Search

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]

Streams and rivers play an important role in the global carbon cycle. The origins of CO2 in streams are often poorly constrained or neglected, which is especially true for CO2 originating from heterotrophic metabolism in streambeds. We hypothesized that sediment movement will have a direct effect on stream metabolism, and thus, the aim of this study was to quantify the effect of moving bedforms on the production of CO2 in sandy streambeds. We conducted flume experiments where we used planar optodes to measure the distributions of O2 and CO2 under various streambed celerities. We combined these measurements with an assessment of bed morphodynamics and modeling to calculate O2 consumption and CO2 production rates. Our results indicate that sediment transport can strongly influence streambed metabolism and CO2 production. We found that bedform celerity controls the shape of the hyporheic zone and exchange flux, and is directly linked to the spatial and temporal distributions of O2 and CO2. It was also found that the most pronounced change in CO2 production occurred when the bed changed from stationary conditions to a slowly moving bed. A more gradual increase in O2 consumption and CO2 production rates was observed with further increase in celerity. Our study also points out that bedform movement causes hydraulic isolation between the moving and the non‐moving fraction of the streambed that can lead to a transient storage of CO2 in deeper sediments, which may be released in bursts during bed scour. Plain Language Summary: Streams play an important role in the global carbon cycle. Carbon is transported in streams and rivers toward the oceans, stored in the streambed, or emitted as the greenhouse gas CO2 to the atmosphere. Many studies try to link CO2 in streamwater and CO2 emission to flow conditions and CO2 import via groundwater while ignoring the role of microbial processes in sediments that also produce CO2. We performed lab experiments using an artificial stream channel and special sensors in the streambed to investigate the influence of moving sandy sediments on the distribution and production of CO2. We found that sediment movement controls the distribution of CO2, and that those distributions are influenced by the flow paths of the water flowing through the sediment. We also found that the production of CO2 and the flux of stream water entering the sediment increased with the speed of the sediment movement. Moreover, our results point out that the movement of the sediment disconnects the lower, non‐moving part of the sediment from the upper, moving part of the sediment. This can build up a storage of high CO2 concentrations in the deeper sediment, which may be released as bursts during events of streambed erosion. Key Points: Bedform celerity controls the spatial and temporal distribution of CO2 in stationary and moving sandy bedformsProduction of CO2 in sediments increases abruptly as the bed starts to move and more gradually with increasing bedform celerityBedform movement causes isolation of the non‐moving fraction of the streambed and increase in transient storage of CO2 in deeper sediments [ABSTRACT FROM AUTHOR]

Soil may be a carbon source or sink under the effect of sediment and soil organic carbon (SOC) erosion, transport and deposition. Partial SOC processes to determine whether soil subjected to water erosion is a carbon sink or source have not been clarified but are essential for increasing and predicting SOC storage. In this review, the associations between SOC mineralization, stability and stock at erosion sites, in transported sediments and at deposition sites are clarified. An overview is given for possible determination standards that define soil carbon sinks and sources. Methods for enhancing SOC sequestration and suggestions for improving SOC prediction are also presented. Particle transport is an important intermediate process that determines the material base for SOC stability and sequestration in each erosion and deposition element. When subject to water erosion, a high probability of SOC physical stabilization and a thick soil layer are essential for soil to function as a SOC sink. Sediment transport weakens SOC physical protection in eroded areas but can promote aggregation factors at deposition sites in some cases while increasing the depth of SOC in the soil. In this circumstance, a good soil environment for plant growth facilitates the occurrence of carbon sinks. A concept for critical erosion intensity is proposed to understand SOC sink determination; the concept is presented as the volume of soil erosion per square kilometre (m3 km−2) for which the maximum reduction in vegetation cover or plant biomass still permits plants to provide enough organic matter to compensate for erosion‐induced SOC loss. If erosion intensity is higher than critical erosion intensity at the erosion site, soil degradation is obvious, and soil is presented as an OC source, and vice versa. Finally, the SOC burial method for increasing the soil carbon storage amounts in regions with thick soil layers is presented to explore the SOC sequestration potential in deep soil. The factors considered for SOC prediction should vary with research scale. We hope our review will have direct implications for the modelling of SOC dynamics under water erosion on both slopes and at large scales. Highlights: Changes in soil texture and soil thickness greatly affect the amount of SOC storage at erosion and deposition sites.Selective transport of soil materials or chemical elements promotes soil aggregation.Possible standards that define soil carbon sinks and sources and suggestions for improving SOC prediction are presented.SOC burial may be a good method for increasing C storage amounts in regions with thick layers of fine soil. [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.

Input of organic matter into stream channels is the primary energy source for headwater ecosystems and ultimately carbon to the oceans and hence is an important component of the global carbon cycle. Here, we quantify organic‐rich fine sediment mobilization, transport, and storage in a Strahler fourth‐order stream during individual intermediate‐sized storm events. By combining measurements of fallout radionuclides (FRNs) 7Be and 210Pb and stable water isotopes with a conceptual model of suspended load trapping by channel margins, we find that the channel bed was consistently a source of suspended load to the channel margins. Relative to storage on the channel margins, suspended load export increased through the spring and summer, perhaps related to the in‐channel decomposition of organic debris as indicated by its FRN exposure age and changing bulk δ13C composition. Trapping of suspended load by riparian margins limits sediment transport distances, which, given sufficient discharge to fully suspend the load, is nearly independent of stream discharge for sub‐bankfull discharges. Limited data indicate that the fractional size of the channel margins where trapping occurs decreases with increasing watershed area. Increasing transport length and decreasing fractional margin area with increasing watershed area results in a systematic downstream decoupling of the channel from local terrestrial organic matter exchange. These findings provide a framework for understanding suspended load dynamics in formerly glaciated regions where sediment production and fluxes are generally low and thus the annual input of organic debris is a major component of suspended load budget. Plain Language Summary: The decomposition of organic‐rich debris (leaves, twigs, etc.) within stream channels serves as an important organic carbon source to stream margins or banks. During moderate storm events, we observed that the channel bed was consistently a source of organic‐rich suspended load that is then trapped by the channel margins. Through spring and summer less of the suspended load is trapped by the margins, increasing the fraction of the suspended load exported. This decreased trapping and increased export may be related to changes in the character of the suspended load due to the in‐channel decomposition of organic debris. Trapping of suspended load by channel margins limits the transport distance of suspended load, systematically decoupling the channel from the channel margins with increasing watershed size. These findings provide a framework for understanding suspended load transport in formerly glaciated regions where the annual input of organic debris is a significant component of suspended load budget. Key Points: We observe seasonal variations in the ratio of suspended load exported from the watershed versus stored in channel marginsGreater fractional area of deposition along margins of headwaters facilitates trapping of suspended load that limits suspended load exportIncreasing transport length with increasing watershed area systematically decouples the channel from terrestrial organic exchange [ABSTRACT FROM AUTHOR]

