Riverine Particles Research Articles

Global patterns of organic carbon transfer and accumulation across the land–ocean continuum constrained by radiocarbon data

Radiocarbon (Δ14C) serves as an effective tracer for identifying the origin and cycling of carbon in aquatic ecosystems. Global patterns of organic carbon (OC) Δ14C values in riverine particles and coastal sediments are essential for understanding the contemporary carbon cycle, but are poorly constrained due to under-sampling. This hinders our understanding of OC transfer and accumulation across the land–ocean continuum worldwide. Here, using machine learning approaches and >3,800 observations, we construct a high-spatial resolution global atlas of Δ14C values in river–ocean continuums and show that Δ14C values of river particles and corresponding coastal sediments can be similar or different. Specifically, four characteristic OC transfer and accumulation modes are recognized: the old–young mode for systems with low river and high coastal sediment Δ14C values; the young–old and old–old modes for coastal systems with old OC accumulation receiving riverine particles with high and low Δ14C values, respectively; and the young–young mode with young OC for both riverine and coastal deposited particles. Distinguishing these modes and their spatial patterns is critical to furthering our understanding of the global carbon system. Specifically, among coastal areas with high OC contents worldwide, old–old systems are largely neutral to slightly negative to contemporary atmospheric carbon dioxide (CO2) removal, whereas young–old and old–young systems represent CO2 sources and sinks, respectively. These spatial patterns of OC content and isotope composition constrain the local potential for blue carbon solutions.

Sources, Transport, and accumulation of Mercury in the northwestern Mediterranean margin sediments during the Industrial Era and influence of turbiditic events

Sources and pathways of the Hg accumulated in the sediments of the Gulf of Lion (GoL) and its adjacent marine areas (Northwestern Mediterranean) have been explored using sediment grab samples, sediment cores, and sediment trap samples. The main source of Hg along this margin is the Rhône River, whose suspended sediments settle mainly in the prodelta area but also along the mid-shelf, then reaching the continental rise via wave resuspension and cascading of dense shelf waters. Seaward, these riverine particles are mixed with carbonated ooze conveyed to the bottom by the biological pump. The Hg is enriched in surface sediments of the GoL with decreasing concentrations westward and seaward from the Rhône prodelta to the continental rise. Dated cores from the Rhône prodelta give access to riverine sediments deposited over the last 400 years and show that the Hg concentration time trend resembles the evolution of coal consumption in France, peaking during in the 1960s. Similar trends were observed in sediment cores collected along the GoL slope and submarine canyons along with the preservation of traces of erosion and sediment instability events. Seaward, on the continental rise, the Hg concentration distribution suggests the deposition of fine planktonic-derived material and particles episodically advected from the shelf during deep cascading pulses. Anthropogenic Hg accumulated in GoL and continental rise sediments during the Industrialized Era is ∼ 150 Mg (tons), two-thirds of which are buried in the Rhône prodelta area. Significant correlations are found between Hg and organic matter in GoL sediments, but the relationships differ between areas and are disrupted by the inputs of anthropogenic Hg and by the Hg availability in the Rhône River watershed during the pre-industrial period. The HgT availability in waters appears to be the limiting factor to the Hg enrichment of the particulate organic matter. Monomethyl mercury (MMHg), which represents on average 0.3 % of the total Hg, was positively correlated to total Hg. Their distributions suggest in situ MMHg formation and a more effective net Hg methylation for surface sediment or particles collected in traps compared with particles buried in the sedimentary column.

