Iron hydroxides play a key role in the preservation of organic matter in soils and sediments, yet the specificity and extent of their interactions with lignin-derived phenols-important source indicators for terrestrial organic matter-remain poorly understood in aquatic systems. In this study, we analyzed surface and downcore sediments along the terrestrial-to-marine continuum of the St. Lawrence Estuary and Gulf, as well as from a boreal lake (Lake Brock), to better characterize iron-lignin associations. Using CuO oxidation (lignin) and buffered dithionite (iron), we quantified lignin oxidation products before and after the reductive dissolution of reactive iron with dithionite and evaluated association patterns through multivariate analyses. Our results show that lignin is consistently associated with reactive iron, with losses upon iron dissolution ranging from ∼20 % for the marine sites to over 40 % in terrestrial and freshwater sediments. Strikingly, 3,5-dihydroxybenzoic acid (3,5-Bd) showed the highest sensitivity to iron reduction, suggesting a unique and possibly distinct origin or sorptive behavior. Despite substantial lignin oxidation products losses, source-indicating S/V and C/V ratios remained relatively stable, and acid-to-aldehyde ratios-proxies for lignin degradation-were not significantly affected by iron binding. Principal component analysis confirmed that compositional shifts following iron hydroxides reduction are small and limited to one terrestrially influenced sample. These findings reveal that iron hydroxides not only shield lignin-rich organic matter from degradation but also act as selective shuttles for phenolic terrestrial compounds across dynamic redox boundaries in aquatic sediments.

This paper presents the Gulf of St. Lawrence and Estuary Dataset (GOSLED), a quality-controlled compilation of biogeochemical observations collected during 21 research cruises in the St. Lawrence Estuary, Gulf of St. Lawrence, and Saguenay Fjord between 2003 and 2023. This dataset integrates hydrographic measurements and a broad suite of discrete biogeochemical variables into a single, standardized compilation suitable for reuse, synthesis, and long-term analysis. GOSLED includes discrete measurements of dissolved oxygen, carbonate-system parameters, macronutrients, dissolved organic carbon, selected biogeochemical gases, stable isotope ratios of carbon and water, and transient and deliberate tracers. Data were compiled from multiple independent research cruises and laboratory archives (2003-2020), including contributions from the Marine Environmental Observation, Prediction and Response Network (MEOPAR) - R & eacute;seau Qu & eacute;bec maritime (RQM) Gulf of St. Lawrence Tracer Release Experiment (TReX; 2021-2023), RQM Odyss & eacute;e Saint Laurent program (2018-2023), and the Fisheries and Oceans Canada (DFO) Atlantic Zone Monitoring Program (AZMP; fall 2022). Sampling was conducted predominantly during the ice-free season, resulting in limited winter coverage across much of the system. All data were harmonized and processed following primary quality-control procedures adapted from GLODAP and CODAP-NA standards. Secondary crossover analysis was not possible due to a lack of deep-water (>1500 m) sampling. This paper documents the data provenance, quality-control procedures, known limitations, and recommended considerations for dataset usage. GOSLED is archived at the Canadian Integrated Ocean Observing System - St. Lawrence Global Observatory (CIOOS-SLGO) and is publicly accessible at 10.26071/d6f3fdfc-788d-48ff (Nesbitt et al., 2026).

Coastal systems have seen increasing pressure from anthropogenic activities since the beginning of the century, affecting the health of these ecosystems. This increased pressure takes the form of an increase discharge of terrestrial organic matter (OM) and nutrients, leading to a higher primary productivity in the surface waters and to higher concentrations of dissolved and particulate OM in the water column. The organic carbon cycle can thus be profoundly modified in these systems, making them more sensitive to further increases in anthropogenic forcings. To improve our understanding of the cycling of organic carbon and heterotrophic bacteria processing of terrestrial and marine OM, the biological lability of OM degraded by bacteria was assessed through the cultivation of bacteria naturally present in sediment in a 13C-enriched medium, and assessing the relationship between the delta 13C values of OM food source and bacterial fatty acids in the St. Lawrence Estuary and Gulf (SLEG). Based on the compound-specific stable isotope analyses (CSIA) of n-alkyl fatty acids (C10:0, C12:0, C14:0, C16:0, C22:0, C24:0, C26:0, C28:0, C30:0) and branched fatty acid biomarkers (iso and anteiso C15:0) at 10 stations along the terrestrial-marine continuum of the SLEG as well as in the cultured cells, the stable carbon isotope composition of the OM degraded by heterotrophs was calculated, suggesting a preferential consumption of the more labile fraction of terrestrial and marine OM sources. The relationship between the bacterial fatty acids and their food source allowed to better understand the dynamics of OM degradation and the factors that control organic carbon cycling in this coastal system.