The processes of seafloor sediment transport from the Chukchi shelf to the western Arctic basin were investigated using a pan‐Arctic sea ice‐ocean model and sediment‐trap measurements at four mooring stations: North of Barrow Canyon, North of Hanna Canyon, Northwind Abyssal Plain, and Chukchi Abyssal Plain. The available sediment‐trap data verified that the sinking flux of lithogenic material (LM) originally resuspended from the seafloor for 2010–2020 was simulated reasonably well in the four mooring areas. The model results were analyzed to quantify the spatiotemporal variability of LM and to reveal its background mechanisms. Analysis indicated that the Barrow Canyon throughflow, Chukchi Slope Current (CSC), and mesoscale eddies played important roles in LM redistribution. The CSC controlled the westward transport of LM from the mouth of Barrow Canyon to the Chukchi Borderland. The mesoscale eddies generated north of Barrow Canyon efficiently transported shelf‐origin LM toward the southern Canada Basin. The sinking flux of particulate organic carbon (POC) averaged from September 2010 to August 2020, which was estimated statistically from the simulated LM flux, was 0.13–0.30 gC m−2 yr−1 at 200‐m depth in the southern Canada Basin. This finding reveals that lateral transport of sediment from the Chukchi shelf bottom has a considerable effect on the sinking flux of POC in the western Arctic basin, and suggests that the western Arctic marine biogeochemical cycle is strongly influenced by shelf‐basin exchange that depends on the relative strength of the CSC and mesoscale eddy activity. Plain Language Summary: The Arctic marine ecosystem and carbon cycle are strongly influenced by exchange between the shallow continental shelf and deep basin. We conducted the first attempt to quantify the spatial and temporal variability in transport of seafloor sediment on the decadal scale using numerical model experiments under a realistic framework. The simulation results suggested that the major ocean currents along the Alaskan coast and shelf‐basin boundary are the primary drivers of the horizontal transport of sediment and its subsequent sinking to intermediate layers. Eddies generated near Point Barrow occasionally supply large amounts of shelf‐origin sediment to the interior of the Canada Basin. Although our model experiments did not include biological processes such as phytoplankton growth and zooplankton grazing, the sinking volume of particulate organic carbon (POC) obtained statistically from the simulated sediment transport was consistent with previous observational estimates. This finding indicates that the transport of sediment originating from the shallow shelf bottom accounts for a large part of the sinking POC in the western Arctic basin, and that its variability depends on topographically controlled ocean currents and eddies. Our study will help to understand how marine ecosystem and carbon distribution are changing in the early 21st century. Key Points: We conducted the first modeling attempt to quantify the transport of seafloor sediment in the western Arctic OceanBarrow Canyon throughflow, Chukchi Slope Current, and mesoscale eddies play important roles in sediment redistributionSediment transport substantially contributes to carbon supply for the Chukchi Borderland and southern Canada Basin [ABSTRACT FROM AUTHOR]

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]

Deep ocean circulation in the Southern Hemisphere plays a central role in global ocean overturning circulation and determines ocean carbon sink variability on multimillion‐year timescales. For this reason, it is important to understand how deep currents that originate in the Southern Ocean responded to past climate fluctuations and changing temperature patterns in the Southern Hemisphere oceans. To investigate this feedback mechanism, we analyze deep‐sea sediment cores formed under the influence of bottom currents from the Mentelle Basin (International Ocean Discovery Program Sites U1513, U1514, and U1516), in the Southeastern Indian Ocean. We show that bottom current activity intensified in the Australian‐Antarctic Basin during the cooling intervals of the late Miocene (ca. 7.8–4.8 Ma) and late Pliocene (ca. 3.59–3.2 Ma). At those intervals, bulk grain‐sizes and terrigenous input increased, via current winnowing. Ferromanganese nodules and stratigraphic hiatuses are also observed within these periods, indicative of intense sediment erosion at the seafloor. Sediment accumulation and preservation in the basin was particularly low at the deep Site U1514 (3,838 m water depth), suggesting that enhanced current transport and sediment erosion occurred preferentially within the deepest parts of the basin. We suggest that bottom water fluxes in the Australian‐Antarctic Basin intensified during late Miocene and Pliocene cooling intervals, likely boosted by renewed Australian‐Antarctic Discordance spreading since the Miocene. Key Points: Deep‐water transport in the Australian–Antarctic Basin intensified during Neogene cooling intervalsIntervals of winnowing favored the preservation of coarser terrigenous packages, the formation of hiatuses and ferromanganese nodulesRenewed rifting in the Australian–Antarctic Discordance in the Miocene may have boosted bottom water fluxes into the Mentelle Basin [ABSTRACT FROM AUTHOR]

Fly ash--the residuum of coal burning--contains a considerable amount of fossilized particulate organic carbon (FOCash) that remains after high-temperature combustion. Fly ash leaks into natural environments and participates in the contemporary carbon cycle, but its reactivity and flux remained poorly understood. We characterized FOCash in the Chang Jiang (Yangtze River) basin, China, and quantified the riverine FOCash fluxes. Using Raman spectral analysis, ramped pyrolysis oxidation, and chemical oxidation, we found that FOCash is highly recalcitrant and unreactive, whereas shale-derived FOC (FOCrock) was much more labile and easily oxidized. By combining mass balance calculations and other estimates of fly ash input to rivers, we estimated that the flux of FOCash carried by the Chang Jiang was 0.21 to 0.42 Mt C·y-1 in 2007 to 2008--an amount equivalent to 37 to 72% of the total riverine FOC export. We attributed such high flux to the combination of increasing coal combustion that enhances FOCash production and the massive construction of dams in the basin that reduces the flux of FOCrock eroded from upstream mountainous areas. Using global ash data, a first-order estimate suggests that FOCash makes up to 16% of the present-day global riverine FOC flux to the oceans. This reflects a substantial impact of anthropogenic activities on the fluxes and burial of fossil organic carbon that has beenmade less reactive than the rocks from which it was derived. [ABSTRACT FROM AUTHOR]

Inland waters receive a significant quantity of carbon (C) from land. The fate of this C during transit, whether it is emitted to the atmosphere, accumulated in sediments or transported to the ocean, can considerably reshape the landscape C balance. However, these different fates of terrestrial C are not independent but are instead linked via several catchment and aquatic processes. Thus, according to mass conservation, any environmental change inducing a shift in a particular C fate should come at the expense of at least one other fate. Nonetheless, studies that have investigated C emission, accumulation and transport concertedly are scarce, resulting in fragmented knowledge of the role of inland waters in the global C cycle. Here, we propose a framework to understand how different C fates in aquatic systems are interlinked and covary under environmental changes. First, to explore how C fates are currently distributed in streams, rivers, reservoirs and lakes, we compiled data from the literature and show that 'C fate allocation' varies widely both within and among inland water systems types. Secondly, we developed a framework that integrates C fates in any inland water system by identifying the key processes underlying their linkages. Our framework places the partitioning between the different C forms, and how this is controlled by export from land, internal transformations and hydrology, as central to understanding C fate allocation. We argue that, by focusing on a single fate, studies could risk drawing misleading conclusions regarding how environmental changes will alter the role of inland waters in the global C cycle. Our framework thus allows us to holistically assess the consequences of such changes on coupled C fluxes, setting a foundation for understanding the contemporary and future fate of land‐derived C in inland water systems. [ABSTRACT FROM AUTHOR]

Abstract Terrestrial organic carbon (OC) plays an important role in the carbon cycle, but questions remain regarding the controls and timescale(s) over which atmospheric CO2 remains sequestered as particulate OC (POC). Motivated by observations that terrestrial POC is physically stored within soils and other shallow sedimentary deposits, we examined the role that sediment storage plays in the terrestrial OC cycle. Specifically, we tested the hypothesis that sediment storage impacts the age of terrestrial POC. We focused on the Efri Haukadalsá River catchment in Iceland as it lacks ancient sedimentary bedrock that would otherwise bias radiocarbon‐based determinations of POC storage duration by supplying pre‐aged “petrogenic” POC. Our radiocarbon measurements of riverine suspended sediments and deposits implicated millennial‐scale storage times. Comparison between the sample types (suspended and deposits) suggested an age offset between transported (suspended sediments) and stored (deposits) POC at the time of sampling, which is predicted by theory for the sediment age distribution in floodplains. We also observed that POC in suspended sediments is younger than the predicted mean storage duration generated from independent geomorphological data, which suggested an additional role for OC cycling. Consistent with this, we observed interparticle heterogeneity in the composition of POC by imaging our samples at the microscale using X‐ray absorption spectroscopy. Specifically, we found that particles within individual samples differed in their sulfur oxidation state, which is indicative of multiple origins and/or diagenetic histories. Altogether, our results support recent coupled sediment storage and OC cycling models and indicate that the physical drivers of sediment storage are important factors controlling the cadence of carbon cycling.