Chemical Alteration of Riverine Particles in Seawater and Marine Sediments: Effects on Seawater Composition and Atmospheric CO2

Numerous studies have shown that riverine particles react with seawater. Reactions include dissolution of reactive silicate minerals (e.g., feldspars) and formation of authigenic clays and carbonates. Previous studies have either focused on mineral dissolution (marine silicate weathering) or authigenic phase formation (reverse weathering). A comprehensive study that assesses all processes affecting the marine alteration of riverine particle has -to our knowledge- not yet been conducted. Our contribution aims to fill this gap. We first quantify cation exchange between seawater and riverine particles that occurs when particles enter the marine realm and show that significant global cation fluxes are induced by this process (-1.3 Tmol Na yr-1, -0.2 Tmol K yr-1, -0.4 Tmol Mg yr-1, +1.2 Tmol Ca yr-1) where the positive sign indicates cation release into seawater while the negative sign denotes uptake on particles. We then use thermodynamic and kinetic modeling to investigate how much of the suspended particle load dissolves in contact with seawater and estimate corresponding global release rates for dissolved cations and silica assuming congruent dissolution (+0.06 Tmol Na yr-1, +0.15 Tmol Ca yr-1, +2.8 Tmol Si yr-1). Subsequently, we investigate rates of mineral dissolution and authigenic clay and carbonate formation in marine sediments applying reactive transport modeling, porewater data and mass balance calculations. Our best estimates for net fluxes across the sediment/water interface (dissolution–mineral formation) result as +1.5 Tmol Na yr-1, -2.5 Tmol K yr-1, -2.0 Tmol Mg yr-1, +2.5 Tmol Ca yr-1, and +1.9 Tmol Si yr-1 where most of the Na and Ca release is induced by plagioclase dissolution, K is taken up in authigenic clays and Mg is removed from solution by authigenic clay and carbonate formation. We conclude that the alkalinity of seawater is not significantly affected by marine silicate alteration since cation release fluxes (Na, Ca) are as high as cation uptake fluxes (K, Mg) on equivalent basis. Moreover, marine silicate weathering and reverse weathering are closely coupled since Al required for clay formation is mostly provided by feldspar dissolution while Al removal in authigenic clay promotes and maintains feldspar dissolution in marine sediments. Authigenic carbonate formation in anoxic subsurface sediments sequesters significant amounts of carbon (2.5 Tmol C yr-1) according to our estimates where most of the Ca and alkalinity required for carbonate formation are provided by the dissolution of Ca-bearing silicate minerals. This hidden sedimentary cycle provides a sink for dissolved inorganic carbon that may drive a slow draw-down of atmospheric CO2 on geological timescales. Marine silicate alteration has an even stronger effect on the geochemical evolution of seawater by generating large fluxes of dissolved K, Mg, Ca and Si.

The impact of riverine particles on the vertical velocities of large microplastics

Microplastics interact with other suspended particles in aquatic systems, which may impact their environmental fate. Little is known about aggregation between suspended sediment and larger microplastics (1–5 mm), and how this impacts the vertical velocities of microplastics, although it was hypothesised these are size limited. Consumer items made of five common polymers: polypropylene (PP), high density polyethylene (HDPE), polyethylene terephthalate (PET), polyvinyl chloride (PVC) and polystyrene (PS), were fragmented by cryomilling and their vertical velocities (rising/settling) measured experimentally before and after 24-hours of aggregation with riverine particles. Microplastic size (microscopy), zeta potential and density (density gradient column) were measured, with aggregation quantified using microscopy. PP had an experimental density of 1052 kg·m−3, and sank in river water, although it is often stated as being buoyant based on literature density values. Aggregation occurred with all five polymers: 39 %–72 % of microplastics were observed to have sediment and/or organic particles adhered, depending on the polymer type. PVC had the least negative zeta potential, −8.0 ± 3.0, and showed a much higher number of adhered sediment particles than all other polymers: on average 4.55 particles, compared with <1.72 particles for other polymers. For four polymers, aggregation did not significantly change vertical velocities. However, PP particles showed a significantly slower settling velocity after aggregation: a decrease of 6.3 % based on mean averages, from 9.7 × 10−3 to 9.1 × 10−3 m·s−1. Theoretical calculations showed the amount of adsorbed sediment or biofilm required to induce a microplastic density change of ∼50 kg·m−3 was much higher than observed experimentally. Overall, this study indicates that the vertical velocities of larger microplastics are less influenced by interactions with natural particles than smaller microplastics.

