Current Issue

The sediment source-to-sink system serves as a critical link connecting active carbon pools (e.g., atmosphere, biosphere, hydrosphere) with the stable lithospheric carbon pool, playing a core buffering role in the global carbon cycle. As the core area of marine sedimentary carbon sinks, delta-shelf regions account for over 80% of the global marine sedimentary organic carbon flux while occupying less than 8% of the global ocean area. The processes and mechanisms of carbon burial in these regions are crucial for global carbon balance. This paper systematically reviews the source composition and sedimentary flux characteristics of terrestrial organic carbon in delta-shelf sedimentary systems, focuses on elaborating organic carbon source-to-sink tracing technologies, remineralization processes and their dominant mechanisms, analyzes the impacts of human activities on sedimentary carbon sinks, and discusses marine negative emission and carbon sequestration enhancement schemes based on sediment management. Studies show that the heterogeneity of terrestrial organic carbon, physicochemical conditions of the sedimentary environment, and human disturbance collectively regulate the migration, transformation, and burial efficiency of organic carbon. Currently, the potential of sedimentary carbon sinks has not been fully exploited; thus, it is urgent to promote the integration of sedimentary carbon sinks into the global climate governance system through methodological innovation, mechanism deepening, and technological development, so as to provide scientific support and feasible paths for achieving the temperature control goals of the Paris Agreement.

Deep-water sedimentary processes are key drivers that shape seafloor topography and actively participate in marine material cycles, thereby playing a crucial role in the formation of depositional systems and material cycling along continental margins and within deep-sea basins. The transport and transformation of carbon elements and carbon-containing substances are essential for sustaining organic life and maintaining climate stability. As an important end-member reservoir in this cycle, deep-sea sediments act as efficient sinks for atmospheric greenhouse gases, exerting significant regulatory effects on climate evolution over geological timescales. This study aims to elucidate the coupling mechanisms between distinctive deep-water sedimentary processes and organic carbon burial, providing a theoretical basis for establishing the “Shelf edge-slope-deep sea basin organic matter continuous transport system” and the “Deep-water organic carbon burial pyramid model”. By comprehensively analyzing representative deep-water organic carbon burial systems in global ocean basins, this research demonstrates that turbidity currents and bottom currents are the main dynamic mechanisms enabling the continuous transport of deep-water organic matter. The (micro)biological carbon pump, turbidity current carbon pump, bottom current carbon pump, and deep stratigraphic carbon pump together form the core framework for deep-water sedimentary carbon burial. Furthermore, the factors influencing deep-water organic carbon burial outcomes exhibit hierarchical characteristics. However, current research on deep-water organic carbon burial is still in its early stages, with limited case studies and mechanistic understanding, underscoring the urgent need to strengthen research on carbon burial processes in deep-water environments.

Black carbon (BC), a refractory organic carbon, is produced during the incomplete combustion of biomass and fossil fuels. Globally, an estimated 3%-10% of the annual BC production ultimately buried in marine sediments. As a critical component of the inert carbon pool, its spatiotemporal distribution and source-to-sink processes are essential for understanding global carbon cycling and climate evolution. Based on published BC data from nearly 1 000 marine sediment samples worldwide, this study reveals that BC contents vary widely, from 0.02 to 9.72 mg/g, with averaging 1.06 mg/g and accounting for an average of 15.1% of total sedimentary organic carbon. Spatial patterns are controlled by sediment grain size, organic carbon content, and depositional environments while temporal variations reflect the combined influence of climate change and human activities. Current knowledge of marine sedimentary BC sources predominantly assumes terrestrial dominance, with riverine transport, atmospheric deposition, and coastal erosion as primary input pathways. However, emerging evidence indicates that BC sinking fluxes in mid- to deep-ocean layers substantially exceed known terrestrial supply. This raises the possibility of potential unidentified sources. In addition, BC degradation and recycling processes within the marine systems remain poorly understood. Future research must prioritize source-to-sink dynamics in key areas (e.g., deep-sea environment) by integrating geochemical and organic molecular isotopic techniques to resolve BC cycling mechanisms and address current budget imbalances.