Terrestrial organic carbon (OC) plays an important role in the carbon cycle, but questions remain regarding the controls and timescale(s) over which atmospheric CO2 remains sequestered as particulate OC (POC). Motivated by observations that terrestrial POC is physically stored within soils and other shallow sedimentary deposits, we examined the role that sediment storage plays in the terrestrial OC cycle. Specifically, we tested the hypothesis that sediment storage impacts the age of terrestrial POC. We focused on the Efri Haukadalsá River catchment in Iceland as it lacks ancient sedimentary bedrock that would otherwise bias radiocarbon‐based determinations of POC storage duration by supplying pre‐aged "petrogenic" POC. Our radiocarbon measurements of riverine suspended sediments and deposits implicated millennial‐scale storage times. Comparison between the sample types (suspended and deposits) suggested an age offset between transported (suspended sediments) and stored (deposits) POC at the time of sampling, which is predicted by theory for the sediment age distribution in floodplains. We also observed that POC in suspended sediments is younger than the predicted mean storage duration generated from independent geomorphological data, which suggested an additional role for OC cycling. Consistent with this, we observed interparticle heterogeneity in the composition of POC by imaging our samples at the microscale using X‐ray absorption spectroscopy. Specifically, we found that particles within individual samples differed in their sulfur oxidation state, which is indicative of multiple origins and/or diagenetic histories. Altogether, our results support recent coupled sediment storage and OC cycling models and indicate that the physical drivers of sediment storage are important factors controlling the cadence of carbon cycling. Key Points: Bulk radiocarbon measurements of riverine organic matter revealed millennial‐scale aging during sediment storage in IcelandXANES identifies interparticle diversity in S redox states indicative of organic matter with multiple origins and/or diagenetic historiesPhysical storage dynamics in river floodplains may influence the carbon cycle [ABSTRACT FROM AUTHOR]

The sources, transport, and transformation of terrestrial dissolved organic matter (DOM) in large river systems are of great interest due to their importance in the regional and global carbon cycles. However, knowledge of these factors is limited, especially for different DOM compositions coupling with hydrodynamics at the basin scale. To investigate the dynamics of different DOM compositions in a large river system, we characterized the dissolved organic carbon (DOC), chromophoric DOM (CDOM), and dissolved lignin at both moderate (September–October 2009) and high (August 2012) discharges in the Changjiang River basin. We found that extensive soil erosion resulted in the increased DOC concentration in the middle to lower reaches in 2012. In 2009, the transformation of DOM was dominated by flocculation and photo degradation due to the low flow velocity and depressed particle suspension. In 2012, the degradation of CDOM and dissolved lignin was coupled and dominated by microbial degradation and photo degradation. In 2012, CDOM (absorbance and fluorescence) and dissolved lignin concentrations were positively correlated, demonstrating the potential of using CDOM to quantitatively estimate dissolved lignin. Mass balance estimations of the DOM fluxes further confirmed that 17.0% (6.1%) of DOC, 24.2% (32.3%) of dissolved lignin, and 14.5% (0.8%) of CDOM were degraded at high (moderate) discharge. This study reveals contrasting transformation processes of DOM compositions with different sources and reactivities along the Changjiang River under different hydrological conditions, and it provides new insights into the linkage between the chemical and optical properties of DOM in large river systems. Key Points: Main controls on DOM removal were flocculation and photo degradation (microbial degradation and photo degradation) at moderate (high) dischargeCDOM can be used to trace dissolved lignin at high discharge, whereas at moderate discharge, CDOM and dissolved lignin were decoupledAt high (moderate) discharge, 17.0% (6.1%) of DOC, 24.2% (32.3%) of dissolved lignin, and 14.5% (0.8%) of CDOM were degraded [ABSTRACT FROM AUTHOR]

Chemical weathering contributes to the regulation of the global carbon cycle and biogeochemical cycles. Accordingly, the identification of the parameters that control weathering reactions and transport of weathering signals at the catchment scale is essential. The use of boron (B) isotopes have been shown to be a useful proxy in tracing weathering reactions due to large isotope fractionation during weathering processes. However, our knowledge of how boron isotopes record the weathering regime at the catchment scale and how that weathering signal is transported from source areas to the depositional environment remains limited. Here we characterize B isotope and major element behavior during chemical weathering and transport by analyzing the B isotopic (δ11B) and element compositions of riverine material (riverbank sands (<63 µm), clay fractions (<2 µm) extracted from sands, and dissolved load) along the course of the Murrumbidgee River (NSW, Australia), its upstream tributaries, and monolithologic subcatchments. In the Murrumbidgee, two distinct weathering regimes are present, one where mineral dissolution is associated with minimal neoformation at higher elevations and another where mineral neoformation dominates at lower elevations and in granitic lithologies. Significant B isotope difference between the clay fraction and the bedrock (Δ11B clay-bedrock) is observed in most monolithological catchments at high mean elevation (excluding granites), which correlates with a large B depletion. Smaller isotope difference between the clay fraction and bedrock is observed in monolithological catchments at lower elevations as well as in granitic catchments at all elevations and is associated with limited B removal. These results suggest that lithology and catchment topography influence B mobility during weathering and the isotopic composition of weathering products. By mass balance calculation, the B isotope and chemical composition of the clay and sand fractions in the Murrumbidgee River can be explained as a mixture of the clays and sands produced throughout the catchment delivered to the main channel by the tributaries. These results indicate that there is little or no chemical and isotopic modification of the river sediment during fluvial transport and that weathering signal produced in the sediment source areas is transferred to the depositional environment without significant modification. The boron content of the clay-sized fraction (∼40 ppm) is several orders of magnitude greater than that of the dissolved load while B isotope compositions of the clay-sized fraction are isotopically much lighter (up to 40‰). Because a maximum isotopic difference of 30‰ between the dissolved and solid phases is expected during adsorption processes, the observed isotope compositions in the dissolved load and the sediment clay fraction cannot be explained by pH-driven B partitioning. These observations suggest that clays are not directly precipitating from solutions compositionally similar to surface waters; deeper soil solutions are expected to play a significant role in clay formation. This research highlights the potential of B isotopes in river sediments to describe the present and past weathering regimes at the catchment scale, including possible paleoenvironment reconstruction as the B isotope signature of riverine material records the conditions of its formation. [ABSTRACT FROM AUTHOR]

Small mountainous rivers (SMR) export globally significant quantities of particulate organic carbon (POC) to the oceans, the composition of which may be distinct from POC delivered by major world rivers. To better constrain the controls on the Δ14C signatures of POC suspended in SMR, we examined the temporal variability of Δ14C and δ13C of POC suspended in the Santa Clara River, a SMR in California, USA. Our data suggest mixing of bedrock and contemporary soil, which can be applied to other SMR in contrasting climate zones and with varying land cover. The data also suggest that chemical denudation is incomplete, but effective to the point that only the most recalcitrant moieties of the parent POC are exported. We further propose sediment yield as a potentially useful proxy for the Δ14C of riverine POC. A preliminary analysis of the existing data for a spectrum of rivers supports this hypothesis.

