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The upper layer of bottom sediments in the northeastern Kara Sea is studied. It is shown that the sediment composition is dominated by silt, while the percentage of sand increases towards the coast. The organic carbon content (average value 1.1 wt %) is close to the average content in Kara Sea sediments, increasing at deep-sea stations. The linear correlation found between the organic carbon content and surface area of sediments shows adsorption on particle surfaces and is the predominant form of organic matter (OM) accumulation in the sediments. The average n-alkane concentration in the analyzed samples is 1.1 μg/g of sediment; the distribution spectrum is dominated by odd high-molecular homologues of terrigenous origin. [ABSTRACT FROM AUTHOR]

The Arctic is warming at a rate twice that of other global ecosystems and changing climate conditions in the Arctic are mobilizing long frozen permafrost stores of organic carbon. In ice-rich regions, permafrost thaw on sloping terrain can cause land subsidence, and the development of thaw-driven mass wasting. The Peel Plateau, Northwest Territories, Canada has extensive thaw-driven landslides called retrogressive thaw slumps that are exposing early Holocene age paleo-thaw layers and Pleistocene age glaciogenic material deposited by the Laurentide Ice Sheet. This study aimed to see if unique retrogressive thaw slump derived permafrost inputs could be readily observed in streams across six diverse thermokarst features via optical and ultrahigh-resolution mass spectrometry. Aquatic samples from water draining thermokarst slump features, and downstream of thermokarst inputs exhibited higher dissolved organic carbon concentrations and lower aromaticity as evidenced by optical parameters (e.g. declining SUVA254, increasing S275-295) and FT-ICR MS metrics (e.g. lower AImod and nominal oxidation state of carbon) versus upstream of thermokarst impacts. Increases in the relative abundances of assigned heteroatomic molecular formulae (e.g. CHON, CHOS, CHONS) were also greater within and downstream of thermokarst features. The unique molecular formulae present in permafrost thermokarst inputs were determined (n = 1844) and subsequently tracked downstream. These permafrost marker formulae were enriched in aliphatics and H/C, as well as heteroatoms and exhibited low aromaticity. A portion of the unique molecular fingerprint persisted downstream, highlighting the potential to not only assess thermokarst inputs but also to follow these inputs and their fate downstream throughout the aquatic network. [ABSTRACT FROM AUTHOR]

Freeze‐thaw is a disturbance process in cold regions where permafrost soils are becoming vulnerable to temperature fluctuations above 0°C. Freeze‐thaw alters soil physical and biogeochemical properties with implications for carbon persistence and emissions in Arctic landscapes. We examined whether different freeze‐thaw histories in two soil systems led to contrasting biogeochemical responses under a laboratory‐controlled freeze‐thaw incubation. We investigated controls on carbon composition through Fourier‐transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS) to identify nominal carbon oxidation states and relative abundances of aliphatic‐type carbon molecules in both surface and subsurface soils. Soil cores (∼60 cm‐depth) were sampled from two sites in Alaskan permafrost landscapes with different in situ freeze‐thaw characteristics: Healy (>40 freeze‐thaw cycles annually) and Toolik (<15 freeze‐thaw cycles annually). FT‐ICR‐MS was coupled with in situ temperature data and soil properties (i.e., soil texture, mineralogy) to assess (a) differences in soil organic matter composition associated with previous freeze‐thaw history and (b) sensitivity to experimental freeze‐thaw in the extracted cores. Control (freeze‐only) samples showed greater carbon oxidation in Healy soils compared with Toolik, even in lower mineral horizons where freeze‐thaw history was comparable across both sites. Healy showed the most loss of carbon compounds following experimental freeze‐thaw in the lower mineral depths, including a decrease in aliphatics. Toolik soils responded more slowly to freeze‐thaw as shown by intermediary carbon oxidation distributed across multiple carbon compound classes. Variations in the response of permafrost carbon chemistry to freeze‐thaw is an important factor for predicting changes in soil function as permafrost thaws in high northern latitudes. Plain Language Summary: As global warming progresses, permafrost (soils frozen for two or more consecutive years) is undergoing thaw. However, the process of permafrost thaw is more than a simple transition from frozen to thawed. Instead, thawing permafrost undergoes repeated freeze‐thaw cycles on an annual, seasonal, and daily basis. Freeze‐thaw cycles impact soils by changing soil nutrient availability and biological activity with implications for carbon decomposition. We investigated two types of freeze‐thaw response: (a) how previous freeze‐thaw history influenced current soil function and (b) how soils responded to experimental freeze‐thaw. We tested both types of soil response to freeze‐thaw cycles by evaluating the chemical composition of soil carbon. We found that soil response to freeze‐thaw differed by site, even in the lower soil depths with little exposure to prior freeze‐thaw. Our findings indicate that as permafrost thaws and begins to undergo freeze‐thaw cycles, the response to those freeze‐thaw cycles may differ by site. Variation in soil response could play a crucial role in predicting greenhouse gas emissions and rates of emission from thawing Arctic landscapes. Key Points: A combination of freeze‐thaw history and soil properties dictated soil response to experimental freeze‐thaw cyclesThe two soils with the least prior freeze‐thaw showed diverging responses to freeze‐thaw, likely due to soil moisture and mineralogyFuture increases in freeze‐thaw in both the active layer and thawing permafrost may result in contrasting carbon responses across sites [ABSTRACT FROM AUTHOR]