Riverine Particulate Matter Enhances the Growth and Viability of the Marine Diatom Thalassiosira weissflogii

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

Paleolimnological records for tracking dam-induced changes in the composition and supply of sediment to middle Yangtze floodplain lakes

A global boom in dam construction has reduced sediment loading of large rivers, as well as their connected floodplains. In order to explore potential effects of hydrological regulation on floodplain ecosystems, this study presents concentrations of inorganic elements, organic matter content and grain size spectra in 210Pb-dated sediment cores of five lakes in the middle Yangtze River floodplain. These lakes had free hydrological connectivity with the Yangtze River before the operation of local sluice gates, and hence K-rich but Al-poor particulates from the upper Yangtze reaches can be transported into lakes during periods of high river discharge. After hydrological regulation, sedimentary K/Al ratios registered substantial decreases in the study lakes, probably due to the declining supply of fine-grained and K-enriched riverine particulates from the Yangtze River. In East Dongting Lake, a hydrologically open lake proximal to the Three Gorges Dam (TGD), the sharp decrease in K/Al ratio after 2003 mainly responded to declining sediment supply from the Yangtze River after the TGD operation. In addition, prolonged water retention time probably promoted aquatic production and greater deposition of organic matter, as indicated by recent increases in organic matter content in the upper strata of the dammed lakes. Taken together, sedimentary K/Al ratios provide essential information on effects of past hydrological changes on sediment supply and aquatic productivity over multi-decadal timescales in these floodplain lakes with scarce monitoring data. The impacts of dam construction during recent decades on sediment composition in middle Yangtze floodplain lakes might be widespread in other similar floodplains worldwide. Thus sedimentary records can be utilized to inform and direct floodplain management.

Erosion of carbonate-bearing sedimentary rocks may close the alkalinity budget of the Baltic Sea and support atmospheric CO2 uptake in coastal seas

High alkalinity values observed in coastal seas promote the uptake of CO2 from the atmosphere. However, the alkalinity budget of coastal areas and marginal seas is poorly understood, even though some of the recently observed alkalinity enhancement can be ascribed to riverine fluxes and anaerobic processes in shelf sediments. Here, we investigate the alkalinity budget of the Baltic Sea to identify previously unrecognized alkalinity sources. We quantify the generation of alkalinity and dissolved calcium (Ca) in this marginal sea applying simple mass balance calculations. Using this approach, we identify alkalinity and Ca sources of approximately 324 Gmol yr-1 and 122 Gmol yr-1, respectively, that cannot be ascribed to the riverine input. The magnitude of the Ca source suggests that a major fraction of the excess alkalinity (244 Gmol yr-1) is induced by the dissolution of calcium carbonate (CaCO3). A review of available field data shows that carbonate-bearing rocks at the coast and the seabed of the Baltic Sea are rapidly eroded and may provide sufficient CaCO3 to close the Ca budget. Hence, dissolution of eroded CaCO3 is the most likely source for the Ca enrichment observed in Baltic Sea water. This hypothesis is supported by mass accumulation rates of sediments derived from radioisotope data that are evaluated to derive a basin-wide rate of mud to muddy sand accumulation at the bottom of the Baltic Sea. The resulting value (139 Tg yr-1) exceeds current estimates of riverine particle fluxes into the Baltic Sea by more than one order of magnitude and confirms that rates of till erosion are sufficiently high to account for the Ca and most of the alkalinity excess in Baltic Sea water. Finally, we show that deliberate addition of CaCO3 to sediments deposited in the Baltic Sea could neutralize significant amounts of CO2 and help to achieve net-zero greenhouse gas emissions in the Baltic region.