Identifying organic matter (OM) distribution and origins in sediments is vital for distinguishing archival records, depositional dynamics, and extent of terrigenous inputs to marine systems. However, degradation and modification of OM before deposition can camouflage and mask its source signals in sedimentary sinks. Also, limited number of samples, matrix complexity, and multiple available tracers for single-proxy mixing models can all challenge source apportionment of sedimentary OM. To overcome these limitations and deconvolute the origins of deposited OM along a land-ocean transect, this study integrates machine learning and multivariate analyses of elemental contents (C and N), biomarker abundances (C17-C27 n-alkanes), and bulk and compound specific stable carbon isotope values (delta C-13) of sediments across the St. Lawrence Estuary and Gulf (SLEG). With pronounced provenance and terrestrial-marine regimes, the most proximal and distal sedimentary endmembers of the SLEG (60 and 780 km away from outlet) retain distinct geochemical information for training Principal Component Analysis and Partial Least Squares Regression model, predicting OM distribution across in-between stations. Results reveal a gradual shift in sedimented OM composition along the continuum due to OM inputs and source-to-sink processes; with a gradient of 2% decrease in terrestrial character of sedimentary OM per each 10 km distance from river outlet, findings reflect that the inputs and signals of terrigenous OM are progressively diluted and transformed offshore. Also, isotopic and molecular signatures exert stronger predicting controls than conventional tracers such as C/N ratios, highlighting that the diagnostic strength of proxies is system-specific, particularly under dynamic conditions. Integrative multivariate approaches can hence capture the complexity and covariances of multiple biogeochemical proxies to better decode camouflaged sedimentary archives and provide more holistic estimations of OM transportation and transformation across systems with strong geo-spatial and input gradients.

Iron(III) (oxyhydr)oxides are the major sink for PO43- in freshwater sediments, helping to control the onset of eutrophic conditions when PO43- is in excess; however, this sink is transient as ferrous iron and PO43- are typically released when sediments become anoxic, triggering a self-sustaining reversible cycle as sediments become an internal source of phosphate. Ferrous iron can then react with dissolved sulfides to form iron sulfide minerals such as mackinawite (FeS), whose interactions with PO43- remain largely unexplored. We investigate the role of FeS in the sequestration of PO43- under anoxic conditions, with or without natural organic carbon (OC). We show that Fes has a high affinity for PO43-, with a maximum sorption capacity comparable to moderately crystalline FeOx minerals. The presence of sorbed OC decreases the FeS affinity toward the PO43- by ~ 75%, which is attributed to the lower surface area of FeS-OC compared to FeS, electrostatic repulsion, and the reduced accessibility to reactive Fe(II) sites. Moreover, we show that upon exposure of FeS to oxic conditions, PO43- is efficiently recaptured by the FeS oxidation products (FeOx), hence highlighting a parallel self-sustaining reversible cycle resulting in the permanent sequestration of a small fraction of PO43-. Overall, these findings highlight FeS as a potentially important phosphate sink in anoxic freshwater sediments.

Abstract Organic carbon (OC) burial in sediments is partly controlled by catabolic biodegradation and remineralization by heterotrophic bacteria, but long‐term records of these bacterially‐mediated processes have not been available. Here, we use stable hydrogen isotope ratio of the saturated C16 fatty acid (δ2HC16:0) as a proxy of the prevalence of heterotrophic bacterial activity to reconstruct the history of Holocene OC biodegradation and remineralization from a sediment core recovered on the southwest Greenland Shelf. A positive shift in the δ2HC16:0 during the Holocene Thermal Maximum indicates increased inputs of heterotrophic bacterial biomass to sediments. This implies efficient biodegradation and remineralization of OC during this warm period that are linked to low sediment accumulation rates (SARs) and elevated primary productivity. We propose that enhanced bacterial catabolism of OC in the absence of increased SARs will reduce OC burial efficiency in warmer, high‐latitude oceans, thereby providing positive feedback to climate warming.