Thermokarst (thaw) lakes of permafrost peatlands are among the most important sentinels of climate change and sizable contributors of greenhouse gas emissions (GHG) in high latitudes. These lakes are humic, often acidic and exhibit fast growing/drainage depending on the local environmental and permafrost thaw. In contrast to good knowledge of the thermokarst lake water hydrochemistry and GHG fluxes, the sediments pore waters remain virtually unknown, despite the fact that these are hot spots of biogeochemical processes including GHG generation. Towards better understating of dissolved organic matter (DOM) quality at the lake water – sediment interface and in the sediments pore waters, here we studied concentration and optical (UV, visual) properties of DOM of 11 thermokarst lakes located in four permafrost zones of Western Siberia Lowland. We found systematic evaluation of DOM concentration, SUVA and various optical parameters along the vertical profile of lake sediments. The lake size and hence, the stage of lake development, had generally weak control on DOM quality. The permafrost zone exhibited clear impact on DOM porewater concentration, optical characteristics, aromaticity and weight average molecular weight (WAMW). The lowest quality of DOM, reflected in highest SUVA and WAMW, corresponding to the dominance of terrestrial sources, was observed at the southern boundary of the permafrost, in the sporadic/discontinuous zone. This suggests active mobilization of organic matter leachates from the interstitial peat and soil porewaters to the lake, presumably via subsurface or suprapermafrost influx. Applying a substitute space for time scenario for future evolution of OM characteristics in thermokarst lake sediments of Western Siberia, we foresee a decrease of DOM quality, molecular weight and potential bioavailability in lakes of continuous permafrost zone, and an increase in these parameters in the sporadic/discontinuous permafrost zone. [Display omitted] • DOM characteristics in porewaters of thermokarst lake sediments depend on the permafrost zone • Weak effect of lake size on DOM concentration and quality in porewaters of thermokarst lake sediments • Maximal DOC concentration, SUVA 254 and aromaticity in the sporadic/discontinuous permafrost • Strong link between peat porewaters and lake sediments via subsurface flow • Possible decrease of DOM quality in lakes of continuous permafrost zone upon climate warming [ABSTRACT FROM AUTHOR]

Brown carbon (BrC) plays an important role in global radiative budget but there have been few studies on BrC in Arctic despite rapid warming and increasing wildfires in this region. Here we investigate the optical properties of BrC from boreal fires in Alaska summer, with two sets of measurements from PILS-LWCC-TOC (Particle-Into-Liquid-Sampler – Liquid-Waveguide Capillary flow-through optical Cell - Total-Organic-Carbon analyzer) and filter measurements. We show that during intense wildfires, the mass absorption coefficient at 365 nm (MAC 365) from water soluble organic carbon (WSOC) remained stable at ∼1 m2 g−1. With all plumes sampled and derived transport time, we show a decrease of MAC 365 with plume age, with a shorter photobleaching lifetime (∼11 h) at 365 nm compared to 405 nm (∼20 h). The total absorption by organic aerosols measured from filters at 365 nm is higher than the absorption by WSOC by a factor 2–3, suggesting a dominant role of insoluble organic carbon. Overall BrC dominates absorption in the near-ultraviolet and visible radiation during wildfire season in Alaska summer. [ABSTRACT FROM AUTHOR]

