Sediments comprise a multitude of inorganic and organic components, with much of the composition of the organics still not fully characterized. Our research targeted n-alkanes, to determine whether compound specific carbon and hydrogen isotope analysis allows for their source identification in coastal sediments. Here, we map the current abundances and sources of straight chain n-alkanes in sediments of the St. Lawrence Estuary and Gulf using molecular (diagnostic ratios) and isotopic fingerprinting (813C, 82H). n-Alkane abundances (117.11 +/- 1.61 to 418.64 +/- 70.20 mu g/g OC), carbon preference index (CPI; 1.95 +/- 0.05 to 5.09 +/- 0.10), average chain length (ACL; 28.36 +/- 0.02 to 28.97 +/- 0.01), proportion of aquatic submerged plants and terrestrial plant inputs (Paq; 0.295 +/- 0.003 to 0.377 +/- 0.002), terrigenous aquatic ratio (TAR; 3.43 +/- 0.16 to 7.99 +/- 0.05), and n-alkane ratio (NAR; 0.169 +/- 0.011 to 0.584 +/- 0.011) values varied along the terrestrial-marine continuum. Large differences in the concentration weighted average (WA) 813C and 82H for odd and even n-alkanes were found, with WA 813C ranging from -30.9 +/- 0.3 to -33.4 +/- 0.09 %o and -28.8 +/- 0.01 to -32.3 +/- 0.2 %o, respectively, and 165.6 +/- 3.6 to -200.8 +/- 2.4 %o and -96.0 +/- 2.8 to -158.7 +/- 2.1 %o for 82H. The diagnostic ratios were shown to misrepresent the input sources of organic matter (OM) and were inaccurate as source indicators when more than one OM source was present. With the addition of compound specific 813C and 82H analysis of n-alkanes, it was determined that the n-alkanes were predominantly derived from natural, rather than anthropogenic sources, with variations being driven by geographic changes in vegetation type and differing ratios of terrestrial and marine OM inputs. Importantly, compound specific isotope analysis of the even numbered n-alkanes would permit identification and tracking of petroleum-derived contaminants. Molecular data alone are ineffective for this, owing to the similarity in CPI values for petroleum-derived contaminants and highly degraded OM which is discharged by the St. Lawrence River into the estuary.

Excessive consumption of petroleum and crude oil for energy purposes has resulted in the contamination of many natural systems and waterways. However, determining the presence and level of contamination has been difficult due to the presence of naturally occurring hydrocarbons and to the complexity of the molecular fingerprint of petroleum and crude oils. Naturally occurring straight-chain n-alkanes and polycyclic aromatic hydrocarbons (PAHs) in sediments are both commonly used to determine organic matter sources through diagnostic and isotope ratios, and these ratios are affected by the presence of petroleum and crude oil. As such, they offer a potential avenue for determining whether crude oil contaminants are present in natural systems. The purpose of this project was to determine whether diagnostic ratios of n-alkanes and of PAHs as well as compound-specific isotope ratios of n-alkanes (∂2Halk and ∂13Calk) could be used to detect crude oil or petroleum contamination, and at what level of contamination the difference becomes significant. This was accomplished by separating the aliphatic and aromatic fractions of the natural and crude oil hydrocarbons by column chromatography, spiking natural sediment hydrocarbons with crude oil hydrocarbons at different levels, and analyzing the samples by GC-MS (Gas Chromatography-Mass Spectrometry) and by GC-IRMS (Isotope-Ratio Mass Spectrometry). The isotopic ratios and the hydrocarbon concentrations were determined by external standard calibration, and the diagnostic ratios were then calculated from the concentrations. Both ratios were then evaluated for their efficiency in detecting the presence of crude oil contamination.

Abstract. Most lakes worldwide are supersaturated with carbon dioxide (CO2) and consequently act as atmospheric net sources. Since CO2 is a major greenhouse gas (GHG), the accurate estimation of CO2 exchanges at air/water interfaces of aquatic ecosystems is vital in quantifying the carbon budget of aquatic ecosystems overall. To date, lacustrine CO2 emissions are poorly understood, and lake carbon source proportions remain controversial, largely due to a lack of integration between aquatic and terrestrial ecosystems. In this paper a new process-based model (TRIPLEX-Aquatic) is introduced incorporating both terrestrial inputs and aquatic biogeochemical processes to estimate diffusive emissions of CO2 from lake systems. The model was built from a two-dimensional hydrological and water quality model coupled with a new lacustrine CO2 diffusive flux model. For calibration and validation purposes, two years of data collected in the field from two small boreal oligotrophic lakes located in Quebec (Canada) were used to parameterize and test the model by comparing simulations with observations for both hydrodynamic and carbon process accuracy. Model simulations were accordant with field measurements in both calibration and verification. Consequently, the TRIPLEX-Aquatic model was used to estimate the annual mean CO2 diffusive flux and predict terrestrial dissolved organic carbon (DOC) impacts on the CO2 budget for both lakes. Results show a significant fraction of the CO2 diffusive flux (~30–45%) from lakes was primarily attributable to the input and mineralization of terrestrial DOC, which indicated terrestrial organic matter was the key player in the diffusive flux of CO2 from oligotropical lake systems in Quebec, Canada.

