Dissolved organic matter (DOM) in marine sediment pore waters derives largely from decomposition of particulate organic matter and its composition is influenced by various biogeochemical and ...oceanographic processes in yet undetermined ways. Here, we determine the molecular inventory of pore water DOM in marine sediments of contrasting depositional regimes with ultrahigh-resolution mass spectrometry and complementary bulk chemical analyses in order to elucidate the factors that shape DOM composition. Our sample sets from the Mediterranean, Marmara and Black Seas covered different sediment depths, ages and a range of marine environments with different (i) organic matter sources, (ii) balances of organic matter production and preservation, and (iii) geochemical conditions in sediment and water column including anoxic, sulfidic and hypersaline conditions. Pore water DOM had a higher molecular formula richness than overlying water with up to 11,295 vs. 2114 different molecular formulas in the mass range of 299–600Da and covered a broader range of element ratios (H/C=0.35–2.19, O/C=0.03–1.19 vs. H/C=0.56–2.13, O/C=0.15–1.14). Formula richness was independent of concentrations of DOC and TOC. Near-surface pore water DOM was more similar to water column DOM than to deep pore water DOM from the same core with respect to formula richness and the molecular composition, suggesting exchange at the sediment–water interface. The DOM composition in the deeper sediments was controlled by organic matter source, selective decomposition of specific DOM fractions and early diagenetic molecule transformations. Compounds in pelagic sediment pore waters were predominantly highly unsaturated and N-bearing formulas, whereas oxygen-rich CHO-formulas and aromatic compounds were more abundant in pore water DOM from terrigenous sediments. The increase of S-bearing molecular formulas in the water column and pore waters of the Black Sea and the Mediterranean Discovery Basin was consistent with elevated HS- concentrations reflecting the incorporation of sulfur into biomolecules during early diagenesis. Sulfurization resulted in an increased average molecular mass of DOM and higher formula richness (up to 5899 formulas per sample). In sediments from the methanogenic zone in the Black Sea, the DOM pool was distinctly more reduced than overlying sediments from the sulfate-reducing zone. Bottom and pore water DOM from the Discovery Basin contained the highest abundances of aliphatic compounds in the entire dataset; a large fraction of abundant N-bearing formulas possibly represented peptide and nucleotide formulas suggesting preservation of these molecules in the life inhibiting environment of the Discovery Basin. Our unique data set provides the basis for a comprehensive understanding of the molecular signatures in pore water DOM and the turnover of sedimentary organic matter in marine sediments.
Redox-sensitive trace metals have been used extensively as geochemical proxies to infer the redox-status of marine sediments at the time of their deposition, and by extension, the concentration of ...oxygen in the overlying water and atmosphere. However, to reliably apply these paleoredox proxies in ancient sediment samples we must calibrate their geochemical behavior in modern sedimentary environments with known redox-status. Here we report a new compilation and analysis of modern trace-metal enrichment data for a range of marine depositional environments, including euxinic basins, continental margin upwelling settings, and normal oxic settings. The enrichments (i.e., concentrations normalized to aluminum content) of vanadium, uranium, molybdenum and rhenium (the VUMoRe database) in the various depositional categories were analyzed using receiver operating characteristic (ROC) curve analysis to systematically identify threshold values that differentiate the various settings. The enrichment of both Mo (> 5 μg g−1/%) and V (> 23 μg g−1/%), but with V not exceeding 46 μg g−1/%, provides strong evidence for a euxinic basin-type depositional environment. Furthermore, the enrichment of V (> 46 μg g−1/%), U (> 5 μg g−1/%) and Mo (> 5 μg g−1/%) is strong evidence for sediments depositing within the anoxic core of perennial OMZ environments, whereas the enrichment of U (> 1 μg g−1/%) coinciding with a low enrichment of V (< 23 μg g−1/%) and Mo (< 5 μg g−1/%) is strong evidence of sediment deposition in the oxic water beneath the core of a perennial OMZ environment. The new method we describe here for determining enrichment thresholds of trace metal-based redox proxies will support the reliable classification of ancient depositional environments and the ongoing development of trace metals as paleoredox proxies.
Microbial transformation of arsenic (As) plays a key role in As biogeochemical cycling and affects the mobility, bioavailability, and toxicity of As. This study aims to investigate the accumulation ...of As in marine sediments at different water depths in the East China Sea and reveal the abundance and diversity of the aioA, arrA, arsC, and arsM genes through quantitative real-time polymerase chain reaction (qPCR) and high-throughput sequencing. Results showed that the As content in sediments ranged from 5.53 mg kg−1 to 17.70 mg kg−1, which decreased with water depth. Abundant As biotransformation genes with low diversity were identified in these sediments, of which arsM and arrA were the most abundant. Significant positive correlation exists between the arsM and arrA gene abundance and between arsC and aioA, indicating the co-occurrence of the As biotransformation genes in microbes in marine sediments. Metagenomics analysis revealed that arsM gene was mainly distributed in Alphaproteobacteria, Solibacteres, Deltaproteobacteria, Clostridia, and Bacilli in these sediments. Among the sediment properties, total N, total S, C/N, and TOC were important factors that shaped the abundance profile of the genes involved in As transformation. This study provides a picture of As biotransformation genes in marine sediments from the East China Sea, which may affect As transformation and the ultimate fate of As in a marine environment.