Twenty-seven sediment samples from the eastern Laptev Sea were analyzed for mercury and total organic carbon as well as grain-size distribution. The average total mercury (THg) concentrations in sediments are 29 ± 14 μg kg−1. A significant correlation of THg content with total organic carbon and clay and silt fractions was shown. The 210Pb-dated sediment core was used to evaluate the contamination degree and flux of THg in sediments from the eastern Laptev Sea. The average sedimentation rate for the all dated intervals was 0.17 cm/year. The THg flux increased from 20 to 28 μg/m2/year in the period of 1892–1950 to 53–59 μg/m2/year in the modern period of 2011–2015. According to various indices, the ecological risk from THg in studied sediment was low. • THg concentrations in surface sediment of the eastern Laptev Sea were 3–73 μg kg−1. • In the modern period the THg flux was from 53 to 59 μg/m2/year. • THg accumulation was associated with the warming and destruction of permafrost. • The studied sediments have low ecological risk from the mercury. [ABSTRACT FROM AUTHOR]

Transport and accumulation of radionuclides in the Arctic depends on many biogeochemical processes, which are changing at accelerated rates due to climate change and human economic activity. We present the results of a study on the features distribution of some natural radionuclides in the marine sediments on the East Siberian Arctic Shelf collected during several expeditions from 2008 to 2019. Average activity concentration of 232Th, 40K and 226Ra under the influence of different sedimentation regime increases from 40.7, 418 and 30.8 Bq/kg to 41.6, 423 and 34.9 Bq/kg respectively from coastal shelf marine sediments (<50% clay) to outer shelf marine sediments (>50% clay). Sediment particle size has a greater impact on radionuclides in the coastal shelf. An increase in the activity concentrations of 232Th and 226Ra with the increasing clay particles were found. On the outer shelf with a change in the sedimentation regime, the influence of the size composition decreased, at the same time, there is a correlation between the organic carbon concentration and the radionuclide activity concentration. Absolute maximums of natural radionuclide activity concentration (232Th = 70.9, 226Ra = 70.4, 40K = 591 Bq/kg) were detected in the Chaun Bay. The highest activity concentration of 226Ra was found in paleo-river valleys marine sediments. A low 232Th/226Ra activity concentration ratio indicates the enrichment of paleo-river valleys marine sediments with 226Ra. In the deep-sea sediments of the shelf slope on the contrary paleo-river valleys, this ratio is greatly increased. • The activity concentration of natural radionuclides increases from coast to outer ESAS. • Sediment particle size has a greater impact on radionuclides in the coastal ESAS. • On the outer ESAS, the influence of biogenic OM on radionuclides increases. • The highest activity concentration of 226Ra was found in paleo-river valleys marine sediments. [ABSTRACT FROM AUTHOR]

Subglacial environments are largely anoxic, contain organic carbon ( OC) overridden by glacier ice during periods of advance, and harbour active microbial communities. This creates favourable conditions for OC degradation via methanogenesis. It has been hypothesized that OC beneath ice sheets is converted to methane ( CH4) and may be released to the atmosphere during retreat. However, there are limited data available to support this contention. Here, we present new data on the abundance, diversity and activity of methanogenic archaea and the amount and character of OC in subglacial sediments from Arctic and Antarctic glacial systems based on different substrate types. We employed long-term laboratory incubations to quantify the CH4 production potential in different subglacial settings. Significant numbers of methanogens (up to 7 × 104 cells g−1) were detected in the samples and clones of Methanomicrobiales and Methanosarcinales were identified in clone libraries. Long lag periods (up to >200 days) were observed before significant CH4 concentrations were measured. We report order of magnitude differences in rates of CH4 production (101-105 fmol g−1 d−1) in different subglacial sediments, reflecting contrasts in the origin of the sediment and the OC character. Hence, we predict that contrasting rates of CH4 production are likely to occur beneath glaciers and ice sheets that overran different types of substrate. We subsequently estimated the potential for CH4 production beneath the Laurentide/Inuitian/Cordilleran and Fennoscandian Ice Sheets during a typical 85 ka Quaternary glacial/interglacial cycle. CH4 production from lacustrine-derived OC is likely to be an order of magnitude higher (~6.3-27 Pg C) than that from overridden soils (~0.55-0.68 Pg C), possibly due to a difference in lability between lacustrine and soil OC. While representing a fraction of the entire OC pool (~418-610 Pg C), this finding highlights the importance of considering the character of different OC pools when calculating subglacial CH4 production. [ABSTRACT FROM AUTHOR]

