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Warming air and sea temperatures, longer open-water seasons and sea-level rise collectively promote the erosion of permafrost coasts in the Arctic, which profoundly impacts organic matter pathways. Although estimates on organic carbon (OC) fluxes from erosion exist for some parts of the Arctic, little is known about how much OC is transformed into greenhouse gases (GHGs). In this study we investigated two different coastal erosion scenarios on Qikiqtaruk – Herschel Island (Canada) and estimate the potential for GHG formation. We distinguished between a delayed release represented by mud debris draining a coastal thermoerosional feature and a direct release represented by cliff debris at a low collapsing bluff. Carbon dioxide (CO2) production was measured during incubations at 4°C under aerobic conditions for two months and were modeled for four months and a full year. Our incubation results show that mud debris and cliff debris lost a considerable amount of OC as CO2 (2.5 ± 0.2 and 1.6 ± 0.3% of OC, respectively). Although relative OC losses were highest in mineral mud debris, higher initial OC content and fresh organic matter in cliff debris resulted in a ∼three times higher cumulative CO2 release (4.0 ± 0.9 compared to 1.4 ± 0.1 mg CO2 gdw–1), which was further increased by the addition of seawater. After four months, modeled OC losses were 4.9 ± 0.1 and 3.2 ± 0.3% in set-ups without seawater and 14.3 ± 0.1 and 7.3 ± 0.8% in set-ups with seawater. The results indicate that a delayed release may support substantial cycling of OC at relatively low CO2 production rates during long transit times onshore during the Arctic warm season. By contrast, direct erosion may result in a single CO2 pulse and less substantial OC cycling onshore as transfer times are short. Once eroded sediments are deposited in the nearshore, highest OC losses can be expected. We conclude that the release of CO2 from eroding permafrost coasts varies considerably between erosion types and residence time onshore. We emphasize the importance of a more comprehensive understanding of OC degradation during the coastal erosion process to improve thawed carbon trajectories and models.

Collapse of permafrost coasts delivers large quantities of particulate organic carbon (POC) to Arctic coastal areas. With rapidly changing environmental conditions, sediment and organic carbon (OC) mobilization and transport pathways are also changing. Here, we assess the sources and sinks of POC in the highly dynamic nearshore zone of Herschel Island‐Qikiqtaruk (Yukon, Canada). Our results show that POC concentrations sharply decrease, from 15.9 to 0.3 mg L−1, within the first 100–300 m offshore. Simultaneously, radiocarbon ages of POC drop from 16,400 to 3,600 14C years, indicating rapid settling of old permafrost POC to underlying sediments. This suggests that permafrost OC is, apart from a very narrow resuspension zone (, Key Points Suspended particulate organic carbon (POC) sharply decreases in age and concentration within the nearshore zoneBeyond a narrow resuspension zone, permafrost‐derived OC dominates surface sediments, whereas much younger OC is found in the water columnPermafrost‐derived POC is rapidly removed from the water column, showing the critical role of nearshore dynamics for OC transport

The cycle of organic carbon through the atmosphere, oceans, continents and mantle reservoirs is a hallmark of Earth. Over geological time, chemical exchanges between those reservoirs have produced a diversity of reduced carbon materials that differ in their molecular structures and reactivity. This reactive complexity challenges the canonical dichotomy between the surface and deep, short-term and long-term organic carbon cycle. Old and refractory carbon materials are not confined to the lithosphere but are ubiquitous in the surface environment, and the lithosphere hosts various forms of reduced carbon that can be very reactive. The biological and geological pathways that drive the organic carbon cycle have changed through time; from a synthesis of these changes, it emerges that although a biosphere is required to produce organic carbon, mortality is required to ensure its export to the lithosphere, and graphitization is essential for its long-term stabilization in the solid Earth. Among the by-products of the organic carbon cycle are the accumulation of a massive lithospheric reservoir of organic carbon, the accumulation of dioxygen in the atmosphere and the rise of a terrestrial biosphere. Besides driving surface weathering reactions, free dioxygen has allowed the evolution of new metabolic pathways to produce and respire organic carbon. From the evolution of photosynthesis until the expansion of biomineralization in the Phanerozoic, inorganic controls on the organic carbon cycle have diversified, tightening the connection between the biosphere and geosphere. A review of the organic carbon cycle explores the interactions between the Earth’s surface and deeper reservoirs, the expanding inorganic controls on the organic carbon cycle, and how these links have strengthened through geological time.

The storage of organic carbon in the terrestrial biosphere directly affects atmospheric concentrations of carbon dioxide over a wide range of timescales. Within the terrestrial biosphere, the magnitude of carbon storage can vary in response to environmental perturbations such as changing temperature or hydroclimate1, potentially generating feedback on the atmospheric inventory of carbon dioxide. Although temperature controls the storage of soil organic carbon at mid and high latitudes2,3, hydroclimate may be the dominant driver of soil carbon persistence in the tropics4,5; however, the sensitivity of tropical soil carbon turnover to large-scale hydroclimate variability remains poorly understood. Here we show that changes in Indian Summer Monsoon rainfall have controlled the residence time of soil carbon in the Ganges-Brahmaputra basin over the past 18,000 years. Comparison of radiocarbon ages of bulk organic carbon and terrestrial higher-plant biomarkers with co-located palaeohydrological records6 reveals a negative relationship between monsoon rainfall and soil organic carbon stocks on a millennial timescale. Across the deglaciation period, a depletion of basin-wide soil carbon stocks was triggered by increasing rainfall and associated enhanced soil respiration rates. Our results suggest that future hydroclimate changes in tropical regions are likely to accelerate soil carbon destabilization, further increasing atmospheric carbon dioxide concentrations.

Erosion and riverine transport of organic carbon is an important component of the global carbon cycle, but the significance of this process for Earth's surface carbon budgets depends on the sources of carbon being mobilised. In this study, we aim to constrain how runoff-driven erosion modulates the contribution of different carbon source endmembers, i.e., bedrock, soil and vegetation, in three forested headwater catchments in the Swiss Prealps. The sources of organic carbon are determined using an inverse model based on bulk carbon isotope signatures and the abundances and distributions of long-chain n-alkane plant wax biomarkers in suspended sediments collected over a range of discharges. Despite landcover differences and contrasting bulk particulate organic carbon (POC) signatures, the increase of soil-sourced organic carbon with discharge is similar in all three studied catchments. This apparent existence of common processes implies that export fluxes of soil organic carbon may be extrapolated to similar catchments. Overall, our analysis shows that runoff-driven soil erosion in these alpine headwater streams is responsible for the export of ca. 0.3 to 0.8 gC m-2 a-1 as POC, which represents ca. 0.1 - 0.3 % of carbon fixed by NPP. Most of this soil OC export occurs during high-discharge events. Our study also shows that despite a significant variability in isotopic and molecular POC signatures at low discharge, all three catchments show a convergence of these signatures at higher discharges. Suspended sediment samples collected at above-average discharges are hence most representative of overall endmember contributions., Journal of Geophysical Research: Biogeosciences, 127 (3), ISSN:0148-0227, ISSN:2169-8953, ISSN:2169-8961

