Transport Of Organic Carbon Research Articles

Nearshore Zone Dynamics Determine Pathway of Organic Carbon From Eroding Permafrost Coasts

Collapse of permafrost coasts delivers large quantities of particulate organic carbon (POC) to Arctic coastal areas. With rapidly changing environmental conditions, sediment and organic carbon (OC) mobilization and transport pathways are also changing. Here, we assess the sources and sinks of POC in the highly dynamic nearshore zone of Herschel Island‐Qikiqtaruk (Yukon, Canada). Our results show that POC concentrations sharply decrease, from 15.9 to 0.3 mg L−1, within the first 100–300 m offshore. Simultaneously, radiocarbon ages of POC drop from 16,400 to 3,600 14C years, indicating rapid settling of old permafrost POC to underlying sediments. This suggests that permafrost OC is, apart from a very narrow resuspension zone (<5 m water depth), predominantly deposited in nearshore sediments. While long‐term storage of permafrost OC in marine sediments potentially limits biodegradation and its subsequent release as greenhouse gas, resuspension of fine‐grained, OC‐rich sediments in the nearshore zone potentially enhances OC turnover.

Spatio-temporal assessment and climatology of atmospheric organic carbon over Pakistan

Organic carbon (OC) is the amount of carbon found in an organic compound. Once entered, the environment OC may cause climate change and effects on public health. The objective of the study was to assess the spatio-temporal trends of OC, climatology, and to develop inventories. The three satellite datasets (MACCity, ACCMIP, RCP 8.5) showed significant different trends in OC levels. Results indicated significantly higher trends in OC distribution during anthropogenic activities and the industrial era as compared with natural sources and the pre-industrial era. The residential and industrial sectors showed higher emission of OC 1.0 × 10−5to 2.5 × 102 Tg/year, respectively as compared with other sectors. Spatial distribution of OC revealed higher levels in bigger cities (Islamabad, Lahore, Karachi, Multan, Punch, and Mardan districts) of Pakistan. The temporal analysis shows irregular increasing trends in OC distribution with the passage of time. Principal component analysis (PCA) revealed that residential OC showed a positive correlation with surface temperature, wind and heat and OC transport, sum, residential, and energy sector with relative humidity in MACCity and RCP 8.5 datasets, respectively. The study indicated that savanna burning and residential sectors were a major contributor to the level of OC. The results will be helpful for the government to devise policies regarding OC management.

Sediment deposition changes the relationship between soil organic and inorganic carbon: Evidence from the Chinese Loess Plateau

Soil erosion has a significant impact on the distribution and sequestration of soil carbon, affecting the global C cycle. Although the transport and transformation of soil organic carbon (SOC) during the erosion process have been widely studied, there is still a lack of understanding of the impacts of soil erosion on soil inorganic carbon (SIC) and its relationships with SOC. In this study, 62 erosional sites (including 16 gullies, 17 grasslands, 16 forestlands and 13 croplands) and 35 depositional sites (check dams) on the Chinese Loess Plateau were selected. Partial least squares regression (PLSR) was carried out to explore the main influential factors of SIC variation. The results showed that, compared with erosional sites, the mean SOC and SIC content in depositional sites (0–25 cm) increased by 24.4 % and 15.4 %, respectively. Both SOC and SIC contents in depositional sites exhibited a decreasing tendency with depth. SIC was significantly negatively correlated with SOC (R2 = 0.25, P < 0.01) across all erosional sites (0–25 cm). In contrast, SIC was significantly positively correlated with SOC in all depositional layers beneath 100 cm (P < 0.05), and R2 increased from 0.23 (100–200 cm) to 0.68 (300–400 cm) and then decreased to 0.46 (500–600 cm). Sediment transport and deposition converted the negative relationship between SOC and SIC in the erosional sites to a positive relationship in deep depositional soil. The PLSR analysis suggested that SOC had an important (VIP > 1) positive impact on predicting SIC in depositional sites, which indicated that SOC increase contributed to enhancing SIC accumulation under the depositional conditions in the semi-arid region. We believe that the influencing mechanism of soil erosion on the relationship between SOC and SIC revealed in this study will contribute to understanding the soil C cycle.