Refining the contribution of riverine particulate release to the global marine Nd budget

The release of neodymium (Nd) from particles along continental margins may contribute to losses in the global of Nd budget. The Changjiang River, which carries a heavy load of total suspended matter, empties into the East China Sea, and a strong particulate–seawater interaction process occurs along the salinity gradient. In the low-salinity region (S < 2.0), strong removal of dissolved rare earth elements (dREEs) occurs, but the Nd isotope values are uniform. At mid- and high-salinity (S = 2.0–28.0 and S > 28.0) areas, the dREE concentrations increase slightly. An Nd isotope mass balance indicates that the release of particulate matter is a source of dREEs in the Changjiang estuary. The release rate of particulate Nd (NdSPM) to the dissolved Nd pool in Changjiang estuary is higher than other estuaries, such as Amazon estuary. Composite all river data available from the previous studies indicate that 5800–9200 Mg per year of Nd is released to global marine waters from riverine particles. This estimated quantity is on the same order of magnitude as the calculated global Nd release flux based on the case study in the Amazon estuary. Our study indicates that to better constrain the global Nd budget, it is required to consider the release rate of NdSPM in different rivers due to the significant difference among various rivers, but with very limited available data as of now.

Dissolved neodymium isotopes in the Mediterranean Sea

The neodymium isotopic composition (εNd) of seawater is one of the most important geochemical tracers to investigate water mass provenance, which can also serve as a proxy to reconstruct past variations in ocean circulation. Nd isotopes have recently also been used to reconstruct past circulation changes in the Mediterranean Sea on different time scales. However, the modern seawater εNd dataset for the Mediterranean Sea, which these reconstructions are based on, is limited and up to now only 160 isotopic measurements are available for the entire basin. The lack of present-day data also limits our understanding of the processes controlling the Nd cycle and Nd isotopic distribution in this semi-enclosed basin. Here we present new εNd data from 24 depth profiles covering all Mediterranean sub-basins, which significantly increases the available dataset in the Mediterranean Sea. The main goal of our study is to better characterize the relationship between the dissolved Nd isotope distributions and major water masses in the Mediterranean Sea and to investigate the impact and relative importance of local non-conservative modifications, which include input of riverine particles and waters, aeolian-derived material and exchange with the sediments at continental margins. This comprehensive εNd dataset reveals a clear εNd – salinity correlation and a zonal and depth gradient with εNd systematically increasing from the western to the eastern Mediterranean basin (average εNd = −8.8 ± 0.8 and −6.7 ± 1 for the entire water column, respectively), reflecting the large-scale basin circulation. We have evaluated the conservative εNd behaviour in the Mediterranean Sea and quantified the non-conservative components of the εNd signatures by applying an Optimum Multiparameter (OMP) analysis and results from the Parametric Optimum Multiparameter (POMP) analysis of Jullion et al. (2017). The results of the present study combined with previously published Nd isotope values indicate that dissolved εNd behaves overall conservatively in the open Mediterranean Sea and show that its water masses are clearly distinguishable by their Nd isotope signature. However, misfits between measured and OMP- and POMP-derived εNd values exist in almost all sub-basins, especially in the eastern Levantine Basin and Alboran Sea at intermediate-deep depths, which can be explained by the influence of detrital lithogenic εNd signatures through interaction with highly radiogenic Nile sourced volcanic fractions and unradiogenic sediments, respectively.The radiogenic signature acquired in the eastern Levantine Basin is carried by the Levantine Intermediate Water and transferred conservatively to the entire Mediterranean at intermediate depths. Our measured εNd values and OMP- and POMP-derived results indicate that non-conservative contributions originating from sediment sources are then propagated by water mass circulation (with distinct preformed εNd) along the Mediterranean Sea through advection and conservative mixing. Mediterranean εNd effectively traces the mixing between the different water masses in this semi-enclosed basin and is a suitable water mass tracer.

