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Lakes are important carbon sinks in terrestrial ecosystems. Currently, most of lake carbon burial research in China only considers organic carbon burial and has neglected inorganic carbon burial, which will lead to an underestimation of the total carbon burial capacity of lakes. This situation is particularly significant in the humid eastern plain region. In order to comprehensively understand the carbon burial characteristics of lakes in the eastern plain and evaluate the contributions of organic and inorganic carbon burial to total carbon burial more reasonably. This research chooses Hongze Lake as study site, which is a representative lake in the eastern plain region. Based on the 210Pb-137Cs chronological sequence, combined with the C/N and δ13C characteristics in sediments, the carbon burial characteristics of Hongze Lake sediments in the past 60 years were explored by analyzing the spatiotemporal changes of organic and inorganic carbon content in typical rock cores (numbered HZH-12) and 11 other surface sediments. C/N and δ13C shows that endogenous organic carbon is the main source of organic carbon inHongze Lake . The significant synergistic changes in the accumulation rates of organic and inorganic carbon over the past 60 years (R2=0.76, p

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Based on the focus on the role of reservoirs as carbon sources and sinks, this article explores their impact on the global carbon cycle. Reservoirs, which are “artificial lakes” created by human activities in rivers, are reviewed in terms of the key links in carbon cycling within reservoir areas, the influencing factors, and research methods related to carbon cycling and carbon emissions. By reviewing the literature on reservoir carbon cycling, we elucidate the mechanisms of carbon inputs, decomposition and transformation, carbon emissions, and carbon burial, summarizing the biogeochemical coupling processes of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), particulate organic carbon (POC), and particulate inorganic carbon (PIC) with hydrological conditions and nutrients. The study concludes that the reservoir carbon cycling process is influenced by the characteristics of the reservoirs themselves, hydrological and hydrodynamic conditions, physicochemical and nutrient conditions, and human activities. Additionally, we further clarify the applications of field sampling and analysis, modeling, remote sensing technology, and isotope techniques in reservoir carbon cycling. In the future, it is recommended to continue monitoring and simulating the reservoir carbon cycle processes on a long-term and large-scale basis, and to deeply explore the multifactorial mechanisms of reservoir construction in the global carbon cycle.

Riverine particulates dominate the transport of vital nutrients such as Si, Fe or P to the ocean margins, where they may increase primary production by acting as slow-release fertilizer. Furthermore, the supply of particulate surface area to the ocean is considered to be a major control of organic carbon burial. Taken together, these observations suggest a close link between the supply of riverine particulate material and the organic carbon cycle. To explore this link, we conducted microcosm experiments to measure the growth of the marine diatom Thalassiosira weissflogii in the presence and absence of different types and concentrations of riverine particulate material. Results demonstrate a strong positive effect of riverine particulate material on diatom growth with increased total diatom concentrations and slowed post-exponential death rates with increasing particulate concentration. Moreover, SEM and optical microscope investigations confirm that riverine particulates facilitate organic carbon burial through their role in the aggregation and sedimentation of phytoplankton. The supply of riverine particulate material has been shown to be markedly climate sensitive with their fluxes increasing dramatically with increasing global temperature and runoff. This pronounced climate sensitivity implies that riverine particulates contribute substantially in regulating atmospheric CO2 concentrations through their role in the organic carbon cycle. [ABSTRACT FROM AUTHOR]

• Reservoirs have carbon source/ sink functions serving as sites for carbon flow and exchange. • The carbon cycle process in reservoirs is influenced by multi-factor mechanisms. • Future research directions for global reservoir carbon cycle include long-term, large-scale monitoring and modeling. Based on the focus on the role of reservoirs as carbon sources and sinks, this article explores their impact on the global carbon cycle. Reservoirs, which are "artificial lakes" created by human activities in rivers, are reviewed in terms of the key links in carbon cycling within reservoir areas, the influencing factors, and research methods related to carbon cycling and carbon emissions. By reviewing the literature on reservoir carbon cycling, we elucidate the mechanisms of carbon inputs, decomposition and transformation, carbon emissions, and carbon burial, summarizing the biogeochemical coupling processes of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), particulate organic carbon (POC), and particulate inorganic carbon (PIC) with hydrological conditions and nutrients. The study concludes that the reservoir carbon cycling process is influenced by the characteristics of the reservoirs themselves, hydrological and hydrodynamic conditions, physicochemical and nutrient conditions, and human activities. Additionally, we further clarify the applications of field sampling and analysis, modeling, remote sensing technology, and isotope techniques in reservoir carbon cycling. In the future, it is recommended to continue monitoring and simulating the reservoir carbon cycle processes on a long-term and large-scale basis, and to deeply explore the multifactorial mechanisms of reservoir construction in the global carbon cycle. [ABSTRACT FROM AUTHOR]

A geometric theory was developed to explain the empirical relationship between carbon burial and lake shape in boreal lakes. The key feature of this model is an attenuation length scale, analogous to models of marine organic carbon fluxes. This length scale is the ratio of how fast carbon is displaced vertically versus how fast it is respired and engenders a simple model with a single easily constrained free parameter. Lake depths are modeled based on fractal area–volume relationships that reflect the approximate scale invariance of Earth’s topography on idealized lake geometries. Carbon burial is estimated by applying the attenuation length scale to these depths. Using this model, we demonstrate the relationship between the dynamic ratio—a metric of lake morphometry calculated by dividing the square root of surface area by the mean depth—and carbon burial. We use scaling relationships to predict how dynamic ratio, and by extension carbon burial, varies across the lake size spectrum. Our model also provides a basis for generalizing empirical studies to the biome scale. By applying our model to a boreal lake census, we estimate boreal lake carbon burial to be 1.8 ± 0.5 g C/m2/yr or 1.1 ± 0.3 Tg C/yr among all boreal lakes.

Introduction: Damming significantly modifies the function of natural river networks and influences sediment dynamics with a reservoir’s operation. The dissolved organic matter (DOM) in reservoir sediments severely affects carbon flow from land to sea. However, the properties of DOM (e.g., quantity and quality) in reservoir sediments and their relationship with carbon cycling remain unclear as complex reservoir construction interrupts the environmental processes.Methods: This study characterizes the optical and molecular properties of sediment water-extractable organic matter (WEOM) in the Daning and Shennong tributaries of the world’s largest reservoir—the Three Gorges Reservoir (TGR)—by applying optical techniques and ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS).Results and Discussion: We first assessed the link between light-absorbing components and the individual molecules in WEOM, which were significantly different than DOM in water and indicated that there might be an intrinsic variation between DOM in sediment and in water. Then, with the unique optical–molecular property linkage assessed, multiple sources (autochthonous and terrestrial) were identified, and a declining trend of terrestrial and recalcitrant WEOM was revealed from the tributaries upstream to downstream. Finally, through covariance analysis of the properties between WEOM and sediment particles, we demonstrated that the WEOM dynamic was most likely regulated by hydrologic sorting-induced particle size and mineral composition variations of sediment. Moreover, assessment between lability and WEOM molecular properties suggested that the WEOM dynamic likely contributes to carbon burial in the reservoir. This study emphasizes the influence of dam construction on organic matter accumulation and riverine carbon cycling.

Abstract Many metals present in trace concentrations in the oceans are sensitive to reduction and oxidation reactions (termed redox‐sensitive trace metals or RSTMs) and can therefore be affected during intervals of expanded oceanic anoxia and euxinia (anoxic and sulfidic waters). These RSTMs are important micronutrients; and their availability plays a significant role in controlling metabolic pathways and therefore ecosystem structure. Understanding the links to ecosystem restructuring and potential collapse is important since past deoxygenation events are associated with biological turnover. The Early Jurassic Toarcian Oceanic Anoxic Event (T‐OAE) was one such event. We focus on the RSTM molybdenum (Mo), important in the nitrogen cycle, from a basin transect where local redox conditions permit the tracking of the marine reservoir of Mo. Mo enrichments start to decline in the Pliensbachian, continue to decline in the early Toarcian, drop precipitously during the T‐OAE interval, and remain low afterward before a protracted increase toward the initial values seen in the Pliensbachian. From a compilation of available data, we estimate that ∼41 Gt of Mo was buried during the T‐OAE using a range of possible flux estimates. Given the known correlations of Mo with total organic carbon, the estimated burial of organic carbon during the T‐OAE interval was ∼244,000 Gt. This requires a minimum of 3.25% of the ocean floor to be covered by euxinic waters. Finally, we suggest that the intervals traditionally classified as OAEs are associated with the greatest extent of euxinia.

