Molecular characterization of dissolved black nitrogen via electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Combustion produces a complex mixture of polycondensed aromatic compounds known as black carbon (BC). Such products can become remobilized from char and soil in the form of dissolved BC (DBC). Ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (ESI–FT-ICRMS) analysis of a variety of soil and char leachates showed that a significant proportion of DBC compounds contained one or more nitrogen atoms. While the presence of black nitrogen (DBN) in dissolved organic matter (DOM) has been reported, its molecular features were uncharacterized. Here we present results of FT-ICRMS characterization of DBN, where assigned formulae were validated on the basis on their ¹³C isotope signatures and fragmentation patterns obtained via collision induced dissociation. Possible chemical structures were assigned for several DBN formulae and suggest that nitrogen was incorporated into the core ring system as a pyrrole-type moiety. Most DBN compounds existed as part of homologous series where homologs differed by a mass corresponding to CO₂, suggesting that they were polysubstituted with carboxylic acid groups. The environmental contribution of such novel, aromatic, combustion-derived nitrogen compounds with respect to global nitrogen cycling remains elusive. The biogeochemical implications of the input of such fire-derived products to aquatic ecosystems as part of climate change therefore need to be assessed.     

应用高分辨率质谱分析苏丹高酸值原油成因

苏丹Muglad和Melut盆地是苏丹乃至整个中、西非剪切带最富含油气的盆地,所发现的原油主要为中质油(重度为20°～34°API),其次为重质油(重度小于20°API),普遍高含沥青质、高含蜡、高酸值、低含硫。为了探讨高酸值原油的成因,选择了苏丹地区18个不同酸值的原油样品,尝试高分辨率质谱分析上述原油有机酸的组成。结果表明,高酸值原油的有机酸主要由环烷酸组成;环烷酸的平均相对分子质量随降解作用程度增加而增大,分子碳原子数分布范围变宽;环烷酸以一环、二环、三环环烷酸为主。生物降解作用是形成高酸值原油的主要原因。     

Effect of secondary oil migration distance on composition of acidic NSO compounds in crude oils determined by negative-ion electrospray Fourier transform ion cyclotron resonance mass spectrometry

The acidic and neutral NSO compounds in a series of Duvernay-sourced oils in Canada, which are believed to have migrated extensively over relatively long distances such as along the Rimbey-Meadowbrook reef trends, were characterized by negative-ion electrospray (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Heteroatomic compounds were characterized according to their class (number of nitrogen, oxygen and sulfur heteroatoms), degree of aromaticity [rings plus double bonds (DBE)] and carbon number distribution. The N₁, N₁O₁, N₁O₂, N₁S₁, O₁ and O₂ classes were identified in Duvernay-sourced oils. With increasing migration distance, the relative abundance of O₂, N₁O₁ and N₁O₂ showed a significant decrease, while the O₁ class increased from < 10% to nearly 30% of the total. With increasing migration along the Rimbey-Meadowbrook reef trend, pyrrolic nitrogen compounds (N₁ class) shows an enrichment of alkylcarbazoles (DBE = 9) relative to alkylbenzocarbazoles (DBE = 12), and of higher homologous relative to the lower homologous. O₁ compounds show a relative enrichment of those with low DBE values. Additionally, the N₁O₁ and N₁S₁ compounds show a relative enrichment of those with high DBE values, and of higher homologues compared to the lower homologues, indicating great potential for developing new migration indices.     

A new approach for molecular characterisation of sediments with Fourier transform ion cyclotron resonance mass spectrometry: Extraction optimisation

Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) can begin to tease apart the molecular character of sedimentary organic matter (SOM). We therefore tested five different solvents (aqueous base, CHCl₃, MeOH, pyridine and water) for their ability to extract a representative fraction from two organic rich lacustrine sediments, Mangrove Lake, Bermuda (MLB) and Mud Lake, Florida (MLF). Following comparison using liquid state nuclear magnetic resonance spectroscopy (NMR) and negative ion mode electrospray ionization mass spectrometry (FTICRMS) we found that pyridine was the optimal solvent, extracting a more diverse (10–100× greater integration for carbonyl, amide and amine groups) and a larger number of peaks on average (1375–1450 vs. 380–1450). Comparison of the pyridine extracts between MLB, MLF and two organic poor sediments from the Mississippi River Delta and Bayou Grande (Pensacola, FL) showed that only 4.9% of the molecular formulae were common to all four and that unique formulae made up the highest proportion of the assignments. The use of pyridine for extracting immature (Holocene) SOM for FTICRMS analysis can therefore be widely applied to immature sediments and produce representative spectra.     

