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Continental marginal seas are key systems in the global carbon cycle. Carbon stocks represent the ability to store carbon, thus quantifying the carbon stocks in marine sediments would help to better understand their importance in the carbon cycle. In this study, 17 sediment cores in the mud areas of the South Yellow Sea and the East China Sea were measured for total organic carbon (TOC) and its stable isotope (d13C), and dry bulk density; and from which the carbon stocks and carbon stock accumulation rate as well as marine/terrestrial carbon stocks/carbon stock accumulation rate were calculated. The carbon stocks in the mud area of the South Yellow Sea showed a decreasing trend during 1855 to 1950 caused by the relocation of the Yellow River Estuary in 1855, but increased after 1950s due to increased sediment input via the enhancement of South Shandong Coastal Current. In the Min-Zhe belt of the East China Sea, carbon stocks showed an overall high marine proportion due to the phytoplankton bloom induced by high nutrient level, but the decreased carbon stocks in recent decades were mainly caused by the construction of reservoirs in the Yangtze River that reduced sediment transports. The average carbon stocks in 1 m sediments from the South Yellow Sea (45.2 t ha-1) and Min-Zhe belt (52.8 t ha-1) were low compared to that of global marine sediments (66.6 t ha-1), while the carbon stock accumulation rate showed much higher values (0.1 t ha-1 yr-1 in South Yellow Sea and 0.31 t ha-1 yr-1 in the Min-Zhe belt) because of higher sedimentation rates. Although carbon stocks of Chinese marginal seas were also lower than that of the tidal flats (70.7 t ha-1) and wetland (123.6 t ha-1) in China, their much larger area could store 0.75 Pg C in marine sediments. Our temporal records suggest that anthropogenic activities have reduced carbon stocks in the marginal seas since 1950, causing carbon to re-enter the atmosphere to impact climate changes. [ABSTRACT FROM AUTHOR]

Shallow coastal muddy sediments are important for global nutrient cycling and carbon storage and provide a habitat for diverse communities of organisms. Bottom shear stress from physical disturbances such as storms and hurricanes resuspend coastal muds, disrupting their cohesive structure. How biological vs. physical processes affect reconsolidation of physically disturbed natural muds, however, is poorly understood. We compare the recovery of sediment properties related to compaction and cohesion of muds following physical disturbance. We hypothesized that recovery of cohesion would take longer than compaction and that changes in surface and subsurface cohesion would occur on different timescales. We collected muddy sediment cores and resuspended the top 5 cm in the lab to simulate storm disturbance. At several timepoints following resuspension (1–30 d), we measured sediment properties providing metrics of compaction (sediment height, porosity, grain size distribution, sound speed) and of surface and subsurface cohesion (erodibility, fracture behavior, exopolymeric substances [EPS]). Compaction and cohesion increased rapidly over the first 1–2 d after resuspension and continued to slowly increase for 2 weeks. Between 2 and 4 weeks, however, subsurface cohesion increased and turbidity decreased, consistent with sediment restabilization. In contrast, subsurface compaction decreased and eroded mass increased, suggesting destabilization. We attribute this apparent destabilization to delayed activity of small‐bodied infauna. Surprisingly, EPS concentration did not explain cohesion changes. These results highlight the importance of including biological parameters when predicting the recovery of sediment structure following a physical disturbance. This work has implications for understanding coastal sediment transport dynamics in frequently disturbed sediments. [ABSTRACT FROM AUTHOR]

Soil erosion substantially implicates global nutrient and carbon cycling of the land surface. Its monitoring is crucial for assessing and managing global land productivity and socio-economy. The Zhuoshui River Basin, the largest catchment, in Taiwan is highly susceptible to soil erosion by water due to extremely high rainfall, rugged terrain, easily eroded soil, and intensively agricultural cultivation over the steep land. Hence, this study examines the annual soil erosion rate for 2005, 2011, and 2019 and the average long-term soil erosion and sediment yield (SY) during 2005–2019. Coupling of the Revised Universal Soil Loss Equation (RUSLE) and sediment delivery ratio (SDR) models is implemented using remote sensing and GIS techniques. The soil erosion rate is classified into five classes, namely mild (0–10 t ha−1 year−1), moderate (10–50 t ha−1 year−1), moderately severe (50–100 t ha−1 year−1), severe (100–150 t ha−1 year−1), and very severe (> 150 t ha−1 year−1). Over one half of the total area is categorized as moderate and moderately severe classes, and one-third of the whole basin as severe and very severe classes. Recently, mild and moderate classes increase, while moderately severe, severe, and very severe decrease. During 2005–2019, the annual soil loss rate ranges from 0.00 to 6,881.88 t ha−1 year−1 with an average rate of 122.94 t ha−1 year−1. Among the SDR models, the RUSLE combined with the SDR model with the length and slope gradient of mainstream shows satisfactory sediment yield estimation. Predictably, the downstream receives a massive sediment delivery from all upper streams (246.06 × 106 t year−1), and the percent bias values for all sub-basins are below ± 39.0%. The study provides a rapid approach to investigate soil erosion and sediment yield, and it can be applied to the other basins in Taiwan. More importantly, information about spatial patterns of soil erosion and SY is critical to establish suitable measures to achieve effective watershed planning and optimize the regional productivity and socio-economy. The proposed approach is potentially to identify risk areas, conduct scenario estimation for management, and perform spatiotemporal comparison of soil erosion, while adjustment in the empirical formulas of the proposed approach may be needed when it is applied to the other regions, especially outside Taiwan. [ABSTRACT FROM AUTHOR]