Factors controlling dissolved 137Cs activities in coastal waters on the eastern and western sides of Honshu, Japan

The distributions of dissolved 137Cs in river, nearshore, and offshore waters on the east and west coasts of the Japanese island of Honshu were studied in 2018–2021, 7–10 years after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. On the east side along the north western North Pacific (Fukushima Prefecture), estuarine processes, including desorption from riverine particles and dissolution into pore water from riverine particles that had settled to the seafloor, contributed to the maintenance of high dissolved 137Cs activities in nearshore and offshore waters. A survey and mass-balance calculation in a semi-enclosed estuarine area, the Matsukawa-ura, in the northern part of Fukushima, provided convincing evidence that rivers contributed to the influx of 137Cs to coastal waters. In contrast, the extremely low activities of dissolved and particulate 137Cs in the Tedori River of Ishikawa Prefecture on the western side of Japan along the Japan Sea suggested that inputs of riverine 137Cs made a negligible contribution to the increase of dissolved 137Cs activities in the nearshore and offshore waters. The relatively high dissolved 137Cs activities observed in the offshore waters of the Japan Sea were due to movement of FDNPP-derived 137Cs into the Japan Sea via the Tsushima Warm Current. Mechanisms controlling the distributions of 137Cs activities in coastal waters of the eastern and western sides of Japan therefore differ.

Particle Triggered Reactions as an Important Mechanism of Alkalinity and Inorganic Carbon Removal in River Plumes

AbstractThe effects of heterogeneous reactions between river‐borne particles and the carbonate system were studied in the plumes of the Mississippi and Brazos rivers. Measurements within these plumes revealed significant removal of dissolved inorganic carbon (DIC) and total alkalinity (TA). After accounting for all known DIC and TA sinks and sources, heterogeneous reactions (i.e., heterogeneous CaCO3 precipitation and cation exchange between adsorbed and dissolved ions) were found to be responsible for a significant fraction of DIC and TA removal, exceeding 10% and 90%, respectively, in the Mississippi and Brazos plume waters. This finding was corroborated by laboratory experiments, in which the seeding of seawater with the riverine particles induced the removal of the DIC and TA. The combined results demonstrate that heterogeneous reactions may represent an important controlling mechanism of the seawater carbonate system in particle‐rich coastal areas and may significantly impact the coastal carbon cycle.

Estuarine processes modify the isotope composition of dissolved riverine barium fluxes to the ocean

Barium (Ba) isotope variations offer the potential to trace environmental processes, including long-term changes in river discharge and marine export production. Riverine inputs are an important source of dissolved Ba to the ocean, which we estimate to be ~10 to 20 Gmol yr−1. A large fraction (~20 to 75%) of this net riverine dissolved Ba flux to the ocean is derived from estuarine processes, in particular the release of exchangeable Ba from riverine suspended particles due to exchange with major cations in seawater. Despite the importance of this process for controlling the input of dissolved Ba to the ocean, its impact on the δ138/134Ba of net riverine dissolved Ba fluxes remains unknown. To address this observational gap, Ba concentration and isotope data from across the estuarine mixing zones of the Amazon (Brazil), Fly (Papua New Guinea) and Johor (Malaysia) Rivers are presented. Desorption from suspended riverine particles releases Ba with δ138/134Ba 0.2 to 0.3‰ lower than corresponding river dissolved loads, modifying the isotope composition of net riverine dissolved Ba fluxes that reach the ocean. This offset likely represents an isotope fractionation accompanying the adsorption of Ba by particulate phases within river catchments, which can explain the systematic enrichment of heavier Ba isotopes in river dissolved loads relative to weathering lithologies. River dissolved loads are also systematically offset to higher δ138/134Ba than the main oceanic Ba sink: burial of BaSO4 in marine sediment. This represents an apparent imbalance in the modern marine Ba isotope budget. Our results suggest that accounting for modification of the δ138/134Ba of net riverine Ba fluxes to the ocean by estuarine processes is likely to play a key role towards balancing the modern marine Ba isotope budget.