Interactions between organic and detrital mineral phases strongly influence both the dispersal and accumulation of terrestrial organic carbon (OC) in continental margin sediments. Yet the complex interplay among biological, chemical, and physical processes limits our understanding of how organo-mineral interactions evolve during sediment transfer and burial. In particular, diverse OC sources and complex hydrodynamic processes hinder the assessment of how the partnership of organic matter and its mineral host evolves during supply and dispersal over continental margins. In this study, we integrate new and compiled sedimentological (grain size, surface area), organic (%OC, OC-δ13C, OC-F14C), and inorganic isotopic (εNd, 87Sr/86Sr) geochemical data for a broad suite of surface sediments spanning the Western Arctic Ocean from the Bering Sea to the Mackenzie River Delta that capture diverse sources and ages of both terrestrial and marine material deposited in contrasting shelf and slope settings. Spatial gradients in sediment properties were used to delineate regional sources and transport processes influencing the dispersion and persistence of OC-mineral particle associations during export and burial. We found strong relationships between physical parameters, aluminum content, and OC-14C suggesting that terrestrial OC remains tightly associated with its detrital mineral carrier during source-to-sink transport. Notably, carbon and neodymium isotopic data yield consistent information regarding organic matter provenance. Results obtained highlight the potential for coupled organic-inorganic tracer measurements to elucidate sediment sources and to constrain physical and geochemical processes during sediment mobilization and transport in the Western Arctic Ocean. Tandem measurements of carbon and Nd isotopes may provide a new way to identify large-scale biogeochemical and ecological changes in the sources, nature, and fate of OC stemming from predicted increases in sea ice loss and fluvial inputs of dissolved and particulate OC to this complex and dynamic high latitude marginal sea. ISSN:0016-7037 ISSN:1872-9533

Rivers transfer terrestrial organic carbon (OC) from mountains to ocean basins, playing a key role in the global carbon cycle. During fluvial transit, OC may be oxidized and emitted to the atmosphere as CO2 or preserved and transported to downstream depositional sinks. The balance between oxidation and preservation determines the amount of particulate OC (POC) that can be buried long term, but the factors regulating this balance are poorly constrained. Here, we quantify the effects of fluvial transit on POC fluxes along an ~1,300 km lowland channel with no tributaries. We show that sediment transit time and mineral protection regulate the magnitude and rate of POC oxidation, respectively. Using a simple turnover model, we estimate that annual POC oxidation is a small percentage of the POC delivered to the river. Modelling shows that lateral erosion into POC-rich floodplains can increase POC fluxes to downstream basins, thereby offsetting POC oxidation. Consequently, rivers with high channel mobility can enhance CO2 drawdown while management practices that stabilize river channels may reduce the potential for CO2 drawdown., Nature Geoscience, 14 (11), ISSN:1752-0908, ISSN:1752-0894

The erosion and weathering of mountain ranges exert a key control on the long-term (105–106 yr) cycling of carbon between Earth’s surface and crust. The net carbon budget of a mountain range reflects the co-existence of multiple carbon sources and sinks, with corresponding fluxes remaining difficult to quantify. Uncertain responses of these carbon fluxes due to the stochastic nature of erosional processes further complicate the extrapolation of short-term observations to longer, climatically relevant timescales. Here, we quantify the evolution of the organic and inorganic carbon fluxes in response to the 2015 Gorkha earthquake (Mw 7.8) in the central Himalaya. We find that the Himalayan erosion acts as a net carbon sink due mainly to efficient biospheric organic carbon export. Our high-resolution time series encompassing four monsoon seasons before and after the Gorkha earthquake reveal that coseismic landslides did not significantly perturb large-scale Himalayan sediment and carbon fluxes. This muted response of the central Himalaya to a geologically frequent perturbation such as the Gorkha earthquake further suggests that our estimates are representative of at least interglacial timescales. Carbon fluxes in the central Himalaya did not change after the 2015 Gorkha earthquake and its accompanying landslides, according to observations of riverine sediment and carbon fluxes over four monsoon seasons spanning the event.

Burial of terrestrial organic carbon (OCterr) in marginal sea sediments is a key component of the carbon cycle, exerting long-term influence on atmospheric CO2 and climate. Assessment of the burial efficiency of OCterr is of key importance, yet remains poorly constrained due to current gaps in our knowledge of mechanistic controls, including the influence of organic matter composition and age on the fate of OCterr in marine sediments. We measured bulk characteristics (δ13C and Δ14C of OC; mineral surface area, SA; grain size; n = 98 samples) and biomarker carbon isotopic compositions (fatty acid δ13C and Δ14C; n = 11) of a suite of surface sediment samples from the Bohai Sea and Yellow Sea (BS-YS). Combining with published results, bulk OC (n = 234) and biomarker (n = 19) carbon isotopic data are used to examine spatial variability in the sources and ages of OCterr in this shallow marginal sea system. Biomarker carbon isotopic values are used to constrain endmember values, and a dual carbon isotope mixing model was applied to all bulk samples to develop a spatially explicit assessment of contributions of different pools of OCterr. Although highly spatially variable, pre-aged OC (OCpre-aged) and fossil OC (OCfossil) when combined (i.e., “non-modern OC”) on average accounted for 51 ± 10% of the OC in BS-YS surface sediments. This was equivalent to the proportion of OCterr (ave., 51 ± 14%) estimated from a δ13COC binary mixing model, suggesting OCterr in the mixed layer of BS-YS sediments is predominantly composed of millennial-aged OC. The burial potential of pool-specific OCterr was then evaluated through comparison of corresponding mineral SA-normalized loadings of surface sediments with those of Yellow River suspended particulate matter. Both the regression slope and the arithmetic mean value approaches reveal high burial potential for all terrestrial OC pools delivered by Yellow River, which is attributed to the aged and refractory nature of OC exported from the Yellow River. However, differing relationships of pool-specific OCterr loadings between BS-YS and Yellow River sediments imply contrasting fates. In particular, we find that the burial potential of OCpre-aged is consistently greater than that of OCfossil. This surprising observation suggests either enhanced degradation of OCfossil during lateral transport (possibly as a consequence of different mineral associations and hydrodynamic sorting effects), or additional sources for OCpre-aged, such as from small rivers or aerosol deposition. Overall, OC age, mineralogical composition and hydrological settings contribute to the complex patterns of OC residing in northern China marginal sea surface sediments. This novel molecular-isotopic approach reveals significant spatial variability in proportions, contents and burial potential among terrestrial OC pools, and underlines the importance of considering organic matter age and composition in understanding and constraining the fate of OCterr in marine sedimentary environments., Geochimica et Cosmochimica Acta, 309, ISSN:0016-7037, ISSN:1872-9533

Estuaries are action zones for organic carbon (OC) degradation and aging. These processes influence the nature of terrestrial OC (OCterr) export and the magnitude of OCterr burial in marginal seas, with important environmental implications such as CO2 release and hypoxia. In this study, we determined the contents and carbon isotopic compositions (C-13 and C-14) of bulk OC and fatty acids (FAs) as well as the sedimentological characteristics of suspended particulate matter (SPM) samples collected from two sites over four seasons and of surface sediment samples from three sites in the Pearl River estuary (PRE) to evaluate processes controlling OCterr degradation and aging along an estuarine gradient. We found that the abundance-weighted average C(24-32)FA C-14 ages increased by an average of similar to 1170 years for SPM and by an average of similar to 3440 years in PR/PRE sediments, along the similar to 60 km PRE transect. These increases in the FA age coincided with an 86% decrease in the corresponding mineral surface area-normalized FA loading along the sediment transport pathway, implying that selective degradation of labile and younger OC resulted in apparent OC aging. These measurements reveal an important shift in the nature of OC, with implications for biogeochemical cycling within estuaries and for regional environmental changes. ISSN:0013-936X ISSN:1520-5851