As a pivotal intertidal blue-carbon ecosystem, the seagrass-mangrove continuum is a focal point of contemporary blue-carbon research. In contrast to individual ecosystems, the continuum facilitates lateral carbon transport and redistribution between systems via tidal forcing—a process that profoundly influences regional and even global assessments of blue carbon budgets. However, the internal carbon cycling within the continuum and the multi-interface, multi-process coupling mechanisms of carbon sequestration remain a black box, representing one of the hot topics in blue carbon research. Here we systematically synthesize current understanding of carbon cycling in the seagrass-mangrove continuum, mapping key processes—from plant photosynthetic carbon sequestration, sediment carbon accumulation to aquatic carbon transformation, and gas exchange—within a novel, dual perspective of “vertical sequestration vs. lateral transport”. Special emphasis is placed on tide-driven lateral carbon fluxes (e.g., litter fall, DOC, POC, DIC), highlighting their central role in the continuum’s carbon dynamics. Given the complexity of intertidal habitats, future research on the coupled carbon cycling mechanisms within the seagrass-mangrove continuum remains a critical and underexplored field. In particular, key processes such as plant carbon fixation mechanisms, sediment carbon accumulation, and elemental exchange urgently require further investigation.

Coastal wetlands have a strong capacity for carbon capture and storage, playing a significant role in mitigating climate warming. Vegetation type is an important factor influencing the carbon storage. In this study, we measured the soil organic carbon (SOC), lignin, stable carbon isotope (δ¹³C), grain size, and iron content in the surface soil of three typical vegetation habitats (Phragmites australis, Phragmites australis-Tamarix chinensis, and Suaeda salsa) in the Yellow River Estuary wetland, and analyzed the content, source, and degradation characteristics of organic carbon. The results showed that the SOC content in the three vegetation habitats of the Yellow River estuary wetland ranged from 0.34% to 1.85%, with the highest in P. australis, which had an average value of 0.94%. The SOC content was jointly affected by vegetation type and clay content. The three-end-member Monte Carlo model calculation found that the soil organic carbon in the three vegetation habitats was mainly from terrestrial (47.7%±13.2%) and plant sources (36.3%±15.0%), with a relatively low marine source (16.0%±14.2%) (S. salsa>P. australis-T. chinensis>P. australis). The soil lignin in the three vegetation habitats all showed a mixture or single source of woody and herbaceous tissues, indicating that part of the soil organic carbon in the P. australis and S. salsa habitats originated from the upstream Loess Plateau. Iron oxides and water in the soil might reduce the degradation of lignin due to their protective effect on organic carbon and their inhibitory effect on aerobic respiration of microorganisms. This study showed that there were significant differences in the distribution, source, and degradation characteristics of soil organic carbon among different vegetation habitats.

Seagrass meadows sediments are important marine carbon reservoirs, and accurately assessing their carbon stocks is fundamental to evaluate the carbon sink function of seagrass ecosystems. However, compared with organic carbon pools, the characteristics of inorganic carbon pools in the sediments of seagrass meadows remain poorly understood. This study focused on the tropical mixed seagrass meadows in Li’an Lagoon, Hainan. Through field sampling, we analyzed the inorganic carbon stock and its sources in sediments from areas dominated by Enhalus acoroides, Cymodocea rotundata, and Thalassia hemprichii, as well as from adjacent bare areas, thereby clarified the influence of seagrass growth on sedimentary carbon burial. The results show that inorganic carbon stocks in sediments in seagrass-covered areas are significantly higher than those in adjacent bare areas (59.84±24.55 Mg·ha^-1). Significant differences also exist among seagrass species: the inorganic carbon stocks in sediments associated with E. acoroides, C. rotundata, and T. hemprichii are 105.66±9.45 Mg·ha^-1, 96.09±31.36 Mg·ha^-1, and 75.05±8.17 Mg·ha^-1, respectively. Moreover, larger seagrass plants correspond to higher sediment inorganic carbon stocks. Carbonate stable-isotope analyses indicate that calcifying benthic organisms are the primary source of inorganic carbon in sediments within seagrass-covered areas, whereas sediments in adjacent bare areas mainly derive their inorganic carbon from calcifying epiphytic algae. Based on these findings, this study indicates that seagrass meadows enhance calcium carbonate production by increasing the abundance of calcifying benthic organisms, while their flow-attenuation and sediment-stabilization effects favor the burial of carbonate minerals. The combined effects of these processes result in a substantial increase in sedimentary inorganic carbon storage. Furthermore, we quantify the offsetting effect of CO₂ release associated with calcium carbonate formation on the organic carbon sink in seagrass meadows, providing a scientific basis for accurately evaluating the carbon-sink capacity of tropical mixed seagrass meadows.

