Low-latitude arc-continent collisions are hypothesized to drive global cooling through exhuming and subsequently weathering fresh mafic/ultramafic rocks. High primary productivity and intense erosion in the tropics could also promote carbon dioxide (CO2) drawdown through enhanced organic carbon (OC) burial. Here we examine OC burial over the Middle-to-Late Pleistocene (similar to 550 ka) offshore North New Guinea at the Australia-Pacific convergent boundary. We reconstructed OC burial using carbon isotopes and Ramped Pyrolysis/Oxidation (RPO) analyses of sediment samples from IODP core U1485. We resolved three biospheric carbon pools-marine OC (28 +/- 14%), fresh terrestrial OC (39 +/- 12%), aged terrestrial OC (23 +/- 5%)-along with a petrogenic pool (10 f 1%) in U1485 sediments. Terrestrial OC burial dominated the total OC pool and persisted throughout the core, likely driven by the intense erosion sustained by the mountain range under tropical hydroclimatic forcing. The burial of biospheric carbon pools increased remarkably after similar to 350 ka, which coincided with the emergence of the Coastal Range and could be attributed to the combined effect of uplift on erosion, nutrient release, and landform evolution. High OC abundances occurred during interglacials, likely caused by a decrease in grain-size and enhanced upwelling at sea-level highstands, with the opposite trend during glacials. Northern New Guinea sediments have 3-5x lower CO2 drawdown capacity from biospheric OC burial than silicate weathering (derived from elemental depletion), yet both rank high globally. Our findings demonstrate that low-latitude arc-continent collisions could promote the burial of both terrestrial-and marine-sourced OC, thereby influencing the composition and magnitude of the global sedimentary carbon reservoir.

Abstract. The Ayeyarwady and Thanlwin Rivers deliver ~1.9 Mt y-1 of particulate terrestrial organic carbon (TerrOC) to the Northern Andaman Sea making them one of the largest sources of TerrOC to the world ocean. Offshore, fluvial material is extensively mixed by tides and estuarine circulation in the Gulf of Martaban, concurrently incorporating marine organic carbon. While previous bulk stable isotope analyses have suggested that frequent resuspension enhances TerrOC degradation, limiting burial on high-energy margins, in this study we use ramped pyrolysis/oxidation (RPO) and radiocarbon analysis to identify high and uniform TerrOC content (by Wt. %) in continental shelf sediments offshore of the Ayeyarwady Delta. By analyzing sediment samples from the river mouths and across the shelf, RPO results demonstrate consistent radiocarbon and stable carbon isotope composition (δ13C) of TerrOC from the Gulf of Martaban to the mid-shelf clinoform depocenter, suggesting that extensive degradation does not occur during shelf transport. Instead, rapid remineralization and processing of labile material likely occurs near-shore, while refractory terrestrial components are efficiently transported to the mid-shelf depocenter. Given δ13C values of refractory TerrOC fractions that mimic marine sources, we suggest that offshore TerrOC content has been substantially underestimated by bulk δ13C-based mixing models. Providing a new conceptual framework for organic carbon preservation in this globally significant offshore delta, these findings indicate the necessity of re-evaluating our understanding of TerrOC processing on high-energy, river-dominated margins, and support recent hypotheses that these systems provide a larger sink for TerrOC than previously suggested.

Abstract The riverine mobilization and export of OC from different compartments of the terrestrial biosphere such as standing biomass, organic‐rich surface soils and deep mineral soils have contrasting impacts on the carbon cycle. Heretofore, the distribution of these OC sources in river sediments remains poorly constrained, hampering attempts to close the carbon budget of river catchments. Here, we use a time series of radiocarbon measurements to suggest that particulate OC in the Amazon River is composed of ca. 60% plant and soil litter contributions characterized by a turnover of <5 years, mixed with millennial‐age soil OC and minor petrogenic OC contributions. This reflects concomitant organic matter stabilization over millennial timescales in soil horizons via mineral protection and mobilization of abundant primary production from the Amazon rainforest. The Amazon Basin thus holds large reservoirs of millennial and decadal carbon that are susceptible to destabilization upon increasing temperature and deforestation, respectively.