Seagrass meadows are important carbon sinks in the global coastal carbon cycle yet are also among the most rapidly declining marine habitats. Their ability to sequester carbon depends on flow–sediment–vegetation interactions that facilitate net deposition, as well as high rates of primary production. However, the effects of seasonal and episodic variations in seagrass density on net sediment and carbon accumulation have not been well quantified. Understanding these dynamics provides insight into how carbon accumulation in seagrass meadows responds to disturbance events and climate change. Here, we apply a spatially resolved sediment transport model that includes coupling of seagrass effects on flow, waves, and sediment resuspension in a seagrass meadow to quantify seasonal rates of sediment and carbon accumulation in the meadow. Our results show that organic carbon accumulation rates were largely determined by sediment accumulation and that they both changed non‐linearly as a function of seagrass shoot density. While seagrass meadows effectively trapped sediment at meadow edges during spring–summer growth seasons, during winter senescence low‐density meadows (< 160 shoots m−2) were erosional with rates sensitive to density. Small variations in winter densities resulted in large changes in annual sediment and carbon accumulation in the meadow; meadow‐scale (hundreds of square meters) summer seagrass dieback due to marine heatwaves can result in annual erosion and carbon loss. Our findings highlight the strong temporal and spatial variability in sediment accumulation within seagrass meadows and the implications for annual sediment carbon burial rates and the resilience of seagrass carbon stocks under future climate change. [ABSTRACT FROM AUTHOR]

Submarine canyons are important conduits of sediment and organic matter to deep-sea environments, mainly during high-energy natural events such as storms, river floods, or dense shelf water cascading, but also due to human activities such as bottom trawling. The contributions of natural and trawling-induced sediment and organic matter inputs into Palamós Canyon (NW Mediterranean) were assessed from three instrumented moorings deployed in the axis and northern flank of the canyon covering the trawling closure (February) and the trawling season (March-December) of 2017. During the trawling closure, large sediment fluxes with high contents of labile marine organic matter content were registered in the canyon axis, associated to storm resuspension on the shelf that coincided with dense shelf water cascading and high surface water productivity. Although no major natural sediment transport events occurred during the following spring and summer months, near-daily trawling-induced sediment gravity flows were recorded in the northern flank mooring, placed directly below a fishing ground, which sometimes reached the canyon axis. Compositionally, the organic matter transferred by trawling resuspension was impoverished in the most labile biomarkers (fatty acids, amino acids, and dicarboxylic acids) and had a high degree of degradation, which was similar to surficial sediment from the adjacent fishing ground. Trawling resuspended particles masked the transfer of organic matter enriched in labile biomarkers that naturally occur during the quiescent summer months. Overall, bottom trawling enhances the magnitude of particle fluxes while modifying its organic carbon composition, increasing the re-exposure and transfer of degraded organic carbon and potentially affecting benthic communities that rely on the arrival of fresh organic matter.

This study proposed that the dissolution of calcium and magnesium minerals in river sediment could sequester CO2 and function as a carbon sink. Based on the published study, "the contents and chemical and mineral compositions of the suspended particulate materials in the Yangtze River and their geological environmental implications" by Ding Tiping, the contents of CaO, MgO, calcite and dolomite in suspended sediment collected from 25 sampling points in the mainstream and 13 sampling points in the tributaries of the Yangtze River in 4 sampling campaigns during 2003–2007 were used to calculate the total inorganic carbon sink (TCS) capacity and nonsubstantial and substantial inorganic carbon sink (NSCS and SCS) capacities of suspended sediment along the river. Due to the reduction in the sediment yield, the TCS, NSCS and SCS of the Cuntan–Datong section during 2006–2019 decreased by 18.52 × 106 tons, 12.24 × 106 tons and 8.72 × 106 tons, respectively, compared to the period before 2002. The average annual sedimentation of the Three Gorges Reservoir (TGR) was 114.5 × 106 tons, and the related TCS and SCS losses were 6.76 × 106 tons and 2.29 × 106 tons, respectively, which were equivalent to 7.9 and 2.7 percent of the 85.8 × 106 tons of CO2 emissions reduced by the clean energy production of the Three Gorges Hydropower Station. The TCS of global rivers was estimated as 757 × 106 tons (the SCS was more than one quarter of the TCS), which is equivalent to 71.6% of the TCS by global rock weathering with 1.06 × 109 tons of sequestered CO2. The collision and erosion of river sediment caused by turbulence in the processes of sediment transport (off-site rock weathering) could promote the dissolution of minerals. Therefore, it is reasonable that the dissolution rate of calcium and magnesium minerals for offsite rock weathering was much higher than that for in situ rock weathering. [ABSTRACT FROM AUTHOR]