Abstract: The remineralization of organic matter (OM) in Hudson Bay was examined using redox element (Mn, Fe, S, Cd, Re, Mo and U) profiles in six sediment cores. Variation in thickness of surface Mn enrichments and increases in Fe and S in subsurface sediments provide a means to sort sediment cores according to metabolic intensity. The data imply a relatively high flux of labile OM in Hudson Strait, weaker labile OC fluxes in inshore regions of Hudson Bay and weakest fluxes in central Hudson Bay. Enrichments of Cd, U, and Re, which precipitate in reducing sediments, are present at depth in all sediments, whereas Mo enrichments occur only in the more strongly reducing sediments. Highest rates of authigenic element accumulation in the inshore regions generally reflect the higher flux of labile OM inferred from Mn profiles. After accounting for variation in water depth, we find Hudson Bay to be the most oligotrophic among ocean margins for which there are comparable data. Redox element profiles in sediments provide a way to compare and thus possibly monitor change in labile OC forcing over different continental margins, including those in Arctic seas, where other monitoring methods are scarce. [Copyright &y& Elsevier]

During the Arctic spring freshet, the Mackenzie River delivers large amounts of dissolved and particulate terrestrial organic carbon to the Canadian Beaufort Sea shelf. This pulse of material and nutrients fuels phytoplankton blooms in late spring and summer and together with under ice primary production contributes to the organic carbon reservoir of the coastal shelf sediments. As a component of the Marine Arctic Ecosystem Study (MARES), samples transecting the Canadian Mackenzie Trough were collected in fall before ice cover to examine the balance of marine and terrestrial carbon in sediments and its relationship with sediment meiobenthic diversity and abundance. Organic biomarker analysis of lipid proxies together with total hydrolyzable amino acids (THAA) defined organic matter sources and potential lability. Across the shelf transect, consistent values observed in sedimentary total organic carbon (1.09%–1.62%) belied a shift in the balance of terrestrial and marine carbon sources. Predominately terrestrial inputs at the shallow site near the delta outflow appeared to constrain meiofaunal populations while the deepest location off shelf reflected lower labile marine organic carbon reaching the sediments that limited benthic production. Surface sediments at the mid-shelf, however, showed a near 3-fold increase (to 1.16 mg C OC-1) in contributions by diatoms and dinoflagellates markers plus a peak in total amino acids (9.53 mg g OC-1) which corresponded with higher meiofauna abundance. The combination of detailed organic analysis together with meiofaunal community measures suggest that mid-shelf sediments receive higher contributions of labile marine carbon which drive increased meiofauna diversity and abundance despite the large background of terrestrial organic material exiting the Mackenzie River outflow. • Sediments transecting the Mackenzie Trough show high inputs of marine carbon to the mid-shelf regions. • Meiofaunal communities were found in higher abundance where more marine related particulates reached the sediment surface. • Bulk isotopes of sedimentary organic matter reflect a predominance of Mackenzie River particulates in the sediments of the western Mackenzie shelf. • Sum of sedimentary organic carbon proxies suggests a combination of physical resuspension and biological production in ice flaw leads, regulate particulates reaching mid-shelf surface sediments. [ABSTRACT FROM AUTHOR]

Black carbon (BC), organic carbon (OC), and total carbon (TC) in snow are important for their climatic and cryospheric effects. They are also part of the global carbon cycle. Atmospheric black and organic carbon (including brown carbon) may deposit and darken snow surfaces. Currently, there are no standardized methods for sampling, filtering, and analysis protocols to detect carbon in snow. Here, we describe our current methods and protocols to detect carbon in seasonal snow using the OCEC thermal optical method, a European standard for atmospheric elemental carbon (EC). We analyzed snow collected within and around the urban background SMEARIII (Station for Measuring Ecosystem-Atmosphere Relations) at Kumpula (60° N) and the Arctic GAW (Global Atmospheric Watch) station at Sodankylä (67° N). The median BC, OC, and TC in snow samples (ntot = 30) in Kumpula were 1118, 5279, and 6396 ppb, and in Sodankylä, they were 19, 1751, and 629 ppb. Laboratory experiments showed that error due to carbon attached to a sampling bag (n = 11) was <0.01%. Sonication slightly increased the measured EC, while wetting the filter or filtering the wrong side up indicated a possible sample loss. Finally, we discuss the benefits and drawbacks of OCEC to detect carbon in snow. [ABSTRACT FROM AUTHOR]