Arctic–Boreal ecosystems hold vast carbon stocks, yet intensifying wildfires threaten to shift them from net sinks to net sources. Fires produce condensed aromatic carbon (ConAC), a chemically stable form of pyrogenic carbon that accumulates in terrestrial and marine reservoirs. However, ConAC mobilization, retention, and export along the riverine continuum remain poorly constrained. Here, we present a comprehensive source-to-sink ConAC budget for the Mackenzie River–Beaufort Sea system, quantifying wildfire production, riverine transport, and shelf accumulation. Between 2001 and 2017, fires burned 9.7% of the basin and generated 36.5 Tg ConAC, while soils stored ~4 Pg ConAC with millennial-scale turnover. Fluvial export averaged 0.42 Tg ConAC yr –1 . The dissolved fraction reflected modern pyrogenic sources, in contrast to particulate ConAC dominated by radiocarbon-depleted carbon derived from sedimentary rocks. On the Beaufort Shelf, waters retained ~1.2 Tg ConAC with a ~6-year residence time, whereas sediments accumulated ~0.14 Tg ConAC yr –1 . This budget reveals that basin properties including lithology, geomorphic routing, and sediment trapping control the composition and efficiency of land-to-ocean ConAC transfer. It establishes a quantitative baseline for assessing how ongoing environmental change will reshape ConAC cycling across high-latitude systems.

Understanding the mechanisms that drive the mobilization and fate of organic carbon (OC) in Arctic landscapes is important for modeling the feedbacks among permafrost thaw, carbon cycling, and climate change. While significant progress has been made toward measuring in situ OC decomposition in permafrost soils and bulk particulate organic carbon (POC) export from Arctic rivers, few studies have distinguished the source and lability of POC across Arctic landscapes, limiting our ability to predict whether mobilized POC will be oxidized to CO2 and CH4 or buried in downstream depositional environments. This study uses ramped pyrolysis/oxidation radiocarbon (RPO-14C) analyses to investigate spatial and temporal variations in the thermochemical stability and radiocarbon content of fluvial POC during downstream transport from mountains to the coast in the Canning River (North Slope, Alaska). Fluvial POC in the headwaters is predominantly comprised of high activation energy, thermally recalcitrant petrogenic OC (OCpetro) derived from shale bedrock. Moving into the foothills and low-relief coastal plains, river bank erosion primarily drives mobilization of labile, low activation energy, soil-derived OC (OCsoil). Fluvial POC in mountainous upstream reaches consisted of similar to 70% OCpetro and just similar to 30% OCsoil, while POC in the downstream coastal plain reaches comprised similar to 85% OCsoil and similar to 15% OCpetro. The high relative lability of POC exported to the coast indicates high susceptibility to oxidation and microbial decomposition, which could enhance CO2 release as the Arctic hydrologic cycle intensifies. However, the persistence of refractory OCpetro in the suspended load indicates the potential for long-term burial of rock organic carbon in marine sediments.

Arc-continent collisions at low latitudes have been proposed to drive late Cenozoic cooling. These collisions built high-relief Southeast Asian Islands (SEAIs), enhancing erosion and weathering of mafic/ultramafic rocks. This hypothesis is challenged by the contrast between the rising seawater strontium (Sr) isotope ratio (87Sr/86Sr) and the low 87Sr/86Sr from mafic/ultramafic weathering. However, arc-continent collisions can also exhume lithologies from evolved continental crust that introduce radiogenic 87Sr/86Sr signals, but the effects of such variable lithologies on regional weathering signals and global seawater 87Sr/86Sr remain untested. Here we characterize geochemical signals of silicate weathering in New Guinea (NG), the largest SEAI, by analyzing Sr, neodymium (Nd), and uranium (U) isotopes in core sediments from the International Ocean Discovery Program (IODP) site U1485 that span the last 550 kyr. The measured Sr-Nd isotopes indicate that U1485 sediments are sourced from both juvenile mafic/ultramafic lithologies and metasedimentary rocks derived from older continental crust. Before the emergence of the Coastal Range at ∼ 350 ka, NG weathering mostly occurred in the Central Range and released radiogenic Sr (87Sr/86Sr > 0.7090) to the oceans. Since then, 87Sr/86Sr ratios shifted from radiogenic to unradiogenic (<0.7050) values, reflecting lithologic controls on weathering signals. Sedimentary residence times derived from (234U/238U) comminution ages reveal long durations and deep weathering zones in the Central Range and vice versa in the Coastal Range, suggesting that less-reactive metasedimentary rocks can still produce high weathering fluxes sustained by prolonged tropical weathering. Element-based proxies, including a modified chemical index of alteration and a Na-enrichment factor, showed marked shifts following the uplift of the Coastal Range and enhanced mafic/ultramafic weathering. Glacial–interglacial cycles introduced additional variations on Sr-Nd-U isotopes, reflecting greater contributions from the Central and Coastal Ranges to weathering fluxes during interglacial and glacial periods, respectively, likely caused by precipitation pattern changes. To test global influences, we constructed a geochemical box model and show that a radiogenic Sr flux from SEAI weathering can persistently explain rising seawater 87Sr/86Sr over the late Cenozoic across various scenarios where different Sr fluxes and isotopic signals change concurrently. Model results also show that the seawater Sr isotopic curve can be explained by either an increase or a decrease in the global weathering flux since 15 Ma, but a decreased flux is favored across a broader parameter space. Overall, our work demonstrates that lithologies, in tandem with tectonic uplift and climatic variations, exert primary controls on tropical weathering signals, highlighting SEAIs as a key player in Earth’s climate and global geochemical cycles

