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The Amazon River mobilizes organic carbon across one of the world's largest terrestrial carbon reservoirs. Quantifying the sources of particulate organic carbon (POC) to this flux is typically challenging in large systems such as the Amazon River due to hydrodynamic sorting of sediments. Here, we analyze the composition of POC collected from multiple total suspended sediment (TSS) profiles in the mainstem at Óbidos, and surface samples from the Madeira, Solimões and Tapajós Rivers. As hypothesized, TSS and POC concentrations in the mainstem increased with depth and fit well to Rouse models for sediment sorting by grain size. Coupling these profiles with Acoustic Doppler Current Profiler discharge data, we estimate a large decrease in POC flux (from 540 to 370 kg per second) between the rising and falling stages of the Amazon River mainstem. The C/N ratio and stable and radiocarbon signatures of bulk POC are less variable within the cross‐section at Óbidos and suggest that riverine POC in the Amazon River is predominantly soil‐derived. However, smaller shifts in these compositional metrics with depth, including leaf wax n‐alkanes and fatty acids, are consistent with the perspective that deeper and larger particles carry fresher, less degraded organic matter sources (i.e., vegetation debris) through the mainstem. Overall, our cross‐sectional surveys at Óbidos highlight the importance of depth‐specific sampling for estimating riverine export fluxes. At the same time, they imply that this approach to sampling is perhaps less essential with respect to characterizing the composition of POC sources exported by the river. Plain Language Summary: The Amazon River transports one of the largest quantities of freshwater organic carbon into the Atlantic Ocean. In this study, we collected suspended particles at different depths within a cross‐section of the Amazon River mainstem during the rising and falling stages of the river's hydrological cycle. We analyzed the organic carbon, nitrogen, grain size, and leaf‐derived compounds in these particles, and integrated water velocity measurements to calculate the quantity of carbon in particle form moving through the river at these two stages. The analyses showed that large, dense particles concentrate with depth in the Amazon River mainstem. The composition of these particles is relatively homogenous, but slight variations in metrics like carbon‐to‐nitrogen ratio, age derived from carbon‐dating, and leaf waxes imply that less degraded sources of organic carbon are found in the deeper and coarser grained particles. Overall, the data suggest that the majority of Amazon River particulate organic carbon comes from a mixture of soil organic carbon washing in from different landscapes and soil depth horizons. A globally significant quantity of this carbon will get buried in the Atlantic Ocean, forming a long‐term carbon sink. Key Points: Amazon River suspended sediments show little variation in organic carbon composition with depth despite hydrodynamic sortingEstimated particulate organic carbon fluxes range from 370 to 540 kg per second during the falling and rising stages, respectivelyThe majority of Amazon River particulate organic carbon exported from the mainstem at Óbidos is soil‐derived [ABSTRACT FROM AUTHOR]

The transfer of carbon from land to the near-coastal ocean is increasingly being recognized in global carbon budgets. However, a more direct transfer of terrestrial carbon to the deep-sea is comparatively overlooked. Among systems that connect coastal to deep-sea environments, the Congo Submarine Canyon is of particular interest since the canyon head starts 30 km into the Congo River estuary, which delivers ~7 % of the total organic carbon from the world's rivers. However, carbon and sediment transport mechanisms that operate in the Congo Canyon, and submarine canyons more globally, are poorly constrained compared to rivers because monitoring of deep-sea canyons remains challenging. Using a novel array of acoustic instruments, sediment traps and cores, this study seeks to understand the hydrodynamic processes that control delivery of particulate organic carbon via the Congo Submarine Canyon to the deep-sea. We show that particulate organic carbon transport in the canyon-axis is modulated by two processes. First, we observe periods where the canyon dynamics are dominated by tides, which induce a background oscillatory flow (speeds of up to 0.15 m/s) through the water column, keeping muds in suspension, with a net upslope transport direction. Second, fast-moving (up to 8 m/s) turbidity currents occur for 35 % of the time during monitoring periods and transport both muddy and sandy particulate organic carbon at an estimated transit flux that is more than ten times the flux induced by tides. Remarkably, organic carbon transported and deposited in the submarine canyon has a similar isotopic composition to organic carbon in the Congo River, and in the deep-sea fan at 5 km of water depth. Episodic turbidity currents, together with background tidal currents thus promote efficient transfer of river-derived particulate organic carbon in the Congo Submarine Fan, leading to some of the highest terrestrial carbon preservation rates observed in marine sediments globally. [ABSTRACT FROM AUTHOR]

As major sites of carbon burial and remineralization, continental margins are key components of the global carbon cycle. However, heterogeneous sources of organic matter (OM) and depositional environments lead to complex spatial patterns in sedimentary organic carbon (OC) content and composition. To better constrain the processes that control OM cycling, we focus on the East Asian marginal seas as a model system, where we compiled extensive data on the OC content, bulk isotopic composition (δ13C and Δ14C), total nitrogen, and mineral surface area of surficial sediments from previous studies and new measurements. We developed a spatial machine learning modeling framework to predict the spatial distribution of these parameters and identify regions where sediments with similar geochemical signatures drape the seafloor (i.e., "isodrapes"). We demonstrate that both provenance (44%-77%) and hydrodynamic processes (22%-53%) govern the fate of OM in this margin. Hydrodynamic processes can either promote the degradation of OM in mobile mud-belts or preserve it in stable mud-deposits. The distinct isotopic composition of OC sources from marine productivity and individual rivers regulates the age and reactivity of OM deposited on the sea-floor. The East Asian marginal seas can be separated into three main isodrapes: hydrodynamically energetic shelves with coarser-grained sediment depleted in OC, OM-enriched mud deposits, and a deep basin with fine-grained sediments and aged OC affected by long oxygen exposure times and petrogenic input from rivers. This study confirms that both hydrodynamic processes and provenance should be accounted for to understand the fate of OC in continental margins. [ABSTRACT FROM AUTHOR]

