Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) has been increasingly employed to characterize dissolved organic matter (DOM) across a range of aquatic environments ...highlighting the role of DOM in global carbon cycling. DOM analysis commonly utilizes electrospray ionization (ESI), while some have implemented other techniques, including dopant-assisted atmospheric pressure photoionization (APPI). We compared various extracted DOM compositions analyzed by negative ESI and positive APPI doped with both toluene and tetrahydrofuran (THF), including a fragmentation study of THF-doped riverine DOM using infrared multiple photon dissociation (IRMPD). DOM compositions followed the same trends in ESI and dopant-assisted APPI with the latter presenting saturated, less oxygenated, and more N-containing compounds than ESI. Between the APPI dopants, THF-doping yielded spectra with more aliphatic-like and N-containing compounds than toluene-doping. We further demonstrate how fragmentation of THF-doped DOM in APPI resolved subtle differences between riverine DOM that was absent from ESI. In both ionization methods, we describe a linear relationship between atomic and formulaic N-compositions from a range of DOM extracts. This study highlights that THF-doped APPI is useful for uncovering low-intensity aliphatic and peptide-like components in autochthonous DOM, which could aid environmental assessments of DOM across biolability gradients.
The Congo and Amazon are the two largest rivers on Earth and serve as major sources of dissolved organic carbon to the ocean. We compared the dissolved organic matter (DOM) composition of both rivers ...using Fourier‐transform ion cyclotron resonance mass spectrometry to investigate seasonal and regional differences in DOM composition exported to the ocean. We found that over a 15‐month observational period in the Congo River, molecular aromaticity and oxygenation between the wet and dry periods varied slightly, but most of the relative abundance of DOM formulae (∼90%) were present in all samples, suggesting that Congo River DOM quality is stable across different hydrological conditions. In contrast, the multi‐year DOM composition in the Amazon River was highly susceptible to changes in hydrology, with clear differences in molecular aromaticity, oxygenation, and heteroatom (N, S, P) content between the wet and dry seasons. Overall, the DOM composition of the Congo River was more terrestrial than Amazon River DOM, which was more characteristic of aquatic DOM. Finally, we compared the relative contribution of island of stability (IOS) formulae between the rivers and found that both rivers export similar amounts of these formulae annually, more than several major rivers combined, and that the Congo is more than twice as efficient in exporting these IOS formulae. With changing precipitation and land use, the quantity and composition of exported DOM will likely reflect the mobilization of additional terrestrial and anthropogenic sources that will also be subjected to downstream land‐to‐ocean cycling.
Plain Language Summary
Almost 16% of the global riverine dissolved organic carbon (DOC) export is delivered by just two rivers: Amazon and Congo. Although both rivers drain immense tropical rainforests, differences in landscape, precipitation, and riverine discharge all contribute to the distinct molecular composition of their dissolved organic matter (DOM). We compared DOM composition of both rivers over several years and found that hydrology is an important driver for determining the DOM composition of both rivers; however, it is more important for the Amazon than Congo, which had DOM that was more similar across the wet and dry seasons. Overall, both rivers had more aromatic and oxygen‐rich DOM than temperate or arctic rivers, but DOM from the Congo had a more terrestrial signature than the Amazon River DOM. Finally, we found that DOM molecular formulae thought to be stable over long periods of time in marine systems were exported in similar amounts by both rivers, even though the Amazon exports about twice as much DOC annually compared to the Congo, suggesting that the Congo is more than twice as efficient per unit volume at exporting molecular formulae that is thought to be stable in the ocean for long periods of time.
Key Points
Congo River dissolved organic matter (DOM) composition remains mostly stable across the year, while Amazon River DOM varies seasonally with discharge
Amazon River DOM is less aromatic, less oxygenated, and more heteroatom‐rich than Congo River DOM
Both rivers export similar amounts of island of stability molecular formulae but the Congo is over twice as efficient
Abstract
Northern high-latitude lakes are critical sites for carbon processing and serve as potential conduits for the emission of permafrost-derived carbon and greenhouse gases. However, the fate ...and emission pathways of permafrost carbon in these systems remain uncertain. Here, we used the natural abundance of radiocarbon to identify and trace the predominant sources of methane, carbon dioxide, dissolved inorganic and organic carbon in nine lakes within the Yukon Flats National Wildlife Refuge in interior Alaska, a discontinuous permafrost region with high landscape heterogeneity and susceptibility to climate, permafrost, and hydrological changes. We find that although Yukon Flats lakes primarily process young carbon (modern to 1290 ± 60 years before present), permafrost-derived carbon is present in some of the sampled lakes and contributes, at most, 30 ± 10% of the dissolved carbon in lake surface waters. Apportionment of young carbon and legacy carbon (carbon with radiocarbon age ⩾5000 years before present) is decoupled among the dissolved inorganic and organic carbon species, with methane showing a stronger legacy signature. Our observations suggest that permafrost-thaw-related transport of carbon through Yukon Flats lacustrine ecosystems and into the atmosphere is small, and likely regulated by surficial sediments, permafrost distribution, wildfire occurrence, or masked by contemporary carbon processes. The heterogeneity of lakes across our study area and northern landscapes more broadly cautions against using any one region (e.g. Yedoma permafrost lakes) to upscale their contribution across the pan-Arctic.