Direct Fourier transform mass spectral analysis of natural waters with low dissolved organic matter

A major obstacle for characterizing dissolved organic matter (DOM) with ultrahigh resolution mass spectrometry has been its low concentrations in natural waters. Many previous mass spectrometric studies of both terrestrial and marine DOM typically have isolated and concentrated the DOM using solid phase C₁₈ extraction disks, ultrafiltration, or XAD resins, all of which are known to discriminate against many different classes of compounds. We have, for the first time, developed an approach to directly analyze natural water samples with less than 6 mg/l DOC (dissolved organic carbon), using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS). We demonstrate the sensitivity and ability of sequential selective ion accumulation (SSIA) to detect the thousands of components in a single freshwater DOM sample without any significant pretreatment. By utilizing SSIA, the baseline noise decreases while signal to noise ratios of the peaks increase, allowing for approximately 40% more formulas to be assigned to peaks in the mass spectra.     

Microanalysis of carbon isotope composition in organic matter by secondary ion mass spectrometry

An analytical procedure has been developed for the in situ measurement of carbon isotope composition of organic matter, with a spatial resolution of 20-30 μm, using a Cameca IMS 1270 ion microprobe. Instrumental mass fractionation (IMF) of carbon isotopes was observed to be independent of primary ion beam intensity and sputtering time, but did depend on vacuum conditions and on the chemical composition of the sample. To evaluate such “matrix effects”, a set of 9 standards representative of the natural chemical variability of organic matter was prepared, with H/C atomic ratios and organic carbon contents (C_org) ranging between 0.04 and 1.74 and between 41 and 100 wt.%, respectively. Under the analytical conditions tested, IMF was not found to be influenced by the presence of silicate mineral impurities in the organic matter, but variations in IMF up to 5‰ were observed over the set of standards with the magnitude of IMF negatively correlated to the H/C ratios of samples. Aliphaticity ratios determined using Fourier transform infrared microspectroscopy provided an in situ estimation of H/C ratios with a spatial resolution barely exceeding that of the ion microprobe and permit a correction for matrix effects with a standard error of ± 0.2‰ (1σ). Taking into account all sources of uncertainty, ion microprobe δ¹³C were accurately determined with a ± 0.7‰ (1σ) total uncertainty. The mechanism for the matrix effect of H/C ratios upon IMF is still to be determined but it is likely related to the difference in proportion of atomic vs. molecular carbon ions observed between samples of different H/C ratios.     

Bacterial reworking of terrigenous and marine organic matter in estuarine water columns and sediments

Amino acids and the bacterial biomarkers muramic acid and d-amino acids were quantified in the ultrafiltered dissolved, particulate and sedimentary organic matter (UDOM, POM and SOM) of the St. Lawrence system (Canada). The main objectives were to better describe the fate of terrigenous and marine organic matter (OM) in coastal zones and to quantify the bacterial contributions to OM composition and diagenesis. Regardless of their origin, the carbon (C) content of the particles substantially decreased with depth, especially near the water-sediment interface. Major diagenetic transformations of organic nitrogen (N) were revealed and important differences were observed between terrigenous and marine OM. Amino acid contents of particles decreased by 66-93% with depth and accounted for 12-30% of the particulate C losses in marine locations. These percentages were respectively 18-56% and 7-11% in the Saguenay Fjord where terrigenous input is important. A preferential removal of particulate N and amino acids with depth or during transport was measured, but only in marine locations and for N-rich particles. This leads to very low amino acid yields in deep marine POM. However, these yields then increased to a level up to three times higher after deposition on sediments, where SOM showed lower C:N ratios than deep POM. The associated increase of bacterial biomarker yields suggests an active in situ resynthesis of amino acids by benthic bacteria. The N content of the substrate most likely determines whether a preferential degradation or an enrichment of N and amino acid are observed. For N-poor OM, such as terrigenous or deep marine POM, the incorporation of exogenous N by attached bacteria can be measured, while the organic N is preferentially used or degraded in N-rich OM. Compared to the POM from the same water samples, the extracted UDOM was poor in N and amino acids and appeared to be mostly made of altered plant and bacterial fragments. Signs of in situ marine production of UDOM were observed in the most marine location. The POM entering the St. Lawrence Upper Estuary and the Saguenay Fjord appeared made of relatively fresh vascular plant OM mixed with highly altered bacterial debris from soils. In contrast, the POM samples from the more marine sites appeared mostly made of fresh planktonic and bacterial OM, although they were rapidly degraded during sinking. Based on biomarker yields, bacterial OM represented on average ∼20% of bulk C and approximately 40-70% of bulk N in POM and SOM, with the exception of deep marine POM exhibiting approximately two times lower bacterial contributions.     