Variations in organic carbon sourcing along a trans-Himalayan river determined by a Bayesian mixing approach

Rivers transfer particulate organic carbon (POC) from eroding mountains into geological sinks. Organic carbon source composition and selective mobilization have been shown to affect the type and quantity of POC export, but their combined effects across complex mountain ranges remain underexplored. Here, we examine the variation in organic carbon sourcing and transport in the trans-Himalayan Kali Gandaki River catchment, along strong gradients in precipitation, rock type and vegetation. Combining bulk stable nitrogen, and stable and radioactive organic carbon isotopic composition of bedrock, litter, soil and river sediment samples with a Bayesian end-member mixing approach, we differentiate POC sources along the river and quantify their export. Our analysis shows that POC export from the Tibetan segment of the catchment, where carbon bearing shales are partially covered by aged and modern soils, is dominated by petrogenic POC. Based on our data we re-assess the presence of aged biospheric OC in this part of the catchment, and its contribution to the river load. In the High Himalayan segment, we observed low inputs of petrogenic and biospheric POC, likely due to very low organic carbon concentrations in the metamorphic bedrock, combined with erosion dominated by deep-seated landslides. Our findings show that along the Kali Gandaki River, the sourcing of sediment and organic carbon are decoupled, due to differences in rock organic carbon content, soil and above ground carbon stocks, and geomorphic process activity. While the fast eroding High Himalayas are the principal source of river sediment, the Tibetan headwaters, where erosion rates are lower, are the principal source of organic carbon. To robustly estimate organic carbon export from the Himalayas, the mountain range should be divided into tectono-physiographic zones with distinct organic carbon yields due to differences in substrate and erosion processes and rates.

Assessing branched tetraether lipids as tracers of soil organic carbon transport through the Carminowe Creek catchment (southwest England)

Abstract. Soils represent the largest reservoir of organic carbon (OC) on land. Upon mobilization, this OC is either returned to the atmosphere as carbon dioxide (CO2) or transported and ultimately locked into (marine) sediments, where it will act as a long-term sink of atmospheric CO2. These fluxes of soil OC are, however, difficult to evaluate, mostly due to the lack of a soil-specific tracer. In this study, a suite of branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are membrane lipids of soil bacteria, is tested as specific tracers for soil OC from source (soils under arable land, ley, grassland, and woodland) to sink (Loe Pool sediments) in a small catchment located in southwest England (i.e. Carminowe Creek draining into Loe Pool). The analysis of brGDGTs in catchment soils reveals that their distribution is not significantly different across different land use types (p&gt;0.05) and thus does not allow land-use-specific soil contributions to Loe Pool sediments to be traced. Furthermore, the significantly higher contribution of 6-methyl brGDGT isomers in creek sediments (isomerization ratio (IR) = 0.48±0.10, mean ± standard deviation (SD); p&lt;0.05) compared to that in catchment soils (IR = 0.28±0.11) indicates that the initial soil signal is substantially altered by brGDGT produced in situ. Similarly, the riverine brGDGT signal appears to be overwritten by lacustrine brGDGTs in the lake sedimentary record, indicated by remarkably lower methylation of branched tetraethers (MBT5ME′=0.46±0.02 in creek bed sediments and 0.38±0.01 in lake core sediments; p&lt;0.05) and a higher degree of cyclization (DC = 0.23±0.02 in creek bed sediments and 0.32±0.08 in lake core sediments). Thus, in this small catchment, brGDGTs do not allow us to trace soil OC transport. Nevertheless, the downcore changes in the degree of cyclization and the abundance of isoprenoid GDGTs produced by methanogens in the Loe Pool sediment do reflect local environmental conditions over the past 100 years and have recorded the eutrophication history of the lake.