To investigate how tidal dynamics influence the composition of dissolved metabolites in seawater, we collected seawater samples across a semidiurnal tidal cycle, spanning both the salinity and turbidity fronts of the Changjiang River Estuary in March 2023. Using high-resolution mass spectrometry-based untargeted metabolomics, a total of 1 379 metabolite molecules were annotated (annotation rate of 6.8%), covering 14 super classes. Among these, organoheterocyclic compounds, benzenoids, lipids and lipid-like molecules were dominant (accounting for 59.1%). The results indicated that the composition of metabolites was significantly influenced by tidal forces, with greater heterogeneity during ebb tide than flood tide, revealing a temporal asynchrony between tidal movement and metabolite compositional variation. Statistical analyses further demonstrated that metabolite compositions were significantly negatively correlated with salinity, and that non-conservative nutrients (nitrate, phosphate, and silicate) exerted strong influences on metabolite variations. These results suggested that terrestrial dissolved organic matter inputs were a primary driver, with their influence further modified by brackish water mixing. In addition, the correlation between turbidity and most metabolites was relatively week. However, substantial percentage changes in lipids and lipid-like substances were observed within the suspended sediment front, indicating that the transformation of dissolved organic matter driven by resuspension processes primarily occurs under strong hydrodynamic conditions. Analysis of differential metabolite during flood and ebb phases further showed that these substances were dominated by secondary metabolites, such as lipids and heterocyclic compounds. These substances might reflect regulation by microbial community interactions. Overall, this study highlights the short-term tidal influence on dissolved organic matter composition and provides new insights into how multi-scale physical processes regulate organic matter cycling in estuarine environments.

The estuarine turbidity maximum (ETM) is a critical hub for the transport of particulate organic carbon (POC) from estuaries to ocean. To investigate the provenance and transport patterns of POC within the ETM, the Jiulong River ETM was selected as a research site. Hourly-resolved hydrological parameters, horizontally transported POC (collected via filtration), and vertically settling POC (collected using sediment traps) were systematically sampled. POC was analyzed for organic carbon isotopes, and the Monte Carlo end-member model was employed to analyze the relative contribution of different endmembers. Then the empirical orthogonal function (EOF) analysis was applied to examine and discuss the transport patterns and their driving mechanisms of POC within the ETM. The results revealed significant spatiotemporal variations in POC sources: surface POC was primarily of riverine sources (35.5%), while bottom POC was dominated by sedimentary sources (35.1%). Settling POC was also mainly derived from sedimentary sources, reaching up to 65% in high-flow flood tide periods. The bottom POC mass concentrations (0.8-8.4 mg·L^-1) showed a significant positive correlation with the magnitude of bottom tidal current velocity (absolute range: 0-0.5 m·s^-1). The peak settling flux of particles (227.1 mg·cm^-2·h^-1) occurred during low-flow periods (profile-averaged velocity <0.2 m·s^-1). Based on the results, it is find that tidal current velocity is a key factor regulating the sources and transport of POC within the Jiulong River Estuary’s ETM. It influences the horizontal transport, resuspension, and vertical mixing processes of POC through its magnitude, direction, and duration, thereby governing the provenance composition and mass concentration of POC. This control manifests specifically in three ways: a significant positive correlation exists between tidal current velocity and POC mass concentration, where high-velocity currents primarily drive the resuspension of sediments, making this process the main source of sedimentary POC; the alternating flow patterns of flood and ebb tides are the dominant control for the shifting predominance between river and marine POC; and velocity stratification (especially during the ebb tide stage) governs the vertical mixing intensity of POC from different sources. Moreover, from the perspective of how tidal current velocity regulates POC sources and transport processes within the ETM, this study summarizes the POC transport models for different tidal stages: During the flood tide, currents drive the input of marine POC. However, the high flow velocities cause significant sediment resuspension, resulting in the overall dominance of sedimentary POC. During the high slack tide, characterized by low flow velocities, particle settlement predominates, and sediment resuspension diminishes. This leads to a relative increase in the proportions of river or marine POC. Although the contribution of sedimentary POC decreases, it remains the dominant source overall. During the ebb tide, the outgoing currents facilitate the input of river POC. Meanwhile, the high flow velocities in the surface layer have a limited effect on adding sedimentary POC. The proportional distribution among the three POC sources is closely linked to the duration of the preceding high-velocity flood tide; generally, a longer duration leads to more pronounced dominance of sedimentary POC. During the low slack tide, which also features low flow velocities, settlement is again the primary process. The contributions of the three POC sources are generally comparable during this stage. These findings provide a valuable reference for a deeper understanding of the source-to-sink processes of POC within the ETM.