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•As content in marine sediments from the East China Sea decreased with water depth.•As biotransformation genes were analyzed by metagenomics in marine sediments.•arsM and arrA were abundant and co-existed in marine sediments.•α-, δ-Proteobacteria, Solibacteres, Clostridia, were main microbes carrying arsM.•Total N, S, C/N and TOC were important factors shaping the diversity of these genes.
This study depicts the As distribution and its biotransformation genes in marine sediments from the East China Sea.
Many studies have examined relationships of microorganisms to geochemical zones in subseafloor sediment. However, responses to selective pressure and patterns of community succession with sediment ...depth have rarely been examined. Here we use 16S rDNA sequencing to examine the succession of microbial communities at sites in the Indian Ocean and the Bering Sea. The sediment ranges in depth from 0.16 to 332 m below seafloor and in age from 660 to 1,300,000 years. The majority of subseafloor taxonomic diversity is present in the shallowest depth sampled. The best predictor of sequence presence or absence in the oldest sediment is relative abundance in the near-seafloor sediment. This relationship suggests that perseverance of specific taxa into deep, old sediment is primarily controlled by the taxonomic abundance that existed when the sediment was near the seafloor. The operational taxonomic units that dominate at depth comprise a subset of the local seafloor community at each site, rather than a grown-in group of geographically widespread subseafloor specialists. At both sites, most taxa classified as abundant decrease in relative frequency with increasing sediment depth and age. Comparison of community composition to cell counts at the Bering Sea site indicates that the rise of the few dominant taxa in the deep subseafloor community does not require net replication, but might simply result from lower mortality relative to competing taxa on the long timescale of community burial.
Recently developed methane clumped isotope techniques provides fresh and novel insights into methane biogeochemistry, which have been unobtainable through other techniques such as conventional stable ...isotope determinations and molecular composition measurements of hydrocarbons. Nonetheless, the governing processes and mechanisms which control the clumped isotope signatures (Δ13CH3D and Δ12CH2D2) of natural methane samples remain an active area of investigation. Here, we present paired clumped isotope measurements in methane hydrate, which is a major methane reservoir widely distributed along the continental margins and which plays an important role in the global carbon cycle and climate system. Our study aims to shed new light into the fundamental processes of methane clumped isotope effects and their potential to answer fundamental questions regarding the source and migration of methane found in naturally occurring gas hydrate accumulations.
Gas hydrate samples were recovered from five shallow marine sediment sites on the eastern margin of the Japan Sea and most of them present Δ13CH3D and Δ12CH2D2 temperatures ranging from ∼15 to ∼170 °C that apparently match expected methane formation temperatures. The distribution of these clumped isotope signatures along the equilibrium line is best explained by the mixing effect of equilibrated thermogenic methane formed at temperatures of 165 ± 15 °C and biogenic methane equilibrated at 1–2 °C, which may result from slow methanogenesis, anaerobic oxidation of methane (AOM), or their combination. The influences of gas migration/diffusion, hydrate formation and dissociation on Δ13CH3D and Δ12CH2D2 values are insignificant. By combining clumped isotope results with other traditional approaches, a thermogenic and two microbial end-members as well as their isotopic compositions were identified and the relative contribution of each end-member was also quantified. The results not only demonstrate the applicability of methane clumped isotope data to identify potential end-members in natural methane samples, but also reveal that more conventional carbon isotope approaches may significantly underestimate the fraction of thermogenic methane present in global gas hydrate reservoirs. Improvements in the accuracy of source apportionment enable us to better understand the formation history and mechanisms of gas hydrate accumulation, as well as the role played by gas hydrate dissociation in past geological events. The estimated formation temperatures of thermogenic end-member can be further applied in reconstruction of the paleo geothermal gradient at the time when the thermogenic methane was formed at marine sedimentary environment.