Preservation of organic carbon (OC) in marine and terrestrial deposits is enhanced by bonding with reactive iron (FeR). Association of OC with FeR (OC‐FeR) provides physical protection and hinders microbiological degradation. Roughly 20% of all OC stored in unconsolidated marine sediments and 40% of all OC present in Quaternary terrestrial deposits is preserved as OC‐FeR, but this value varies from 10% to 80% across global depositional environments. Here, we provide a new assessment of global OC‐FeR burial rates in both marine and terrestrial environments, using published estimates of OC associated with FeR, carbon burial, and probabilistic modeling. We estimate the marine OC‐FeR sink between 31 and 70 Mt C yr−1 (average 52 Mt C yr−1), and the terrestrial OC‐FeR sink at between 146 and 917 Mt C yr−1 (average 446 Mt C yr−1). In marine environments, continental shelves (average 17 Mt C yr−1) and deltaic/estuarine environments (average 11 Mg C yr−1) are the primary settings of OC‐FeR burial. On land, croplands (279 Mt C yr−1) and grasslands (121 Mt C yr−1) dominate the OC‐FeR burial budget. Changes in the Earth system through geological time impact the OC‐FeR pools, particularly in marine settings. For example, periods of intense explosive volcanism may lead to increased net OC‐FeR burial in marine sediments. Our work highlights the importance of OC‐FeR in marine carbon burial and demonstrates how OC‐FeR burial rates may be an order of magnitude greater in terrestrial environments, but here OC‐FeR stocks are most sensitive to the anthropogenic impacts of climatic change. Plain Language Summary: Most biomass, which is organic carbon (OC) accumulated in living organisms, decomposes to carbon dioxide and enters Earth's atmosphere, but some escapes complete decomposition, becoming stored in soils on land, and in sediments under the ocean. This storage of OC is part of a global carbon sink that helps to balance carbon dioxide levels in Earth's atmosphere over time, and any process that impacts OC storage may alter Earth's climate. Iron oxide minerals protect OC from decomposition, enhancing carbon storage in soils and sediments. Here, we collated measurements of OC associated with iron oxides from hundreds of observations world‐wide. Combining these data with estimates of the global carbon sink and modeling, we evaluate the importance of iron minerals for OC burial. Half a billion tonnes of OC is buried every year bound to iron minerals or 14% of the global carbon sink. The largest sinks for iron‐bound OC in the ocean are continental shelves and deltas, whilst the greatest proportions of iron‐bound OC are found in volcanic ash deposits. Croplands and grasslands are the largest contributors to iron‐bound OC burial on land, locations vulnerable to changes in land use and climate. Key Points: Annual burial rates of organic carbon (OC) with reactive iron (FeR) estimated by probabilistic modeling of compiled dataFeR minerals bury c. 52 Mt C yr−1 in marine sediments, and c. 446 Mt C yr−1 in terrestrial soilsTerrestrial stocks of OC bound to FeR (OC‐FeR) vulnerable to changing climate, while oceanic FeR inputs influence long term OC burial [ABSTRACT FROM AUTHOR]

Estimates of sedimentary organic carbon burial fluxes based on inventory and isotope mass balance methods have been divergent. A new calculation of the isotope mass balance using a revised assessment of the inputs to the ocean-atmosphere system resolves the apparent discrepancy. Inputs include weathering of carbonate and old kerogen, geogenic methane oxidation, and volcanic and metamorphic degassing. Volcanic and metamorphic degassing comprise ≈23% of the total C input. Inputs from isotopically light OCpetro and CH 4-geo drive the mean δ 13 C of the input to =-8.0 ± 1.9‰, notably lower than the commonly assumed volcanic degassing value. The isotope mass balance model yields a modern burial flux =15.9 ± 6.6 Tmol y -1 . The impact of the mid-Miocene Climatic Optimum isotope anomaly is an integrated excess deposition ≈ 4.3 × 10 6 Tmol between 18 and 11 Ma, which is both longer and larger than estimates for the total degassing by the Columbia River Basalt eruptions, implying a complex carbon system response to large eruptive events. Monte Carlo evaluation finds that late Cenozoic net growth of the carbonate reservoir is very likely while net growth of the C org reservoir is less certain but more likely than not. At present, subduction does not appear to keep up with net sedimentation and the overall masses of sedimentary carbonate and organic carbon are likely increasing. Growth in the sedimentary C org reservoir implies oxidation of the surface environment and likely increases in atmospheric pO 2 ., Competing Interests: Competing interests statement:The author declares no competing interest.

Estuaries are critical zones for understanding the dynamics of organic carbon (OC) burial, particularly in the context of human intervention, yet detailed information remains limited. This study presents comprehensive field observations aimed at assessing the spatial and temporal variations in sedimentary OC sources, composition, and distribution within the Yangtze Estuary. By employing a three-endmember estimation, we determined the contributions of fluvial, marine, and salt marsh plants to OC in the mouth bar as 52 ± 6 %, 28 ± 12 %, and 20 ± 9 %, respectively. To address the complexities introduced by human activities and dispersal processes, we utilized a PCA-MC four-endmember model, which revealed that redispersal terrigenous organic matter from the outer estuary, categorized as "marine origin", accounted for 26 ± 10 % re-deposited within the estuary. During the construction of the Deep Channel Navigation Project (DCNP), we observed a shift in terrigenous OC deposition from the North Passage (NP) to the South Passage (SP), along with an increased contribution from salt marsh plants. Post-DCNP completion, sedimentary OC compositions exhibited significant differences between the NP and SP, with the SP enriched in fresh terrigenous OC and the NP showing degraded signals due to dredging activities and source discrimination. Human interventions not only altered the estuarine geomorphology and hydrodynamics but also impacted OC burial and sequestration by modifying sources and compositions. This study highlights the transition of estuarine regions from carbon sinks to potential sources, underscoring the necessity for a comprehensive understanding of carbon cycle dynamics in Anthropocene-influenced estuarine environments., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier Ltd.)

Oceanic productivity is a vital component of the global carbon cycle. The productivity in the tropical oceans is strongly modulated by the monsoon winds. The monsoon winds are driven by the temperature. Therefore, the oceanic productivity is most likely to be influenced by the increasing global warming. However, the projected response of oceanic productivity to global warming is ambiguous. The past records can help to better constrain the future response of oceanic productivity to different warming scenarios. Here, we reconstruct decadal monsoon and productivity changes from the central-western Bay of Bengal, covering the entire Holocene. We use both foraminiferal (relative abundance of Globigerina bulloides , stable oxygen isotopic ratio of surface dwelling Globigerinoides ruber) and geochemical [calcium carbonate (CaCO 3), organic carbon (C org), organic carbon/total nitrogen (C org /TN)] proxies to reconstruct productivity and associated parameters. All the proxy data suggests a large variation in productivity and carbon burial during the Holocene. A very intense upwelling, concomitant with a phase of increased precipitation and CaCO 3 burial is inferred during the beginning of Greenlandian (~12,000 - ~10,000 yrs BP). The intense upwelling, however, did not result in higher organic carbon burial. The beginning of Northgrippian (~8000–7000 yr BP) was a significantly wet phase followed by gradual decrease in precipitation. Incidentally, the reduced upwelling during the Northgrippian was synchronous with a large increase in organic carbon burial. The opposite trend in upwelling intensity and %C org during the Meghalayan as well, suggests decoupling of upwelling induced productivity and organic carbon burial in the western Bay of Bengal. Towards the late Meghalayan (after ~1500 yr BP), we report a relatively enhanced precipitation with continual increase in CaCO 3 but with reduced C org burial. Further, these long-term variations were accompanied by significant short-term periodic changes, closely resembling Gleissberg cycle as well as Seuss cycle, suggesting a possible link with the solar variability. [ABSTRACT FROM AUTHOR]

Evidence suggests that boreal-lake ecosystems are changing rapidly, but with variable ecological responses, due to climate warming. Paleolimnological analysis of 27 undeveloped northern forested lakes showed significant and potentially climate-mediated shifts in diatom communities and increased carbon and biogenic silica burial. We hypothesize the sensitivity of northern forested lakes to climate change will vary along two physical gradients: one reflecting direct, in-lake climate effects (propensity to thermally stratify), the other reflecting indirect watershed effects (watershed to lake-surface area ratio). We focus on the historical response of algal communities to test our two-dimensional sensitivity framework. Historical algal response was summarized by measures of diatom community turnover, changes in species and diagnostic species groups, and measures of siliceous algal and overall primary production (biogenic silica, carbon burial). Measures of algal production increased across all lake types, with carbon burial proportionately higher in polymictic lakes. Greater diatom community change occurred in deep, stratified lakes with smaller watersheds, whereas diatom species groups showed variable responses along our two-dimensional sensitivity framework. Physical characteristics of lakes and watersheds could serve as predictors of sensitivity to climate change based on paleo-indicators that are mechanistically linked to direct and indirect limnological effects of climate change. [ABSTRACT FROM AUTHOR]