Elucidating the chemical structure of pyrogenic organic matter by combining magnetic resonance,mid-infrared spectroscopy and mass spectrometry

Fire-derived organic matter (pyrogenic organic matter, or PyOM), despite its apparent long term stability in the environment, has recently been reported to degrade faster than previously thought. Current studies have suggested that the composition and structure of PyOM can provide new insights on the mechanisms by which C and N from pyrolyzed biomaterials are stabilized in soils. To better understand the chemical structure of PyOM produced under typical fire conditions in temperate forests, samples of dual-enriched (¹³C/¹⁵N) Pinus ponderosa wood and the charred material produced at 450 °C were analyzed by solid state nuclear magnetic resonance (ssNMR), electron paramagnetic resonance (EPR), diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy, and both isotopic and elemental composition (C, H, O, and N). Notably, the use of high magnetic field strengths in combination with isotopic enrichment augmented the NMR detection sensitivity, and thus improved the quality of molecular information as compared with previously reported studies of pyrogenic materials. The key molecular groups of pine wood and the corresponding PyOM materials were determined using magic-angle spinning (MAS) ¹³C, ¹⁵N, and ¹H NMR. Together with DRIFT and EPR measurements, ssNMR revealed the formation of a free radical-containing disordered blend of nitrogenous aromatics and heat resistant aliphatics in the PyOM due to incomplete combustion of the precursor wood. ¹³C ssNMR and DRIFT analyses showed the removal of oxygenated aliphatics due to pyrolysis of the precursor wood and the dominant contribution of multiply-bonded and oxygenated aromatic structures in the resulting PyOM. However, the ¹⁸O isotopic analyses indicated selective retention of ligneous moieties during charring at 450 °C. ¹⁵N ssNMR studies implied that the nitrogenous species in PyOM corresponded to thermally altered rather than heat resistant domains of the pine wood precursor. Our molecular characterization suggests that biomaterials pyrolyzed near 450 °C may degrade in soils faster than those pyrolyzed at higher temperatures and may not represent a stable C sink in terrigenous ecosystems.     

Nature and dynamics of phosphorus-containing components of marine dissolved and particulate organic matter

The molecular sources, dynamics and analytical characterizations of the phosphorus (P) containing components of marine dissolved and particulate organic matter (OM) are reviewed. Using a combination of ¹³C and ³¹P nuclear magnetic resonance spectroscopy on samples collected from a depth profile (20-4000 m) at Station Aloha in the North Pacific subtropical gyre, the biomolecular associations of P functional groups in marine OM are identified. Compositional differences between ultrafiltered dissolved organic matter (UDOM; 1-100 nm size fraction) and ultrafiltered particulate organic matter (UPOM; 0.1-60 μm size fraction) as reflected by NMR and elemental analyses indicate that UDOM does not originate from simple solubilization of UPOM, and the aggregation of UDOM is not the primary source of UPOM. Regression analyses indicated a large fraction of the P in UDOM is associated with carbohydrates and amino acids, but not with lipids. Similar analyses for UPOM indicated that P is associated with carbohydrates, amino acids and lipids. The P functional groups also appear to be balanced in their distribution among molecular classes, because they remain in relatively constant proportion throughout the ocean.     