While it is accepted that the tropical hydrological cycle has intensified during past interglacial periods due to changes in insolation, greenhouse gases, and ice volume, variations in the intensity and spatial distribution of rainfall in the South Asian monsoon domain, as well as the respective influence of these forcings during past warm periods, remain uncertain. Here, we present a pollen record from the Bay of Bengal (IODP Site U1446, located off the Mahanadi river exit, outside the influence of the Bengal fan) that allows reconstruction of vegetation changes in the core monsoon zone of India during two warm periods, the current and last interglacial periods. We compare the data with numerical model simulations (HadCM3 and LOVECLIM1.3) to assess the influence of different forcing mechanisms on the response of summer monsoon rainfall during past interglacials characterized by different levels of warming (Clément et al., 2024). We also present a pollen record from cores (SO93) taken at 16°N from the Ganges-Brahmaputra-Meghna (G-B-M) river-fed Bengal fan, covering the current interglacial period.Results from IODP Site U1446 show tropical forest expansion between 11.7-5 ka and 127-120 ka, defining two Indian humid periods, with the last interglacial showing the strongest monsoon activity, consistent with salinity reconstructions. During the last five millennia of both interglacial periods, moist tropical forest largely declined in favor of savanna marking a significant decrease in summer monsoon rainfall. Although the pollen assemblages from sites SO93 and U1446 show substantial differences in Holocene vegetation cover between the basins, the maximum expansion of the evergreen component of the tropical forest is recorded contemporaneously in both sequences. This suggests a similar Holocene evolution of the summer monsoon from central to northern India. The model-data comparison highlights boreal summer insolation as the primary driver of vegetation dynamics and monsoon intensity during interglacial periods, with CO2 and ice-sheets having a limited effect. These results also show that vegetation remains unaffected by pre-industrial CO2 variations above 250 ppmv, a threshold value that characterizes most interglacials of the last million years.Clément, C., Martinez, P., Yin, Q., Clemens, S., Thirumalai, K., Prasad, S., Anupama, K., Su, Q., Lyu, A., Grémare, A., Desprat, S., 2024. Greening of India and revival of the South Asian summer monsoon in a warmer world. Commun. Earth Environ. 5, 685.

The Amazon River mobilizes one of the largest fluxes of particulate organic carbon (POC) from land to coastal ocean sediments, playing an important role in the long‐term sequestration of biospheric organic carbon in the ocean. Ramped oxidation (RPO) analyses of suspended sediments collected from the Amazon River mainstem, Solimões River, Madeira River, and Tapajós River presented an opportunity to parse riverine POC by thermal reactivity, extract the activation energy distributions of specific biomolecular pools in these samples, and characterize the molecular diversity of POC across the floodplain. The thermal reactivity data imply that POC from the Amazon River basin spans a wide but relatively homogenous activation energy range across samples, suggesting that the degradation history of the organic carbon comprising riverine suspended particles is relatively constant across depths within the mainstem and different tributary locations. Coupling activation energy distributions to stable and radiocarbon isotopic analyses shows that ca. 85% of mainstem POC derives from a range of partially degraded terrestrial sources, likely organic matter from mineral soil horizons, and that a similar range of soil sources influences the biomolecular diversity in tributary samples. In agreement with earlier assessments, ca. 10% of the riverine POC flux is fresh vegetation and up to 5% of it is petrogenic organic matter. Expanded RPO analyses of samples across the Amazon river‐to‐ocean continuum would provide an opportunity to track the fate of these different organic matter pools downstream that is uniquely different from, but complementary to, past compound‐specific and bulk analyses of riverine POC.