As climate‐driven El Niño Southern Oscillation (ENSO) events are projected to increase in frequency and severity, much attention has focused on impacts regarding ecosystem productivity and carbon ...balance in Amazonian rainforests, with comparatively little attention given to carbon dynamics in fluvial ecosystems. In this study, we compared the wet 2012 La Niña period to the following normal hydrologic period in the Amazon River. Elevated water flux during the La Niña period was accompanied by dilution of inorganic ion concentrations. Furthermore, the La Niña period exported 2.77 Tg C yr−1 more dissolved organic carbon (DOC) than the normal period, an increase greater than the annual amount of DOC exported by the Mississippi River. Using ultra‐high‐resolution mass spectrometry, we detected both intra‐ and interannual differences in dissolved organic matter (DOM) composition, revealing that DOM exported during the dry season and the normal period was more aliphatic, whereas compounds in the wet season and following the La Niña event were more aromatic, with ramifications for its environmental role. Furthermore, as this study has the highest temporal resolution DOM compositional data for the Amazon River to‐date we showed that compounds were highly correlated to a 6‐month lag in Pacific temperature and pressure anomalies, suggesting that ENSO events could impact DOM composition exported to the Atlantic Ocean. Therefore, as ENSO events increase in frequency and severity into the future it seems likely that there will be downstream consequences for the fate of Amazon Basin‐derived DOM concurrent with lag periods as described here.
Plain Language Summary
Increases in atmospheric carbon concentrations originate from many sources and pose a serious threat to global ecosystem health and humanity. The Amazon River delivers one‐fifth of global discharge and represents the largest single flux of dissolved organic carbon (DOC) from land to ocean. As climate change is projected to increase precipitation anomalies throughout the Amazon, flooding and droughts will become more frequent and severe, disrupting the natural seasonal rhythm of the Amazon River. We demonstrate that precipitation anomalies in South America (caused by La Niña) exported an additional amount of DOC from the Amazon River to the Atlantic Ocean than the Mississippi River exports annually. Organic compounds mobilized during the La Niña were more aromatic, presumably from terrestrial sources. These compositions, measured near the mouth of the Amazon River, arrived six months after Pacific sea‐surface temperature and pressure anomalies indicated the onset of La Niña, highlighting the lag time that events in the Pacific take to impact the Atlantic Ocean.
Key Points
The Amazon River during a La Niña year exports an additional amount of dissolved organic carbon than the Mississippi exports annually
The Amazon River dissolved organic matter composition in a La Niña year was more oxidized and aromatic than during a non‐ENSO year
The dissolved organic matter composition in the Amazon River correlates to a 6‐month lag with La Niña indices
Redox-active functional groups in dissolved organic matter (DOM) are crucial for microbial electron transfer and methane emissions. However, the extent of aquatic DOM redox properties across northern ...high-latitude lakes and their relationships with DOM composition have not been thoroughly described. We quantified electron donating capacity (EDC) and electron accepting capacity (EAC) in lake DOM from Canada to Alaska and assessed their relationships with parameters from absorbance, fluorescence, and ultrahigh resolution mass spectrometry (FT-ICR MS) analyses. EDC and EAC are strongly tied to aromaticity and negatively related to aliphaticity and protein-like content. Redox-active formulae spanned a range of aromaticity, including highly unsaturated phenolic formulae, and correlated negatively with many aliphatic N and S-containing formulae. This distribution illustrates the compositional diversity of redox-sensitive functional groups and their sensitivity to ecosystem properties such as local hydrology and residence time. Finally, we developed a reducing index (RI) to predict EDC in aquatic DOM from FT-ICR MS spectra and assessed its robustness using riverine DOM. As the hydrology of the northern high-latitudes continues to change, we expect differences in the quantity and partitioning of EDC and EAC within these lakes, which have implications for local water quality and methane emissions.
•Extraction efficiency increased with aromaticity across eight different sorbents.•Silica sorbents extracted more N-containing formulae than SDVB sorbents.•The C18 sorbent was comparable to PPL in ...terms of extracted DOM composition.•Sorbents shared > 90 % of their molecular relative abundance with PPL.•H/C increases and O/C decreases as extraction pH is adjusted from 2 to 10.