Strong earthquakes at active ocean margins can remobilize vast amounts of surficial slope sediments and dynamically strengthen the margin sequences. Current process understanding is obtained from resulting event deposits and low‐resolution shear strength data, respectively. Here we directly target a site offshore Japan where both processes are expected to initiate, that is, at the uppermost part (15 cm) of a sedimentary slope sequence. Based on a novel application of short‐lived radionuclide data, we identified, dated, and quantified centimeter‐scale gaps related to surficial remobilization. Temporal correlation to the three largest regional earthquakes attest triggering by strong earthquakes (Mw >8). Also, extremely elevated shear strength values suggest a strong influence of seismic strengthening on shallow sediments. We show that despite enhanced slope stability by seismic strengthening, earthquake‐induced sediment transport can occur through surficial remobilization, which has large implications for the assessment of turbidite paleoseismology and carbon cycling at active margins. Plain Language Summary: During strong earthquakes, large amounts of sediment are stripped from ocean slopes and transported into the deeper basin. The resulting event deposits can provide information on the recurrence of past earthquakes, which is crucial for seismic hazard assessment. To unravel the information stored within this archive, it is important to understand the effect of earthquakes on surficial slope sediments where the removal of sediment is supposed to take place. We discovered—for the first time—several centimeter‐scale gaps in a slope sequence offshore Japan, which can be linked to the largest regional earthquakes. Also, we found that seismic shaking can strengthen and stabilize a slope, therefore reducing the probability for large landslides and associated destructive tsunamis. This study shows that strong earthquakes can both stabilize and remobilize young sediments at ocean slopes. Key Points: Slope stratigraphy reveals direct impact of strong earthquakes on young sediments of the active margin offshore JapanRemobilization of surficial slope sediments at site occurs during Mw 8+ earthquakes, allowing turbidite paleoseismology in terminal basinsSeismic strengthening is most effective on uppermost sediments, progressively reducing submarine landslide potential [ABSTRACT FROM AUTHOR]

Carbonate mudstones are key geochemical archives for past seawater chemistry, yet the origin of carbonate mud remains a subject of continued debate and uncertainty. Prevailing hypotheses have settled on two mechanisms: (1) direct precipitation in the water column and (2) postmortem dispersal of mud‐sized algal skeletal components. However, both mechanisms conflict with geochemical observations in modern systems and are problematic in deep time. We tested the hypothesis that abrasion of carbonate sand during sediment transport might produce carbonate mud using laboratory experiments and a sediment transport model. We documented experimental mud production rates up to two orders f magnitude faster than rates estimated for other mechanisms. Combined with model calculations, these results illustrated that transport and abrasion of carbonate sand is a major source of carbonate mud. Plain Language Summary: Carbonate mudstones are widely used as archives of ancient seawater chemistry, under the assumption that the compositions of mud‐sized (<62.5 μm in diameter) carbonate particles that make up these mudstones provide reliable records of seawater at the time the particles were formed and deposited. This assumption relies on understanding how carbonate mud forms—current ideas center on direct mineral precipitation from seawater and the disintegration of algae mineral skeletons—but these mechanisms conflict with some geochemical observations in modern systems. We used experiments to demonstrate that when carbonate sand grains are transported by currents, collisions cause mud‐sized carbonate particles to be released from grain surfaces via abrasion. The rapid rates of carbonate mud production observed in our experiments suggest that abrasion has been a significant source of carbonate mud throughout Earth history, which is important for interpreting geochemical records from carbonate mudstones because the material abraded from sand grains may not be instantaneous records of seawater chemistry. Key Points: Experiments demonstrate that abrasion of carbonate sand generates carbonate mud at geologically significant ratesAn abrasion rate model can be used to estimate fluxes of carbonate mud produced via abrasion in modern and ancient environmentsAbrasion‐produced mud may reflect a mixture of carbonate particles of varying age and composition [ABSTRACT FROM AUTHOR]

• Demonstration of rise and fall of carbon discharge with time. • Demonstration of enrichment of sand in loose layer above soil. • Demonstration of no carbon enrichment is sediment at the steady state. Soil carbon losses from sheet and interrill erosion areas have an impact on the global carbon cycle. Sediment discharged in many experiments where artificial rainfall has been applied to sloping soil surfaces show greater proportions of carbon in the sediment than found in the cohesive soil source. In many cases, conclusions drawn from these experiments fail to recognise that the enrichment of carbon in the sediment discharge is a result of unsteady conditions that occur before the steady state develops. It is known that particles of sand of different sizes move at different rates when raindrop induced salation occurs with the result that initially the particle size distribution is finer than the source but tends towards that of the source as time progresses. The presence of carbon in aggregates influences the density of aggregates. Here a qualitative model of sediment transport of sand and coal particles by raindrop induced saltation is used to demonstrate time varying enrichment of coal resulting from the effect of carbon on particle settling velocity. The model also demonstrates how the layer of loose particles that develops on top of the cohesive soil surface influences the relative amounts of carbon and sand in the sediment discharges. The steady state is not achieved until the slowest moving particle detached at the farthest distance from the downslope boundary is discharged. That may not be achieved in many of the experiments reported in the literature. [ABSTRACT FROM AUTHOR]

The Yellow River has experienced prolific environmental changes from dam regulations that have significantly altered the downstream hydrologic regime, and transport of sediment and POC within the river-coast continuum. Here, we investigated the contrasting changes in sources and transport of POC from the lower Yellow River to the river mouth, at two different stages (e.g., Water vs. Sediment regulation) of dam operation - during the period of Water-Sediment Regulation Scheme (WSRS). Riverbed erosion downstream the Xiaolangdi Dam contributed the POC with more enriched δ13C stable isotopic values, along with those more isotopically depleted POC from riparian floodplains and the reservoir at the Water-regulation stage, when clear water was released from the reservoir. At the Sediment-regulation stage, muddy floodwaters were released, while soil-derived POC and fine-grained sediment released from the reservoir became dominant with evidently increasing POC content. The dispersal and distribution of river-laden POC at the river mouth were primarily modulated by the plume dynamics, as remotely impacted by dam regulation, in combination with the dynamic shear front. As a result, the POC distribution was confined within 10 km off the river mouth due to the rapid deposition of coarse-grained sediments. In contrast, the fine-grained sediment and more isotopically depleted POC, from modern sources (e.g., C 3 vascular plant), were delivered to the far-field plume region. POC transport along the Yellow River-Estuary continuum exhibited complex behaviors, coupled with natural and dam-regulated drivers, on a variety of spatial-temporal scales. These findings provide important insights into dynamic linkages on how coastal carbon cycling can be significantly altered, remotely from the upstream dam regulations, in a large global river system. • Dam operations and hydrological changes induced contrasting features in downstream POC delivery. • Dam-impacted plume dynamics and the shear front dominated the preferential transport of POC at river mouth. • POC transport within the river-estuary continuum exhibited complex and dynamic behaviors. [ABSTRACT FROM AUTHOR]