Glycerol Dialkyl Glycerol Tetraethers (GDGTs) are ubiquitous biomolecules whose structural diversity or isotopic composition is increasingly used to reconstruct environmental changes such as air temperature or pCO2. Isoprenoid GDGTs (iGDGT), in particular GDGT-0, are biosynthesized by a large range of Archaea. To assess the potential of GDGT-0 as a tracer of past methane cycle variations, three sediment cores from the Mackenzie River Delta have been studied for iGDGT and diploptene concentration, distribution and stable carbon signature. The absence of crenarchaeol, high GDGT-0 vs crenarchaeol ratio values, and 13C-enriched carbon signature of GDGT-0 indicate production by acetoclastic methanogens as well as heterotrophic Archaea. The oxidation of methane seems to be dominated by bacteria as indicated by the high abundance of 13C-depleted diploptene. Branched GDGTs (brGDGT), thought to be produced by heterotrophic bacteria, are dominated by hexa- and penta-methylated 5- and 6-methyl compounds. The presence of 5,6-methyl isomer IIIa’’ points towards in situ production of brGDGTs, with only a minor input from soil branched GDGT brought by the Mackenzie River. Carbon isotopic compositions of brGDGTs are in agreement with heterotrophic producers, likely living during summer. The reconstructed temperatures using a global lake calibration reflect recorded summer air temperature (± 2 °C) during the last 60 years, and further highlight the absence of warming in summer in this region during the last 200 years. Oxygen availability and connection time to the Mackenzie River also seem to control the distribution of brGDGT with an increase in 6-methyl and 5,6-methyl isomers during periods of increased anoxia., Organic Geochemistry, 157, ISSN:0146-6380

Tropical rivers deliver ∼60% of particulate organic carbon to the world ocean. However, compositions and loadings of sedimentary organic matter (OM) from tropical small mountainous rivers are largely unknown. Here, we provide an initial constrain on sources of sedimentary OM from 28 fluvial systems across Luzon in the Philippines by measuring total organic carbon (TOC), total nitrogen (TN), stable carbon isotope (δ13C) and radiocarbon activity of TOC (expressed as fraction modern—Fm), as well as grain size and mineral surface area (SA) of sediments. Results indicate that sediments in Luzon rivers contain both contemporary and 14C-depleted OM (Fm: 0.71–1.06, mean 0.97 ± 0.07) with a wide range of δ13C values (−28.3‰ to −17.7‰, −24.9 ± 2.2‰). This is attributed to the OM sources originated from modern surface soil and 14C-depleted subsoil and deep soil vegetated by C3 and C4 plants, with mean fraction of C3 plant at 80% ± 11%. Minor input from bedrock may also contribute to the 14C-depleted OM in sediments, accounting for 6% ± 6%. Sediments in most rivers are featured by low OC loadings (OC/SA ratio < 0.4 mg C m−2), owing either to a less OM input or intensive OM degradation. The estimated yields of particulate OC from Luzon vary between 3.2 and 3.7 t km−2 yr−1, which is higher than most tropical large rivers. © 2021 American Geophysical Union ISSN:0148-0227 ISSN:2169-8953 ISSN:2169-8961

ISSN:0148-0227 ISSN:2169-8953 ISSN:2169-8961

Perylene is a frequently abundant, and sometimes the only polycyclic aromatic hydrocarbon (PAH) in aquatic sediments, but its origin has been subject of a longstanding debate in geochemical research and pollutant forensics because its historical record differs markedly from typical anthropogenic PAHs. Here we investigate whether perylene serves as a source-specific molecular marker of fungal activity in forest soils. We use a well-characterized sedimentary record (1735-1999) from the anoxic-bottom waters of the Pettaquamscutt River basin, RI to examine mass accumulation rates and isotope records of perylene, and compare them with total organic carbon and the anthropogenic PAH fluoranthene. We support our arguments with radiocarbon (

Constraints on timescales of lateral transport of sedimentary organic carbon (OC) over continental shelves and associated influences on the distribution and abundance of OC remain sparse. Preferential degradation of labile, young OC during lateral transport results in apparent “diagenetic aging” of OC. Additionally, sediment translocation can also result in “transport time-associated aging” of associated organic matter (OM) as a function of the lateral transport time (LTT). Here, we use a coupled thermal decomposition and radiocarbon (14C) approach to constrain timescales of lateral transport and concomitant loss of OC associated with different grain size fractions of sediments collected from two locations ∼275 km apart along a dispersal pathway on the inner shelf of the East China Sea. The 14C age contrasts between corresponding thermal fractions are used to distinguish these two components of sedimentary OM “aging”. To minimize interferences from hydrodynamic sorting and diagenetic aging of OC accompanying lateral transport, we assess 14C age differences of decomposition products from the most thermally-refractory OC components associated with specific grain size fractions between locations. We show that LTTs vary among different grain size fractions, and examine relationships between LTTs and sedimentary OC loss in order to assess the decomposition of OC as a consequence of lateral transport. We suggest that the decomposition of OC associated with protracted lateral transport exerts a strong influence on OC burial efficiency, with broad implications for carbon cycling over continental shelves., Organic Geochemistry, 128, ISSN:0146-6380

Natural and human-induced hydrological changes can influence organic carbon (OC) composition in fluvial systems, with biogeochemical consequences in both terrestrial and marine environments. Here, we use bulk and molecular carbon isotopes (13C and 14C) to examine spatiotemporal variations in particulate OC (POC) composition and age from two locations along the course of the Yellow River during 2015 and 2016. Dual carbon isotopes enable deconvolution of modern, pre-aged (millennial age) soil, and fossil inputs, revealing heterogeneous OC sources at both sites. Pre-aged OC predominated at the upstream site (Huayuankou) throughout the study period, mostly reflecting the upper riverine OC. Strong downstream (Kenli) intra-annual variations in modern and pre-aged OC were caused by increased contributions from modern aquatic OC production under the drier and less turbid conditions during this El Nino year. The month of July, which included the human-induced water and sediment regulation (WSR) event at Kenli, accounted for 82% of annual POC flux, with lower modern OC contribution compared with periods of natural seasonal variability. Both natural and human-induced hydrological events clearly exert strong influence on both fluxes and composition of Yellow River POC which, in turn, affect the balance between OC remineralization and burial for this major fluvial system.