Rapidly-migrating and internally-generated knickpoints can control submarine channel evolution

Submarine channels are the primary conduits for terrestrial sediment, organic carbon, and pollutant transport to the deep sea. Submarine channels are far more difficult to monitor than rivers, and thus less well understood. Here we present 9 years of time-lapse mapping of an active submarine channel along its full length in Bute Inlet, Canada. Past studies suggested that meander-bend migration, levee-deposition, or migration of (supercritical-flow) bedforms controls the evolution of submarine channels. We show for the first time how rapid (100–450 m/year) upstream migration of 5-to-30 m high knickpoints can control submarine channel evolution. Knickpoint migration-related changes include deep (>25 m) erosion, and lateral migration of the channel. Knickpoints in rivers are created by external factors, such as tectonics, or base-level change. However, the knickpoints in Bute Inlet appear internally generated. Similar knickpoints are found in several submarine channels worldwide, and are thus globally important for how channels operate.

Long-term data reveal highly-variable metabolism and transitions in trophic status in a montane stream

AbstractIn streams, gross primary production (GPP) and ecosystem respiration (ER) (i.e., stream metabolism) control the transport and fate of nutrients and organic carbon and vice versa. The import...

Evidence for glacial geological controls on the hydrology of Maine (USA) peatlands

Abstract Freshwater pools commonly form eccentric crescent patterns in peatlands, an important atmospheric methane (CH4) source, and show an apparent spatial association with eskers in some deglaciated regions. However, the role of underlying permeable glacial deposits such as eskers in regulating hydrogeology, and perhaps even carbon cycling, in peatlands is rarely considered. In this study, ground-penetrating radar imaging and direct coring confirmed that clustered pools coincide with buried esker crests in contact with peat soil in Caribou Bog and Kanokolus Bog in Maine (USA). Hydraulic head and geochemical data combined with lidar indicate vertical water flow from shallow peat toward the permeable esker crests, suggesting enhanced downward transport of labile organic carbon that presumably accelerates rates of methanogenesis in deep peat. Eskers might therefore serve as proxies for enhanced CH4 production in deep peat, as supported by differences in dissolved CH4 profiles depending on proximity to pools. Geographic data compiled from multiple sources suggest that many peatlands with eccentric pools appear to be located proximal to esker systems in Maine and Fennoscandia. These geological factors may be important, previously unrecognized controls on water and the carbon cycle in peatlands.

Offshore transport of organic carbon by upwelling filaments in the Canary Current System

A coupled physical-biogeochemical model (ROMS-PISCES) forced by climatological fields is used to examine the role of upwelling filaments in the offshore exchange of particulate (POC) and dissolved (DOC) organic carbon in the Canary Current eastern boundary upwelling system (CanC EBUS). In this region, mesoscale filaments at Capes Ghir (∼30.5 °N) and Juby (∼27.5 °N) have been frequently described using both observational and numerical data. Due to their semi-permanent presence and unique dynamical characteristics, studies focusing on filaments often provide an incomplete picture of the physical and biological processes at work, and their effects on coast-to-ocean export. The present model experiment confirms the complex three-dimensional structure of the filaments that comprises both offshore and onshore flow components. The model shows strong seasonal variability in the offshore transport mediated by the filaments. Recirculation at the edges of the filaments returns water towards the shore, especially in autumn when they are diverted northwards by the large scale boundary circulation. By contrast, offshore transport peaks during late spring - early summer when onshore recirculation is limited. Overall, the estimated net annual offshore flux of excess total organic carbon (e-TOC, the non-refractory pools of DOC and POC) averages 2.0 ×109 kg C y−1, and may increase up to 4.3 ×109 kg C y−1 during the peak upwelling season, each filament contributing to export of up to 22.6% of the organic carbon within the first 100 km from shore along the CanC EBUS (between 9.5 and 32 ° N). These results strongly support the inclusion of offshore transport estimates by coastal filaments in regional carbon budgets.