Abstract: The distribution, transport, and accumulation of wildfire‐generated pyrogenic carbon (PyC) has important consequences for contaminant transport and carbon cycling, but a conceptual model for PyC accumulation and loss that includes geomorphic processes is lacking. In this study we quantified PyC concentration in soil samples collected from the Jemez Mountains of New Mexico before and after the 2013 Thompson Ridge (TR) fire, and developed a conceptual model describing PyC redistribution. Pre‐fire samples were fortuitously collected 4 years before the TR burn and post‐fire samples were collected at the same locations 15 months following the TR fire. Samples were collected from the O and A horizon, with sites representing a range of slope angle, aspect, burn severity, and geomorphic setting. PyC was determined by a modified chemo‐thermal oxidation method to compare PyC to total organic carbon (TOC). The mean PyC/TOC ratio was significantly higher post‐fire than pre‐fire (0.14 vs 0.12), indicating increased PyC sequestration. O horizon PyC concentrations were more variable and more responsive to fire than the A horizon. Soil horizon, watershed, and geomorphic setting proved to be the most influential factors in predicting PyC concentration changes. PyC concentrations increased most on hillslopes and in low‐severity burn areas, suggesting higher rates of PyC production or post‐fire accumulation. Burn patchiness appears to facilitate PyC accumulation, with lower severity patches trapping PyC mobilized from high severity patches. While PyC content had greater point scale variance following the fire, the fire also homogenized pre‐fire PyC differences between soil horizons and among watersheds within the burn perimeter, differences that appear to develop over time between fires. The O horizon is a larger sink for PyC in the short term following fire, but based on pre‐fire concentrations the A horizon appears to be a more stable sink for PyC. Copyright © 2018 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Continental margin systems collectively receive and store vast amounts of organic carbon (OC) derived from primary productivity both on land and in the ocean, thereby playing a central role in the global carbon cycle. The land-ocean interface is however extremely heterogeneous in terms of terrigenous input, marine primary productivity, sediment transport processes and depositional conditions (e.g. such as bottom water oxygen level). Continental margins are also highly dynamic, with processes occurring over a broad range of spatial and temporal scales. The rates of OC burial and oxidation are consequently variable over both space and time, hindering our ability to derive a global picture of OC cycling at the land-ocean interface. Here, we review the processes controlling the fate of organic matter in continental margin sediments, with a special emphasis on “hot spots” and “hot moments” of OC burial and oxidation. We present a compilation of compositional data from a set of illustrative settings, including fjords, small mountainous river margins, large deltaic systems and upwelling areas. Bulk OC stable isotope and radiocarbon compositions reveal the diversity and complexity characteristic of OC buried in marginal seas. This primarily relates to differences in marine and terrestrial inputs, the composition of the terrestrial component (e.g. vascular plant OC, soil, and petrogenic OC inputs), and processes modulating the fate of OC within the marine environment (e.g. priming). This widely contrasting behavior of OC among these systems illustrates that the reactivity of OC is a product of its chemical composition and regional conditions. Interpreted in the context of bulk compositional data as well as that obtained on specific molecular markers (e.g. lignin-derived phenols), the possibility exists to tease apart complex mixtures of terrestrial and marine inputs, and to shed light on the role of the myriad of depositional and post-depositional processes. Finally, we discuss a set of hot topics that warrant further investigation – such as the role of photochemistry, fungi, halogenation and reactive iron in dictating the fate of OC in the (changing) coastal ocean. [ABSTRACT FROM AUTHOR]

Burial in sediments removes organic carbon (OC) from the short-term biosphere-atmosphere carbon (C) cycle, and therefore prevents greenhouse gas production in natural systems. Although OC burial in lakes and reservoirs is faster than in the ocean, the magnitude of inland water OC burial is not well constrained. Here we generate the first global-scale and regionally resolved estimate of modern OC burial in lakes and reservoirs, deriving from a comprehensive compilation of literature data. We coupled statistical models to inland water area inventories to estimate a yearly OC burial of 0.15 (range, 0.06-0.25) Pg C, of which ~40% is stored in reservoirs. Relatively higher OC burial rates are predicted for warm and dry regions. While we report lower burial than previously estimated, lake and reservoir OC burial corresponded to ~20% of their C emissions, making them an important C sink that is likely to increase with eutrophication and river damming. [ABSTRACT FROM AUTHOR]

Terrestrial organic carbon (OC) burial in marginal seas comprises an important marine carbon sink. However, controls on the evolution of OC during sediment transport within and beyond river basins remain poorly constrained. OC contents and isotopic compositions (δ13C and Δ14C), and specific surface areas (SSA) of river particles of the Changjiang (Yangtze River) and Huanghe (Yellow River) and surface sediments in the Eastern China Marginal Seas (ECMS) were determined to better understand controls of OC composition and age along these two major river sedimentary systems of China. Although petrogenic OC (OC petro) content (0.02%) of particulate organic carbon (POC) in the Huanghe is lower than the Changjiang (0.09%), the radiocarbon age of Huanghe POC (4500 ± 420 yr) is significantly older than Changjiang POC (3230 ± 315 yr). Biospheric OC (OC bio) of POC in the Huanghe (0.32 ± 0.08%) is primarily derived from pre-aged soils in contrast to OC bio of Changjiang POC (0.87 ± 0.23%) - mainly from modern litter biomass. However, the age of OC in the Bohai Sea and Yellow Sea surface sediments is younger than those in the East China Sea (2335 ± 590 yr vs. 4135 ± 725 yr). Removal of most terrestrial OC bio during transport results in diagenetic aging in the Changjiang estuary (via frequent physical reworking in highly oxidized mobile-muds) and the remaining terrestrial OC bio experiences further aging during along-shore transport (i.e., transport time-associated aging). However, the continuous addition of young marine OC with little loss of the terrestrial OC bio occurs along the Huanghe system, which is mainly due to the relatively weak sediment dynamics and the high proportion of pre-aged OC in Huanghe POC. Compared with other globally significant river sedimentary systems, the evolution pattern of OC in the Huanghe system is similar to those of small mountain river-active margin systems, although it belongs to the passive margin system. By contrast, the Changjiang system acts like other large river-passive margins. Our findings further demonstrate that the alteration of OC composition and age in river sedimentary systems can be controlled by similar sediment dynamics and OC reactivities – across divergent continental margins. • Ages of OC in coastal sediments are decoupled with OC in riverine particles. • Sedimentary OC becomes younger along the Huanghe system from additions of marine biospheric OC. • Sedimentary OC becomes older along the Changjiang system from the removal of terrestrial biospheric OC. • OC reactivities and sediment dynamics determine the evolution of OC composition and age. [ABSTRACT FROM AUTHOR]