Relatively little is known about the amount of time that lapses between the photosynthetic fixation of carbon by vascular land plants and its incorporation into the marine sedimentary record, yet the dynamics of terrestrial carbon sequestration have important implications for the carbon cycle. Vascular plant carbon may encounter multiple potential intermediate storage pools and transport trajectories, and the age of vascular plant carbon accumulating in marine sediments will reflect these different pre-depositional histories. Here, we examine down-core 14C profiles of higher plant leaf wax-derived fatty acids isolated from high fidelity sedimentary sequences spanning the so-called “bomb-spike” and encompassing a ca. 60-degree latitudinal gradient from tropical (Cariaco Basin), temperate (Saanich Inlet), and polar (Mackenzie Delta) watersheds to constrain integrated vascular plant carbon storage/transport times (“residence times”). Using a modeling framework, we find that, in addition to a “young” (conditionally defined as < 50 y) carbon pool, an old pool of compounds comprises 49 to 78 % of the fractional contribution of organic carbon (OC) and exhibits variable ages reflective of the environmental setting. For the Mackenzie Delta sediments, we find a mean age of the old pool of 28 ky (±9.4, standard deviation), indicating extensive pre-aging in permafrost soils, whereas the old pools in Saanich Inlet and Cariaco Basin sediments are younger, 7.9 (±5.0) and 2.4 (±0.50) to 3.2 (±0.54) ky, respectively, indicating less protracted storage in terrestrial reservoirs. The “young” pool showed clear annual contributions for Saanich Inlet and Mackenzie Delta sediments (comprising 24% and 16% of this pool, respectively), likely reflecting episodic transport of OC from steep hillside slopes surrounding Saanich Inlet and annual spring flood deposition in the Mackenzie Delta, respectively. Contributions of 5–10 year old OC to the Cariaco Basin show a short delay of OC inflow, potentially related to transport time to the offshore basin. Modeling results also indicate that the Mackenzie Delta has an influx of young but decadal material (20–30 years of age), pointing to the presence of an intermediate reservoir. Overall, these results show that a significant fraction of vascular plant C undergoes pre-aging in terrestrial reservoirs prior to accumulation in deltaic and marine sediments. The age distribution, reflecting both storage and transport times, likely depends on landscape-specific factors such as local topography, hydrographic characteristics, and mean annual temperature of the catchment, all of which affect the degree of soil buildup and preservation. We show that catchment-specific carbon residence times across landscapes can vary by an order of magnitude, with important implications both for carbon cycle studies and for the interpretation of molecular terrestrial paleoclimate records preserved in sedimentary sequences.

Significance Terrestrial organic-carbon reservoirs (vegetation, soils) currently consume more than a third of anthropogenic carbon emitted to the atmosphere, but the response of this “terrestrial sink” to future climate change is widely debated. Rivers export organic carbon sourced over their watersheds, offering an opportunity to assess controls on land carbon cycling on broad spatial scales. Using radiocarbon ages of biomolecular tracer compounds exported by rivers, we show that temperature and precipitation exert primary controls on biospheric-carbon turnover within river basins. These findings reveal large-scale climate control on soil carbon stocks, and they provide a framework to quantify responses of terrestrial organic-carbon reservoirs to past and future change., Terrestrial vegetation and soils hold three times more carbon than the atmosphere. Much debate concerns how anthropogenic activity will perturb these surface reservoirs, potentially exacerbating ongoing changes to the climate system. Uncertainties specifically persist in extrapolating point-source observations to ecosystem-scale budgets and fluxes, which require consideration of vertical and lateral processes on multiple temporal and spatial scales. To explore controls on organic carbon (OC) turnover at the river basin scale, we present radiocarbon (14C) ages on two groups of molecular tracers of plant-derived carbon—leaf-wax lipids and lignin phenols—from a globally distributed suite of rivers. We find significant negative relationships between the 14C age of these biomarkers and mean annual temperature and precipitation. Moreover, riverine biospheric-carbon ages scale proportionally with basin-wide soil carbon turnover times and soil 14C ages, implicating OC cycling within soils as a primary control on exported biomarker ages and revealing a broad distribution of soil OC reactivities. The ubiquitous occurrence of a long-lived soil OC pool suggests soil OC is globally vulnerable to perturbations by future temperature and precipitation increase. Scaling of riverine biospheric-carbon ages with soil OC turnover shows the former can constrain the sensitivity of carbon dynamics to environmental controls on broad spatial scales. Extracting this information from fluvially dominated sedimentary sequences may inform past variations in soil OC turnover in response to anthropogenic and/or climate perturbations. In turn, monitoring riverine OC composition may help detect future climate-change–induced perturbations of soil OC turnover and stocks.

Ongoing warming of the Northern high latitudes has intensified abrupt thaw processes throughout the permafrost zone. The resulting terrain disturbances are prone to release large amounts of particulate organic matter (OM) from deeper permafrost soils with thus far poorly constrained decay kinetics. Organo-mineral interactions may inhibit OM decomposition, thereby mediating the release of carbon to the atmosphere. Yet how these interactions evolve upon release and during transport along the fluvial continuum is still insufficiently understood. Here we investigate the mobilization of particulate OM from disturbed permafrost soils to the aquatic environment in the Zackenberg watershed in Northeastern Greenland. We collected soil samples in a thermo-erosion gully and a retrogressive thaw slump, as well as suspended solids and stream sediments along the glacio-nival Zackenberg River, including its tributaries, and a small headwater stream (Grænselv) affected by abrupt permafrost thaw. To evaluate the organic and mineral material transported, we compare mineral element and organic carbon (OC) concentrations, bulk carbon isotopes (13C and 14C), together with source-specific molecular biomarkers (plant-wax lipids and branched glycerol dialkyl glycerol tetraethers, brGDGTs) for the suspended load with their soil and sediment counterparts. Preliminary results show large contrasts in OC concentrations as well as Δ14C between the glacio-nival river and the headwater stream, as well as between the different thaw features. The retrogressive thaw slump mobilizes relatively OC-poor material with very low Δ14C signatures suggesting a petrogenic contribution, while soil samples from the thermo-erosion gully had higher OC concentrations and Δ14C values. For Grænselv, Δ14C values of the particulate OC were lower close to the eroding stream bank, whereas the Zackenberg main stem displayed fairly constant Δ14C values, with some of the Zackenberg tributaries delivering relatively organic-rich particles low in Δ14C. Molecular biomarker analyses will provide additional information on specific OM sources, while X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) analyses on the soils, sediments and suspended mineral load will give more detailed insights into the composition of the mineral matrices. By combining these analytical methods, we aim to improve our understanding of the interactions between minerals and OM and thereby help to constrain the fate of mobilized OM upon permafrost thaw., EGUsphere

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., Journal of Geophysical Research: Biogeosciences, 126 (3), ISSN:0148-0227, ISSN:2169-8953, ISSN:2169-8961

Summary Organic matter (OM) in marine sediment is usually associated with mineral surfaces because it offers protection from degradation. As the amount of OM on a particle increases, so does its grain size, but its density decreases. The reduced density of the surface sediment results in a high propensity for resuspension, therefore aggregate transport represents an efficient way to relocate OM to offshore sites and depocentres on the continental slope. This process has significant effects on reconstructing past environments as OM is laterally displaced from where it was produced. OM abundance in sediments is negatively correlated to oxygenation state; hence, oxygen minimum zones are often characterised by a soft, soupy sediment surface. We study samples from the Benguela upwelling system offshore Namibia, where widespread lateral transport has been described and significant age differences are reported between compounds (alkenones, crenarchaeol), calcareous shells, and bulk organic carbon. Working with the comprehensive dataset on environmental tracers on the Benguela upwelling system, we use bulk molecular tracers on core-top samples and their respective grain size and density fractions to trace resuspension effects on sedimentary OC ageing in anoxic and oxic water condition.