There is a great divide between the microbes active in natural environments and the organisms that may be grown in a laboratory setting. In this work we set out to cultivate representatives of the marine myxobacterial clade, a highly diverse, largely uncultured group of Gram-negative bacteria believed to have extensive biosynthetic potential. Sediment samples were collected from the St. Lawrence Estuary and Gulf and the presence of active marine myxobacteria was established through qPCR analysis of 16S rRNA gene and transcript abundances. In the expectation that the marine myxobacteria would exhibit predatory behaviour like their terrestrial counterparts, the sediment samples were then streaked on agar plates that contained common marine bacteria as the sole carbon source. Unexpectedly, in place of myxobacteria we isolated Pseudomonas, Bacillus, and Stenotropomonas spp., among others, revealing a generalized ability for these strains to break down living organic matter and suggesting that "bait" bacteria may be an effective approach for the cultivation of novel marine saprophytes.

Geochemical data compiled from dried sediments from three water reservoirs at the ancient Maya city of Ucanal, Pet & eacute;n, Guatemala, reveal low to undetectable fecal biomarker concentrations. These low concentrations may be the result of the aerobic decay of sterols combined with well-managed waste disposal practices. Despite generally low concentrations of fecal biomarkers, evidence was detected for increased fecal residues during the Late Classic and Terminal Classic periods, corresponding with population increases at the site. Together, the evidence suggests that Maya inhabitants of Ucanal created an urban environment that prevented, to some degree, fecal waste pollution of their water resources.

While iron oxides have been thoroughly explored in terms of their ability to sorb and sequester organic carbon (OC) in sediments, the role of iron sulfide (Fe-S) minerals in the long-term sequestration of OC remains poorly defined. In this study, we assessed the affinity of different types of natural organic matter (NOM) towards synthetic Fe-S minerals using sorption isotherms. We found affinities and sorption capacities varying in the following order: plankton NOM > corn leaves NOM > aged terrestrial NOM. Scanning electron microscopy showed that NOM increases the size and surface area of Fe-S aggregates, likely also influencing their surface reactivity. High NOM contents in Fe-S minerals protected Fe(II) from oxidation after exposure to atmospheric oxygen. Analysis of the synthetically prepared Fe-S-NOM complexes by synchrotron scanning transmission X-ray microscopy (STXM) coupled to near-edge X-ray absorption fine-edge structure (NEXAFS) spectroscopy revealed strong interactions between Fe-S minerals and NOM extracted from plankton, specifically with amide and carboxylic functional groups. We also attempted to identify and characterize interactions between OC and Fe-S minerals in natural sulfidic sediments from the St. Lawrence Estuary and the Saguenay Fjord, hence linking our work on synthetic iron sulfides to what is occurring in natural environments, although this effort proved more challenging owing to the presence of Fe(III) minerals even at depth and the difficulty in distinguishing FeS from other Fe(II) minerals. We present depth concentration profiles of dissolved OC, iron, and sulfur in the liquidphase (pore water) along with speciation data from sequential extractions of sulfur in the solid-phase collected from sediment cores. We found a clear association between mixed Fe(II)/Fe(III) minerals and OC in sediments, which, combined with the results of the synthetic FeS experiment, suggests that Fe-S minerals can promote OC sequestration in sediments.

Estuarine and fjord systems host large amounts of buried organic carbon with highly heterogeneous sources in their sediments. The age of this buried carbon is important because it determines to what extent it represents a short-term atmospheric carbon sink on decadal to centennial timescales. Here, we utilized molecular (fatty acids and n-alkanes) and bulk radiocarbon (14C), stable isotope, and elemental analyses and a mixing model to apportion the source of organic carbon buried in the Lower St. Lawrence Estuary, the world's largest estuary, and the linked Saguenay Fjord, differentiating between modern, millennial aged, and fossil carbon sources. The 14C ages of long-chain (C 24 +26 ) fatty acids indicate an average terrestrial storage time of-1700 +/- 284 yr (before present) for soil organic carbon prior to re-deposition in these sediments. A three-tracer source model for bulk organic carbon indicates that 64 +/- 0.8 % of organic carbon buried in the Saguenay Fjord and Lower St. Lawrence Estuary was modern marine- and terrestrially-derived carbon, representing direct atmospheric carbon sinks, while 36 +/- 4 % was pre-aged soil and fossil petrogenic organic carbon. Comparison with a similar dataset from the subtropical Pearl River Estuary in China indicates that burial of soil and petrogenic organic carbon is significantly lower in the Lower St. Lawrence Estuary on both a fractional and flux basis, probably as a result of greater topographic relief and human land use in the Pearl River catchment, which generates greater erosional inputs of soil and petrogenic carbon.

