Search

The Arctic rapidly warms and sea ice retreats, a large fraction of organic carbon (OC), currently stored in coastal permafrost will be released into the marine system. Once reintroduced into the active carbon cycle, this material will either be decomposed or buried on the shelf depending on its hydrodynamic and chemical properties. Currently, carbon estimates are based on bulk measurements, which does not take the hydrodynamic pathway of different fractions into account. Therefore, eight coastal permafrost locations have been sampled along the Canadian Beaufort Sea Coast, hydrodynamically fractionated and analyzed for their C, N, 13C and 14C content. We found that the matrix‐free fraction (low density <1.8 g/cm3, and high‐density >1.8 g/cm3; <38 μm) account for 77%–98% of the OC. By using a coastal classification combined with field data, our results showed that short coastal segments can become key players in delivering matrix‐free, easily degradable OC to the marine system. Arctic coastlines consisting out of permafrost, permanently frozen ground, experience accelerated erosion due to rising global temperatures. This erosion poses a threat to local communities and releases thawed permafrost material into the Arctic Ocean. Permafrost coasts store significant amounts of organic carbon (OC), and as they thaw and erode, this carbon is released, impacting the climate through the potential release of CO2. Current estimates rely on bulk measurements, but the behavior of released OC in the ocean remains poorly understood. To address this, we used hydrodynamic fractionation on eight sampled coastal permafrost sites along the Canadian Beaufort Sea. By doing so we separate the material based on density and grain size. The results show that a significant portion of OC (77%–98%) falls into the low‐density and the high‐density fine (<38 μm) fraction, containing easily degradable plant‐rich material. Due to these two fractions short coastal segments can become key players for the delivery of degradable OC into the ocean. Coastal types vary in size and density distribution, high‐density prevailing in weight and low‐density dominating organic carbon contentBetween 77% and 98% of eroded organic carbon resides in the matrix‐free fraction, prone to degradation and CO2releaseHydrodynamic fractionation and site‐specific properties contribute to integration of results into land‐ocean flux models Coastal types vary in size and density distribution, high‐density prevailing in weight and low‐density dominating organic carbon content Between 77% and 98% of eroded organic carbon resides in the matrix‐free fraction, prone to degradation and CO2release Hydrodynamic fractionation and site‐specific properties contribute to integration of results into land‐ocean flux models

We examine instrumental and methodological capabilities for microscale (10–50 μg of C) radiocarbon analysis of individual compounds in the context of paleoclimate and paleoceanography applications, for which relatively high-precision measurements are required. An extensive suite of data for 14C-free and modern reference materials processed using different methods and acquired using an elemental-analyzer–accelerator-mass-spectrometry (EA-AMS) instrumental setup at ETH Zurich was compiled to assess the reproducibility of specific isolation procedures. In order to determine the precision, accuracy, and reproducibility of measurements on processed compounds, we explore the results of both reference materials and three classes of compounds (fatty acids, alkenones, and amino acids) extracted from sediment samples. We utilize a MATLAB code developed to systematically evaluate constant-contamination-model parameters, which in turn can be applied to measurements of unknown process samples. This approach is computationally reliable and can be used for any blank assessment of small-size radiocarbon samples. Our results show that a conservative lower estimate of the sample sizes required to produce relatively high-precision 14C data (i.e., with acceptable errors of <5% on final 14C ages) and high reproducibility in old samples (i.e., F14C ≈ 0.1) using current isolation methods are 50 and 30 μg of C for alkenones and fatty acids, respectively. Moreover, when the F14C is >0.5, a precision of 2% can be achieved for alkenone and fatty acid samples containing ≥15 and 10 μg of C, respectively.