Over the Tertiary, the uplift of the Himalaya combined to the development of the monsoon generated the largest erosion basins of the planet. More than 80% of the erosion is exported to the Bay of Bengal by the Ganga-Brahmaputra river system and generates turbidity currents which convey detrital sediment building the Bengal Fan. In the modern Himalaya, the monsoon rainfall and tectonic processes shape the erosion pattern. The monsoon seasonal precipitation ensures efficient transport of sand-rich sediments in the basin despite long distances through a very flat floodplain and delta. Rapid transport also acts as a limiting factor for weathering as it reduces residence time in the floodplain but favors efficient carbon burial. The IODP Expedition 354 drilled the Bengal Fan with seven sites over a 320 km E-W transect at 8°N. This construcs a composite sedimentary record of Himalayan erosion over the Neogene and Quaternary. Sediments are predominantly composed of turbidites generated from the Ganga-Brahmaputra delta. Turbiditic sediments show mineralogical, geochemical and isotopic characteristics which reveal a close analogy with those of the modern Ganga-Brahmaputra river. Sand deposition is dominant and is present in several meters thick sand lobe as well as in levee turbidite (Bergmann et al. 2020). Sand was used to determine average erosion rates of the Himalaya using quartz in situ concentrations of cosmogenic 10Be. Those show stable rate in spite of the onset of a more unstable climate from the Pliocene to the Pleistocene (Lenard et al. 2020). Major element concentrations and Sr-Nd isotopic compositions of turbidite samples reflect combined effects of geological sources exposed to erosion, weathering and mineral sorting during transport. Deciphering these controls, based on the comparison between turbidite samples and modern river sediments of the Ganga and Brahmaputra basin reveals evolution from Miocene to present. Changes appear in the abundance of detrital carbonates likely reflecting decreasing exposition of the Tethys Himalaya to erosion since Miocene. Clear increase in the silicate Na and Ca concentrations from Miocene to Pleistocene indicates major change in the weathering conditions in the basin which can be related to longer residence time of the sediment in the floodplain and lower erosion ratesin the Miocene. Bergmann et al. 2020, G. cube 10.1029/2019gc008702Lenard et al. Nat Geosc. 2020, doi:10.1038/s41561-020-0585-2

Rivers and streams are an important pathway in the global carbon cycle, releasing carbon dioxide (CO2) and methane (CH4) from their water surfaces to the atmosphere1,2. Until now, CO2 and CH4 emitted from rivers were thought to be predominantly derived from recent (sub-decadal) biomass production and, thus, part of ecosystem respiration3-6. Here we combine new and published measurements to create a global database of the radiocarbon content of river dissolved inorganic carbon (DIC), CO2 and CH4. Isotopic mass balance of our database suggests that 59 ± 17% of global river CO2 emissions are derived from old carbon (millennial or older), the release of which is linked to river catchment lithology and biome. This previously unrecognized release of old, pre-industrial-aged carbon to the atmosphere from long-term soil, sediment and geologic carbon stores through lateral hydrological routing equates to 1.2 ± 0.3 Pg C year-1, similar in magnitude to terrestrial net ecosystem exchange. A consequence of this flux is a greater than expected net loss of carbon from aged organic matter stores on land. This requires a reassessment of the fate of anthropogenic carbon in terrestrial systems and in global carbon cycle budgets and models.