Enhanced vertical carbon transport (gravitational sinking and subduction) at mesoscale ocean fronts may explain the demonstrated imbalance of new production and sinking particle export in coastal upwelling ecosystems. Based on flux assessments from 238U:234Th disequilibrium and sediment traps, we found 2 to 3 times higher rates of gravitational particle export near a deep-water front (305 mg C·m-2·d-1) compared with adjacent water or to mean (nonfrontal) regional conditions. Elevated particle flux at the front wasmechanistically linked to Fe-stressed diatoms and highmesozooplankton fecal pellet production. Using a data assimilative regional ocean model fit to measured conditions, we estimate that an additional ~225 mg C·m-2·d-1 was exported as subduction of particlerich water at the front, highlighting a transport mechanism that is not captured by sediment traps and is poorly quantified by most models and in situ measurements. Mesoscale fronts may be responsible for over a quarter of total organic carbon sequestration in the California Current and other coastal upwelling ecosystems. [ABSTRACT FROM AUTHOR]

Small mountainous rivers (SMRs) play a crucial role in the global sediment and carbon cycle by rapid transfer of huge particulate and dissolved loads into the sea. Gaoping river-sea system is a good representative of SMRs in Oceania, having an extreme high denudation rate (15,000 t km − 2 yr − 1 ) and a high burial efficiency of terrestrial particulate organic carbon (POC) in the Gaoping submarine canyon (GPSC). However, it is contentious whether silicate weathering or POC burial plays more important role in long-term carbon cycling in such a high active orogen. Clay minerals of fluvial sediments not only act as an important carrier of POC, but also encode valuable information of weathering regimes on land and dispersal routes in the seas. To uncover these informations, surface and core sediments are systematically collected from the Gaoping river-sea system for clay-mineral analysis. Comparative studies are carried out on the basis of new results and vast published data to better understand weathering processes and products in Oceanian islands and Southeast Asian continent. We find that clay-mineral compositions in the Gaoping River (GPR) and other Taiwan rivers are a legacy from provenance rocks in the alpine reach, characterized by predominant illite and chlorite and limited compositional change throughout the river courses. These sharply contradict the downstream increase in compositional complexity (rich in kaolinite and smectite) and chemical weathering intensity in Southeast Asian large rivers because of strong hydrolysis and active sediment recycling at the lowland reach with extensive floodplains. High consistency of clay-mineral compositions is also observed in the interior and exterior of GPSC. Careful examination shows that a slight offshore decrease in cumulative illite and chlorite content is accompanied by a slight increase in exotic clay-mineral inputs by ocean circulations. The combination of this finding with previously reported data on the offshore change in δ 13 C org and Δ 14 C org values suggests that petrogenic POC predominates in the river-dominated Gaoping continental margin with high preservation efficiency due to its tight bandage with clay minerals. Moreover, a tardy response of chemical weathering fluxes to high elevated physical erosion rates is remarkably examined in the Oceanian and global river gauging data. We therefore postulate that rapid transfer and high burial efficiency of terrestrial POC in the tectonically active high-standing islands and their adjacent continental margins play much more important role in the long-term carbon cycle than silicate weathering. [ABSTRACT FROM AUTHOR]

The wave bottom boundary layer is a major conduit delivering fine terrestrial sediments to continental margins. Hence, studying fine sediment resuspensions in the wave boundary layer is crucial to the understanding of various components of the earth system, such as carbon cycles. By assuming the settling velocity to be a constant in each simulation, previous turbulence-resolving numerical simulations reveal the existence of three transport modes in the wave boundary layer associated with sediment availabilities. As the sediment availability and hence the sediment-induced stable stratification increases, a sequence of transport modes, namely, (I) well-mixed transport, (II) formulation of lutocline resembling a two-layer system, and (III) completely laminarized transport are observed. In general, the settling velocity is a flow variable due to hindered settling and particle inertia effects. Present numerical simulations including the particle inertia suggest that for a typical wave condition in continental shelves, the effect of particle inertia is negligible. Through additional numerical experiments, we also confirm that the particle inertia tends (up to the Stokes number St = 0.2) to attenuate flow turbulence. On the other hand, for flocs with lower gelling concentrations, the hindered settling can play a key role in sustaining a large amount of suspended sediments and results in the laminarized transport (III). For the simulation with a very significant hindered settling effect due to a low gelling concentration, results also indicate the occurrence of gelling ignition, a state in which the erosion rate is always higher than the deposition rate. A sufficient condition for the occurrence of gelling ignition is hypothesized for a range of wave intensities as a function of sediment/floc properties and erodibility parameters. [ABSTRACT FROM AUTHOR]

A fundamental goal of the Earth Science community is to understand how perturbations on Earth's surface are preserved in the stratigraphic record. Recent Source to Sink (S2S) studies of the Waipaoa Sedimentary System (WSS), New Zealand, are synthesized herein to provide a holistic perspective of the processes that generate, transport and preserve sedimentary strata and organic carbon on the Waipaoa margin in the late Quaternary. Rapid uplift associated with subduction processes and weak sedimentary units have conspired to generate rapid rates of incision and erosion in the Waipaoa catchment since the Last Glacial Maximum (LGM). We show that although much of the sediment exported offshore during this time interval originated from valley excavation, a substantial portion emanated from hillslopes, mostly through deep-seated landslide and earthflow processes that were vigorous during periods of rapid fluvial incision just prior to the Pleistocene–Holocene transition. Lacustrine sediments deposited in naturally-dammed 7-ky-old Lake Tutira provide a record of Holocene environmental controls on upper catchment sedimentation in the WSS, with 1400 storms identified. Storm frequency is modulated by the waxing and waning of atmospheric teleconnections between the tropics and Antarctica. Furthermore, clear long-term changes in sediment yield are evident from the Lake Tutira record following human settlement as conversion to pasture is accompanied by a 3-fold increase in the long-term lake sediment accumulation rate. Whereas there is ample evidence that Waipaoa River flood deposits are routinely deposited offshore in the sheltered confines of Poverty Bay, over the longer term, waves and currents subsequently resuspend and transport these deposits both landward (sandy fraction) and seaward (finer fraction). Thus, the timing of sediment supply to areas of net sediment accumulation is more often driven by wave events that are not associated with river flooding. Therefore, we conclude that asynchronicity of river-sediment delivery and of wave resuspension in most instances precludes the direct preservation of flood events in the stratigraphic record of the Waipaoa Shelf. Over the longer term, the sediment package preserved on the shelf and slope since the LGM can be explained in large measure by sequence-stratigraphic models forced by varying sea level and ongoing tectonic deformation of the margin. As sea level rose, sediment supply to the slope was reduced by about a factor of 5 due to shelf trapping. Despite this reduction, turbidites are found at similar frequency throughout the LGM–Present, as the dominant trigger appears to be subduction earthquakes, with large ones having a return interval of about 200 ± 100 years. Sediment-budget exercises that consider both modern (river discharge versus centennial accumulation rates) and post-LGM (terrestrial production versus offshore isopachs) mass balances indicate that about half of the total sediment production from the Waipaoa escapes the study area. Moreover, a coupled sediment transport-hydrodynamic model and observations of textural trends on the shelf indicate that a large fraction of the sediment is carried outside the study area along the shelf to the northeast by the river plume or by combined current/wave activity. Therefore, we conclude that the WSS is an open system with sediment escape from the present day through the LGM. The organic matter associated with sediment as it moves from upland source to marine sink is a product of particle history, and provides a record of materials that have cycled over timescales of days to millions of years. The ubiquity of fossil Organic Carbon (OC) in both the terrestrial and marine realms of the Waipaoa attests both to the chronic nature of its source, crumbling mudstones further destabilized by land use, and its biogeochemical recalcitrance. Modern OC persists by virtue of its continual production along the S2S transit, and is buried and preserved within the adjacent marine depocenters. The Waipaoa contrasts with dispersal systems on wide, energetic shelves (e.g., the Amazon and Fly Rivers) where sediment is extensively refluxed in oxygenated overlying water resulting in the biogeochemical incineration of particulate OC. The Waipaoa, like other small mountainous rivers on active margins, exhibits a high riverine OC preservation efficiency (> 50%) in its marine depocenters because of the relatively rapid, event-driven accumulation of sediment. [ABSTRACT FROM AUTHOR]