Late Quaternary sporadic development of Desmophyllum dianthus deep-coral populations in the southern Labrador Sea with specific attention to their 14C- and 230Th-dating

About 70 fossil and live-collected specimens of Desmophyllum dianthus were collected from steep rocky surfaces and time-averaged assemblages at their base (“coral graveyards”) in the southern Labrador Sea at depths ranging from ~1700 m (Orphan Knoll) to ~2200 m (Flemish Cap). Aside from stable isotope and mineralogical analysis, U-series and 14C age determinations were performed on the samples. Fossil corals from the coral graveyards display two principal age clusters corresponding respectively to the Holocene and Marine Isotope Stage (MIS) 5c intervals. Three samples lay outside these clusters, at ~13.5 ka (Bølling-Allerød), ~ 64 ka (MIS 3) and at ~181 ka (MIS 7a). The clusters are thought to record intervals with both i) high food availability, either through pelagic primary production in the overlying water column or more effectively in relation with particulate and dissolved organic carbon transport via an active Western Boundary Undercurrent (WBUC), and ii) high carbonate micrite advection by this current from more open North Atlantic coccolithophorid production areas, maintaining an aragonite saturation horizon (now at 2300 m) slightly deeper than the coral habitat (~ 2200 m). Attempts at estimating ventilation ages from 230Th vs 14C age offsets in Holocene samples failed. This is due to uncertainties about the initial 14C-apparent age of coral skeletons and to their high initial excesses in 230Th. This initial excess relates mostly to the scavenging of 230Th produced by the dissolved marine U by the coral-feeding organic matter carried along the WBUC trajectory. In addition, discrete diagenetic U-fluxes can be documented in fossil samples. They are well illustrated in specimens from MIS 5c age, which depict variable excesses in 234U vs 238U, but similar 230Th/238U activity ratios, pointing to a recent but fractionating redistribution of uranium in the samples. These are also documented by the development of FeMn coatings of specimens exposed at the sea floor, due to the winnowing of embedding slope sediments, with the mid- to late Holocene intensified WBUC. Such diagenetic processes are to be notably taken into consideration for the dating of pre-Holocene samples. Thus, both the initial isotopic properties of corals from this WBUC-influenced, mid-continental slope, and subsequent diagenetic effects impact their calculated ages, and blur estimates of “ventilation ages” based on their 14C vs 230Th-ages offsets.

Modeling Spatiotemporal Patterns of Ecosystem Metabolism and Organic Carbon Dynamics Affecting Hypoxia on the Louisiana Continental Shelf.

The hypoxic zone on the Louisiana Continental Shelf (LCS) forms each summer due to nutrient enhanced primary production and seasonal stratification associated with freshwater discharges from the Mississippi/Atchafalaya River Basin (MARB). Recent field studies have identified highly productive shallow nearshore waters as an important component of shelf-wide carbon production contributing to hypoxia formation. In this study we present results from a three-dimensional hydrodynamic-biogeochemical model named CGEM (Coastal Generalized Ecosystem Model) applied to quantify the spatial and temporal patterns of hypoxia, carbon production, respiration, and transport between nearshore and middle shelf regions where hypoxia is most prevalent. We first demonstrate that our simulations successfully reproduced spatial and temporal patterns of carbon production, respiration, and bottom-water oxygen gradients compared to field observations. We then used interannual simulations to identify transport of particulate organic carbon (POC) from nearshore areas where riverine organic matter and phytoplankton carbon production are greatest. The spatial disconnect between carbon production and respiration in our simulations was driven by westward and offshore POC flux, a pattern that supported heterotrophic respiration on the middle shelf where hypoxia is frequently observed. These results validate the importance of offshore carbon flux to hypoxia formation, particularly on the west shelf where hypoxic conditions are more variable.