Solid phase extraction (SPE) of a variety of diverse dissolved organic matter (DOM) endmembers through eight commercially available sorbents was examined (ENV, PLEXA, PPL, HLB, Isolute 101, C18/ENV+, C18, EnvirElut) representing styrene divinylbenzene polymer (SDVB) and silica-based sorbents. We assessed dissolved organic carbon (DOC) recovery and DOM composition via 21T Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). DOC recoveries and SPE-DOM composition differed more by endmember type than by sorbent. Silica-based sorbents retained DOM with many N-containing formulae, while SDVB-based sorbents retained DOM with more S-containing formulae. Extraction pH exerted a greater influence on DOM composition, notably through the presence of strong groupings composed of saturated and lowly oxygenated formulae at basic pH, and of aromatic and highly oxygenated formulae at pH 2, irrespective of endmember or sorbent. There was above 25% DOC recovery, regardless of sorbent or endmember; >90% of the relative abundance (RA) of molecular formulae were shared with PPL, which is currently the most commonly utilized sorbent for DOM. This clearly highlights the ability of the selected sorbents to retain representative DOM across diverse endmembers. Such findings may be useful for future targeted DOM studies (e.g., bioincubations, wastewater and drinking water applications) interested in focusing on specific compositional changes and will provide a better understanding of how organic carbon cycling is impacted by anthropogenic processes.
Arctic lakes store, modify, and transport large quantities of carbon from terrestrial environments to the atmosphere; however, the spatial and temporal relationships between quantity and composition ...of dissolved organic matter (DOM) have not been well characterized across broad arctic regions. Moreover, most arctic lake DOM compositions have been examined during the ice‐free summer, whereas DOM cycling between the ice‐covered winter months and summer have not been addressed. To resolve these spatial and seasonal uncertainties in DOM cycling, we sampled a series of arctic lakes from the North Slope of Alaska across a latitudinal gradient in the winter and summer over 3 years. Samples were analyzed for dissolved organic carbon concentration and DOM composition was characterized using optical and fluorescence properties combined with molecular‐level analysis using Fourier transform‐ion cyclotron resonance mass spectrometry. Tundra lake DOM properties including aromaticity and molecular stoichiometries were similar to other northern high‐latitude lakes, but optical parameters related to aromaticity and molecular weight were greater in major arctic rivers and in coastal lakes in the North Slope region. DOM composition was highly seasonal, with ice exclusion concentrating microbially processed DOM in the winter water columns, potentially influencing DOM cycling the following summer. However, the greatest variations in DOM composition were related to lake depth and likely other physical features including morphology and bathymetry. As the Arctic warms, we expect changes in hydrology and ice cover to enhance under‐ice microbial DOM processing, early summer photodegradation, and ultimately carbon fluxes to the atmosphere after ice‐out.
Plain Language Summary
Arctic lakes contain large amounts of both inorganic and dissolved organic carbon (DOC) that is stored, processed, and transferred to the atmosphere. This cycling has been enhanced by recent arctic warming via anthropogenic climate change, making the fate and distribution of these different pools uncertain on both seasonal and spatial scales. We demonstrate that the amount of DOC and the composition of dissolved organic matter (DOM) vary significantly across tundra lakes between summer and winter months when the lakes are entirely covered by ice. Furthermore, these seasonal trends are consistent across the North Slope of Alaska with lakes closer to the coast of the Arctic Ocean differing in bulk optical and molecular‐level properties, suggesting DOM is supplied by different sources and influenced by maritime effects. Finally, while seasonality was an important factor in explaining DOM compositions across the tundra, the greatest differences between lakes were attributed to other factors, likely related to the variations in lake geomorphology including lake depth and area across the region. As temperatures and precipitation change across the Arctic are expected, lake morphology will respond to a thawing landscape and influence the composition of DOM as well as how it is cycled from lakes to the atmosphere.