Post-depositional degradation of organic matter (OM) in freshwater sediments is crucial for driving the biogeochemical dynamics and influencing the carbon burial. This process also often causes diagenetic alteration on paleoenvironmental proxies. Yet, mechanisms behind degradation of sedimentary OM and depth-related variations in stable isotope ratios can so far only be explained in part. Degradation of sedimentary OM in two drinking water reservoirs with contrasting eutrophic and mesotrophic states and different catchment land use (agriculture versus forestry) was studied. A 4-step procedure was used to chemically separate sedimentary OM in terms of biochemical composition. Here we presented depth profiles of biochemical composition of sedimentary OM that helped to quantify preferential degradation of aquatic proteins and carbohydrates and the following removal of aquatic lipids. Sediment in the eutrophic reservoir, which reflected a larger contribution of algal-derived OM than the mesotrophic reservoir with a forest dominated catchment, was therefore subject to more intensive degradation of sedimentary OM along with δ13C and δ15N alterations. In addition, changes in the relative proportions of biochemical components in sedimentary OM had more pronounced impact on δ15N values relative to δ13C. Our findings suggest that the lability of algae-derived OM leads to uncertainties for the estimation of carbon burial in water bodies and obscures paleo-limnological information derived from isotopic proxies. Post-depositional modifications are more pronounced in eutrophic freshwaters that accumulated more readily degradable OM of algal origin in their sediments. Recognition of these modifications will help constrain carbon burial rates of productive lakes and reservoirs and assess the role of reservoirs in carbon cycling. [ABSTRACT FROM AUTHOR]

The delivery and burial of terrestrial particulate organic carbon (OC) in marine sediments is important to quantify, because this OC is a food resource for benthic communities, and if buried it may lower the concentrations of atmospheric CO2 over geologic timescales. Analysis of sediment cores has previously shown that fjords are hotspots for OC burial. Fjords can contain complex networks of submarine channels formed by seafloor sediment flows, called turbidity currents. However, the burial efficiency and distribution of OC by turbidity currents in river‐fed fjords had not been investigated previously. Here, we determine OC distribution and burial efficiency across a turbidity current system within Bute Inlet, a fjord in western Canada. We show that 62% ± 10% of the OC supplied by the two river sources is buried across the fjord surficial (30–200 cm) sediment. The sandy subenvironments (channel and lobe) contain 63% ± 14% of the annual terrestrial OC burial in the fjord. In contrast, the muddy subenvironments (overbank and distal basin) contain the remaining 37% ± 14%. OC in the channel, lobe, and overbank exclusively comprises terrestrial OC sourced from rivers. When normalized by the fjord's surface area, at least 3 times more terrestrial OC is buried in Bute Inlet, compared to the muddy parts of other fjords previously studied. Although the long‐term (>100 years) preservation of this OC is still to be fully understood, turbidity currents in fjords appear to be efficient at storing OC supplied by rivers in their near‐surface deposits. Plain Language Summary: Plants on land use CO2 from the atmosphere to produce organic carbon, which promotes their growth. Rivers transport organic carbon to the sea, where it is either eaten by fauna or buried in the seafloor, thus decreasing atmospheric CO2 levels on Earth over thousands to millions of years. Fjords are recognized as global organic carbon sinks; trapping 18 million tons of organic carbon in their seafloor sediments each year. However, the complex morphology of fjord seafloors was not considered in the calculation of this organic carbon flux. In this study, we determine the distribution and abundance of terrestrial organic carbon across a fjord (Bute Inlet, Canada), which contains a submarine channel network terminating onto a large accumulation of sand (called a lobe). We show that 62% of the organic carbon supplied by the two rivers connected to the fjord is buried across the fjord; the majority of this carbon being held in the lobe. In total, Bute Inlet buries at least 3 times more organic carbon per unit of surface area than other fjords previously studied. Submarine channels in fjords thus appear to promote the storage of land‐derived organic carbon in the seafloor, potentially impacting CO2 levels and food resources for marine fauna. Key Points: Bute Inlet, a river‐fed fjord incised by turbidity currents, has a contemporary terrestrial organic carbon burial efficiency of 62% ± 10%Sandy surficial deposits are responsible for 63% ± 14% of the total terrestrial organic carbon burial budget in Bute Inlet, but only cover 17% of the seafloor areaGlobal estimates based only on the muddy parts of fjords may significantly underestimate organic carbon burial rates by a factor >3 [ABSTRACT FROM AUTHOR]

High‐latitude, coastal wetland biogeochemistry is dynamic in response to climate change, and yet we do not understand, and thus cannot fully predict, how crucial aspects of these systems will change in the future. Temperatures in the Northern Hemisphere have disproportionately increased 4°C in 30 years causing the rate of deglaciation to increase significantly in global high‐latitude river deltas. This will have a prolonged effect on local microbiome metabolism and biodiversity of the subsurface, influencing solid, liquid, and gaseous compounds in the system. Using sediment geochemical analyses, autonomous sampling techniques, and 16S rRNA gene sequencing, we have identified key processes that occur in the Copper River Delta, AK, a model system to study high‐latitude watersheds during rapid climate change. We calculated carbon accumulation rates upwards of 520 ± 60 g C m−2 yr−1 in outwash pond sediments nearest to the glaciers, which co‐occurred with pronounced suboxic peaks in Fe (III) and Mn (II). Sediment microbial communities across the outwash ponds are structured on the basis of total iron and manganese concentrations, proximity to glaciers, and organic matter content. Additionally, we revealed no methane accumulation in the ponds during ice‐cover, despite high organic matter content. High‐latitude wetland ecosystems are not only influenced by the changing climate, but also have the potential to impact carbon cycling considering high carbon burial rates. These findings show the importance of understanding changing biogeochemical processes in high‐latitude wetlands, as they have the potential to influence carbon cycling. Plain Language Summary: Climate change is reshaping how ecosystems function. Average temperatures are rising most rapidly near Earth's poles, causing these cold regions to lose snow and ice. Water from melting glaciers carries minerals to wetlands where the activities of naturally occurring microorganisms may be altered. Our study focused on describing microbial communities and the chemistry in ponds on the Copper River Delta in Alaska, a glacially influenced coastal wetland, along a gradient between the glaciers and ocean. We sampled water and sediments from wetland ponds over several years and identified that ponds near the glaciers bury carbon in sediments faster than other recorded coastal wetlands. Our findings suggest that iron minerals from the glaciers and the microbes in the system may be associated with this carbon burial and how greenhouse gases are produced and consumed in this watershed. Our findings advance the understanding of the role of microbial communities and the associated biogeochemical processes that store or release carbon in a dynamic, high‐latitude setting. Key Points: Freshwater glacial outwash ponds in southcentral AK exhibit carbon accumulation rates up to 520 g C m−2 yr−1Microbial communities in glacial outwash sediments are shaped by iron, manganese, organic matter, and sediment load from melting glaciersMethane did not buildup during ice cover despite high organic matter content and anoxic conditions [ABSTRACT FROM AUTHOR]

Organic matter burial in the deep-sea fan sediments is an important component of the long-term carbon cycle. Although there is increasing recognition of the importance of organic matter in deep-sea sediments, a major focus has been on mudstones, commonly interpreted as the background sediments, deposited by pelagic or hemipelagic vertical suspension fallout in low-energy fan environments. Emerging evidence suggests that relatively coarse-grained sediment gravity flow deposits (e.g., turbidites and hybrid event beds) can also store a significant quantity of organic carbon, implying that a wide range of depositional processes can result in the concentration and enrichment of organic matter in submarine fans. However, the role of these processes on carbon burial is still not fully understood. This review aims to discuss the impact of three widely documented deep-sea depositional mechanisms/processes, namely vertical suspension settling, grain-by-grain (incremental aggradation), and the en-masse deposition on distribution, burial, and preservation of organic matter in deep-marine deposits. Organic matter accumulated from slowly settling suspension in mud caps (Te or H5 divisions of turbidites and hybrid beds, respectively) is prone to higher oxidation compared to the carbon buried in sandy components of turbidity currents (Ta-Tc units) and hybrid beds (H2/H3 divisions). The burial of organic matter in sandy parts of the deposits has important implications for understanding the fundamental physical processes that control carbon accumulation and preservation in deep-marine rock record. [ABSTRACT FROM AUTHOR]