Tracer‐Aided Modeling in the Low‐Relief, Wet‐Dry Tropics Suggests Water Ages and DOC Export Are Driven by Seasonal Wetlands and Deep Groundwater

AbstractOur understanding of how wet‐dry tropical catchments process water and solutes remains limited. In this study, we attempt to gain understanding of water and dissolved organic carbon (DOC) transport, storage, and mixing in a 126 km2 catchment of northern Australia. We developed a coupled, tracer‐aided, conceptual rainfall‐runoff model (SAVTAM) that simultaneously calculates water, isotope, and DOC‐based processes at a daily time step. The semidistributed model can account for the marked hydrological distinction between savanna woodlands and adjacent seasonal wetlands. Using the calibrated model, we tracked the fluxes and derived the age of water in fluxes and storages. Model output matched the seasonal variability, controlled by seasonal rainfall, which switched on and off water and carbon flow pathways from the savanna to seasonal wetlands and ultimately to the perennial river. Such hydrological connectivity is modulated by the karst aquifer system that continuously contributes older waters (decades to century old) to maintain relatively stable and older streamflow during the dry season (average stream water age = 9.7 to 16.2 years). Such older waters occur despite a rapid, monsoon‐driven streamflow response. The DOC fluxes were largely sourced from the wetland and riparian forest that transported DOC in the order of 1.9 t C km−2 year−1 to the stream, which was on average 90% of the total simulated DOC exports of 2 t C·km−2·year−1. We conclude that coupled simulation of water and biogeochemistry is necessary to generate a more complete picture of catchment functioning, particularly in the tropics.

Selective Adsorption of Terrestrial Dissolved Organic Matter to Inorganic Surfaces Along a Boreal Inland Water Continuum

AbstractDifferent processes contribute to the loss or transformation of dissolved organic matter (DOM) and change DOM concentration and composition systematically along the inland water continuum. Substantial efforts have been made to estimate the importance of microbial and photochemical degradation for DOM concentration and composition and, to some extent, also DOM losses by flocculation, whereas the significance of DOM adsorption to inorganic surfaces has received less attention. Hence, knowledge on the possible extent of adsorption, its effect on DOM loads and composition and on where along the aquatic continuum it might be important, is currently limited or lacking altogether. Here we experimentally determine DOM adsorption onto mineral particles in freshwater ecosystems covering a water residence time gradient in boreal landscape Sweden. We hypothesized that adsorption would gradually decrease with increasing water residence time but actually found that DOM is highly susceptible to adsorption throughout the aquatic continuum. Mass spectrometry and fluorescence analysis on DOM suggest that freshly produced aquatic DOM is less susceptible to adsorption than more terrestrial material. Moreover, the percentage DOM adsorbed in the experiments greatly exceeds the actual adsorption taking place in boreal inland waters across all studied systems. These results illustrate the potential impact of mineral erosion, for example, as a result of agriculture, mining or forestry practices, on the availability, transport, and composition of organic carbon in inland waters.

Impact of origin and structure on the aggregation behavior of natural organic matter

The intermolecular interactions of natural organic matter (NOM) play a key role in the fate and transport of organic carbon and pollutants in environmental and engineered systems. In this study, the impact of origin and structure on the aggregation behavior of NOM was investigated in the presence of naturally abundant cations. The physicochemical properties of NOM were quantified using a range of indices. Thermodynamic analysis suggests that the colloidal stability of NOM was mainly determined by its hydrophobicity (i.e., Lewis acid-base interactions). All NOM can be coagulated by Ca2+ owing to the strong cation-NOM interactions, which lead to bridging effect and lower Lewis acid-base interactions. Terrestrial NOM can be coagulated by Mg2+ while aquatic NOM cannot, owing to their different hydrophobicity. The critical coagulation concentrations of tested terrestrial NOM in the presence of Ca2+ (CCC–Ca) were quite similar at 1.94–4.88 mM despite their different structural properties. The CCC-Ca of tested aquatic NOM varied significantly from 46.89 mM to 110.40 mM depending on their structure. The optical indices including E2/E3, FI, and HIX can be potentially used as convenient indicators for assessing the colloidal stability of aquatic NOM for water treatment and risk assessment purposes.

