 1.2 mg m-2 h-1 and were significantly greater closer to plant stems. In addition, pH, ranging from 3.95 to 4.16, was significantly greater closer to stems. A three pool model of organic matter thermal stability (labile, intermediate and passive pools) indicated a large labile pool in surface peat (35-42%), with equivalent carbon stocks of 2236-3065 g m-2. Methane fluxes were driven by overall substrate availability rather than any specific carbon pool. No peat properties correlated with carbon dioxide fluxes, suggesting a significant role for root respiration, aerobic decomposition and/or methane oxidation. These results demonstrate how vegetation type and inputs, and peat organic matter properties are important determinants of small scale spatial variation of methane fluxes in tropical peatlands that are affected by climate and land use change.

The use of solid-phase fluorescence spectroscopy in the characterisation of organic matter transformations.

Given its high sensitivity and non-destructive nature, fluorescence excitation-emission matrix (EEM) spectroscopy is widely used to differentiate changes and transformations of dissolved or water-extracted organic matter (OM) in natural environments. The same technique applied directly on solid samples (solid-phase fluorescence spectroscopy, SPF-EEM) provides accurate results when used with pharmaceutical products or food samples, but only a few studies have considered natural OM. This study reports on the use of SPF-EEM on solid compost samples and emphasises the way the different maturation phases can be distinguished with fluorophores closely resembling those found in dissolved samples. A very good correlation has been found with data from Rock-Eval pyrolysis, nuclear magnetic resonance ((13)C CPMAS NMR), and humic-fulvic acid ratios determined by conventional NaOH-extraction. SPF-EEM appears as a much simpler method than the conventional ones to detect transformations in natural OM samples with low mineral contents. However, direct application to soil samples requires some additional studies.

The microbial community of a passive biochemical reactor treating arsenic, zinc, and sulfate-rich seepage.

Sulfidogenic biochemical reactors (BCRs) for metal removal that use complex organic carbon have been shown to be effective in laboratory studies, but their performance in the field is highly variable. Successful operation depends on the types of microorganisms supported by the organic matrix, and factors affecting the community composition are unknown. A molecular survey of a field-based BCR that had been removing zinc and arsenic for over 6 years revealed that the microbial community was dominated by methanogens related to Methanocorpusculum sp. and Methanosarcina sp., which co-occurred with Bacteroidetes environmental groups, such as Vadin HA17, in places where the organic matter was more degraded. The metabolic potential for organic matter decomposition by Ruminococcaceae was prevalent in samples with more pyrolyzable carbon. Rhodobium- and Hyphomicrobium-related genera within the Rhizobiales order that have the metabolic potential for dark hydrogen fermentation and methylotrophy, and unclassified Comamonadaceae were the dominant Proteobacteria. The unclassified environmental group Sh765B-TzT-29 was an important Delta-Proteobacteria group in this BCR that co-occurred with the dominant Rhizobiales operational taxonomic units. Organic matter degradation is one driver for shifting the microbial community composition and therefore possibly the performance of these bioreactors over time.

Explaining PAH desorption from sediments using Rock Eval analysis.

Here, we provide Rock Eval and black carbon (BC) characteristics and polycyclic aromatic hydrocarbon (PAH) distribution coefficients (KD) for sediments from the Danube, Elbe, Ebro, and Meuse river basins. PAH desorption kinetic parameters were determined using sequential Tenax extractions. We show that residual carbon (RC) from Rock Eval analysis is an adequate predictor of fast, slow, and very slow desorbing fractions of 4-ring PAHs. RC correlated better than BC, the latter constituting only 7% of RC. A dual domain sorption model was statistically superior to a single domain model in explaining KD for low molecular weight PAHs, whereas the opposite was observed for high molecular weight PAHs. Because particularly the 4-ring PAHs are bioavailable and relevant from a risk assessment perspective and because their fast desorbing fractions correlate best with RC, we recommend RC as a relevant characteristic for river sediments.