Importance of desorption process from Abukuma River's suspended particles in increasing dissolved 137Cs in coastal water during river-flood caused by typhoons

Desorption of radiocesium (137Cs) from riverine particles into seawater strongly influences 137Cs concentrations in coastal seawater. This process is important for quantifying the input of radionuclides to marine environments. Here we quantify the particulate 137Cs flux from the Abukuma River, Japan, during typhoon Hagibis and following typhoons in 2019 and estimate the resulting increased dissolved 137Cs levels in coastal seawater. Particulate 137Cs export flux, 1.1 × 1012 Bq, from the Abukuma River during the 4-day period of typhoon Hagibis (12–15 October 2019) equaled two-thirds of the annual flux during 2012–2015, the period of high 137Cs levels following the Fukushima Daiichi Nuclear Power Plant accident. The flux of the desorbed fraction from the Abukuma River during typhoon Hagibis was 0.061–0.12 × 1012Bq, and its daily flux to the surrounding coastal seawater (1.5–3.0 × 1010 Bq/d) was one to two orders of magnitude greater than the estimated input to the coastal seawater during the pre-typhoon period (1.3× 108–1.0 × 109 Bq/d). Simulated results suggest that the massive influx of riverine particles and subsequent desorption of 137Cs increased dissolved 137Cs levels in the coastal seawater by an order of magnitude, from 3.3 mBq/L (pre-typhoon level) to 45–126 mBq/L. This found pathway opens up new scenarios involving radionuclide dynamics in the boundary area of river-sea system.

Sources and radiocarbon ages of organic carbon in different grain size fractions of Yellow River-transported particles and coastal sediments

The elemental (TOC, TN and C/N) and carbon isotope (Δ14C and δ13C) compositions of organic matter were measured in different grain size fractions of particles transported by the Yellow River and surface sediments in the Bohai Sea and the Yellow Sea. In the riverine particle and sediment samples, high OC contents were associated with small grain size fractions consisting mainly of clay minerals. The δ13C and Δ14C values of the bulk riverine particulate organic carbon (POC) collected from the Lijin and Xiaolangdi sites were relatively constant but varied significantly (−21.9‰ to −26.0‰ and −325‰ to −620‰, respectively) among the different size fractions. In comparison, large spatial variations in δ13C (−20.6‰ to −24.5‰) and Δ14C (−188‰ to −646‰) values, which increased seaward due to the difference in source carbon, were found for bulk TOC Δ14C preserved in the surface sediments, but no significant differences were observed among the values in the different size fractions in most sediments. The different carbon isotopic values of the riverine POC and sedimentary TOC reflect differences in the sources, degradation and cycling time scales of the OC. The Yellow River exports very old (5220 ± 295 yrs) POC that is much older than the TOC (2457 ± 676 yrs) preserved in the surface sediments in the Bohai and Yellow seas. Calculations using a dual-isotope three end-member model indicate that pre-aged soil OC and ancient fossil OC represent major proportions (57 ± 16% and 30 ± 8%, respectively) of the riverine POC and that terrestrial biomass OC represents a minor proportion (13 ± 11%). The drainage environment of the river plays important roles in controlling the sources and ages of the riverine POC. In contrast, the TOC in the grain size fractions of the surface sediments in the Bohai and Yellow seas is dominated by marine-derived modern OC (47 ± 13%), followed by pre-aged soil OC (29 ± 9%) and ancient fossil OC (25 ± 14%). The ages of the TOC are determined mainly by the source input, rapid sedimentation, sediment minerology and decomposition of OC during early diagenesis in these large river-influenced marginal seas.

Iron isotopes reveal the sources of Fe-bearing particles and colloids in the Lena River basin