Hydrological regulation and land uses are known to alter lake organic carbon stock, while inorganic carbon cycling in karst landscapes may as well be shifted due to its rapid kinetic rate. To date, the synchronous pattern in the variations of sediment organic carbon (TOC) and inorganic carbon (TIC) burial in karst lakes largely remains to be quantified along the gradient of catchment development. Two karst lakes with contrasting nutrient level and water depth from Southwest China, both of which were converted to medium-sized reservoirs in ∼1985, were selected for sediment multi-proxy analyses on the temporal and spatial variation of carbon burial in the last ∼80 years. While hydrological fluctuations were well recorded by the sediment variation of particle size, the gradual decrease in median grain size showed a trend of increasing water depth that is consistent with the damming events for both lakes. Meanwhile, the nutrient variables revealed an acceleration of eutrophication since ∼2000 when a rapid expansion of catchment croplands was documented for both lakes. While diatom assemblage showed a significant response to hydro-climatic fluctuation and nutrients at both lakes, algal production (i.e. chlorophyll and cyanobacterial pigments) significantly increased with eutrophication at both lakes. The TIC content in the mesotrophic Taiping Lake and the TOC content in the eutrophic lake of Sanjiaohai both showed a significant increase with agricultural expansion, however, the contents of TOC in Taiping and of TIC in Sanjiaohai remained relatively stable over time. While a significant increase in the sediment flux with decreasing bulk sediment C:N ratios suggests elevated autochthonous input, the temporal change of TIC and TOC flux showed a significant synchrony at both lakes. Meanwhile, the sediment content and flux of TIC is significantly higher than those of TOC in Taiping Lake with a higher rate of carbon turnover, but lower than those of TOC in Sanjiaohai. Thus, the accumulation of TIC was disproportionally promoted in deep lakes while the burial of TOC is more favorable in eutrophic and shallow lakes. Overall, our sediment evidence highlights that agricultural expansion and hydrological regulation could interact in altering the sediment stock of TIC and TOC, and their synchronous variation in Karst landscapes.

In this article, we discuss knowledge and gaps regarding blue carbon ecosystems (BCEs) in Brazil, considering the urgency to apply protection actions and policies to safeguard their biodiversity and associated ecosystem services. We also indicate areas of further research to improve carbon stocks and sequestration rate estimates. We call attention to the shortage of studies on Brazilian BCEs relative to the growing knowledge on the Blue Carbon Framework accumulated worldwide over the last decade. Considering the extensive Brazilian Economic Exclusive Zone (known as “Blue Amazon”), knowledge concerning blue carbon stocks is vital at regional and global scales for mitigating global increases in atmospheric carbon dioxide (CO2). The Blue Amazon has at least 1,100,000 ha of vegetated and non-vegetated coastal ecosystems (mangroves, salt marshes, seagrass meadows, and hypersaline tidal flats) that collectively contain vast amounts of stored carbon, making Brazil an ideal place to test mechanisms for evaluating, conserving, and restoring BCEs. Other poorly understood potential sinks and sources of carbon are macroalgal and rhodolith beds, mudflats, continental shelf sediments, and marine animal forests in shallow, mesophotic, and deep waters. The carbon fluxes between diverse environmental compartments, such as soil–air, soil–water, groundwater–water–surface water, air–water, and land–ocean, in BCEs across the Blue Amazon must be studied. We emphasize the importance of assessing the total carbon stock and the recent dismantling of environmental laws that pose great risks to these important BCEs. The conservation and recovery of these areas would enhance the carbon sequestration capacity of the entire country. Furthermore, we highlight priorities to improve knowledge concerning BCEs and their biogeochemical cycles in the Blue Amazon and to provide information to assist in the reduction of atmospheric levels of CO2 for the United Nations Decade of Ocean Science (2021–2030).

Eutrophication is one of the most important factors for the increasing production of organic carbon in inland waters. Variations in primary productivity across lakes due to eutrophication complicate predictions regarding future carbon burial and management of carbon sequestration in lakes. Phosphorus, an essential nutrient for plants and notorious for eutrophication, often limits primary productivity in lakes. It is crucial to normalize organic carbon burial to the content of total phosphorus (TP) in the water column to compare the inherent carbon burial capacity across lakes. However, the absence of long-term observational data of TP in lake water columns has hindered the ability to do so. In this study, we first verified that TP in sediments of Chinese lakes could substitute for that in the water column to reflect the annual primary productivity of lakes. Then we defined the inherent carbon burial potential of lakes as the total organic carbon to TP atomic ratio (TOC/TP) in the sediments. We quantified the inherent carbon burial potential of 90 lakes across China, exploring its spatial distribution, temporal trends, and potential controlling factors. The inherent carbon burial potential, based on surface sediments, decreased from western to eastern China, ranging from 466.1 to 24.3 with a mean of 137.0 ± 87.4. Mean annual precipitation was negatively related, while mean lake depth was positively related to the inherent carbon burial potential of the lakes across China. Analysis of sediment cores revealed that the mean inherent carbon burial potential of the lakes across China increased from 75.1 to 122.9 since the 1950s. Inherent carbon burial potential of the lakes in southwestern and eastern China, however, decreased since the 1990s. Changes in the extent of cultivated land in the watersheds were positively related to variations in the inherent carbon burial potential of Chinese lakes in the past few decades., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Norwegian fjords have been recently recognized as hot spots for carbon burial due to the large amounts of terrestrial organic matter delivered to fjord sediments, as well as the high sediment accumulation rates. Here, we present the first data on the contribution of benthic foraminiferal inorganic carbon to the sediments of three Norwegian fjords. Our study shows that calcareous foraminifera, which are among the most abundant calcifying organisms in the modern global oceans, can constitute between 15% and 33% of inorganic carbon accumulated in the sediments of the two studied southern Norwegian fjords (Raunefjorden and Hjeltefjorden). In a northern Norwegian fjord (Balsfjorden), the contribution of calcareous foraminifera to the inorganic carbon pool is smaller (<1%) than the one observed in southern fjords. We also found that the amount of foraminifera‐derived carbon is primarily dependent on the species composition of the foraminifera community. Large calcareous foraminifera species, despite a lower number of individuals, constitute, on average, 13%–29% of the inorganic carbon in the two southern Norwegian fjords, while the contribution of small, highly abundant species does not exceed 4% of the inorganic carbon pools in the sediments. Calcareous foraminifera species that are indicative of dysoxic conditions have been found to have low inorganic carbon contents per specimen compared to other analysed similar‐sized calcareous foraminifera species. This relationship most likely exists due to the thin test walls of these foraminifera species, which may facilitate gas exchange. The results of our case study suggest that the climate‐driven formation of near‐bottom low‐oxygen zones may lead to the dominance of foraminifera associated with dysoxic conditions and, in consequence, to the decrease of foraminifera‐derived inorganic carbon. However, to properly analyse the contribution of carbon from thin‐walled foraminifera to the sedimentary carbon pool, further studies analysing a broader range of these species is needed. [ABSTRACT FROM AUTHOR]

Lake carbon burial is of vital significance in global carbon cycle and carbon budget, particularly in the large deepwater lakes. However, carbon burial in large deepwater lakes is hard to estimate due to the difficulty in obtaining high spatial-resolution samples. In this study, we investigated distributions of total organic carbon (TOC) and inorganic carbon (TIC), two main carbon components in lake sediments, based on dozens of surficial sedimentary samples (n = 26) covering whole Lake Qinghai, the largest saline lake in China. The results showed that the TOC content, with a range of 1.4–4.8%, was significantly higher in the lake area near the northern lakeshore where human activities are concentrated and lower in the lake areas near the Buha River mouth and the eastern lake area. In contrast, the TIC content, ranging from 1.5 to 3.8%, increased from the northwestern and southeastern lake areas toward the lake center, and mainly depended on hydro-chemical and hydraulic characteristics. The inorganic carbon burial (47.77 ± 19.73 Gg C yr−1) was approximately equal to organic carbon burial (47.50 ± 22.68 Gg C yr−1) and accounted for about 50% of the total carbon burial (95.27 ± 37.74 Gg C yr−1), suggesting that saline lakes constitute a large inorganic carbon pool in addition to an organic carbon pool. Because of saline water body type in arid and semiarid regions and alpine Qinghai–Tibet Plateau, lakes in these regions have huge inorganic carbon burial potential and important contributions to the global carbon budget.