A synthesis of published and newly acquired stable and radiocarbon isotope data from soil, river, and marine particulate organic carbon (OC) from the South China Sea drainage and sedimentary basin reveals that OC derived from bedrock‐erosion (petrogenic OC) and marine productivity comprises the major contributors to bulk OC in particulate matter reaching abyssal depths, while soil‐derived OC appears negligible. Aluminum‐radiocarbon relationships of sediments suggest that soil OC initially associated with detrital terrestrial minerals is lost and replaced by marine OC during transport beyond the continental shelf. We estimate that petrogenic OC sinking to a ~30,000 km2region of the deep northeastern South China Sea accounts for 0.6% of global petrogenic OC burial. The basin‐wide OC isotope patterns coupled with sediment trap observations highlight both the spatial variabilities of OC components as they propagate from source to sedimentary sink and the significance of petrogenic OC to deep ocean sediments. Sediment traps deployed in the ocean intercept settling particulate organic matter of marine and terrestrial origin. Terrestrial organic matter includes contributions from soils as well as bedrock‐derived organic matter mobilized by erosional processes and is exported by rivers into the ocean, where it contributes to sinking sediment fluxes. In this sediment trap study from the northeastern South China Sea, we constrain the flux and type of organic carbon exported to the deep ocean using stable and radiocarbon isotopes. We find that bedrock‐derived and marine organic matter comprise the dominant types of sedimentary organic matter reaching the deep South China Sea, whereas soil organic matter is conspicuously absent. Both bedrock‐derived carbon and marine carbon are associated with terrestrial mineral particles as revealed by the high aluminum contents of all collected sediments, implying that soil organic matter must be lost from mineral surfaces and replaced with marine carbon. These findings raise fundamental new questions concerning the role of organic matter‐mineral interactions in the ocean. Marine and petrogenic OC dominantly contribute to bulk POC in abyssal sedimentsSoil OC is absent in deep‐sea sediments due to preferential loss and deposition on proximal shelfAl‐14C trend indicates loss of soil OC from lithogenic particles with lowest 14C values thus far reported for deep ocean POC

On‐going shrinkage of Greenland's icecap, permafrost thaw, and changes in precipitation are exposing its landscapes to erosion and remobilization of ancient organic carbon (OC) held in soils and sedimentary rocks. The fate of this OC and potential feedbacks to climate are still unclear. Here, we show that the glacial Zackenberg river (Northeastern Greenland) exports aged particulate OC (POC, uncalibrated radiocarbon ages of ∼4,000 years). Many of the smaller periglacial streams affected by abrupt permafrost thaw transport substantially older POC (up to 32,000 years), especially with enhanced discharge following intense precipitation. Mineralogical analysis, and density and size fractionation of soils and both glacial and nonglacial river sediments reveal that OC is largely associated with phyllosilicate minerals, suggesting stabilization against microbial processing. Enhanced export of ancient, mineral‐associated OC as a consequence of summer rainfall may accelerate translocation of OC from terrestrial to marine environments, but could have limited consequences for climate. On‐going Arctic warming leads to shrinking ice sheets and permafrost thaw, which promotes erosion of ancient carbon stocks. The decomposition of this remobilized carbon to CO2or CH4would in turn fuel further climate warming. Here we show that the glacial Zackenberg river in Northeastern Greenland likely exports a mixture of aged soil organic matter, recent bacteria and ice algae biomass residues, and deposited organic aerosols. In contrast, many of the smaller, nonglacial streams in the study area carried significantly older carbon from deeper soil layers or sedimentary rocks. More intense rainfall will likely increase these carbon fluxes. However, close interactions of organic matter with mineral surfaces may hamper greenhouse gas production and instead promote burial in marine sediments. Therefore, the impact on atmospheric CO2and climate may be limited. Particulate organic carbon (POC) carried by glacial Zackenberg river shows uniform composition despite diverse inputs from tributariesGeology and hydrologic conditions control POC composition in small periglacial streams with rain events eroding more ancient carbonOrganic matter in soils and river sediments throughout Zackenberg valley is largely associated with minerals Particulate organic carbon (POC) carried by glacial Zackenberg river shows uniform composition despite diverse inputs from tributaries Geology and hydrologic conditions control POC composition in small periglacial streams with rain events eroding more ancient carbon Organic matter in soils and river sediments throughout Zackenberg valley is largely associated with minerals