Large Arctic rivers are important suppliers of iron to the Arctic Ocean. However, the sources of Fe-bearing particles in permafrost-dominated systems and the mechanisms driving this supply of Fe are poorly resolved. Here, Fe isotope ratios were used to determine the sources of Fe-bearing particles and colloids in the Lena River and tributaries.In samples collected after the spring floods, Fe-bearing particles (>0.22 µm) carried ∼70% of the Fe and have isotope ratios that are lower than, or similar to that of the continental crust. These particles are composed of a leachable Fe fraction of largely ferrihydrite, with isotope values of −1.40‰ to −0.12‰, and a fraction of clays and Fe oxides with continental crust values. Co-existing Fe-bearing colloids (<0.22 µm), composed mainly of ferrihydrite, have higher isotope values, of −0.22‰ to +1.83‰. A model is proposed in which soil mineral weathering generates aqueous Fe with lower δ56Fe values. During transport, a small fraction of the dissolved Fe is precipitated as colloidal ferrihydrite with higher δ56Fe values. Most of the Fe is precipitated onto mineral grains in oxic riparian zones, with the δ56Fe values largely generated during weathering. Groundwater discharge and riparian erosion supply the colloids and coated particles to the rivers.The differences between δ56Fe values in leachates and detrital grains in Fe-bearing particles agree with values determined in mineral dissolution experiments and in Fe accumulation horizons in soils. The difference in δ56Fe values between leachates and colloids reflects isotope fractionation during incremental Fe(III)aq precipitation and Fe-OC complexation during transport towards the riparian zone. Overall, the Fe isotope values of riverine particles and colloids reflect processes that occur during mineral dissolution, transport, and secondary mineral formation in permafrost soils.

The contribution of 137Cs export flux from the Tone River Japan to the marine environment

The contribution of 137Cs transport to the marine environment via the Tone River, Japan was investigated. This river has the largest discharge among rivers on the North Pacific side of eastern Japan. The sampling site was located upstream near the river mouth and dissolved and particulate 137Cs in the river water was measured during 2014–2015, three years after the Tokyo Electric Power Corporation Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Annual fluxes of total (dissolved and particulate) 137Cs with considering desorption of 137Cs from riverine particles by change of salinity from the Tone River were similar in both years (78–107 × 109 Bq/y), indicating that about 0.03–0.06% of the estimated total amount of 137Cs deposited in the catchment (1.9–2.8 × 1014 Bq) was transported to the marine environment each year. Although the annual flux was about one order of magnitude lower than the daily direct discharge into the ocean from the FDNPP (800 × 109 Bq/y) during the corresponding period, continuous monitoring of rivers in the southern coastal area of east Japan on the North Pacific side are needed for the effect of 137Cs release via the rivers in the Kanto area over the long-term.

Particle aggregation, pH changes and metal behaviour during estuarine mixing: review and integration

Estuaries are dynamic mixing zones where river water interacts with seawater, resulting in large and complex geochemical changes. How two key factors, particle aggregation and pH, affect metal behaviour in estuaries is reviewed and integrated in this paper. Riverine particles are coated with organic matter and electrostatic repulsive forces restrict aggregation. In estuaries, increased concentrations of divalent cations reduce the repulsive forces between particles at low salinities, resulting in their rapid coagulation and removal of particulate-associated metals (e.g. Fe and Pb). However, truly dissolved metals may mix conservatively, and metals associated more with colloidal and dissolved organic material (e.g. Cu and Zn) can show variable behaviour. In many field studies and modelling of river inputs with different compositions, pH decreases slightly at low salinity. Geochemical model simulations of dissolved metal speciation indicated that Zn would be desorbed from iron oxide binding surfaces due to these pH and cation concentration changes, with Cu also showing less binding to dissolved organic matter (DOM). DOM, pH and particle surfaces can influence individual metal behaviour at various spatial and temporal scales. Further integrated field and laboratory research in estuaries where key geochemical processes affecting metal concentrations are measured and modelled is needed.