Terrestrial sediment supply and its composition, aquatic productivity and coastal hydrodynamics together control the nature of organic matter (OM) that buried in ocean margins. Distinguishing their individual roles in river-dominated continental margins is challenging, but crucial for understanding (i) the regional carbon cycle in modern times and (ii) the OM burial-climate link on shorter timescales. Here we investigate the contents of total organic carbon (TOC) and total nitrogen (TN) and stable carbon isotopic composition of TOC (δ13C), along with grain size and calcium carbonate, in three short, 210Pb- and 137Cs-dated, sediment cores (DH1-1, DH3-3 and DH5-1) from the Yangtze distal mud belt in the East China Sea to delineate the spatiotemporal distribution, origin and burial flux of sedimentary OM for the last ca. 130 years. We also use selected inorganic elements (Si, Al, Ca, Na, K, Rb and Sr) and geochemical proxies derived from them (i.e. chemical index of alteration-CIA, molar Al/Si, molar K/Na, K/Rb and Rb/Sr) to elucidate the importance of detrital composition and chemical weathering on the supply, transport and burial of OM. Bulk organic geochemical and grain size data suggest a longer transport of fine sediments-laden OM, indicating the role of China Coastal Current on hydrodynamic sorting in the study area. Spatial distribution of Al/Si, K/Na, Rb/Sr and CIA in all cores corroborates the selective transport of fine-grained minerals from the Yangtze estuary to the distal mud belt. Less variable molar C/N ratios (5.6–7.7) and δ13C values (–22.4 to –21.2‰) in our cores and the results of δ13C mixing model indicate the predominance of marine OM burial over the last century in the study area. Relationships of TOC, δ13C and mean grain size and spatial evolution of δ13C along the river-mud belt continuum show that adsorption of marine OM on the Yangtze-derived fine sediments during their southward transport. Estimated mean burial fluxes of terrestrial and marine OC (0.73 ± 0.21 and 1.88 ± 0.29 Tg C yr−1) indicate that the Yangtze distal mud belt, despite covering ~18% of the shelf area, acts as a key OM sequestration center (>50%) in the East China Sea. Comparison of records of riverine OC burial and geochemical weathering and grain size proxies of our cores with East Asian winter monsoon index, and sediment and water discharges in the Yangtze River confirms that hydrodynamics associated with the winter monsoon winds-driven China Coastal Current predominantly controlled the sediment accumulation and OM burial along the mud belt before 1970s. Nonetheless, human activities especially the construction of Three Gorges Dam have reduced the burial flux of terrestrial OC from 0.93 to 0.54 Tg yr−1, significantly affecting the supply and burial of OM and thus the regional carbon cycle since 1970s. [ABSTRACT FROM AUTHOR]

Understanding the exact origin of sedimentary organic matter (OM) was crucial for assessing carbon cycle processes and environmental contamination in continental margin. However, such studies are less performed on shelves far away from the mainland. Here, the abundance and composition of lipid biomarkers were determined in surface sediments from 58 sites at the eastern Sunda Shelf, South China Sea, to identify the sedimentary OM sources and characterize the spatial variation of OM input from specific sources. The results indicated that the sedimentary OM were derived mainly from algae, bacteria, seagrass, terrestrial plants, mangroves, petroleum, and rock erosion products, and the relative significance of these sources varied between different sites. Algal OM input, as indicated by the abundances of short-chain n -alkanes, cholest-2-ene, dinosterol, brassicasterol, and C 30 1,15 alkyl diol, was relatively high at five sites (ZW 47, 51, 55, 82, and 86). Conversely, long-chain n -alkane and long-chain n -alkanol proxies suggested terrestrial plant input was most predominant at three other sites (ZW68, 103, and 106), and high abundances of taraxerol and β-amyrin in these sites imply the dominance of mangrove vegetation in the terrestrial OM. UCM (unresolved complex mixture) and αβ-hopanes indicated a relatively high input of petroleum-derived OM at another five sites (ZW3, 8, 25, 51, and 97). The analysis of abundance variations in the biomarkers suggested a substantial spatial heterogeneity in terrestrial plant OM input and a much lower heterogeneity in algal OM input, probably resulting from great variation in riverine transport intensity and relatively homogeneous water environment across the studied region. Correlation analysis indicated that both algal and terrestrial plant OM significantly contributed to the OM buried in the sediments. These results can form a basis for understanding the carbon cycle processes in Sunda Shelf. The finding of petroleum contamination in several sites can benefit for environmental protection and management of the shelf. • Sedimentary OM source and spatial variation in OM input were studied in eastern Sunda Shelf. • Both algae and terrestrial plants contributed significantly to the sediment OM. • Mangroves may be the dominant component of terrestrial OM in some sites. • Input of petroleum-derived OM was prominent at a few sites of the shelf. • Input of terrestrial plant OM had a much greater spatial heterogeneity than algal OM. [ABSTRACT FROM AUTHOR]

[Display omitted] • The sediment provenance of North Manila Trench is mainly from Taiwan Island. • Turbidity currents carry biogenic debris and fresh organic carbon into deep ocean. • Turbidity currents bury organic carbon as much as millennium sedimentation. Turbidity currents play a significant role in sediment transport in deep ocean, profoundly impacting sedimentation in the abyssal sea. This study focused on a 375 cm long sediment core collected from the Manila Trench, revealing a distinctive 15 cm turbidite layer characterized by coarse sediment, an upward fining pattern, and a basal erosion surface. Analysis of rare earth elements and Nd isotope indicates that this turbidite layer originated from Taiwan Island. Initiated in the estuarine shelf region of southwestern Taiwan Island, the turbidity current transported substantial biogenic debris and marine fresh organic carbon into the Manila Trench. Consequently, the turbidite layer exhibited an elevated CaO content and more positive δ13C values. Despite lower TOC content in the turbidite layer due to coarse particle size, the mixture of marine fresh organic matter significantly lowered the carbon age of this layer compared to the strata both below and above it. The rapid sedimentation of the turbidite deposit led to an organic carbon burial flux comparable to the rate at which normal deep-sea sediment buries organic carbon over seven hundred years. Notably, finer sediment overlaying the turbidite layer acted as a cap, enhancing carbon sequestration within the trench. In contrast to nearby core GEO6, which documented over 11 turbidity currents, core S02 recorded only one event. Detailed bathymetric analysis highlights the crucial role of local topography in detecting and preserving turbidity currents. The insights gained from this study would advance our understanding of how turbidity currents shape and influence the deep ocean sedimentation. [ABSTRACT FROM AUTHOR]

Salt marsh vertical accretion that keeps pace with relative sea-level rise (RSLR) promotes habitat resilience and a continuously growing carbon stock. Vertical accretion, however, hinges on various salt marsh biogeomorphic feedback loops that respond to changing sediment supply, storms events, accelerating RSLR, elevation, and vegetation density. During the latter half of the 20th century, a large proportion of watersheds on the lower coastal plain of North America experienced land cover change. Many salt marshes show slow rates of vertical accretion over that period suggesting degradation and destabilization attributed to reduced upstream sediment loads and changing proportions of mineral sediment to organic matter. Here, we investigated a potential dichotomy observed at many sites in coastal North America between previously published accretion rates of the subtidal bay bottoms and adjacent salt marshes that were exceeding and lagging behind RSLR since 1950 CE, respectively. In 12 small coastal watersheds that formed within tributary and coastal prism incised valleys, we coupled 210Pb-derived accumulation rates of fringing salt marshes and previously published data from adjacent tidal creek bay bottoms with geospatial analysis of watershed land cover change since 1959, inundation duration, and vegetation density. Accumulation rates were higher at marsh sites within tributary incised valleys with lower relief watersheds, smaller tidal ranges, shorter inundation times, higher vegetation densities, and where land cover changes were dominated by increased cleared forest area. Additionally, mass accumulation rates (MAR) accelerated at 8 of the 12 marsh sites after a Major Land Cover Change (MLCC). Despite this acceleration in MAR, only 2 marsh sites recorded average sediment accumulation rates (SAR) greater than RSLR after a MLCC, which suggests that most of the marsh sites are drowning or being converted to subtidal habitat. This may be misleading, however, because modern marsh platform elevations at all but one marsh site were above local mean sea level, so the marshes could be accommodation limited. This work supports the disconnect found in previous research suggesting that estuaries are accumulating sediment at sustainable rates while their fringing salt marshes are not, which illuminates the complexities associated with sedimentation regimes in small local watersheds and their fringing salt marshes. • Land-cover change caused an acceleration of mass accumulation rates (MAR) at most marsh sites in this study. • Despite accelerating accumulation rates after a land-use change, most saltmarsh sites in this study are accreting slower than the rate of relative sea-level rise. • Accumulation rates at marsh sites in this study suggest vulnerability not seen in rates previously recorded in the adjacent subaqueous creek bed. [ABSTRACT FROM AUTHOR]