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 hydroclimate.sup.1, potentially generating feedback on the atmospheric inventory of carbon dioxide. Although temperature controls the storage of soil organic carbon at mid and high latitudes.sup.2,3, hydroclimate may be the dominant driver of soil carbon persistence in the tropics.sup.4,5; 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 records.sup.6 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. Over the past 18,000 years, the residence time and amount of soil carbon stored in the Ganges-Brahmaputra basin have been controlled by the intensity of Indian Summer Monsoon rainfall, with greater carbon destabilization during wetter, warmer conditions., Author(s): Christopher J. Hein [sup.1] , Muhammed Usman [sup.2] [sup.3] , Timothy I. Eglinton [sup.2] , Negar Haghipour [sup.2] [sup.4] , Valier V. Galy [sup.5] Author Affiliations: (1) Department of [...]

Compound- and compound class-specific radiocarbon analysis of source-diagnostic 'biomarker' molecules has emerged as a powerful tool to gain insights into terrestrial carbon cycling. While most studies thus far have focused on higher plant biomarkers (i.e. plant leaf-wax n-alkanoic acids and n-alkanes, lignin-derived phenols), tracing paedogenic carbon is crucial given the pivotal role of soils in modulating ecosystem carbon turnover and organic carbon (OC) export. Here, we determine the radiocarbon (14C) ages of glycerol dialkyl glycerol tetraethers (GDGTs) in riverine sediments and compare them to those of higher plant biomarkers as well as markers of pyrogenic (fire-derived) carbon (benzene polycarboxylic acids, BPCAs) to assess their potential as tracers of soil turnover and export. GDGT Δ14C follows similar relationships with basin properties as vegetation-derived lignin phenols and leaf-wax n-alkanoic acids, suggesting that the radiocarbon ages of these compounds are significantly impacted by intermittent soil storage. Systematic radiocarbon age offsets are observable between the studied biomarkers, which are likely caused by different mobilization pathways and/or stabilization by mineral association. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'. [ABSTRACT FROM AUTHOR]

Sequential thermal analysis allows for deconvoluting the refractory nature and complexity of carbon mixtures embedded in mineral matrices for subsequent offline stable carbon and radiocarbon (14C) isotope analyses. Originally developed to separate Holocene from more ancient sedimentary organic matter to improve dating of marine sediments, the Ramped Pyrolysis and Oxidation (RPO) apparatus, or informally, the "dirt burner" is now used to address pressing questions in the broad field of biogeochemistry. The growing interest in the community now necessitates improved handling and procedures for routine analyses of difficult sample types. Here we report on advances in CO2 purification during sample processing, modifications to the instrumentation at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility, and introduce sodium bicarbonate procedural standards with differing natural abundance 14C signatures for blank assessment. Measurements from different environmental samples are used to compare the procedure to the different generations of sequential thermal analyses. With this study, we aim to improve the standardization of the procedures and prepare this instrumentation for innovations in online stable carbon isotopes and direct AMS-interface measurements in the future. [ABSTRACT FROM AUTHOR]

Soils of the northern high latitudes store carbon over millennial timescales (thousands of years) and contain approximately double the carbon stock of the atmosphere (1-3). Warming and associated permafrost thaw can expose soil organic carbon and result in mineralization and carbon dioxide (C[O.sub.2]) release (4-6). However, some of this soil organic carbon may be eroded and transferred to rivers (7-9). If it escapes degradation during river transport and is buried in marine sediments, then it can contribute to a longer-term (more than ten thousand years), geological C[O.sub.2] sink (8-10). Despite this recognition, the erosional flux and fate of particulate organic carbon (POC) in large rivers at high latitudes remains poorly constrained. Here, we quantify the source of POC in the Mackenzie River, the main sediment supplier to the Arctic Ocean (11, 12), and assess its flux and fate. We combine measurements of radiocarbon, stable carbon isotopes and element ratios to correct for rock-derived POC (10, 13, 14). Our samples reveal that the eroded biospheric POC has resided in the basin for millennia, with a mean radiocarbon age of 5,800 ± 800 years, much older than the POC in large tropical rivers (13, 14). From the measured biospheric POC content and variability in annual sediment yield (15), we calculate a biospheric POC flux of [2.2.sup.+1.3.sub.-0.9] teragrams of carbon per year from the Mackenzie River, which is three times the C[O.sub.2] draw down by silicate weathering in this basin (16). Offshore, we find evidence for efficient terrestrial organic carbon burial over the Holocene period, suggesting that erosion of organic carbon-rich, high-latitude soils may result in an important geological C[O.sub.2] sink., Photosynthesis and the production of organic carbon by the terrestrial biosphere (O[C.sub.biosphere]) is a major pathway of atmospheric C[O.sub.2] drawdown. Over millennial timescales, some O[C.sub.biosphere] escapes oxidation and contributes to [...]

Author(s): Valier Galy (corresponding author) [1]; Bernhard Peucker-Ehrenbrink [1]; Timothy Eglinton [1, 2] Riverine export of particulate organic carbon (POC) to the ocean affects the atmospheric carbon inventory over a [...]