Hydrogen isotope ratios of leaf waxes are used to reconstruct past hydroclimate because they are correlated to meteoric/growth water hydrogen isotopes. The interpretation of these signatures from ancient sedimentary archives relies on a thorough understanding of the drivers of modern isotope variability. Studies in the high latitudes, regions that are particularly valuable in light of their vulnerability to rapid climate change, are scarce. We studied modern vegetation (22 plants) in two areas in the Northwestern Territories (Canada): Herschel Island and Peel Plateau, to understand the stable isotope variability found in plants of Arctic regions. Bulk biomass stable carbon isotope and radiocarbon composition have been measured as well as fatty acids (wax lipids coating the plant leaves) stable carbon and hydrogen isotopes. Furthermore, lake surface sediments and river bank sediments from the Mackenzie River Delta (surrounded by the same plants) have been similarly studied. Bulk carbon isotope composition of the plants show strong difference between plant type, i.e. herbs, shrubs, lichen and moss, as shown in previous studies. Whereas the commonly used average chain length (ACL) is not useful to differentiate the plants. In term of compound-specific isotope ratios, herbs are generally 2H-enriched in comparison to shrubs as shown in other regions of the world, and the C28 fatty acid present the most differences amongst plant type (from ~ -207‰ for herbs to ~ – 240‰ for shrubs). No major difference between the areas is noted indicating that the ~ 250 km (Herschel Island 69.5⁰N and Peel Plateau 67.3⁰N) have no impact on the hydrogen isotope composition of the fatty acids. As such we decided to compare the plant with the lake surface sediments (from the Mackenzie Delta, located between Herschel Island and the Peel Plateau). Short-chain fatty acids, sourced from organisms growing in the lake, from isolated lakes shows 2H-enriched isotopic values indicating the effect of increased evaporation in the lake during summer plant growth. Whereas long-chain fatty acids do not show any differences and are enriched compared to the shorter-chain (~ -260‰ for long-chain vs ~ -260‰ to - 280‰ for short-chain). In conclusion, differences between plant fatty acids seems to be best represented by the C28 fatty acid, indicating the potential to reconstruct past vegetation and hydrological conditions in the region using lacustrine archives., EGUsphere

Climate-driven thawing of Arctic permafrost renders its vast carbon reserves susceptible to microbial degradation, serving as a potentially potent positive feedback hidden within the climate system. While seemingly intuitive, the relationship between thermally driven permafrost losses and organic carbon (OC) export remains largely unexplored in natural settings. Filling this knowledge gap, we present down-core bulk and compound-specific radiocarbon records of permafrost change from a sediment core taken within the Alaskan Colville River delta spanning the last c. 2,700 years. Fingerprinted by significantly older radiocarbon ages of bulk OC and long-chain fatty acids, these data expose a thermally driven increase in permafrost OC export and/or deepening of mobilizable permafrost layers over the last c. 160 years after the Little Ice Age. Comparison of OC content and radiocarbon data between recent and Roman warming episodes likely implies that the rate of warming, alongside the prevailing boundary conditions, may dictate the ultimate fate of the Arctic's permafrost inventory. Our findings highlight the importance of leveraging geological records as archives of Arctic permafrost mobilization dynamics with temperature change., AGU Advances, 2 (2), ISSN:2576-604X

Permafrost thaw in Arctic watersheds threatens to mobilize hitherto sequestered carbon. We examine the radiocarbon activity (F14C) of dissolved organic carbon (DOC) in the northern Mackenzie River basin. From 2003–2017, DOC‐F14C signatures (1.00 ± 0.04; n = 39) tracked atmospheric 14CO2, indicating export of “modern” carbon. This trend was interrupted in June 2018 by the widespread release of aged DOC (0.85 ± 0.16, n = 28) measured across three separate catchment areas. Increased nitrate concentrations in June 2018 lead us to attribute this pulse of 14C‐depleted DOC to mobilization of previously frozen soil organic matter. We propose export through lateral perennial thaw zones that occurred at the base of the active layer weakened by preceding warm summer and winter seasons. Although we are not yet able to ascertain the broader significance of this “anomalous” mobilization event, it highlights the potential for rapid and large‐scale release of aged carbon from permafrost., Key Points A widespread pulse of aged dissolved organic carbon (DOC) occurred in the Mackenzie River and its tributaries in June 2018Export of aged DOC is consistent with a prolonged warming period and the formation of supra‐permafrost taliksMobilization of aged DOC and nitrate suggests percolation of supra‐permafrost groundwater through previously frozen soil layers

A novel technique making use of laser ablation coupled online to accelerator mass spectrometry (LA-AMS) allows analyzing the radiocarbon (14C) concentration in carbonate samples continuously at high spatial resolution within very short analysis times. This new technique can provide radiocarbon data similar to the spatial resolution of stable carbon (C) isotope measurements by isotope-ratio mass spectrometry (IRMS) and, thus, can help to interpret δ13C signatures, which otherwise are difficult to understand due to numerous processes contributing to changes in C-isotope changes ratios. In this work we analyzed δ13C and 14C on the Holocene stalagmite SPA 127 from the high-alpine Spannagel Cave (Austria). Combined stable carbon and radiocarbon profiles allow to identify three growth periods characterized by different δ13C signatures: (i) the period > 8 ka BP is characterized by relatively low δ13C values with small variability combined with a comparably high radiocarbon reservoir effect (expressed as dead carbon fraction, dcf) of around 60 %. This points towards C contributions of host rock dissolution and/or from an old organic matter (OM) reservoir in the karst potentially mobilized due to the warm climatic conditions of the early Holocene. (ii) Between 3.8–8 ka BP a strong variability in δ13C reaching values from −8 to +1 ‰ with a generally lower dcf was observed. The δ13C variability is most likely caused by changes in gas exchange processes in the cave, which are induced by reduced drip rates as derived from reduced stalagmite growth rates. Additionally, the lower dcf indicates that the OM reservoir is contributing less to stalagmite growth in this period possibly as a result of reduced precipitation or because it is exhausted. (iii) In the youngest section between 2.4–3.8 ka BP, comparably stable and low δ13C values combined with an increasing dcf reaching up to 50 % are again hinting towards a contribution of an aged OM reservoir in the karst., Climate of the Past Discussions, ISSN:1814-9340, ISSN:1814-9359

Despite the potential importance in the oceanic carbon cycle and benthic ecosystem, global feature of lateral supply of aged organic matter hosted on lithogenic particles derived from sediment resuspension has not been systematically examined. We compiled concentrations and fluxes of lithogenic material in the ocean in a global-scale by using literature data of sediment trap studies to understand the contribution of resuspended sediment to sinking particulate matter. We find that these contributions are significant in various oceanic settings, particularly over continental margins. Lithogenic material flux decreased with increasing distance from the margins and above the seafloor. Examination of Δ14C values of sinking POC revealed strong relationships with parameters that represent contribution of resuspended sediment. We then derive estimates for the contribution of aged POC from sediment resuspension to sinking POC based on these relationships and global lithogenic material flux data.

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. ISSN:0094-8276 ISSN:1944-8007

The biological carbon pump in the southwestern East Sea (Japan Sea, EJS hereafter) was investigated based on examination of sinking particulate matter samples intercepted by bottom-tethered sediment traps deployed on a mooring at three depths (500, 1000, and 2000 m) between 2011 and 2017. The total particle flux increased as the sampling depth increased, while particulate organic carbon (POC) flux was greatest at 500 m. The lithogenic material content was high at all depths, and accounted for an average of ~42 % of the particulate matter. The total particle flux at all sampling depths consistently shifted towards much higher values in 2014-2016. During this period, the POC flux at 500 m increased by 32 % while net primary production (NPP) increased only slightly. Consequently, the POC flux/NPP ratio increased significantly, indicating greater biological pump efficiency than in earlier years of the study. The flux of lithogenic material derived primarily from sediment resuspension was much greater at 500 m in 20142016 compared with previous years, implying its potential role as a ballast mineral in enhancing particle export and transfer to the deep sea interior. The radiocarbon isotope ratio of POC was higher, and the excess Mn content values were lower at 500 m in 2014-2016, suggesting that the resuspended sediment at 500 m likely originated from a shallow region during this period, and differed in provenance from the lithogenic material intercepted at 2000 m. The period of enhanced particle flux coincided with the 2015/16 El Nino and a mesoscale warm (anticyclonic) eddy that persisted for two years in the study region. The East Korean Warm Current and the eddy may have facilitated the transport of resuspended particles entrained in the Korea Strait and/or the western shelf and upper slope of the basin to the study site.