Rare earth elements (REEs) including Yttrium (Y) are commonly used as tracers of estuarine and oceanic mixing. The lanthanide series and yttrium are usually referred to as REYs. The geochemical behavior of REYs in estuarine environments is generally described as being non-conservative, with large-scale removal by particle scavenging. During mixing, partitioning of these elements occurs according to their source function and the stability of natural complexes, with heavy REEs typically forming more stable complexes than light REEs in solution. In this study, we compare the concentrations and partitioning of the 0.7 11m-filtered and 0.05 11m-filtered fractions of the dissolved REYs collected during the summers of 2017 and 2021 in the surface waters (< 3 m) of the St. Lawrence estuarine system (river, estuary and gulf) with those of the Saguenay Fjord, a tributary of the latter that drains the Mesoproterozoic rocks of the Canadian Shield. Whereas REYs do not mix conservatively in the St. Lawrence Estuary (SLE) in the summer, they nearly do so in the Saguenay Fjord (SF). REY concentrations are 2.5 to 6 times greater in the surface waters of the SF than those of the SLE at the same salinity and, in contrast to most estuaries including the SLE, the fjord waters are enriched in LREEs. The 0.05 11m-filtered REY concentrations are positively correlated with dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) concentrations in the SF but independent of both DOC and CDOM concentrations in the SLE. The CDOM in the fjord differs from that of the estuary as it is more aromatic and has a higher molecular weight. The formation of strong REE-humate complexes stabilizes REY ions in the SF surface waters and impedes their adsorption to and scavenging by solid surfaces during estuarine mixing. The LREE enrichment in the SF surface waters most likely reflects the geology of the fjord's drainage basin, more specifically the exposed Mesoproterozoic granites and gneisses of the Canadian Shield that are enriched in LREE relative to the younger Paleozoic sedimentary rocks exposed along the St. Lawrence Lowlands.

Abstract Marine sediments in glacially-carved fjords at high latitudes feature high organic carbon (OC) burial rates, but there are fewer data on the role of glacial activity on high-latitude OC burial rates outside of fjords. Here, we investigate the relationship between sediment OC burial rates in the deep troughs and basins of the southwest Greenland shelf and Holocene glacial dynamics. Since the onset of prominent Neoglacial advances ~2500 years ago, the nature of the OC buried in the deep troughs and basins of the shelf was influenced by the glacier-driven increase in sediment accumulation rates (SAR), reactive iron (oxyhydr)oxide concentrations and fine-grain sediment, while OC burial rates were primarily enhanced by increasing SAR. Peak OC burial rates (~18.5 ± 5.7 g m−2 a−1) in the deep troughs and basins of the shelf during the past ~1300 years are comparable to those of many high-latitude fjords, and the inferred total annual OC burial in these trough and basin areas is equivalent to ~5% of the annual CO2 uptake by the Labrador Sea deep convection.