Nature and reactions of dissolved organic matter in the interstitial waters of marine sediments

Two organic rich sediments, an oxic muddy sand and a silty mud containing sulphate reducing and methane producing metabolic zones, were sampled from Loch Duich, a fjord type estuary in the N.W. coast of Scotland. Dissolved organic carbon (DOC), as measured by dry combustion and UV absorption, remained constant (8.3–15.8 mg C/l) with depth in the oxic pore waters at a concentration at least twice that of the overlying seawater. DOC in the anoxic pore waters increased linearly with depth from 13.6 at the surface to 55.9–70.5 mg C/l at 80cm. Most of the DOC was present in the high molecular weight (HMW) fraction as separated by ultrafiltration; the low molecular weight (LMW) fraction remained constant (10.0 mg C/l) in both oxic and anoxic pore waters. Spectroscopic data showed the ‘humic’ fraction of the HMW dissolved organic matter was mainly fulvic acid, a small proportion (approx 1%) of humic acid, and a third fraction, possibly melanoidins, which increased relative to fulvic acid with depth. These data confirm the pathway of humification (NissenBaum et al, 1971; nissenbaum and Kaplan, 1972) where HMW organic matter accumulates in pore waters as condensation products of LMW organic substances.     

Chemical and spectroscopic characterization of marine dissolved organic matter isolated using coupled reverse osmosis-electrodialysis

The coupled reverse osmosis-electrodialysis (RO/ED) method was used to isolate dissolved organic matter (DOM) from 16 seawater samples. The average yield of organic carbon was 75 ± 12%, which is consistently greater than the yields of organic carbon that have been commonly achieved using XAD resins, C₁₈ adsorbents, and cross-flow ultrafiltration. UV-visible absorbance spectra and molar C/N ratios of isolated samples were consistent with the corresponding properties of DOM in the original seawater samples, indicating that DOM samples can be isolated using the coupled RO/ED method without any bias for/against these two properties. Five of the samples were desalted sufficiently that reliable measurements of their ¹³C and ¹HNMR spectra and their Fourier transform ion cyclotron resonance (FTICR) mass spectra could be obtained. The ¹³C and ¹HNMR spectra of RO/ED samples differed distinctly from those of samples that have been isolated in much lower yields by other methods. In particular, RO/ED samples contained a relatively lower proportion of carbohydrate carbon and a relatively greater proportion of alkyl carbon than samples that have been isolated using cross-flow ultrafiltration. From the FTICR mass spectra of RO/ED samples, samples from the open ocean contained a much lower proportion of unsaturated compounds and a much higher proportion of fatty acids than coastal samples.     

Chemical characterization of high molecular weight dissolved organic matter in fresh and marine waters

The high molecular weight fraction of dissolved organic matter in a suite of lakes, rivers, seawater, and marine sediment interstitial water samples was collected by ultrafiltration and characterized by molecular level and spectroscopic techniques. Proton nuclear magnetic resonance spectra of all samples show a high degree of similarity, with major contributions from carbohydrates, bound acetate, and lipids. Molecular level analyses of neutral sugars show seven monosaccharides, rhamnose, fucose, arabinose, xylose, mannose, glucose, and galactose, to be abundant, and to occur in comparable relative amounts in each sample. Previous studies have emphasized the distinctive composition of dissolved humic substances in fresh and marine waters, and have attributed these differences to sources and transformations of organic matter unique to each environment. In contrast we find a large fraction of freshwater high molecular weight dissolved organic matter (HMWDOM; > 1kD) to be indistinguishable from marine HMWDOM in bulk and molecular-level chemical properties. Aquatic HMWDOM is similar in chemical composition to biologically derived acylated heteropolysaccharides isolated from marine algal cultures, suggesting a biological source for some fraction of persistent HMWDOM. High molecular weight DOC contributes 51 ± 26% of the total DOC, and monosaccharides 18 ± 8% of the total HMWDOC in our freshwater samples. These contributions are on average higher and more variable, but not significantly different than for surface seawater (30% and 16% respectively). Biogeochemical processes that produce, accumulate, and recycle DOM may therefore share important similarities and be broadly comparable across a range of environmental settings.     