Organic matter burial in the deep-sea fan sediments is an important component of the long-term carbon cycle. Although there is increasing recognition of the importance of organic matter in deep-sea sediments, a major focus has been on mudstones, commonly interpreted as the background sediments, deposited by pelagic or hemipelagic vertical suspension fallout in low-energy fan environments. Emerging evidence suggests that relatively coarse-grained sediment gravity flow deposits (e.g., turbidites and hybrid event beds) can also store a significant quantity of organic carbon, implying that a wide range of depositional processes can result in the concentration and enrichment of organic matter in submarine fans. However, the role of these processes on carbon burial is still not fully understood. This review aims to discuss the impact of three widely documented deep-sea depositional mechanisms/processes, namely vertical suspension settling, grain-by-grain (incremental aggradation), and the en-masse deposition on distribution, burial, and preservation of organic matter in deep-marine deposits. Organic matter accumulated from slowly settling suspension in mud caps (Te or H5 divisions of turbidites and hybrid beds, respectively) is prone to higher oxidation compared to the carbon buried in sandy components of turbidity currents (Ta-Tc units) and hybrid beds (H2/H3 divisions). The burial of organic matter in sandy parts of the deposits has important implications for understanding the fundamental physical processes that control carbon accumulation and preservation in deep-marine rock record.

Global vegetated coastal habitats (VCHs) represent a large sink for organic carbon (OC) stored within their soils. The regional patterns and causes of spatial variation, however, remain uncertain. The sparsity and regional bias of studies on soil OC stocks from Chinese VCHs have limited the reliable estimation of their capacity as regional and global OC sinks. Here, we use field and published data from 262 sampled soil cores and 181 surface soils to report estimates of soil OC stocks, burial rates and losses of VCHs in China. We find that Chinese mangrove, salt marsh and seagrass habitats have relatively low OC stocks, storing 6.3 ± 0.6, 7.5 ± 0.6, and 1.6 ± 0.6 Tg C (±95% confidence interval) in the top meter of the soil profile with burial rates of 44 ± 17, 159 ± 57, and 6 ± 45 Gg C/year, respectively. The variability in the soil OC stocks is linked to biogeographic factors but is mostly impacted by sedimentary processes and anthropic activities. All habitats have experienced significant losses, resulting in estimated emissions of 94.2–395.4 Tg CO2e (carbon dioxide equivalent) over the past 70 years. Reversing this trend through conservation and restoration measures has, therefore, great potential in contributing to the mitigation of climate change while providing additional benefits. This assessment, on a national scale from highly sedimentary environments under intensive anthropogenic pressures, provides important insights into blue carbon sink mechanism and sequestration capacities, thus contributing to the synchronous progression of global blue carbon management. [ABSTRACT FROM AUTHOR]

Climate conditions and instantaneous depositional events can influence the relative contribution of sediments from terrestrial and marine environments and ultimately the quantity and composition of carbon buried in the sediment record. Here, we analyze the elemental, isotopic, and organic geochemical composition of marine sediments to identify terrestrial and marine sources in sediment horizons associated with droughts, turbidites, and floods in the Santa Barbara Basin (SBB), California, during the last 2,000 years. Stable isotopes (δ13C and δ15N) indicate that more terrestrial organic carbon (OC) was deposited during floods relative to background sediment, while bulk C to nitrogen (C/N) ratios remained relatively constant (~10). Long‐chain n‐alkanes (C27, C29, C31, and C33), characteristic of terrestrial OC, dominated all types of sediment deposition but were 4 times more abundant in flood layers. Marine algae (C15, C17, and C19) and macrophytes (C21 and C23) were also 2 times higher in flood versus background sediments. Turbidites contained twice the terrestrial n‐alkanes relative to background sediment. Conversely, drought intervals were only distinguishable from background sediment by their higher proportion of marine algal n‐alkanes. Combined, our data indicate that 15% of the total OC buried in SBB over the past 2,000 years was deposited during 11 flood events where the sediment was mostly terrestrially derived, and another 12% of deep sediment OC burial was derived from shelf remobilization during six turbidite events. Relative to twentieth century river runoff, our data suggest that floods result in considerable terrestrial OC burial on the continental margins of California. Key Points: Terrestrial organic carbon is the dominant source of carbon to the SBB with deposition significantly increasing during flood eventsEpisodic flood and turbidite remobilization events were responsible for over 25% of the OC buried in the SBB over the past 2,000 yearsDrought sedimentation had significantly lower sedimentation rates and had an n‐alkane composition consistent with increased marine inputs [ABSTRACT FROM AUTHOR]

Blue carbon habitats like salt marshes and mangroves bury large amounts of carbon with limited area; however, they also are increasingly susceptible to current climate change. Combined effects of rising temperatures, decreasing freeze frequencies, and increasing sea level rise rates are resulting in mangrove replacement of salt marshes along the southern United States. Surface soils analyzed here from wetlands along northern Florida Atlantic and Gulf Coasts showed higher apparent sedimentation rates in mangrove‐dominated sites (1.5–3.2 mm yr−1) and where mangroves are migrating into the marsh (termed transition sites, 2.3–3.8 mm yr−1). Average carbon burial rates were higher in transition sites for both coasts (27–47 g C·m−2·yr−1) compared to the respective mangrove (10–22 g C·m−2·yr−1) and salt marsh (4–7 g C·m−2·yr−1) sites. Lignin biomarker data (Λ‐6, Λ‐8, and C/V) indicated that mangrove and transition sites had higher lignin inputs from woody vascular plants compared to salt marsh sites, which may slow decadal‐ to centennial‐scale decay. Higher amino acid concentrations in mangrove soils relative to mangrove biomass (1.8–2.3 mmol g C−1 vs. 0.2–0.9 mmol g C−1) and lower C/N indicated that these mangrove sites receive higher algal inputs than the transition and salt marsh sites, attributed to greater tidal inundation in the mangrove sites given their position near the shoreline. Overall, increased accretion, carbon burial, and lignin in mangrove transition sites indicate that this migration may increase carbon burial and increase the stability and residence time of buried soil carbon. Future studies on mangrove migration in northern Florida can verify this through replication and elevation analysis. Key Points: Mangrove migration increases lignin‐rich woody material in the soil that relates to increased organic carbon preservationNorthern Florida sites with mixed mangrove and marsh vegetation had higher organic carbon burial than adjacent salt marshes and mangrovesAmong our mangrove sites, wave action and tidal pumping likely cause lesser organic carbon burial in shoreline mangroves [ABSTRACT FROM AUTHOR]

Carbon sequestration by sediments and vegetated marine systems contributes to atmospheric carbon drawdown, but little empirical evidence is available to help separate the effects of climate change and other anthropogenic activities on carbon burial over centennial timescales. We used marine sediment organic carbon to determine the role of historic climate variability and human habitation in carbon burial over the past 5,071 years. There was centennial‐scale sensitivity of carbon supply and burial to climatic variability, with Little Ice Age cooling causing an abrupt ecosystem shift and an increase in marine carbon contributions compared to terrestrial carbon. Although land use changes during the late 1800s did not cause marked alteration in average carbon burial, they did lead to marked increases in the spatial variability of carbon burial. Thus, while carbon burial by vegetated systems is expected to increase with projected climate warming over the coming century, ecosystem restructuring caused by abrupt climate change may produce unexpected change in carbon burial whose variability is also modulated by land use change. [ABSTRACT FROM AUTHOR]