SummaryA suite of four samples of xylem tissue from Ginkgo (Ginkgo biloba L.) shoots grown in a medium containing coniferin 13C-labeled at differing side-chain carbon atoms were studied using thermal and chemical degradation methods in combination with molecular-level isotopic analyses. The aims of the study were threefold: (1) to verify conclusions drawn from Nuclear Magnetic Resonance experiments previously performed on the same tissue samples, (2) to investigate degradation mechanisms and (3) to quantify the proportion of labeled material in each sample.Isotopic analysis of specific degradation products revealed the presence of the label exclusively within lignin-derived (phenolic) products and that the label is retained in its original position on the side-chain. These two results clearly indicate that there is no “scrambling” of carbon atoms as a result of thermal or chemical degradation, and thus lend strong support to analytical pyrolysis and chemolysis as viable approaches for structural investigations of the lignin macromolecule. Indeed, the isotopic enrichment of specific degradation products provides new evidence for certain types of linkages within the lignin polymer.The distribution and isotopic composition of the degradation products also strongly suggest an origin from newly-formed lignin as opposed to DHP-type products or unreacted substrate. As such, the data provides added confidence in the selective labeling approach for elucidation of the structure and biosynthesis of lignin. Isotopic mass balance calculations reveal that certain pyrolysis and CuO oxidation products show enhanced labeling which may be indicative of preferential incorporation of their specific precursors into the growing lignin macromolecule or heterogeneous lignin deposition.

The sediments of Loe Pool, a eutrophic coastal lake in south west England, consist largely of laminated clays and clay-gyttjas. Studies of the diatom microstratigraphy of frozen sediment cores from the Pool indicate that the laminations are annual, and that they contain pairs of light and dark bands formed by seasonal variations in the supply of sediment to the Pool from its catchment. Analysis of the magnetic properties of individual laminations demonstrates the presence of physical and mineralogical microstratigraphic variations, which may also be related to seasonality.

AbstractConcentrations of organic acids ranging up to several thousand parts per million have previously been found in oil-field waters. These acids are of interest because of their potential to enhance porosity by the dissolution of carbonates and aluminosilicates. They are believed to be generated from organic geopolymers (kerogen) in the late-diagenetic-early-catagenetic stage of thermal maturation.During the course of artificial maturation experiments in which kerogens of varying type were heated in the presence of water (so-called ‘hydrous pyrolysis’) and different minerals, the distribution and abundance of low molecular weight water-soluble acids were determined by gas chromatography and gas chromatography-mass spectrometry. Preliminary results suggest that significant quantities of mono- and di-carboxylic acids are produced during hydrous pyrolysis. The amounts and types of acid appear to vary as a function of kerogen type, maturity and mineralogy. Implications of these findings regarding the development of secondary porosity are discussed.