Understanding the enhanced aqueous solubility of styrene by terrestrial dissolved organic matter using stable isotope mass balance and FTIR

We present a new stable isotope mass balance method for measuring the enhanced aqueous solubility of specific organic compounds in the presence of natural dissolved organic matter (DOM). It involves interfacing a standard dissolved organic carbon (DOC) analyzer with a stable isotope ratio monitoring system, is applicable to a wide range of model organic compounds and can be tuned to provide maximum precision for a given range of compound solubility and initial natural DOC concentration. Using ¹³C-labeled styrene as a model compound, we applied the method to investigate the reactivity of Dismal Swamp DOM as a function of season, nominal molecular size and chemical composition as determined using Fourier Transform Infrared Spectroscopy (FTIR). The solubility enhancement of styrene ranged from 23% to 118% relative to deionized water, while DOC-normalized enhancements varied from about 0.04 to 0.35 μM styrene/μM DOC as a function of season and nominal molecular weight. Statistical analysis of FTIR spectra reveals a strong positive correlation between the styrene concentration and the carboxyl content of the natural DOM. Reactivity differences between high molecular weight (HMW), low molecular weight (LMW) and total DOM samples are consistent with potential variations in their higher order structures.     

Molecular characterization of Type I kerogen from the Green River Formation using advanced NMR techniques in combination with electrospray ionization/ultrahigh resolution mass spectrometry

This study describes a new approach for characterizing high molecular weight compounds in Type I kerogen, involving both nuclear magnetic resonance (NMR) spectroscopy and Fourier transform ion cyclotron mass spectrometry (FTICR-MS). Kerogen isolated from the Mahogany zone of the Green River Formation was examined directly using high resolution magic angle spinning (HRMAS) NMR to obtain liquid-like multidimensional spectra. It was then successively extracted with n-pentane, dichloromethane and pyridine. Pyridine extraction was also performed for comparison with the successive extractions. Using solid-state NMR, we show that the sum of the successive extracts and the single pyridine extract are quantitatively representative of the unextracted kerogen. This suggests that a non-invasive characterization of Green River kerogen can be obtained by examining the soluble extracts, all of which were subjected to ESI-FTICR-MS to identify a wide series of compounds. Series of polar CHO, CHOS and CHON compounds between C₁₂ and C₅₀₊ were found. In all the extracts the two homologous series of acids (C_nH_2nO₂ and C_nH_2n−2O₄) dominated. Collision-induced dissociation was also employed to identify the different functional groups comprising the different series. The CHO series contained carboxylic acid and alkoxyl groups, whereas the CHOS and CHON series contained sulfoxide groups and nitrile-type compounds. The results also show that pyridine extraction can be used either for screening a large series of samples or for the specific study of CHO compounds. However for a detailed and complete study of the different homologous series we recommend using the successive extraction protocol.     

Large-volume ultrafiltration for the study of radiocarbon signatures and size vs. age relationships in marine dissolved organic matter

In recent decades, tangential-flow ultrafiltration (UF) technology has become a primary tool for isolating large amounts of “ultrafiltered” marine dissolved organic carbon (UDOC; 0.1 μm to ∼1 nm) for the detailed characterization of DOC chemical composition and radiocarbon (Δ¹⁴C) signatures. However, while total DOC Δ¹⁴C values are generally thought to be quite similar in the world ocean, previous studies have reported widely different Δ¹⁴C values for UDOC, even from very similar ocean regions, raising questions about the relative “reactivity” of high molecular weight (HMW) DOC. Specifically, to what degree do variations in DOM molecular weight (MW) vs. composition alter its relative persistence, and therefore HMW DOC Δ¹⁴C values?In this study we evaluate the effects of varying proportions of HMW vs. low molecular weight (LMW) DOC on UDOC Δ¹⁴C values. Using concentration factor (CF) as a proxy for MW distributions, we modeled the retention of both OC and Δ¹⁴C in several very large CF experiments (CF >3000), from three depths (20, 670, and 915 m) in the North Pacific Subtropical Gyre (NPSG). The resulting DOC and Δ¹⁴C UF permeation coefficients generally increase with depth, consistent with mass balance trends, indicating very significant permeation of LMW, ¹⁴C-depleted DOC at depth, and higher recoveries of Δ¹⁴C-enriched, HMW DOC in the surface. In addition, changes in CF during sample concentration and ionic strength during sample diafiltration had very large and predictable impacts on UDOC Δ¹⁴C values.Together these results suggest that previously reported disparities in UDOC Δ¹⁴C values are reconciled by linked trends of Δ¹⁴C content vs. MW. At low CFs, UDOC samples have similar Δ¹⁴C values to total DOC. In contrast, UDOC samples collected at extremely high CFs (and after diafiltration) have more positive Δ¹⁴C values. We demonstrate that the observed relationships between UDOC Δ¹⁴C and CF derived from our data can directly explain offsets in all previously published UDOC Δ¹⁴C values for the NPSG. While CF is not traditionally considered in UF studies, our results indicate it can substantially influence the interpretation of UDOC ¹⁴C “age”, and thus reactivity, in the marine environment. In addition, our results indicate that CF can in fact be used as a proxy for average MW. We suggest that a variable-CF-UF approach, coupled with molecular-level Δ¹⁴C analyses, presents a new tool for studying relationships between molecular size, age, and “labile” DOC distributions in the ocean.     