Under the dual pressures of global change and human activities, the impact of extreme storm events on estuarine sedimentary processes and carbon sink capacity has become increasingly prominent. However, traditional single-core methods face challenges in quantitatively characterizing the erosion-deposition processes and net carbon burial effects induced by storm events. In this study, we conducted repetitive coring at a fixed station near the Dayushan Island in the Hangzhou Bay, China in 2021 (before Typhoon Chanthu) and 2022. Through analysis of sedimentary structures, grain size and radionuclides, we identified an intense erosion-deposition event triggered by Typhoon Chanthu. Combined with elemental composition and organic geochemical indicators, we further quantitatively assessed its net impact on carbon burial. The results demonstrate that despite the efficient organic carbon preservation within the storm layer, the related intense erosion resulted in a net organic carbon deficit of 1 950±523 g·m^-2. This finding highlights a preservation bias of the storm layers in sedimentary records, leading to a systematic overestimation of the storm contribution to estuarine carbon burial in studies relying on single cores. The “repetitive coring-radionuclides tracing” methodology developed in this study provides a new paradigm for accurately assessing estuarine sedimentary processes and carbon cycling under non-steady-state conditions.

River deltas are critical global sinks for organic carbon (OC). To elucidate their evolutionary patterns under natural and anthropogenic influences, this study systematically reconstructs the OC burial history of the Changjiang Delta since the mid-to-late Holocene (8 ka BP), based on chronological, sedimentological, and organic geochemical data from 50 boreholes. The results revealed that the sediment accumulation rate was the core driver controlling the OC burial flux, with the two showing a strong positive correlation (r²=0.87). However, a significant decoupling existed between the OC burial flux and the total OC content, with the latter remaining stable within a low range of 0.41%-0.52% throughout the study period. This was primarily constrained by the dual effects of clastic dilution and particle size sorting. The provenance of OC showed a distinct phased evolution: from 8 to 2 ka BP, source variations were mainly driven by natural factors, with sea-level rise (8-5 ka BP) and the weakening of the East Asian Summer Monsoon (5-4 ka BP) successively leading to a decrease in the terrestrial OC fraction. Since 2 ka BP, human activities had become the dominant factor, profoundly reshaping the delta’s geochemical signals by altering sediment provenance zones within the catchment. This study unveils the complete process of the Changjiang Delta’s carbon sink function transitioning from a dynamic equilibrium under a natural background to being intensely disturbed in the Anthropocene, providing crucial scientific insights for understanding and predicting the vulnerability of deltaic carbon reservoirs in the context of global change.

Estuarine and coastal sediments serve as significant reservoirs for organic carbon storage, play a crucial role in the long-term carbon burial process and influence atmospheric CO₂ content variations. The lability (degradability) of organic carbon is one of the key factors determining its preservation efficiency in marine sedimentary environments. However, systematic understanding of the distribution characteristics and controlling mechanisms of labile organic carbon in marine sediments remain limited. Six sediment cores collected from the Changjiang (Yangtze River) Estuary and the adjacent East China Sea continental shelf were selected in this study. Using chemical oxidation methods, the study distinguished and quantified the content characteristics of labile and refractory organic carbon to reveal their evolution with sedimentary depth. By integrating chlorophyll-a content, CT scan data, and physicochemical parameters (pH, water content), the study systematically examined the primary environmental control factor influencing the chemical stability of sedimentary organic carbon. Results showed that the total organic carbon content in the sediment cores ranged from 0.16% to 0.90%, predominantly consisting of labile components (accounting for 51%-88%), primarily derived from marine organic matter. Sedimentary particle composition dominated the distribution of total organic carbon through the adsorption of labile organic carbon. Within the study area, finer grain size, moderate water content (70%), slightly alkaline pH (7.6-8.0), and weaker bioturbation intensity (0-2%) were favorable for the preservation of labile organic carbon.

Based on samples collected during the cruise in the Pearl River Estuary-northern South China Sea, this study combines laboratory-based experimental analysis and data analysis methods to illustrate the spatial distribution pattern and key influencing factors of iron-bound organic carbon, and to explore the preservation of organic carbon by reactive iron in the sediments. The average organic carbon content in the surface sediments of the Pearl River Estuary-northern South China Sea is 0.53%±0.36%, while the average iron-bound organic carbon content is 0.14%±0.12%, with the proportion of iron-bound organic carbon reaching approximately 25%. The spatial distribution of iron-bound organic carbon content correlates with that of organic carbon content and reactive iron content, exhibiting higher content in the Pearl River Estuary, western land slope, and abyssal plain, and lower content in the shelf and eastern land slope. The reactive iron content and its binding mode with organic carbon are the primary factors determining the spatial distribution and preservation mechanism of iron-bound organic carbon. Compared to the eastern China Sea, reactive iron exhibits a stronger preservation capacity for organic carbon in the South China Sea, indicating the significant role of reactive iron in carbon preservation in the tropical sea.