Climate change is dramatically altering Arctic ecosystems, leading to shifts in the sources, composition, and eventual fate of riverine dissolved organic matter (DOM) in the Arctic Ocean. Here we examine a 6‐year DOM compositional record from the six major Arctic rivers using Fourier‐transform ion cyclotron resonance mass spectrometry paired with dissolved organic carbon isotope data (Δ14C, δ13C) to investigate how seasonality and permafrost influence DOM, and how DOM export may change with warming. Across the pan‐Arctic, DOM molecular composition demonstrates synchrony and stability. Spring freshet brings recently leached terrestrial DOM with a latent high‐energy and potentially bioavailable subsidy, reconciling the historical paradox between freshet DOM's terrestrial bulk signatures and high biolability. Winter features undiluted baseflow DOM sourced from old, microbially degraded groundwater DOM. A stable core Arctic riverine fingerprint (CARF) is present in all samples and may contribute to the potential carbon sink of persistent, aged DOM in the global ocean. Future warming may lead to shifting sources of DOM and export through: (1) flattening Arctic hydrographs and earlier melt modifying the timing and role of the spring high‐energy subsidy; (2) increasing groundwater discharge resulting in a greater fraction of DOM export to the ocean occurring as stable and aged molecules; and (3) increasing contribution of nitrogen/sulfur‐containing DOM from microbial degradation caused by increased connectivity between groundwater and surface waters due to permafrost thaw. Our findings suggest the ubiquitous CARF (which may contribute to oceanic carbon sequestration) underlies predictable variations in riverine DOM composition caused by seasonality and permafrost extent. Climate change is causing the Arctic to warm, leading to the thaw of permafrost (permanently frozen ground) and shifting the distribution of plants, which in turn impact the quantity and quality of dissolved organic matter (DOM) in rivers. The fate of the carbon found in river DOM depends on its source as this determines its composition – a smorgasbord of organic substrates with some destined to be rapidly turned into greenhouse gases through respiration, while others may be exported to and sequestered in the ocean for millennia. Understanding the sources and composition of DOM presently in Arctic rivers will help predict future changes in the global carbon cycle. In this study we examine six years of data on the composition of DOM found in the six largest Arctic rivers (sampled by the Arctic Great Rivers Observatory). Overall, our work suggests that DOM in large Arctic rivers contains a universal core Arctic riverine fingerprint (which may contribute to long‐term ocean carbon sequestration), and that there are predictable, pan‐Arctic seasonal changes in DOM composition. Finally, we explore how future warming may shift DOM sources in Arctic rivers, particularly due to increasing groundwater inputs to rivers caused by thawing permafrost in the region. Synchrony: Ultrahigh‐resolution mass spectrometry shows seasonality controls dissolved organic matter composition in six large Arctic riversStability: A core Arctic riverine fingerprint was found in all rivers and months for six years and may add stable carbon to the global oceanShifting sources: Permafrost thaw may increase old, stable carbon load via increasing groundwater inputs and microbial processing time Synchrony: Ultrahigh‐resolution mass spectrometry shows seasonality controls dissolved organic matter composition in six large Arctic rivers Stability: A core Arctic riverine fingerprint was found in all rivers and months for six years and may add stable carbon to the global ocean Shifting sources: Permafrost thaw may increase old, stable carbon load via increasing groundwater inputs and microbial processing time

The Arctic is undergoing accelerated changes in response to ongoing modifications to the climate system, and there is a need for local to regional scale records of past climate variability in order to put these changes into context. The Mackenzie Delta region in northern Canada is populated by numerous small shallow lakes. They are classified as no‐, low‐, and high‐closure (NC, LC, and HC, respectively) lakes, reflecting varying degrees of connection to the river main stem, and have different sedimentation characteristics. This study examines sedimentological (mineral surface area, grain size), carbon isotopic (bulk and molecular‐level) and inorganic isotopic (neodymium) characteristics of sediment cores from three lakes representing each class. We find that HC lake sediments exhibit strikingly different properties from the other lake sediments. Specifically, they are characterized by higher organic carbon loadings per unit mineral surface area and with relatively minor influence from allochthonous, petrogenic (rock‐derived) organic carbon. In contrast, LC and NC lakes have the potential to record basin‐scale climatic changes at a high resolution by virtue of enhanced detrital sedimentation. Overall the delta lakes have the capacity to bury about 2 MtC year−1, with little changes in the last 200 years. However, in the (near) future, an increased number of high closure lakes might change the carbon burial efficiency of the Mackenzie Delta as they seem to retain less carbon than NC and LC lakes. The Arctic is rapidly warming compared to other parts of the world, yet the consequences on regional aquatic carbon cycling are unknown. We studied the properties of sediment cores from three different types of lakes within the Mackenzie River Delta to examine relationships between the degree of lake connectivity with the main river channel network and carbon inputs and burial. “High‐closure” lakes that are rarely connected to the river mainstem are markedly different from those that are frequently inundated by the river. Their organic carbon is of local origin, in contrast to the carbon in more connected lakes. Moreover, despite higher carbon contents, high closure lakes bury carbon less efficiently, suggesting that an increase in the abundance of these lakes would lead to a reduction in carbon burial in the delta. No significant changes in carbon burial are, however, apparent in lake sediment records spanning the last ∼200 years. High‐closure lake bury very little petrogenic hydrocarbons in contrast to no‐ and low‐closure lakesHigh closure lakes have higher organic carbon loadings than usually found in freshwater systemsWe found no evidence for a significant change in carbon burial in the lakes over the last 200 years High‐closure lake bury very little petrogenic hydrocarbons in contrast to no‐ and low‐closure lakes High closure lakes have higher organic carbon loadings than usually found in freshwater systems We found no evidence for a significant change in carbon burial in the lakes over the last 200 years