Determination of Hg and MeHg complexation with dissolved organic matter by fluorescence quenching titration

In natural waters inorganic mercury （Hg） and methylmercury （MeHg） are complexed with a variety of inorganic and organic ligands, such as OH^-, Cl^-, sulfide, thiols, and dissolved organic matter （DOM）. The bioavailability and toxicity of Hg and MeHg are related to their speciation, instead of their total concentrations. Among these species, Hg-DOM and MeHg-DOM complexes are the least known, due to the complexity and site-specificity of DOM in natural waters. Here we report the complexation between DOM and Hg or MeHg using fluorescence spectroscopy. The Suwannee River fulvic acid, peat humic acid, amino acid typotophan, and natural organic matter from the Suwannee River, Nordic River, and Delta Marsh were studied at their respective excitation/emission maxima,     

Enhanced trapping of molybdenum by sulfurized marine organic matter of marine origin in Mesozoic limestones and shales

There have been many studies devoted to trace metals and their value in assessing the paleoredox conditions of ancient marine deposition. Among them, molybdenum (Mo) is frequently cited as an effective proxy for sediments and sedimentary rocks. Recently, Helz et al. (Helz, G.R., Miller, C.V., Charnock, J.M., Mosselmans, J.L.W., Pattrick, R.A.D., Garner, C.D., Vaughan, D.J., 1996. Mechanisms of molybdenum removal from the sea and its concentration in black shales: EXAFS evidences. Geochim. Cosmochim. Acta, 60, 3631-3642) and Adelson et al. (Adelson, J. M., Helz, G. R., Miller, C. V., 2001. Reconstructing the rise of recent coastal anoxia; molybdenum in Chesapeake Bay sediments. Geochim. Cosmochim. Acta, 65, 237-252.) suggested that Mo does not behave conservatively in the water column when H₂S reaches a threshold concentration. Above this concentration, a “switch” operates, and Mo is scavenged by forming bonds with metal-rich (notably iron) particles, sulfur-rich organic molecules and pyrite. In this paper, Mo-trapping by sulfur-rich organic matter (OM) in ancient marine deposits is emphasized. The following Mesozoic geological formations were selected for study because of their relatively high concentration of sulfurized OM: the Akkuyu Formation (Turkey), the Calcaires d'Orbagnoux (France) and Kimmeridge Clay (UK) and its timeequivalent in Boulonnais (France), the Kashpir oil shales (Russia), and the La Luna Formation (Venezuela). The sulfur-rich OM is identified by either measured organic-S abundance or kerogen microscope observation. Our results show that Mo is systematically more enriched relative to the other redox-sensitive/sulfide-forming elements studied (U, V, Ni, Cu, Zn, Cr), and Mo enrichment is positively correlated with the amount of sulfurized OM but not with pyrite abundance. These results illuminate the role played by sulfurized OM in geologic-scale Mo capture and retention, but they also underline the role played by reactive iron. Significant OM sulfurization is only possible when reactive iron is limited. Nevertheless, pyrite formation, though limited, could act as an initial Mo trap, prior to Mo uptake by OM that is sulfurized after the pyritization step. In future paleoenvironmental reconstructions, attention must be paid to this enhanced Mo enrichment in the presence of sulfurized organic matter. In such cases, the use of Mo could lead to overestimation of the reducing conditions of the depositional environment.     