Fine-grained muds produced largely from rock weathering at the Earth's surface have great influence on global carbon cycling. Mud binds and protects organic carbon (OC) from remineralization, and its organic loading controls the amounts, timescales and pathways of OC sequestration in sediments and soils. Human activities have resulted in marked changes (both increases and decreases) in mud accumulation and associated OC (mud-OC) loadings in different environments via altering organic matter inputs and reactivity. Such impacts on mud and mud-OC can be directly caused by activities such as damming and levee building, or indirectly result from human-induced climate change. Here we present a synthesis of impacts of human activities on the production, transfer and storage of mud-OC. In general, we find that anthropogenic climate warming has increased net fluxes of mud-OC in most of the systems discussed here (for example, mountain glaciers, land erosion, dam burial, river export, permafrost thaw, ice-sheet erosion and burial in margins), with uncertainties for tidal flats and floodplains, and probably net losses for coastal wetlands. Whether the anthropogenic mobilization of mud-OC results in more or less sequestration of OC is not known with the current data, as it is dependent on timescales that involve complex transient effects. Human activities have altered the production, transport and fate of mud and associated organic carbon, with important implications for global carbon cycling.

River particulate organic carbon (POC) plays a central role in the global carbon cycle, while its geochemical composition has provided insight on the functioning of the terrestrial biosphere and soils. Here we review the isotopic composition of river POC (stable isotopes and radiocarbon) and organic biomarkers found in river sediments and discuss how these reflect varied sources of organic matter delivered to river systems by hydrological and geomorphic processes. Subsequent hydrodynamic sorting in river channels, alongside biogeochemical processing and sediment deposition also impact the fluxes and composition of river POC. A first order separation of POC into biospheric and rock-derived components allows us to quantify global export by rivers to the oceans (biospheric POC = 0.18+0.07/−0.05 PgC year−1, petrogenic POC = 0.04+0.06/−0.03 PgC year−1) and evaluate impacts on carbon cycling. However, we emphasize that biospheric POC is itself a diverse mixture of organic matter sourced from plants, soils and aquatic productivity. A recent shift in paradigm has shown that biospheric POC can be highly reactive and turnover on annual to decadal timescales, contributing actively to the contemporary carbon cycle. Here we discuss the tectonic, climatic and anthropogenic drivers of POC erosion, supply and river export. River POC plays a key role in the sedimentary cycle of carbon over 103–106 timescales, while we suggest riverine transfer of biospheric POC could amount to an important component of the anthropogenic C budget.

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 like 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 to Acoustic Doppler Current Profiler discharge data, we estimate a large decrease in POC flux (from 540 to 370 kilograms per second) between the rising and falling stages of Amazon River mainstem. The C/N ratio, 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.

Abstract Ratios of glycerol dialkyl glycerol tetraethers (GDGT), which are membrane lipids of bacteria and archaea, are at the base of several paleoenvironmental proxies. They are frequently applied to soils as well as lake‐ and marine sediments to generate records of past temperature and soil pH. To derive meaningful environmental information from these reconstructions, high analytical reproducibility is required. Based on submitted results by 39 laboratories from across the world, which employ a diverse range of analytical and quantification methods, we explored the reproducibility of brGDGT‐based proxies (MBT′5ME, IR, and #ringstetra) measured on four soil samples and four soil lipid extracts. Correct identification and integration of 5‐ and 6‐methyl brGDGTs is a prerequisite for the robust calculation of proxy values, but this can be challenging as indicated by the large inter‐interlaboratory variation. The exclusion of statistical outliers improves the reproducibility, where the remaining uncertainty translates into a temperature offset from median proxy values of 0.3–0.9°C and a pH offset of 0.05–0.3. There is no apparent systematic impact of the extraction method and sample preparation steps on the brGDGT ratios. Although reported GDGT concentrations are generally consistent within laboratories, they vary greatly between laboratories. This large variability in brGDGT quantification may relate to variations in ionization efficiency or specific mass spectrometer settings possibly impacting the response of brGDGTs masses relative to that of the internal standard used. While ratio values of GDGT are generally comparable, quantities can currently not be compared between laboratories.

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 organic carbon to the deep sea is comparatively overlooked. Among systems that connect coastal to deep-sea environments, the submarine Congo Canyon is of particular interest since the canyon head starts 30 km into the Congo River estuary, which delivers similar to 7 % of the dissolved and particulate organic carbon from the world's rivers. However, sediment and particulate organic carbon 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 submarine Congo 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 ms-1) through the water column, keeping muds in suspension, with a net upslope transport direction. Second, fast-moving (up to 8 ms-1) turbidity currents occur for 35 % of the time during monitoring periods and transport particulate organic carbon with mud and sand at an estimated transit flux that is more than 3 to 6 times the flux induced by tides. 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 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.