Detailed organic geochemical and carbon isotopic (δ13C and Δ14C) analyses are performed on permafrost deposits affected by coastal erosion (Herschel Island, Canadian Beaufort Sea) and adjacent marine sediments (Herschel Basin) to understand the fate of organic carbon in Arctic nearshore environments. We use an end‐member model based on the carbon isotopic composition of bulk organic matter to identify sources of organic carbon. Monte Carlo simulations are applied to quantify the contribution of coastal permafrost erosion to the sedimentary carbon budget. The models suggest that ~40% of all carbon released by local coastal permafrost erosion is efficiently trapped and sequestered in the nearshore zone. This highlights the importance of sedimentary traps in environments such as basins, lagoons, troughs, and canyons for the carbon sequestration in previously poorly investigated, nearshore areas. Plain Language Summary: Increasing air and sea surface temperatures at high latitudes leads to accelerated thaw, destabilization, and erosion of perennially frozen soils (i.e., permafrost), which are often rich in organic carbon. Coastal erosion leads to an increased mobilization of organic carbon into the Arctic Ocean, which there can be converted into greenhouse gases and may therefore contribute to further warming. Carbon decomposition can be limited if organic matter is efficiently deposited on the seafloor, buried in marine sediments, and thus removed from the short‐term carbon cycle. Basins, canyons, and troughs near the coastline can serve as sediment traps and potentially accommodate large quantities of organic carbon along the Arctic coast. Here we use biomarkers (source‐specific molecules), stable carbon isotopes, and radiocarbon to identify the sources of organic carbon in the nearshore zone of the southern Canadian Beaufort Sea near Herschel Island. We quantify the contribution of coastal permafrost erosion to the sedimentary carbon budget of the area and estimate that more than a third of all carbon released by local permafrost erosion is efficiently trapped in marine sediments. This highlights the importance of regional sediment traps for carbon sequestration. Key Points: Permafrost organic matter derived from coastal erosion dominates nearshore marine sedimentsNearshore basins trapping sediments in the Arctic Ocean efficiently sequester permafrost organic carbon [ABSTRACT FROM AUTHOR]

Rates of organic carbon (OC) burial in some coastal wetlands appear to be greater in recent years than they were in the past. Possible explanations include ongoing mineralization of older OC or the influence of an unaccounted‐for artifact of the methods used to measure burial rates. Alternatively, the trend may represent real acceleration in OC burial. We quantified OC burial rates of mangrove and coastal freshwater marshes in southwest Florida through a comparison of rates derived from 210Pb, 137Cs, and surface marker horizons. Age/depth profiles of lignin: OC were used to assess whether down‐core remineralization had depleted the OC pool relative to lignin, and lignin phenols were used to quantify the variability of lignin degradation. Over the past 120 years, OC burial rates at seven sites increased by factors ranging from 1.4 to 6.2. We propose that these increases represent net acceleration. Change in relative sea‐level rise is the most likely large‐scale driver of acceleration, and sediment deposition from large storms can contribute to periodic increases. Mangrove sites had higher OC and lignin burial rates than marsh sites, indicating inherent differences in OC burial factors between the two habitat types. The higher OC burial rates in mangrove soils mean that their encroachment into coastal freshwater marshes has the potential to increase burial rates in those locations even more than might be expected from the acceleration trends. Regionally, these findings suggest that burial represents a substantially growing proportion of the coastal wetland carbon budget. Plain Language Summary: Coastal mangroves and marshes use photosynthesis to change carbon dioxide into plant material like leaves, wood, and roots. As these materials die over time, they do not decompose the same way they might in a dry soil environment. Instead, daily ocean tides flood the soil, removing the oxygen and allowing dead plant materials to be buried and preserved for hundreds to thousands of years or more. This "carbon burial" process is a global benefit because it means that carbon dioxide, a greenhouse gas, is stored belowground where it can no longer warm the atmosphere. Mangroves and marshes bury carbon more quickly than most other ecosystems, but little is known about how burial rates change over time. In this study, we quantify how rates of carbon burial have changed in southwest Florida coastal wetlands in the past century. We find that carbon burial is 2.5–4.5 times greater today than it was a century ago in mangroves and 1.9–2.3 times greater in marshes. We propose sea‐level rise as the most likely cause of increased carbon burial. Rising seas provide the means and opportunity for wetlands to bury more carbon, but if sea level rises too quickly, these wetlands may drown. Key Points: Organic carbon burial rates increased by factors of 2.5 to 6.2 for mangroves and 1.4 to 2.4 for coastal freshwater marshes in the past ~120 yearsIncreasing rates represent acceleration rather than a record influenced by an artefact of the dating method or postdepositional degradationAcceleration has increased soil carbon stocks by 5.7–8.0 kg m−2 for mangroves and 4.2–4.5 kg m−2 for coastal freshwater marshes [ABSTRACT FROM AUTHOR]

Globally, deltaic wetlands sequester large volumes of terrestrial and marine-derived organic carbon. Diminishing sediment supply via river diversions, dams, and/or sea level rise threatens this stored carbon by enhancing erosion, thereby potentially releasing CO2 back to the atmosphere during remineralization of organic matter. The Yellow River delta, located in the Bohai Sea, China, has undergone intense anthropogenic manipulation since the 1950s including rerouting of the river mouth to expand the delta for oil exploitation. The goal of this study is to identify the impacts of river course diversions on sources and rates of carbon burial in the modern and abandoned delta lobes of the Yellow River delta. In 2016, we collected four cores total in abandoned and modern deltaic lobes and measured vertical accretion, total carbon, total nitrogen, δ13C, and n-alkanes. The highest average mass accretion rate of 12,470.1 g m−2 year−1 is observed in the abandoned delta, although it no longer receives direct river sediment input. The modern and abandoned deltas are currently outpacing sea level rise, but vertical accretion rates are influenced by sediment trapping practices in the modern delta and redistribution of eroded sediments in the abandoned. Average carbon burial rates across both delta sites vary between 7.2 and 14.9 g OC m−2 year−1. Sediment-associated carbon at both sites is dominantly sourced from the Loess Plateau. To conserve wetlands across the Yellow River delta, sediment management practices that periodically reintroduce sediment-laden river water to former river courses, such as in the Mississippi delta, are suggested. [ABSTRACT FROM AUTHOR]

Sequestration of soil organic carbon (SOC) has been recognized as an opportunity to off‐set global carbon dioxide (CO2) emissions. Flipping (full inversion to 1–3 m) is a practice used on New Zealand's South Island West Coast to eliminate water‐logging in highly podzolized sandy soils. Flipping results in burial of SOC formed in surface soil horizons into the subsoil and the transfer of subsoil material low in SOC to the "new" topsoil. The aims of this study were to quantify changes in the storage and stability of SOC over a 20‐year period following flipping of high‐productive pasture grassland. Topsoils (0–30 cm) from sites representing a chronosequence of flipping (3–20 years old) were sampled (2005/07) and re‐sampled (2017) to assess changes in topsoil carbon stocks. Deeper samples (30–150 cm) were also collected (2017) to evaluate the changes in stocks of SOC previously buried by flipping. Density fractionation was used to determine SOC stability in recent and buried topsoils. Total SOC stocks (0–150 cm) increased significantly by 69 ± 15% (179 ± 40 Mg SOC ha‐1) over 20 years following flipping. Topsoil burial caused a one‐time sequestration of 160 ± 14 Mg SOC ha‐1 (30–150 cm). The top 0–30 cm accumulated 3.6 Mg SOC ha‐1 year‐1. The chronosequence and re‐sampling revealed SOC accumulation rates of 1.2–1.8 Mg SOC ha‐1 year‐1 in the new surface soil (0–15 cm) and a SOC deficit of 36 ± 5% after 20 years. Flipped subsoils contained up to 32% labile SOC (compared to <1% in un‐flipped subsoils) thus buried SOC was preserved. This study confirms that burial of SOC and the exposure of SOC depleted subsoil results in an overall increase of SOC stocks of the whole soil profile and long‐term SOC preservation. [ABSTRACT FROM AUTHOR]

The inland water system is a key conduit in transporting carbon from land to ocean, but this conduit is not a neutral pipeline because it releases greenhouse gases into the atmosphere and also stores carbon in sediment. Currently, the role of inland waters in the global carbon cycle is modified and complicated by the huge number of reservoirs that are associated with climatic alteration due to their carbon sequestration and greenhouse gas emission functions. Moreover, reservoirs are distinct from natural rivers and lakes in various ways, promoting the emission of greenhouse gases from their surfaces and carbon accumulation in sediment through high sedimentation rates and larger ratios of watershed areas to reservoir areas. Consequently, their contributions to the global carbon cycle will increase in the coming decade as a result of an increase in the number of reservoirs. This assessment (1) demonstrates that reservoirs play a significant role in carbon burial and greenhouse gas emissions, (2) defines factors that influence the carbon budget of reservoirs, and (3) explains how reservoirs affect global climatic change. According to previous findings, reservoirs could serve as carbon sinks or sources, depending on the age, location, and climate of the given reservoir. Further, the magnitude of carbon burial and greenhouse gas emissions in reservoirs depends on productivity, land use, geology, water body type, and watershed morphometry. So far, few studies have examined both carbon storage and greenhouse gas emissions in reservoirs, and many efforts to quantify burial and emissions have been compromised by limited data availability. In this review, we mainly focus on recent literature on carbon burial and greenhouse gas emissions in reservoirs and global estimates of terrestrial carbon input to inland waters. [ABSTRACT FROM AUTHOR]