About one-third to half of the global soil carbon is held in the top 1-3 m of tundra+taiga permafrost PF (~1000 Pg-C) with deeper layers below as Deep-PF (~400 Pg-C) and in Pleistocene Ice Complex Deposit permafrost (ICD-PF, ~400 Pg-C), lining 4000 km of the East Siberian Arctic coast. In order to overcome the landscape heterogeneity and the stochastic nature of e.g. erosional release processes, we use the East Siberian Arctic Shelf (ESAS) in an inverse approach – as a natural integrator of the TerrOM releases from both the river drainage basins and from the erosion of ICD-containing bluffs. We are exploring how source-dependent transport and translocated degradation affect the released TerrOM. The sources of released terrOM have been increasingly constrained using great rivers and the ESAS as natural integrators through a combination of biomarkers and δ13C/Δ14C on bulk-C and on compound level. There are significant gradients in sources both E-W and S-N across each shelf sea and between water column DOM, POM and sedimentary OM. The largest source of OC to ESAS sediments is not rivers or marine plankton – it is coastal erosion of old ICD. Our initial limited dataset has now been much expanded, as has the end-member database while the statistical source apportionment method has been refined. They combine to show more efficient cross-shelf transport of river-borne “topsoil-PF” compared to ICD-PF and a clear distinction in sources of TerrOM between western and eastern ESAS regimes separated roughly along 165E, consistent with the local oceanography.There have been good strides also in understanding degradation of TerrOM exported to ESAS. Studies are demonstrating continuous offshoreward degradation of all TerrOM, yet with large differences between compound classes. Physical association of TerrOM with different sediment components, and sorting of the sediments exert first-order control on TerrOM distribution and degradation. An expanded dataset on specific surface area (SSA) and CuO oxidation products reveals spatial patterns across ESAS. The combination of compound-specific radiocarbon analysis of terrestrial biomarkers with SSA-normalized TerrOM signals constrains the ambient degradation rates and fluxes during the 3-4000 year timescale of cross-shelf transport. The degradation of TerrOM also causes severe ocean acidification of the ESAS.Investigations of sources and fate of TerrOM on the ESAS – the World’s largest shelf sea– provides a window to constrain permafrost-C remobilization and to study mechanisms of transport and degradability of different components of released terrestrial organic matter.

Soil temperatures in permafrost (i.e. perennially frozen ground) are rising globally. The increasing temperatures accelerate permafrost thaw and release of organic carbon, that has been locked in permafrost soils since the last glacial period, to the contemporary carbon cycle. The potential remineralisation of organic carbon to greenhouse gases can contribute to further climate warming. Particulate organic carbon (POC) in the Kolyma River is older than dissolved organic carbon (DOC) thus serves as a good tracer for abrupt permafrost thaw (i.e. river bank erosion and thermokarst) that dominantly releases old POC. While dissolved organic carbon (DOC) mobilised from the old Yedoma outcrops on the banks of the Kolyma River is shown to be highly labile, vulnerability of POC to biodegradation is not yet known. In this study we aim to constrain degradation rates for POC in the Kolyma River. To capture seasonal variability of the POC pool and its degradation rate the incubation was conducted both during the spring freshet and in late summer (2019 and 2018, respectively). We incubated whole-water samples over 9 to 15 days and quantified POC (and DOC) loss over time, as well as dissolved inorganic carbon (DIC). The incubation was carried out in the dark. We also tracked changes in POC composition and age with carbon isotopes (d13C-OC, d13C-DIC, ∆14C). Preliminary results from 2018 suggest a decrease in POC concentrations of up to 30 % while those of DOC decrease by up to 11 %. The rate of POC degradation is nearly three times faster than DOC though the absolute amounts of DOC are in turn higher than those of POC (< 1 mg L-1 for POC and ~3 mg L-1 for DOC). Furthermore, the changes in d13C of POC, DOC and DIC suggest ongoing microbial degradation and conversion of organic carbon into inorganic carbon. These first estimates show that POC degrades fairly rapidly while transported in the Kolyma River. A better understanding of POC degradation along lateral flow paths is critical for improving our knowledge of permafrost thaw and its possible climate impacts in the future., EGUsphere

Ongoing rapid arctic warming leads to extensive permafrost thaw, which in turn increases the hydrologic connectivity of the landscape by opening up subsurface flow paths. Suspended particulate organic matter (POM) has proven useful to trace permafrost thaw signals in arctic rivers, which may experience higher organic matter loads in the future due to expansion and increasing intensity of thaw processes such as thermokarst and river bank erosion. Here we focus on the Kolyma River watershed in Northeast Siberia, the world's largest watershed entirely underlain by continuous permafrost. To evaluate and characterize the present‐day fluvial release of POM from permafrost thaw, we collected water samples every 4–7 days during the 4‐month open water season in 2013 and 2015 from the lower Kolyma River mainstem and from a small nearby headwater stream (Y3) draining an area completely underlain by Yedoma permafrost (Pleistocene ice‐ and organic‐rich deposits). Concentrations of particulate organic carbon generally followed the hydrograph with the highest concentrations during the spring flood in late May/early June. For the Kolyma River, concentrations of dissolved organic carbon showed a similar behavior, in contrast to the headwater stream, where dissolved organic carbon values were generally higher and particulate organic carbon concentrations lower than for Kolyma. Carbon isotope analysis (δ13C, Δ14C) suggested Kolyma‐POM to stem from both contemporary and older permafrost sources, while Y3‐POM was more strongly influenced by in‐stream production and recent vegetation. Lipid biomarker concentrations (high‐molecular‐weight n‐alkanoic acids and n‐alkanes) did not display clear seasonal patterns, yet implied Y3‐POM to be more degraded than Kolyma‐POM., Key Points A permafrost‐underlain large arctic river and a first‐orderstream in NE Siberia were sampled every 4–7 days during the open water seasonContrasting seasonal patterns and concentrations of particulate and dissolved organic carbon are observed for the two catchmentsKolyma particulate organic matter is generally older, yet according to molecular proxies less degraded thanmaterial from the small stream

Soil erosion in high mountain ranges plays an important role in redistributing soil organic carbon across landscapes and may influence the global climate on different timescales [1, 2]. Here, we investigate the dynamics of soil organic matter export in the steep mountain belt of the Himalaya by tracing the provenance of soil-derived lipids in riverine sediments from nested catchments with areas ranging from 370 to 57700 km2. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are a suite of lipids that occur ubiquitously in soils [3, 4]. Their isomer distribution depends on environmental parameters such as the mean annual temperature of the local environment [3]. In this study, we explore the use of brGDGT distributions as a proxy for the altitudinal provenance of soil organic matter in riverine sediments of the Central Himalaya of Nepal. BrGDGT distributions in soils collected along an altitudinal profile, spanning elevations from 200 to 4450 m asl, yield a robust calibration of soil signatures as a function of elevation. This calibration is then used to trace the provenance of soil organic matter exported from their catchments and entrained in suspended sediments of rivers draining the Central Himalaya. We show that brGDGT compositions of fluvial sediments accurately reflect the mean elevation of the soil-cover in their respective watersheds. The type of land-cover does not seem to have a significant influence on the export of organic matter at a catchment scale. We, therefore, conclude that soil organic matter mobilization in the Himalaya occurs pervasively, and is currently insensitive to anthropogenic perturbations., EGUsphere