The priming effect (PE) refers to the enhanced remineralization of recalcitrant organic carbon (OC) driven by the respiration of labile OC, potentially increasing CO2 fluxes from aquatic ecosystems. Patterns of PE induced by marine and terrestrial OC inputs can be explored through sedimentary contributions to the degraded OC pool. In this study, coastal sediments (δ13Cbulk = -25.26 ± 0.06 ‰, 1.63 ± 0.07 % OC) were spiked with isotopically distinct marine and terrestrial OC sources (Nannochloropis phytoplankton, δ13C = -43.18 ± 0.31 ‰; and C4 corn leaves, δ13C = -13.90 ± 0.09 ‰). Source contributions to respired OC were investigated using n-alkane concentration profiles and stable carbon isotopes (C15-C30) across 30 microcosms. Elevated concentrations of 13C-enriched high molecular weight n-alkanes (e.g., δ13CC29 = -26.3 ± 0.5 ‰) were observed in corn leaf amendments, whereas the phytoplankton spike exhibited a higher abundance of 13C-depleted low molecular weight n-alkanes (e.g., δ13CC17 = -46.8 ± 0.4 ‰). Mixing models indicate the sedimentary OC contribution to the degraded biomarkers, for which an increasing trend suggests a PE. Phytoplankton-amended microcosms showed a sediment OC contribution of 10.3 ± 1.5 % to the degradation of the C17 n-alkane. The corn leaf spike resulted in consistently higher contributions of 30.4 ± 3.6 % for the lost C29 n-alkane, documenting the effect of carbohydrate rich organic matter on sedimentary OC remineralization. A synergistic interaction emerged when sediments received a mix of marine and terrestrial OC, exhibiting contributions to n-alkane loss of 48.3 ± 5.3 % for C17, and 35.2 ± 5.2 % for C29. Following biochemical fractionation that leads to the selective breakdown of certain biochemical structures, our data indicate greater sedimentary degradation during induced terrestrial runoff compared to an algal bloom, providing a quantitative measure of OC remineralization.

Coastal sediments are the main deposition center for allochthonous and autochthonous organic carbon (OC). The discharge of terrestrial biomass, anthropogenic activities, oceanic primary productivity, and natural events contribute to this carbon pool. The OC buried in sediments undergoes alteration through physical, biological and chemical processes, becoming progressively refractory and more likely to be preserved on geological time scales. However, little is known about the rate of bulk OC alteration post weathering and bloom. We incubated coastal sediment slurries with isotopically distinct spikes of C4 corn leaves and cultured phytoplankton, individually and in 1:1 mixture. OC isotopic values and concentrations were probed at different time points to track degradation and incorporation in solid and liquid phases. Both amendments were composed of fresh OC with a high proportion of labile biochemicals (e.g. polysaccharides and proteins). Despite the small differences in their lability, corn leaves were incorporated into the sediments at a slower rate compared to phytoplankton. Following combined spiking of the terrestrial and marine amendments, no sign of synergistic effects was observed in system's response. Despite sediment sensitivity to OC input and the rapid alterations in its properties within the initial days of incubation, swiftly transitioning to a state of minimal change is indicative of a relatively stable system that retained the isotopic imprint of the OC spike for a long time (> 32 days). This isotopic remanence is likely due to heterotrophic bacteria that degrade OC to synthesize their biomass (food stock for successive generations) and incorporate its stable isotope characteristics. Hence, our work sheds light on the kinetics of biogeochemical changes, and recovery time of the system for returning to its pre-perturbation state.

The global carbon and iron cycles are intimately linked as redox-sensitive iron oxides readily bind organic carbon in a variety of environmental settings, including marine and lacustrine sediments. While these iron-organic carbon complexes sequester vast quantities of organic carbon, the composition of the organic matter within them remains unknown for lacustrine environments. Here we present C K1s and Fe L3,2 edge Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of surface sediments and authigenic iron complexes from adjacent basins of a pristine boreal lake located in Quebec, Canada, with contrasting oxygen exposure regimes. We demonstrate differences in organic carbon speciation in sediments from both basins, as well as co-localization of organic carbon and iron on a sub-micron scale in 100 nm thick samples. Differences in redox cycling across these two basins allow for a direct comparison of the effect of oscillating redox conditions on the composition of organic carbon sequestered by iron. Our results suggest that reactive organic molecules, which may be polysaccharides, were found preferentially associated with iron in the perennially oxic sediments compared to more phenol rich organics in the seasonally anoxic sediments, highlighting the importance of iron oxides in the protection and preservation of labile organic compounds. Traces of aliphatic carbon were observed in sediments from the anoxic basin, alongside carboxyl and aromatic functionalities. This carboxyl-rich aliphatic material could possibly interact with the sediment mineral matrix either through a ligand exchange mechanism between the mineral phases and the carboxyl functionalities, or via non-specific hydrophobic interactions involving the aliphatic moieties. Finally, our work also shows that OC:Fe ratios should be used with caution when inferring a binding mechanism between OC and iron oxides.