About one-fifth of organic carbon in sediments is bound to reactive iron phases, which are metastable over geological timescales and may therefore serve as a sink for the long-term storage of organic carbon. It is well known that solid iron phases can preserve organic carbon in soils, but it remains uncertain whether significant amounts of organic carbon can be preserved by iron in sediments. Yves Gélinas et al. study a range of freshwater and marine sediments and find that almost one-quarter of the organic carbon in the sediments tested is directly bound to reactive iron phases. They further estimate that about 22% of the total surface marine sedimentary organic carbon is preserved by its association with iron, which suggests that reactive iron phases are a key factor in the long-term storage of organic carbon. This 'rusty sink' links the global cycles of carbon, oxygen and sulphur. The biogeochemical cycles of iron and organic carbon are strongly interlinked. In oceanic waters, organic ligands have been shown to control the concentration of dissolved iron1. In soils, solid iron phases shelter and preserve organic carbon2, but the role of iron in the preservation of organic matter in sediments has not been clearly established. Here we use an iron reduction method previously applied to soils3 to determine the amount of organic carbon associated with reactive iron phases in sediments of various mineralogies collected from a wide range of depositional environments. Our findings suggest that 21.5 ± 8.6 per cent of the organic carbon in sediments is directly bound to reactive iron phases. We further estimate that a global mass of (19–45) × 1015 grams of organic carbon is preserved in surface marine sediments as a result of its association with iron4. We propose that these associations between organic carbon and iron, which are formed primarily through co-precipitation and/or direct chelation, promote the preservation of organic carbon in sediments. Because reactive iron phases are metastable over geological timescales, we suggest that they serve as an efficient ‘rusty sink’ for organic carbon, acting as a key factor in the long-term storage of organic carbon and thus contributing to the global cycles of carbon, oxygen and sulphur5.

Most natural freshwater lakes are net greenhouse gas (GHG) emitters. Compared to natural systems, human perturbations such as watershed wood harvesting and long‐term reservoir impoundment lead to profound alterations of biogeochemical processes involved in the aquatic cycle of carbon (C). We exploited these anthropogenic alterations to describe the C dynamics in five lakes and two reservoirs from the boreal forest through the analysis of dissolved carbon dioxide (CO2), methane (CH4), oxygen (O2), and organic carbon (DOC), as well as total nitrogen and phosphorus. Dissolved and particulate organic matter, forest soil/litter and leachates, as well as dissolved inorganic carbon were analyzed for elemental and stable isotopic compositions (atomic C:N ratios, δ13Corg, δ13Cinorg and δ15Ntot). We found links between the export of terrestrial organic matter (OM) to these systems and the dissolved CO2 and O2 concentrations in the water column, as well as CO2 fluxes to the atmosphere. All systems were GHG emitters, with greater emissions measured for systems with larger inputs of terrestrial OM. The differences in CO2 concentrations and fluxes appear controlled by bacterial activity in the water column and the sediment. Although we clearly observed differences in the aquatic C cycle between natural and perturbed systems, more work on a larger number of water bodies and encompassing all four seasons should be undertaken to better understand the controls, rates, and spatial as well as temporal variability of GHG emissions, and to make quantitatively meaningful comparisons of GHG emissions (and other key variables) from natural and perturbed systems.

degree of control over the other. In the oceans, organic ligands control the concentration of dissolved Fe in the water column. In soils, Fe and OC concentrations are typically correlated, suggesting that they are closely associated. Nevertheless, until now, the role of Fe in the preservation of sedimentary OC has not been clearly established. We determined that 20 to 40% of the total OC in marine and freshwater sediments is closely associated to solid reactive Fe phases (operationally defined as the solid iron phases that are reductively dissolved with sodium dithionite). The highest Fe-bound OC concentrations are found in surface sediments where authigenic Fe oxides accumulate. Even in mature sediments (>500 years), about 20% of the total OC is bound to Fe, suggesting a strong association that results in the protection and preservation of OC from bacterial degradation. Our measurements show that for young and mature sediments collected around the world, solid reactive Fe phases does not provide sufficient surface area for chemisorption of OC onto Fe oxides. Alternatively, we propose that high OC:Fe ratios reflect the existence of largely organic Fe-OC macromolecular structures (through chelation and co-precipitation), attached only minimally to the surface of clay mineral grains. The organic matter in these Fe-OM chelates is ‘glued’ together by iron ions or nanophases of iron oxide crystals. In most cases, we found isotopic and elemental fractionation between Fe‐associated OC and the rest of the sedimentary OC pool, with 13C and nitrogen-enriched OC preferentially bound to Fe. 13C-rich OC components such as proteins and carbohydrates have oxygen and nitrogen functionalities that favor the formation of inner-sphere complexes with Fe. The formation of these complexes may explain why these intrinsically labile OC compounds can be preserved on long time-scales in sediments. The sorptive stabilization mechanism, which hypothesizes that particle surfaces have a preservative effect on OC, would not accurately describe the mode of stabilization for all OC in sediments. Since reactive iron phases are metastable in sediments over geological timescales, OC bound to these phases may be preserved from degradation, contributing to the balance of the atmospheric oxygen and carbon dioxide.

Portable reverse osmosis (RO) systems are increasingly being used for isolating dissolved organic matter (DOM) from freshwater aquatic systems because of their high volume processing capacity and high absolute DOM recoveries. However, obtaining complete recoveries implies the rinsing of the reverse osmosis system with a solution of dilute NaOH and combining the rinse solution and the DOM concentrate. Because of the potential chemical alterations that can affect the integrity of the organic pool leached from the RO system at high pHs, this approach is not compatible with studies based on the molecular-level analysis of DOM. The potential for elemental, isotopic, and chemical fractionation was thus evaluated on a series of freshwater DOM samples concentrated in the field with a portable RO system when the concentrate and the rinse solution are not combined. DOC recoveries in the concentrate varied between 81.6 and 88.8%, and total balance calculations showed total recoveries of dissolved and particulate organic carbon ranging between 96.4 and 106.9%. Despite similar delta13C signatures, differences in N content and FTIR-based chemical composition between the concentrate and the rinse DOM solutions suggest some degree of chemical fractionation.