Relative influence of forest and cropland on fluvial transport of soil organic carbon and nitrogen in the Nen River basin, northeastern China

Riverine transport of carbon and nitrogen strongly impacts terrestrial and aquatic ecosystem functions but is itself affected by changes in land cover and land use. China’s Nen River basin has experienced widespread conversion from forest to cropland in the past century, making it representative of similar global land use changes. We assessed organic carbon and nitrogen transport in this basin during 2018 and 2019, using water samples collected at one-hour intervals during the twelve flood events in two sub-basins (the Yalu and Yin rivers) dominated by forests and cropland, respectively. We also assessed dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations from 19 sampling sites distributed throughout the Nen River basin, in combination with hydrological records, to investigate the impacts of land cover on riverine carbon and nitrogen fluxes. The forested Yalu river basin showed significantly higher DOC content and lower TDN content than Yin river basin. Riverine DOC concentrations during non-flood periods in the forested Yalu river basin were significantly higher than in the cultivated Yin river basin, but riverine TDN concentrations were lower. Throughout the Nen River basin and its sub-basins, riverine TDN content was proportional to cropland area. During the twelve flood events, riverine DOC and TDN varied widely over time in the Yin and Yalu river basins, possibly due to dilution effects of water originating from different land covers within the basins. Our results demonstrate that land cover is the principal cause of changes in riverine DOC and TDN content in this region.

Contrasting depth distribution of colloid-associated phosphorus in the active and abandoned sections of an alluvial fan in a hyper-arid region of the Atacama Desert

Colloids and their subset nanoparticles are key soil constituents for nutrient and Organic Carbon (OC) storage and transport, yet little is known about their specific role in overall transfer of elements under hyper-arid conditions. We analyzed the Water Dispersible Colloids (WDCs) of two adjacent soil profiles, located either on the active (named: Fan) or passive (named: Crust) sections of an alluvial fan. Colloidal particles (<500 nm) were fractionated using Asymmetric Field-Flow-Field Fractionation (AF4), which was coupled online to an Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) and an Organic Carbon Detector (OCD) to detect the composition of size-fractionated colloids. Three size categories of particles were identified: nanoparticles (0.6–24 nm), fine colloids (24–210 nm), and medium colloids (210–500 nm). The two profiles differed distinctively in vertical WDC distribution and associated phosphorus (P) content. Fractograms of the Crust profile predominantly showed fine colloids, whereas the medium-sized colloids dominated those of the Fan. Furthermore, the highest colloid content in the Crust profile was found at the surface, while in the Fan, colloids accumulated at 10–20 cm depth, thus overall reflecting the different genesis and infiltration capacities of the soils. Despite very low concentration of colloidal P in these hyper-arid soils, a strong correlation between colloidal P and calcium (Ca), Silica (Si), aluminum (Al), iron (Fe), and OC content were found. This also revealed Ca-phosphates as the primary P retention from, with the association of P to phyllosilicates and Fe/Al (hydr-) oxides as the main soil colloidal fractions. Overall, our results did highlight that small local scale differences in topographic-derived distribution of water flow pathways, defined the formation of the crust-like surfaces, and ultimately the overall movement and distribution of nanoparticles and colloids in soil profiles under hyper-arid conditions.

Modelling peak exposure of pesticides in terrestrial and aquatic ecosystems: importance of dissolved organic carbon and vertical particle movement in soil

ABSTRACTIn the present work, an existing vegetation/air/litter/soil model (SoilPlusVeg) was modified to improve organic chemical fate description in terrestrial/aquatic ecosystems accounting for horizontal and vertical particulate organic carbon (POC) transport in soil. The model was applied to simulate the fate of three pesticides (terbuthylazine, chlorpyrifos and etofenprox), characterized by increasing hydrophobicity (log KOW from about 3 to 7), in the soil compartment and more specifically, their movement towards surface and groundwater through infiltration and runoff processes. The aim was to evaluate the role of dissolved organic carbon (DOC) and POC in the soil in influencing the peak exposure of pesticides in terrestrial/aquatic ecosystems. Simulation results showed that while terbuthylazine and chlorpyrifos dominated the free water phase (CW-FREE) of soil, etofenprox was mainly present in soil porewater as POC associated chemical. This resulted in an increase of this highly hydrophobic chemical movement towards groundwater and surface water, up to a factor of 40. The present work highlighted the importance of DOC and POC as an enhancer of mobility in the water of poor or very little mobile chemicals. Further studies are necessary to evaluate the bioavailability change with time and parameterize this process in multimedia fate models.