The isomer distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils has been shown to correlate to the local mean annual temperature. Here, we explore the use of brGDGT distributions as proxy for the elevation at which soil organic carbon is preferentially mobilized in the Central Himalaya. Soil brGDGT distributions collected along an altitudinal profile, spanning elevations from 200 to 4,450 m asl, are linearly correlated to elevation. We use this calibration to trace the provenance of soil organic matter in suspended sediments of rivers draining the Himalaya. BrGDGT distributions of fluvial sediments reflect the mean elevation of the soil cover in most catchments. Inverse modeling of the brGDGT data set suggests similar relative contribution to soil organic carbon mobilization from different land covers within a factor 2. We conclude that riverine soil organic carbon export in the Himalaya mostly occurs pervasively and is at the catchment scale insensitive to anthropogenic perturbations. Soil degradation influences the global climate and has direct impacts on the ability of soils to deliver economic and ecological services. Identifying the source of eroded soil‐derived carbon entrained in sediments of large rivers remains challenging. In this contribution, we use the temperature‐sensitive signature of soil microbe‐derived organic molecules to trace the source of soil carbon exported in river sediments of the rapidly eroding Central Himalaya of Nepal. Molecular signatures of soils, sampled over a wide range of elevations, and hence temperatures are plotted on an elevation profile. River sediments are then compared to soil signatures in order to identify elevations of preferential soil export through rivers. Our data shows that soil‐derived carbon is sampled uniformly over most of the catchments, indicating that soil export by Himalayan rivers is not driven by localized human perturbations such as agriculture or deforestation. A calibration for soil branched glycerol dialkyl glycerol tetraether (brGDGT) signals against an elevation profile is developedThe provenance of soil organic matter in riverine suspended sediments is traced based on corresponding brGDGT distributionsSoil organic carbon in most river sediments originates from the entire soil‐covered catchment, indicating uniform soil organic matter export

The stability and potential vulnerability of soil organic matter (SOM) to global change remain incompletely understood due to the complex processes involved in its formation and turnover. Here we combine compound‐specific radiocarbon analysis with fraction‐specific and bulk‐level radiocarbon measurements in order to further elucidate controls on SOM dynamics in a temperate and subalpine forested ecosystem. Radiocarbon contents of individual organic compounds isolated from the same soil interval generally exhibit greater variation than those among corresponding operationally defined fractions. Notably, markedly older ages of long‐chain plant leaf wax lipids (n‐alkanoic acids) imply that they reflect a highly stable carbon pool. Furthermore, marked 14C variations among shorter‐ and longer‐chain n‐alkanoic acid homologues suggest that they track different SOM pools. Extremes in SOM dynamics thus manifest themselves within a single compound class. This exploratory study highlights the potential of compound‐specific radiocarbon analysis for understanding SOM dynamics in ecosystems potentially vulnerable to global change. Soil carbon forms the largest amount of organic carbon stored on land. In the context of climate change, it is important to know how stable the carbon in this large reservoir is. In this paper we try to attain a better understanding of the stability of soil carbon in a warm and cold area by looking at specific molecules (soil lipids). By measuring the age of these molecules (using radiocarbon) and comparing it to all environmental information, we attain more insight into the soil carbon stability. We found that the molecules show a wide range of ages, indicating they reflect a wide range of sources. These molecular markers may constitute a cleaner method to assess carbon stability than methods that were used previously. They can also indicate the contribution of extremely old (fossil) carbon derived from carbon‐holding rocks. Altogether, this paper presents a new approach to tackle soil carbon stability and showcases new insights gained from this approach. Lipid biomarkers exhibit a larger spread in radiocarbon signatures than observed across operationally defined soil organic matter poolsSpecific compounds reflect inputs from specific biological sources (microbial, plant derived)Biomarker isotopic signatures reveal geogenic and biogenic carbon contributions to soil organic matter