Two months (Oct.–Nov. 2015) of online elemental carbon (EC) and organic carbon (OC) measurements were performed by two field OC/EC Sunset Laboratory analysers working in parallel with an 8-h time resolution at a subarctic remote station in Finland. During such a time resolution, thermal OC and EC concentrations were usually close to or below the recent limits of detection for the analyser. Thus, this work highlights the possible issues to consider during the online measurements of OC/EC analysers working at the threshold of their analytical possibilities. A detailed analysis of the data showed that even though the OC concentrations were always above the detection limit of the analyser, up to 70% of the given concentration was due to the dynamic blank (gaseous phase). The majority of the OC aerosol was present in the less volatile fractions that evolved during steps at 300–450 °C and 450–650 °C. The main bias at low EC concentrations originated from the automatically determined split point between OC and EC. The results from the RTCalc526 software often led to an early determination of the OC-EC split point, resulting in an overestimation of the EC concentrations. However, the comparison of the results was performed after processing the OC/EC raw data in the different RTCalc software versions 526 and 703. The results showed that the EC values from the newer version of the RTCalc703 software were more consistent with the equivalent black carbon (eBC) results from optical instruments (aethalometer and MAAP) measured in parallel in comparison with results from the older RTCalc526 software. Equivalent BC data from the laser attenuation measurement of the Sunset Lab. analyser also provided better agreement with the newer version of the RTCalc software (RTCalc703). Image 1 • A major part of the organic carbon (OC) was present in the less volatile fractions. • Up to 70% of the OC concentration was caused by a contribution of the dynamic blank (gaseous phase). • The elemental carbon (EC) results were overestimated in the earlier RTCalc526 evaluation software. • The updated RTCalc703 software provides reasonable results for very low EC concentrations. • The EC results are comparable to the parallel measurements of the equivalent black carbon. [ABSTRACT FROM AUTHOR]

Arctic rivers export a large amount of organic carbon (OC) and mercury (Hg) to Arctic oceans. Because there are only a few direct calculations of OC and Hg exports from these large rivers, very little is known about their response to changes in the active layer in northern permafrost-dominated areas. In this study, multiyear data sets from the Arctic Great Rivers Observatory (ArcticGRO) are used to estimate the export of dissolved organic carbon (DOC), particulate organic carbon (POC), total mercury (THg) and methylmercury (MeHg) from the six largest rivers (Yenisey, Lena, Ob, Mackenzie, Yukon and Kolyma) draining to the Arctic Ocean. From 2003 to 2017, annual DOC and POC export to the Arctic Ocean was approximately 21612 Gg and 2728 Gg, and the exports of Hg and MeHg to the Arctic Ocean were approximately 20090 kg and 110 kg (0.002% of the total Hg stored in the northern hemisphere active layer). There were great variations in seasonal OC and Hg concentrations and chemical characteristics, with higher fluxes in spring and lower fluxes in winter (baseline). DOC and Hg concentrations are significantly positively correlated to discharge, as discharge continues to increase in response to a deepening active layer thickness during recent past decades. This study shows that previous results likely underestimated DOC exports from rivers in the circum-Arctic regions, and both OC and Hg exports will increase under predicted climate warming scenarios., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The export of organic carbon export by the rivers to the oceans either as particulate organic carbon (POC) or dissolved organic carbon (DOC) is very sensitive to climate change especially in permafrost affected catchments where soils are very rich in organic carbon. With global warming, organic carbon export in both forms is expected to increase in Arctic regions. It should affect contemporary biogeochemical cycles in rivers and oceans and therefore modify the whole food web. This study tries to understand complex processes involved in sediment, POC and DOC riverine transport in the Yenisei River basin and to quantify their respective fluxes at the river outlet. The SWAT (Soil and Water Assessment Tool) hydrological model is used in this study to simulate water and suspended sediment transfers in the largest Arctic river. POC and DOC export have been quantified with empirical models, adapted from literature for the study case. First, the hydrological model has been calibrated and validated at a daily time step for the 2003-2008 and the 2009-2016 periods respectively, and its output has been compared with field data for water and sediment fluxes. Based on conceptualization of transfer processes, calibration on climate and soil properties has been performed in order to correctly represent hydrology and sediment transfer in permafrost basins. Second, calibration of empirical models for DOC/POC transport have been performed by comparing their output with field data, available from 2003 to 2016. Our study reveals that SWAT is capable of correctly representing hydrology, sediment transfer, POC and DOC fluxes and their spatial distribution at a daily timescale, and outlines the links between these fluxes and permafrost features. Our simulation effort results in specific sediment, POC and DOC fluxes of 2.97 t km -2 yr -1 , 0.13 t km -2 yr -1 and 1.14 t km -2 yr -1 for the period 2003-2016 which are in the range of previous estimates. About 60% of the total fluxes of sediment, DOC and POC to the Arctic Ocean are exported during the two months of the freshet. Spatial analysis show that permafrost-free areas have returned higher daily organic carbon export than permafrost affected zones, highlighting the thawing permafrost effect on carbon cycle in climate change feedback., (Copyright © 2019 Elsevier Ltd. All rights reserved.)