Chromium (Cr) isotope fractionation is sensitive to redox changes and the Cr isotopic composition (δ53Cr) of sedimentary rocks has been used to reconstruct marine redox conditions and atmospheric ...oxygenation in the past. However, little is known about the behaviour of chromium isotopes across modern marine redox boundaries. We investigated Cr concentrations and δ53Cr variations in seawater and sediment across the Peruvian oxygen minimum zone (OMZ) to provide a better understanding of Cr cycling in the ocean. We found that seawater δ53Cr values ranged from 0.02 ± 0.16‰ to 0.59 ± 0.11‰ (2SD) and sediment values from 0.31 ± 0.07 to 0.92 ± 0.12‰. Neither Cr concentrations nor δ53Cr values in the water column revealed significant shifts across the oxic-anoxic boundaries. Instead, processes that operate at a local scale, such as Cr scavenging by Fe-rich particles and Cr release from reducing sediments, are identified as the main controls on Cr concentrations and isotope compositions in the water column. The δ53Cr values of sediments deposited in permanently anoxic waters (0.77 ± 0.19‰, n = 5) are significantly different from the δ53Cr values of sediments deposited in oxic bottom waters (0.46 ± 0.19‰, n = 4). This suggests that sediment Cr concentrations and δ53Cr values are to some extent influenced by water column redox (e.g. reductive dissolution and transport of Fe oxides) and/or early diagenetic (e.g. redistribution of Cr during phosphogenesis) processes as well as biologic activity. Our data demonstrate that local scale water column redox gradients and sediment exchange can lead to a large range of δ53Cr values in sediments, comparable to the range found in the entire geologic record to date. Given the increasing prominence of Cr isotope measurements in constraining atmospheric oxygenation in deep time, we argue that the processes influencing Cr cycling under different conditions and from the water column to the sediment need to be better resolved to verify the utility of such measurements as paleoenvironmental proxies.
Veterinary antibiotics are emerging contaminants of concern. A total of 139 samples comprising 104 marine sediments and 35 estuarine sediments were collected from the Bohai Sea area and analyzed for ...seventeen antibiotics. The results reveal that the presence and concentration of antibiotics were generally higher in the estuaries than in the sea. The highest antibiotic concentration, 4695μgkg−1 of oxytetracycline, occurred in the estuarine sediment from Ziya New River. Bohai Bay and Laizhou Bay and the surrounding estuaries had higher concentrations of antibiotics. However, low levels of antibiotics detected were detected in Liaodong Bay in contrast to the high concentrations present in the surrounding estuaries. Spatial heterogeneity and principal component analysis suggest a large impact of terrestrial sources of the antibiotics contaminating the Bohai Sea. Risk quotients indicate that current levels of norfloxacin and oxytetracycline might be potentially hazardous to sensitive biota both in the Bohai Sea and in its surrounding estuaries.
•First antibiotics investigation for sediments covering the whole Bohai Sea•Higher antibiotics concentration occurred in estuarine sediment than marine sediment.•High spatial heterogeneity suggested local sources impacts on antibiotics pollution.•Oxytetracycline and norfloxacin are the most two risky compounds in this area.
Microbial methane oxidation has a significant impact on the methane flux from marine gas hydrate areas. However, the environmental fate of methane remains poorly constrained. We quantified the ...relative contributions of aerobic and anaerobic methanotrophs to methane consumption in sediments of the gas hydrate-bearing Sakata Knoll, Japan, by in situ geochemical and microbiological analyses coupled with 13C-tracer incubation experiments. The anaerobic ANME-1 and ANME-2 species contributed to the oxidation of 33.2 and 1.4% methane fluxes at 0–10 and 10–22 cm below the seafloor (bsf), respectively. Although the aerobic Methylococcaceae species consumed only 0.9% methane flux in the oxygen depleted 0.0–0.5 cmbsf zone, their metabolic activity was sustained down to 6 cmbsf (based on rRNA and lipid biosyntheses), increasing their contribution to 10.3%. Our study emphasizes that the co-occurrence of aerobic and anaerobic methanotrophy at the redox transition zone is an important determinant of methane flux.
In this paper, a series of experiments was conducted on remoulded hydrate-bearing marine sediments from a gas hydrate core drilling site located in the Pearl River Estuary of the South China Sea. The ...composition and particle size distribution of marine sediments were analysed, and triaxial loading experiments were performed to study the mechanical properties of hydrate-bearing marine sediments. The strength properties of hydrate-bearing marine sediments were compared with those of hydrate-bearing kaolin clay, ice-marine sediment mixtures and hydrate-dissociated marine sediments under the same porosity (40%), temperature (8 °C), confining pressures (2, 3, and 4 MPa), and strain rate (1%/min) conditions. Several important results were obtained: The composition and particle size distribution analysis indicated the marine sediment and kaolin clay are both sandy clays, and the stress-strain behaviours and strength properties of hydrate-bearing marine sediments are similar to those of hydrate-bearing kaolin clay. Under different confining pressures, the failure strengths of all ice-marine sediment mixtures were lower than those of methane hydrate-bearing sediments, confirming that hydrate particles enhance the cementation of sediment particles. Hydrate dissociation leads to strength reduction of gas hydrate-bearing marine sediments, as was demonstrated in our previous research of hydrate-bearing kaolin clay. Through the analysis of Mohr circles and failure envelopes of three types of mixtures under different confining pressures, it can be inferred that the cohesive strength played an important role in the failure strength of hydrate-bearing marine sediments following hydrate dissociation.
•Triaxial strength experiments were conducted on hydrate-bearing marine sediments of South China Sea.•Strength property of hydrate-bearing marine sediment was found to be similar to hydrate-bearing kaolin clay.•Hydrate between sediment particles enhances the cohesion of sediment.•Cohesion reduction accompanying with hydrate dissociation greatly affects the failure strength of hydrate-bearing sediment.
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