The Yellow River is the second largest river in China. Carbon transport by the Yellow River has significant influence on riverine carbon cycles in Asia. In order to monitor seasonal and spatial variations of carbon concentrations and to estimate carbon exports, water and suspended solids were sampled every 10 days at three representative stations (Qingtongxia, Tongguan, and Luokou) along the mainstream of the Yellow River. Results showed that riverine carbon was mainly in dissolved form, except during flood period and water and sediment regulation (WSR) scheme, when particulate organic carbon (POC) dominated. Concentration of dissolved inorganic carbon was mostly 5 to 10 times higher than that of dissolved organic carbon (DOC). DOC was mainly related to a natural process (leaching effect) in the upstream and anthropogenic activities in the midstream (domestic sewage and fertilizer application) and downstream (industrial wastewater). POC was connected with high suspended solids. Annually carbon delivered to the Bohai Sea was 1.34 × 1012 g/yr, accounting for 0.15% of the global total riverine carbon flux. Mean DOC exported accounted for 0.12% of the Asian rivers' DOC flux. WSR played an important role in the carbon transport, which accounted for 1/5 to 1/3 of the corresponding annual fluxes. [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]

ABSTRACT The fluvial flux of carbon (C) from terrestrial to marine environments represents an important component of the global C-cycle, which can transfer C from the atmosphere to sedimentary storage. Fluvial fluxes of C are also an essential resource for freshwater ecosystems, critical for habitat heterogeneity and function. As such it is crucial that we are able to quantify this flux accurately. However, at present there are a number of different techniques used to quantify concentrations of fluvial C, and these techniques vary in their accuracy. In this article, we compare particulate organic carbon (POC) measurements derived from two commonly-used techniques; a simple combustion and loss-on-ignition (LOI) technique, and an oxidative-combustion and carbon dioxide (CO2) detection technique. The techniques were applied to water samples collected from 10 contrasting reference-condition, temperate river ecosystems. The POC measurements derived from the LOI technique were up to 16 times higher (average four times higher), than those derived from the oxidative-combustion and CO2 detection technique. This difference was highly variable both across the different river ecosystems and within each river ecosystem over time, suggesting that there is no simple way of converting the mass measured by LOI to estimates of fluvial POC. It is suggested that the difference in POC measured by these two techniques is a consequence of: (1) the loss of inorganic carbon at LOI combustion temperatures of > 425 °C, (2) the potential during the LOI combustion stage to lose hygroscopic and intercrystalline water, not completely driven off by the drying stage at temperatures of < 150 °C, and (3) the variable C content of fluvial organic matter, meaning that the simple application of a fixed correction factor to values obtained from the LOI technique may not be appropriate. These findings suggest that oxidative-combustion and CO2 detection techniques are preferential for quantifying fluvial POC. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Submarine canyons transport large amounts of sediment and organic matter (OM) from the continental shelf to the abyssal plain. Three carbon-based food web models were constructed for the upper (300–750m water depth), middle (2700–3500m) and lower section (4000–5000m) of the Nazaré canyon (eastern Atlantic Ocean) using linear inverse modeling to examine how the food web is influenced by the characteristics of the respective canyon section. The models were based on an empirical dataset consisting of biomass and carbon processing data, and general physiological data constraints from the literature. Environmental conditions, most notably organic matter (OM) input and hydrodynamic activity, differed between the canyon sections and strongly affected the benthic food web structure. Despite the large difference in depth, the OM inputs into the food webs of the upper and middle sections were of similar magnitude (7.98±0.84 and 9.30±0.71mmolCm−2 d−1, respectively). OM input to the lower section was however almost 6–7 times lower (1.26±0.03mmolCm−2 d−1). Carbon processing in the upper section was dominated by prokaryotes (70% of total respiration), though there was a significant meiofaunal (21%) and smaller macrofaunal (9%) contribution. The high total faunal contribution to carbon processing resembles that found in shallower continental shelves and upper slopes, although the meiofaunal contribution is surprisingly high and suggest that high current speeds and sediment resuspension in the upper canyon favor the role of the meiofauna. The high OM input and conditions in the accreting sediments of the middle canyon section were more beneficial for megafauna (holothurians), than for the other food web compartments. The high megafaunal biomass (516mmolCm−2), their large contribution to respiration (56% of total respiration) and secondary production (0.08mmolCm−2 d−1) shows that these accreting sediments in canyons are megafaunal hotspots in the deep-sea. Conversely, carbon cycling in the lower canyon section was strongly dominated by prokaryotes (86% of respiration) and the food web structure therefore resembled that of lower slope and abyssal plain sediments. This study shows that elevated OM input in canyons may favor the faunal contribution to carbon processing and create hotspots of faunal biomass and carbon processing along the continental shelf. [Copyright &y& Elsevier]

Abstract: With the current emphasis on the role of carbon in the environment, agricultural systems and their impacts on the carbon cycle are important parts of the overall issue. Organic carbon lost to streams and rivers can promote bacterial production and microbial respiration of CO2 to the atmosphere. Although pastures and grasslands are major land uses in the humid U.S., row crop agriculture has received most of the carbon research focus. The objective of this study at the North Appalachian Experimental Watershed near Coshocton, Ohio, was to assess organic carbon transported from a pasture system, particularly on a runoff event basis. A beef cow–calf herd rotationally grazed a paddock during the growing season and was fed hay in this paddock during the dormant season (November–April). Surface runoff and sediment loss was measured and sampled throughout the year from the small watershed in the paddock. Most of the sediment samples were collected during the dormant season. With continuous winter occupancy, the percent vegetative cover was often ≤50% in the late winter/early spring, which increased the potential for soil loss. During the 10-year period of sample collection, there were 376 runoff events and 115 events with sediment loss ≥10kgha−1 from the watershed in the winter feeding area. The largest 6 events carried nearly 50% of the total sediment and sediment-attached C lost during this period. Annual losses of sediment and sediment-C varied considerably but averaged 2642 and 140kgha−1, respectively. There was no significant correlation between the amount of sediment transported during individual events and the C concentration on the associated sediment. The pasture sediments have a C enrichment ratio of 1.2–1.5 compared with the 0–2.5cm soil layer. Pasture sediment-C concentrations were >2x the C concentrations on sediments from nearby row crop watersheds. [Copyright &y& Elsevier]