Sources and distribution of terrigenous organic matter delivered by the Atchafalaya River to sediments in the northern Gulf of Mexico

Suspended sediments (SS) from the Atchafalaya River (AR) and the Mississippi River and surficial sediment samples from seven shallow cross-shelf transects west of the AR in the northern Gulf of Mexico were examined using elemental (%OC, C/N), isotopic (δ¹³C, Δ¹⁴C), and terrigenous biomarker analyses. The organic matter (OM) delivered by the AR is isotopically enriched (∼−24.5‰) and relatively degraded, suggesting that soil-derived OM with a C4 signature is the predominant OM source for these SS. The shelf sediments display OC values that generally decrease seaward within each transect and westward, parallel to the coastline. A strong terrigenous C/N (29) signal is observed in sediments deposited close to the mouth of the river, but values along the remainder of the shelf fall within a narrow range (8-13), with no apparent offshore trends. Depleted stable carbon isotope (δ¹³C) values typical of C3 plant debris (−27‰) are found near the river mouth and become more enriched (−22 to −21‰) offshore. The spatial distribution of lignin in shelf sediments mirrors that of OC, with high lignin yields found inshore relative to that found offshore (water depth > 10 m).The isotopic and biomarker data indicate that at least two types of terrigenous OM are deposited within the study area. Relatively undegraded, C3 plant debris is deposited close to the mouth of the AR, whereas more degraded, isotopically enriched, soil-derived OM appears to be deposited along the remainder of the shelf. An important input from marine carbon is found at the stations offshore from the 10-m isobath. Quantification of the terrigenous component of sedimentary OM is complicated by the heterogeneous composition of the terrigenous end-member. A three-end-member mixing model is therefore required to more accurately evaluate the sources of OM deposited in the study area. The results of the mixing calculation indicate that terrigenous OM (soil-derived OM and vascular plant debris) accounts for ∼79% of the OM deposited as inshore sediments and 66% of OM deposited as offshore sediments. Importantly, the abundance of terrigenous OM is 40% higher in inshore sediments and nearly 85% higher in offshore sediments than indicated by a two-end-member mixing model. Such a result highlights the need to reevaluate the inputs and cycling of soil-derived OM in the coastal ocean.     

Degradation of dissolved organic monomers and short-chain fatty acids in sandy marine sediment by fermentation and sulfate reduction

The decay of a wide range of organic monomers (short-chain volatile fatty acids (VFA’s), amino acids, glucose and a pyrimidine) was studied in marine sediments using experimental plug flow-through reactors. The reactions were followed in the presence and absence of 10 mM SO₄²⁻. Degradation stoichiometry of individual monomers (inflow concentration of 6 mM organic C) was traced by measuring organic (VFA’s, amino acids) and inorganic (CO₂, NH₄⁺, SO₄²⁻) compounds in the outflow. Fermentation of amino acids was efficient and complete during passage through anoxic sediment reactors. Aliphatic amino acids (alanine, serine and glutamate) were primarily recovered as CO₂ (24-34%), formate (3-22%) and acetate (41-83%), whereas only ∼1/3 of the aromatic amino acid (tyrosine) was recovered as CO₂ (13%) and acetate (20%). Fermentation of glucose and cytosine was also efficient (78-86%) with CO₂ (30-35%), formate (3%) and acetate (28-33%) as the primary products. Fermentation of VFA’s (acetate, propionate and butyrate), on the other hand, appeared to be product inhibited. The presence of SO₄²⁻ markedly stimulated VFA degradation (29-45% efficiency), and these compounds were recovered as CO₂ (17% for butyrate to 100% for acetate) and acetate (51% and 82% for propionate and butyrate, respectively). When reaction stoichiometry during fermentation is compared with compound depletion during sulfate reduction, the higher proportion CO₂ recovery is consistent with lower acetate and formate accumulation. Our results therefore suggest that fermentation reactions mediate the initial degradation of added organic compounds, even during active sulfate reduction. Fermentative degradation stoichiometry also suggested significant H₂ production, and >50% of sulfate reduction appeared to be fuelled by H₂. Furthermore, our results suggest that fermentation was the primary deamination step during degradation of the amino acids and cytosine.