The biogeochemical cycles of iron and organic carbon (OC) are closely interconnected in terrestrial and aquatic systems. In ocean waters, the concentration of reactive Fe is tightly controlled by soluble organic ligands. In soils, Fe stabilizes OC by forming aggregates that shield OC from degradation. In lake sediments however, the role of Fe in the preservation of OC has not been explored as extensively yet. We investigated Fe-OC interactions in sediment collected from Lake Tantare, in which two basins are characterized by contrasting redox conditions. These contrasting redox conditions provide an opportunity to assess their importance in the formation of stable Fe-OC complexes. On average, 30.1 +/- 6.4 % of total OC was liberated upon reductively dissolving reactive iron. The Fe-associated and the non-Fe-associated OC pools were characterized at the elemental (OC, TN), isotopic (delta 13C, delta 15N) and functional group (FTIR) levels. Large differences in OC:Fe and TN:Fe ratios between the two basins were found which were not linked to OM chemical composition but rather to differences in reactive iron concentrations stemming from the higher abundance of iron sulfides in the anoxic basin. Nevertheless, since the affinity of OM for iron sulfides is lower than that for iron hydroxides, using OC:Fe and TN:Fe ratios as a diag- nostic tool for the type of OM -Fe interactions should be done with care in anoxic environment. Same caution should be considered for oxic sediments due to the variation of the proportion of iron hydroxides associated with OM from sample to sample.

Many processes can contribute to the attenuation of the frequently detected and toxic herbicides atrazine and metolachlor in surface water, including photodegradation. Multi-element compound-specific isotope analysis has the potential to decipher between these different degradation pathways as Cl is a promising tool for both pathway identification and a sensitive indicator of degradation for both atrazine and metolachlor. In this study, photodegradation experiments of atrazine and metolachlor were conducted under simulated sunlight in buffered solutions (direct photodegradation) and with nitrate (indirect photodegradation by OH radicals) to determine kinetics, transformation products and isotope fractionation for C, N and for the first time Cl. For metolachlor, the C–Cl dual isotope slope (ΛC/Cl = 0.46 ± 0.19) is identical to previously reported values for hydrolysis and biodegradation in soils, suggesting the same reaction mechanism (C–Cl bond breakage by SN2 nucleophilic substitution). For atrazine, both direct and indirect photodegradation resulted in a pronounced inverse isotope effect for chlorine (εCl = 6.9 ± 3.3 ‰, and εCl = 2.3 ± 1.2 ‰, respectively), leading to characteristic dual isotope slopes (ΛC/Cl = −0.49 ± 0.17 and ΛC/Cl = −0.31 ± 0.10, respectively). These values are distinct from those previously reported for abiotic hydrolysis, biotic hydrolysis and oxidative dealkylation which are all relevant processes in surface water, opening the path for pathway identification in future field studies.

Abstract Palmitic acid (PA) is ubiquitous in the biosphere and its hydrogen isotopic composition (δ2HPA) was proposed as a potential paleoenvironmental proxy for salinity, with δ2HPA values increasing with salinity. In this study, we analyzed 40 surface sediment samples from Baffin Bay and the Labrador Sea to examine the isotopic composition of PA in relation to local environmental variables, including salinity. In contrast to expectations, our results show a negative relationship between the δ2HPA and sea‐surface salinity, raising questions about its pertinence and usefulness as a salinity proxy. Instead, our results suggest that the relative abundance of distinct organisms that employ different metabolisms is the key in determining the hydrogen isotopic fractionations in PA. While we show that PA is mostly produced through photoautotrophic metabolisms by diatoms and dinoflagellates, varying contributions from heterotrophic metabolisms may obscure the stable isotope composition of PA. Surprisingly, we found no correlation between the stable carbon isotopic composition of the sedimentary organic matter (δ13Corg) and PA (δ13CPA), implying major differences in either the dominant organisms producing sedimentary PA or in carbon isotope fractionation during lipid biosynthesis. We also found that the presence of extended sea‐ice cover leads to enriched carbon and hydrogen isotopic compositions in PA. These enriched values suggest heterotrophic biodegradation in the water column and/or in the sediment as well as an increase in grazing activities. We propose that sea‐ice cover and surface water oxygenation modulate the relative impact of phototrophic and heterotrophic metabolisms, and therefore the isotopic composition of marine sedimentary PA.