Abstract: The processes controlling the production, transport and ultimate fate of particulate organic carbon (POC) in marine systems play a major role in the global carbon cycle. Direct measurements of POC concentrations in oceanic waters have been used extensively to investigate these processes, but because of the time-intensive effort associated with manual collection and filtration of water samples, this approach often results in limited temporal or spatial coverage. To increase the frequency of POC measurements, we designed and built a semi-automated filtration system (SAFS) that can be linked to a ship''s underway surface water flow system and/or a towed profiling vehicle with pumping capabilities. Our results show that, compared to manual filtration, SAFS provides the ability to easily perform filtrations at high enough frequency to measure POC concentrations at spatial and temporal resolutions that are closer to those possible with in situ sensors. When combined with optical measurements, SAFS allows for the characterization of several distinct pools of particles within different geographical and depth regions of the water column off the Oregon Coast. These initial applications yielded data that provide considerable insight into the sources and character of particles in the water column and prove that this approach can be used to gain valuable information on the biogeochemistry of this and other similar ocean margins. [Copyright &y& Elsevier]

Abstract: A fluorescent labelling method is presented as a new tool for the investigation of organic particle transport and biogenic carbon cycling processes in sandy littoral interstices at Lake Tegel, Berlin, Germany. Passive particle transport through the pore system was studied by in situ exposition of 2.4μm monodisperse polymeric resin microparticles stained with 7-amino-4-methylcoumarin (AMC). Uptake of fluorescein-5-isothiocyanate (FITC)-labelled Chlorella vulgaris and fine particulate organic matter (FPOM) by the interstitial fauna was investigated in laboratory and field experiments. The major portion (>85%) of the FITC-labelled particles added to sediment cores was recovered from the topmost centimetre of sediment during the study period of 14 days. Uptake of FITC-labelled FPOM was observed in several benthic groups, e.g. chironomids, microcrustaceans, oligochaetes and tardigrads, whereas C. vulgaris was ingested by oligochaetes only. There is evidence to suggest that both are suitable materials for investigating the fragmentation and ingestion of organic material by herbivorous and detritivorous fauna. Field experiments with inert microparticles and FITC-labelled FPOM (<1mm) prepared from dried alder leaves were carried out in plexiglass tubes as in situ whole core technique. Within the investigation period of two weeks, the transport of FPOM was restricted to the topmost 2–3cm of sediment in conjunction with a distinct fragmentation to finer size classes with respect to increasing sediment depth. Vertical FPOM transport was hindered by a high interstitial concentration of natural POM and an intensive settlement of the interstices by algae (mainly epispammic algae, 65–96% of algae cell number) and extra-cellular polymeric substances (EPS), which formed a dense three-dimensional structure. [Copyright &y& Elsevier]

Abstract: Changing environmental conditions in the Arctic will affect patterns of coastal erosion processes and thus modify the carbon cycle in the Arctic Ocean. To address this issue, a coastal classification of the Arctic was established to provide the first reliable estimate of organic carbon input from coastal erosion to the Arctic Ocean. The calculation relies on geomorphic parameters and the length of the coastline in the form of a line dataset used in geographical information systems (the World Vector Shoreline). The statistical self-similarity of Arctic shorelines (i.e. the fact that they exhibit similar features and hence different lengths at different scales) hampers the calculation process. Delineating the same section of shoreline at different spatial scales produces changes in the calculated length of the coastline and therefore in the volume of sediment released by up to 30% in some cases. The amount of change differs depending on the type and morphology of the coastline. The length of the World Vector Shoreline does not correlate well to any one scale and is inappropriate for use at the global level. Computations of erosion based on areas instead of lengths (i.e. buffers instead of shoreline lengths) provide a valuable yet simple substitute to the length-based method. Differences in quantities of eroded sediment are, on average, 70% less affected by scale changes when areas are used. Area-based methods are therefore recommended for circum-polar, computation-demanding, shoreline-based erosion calculations. [Copyright &y& Elsevier]

The article presents a case study on the eroded soil organic carbon (SOC) in the Mississippi Basin (MS). The soil erosion, translocation, redeposition, oxidation, and carbon dioxide sequestration across the five major river subsystems in the MS was quantitatively analyzed to study the role of soil and soil erosion in the carbon cycle. A significant. portion of eroded SOC is exposed to greater oxidation rates during and after erosion creased oxidation due to the breakdown of aggregates, exposing more SOC to the atmosphere and thus reducing organic carbon of eroded soil. The SOC was analyzed using the State Soil Geographic Database. The major medium for eroded material transport is water, so the river gaging station near the downstream boundary of each subsystem is the monitoring point for transport. The problem in the analysis of MS Basin sediment and organic carbon (OC) budget is that erosion estimates are model-based estimates with inherent biases and do not include erosional processes such as gullying and river-bank erosion. Though the water bodies and alluvial soils are carbon dioxide sources, but they must be compared to the locations from which the decomposing OC came. The soil erosion in the MS Basin results in soil degradation due to the removal of organic matter (OM) from upland soils and relocation to depositional sites rather than by net loss of OM to oxidation.

• We model organic carbon adsorption on sediment in the dammed upper Yangtze River. • Our model integrates sediment properties and environmental factors. • Cascade dams reduce suspended sediment and increase particulate organic carbon. • Dam impacts are pronounced locally and further upstream and downstream. Understanding the behaviour of particulate organic carbon (POC) with sediment transport allows for a more accurate estimation of global carbon cycling and the conditions of aquatic ecosystems. River damming alters POC dynamics profoundly by the retention of sediments on which organic carbon (OC) is adsorbed. In this study, we developed a mechanism-based approach to investigate organic carbon (OC) adsorption on river sediment, integrating sediment particle properties (particle size, particle density, surface site density, and particle morphology) and environmental factors (dissolved OC concentration, pH, and suspended sediment concentration). We used this approach to assess the POC concentration in the dammed upper Yangtze River and to compare it with observed POC values in literature; model results and observations correlated very well (R2 = 0.89; NSE = 0.83; p < 0.001). OC adsorption on sediment was found to correlate positively with dissolved organic carbon concentration and negatively with pH and suspended sediment concentration. We found that hydroelectric cascade development contributed to a decrease in suspended sediment concentration, with a significant increase in POC concentration both at reservoir sites, and upstream and downstream. The average suspended sediment concentration near the watershed outlet decreased from 2.08 kg m−3 (individual reservoir period) to 1.57 kg m−3 (early stage of reservoir construction/operation) and then to 0.01 kg m−3 (cascade reservoir period). In contrast, the average POC concentration in the dammed river increased from less than, or around 1%, to 1% and finally to 3% during each of these three periods, respectively. Our results highlight the pronounced impacts of cascade reservoirs on river sediment and POC dynamics. By providing a method for assessing OC adsorption on sediment and the dynamics of POC in aquatic systems, this work advances our understanding of carbon cycling in aquatic systems in times of global change. Image, graphical abstract [ABSTRACT FROM AUTHOR]

This special issue of Geo-Marine Letters presents selected contributions from the international conference Particles in Europe (PiE) 2010 organized by Sequoia Scientific, Inc., and the Laboratoire d'Océanographie de Villefranche (LOV) on 15-17 November 2010 in Villefranche-sur-Mer, France, and guest-edited by Ole Mikkelsen, Malik Chami and David Doxaran. PiE was initiated in 2008, in order to promote and further our understanding of the importance of suspended particulate matter (SPM) for a very wide range of processes in the aquatic environment-from optics and acoustics, over sediment transport, to the global carbon balance. The papers in this special issue are in particular concerned with the interaction between SPM and water optical properties, as well as how to use optical proxy measurements to understand SPM processes. The next PiE conference is scheduled for 17-19 October 2012 in Barcelona, Spain. [ABSTRACT FROM AUTHOR]