Stream ecosystem metabolism contributes to global carbon cycling, yet predicting metabolism across ecosystems remains elusive. Even within stream segments, spatial variation in metabolic rates and their controls can be substantial, exhibiting sudden rather than continuous changes. We measured metabolism at 6 sites along a 75-km mainstem segment of Marsh Creek, Idaho. This agricultural stream lacks major geomorphic transitions such as tributaries or changes in valley width, but possesses patchy patterns of turbidity. We asked: (1) How variable is metabolism along this segment, and (2) How do the controls on metabolism vary along this segment? Metabolism varied several-fold along this stream segment. Average rates of gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP) among sites did not correlate with water quality or aquatic macrophyte cover. Rather, reaches along this segment appear to represent different process domains that were characterized by turbidity: More turbid reaches saw negative effects of turbidity on GPP and NEP and positive effects on ER. Turbidity was associated with increased respiration and decoupled ER from GPP at turbid sites. Less turbid sites had stronger coupling between GPP and ER, and GPP was predicted by light and temperature. Heterogeneous sediment supply and transport capacity along Marsh Creek create patchy patterns of turbidity, which affects the controls on metabolism. Geomorphic controls on ecosystem processes are complex but not random. Understanding how metabolism and its controls vary across process domains is needed to scale up process rates and understand how process rates might respond to future changes. [ABSTRACT FROM AUTHOR]

Rivers regulate the global carbon cycle by transferring particulate organic carbon (POC) from terrestrial landscapes to marine sedimentary basins, but the processes controlling the amount and composition of fluvially exported POC are poorly understood. We propose that hydrodynamic sorting processes modify POC fluxes during fluvial transit. We test this hypothesis by studying POC transported along a ∼1,200 km reach of the Rio Bermejo, Argentina. Nanoscale secondary ion mass spectrometry revealed that POC was either fine, mineral‐associated organic matter, or coarse discrete organic particles. Mineral‐associated POC is more resistant to oxidation and has a lower particle settling velocity than discrete POC. Consequently, hydraulic sorting and downstream fining amplify the proportion of fine, mineral‐associated POC from ∼55% to ∼78% over 1,220 km of downstream transit. This suggests that mineral‐associated POC has a greater probability of export and preservation in marine basins than plant detritus, which may be oxidized to CO2 during transit. Plain Language Summary: The carbon cycle regulates Earth's climate by determining the amount of carbon in our atmosphere. Rivers are important for the carbon cycle because they deliver large amounts of organic carbon from mountains to the oceans. If this carbon is quickly buried in ocean sediment, atmospheric CO2 concentrations decrease. In order to predict the fluxes of carbon to the oceans, we need to understand the processes controlling organic carbon transport from source to sink. In a 1,300‐km‐long river in Argentina, we measured the amount of sediment and carbon in river water at multiple stations along the source‐to‐sink pathway. Using geochemical methods, we identified two types of organic carbon: (a) plant fragments, which are easily oxidized to CO2, and (b) organic carbon attached to minerals, which is resistant to oxidation. We calculated the mass fluxes of these two types of organic carbon and found that export of mineral‐protected carbon is greater than the export of plant fragments. This suggests that most riverine organic carbon may be preserved upon delivery to the ocean. Our data show that we need to differentiate between plant fragments and mineral‐attached organic carbon to predict the amount of organic carbon moving between rivers, the oceans, and the atmosphere. Key Points: In lowland meandering rivers, downstream particle size fining and hydraulic sorting can modulate particulate organic carbon (POC) fluxesNano‐scale secondary ion mass spectrometry and biomarker isotope data show that mineral‐associated and discrete POC differ in composition and particle settling velocityDownstream particle size fining enhances fluvial export of recalcitrant, mineral‐associated POC that is more resistant to oxidation [ABSTRACT FROM AUTHOR]

Marine nepheloid layer is widely distributed in the oceans and marginal seas. The concentration of suspended particles in the nepheloid layer is significantly higher than that of the adjacent layers. Marine nepheloid layers include the surface nepheloid layer (SNL), intermediate nepheloid layer (INL), and bottom nepheloid layer (BNL). As a transport pathway for the particulate matter in the oceans, nepheloid layer is important to the carbon cycle and the source–sink system at the continental margin. This paper focused on the characteristics of the marine nepheloid layer and the mechanism of internal solitary waves that form INL and BNL, providing a reference for further research on the marine ecological environment dynamic process and source–sink system. BNL is formed by suspended seabed sediments with strong currents near the bottom, but the fate of BNL remains unknown. Marine nepheloid layer significantly affects the transportation of materials within the ocean. The transport of sediments by ISWs dramatically affects the formation of deep seabed sedimentary landform. However, the formation process and the transport contribution of INL and BNL are still unclear. The systematic and long-term research on the marine nepheloid layer in the world oceans is still limited. [ABSTRACT FROM AUTHOR]