Colloidal Organic Matter and Metal(loid)s in Coastal Waters (Gulf of Trieste, Northern Adriatic Sea)

Large volumes of seawater were sampled in the Gulf of Trieste (northern Adriatic Sea) in order to study the interactions between colloidal organic matter (COM) and metal(loid)s (Me) in coastal waters. COM (> 5 kDa) was isolated by ultrafiltration and characterized using 1H NMR spectroscopy and elemental Corg. and Ntot. analyses. COM in the gulf represents about one quarter of the dissolved organic carbon (DOC), and according to 1H NMR analysis, it is composed of polysaccharides (30–45%), lipids (30–55%), proteins and carboxyl-rich alicyclic molecules (CRAM) (15–20%), and humics (< 1%). An accumulation of COM was observed in the late spring–early summer. The polysaccharide and lipid fractions increased up to twofold and the protein fraction decreased, reflected in a higher Corg./Ntot. (28, molar) ratio. Higher concentrations of humics were observed due to local freshwater discharges in spring. COM from the Isonzo/Soca River differed from the marine COM exhibiting higher protein/CRAM and higher humic contents. COM from the Isonzo/Soca mouth at salinities 16–33 was compositionally similar to marine COM. Analysis of Me, performed by ICP-MS and CVAFS (Hg), showed that Hg (nearly 100%), Cu (20%), Cr (10%), and Se (10%) have the highest Me affinity to colloids. Similar to COM, Hg and Cu rapidly increased till summer due to their sequestration in accumulated COM (transfer to particulate phase). The observed Me/Corg. ratios (Co, Cd, Hg < U, Cr, Ni, Mn < As, Zn, Cu, V < Se, Al, Fe) differ somewhat from those of the Irving–Williams series and can be explained by the composition of COM and variable background concentrations of studied Me in the northern Adriatic. Data from the salinity gradient in the metal-contaminated (especially Hg, Pb, Zn) Isonzo/Soca mouth showed flocculation of Al and Ba and desorption of V, Co, As, Se, Cs, U, and Hg, from the riverine particles with increasing ionic strength, while Fe, Mn, Cu, Cr, Ni, Zn, Cd, and Pb did not correlate with salinity.

Carbon and silica megasink in deep-sea sediments of the Congo terminal lobes

Carbon and silicon cycles at the Earth surface are linked to long-term variations of atmospheric CO2 and oceanic primary production. In these cycles, the river-sea interface is considered a biogeochemical hotspot, and deltas presently receive and preserve a major fraction of riverine particles in shallow water sediments. In contrast, periods of glacial maximum lowstand were characterized by massive exports of sediments to the deep-sea via submarine canyons and accumulation in deep-sea fans. Here, we calculate present-day mass balances for organic carbon (OC) and amorphous silica (aSi) in the terminal lobe complex of the Congo River deep-sea fan as an analogue for glacial periods. We show that this lobe complex constitutes a megasink with the current accumulation of 18 and 35% of the OC and aSi river input, respectively. This increases the estimates of organic carbon burial by 19% in the South Atlantic Ocean in a zone representing less than 0.01% of the basin. These megasinks might have played a role in carbon trapping in oceanic sediments during glacial times.

Enhancement of cyanobacterial growth by riverine particulate material

Particulate material plays a major role in the transport of sparingly soluble nutrients such as P and Fe in natural surface waters. Microbes might gain access to these nutrients either indirectly through particulate dissolution or directly through microbial attack. As such, it seems reasonable to expect a link between the particulate material concentration and bacterial growth in natural surface waters. To explore this link, a series of microcosm growth experiments were performed in the presence of a typical freshwater cyanobacteria, Synechococcus sp., grown in dilute BG-11 culture media in the presence and absence of basaltic and continental riverine particulate material. Results demonstrate that riverine particulates can increase bacterial biomass by 1) triggering bacterial growth in otherwise unfavourable conditions, 2) increasing total maximum biomass concentration, and 3) inducing bacteria growth during the post-exponential phase. These effects are found to be enhanced by increasing particulate concentration. Results also indicate a positive feedback between the nutrient release from the particulates and growing bacteria, where dissolving particulates enhance bacterial growth, which further promotes particulate dissolution by altering fluid pH. Microscopic analysis showed direct physical contact between particulates and cyanobacteria, suggesting that bacteria attach directly on mineral surfaces to gain required nutrients. Furthermore, frequent bacteria clusters were observed associated with particulates, indicating an increasing aggregation of bacteria in the presence of particulate material, which may facilitate a higher burial efficiency of organic carbon.