Arctic rivers will be increasingly affected by the hydrological and biogeochemical effects of thawing permafrost. During transport, permafrost thaw-derived organic carbon (OC) can be degraded into greenhouse gases and potentially add to further climate warming, or transported to the shelf seas and buried in marine sediments, attenuating this ‘permafrost carbon feedback’. To assess the transport pathways and fate of permafrost-OC, we focus on the river-shelf continuum of the Kolyma River, the largest river on Earth completely underlain by continuous permafrost. Three pools of riverine OC were investigated: dissolved OC (DOC), suspended particulate OC (POC), and river sediment OC (SOC). Preliminary results of bulk carbon isotopes (δ13C, Δ14C) and molecular biomarkers (lignin phenols, leaf wax lipids) show contrasts in composition and degradation state for these carbon pools. Old permafrost-OC seems to be mostly associated with SOC, and less dominant in POC. However, while SOC shows the oldest Δ14C signal, lignin phenol results (e.g., acid to aldehyde ratios) suggest this material is the least degraded. In contrast, DOC shows more degraded signal, even at the outflow of an active permafrost thaw site. Our study serves as a terrestrial extension to earlier investigated marine sediments from the Kolyma paleoriver transect in the East Siberian Sea. It also highlights the value of connecting terrestrial and marine observations to gain insight into the complete pathway of permafrost-OC, from the moment of thaw, via aquatic transport and degradation, towards storage in marine sediments.

With Arctic warming, both gradual and abrupt thaw of permafrost may trigger a positive feedback loop, since large amounts of organic matter (OM) are released into rivers and thus exposed to mineralization along the fluvial continuum. Both dissolved (DOM) and particulate organic matter (POM) mineralization during lateral transport generates greenhouse gases that may fuel further global warming. In addition to glacier retreat, the extent of permafrost thaw is predicted to increase across the Arctic, which will change the release of DOM and POM to aquatic environments. However, the fate of DOM and POM will likely differ during transport in surface waters due to POM-DOM exchange and biodegradation control from organo-mineral interactions. The contrasting behavior between POM and DOM may affect the strength of the permafrost-carbon feedback to climate but is currently afflicted with high uncertainties. This study characterizes the export of DOM and POM along the fluvial continuum at time of maximum thaw depth and investigates the impacts of permafrost thaw on OM composition and age in the Zackenberg watershed (Northeastern Greenland). In August 2019, streams were sampled twice, before and after a rain event. We collected water and suspended sediments from rivers, the river delta, snow patches and permafrost ice from thermokarst features. Besides in situ river chemistry, we analyzed stable water isotopes (δ18O, δ2H) and dissolved organic carbon (DOC) concentrations. The composition of DOM was characterized using absorbance and fluorescence spectroscopy and both DOM and POM were analyzed for radiocarbon (Δ14C). DOC concentrations increase from 3.1 mg L-1 upstream to 15.6 mg L-1 after the confluence with the main tributaries, which are characterized by a nival river regime, and decreased to 4.3 mg L-1 at the outlet. Optical properties of DOM highlight that low molecular weight microbial-derived organic compounds contribute most to the fluorescent DOM (fDOM) in the upstream part of the river, likely originating from glacial waters. The contribution of soil and plant derived fDOM increases downstream, and corresponding Δ14CDOC values increase from upstream (-240‰, i.e. ~2200 yr) to downstream (-30‰, i.e. ~200 yr) resulting from the increasing tributary inputs. Interestingly, POM displays more depleted Δ14C (older ages) than DOC.We observed contrasting patterns in river chemistry before and after the rain event with temperature decreasing and pH and EC increasing. δ18O and δ2H compositions were less depleted after the rain event, DOC concentrations were lower and DOM displayed a greater contribution of soil and plant derived fDOM. This evidence illustrates the increasing contribution of rain fed streams draining organic-rich top soil and the dilution of the glacial inputs after the rain event. We conclude that, in this glacio-nival Arctic watershed, affected by both permafrost degradation and glacier retreat, old DOM and POM is released and evolves differently in the fluvial continuum.

Weathering, erosion, and redeposition of exhumed rock-derived or “petrogenic” organic carbon (OC) co-occurs with the burial of biospheric OC within sediments, modulating atmospheric CO2 and O2 over geologic time. Disentangling the geochemical fingerprint of petrogenic OC from biospheric OC in sedimentary organic matter, as well as quantifying the influence of its remineralization and burial on atmospheric CO2/O2, has been the focus of numerous observational and geochemical modeling studies. In 1938, Matti Sauramo recognized that petrogenic OC is entrained in a “simple carbon” cycle operating alongside the “complicated” greater rest of the carbon cycle. Sauramo9s achievements were preceded by Charles Lyell9s thoughts on the subject a century earlier, and by observations of reworked palynomorphs in the modern environment made by palynologists in the 19th Century. Towards the present, palynologists, organic petrologists, and geochemists have all made key advances, while their impact often did not radiate beyond their respective bodies of literature. This highlights the importance not only of further investigations focused on the continued pursuit of new information, but also on studies of the history of relevant disciplines in order to place new findings in appropriate context. Petrogenic OC cycling has emerged as a key process for constraining global carbon budgets, long-term biogeochemical cycles and associated variations in atmospheric chemistry. While petrogenic OC is now recognized as a significant component of bulk sedimentary OC in modern systems, its cycling throughout Earth9s history - including during pivotal episodes such as supercontinent amalgamation and late Proterozoic Snowball Earth events followed by greenhouse conditions - remains largely unexplored.

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 km2 region 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., Geophysical Research Letters, 45 (17), ISSN:0094-8276, ISSN:1944-8007

Understanding the effects of hydrodynamic forcing on organic matter (OM) composition is important for assessment of organic carbon (OC) burial in marginal seas on regional and global scales. Here we examine the relationships between regional oceanographic conditions (bottom shear stress), and the physical characteristics (mineral surface area and grain size) and geochemical properties (OC content [OC%] and carbon isotope compositions [13C, 14C]) of a large suite of surface sediments from the Chinese marginal seas to assess the influence of hydrodynamic processes on the fate of OM on shallow continental shelves. Our results suggest that 14C content is primarily controlled by organo‐mineral interactions and hydrodynamically driven resuspension processes, highlighted by (i) positive correlations between 14C content and OC% (and surface area) and (ii) negative correlations between 14C content and grain size (and bottom shear stress). Hydrodynamic processes influence 14C content due to both OC aging during lateral transport and accompanying selective degradation of OM associated with sediment (re) mobilization, these effects being superimposed on the original 14C characteristics of carbon source. Our observations support the hypotheses of Blair and Aller (2012, https://doi.org/10.1146/annurev‐marine‐120709‐142717) and Leithold et al. (2016, https://doi.org/10.1016/j.earscirev.2015.10.011) that hydrodynamically driven sediment translocation results in greater OC 14C depletion in broad, shallow marginal seas common to passive margin settings than on active margins. On a global scale, this may influence the extent to which continental margins act as net carbon sources and sinks. Our findings thus suggest that hydrodynamic processes are important in shaping the nature, dynamics, and magnitude of OC export and burial in passive marginal seas.