Glacial-interglacial variations in CO2 and methane in polar ice cores have been attributed, in part, to changes in global wetland extent, but the wetland distribution before the Last Glacial Maximum (LGM, 21 ka to 18 ka) remains virtually unknown. We present a study of global peatland extent and carbon (C) stocks through the last glacial cycle (130 ka to present) using a newly compiled database of 1,063 detailed stratigraphic records of peat deposits buried by mineral sediments, as well as a global peatland model. Quantitative agreement between modeling and observations shows extensive peat accumulation before the LGM in northern latitudes (>40°N), particularly during warmer periods including the last interglacial (130 ka to 116 ka, MIS 5e) and the interstadial (57 ka to 29 ka, MIS 3). During cooling periods of glacial advance and permafrost formation, the burial of northern peatlands by glaciers and mineral sediments decreased active peatland extent, thickness, and modeled C stocks by 70 to 90% from warmer times. Tropical peatland extent and C stocks show little temporal variation throughout the study period. While the increased burial of northern peats was correlated with cooling periods, the burial of tropical peat was predominately driven by changes in sea level and regional hydrology. Peat burial by mineral sediments represents a mechanism for long-term terrestrial C storage in the Earth system. These results show that northern peatlands accumulate significant C stocks during warmer times, indicating their potential for C sequestration during the warming Anthropocene. [ABSTRACT FROM AUTHOR]

Abstract Significant Ediacaran paleoenvironmental changes recorded in the mixed carbonate-siliciclastic Bambuí Basin, east-central Brazil, outcrop in one of the best-preserved sites, the Januária paleo-high. From a detailed stratigraphic and chemostratigraphic approach (carbon and oxygen isotopes, total organic carbon, total sulfur and selected elements abundances), we identified transgressive-regressive intervals and depositional settings within the Bambuí Group stratigraphy and developed a new model for the origin of the extremely positive Middle Bambuí Excursion (MIBE) present in the middle section of the basin. A post-Marinoan cap carbonate interval is recorded in the first ~10 m, preserving aragonite pseudomorph fans, barite-rich layers and negative δ13C anomaly. A hiatus separates the cap carbonate from the late Ediacaran succession that makes up the remaining Bambuí Group. The younger intervals preserve both siliciclastic (middle Serra de Santa Helena Formation) and carbonate (middle/upper Sete Lagoas and Lagoa do Jacaré formations) shallow-water depositional settings, requiring tectonic influence or climatic changes in source areas. The MIBE yields δ13C values as high as +14‰ and extends for about 350 m, from the upper Sete Lagoas Formation to the lower Serra da Saudade Formation. We suggest a model of a restricted basin setting that favored local carbon isotopic signals in the δ13C record of sedimentary carbonates through preferential weathering of ancient carbonate platforms on the continent and higher burial rate of authigenic carbonate. This scenario caused the global δ13C budget to be affected by an offset of, at least, +4‰ in sedimentary carbonates deposited on the São Francisco craton and along its margin during the terminal Ediacaran. Many previous studies suggested a restricted nature for the middle Bambuí Basin, which probably resulted from the central position of the São Francisco paleocontinent within the mosaic of collisional blocks during the SW Gondwana amalgamation. Highlights • A complete stratigraphic and chemostratigraphic data of the Ediacaran Bambuí Basin of Brazil. • Peritidal carbonates and sandstones to deep-water shales make up the mixed carbonate-siliciclastic sedimentation. • The MIddle Bambuí Excursion (MIBE) yields extremely heavy δ13C values as high as +14‰. • Paleogeography in the terminal Ediacaran probably had an important role in basin restriction and positive isotope excursion. [ABSTRACT FROM AUTHOR]

Abstract Hydrological control of lakes has been increasingly practiced in many parts of the world, however, the long-term ecological impact of hydrological regulation and their dependence on lake impoundment intensity has been rarely examined. We combined a spatial survey of surface sediments with sediment core analyses to quantify the limnological changes over the last two centuries for an oligo-mesotrophic lake, which was dammed in 1957 and reinforced during 1987–1990, respectively. A water depth inference model constructed from surface sediment clay components was applied to a well-dated sediment core for water level reconstruction. The inferred water depth increased from 6.2 ± 0.9 m to 8.7 ± 1.7 m after dam construction and further to 13.6 ± 2.6 m after dam enforcement, resulting in an increase in the magnitude of water level fluctuation (WLF). Accordingly, bulk sediment C/N ratio and median grain size spiked in ∼1957 and ∼1990, respectively, reflecting a large input of terrestrial sources due to impoundment. With a consistent loss of littoral zone and benthic diatoms over time, a significant decrease in C/N ratio and an abrupt depletion of carbon isotopic signal suggested a shift of carbon transfer towards a pelagic pathway after ∼1990. While there was a significant increase in algal production since ∼1990, the accumulation rate of carbon and nitrogen burial displayed an accelerating drop since ∼1957, reflecting a diluting effect derived from expanding water storage. Furthermore, there was a significant increase in both the ratio between inorganic and organic carbon fluxes and sediment burial of inorganic carbon, reflecting enhanced degradation and low storage of aquatic organic carbon in stratified deep waters since ∼1990. Hydro-morphological variables were found to exert strong impact on diatom communities, with an increasing interplay with nutrient and climate variables over time. While there existed a significant shift of diatom composition in ∼1960, species richness and community dissimilarity showed a significant decrease when water depth was raised to above ∼10 m or the magnitude of WFL was above ∼2 m. Thus, our sediment surveys provide evidence on the significant impact of lake regulation on hydro-morphology, carbon burial and ecological shift over time, as well as its stronger interaction with other forcing with increased impoundment intensity. Graphical abstract Image 1 Highlights • Lake impoundment caused a loss of littoral zone and benthic trophic pathway. • Lake level increase significantly decreased sediment carbon accumulate rate. • There existed a depth threshold for the temporal decline of diatom biodiversity. • Large hydrologic variation caused species loss and community homogeneity. • Intensifying impoundment enhanced ecological forcing of nutrients and climate. [ABSTRACT FROM AUTHOR]

• Long-term changes of OM input in Lake Chaohu sediments were assessed. • Anthropogenic activities influenced OM input via shaping local environment and ecosystems. • Agricultural activities enhanced terrestrial allochthonous OM input. • Dam installation inhabited OM input from submerged macrophytes. • Industrialization and urbanization promoted OM input from phytoplankton. Characterizing how and to what extent human activities have influenced organic matter (OM) input in lake sediments is essential for a thorough study of the carbon cycle and carbon burial in lake systems, but this issue has not yet been addressed. Here, the source and composition of OM in sediments of Lake Chaohu, Eastern China, over the past 166 years, were determined by analyzing stratigraphic variations in aliphatic hydrocarbons and OM pyrolysis parameters in a dated sediment core. These data, coupled with documental records, were further used to elucidate the pattern and controlling factors of OM input in the sediments. The lowest hydrogen index (HI), lowest long-chain n -alkane concentrations, and highest Paq (proxy of aquatic macrophyte input) in sediments prior to ca. 1956 indicate a minimal input of OM from phytoplankton and terrestrial plants and a larger input from submerged macrophytes. This may represent the features of OM input in primitive conditions given the mild human activities around the lake before the 1950s. In sediments from ca. 1962 to 1985, elevated HI and C 17 /C 16 n -alkane ratios indicate a greater input from phytoplankton, while the high abundances of long-chain n -alkanes and αβ-hopanes reflect an anomalously high input from terrestrial plant and fossil OM. These variations might have been caused by the rapid development of agriculture during this period, which accelerated catchment erosion and petroleum contamination and promoted the transport of allochthonous OM and nutrients to the lake. The lowest Paq in sediments during this period indicates reduced input from submerged macrophytes, probably resulting from a dam installation that modified the water to an unfavorable plant depth. In sediments after ca. 1989, the highest values of HI, short-chain n -alkane abundances, and C 17 /C 16 n -alkane ratios indicate a further increase in OM input from phytoplankton, a response to domestic and industrial discharge and subsequent lake eutrophication. [ABSTRACT FROM AUTHOR]