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 (delta 13C and Delta 14C), 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. Plain Language Summary This study focuses on carbon cycle processes occurring in marine sediments of the East Asian marginal seas. We compiled extensive data on the organic carbon content and composition of surface sediments in these seas and developed a machine learning model to predict their spatial patterns and identify the environmental conditions that drive their distribution. We found that the spatial distribution of organic matter is governed by the resuspension of different grain size fractions due to water current intensity as well as the contrasting origin of the organic matter (marine, terrestrial, and rock-derived) that influences its reactivity. We also identified three main areas where sediments with similar composition drape the seafloor: shelves with strong bottom current with less organic matter, mud deposits rich in organic matter, and a deep basin with aged organic matter. Understanding the factors that control the distribution of organic matter in these areas is important for accurately assessing their contribution to the global carbon cycle.

AbstractOrganic carbon (OC) sedimentation in marine sediments is the largest long‐term sink of atmospheric CO2 after silicate weathering. Understanding the mechanistic and quantitative aspects of OC delivery and preservation in marine sediments is critical for predicting the role of the oceans in modulating global climate. Yet, estimates of the global OC sedimentation in marginal settings span an order of magnitude, and the primary controls of OC preservation remain highly debated. Here, we provide the first global bottom‐up estimate of OC sedimentation along the margins using a synthesis of literature data. We quantify both terrestrial‐ and marine‐sourced OC fluxes and perform a statistical analysis to discern the key factors influencing their magnitude. We find that the margins host 23.2 ± 3.5 Tmol of OC sedimentation annually, with approximately 84% of marine origin. Accordingly, we calculate that only 2%–3% of OC exported from the euphotic zone escapes remineralization before sedimentation. Surprisingly, over half of all global OC sedimentation occurs below bottom waters with oxygen concentrations greater than 180 μM, while less than 4% occurs in settings with <50 μM oxygen. This challenges the prevailing paradigm that bottom‐water oxygen (BWO) is the primary control on OC preservation. Instead, our statistical analysis reveals that water depth is the most significant predictor of OC sedimentation, surpassing all other factors investigated, including BWO levels and sea‐surface chlorophyll concentrations. This finding suggests that the primary control on OC sedimentation is not production, but the ability of OC to resist remineralization during transit through the water column and while settling on the seafloor.

Carbon (C) in soils persists on a range of timescales depending on physical, chemical, and biological processes that interact with soil organic matter (SOM) and affect its rate of decomposition. Together these processes determine the age distribution of soil C. Most attempts to measure this age distribution have relied on operationally defined fractions using properties like density, aggregate stability, solubility, or chemical reactivity. Recently, thermal fractionation, which relies on the activation energy needed to combust SOM, has shown promise for separating young from old C by applying increasing heat to decompose SOM. Here, we investigated radiocarbon (C-14) and C-13 of C released during thermal fractionation to link activation energy to the age distribution of C in bulk soil and components previously separated by density and chemical properties. While physically and chemically isolated fractions had very distinct mean C-14 values, they contributed C across the full temperature range during thermal analysis. Thus, each thermal fraction collected during combustion of bulk soil integrates contributions from younger and older C derived from components having different physical and chemical properties but the same activation energy. Bulk soil and all density and chemical fractions released progressively older and more C-13-enriched C with increasing activation energy, indicating that each operationally defined fraction itself was not homogeneous but contained a mix of C with different ages and degrees of microbial processing. Overall, we found that defining the full age distribution of C in bulk soil is best quantified by first separating particulate C prior to thermal fractionation of mineral-associated SOM. For the Podzol analyzed here, thermal fractions confirmed that similar to 95 % of the mineral-associated organic matter (MOM) had a relatively narrow C-14 distribution, while 5 % was very low in C-14 and likely reflected C from the < 2 mm parent shale material in the soil matrix. After first removing particulate C using density or size separation, thermal fractionation can provide a rapid technique to study the age structure of MOM and how it is influenced by different OM-mineral interactions.