The long-term effects of deforestation on tropical forest soil carbon reservoirs are important for estimating the consequences of land use on the global carbon cycle, but are poorly understood. The Maya Lowlands of Mexico and Guatemala provide a unique opportunity to assess this question, given the widespread deforestation by the ancient Maya that began ~4,000 years ago. Here, we compare radiocarbon ages of plant waxes and macrofossils in sediment cores from three lakes in the Maya Lowlands to record past changes in the mean soil transit time of plant waxes (MTTwax). MTTwax indicates the average age of plant waxes that are transported from soils to lake sediments, and comparison of radiocarbon data from soils and lake sediments within the same catchment indicates that MTTwax reflects the age of carbon in deep soils. All three sediment cores showed a decrease in MTTwax, ranging from 2,300 to 800 years, over the past 3,500 years. This decrease in MTTwax, indicating shorter storage times for carbon in lake catchment soils, is associated with evidence for ancient Maya deforestation. MTTwax never recovered to pre-deforestation values, despite subsequent reforestation, implying that current tropical deforestation will have long-lasting effects on soil carbon sinks.

Sedimentary high molecular weight (HMW) n-alkyl lipids derived from the waxes of terrestrial plants are common target compounds in biogeochemical and paleoenvironmental research. These plant waxes derive predominantly from the epicuticular cover of vascular plant leaves and their relative and absolute abundances and stable isotopic composition can be used as proxies to decipher, e.g., continental climate and land-ocean carbon transfer processes. In marine sediments, however, compound-specific radiocarbon analysis has revealed that plant waxes are often not syn-depositional, but instead are substantially 14C-depleted (‘pre-aged’) upon deposition. This 14C-depletion can be caused by various processes that either promote retention of plant waxes during transport from source to sink such as storage in soils or entrainment in deposition-resuspension loops in rivers and on continental shelves or, alternatively, by processes that add HMW n-alkyl lipids from other sources (e.g., petrogenic inputs). Here, we review the intrinsic and extrinsic processes affecting the sedimentary plant wax 14C composition (ranging from chemical processes to continental-scale environmental conditions), how plant wax 14C compositions translate into mean ages, and which processes control plant wax mean ages in marine sediments. Finally, we use a compilation of available and new compound-specific plant wax 14C data to provide a synthesis and evaluate the major controls on plant wax mean ages in marine sediments at the global scale.

Humans have interacted with fire for thousands of years, yet the utilization of fossil fuels marked the beginning of a new era. Ubiquitous in the environment, pyrogenic carbon (PyC) arises from incomplete combustion of biomass and fossil fuels, forming a continuum of condensed aromatic structures. Here, we develop and evaluate 14C records for two complementary PyC molecular markers, benzene polycarboxylic acids (BPCAs) and polycyclic aromatic hydrocarbons (PAHs), preserved in aquatic sediments from a suburban and a remote catchment in the United States (U.S.) from the mid-1700s to 1998. Results show that the majority of PyC stems from local sources and is transferred to aquatic sedimentary archives on subdecadal to millennial time scales. Whereas a small portion stems from near-contemporaneous production and sedimentation, the majority of PyC (∼90%) experiences delayed transmission due to “preaging” on millennial time scales in catchment soils prior to its ultimate deposition. BPCAs (soot) and PAHs (precu...

The Arctic Ocean receives an estimated amount of 40 × 1012 g organic carbon (OC) through inflow of rivers every year (Holmes et al., 2012; McClelland et al., 2016). A large part of the Arctic Ocean watershed is underlain by permafrost that experiences widescale warming exposing more OC to thaw and degradation (Biskaborn et al., 2019). Arctic Rivers will be increasingly affected by the hydrological and biogeochemical effects of thawing permafrost. During transport, permafrost-OC can be degraded into greenhouse gasses and potentially add to further climate warming (Schuur et al., 2015). However, a significant amount of this OC is transported all the way to the shelf and is buried in marine sediments, attenuating greenhouse gas emissions (Vonk & Gustafsson, 2013). The East Siberian Arctic Shelf is the largest and shallowest shelf in the Arctic Ocean (Stein & MacDonald, 2004). It receives significant amounts of terrestrial OC, delivered through coastal erosion and fluvial input (Gustafsson et al., 2011; Sánchez-García et al., 2011; Vonk et al., 2012). This region is also warming rapidly, and is strongly affected by the loss of sea ice, increasing the open water extent and wave, storm and tidal impact on the coast (Biskaborn et al., 2019; IPCC, 2014). In this study, we focus on the biogeochemical cycling of OC along the river-shelf continuum of the Kolyma river, the largest river completely underlain by continuous permafrost. To quantify the flux of OC and to link its source to its potential sink in the East Siberian Sea, we have sampled dissolved OC, particulate OC and sediment OC along a transect from an active permafrost thaw site near Cherskiy towards the outer Kolyma delta near Ambarchik. On all of these samples isotopic (δ13C, Δ14C) and molecular (lipid biomarkers, lignin phenols) analyses will be used to characterise the OC and assess its degradation status. This, in combination with published data from the East Siberian Shelf close to the Kolyma delta and earlier studies on the Kolyma river, will give us insight in the complete pathway of permafrost-OC from the moment of thaw to degradation during transport and ultimate storage in marine sediments.

Speleothem organic matter can be a powerful tracer for past environmental conditions and karst processes. Carbon isotope measurements (δ13C and 14C) in particular can provide crucial information on the provenance and age of speleothem organic matter, but are challenging due to low concentrations of organic matter in stalagmites. Here, we present a method development study on extraction and isotopic characterization of speleothem organic matter using a rapid procedure with low laboratory contamination risk. An extensive blank assessment allowed us to quantify possible sources of contamination through the entire method. Although blank contamination is consistently low (1.7 ± 0.34 – 4.3 ± 0.86 μg C for the entire procedure), incomplete sample decarbonation poses a still unresolved problem of the method, but can be detected when considering both δ13C and 14C values. We test the method on five stalagmites, showing reproducible results on samples as small as 7 μg C for δ13C and 20 μg C for 14C. Furthermore, we find consistently lower non-purgeable organic carbon (NPOC) 14C values compared to the carbonate 14C over the bomb spike interval in two stalagmites from Yok Balum Cave, Belize, suggesting overprint of a pre-aged or even fossil source of carbon on the organic fraction incorporated by these stalagmites.

Subsoil contains more than half of soil organic carbon (SOC) globally and is conventionally assumed to be relatively unresponsive to warming compared to the topsoil. Here, we show substantial changes in carbon allocation and dynamics of the subsoil but not topsoil in the Qinghai-Tibetan alpine grasslands over 5 years of warming. Specifically, warming enhanced the accumulation of newly synthesized (14 C-enriched) carbon in the subsoil slow-cycling pool (silt-clay fraction) but promoted the decomposition of plant-derived lignin in the fast-cycling pool (macroaggregates). These changes mirrored an accumulation of lipids and sugars at the expense of lignin in the warmed bulk subsoil, likely associated with shortened soil freezing period and a deepening root system. As warming is accompanied by deepening roots in a wide range of ecosystems, root-driven accrual of slow-cycling pool may represent an important and overlooked mechanism for a potential long-term carbon sink at depth. Moreover, given the contrasting sensitivity of SOC dynamics at varied depths, warming studies focusing only on surface soils may vastly misrepresent shifts in ecosystem carbon storage under climate change.