Changes in riverine organic carbon input to the ocean from mainland China over the past 60 years

Compared to rivers in Europe and North America, Chinese rivers that discharge into oceans have different organic carbon (OC) transport characteristics. Out of the top 25 largest rivers worldwide, three (Changjiang, Huanghe, and Zhujiang rivers) are located in China, along with numerous small rivers. Thus, synthesized estimates of total riverine OC flux from Chinese rivers into marginal seas are critical but remain deficient. In this study, we developed relationships between riverine OC (dissolved OC, or DOC, and particulate OC, or POC) and basin characteristic variables (basin population density, precipitation, and riverine suspended sediment concentration) to estimate annual riverine DOC and POC fluxes during 1953–2016. The results showed that rivers in mainland China transported 9.63 Tg C of OC to the marginal seas in 2008, with 4.61 Tg C of DOC and 5.02 Tg C of POC. Of this transported OC, 14.28% DOC and 17.49% POC were transported by small southeastern rivers, whose drainage areas covered only 6.68% of the total. Because of intensifying human activities, DOC export increased but POC export decreased during 1953–2016. Additionally, basin population growth and reservoir water capacity were the major factors for increasing DOC flux and decreasing POC flux, respectively. Overall, the DOC/POC ratio increased for OC transport in Chinese rivers. Therefore, this study is important for understanding human-induced impacts on environmental change and the carbon cycle in marginal seas.

Offshore transport of particulate organic carbon in the California Current System by mesoscale eddies

The California Current System is characterized by upwelling and rich mesoscale eddy activity. Cyclonic eddies generally pinch off from meanders in the California Current, potentially trapping upwelled water along the coast and transporting it offshore. Here, we use satellite-derived measurements of particulate organic carbon (POC) as a tracer of coastal water to show that cyclones located offshore that were generated near the coast contain higher carbon concentrations in their interior than cyclones of the same amplitude generated offshore. This indicates that eddies are in fact trapping and transporting coastal water offshore, resulting in an offshore POC enrichment of 20.9 ± 11 Gg year−1. This POC enrichment due to the coastally-generated eddies extends for 1000 km from shore. This analysis provides large-scale observational-based evidence that eddies play a quantitatively important role in the offshore transport of coastal water, substantially widening the area influenced by highly productive upwelled waters in the California Current System.

Pathways of Organic Carbon Downward Transport by the Oceanic Biological Carbon Pump

The oceanic biological carbon pump (BCP) regulates the Earth carbon cycle by transporting part of the photosynthetically fixed CO2 into the deep ocean. Suppressing this mechanism would result in an important increase of atmospheric CO2 level. The BCP occurs mainly in the form of (1) organic carbon (OC) particles sinking out the surface ocean, of (2) neutrally buoyant OC (dissolved or particulate) entrained by downward water masses movements and/or mixing, and of (3) active transport of OC by migrating animals such as zooplankton and fishes. These various pools of OC differ in size since their sinking, production and decomposition rates vary spatially and temporally. Moreover, the OC transported to depths via these various export pathways as well as their decomposition pathways all have different ecological origins and therefore may response differently to climate changes. Currently, most ocean biogeochemical models do not resolve these various of OC pathways explicitly; rather, they imply that OC is therein created and destroyed equally. In addition, the organic composition of these various pools is largely unknown, especially at depths below 500m. Here known processes of OC export from the surface ocean to the mesopelagic zone (100-1000m) are briefly reviewed. Three OC export pathways and some of their sub-categories are considered. I refer to published studies of OC fluxes associated with the specific downward export pathways and identify gaps that need to be addressed to better understand the OC fluxes associated with the BCP.