Key Points
Dissolved organic carbon concentration and dissolved organic matter (DOM) composition differ seasonally due to ice exclusion and microbial processing in the winter months
Lake geomorphology is likely a major control for the overall lake DOM composition across the arctic tundra
Tundra lake DOM composition differs in optical properties and heteroatomic content between coastal and inland lakes
Determining the concentration and isotopic composition of elemental sulfur in modern and ancient environments is essential to improved interpretation of the mechanisms and pathways of sulfur ...utilization in biogeochemical cycles. Elemental sulfur can be extracted from sediment or water samples and quantified by converting to hydrogen sulfide. Alternatively, elemental sulfur concentrations can themselves be analyzed using HPLC and other methodologies; however, the preparation and analysis times can be long and these methods are not amenable to stable isotopic analysis. Current reduction methods involve the use of costly and specialized glassware in addition to toxins such as chromium chloride or cyanide to reduce the sulfur to hydrogen sulfide. The novel reduction method presented here uses dithiothreitol (DTT) as a less toxic reducing agent to obtain both elemental sulfur concentrations and isotopic composition from the same sample. The sample is dissolved in an aqueous or organic liquid medium and upon reaction with DTT, the elemental sulfur is volatilized as hydrogen sulfide and collected in a sulfide trap using an inexpensive gas extraction apparatus. The evolved sulfide concentrations can easily be measured for concentration, by absorbance spectrophotometery or voltammetry techniques, and then analyzed for sulfur isotopic composition. The procedure is quantitative at >93% recovery to dissolved elemental sulfur with no observed sulfur isotope fractionation during reduction and recovery. Controlled experiments also demonstrate that DTT is not reactive to sulfate, sulfite, pyrite, or organic sulfur.
Arctic-boreal wetlands, important ecosystems for biodiversity and ecological services, are experiencing hydrological changes including permafrost thaw, earlier snowmelt, and increased wildfire ...susceptibility. These changes are affecting wetland productivity, species diversity, and biogeochemical cycles. However, given the diverse forms and structures of wetland vegetation communities, traditional wetland maps generated from lower spatial and spectral resolution satellite imagery lack community-level vegetation classification and miss spatially complex patterns. In this study, we built a cloud-based workflow to map wetland vegetation community of the Peace-Athabasca Delta (PAD), Canada, by leveraging high-resolution (5-m) airborne multi-sensor datasets, namely NASA's Airborne Visible/Infrared Imaging Spectrometer-Next Generation (AVIRIS-NG) and Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR), and a historical LiDAR archive. Validation of our classifications using ground references indicates that classifications derived from AVIRIS-NG have higher accuracies (≥87.9%) than either UAVSAR (65.6%) or LiDAR (75.9%) for mapping wetland vegetation communities. We also show improved classification accuracy when combining information from multiple sensors. In particular, incorporating AVIRIS-NG and UAVSAR datasets substantially reduced omission errors of wet graminoid and wet shrub classes from 29.6% to 20.5% and from 10.8% to 7.5%, respectively. Combining AVIRIS-NG and LiDAR datasets further improves overall accuracy (+2.2%) for most classifications, especially emergent vegetation, wet graminoid, and wet shrub. The best performing model, using features derived from all three sensors, achieved an overall accuracy of 93.5%. The framework established here can be used to leverage extensive airborne AVIRIS-NG and UAVSAR datasets collected across Alaska and northwest Canada to understand the spatial distribution of Arctic-Boreal wetland vegetation communities.
•Co-located airborne three-sensor data is used to map wetland vegetation communities.•Inclusion at least 2 of 3 multi-sensor data increases accuracy to any one of them.•Feature importance of three-sensor model is evaluated.•Comparison to existing maps shows potential to support ecological applications.•Cloud-based workflow enables efficient processing of airborne hyperspatial imagery.
ABSTRACT
The Ruki is a pristine blackwater tributary in the Congo Basin draining tropical lowland forest. Daily discharge and fortnightly concentrations, isotopic ratios, and molecular composition of ...carbon and organic matter were measured for 1 yr (2019–2020). Like the Congo River, discharge peaked from November–January, with a smaller secondary peak in June. Dissolved organic carbon (DOC), inorganic carbon (DIC), carbon dioxide (pCO2), and methane (pCH4) concentrations were high (21.3 ± 4.8 mg C L−1, 5.8 ± 0.9 mg C L−1, 6370 ± 1740 ppm, and 250 ± 100 ppm, respectively) and positively correlated with discharge, indicating transport limitation. Total suspended solids and particulate organic carbon (POC) concentrations were generally low (3.68 ± 1.61 mg L−1, 0.88 ± 0.33 mg C L−1, respectively) and varied inversely with discharge, indicating source limitation. The Ruki exported a total of 3.25 Tg C yr−1, of which DOC, DIC, and POC comprised 76%, 20%, and 3%, respectively. This DOC flux represents ~ 20% of the annual Congo Basin flux from about 5% of its area, highlighting the high yield. Isotopic ratios of DOC and POC indicate modern C3 forest vegetation as a source, except for a few older samples potentially indicating peat inputs. The bulk molecular composition of dissolved organic matter was seasonally consistent; however, a more oxidized and aromatic assemblage occurred at high discharge, corresponding with forest vegetation, while a more aliphatic, nitrogen‐, and sulfur‐enriched assemblage was found during low discharge, corresponding with soil‐derived organic matter. Overall, these results underscore how hydrology controls C concentrations in the Ruki River and how this blackwater river contributes disproportionately to C export per unit area within the Congo Basin hydrosystem.
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