Sulfate Flux Research Articles

Rainwater chemistry in a subtropical high-altitude mountain site, South China: Seasonality, source apportionment and potential factors

Long-term and frequent observation of rainwater chemistry provides an opportunity to explore the evolution of air pollutant emissions and the effectiveness of the emission control measures. This study presents variations in rainwater chemistry, possible sources of air pollutant emissions, and wet deposition fluxes of ionic species in the rainfall samples during 2018–2020 at Mt. Lushan located in South China. Results showed that the annual volume-weighted mean (VWM) pH of rainwater was 5.8. Moreover, the VWM of 87.7% rainwater samples had pH values above 5.6, which indicated that the atmospheric pollution at Mt. Lushan had less effect during the study period. The polytonic nitrate ion (NO3−), the doubly charged positive calcium ion (Ca2+) and the ammonium ion (NH4+) were the dominant ions in rainwater where Ca2+ and NH4+ were the main neutralizing ionic species for rainwater acidity. The highly seasonal variability of ionic concentrations in rainwater was observed among different ionic species, which is closely associated with meteorological conditions and anthropogenic emissions present in the various seasons. Annual wet deposition flux showed an increasing trend during the entire periods. Hence, the wet flux of NO3− (76.3 kg ha−1 yr−1) was three times higher than the annual wet deposition flux of sulfate (SO42−, 21.7 kg ha−1 yr−1), which points to the fact that acid deposition is still a severe environmental issue in the Mt. Lushan region. Positive matrix factorization (PMF) and correlation matrix were used to identify five sources for ionic species, including aging sea salt, fossil fuel combustion, agriculture, dust, and biomass burning. These findings could be helpful in establishing emission control policies to protect and manage pollution control in the eco-environment in South China.

Chemical weathering and CO2 consumption in a multi-lithological karstic critical zone: Long term hydrochemical trends and isotopic survey

The chemical weathering (CW) of rocks at the Earth's surface plays a key role in the global carbon cycle along multiple pathways. Although karst systems are hotspot carbonated areas, they are not always monolithological. It is therefore challenging to estimate the CW of these complex areas. The interannual, seasonal, and spatial variations of CW rates and CO2 consumption were investigated using a long-term hydrogeochemical database (1994–2019) from a mountainous karstic catchment in southwestern France (Baget Catchment). A geochemical and isotopic spatial sampling allowed the identification of the main mineral or lithological sources in the catchment, which controlled the water chemistry. The CW budget showed that the (Ca2+ + Mg2+) fluxes originated from carbonate dissolution (1.14 mol·m−2·yr−1 equivalent to 74%) and silicate weathering (18%) by carbonic acid solutions. Gypsum dissolution and carbonate weathering by sulphuric acid from pyrite oxidation contribute equally to 4%, although the former accounts for 66% of the dissolved sulphate fluxes. During a summer sampling survey, an innovative sulphur isotopic approach based on δ34SSO4, allowed us to demonstrate that the ore-nature sulphuric acid drove 9.0% of total carbonate dissolution and represented only 16.8% of the dissolved sulphate stream fluxes. Hydrological conditions, temperature, vegetation, the epikarst (quasi-permanent shallow and discontinuous saturated layer under the soil), and the water dynamics were the key factors influencing the inter-annual and inter-seasonal variations of the CW rates and CO2 consumption. In addition, the carbon isotopic signature evidenced geochemical processes such as CO2 outgassing and calcite precipitation processes. The latter could remove up to 74% of HCO3− from streamwaters, depending on the hydrological conditions at the outlet between 2016 and 2019. Finally, this study highlights that CW rates and CO2 consumption may vary over inter-annual and inter-seasonal scales, and spatially even for a small catchment. Furthermore, the global CO2 consumption appears to be mainly driven by the runoff intensity in karst hydrosystems, where carbonate dissolution was found to consume 71% of the total weathering CO2 uptake.

Global syndromes induced by changes in solutes of the world\u2019s large rivers

Solute-induced river syndromes have grown in intensity in recent years. Here we investigate seven such river syndromes (salinization, mineralization, desalinization, acidification, alkalization, hardening, and softening) associated with global trends in major solutes (Ca2+, Mg2+, Na+, K+, SO42−, Cl−, HCO3−) and dissolved silica in the world’s large rivers (basin areas ≥ 1000 km2). A comprehensive dataset from 600 gauge stations in 149 large rivers reveals nine binary patterns of co-varying trends in runoff and solute concentration. Solute-induced river syndromes are associated with remarkable increases in total dissolved solids (68%), chloride (81%), sodium (86%) and sulfate (142%) fluxes from rivers to oceans worldwide. The syndromes are most prevalent in temperate regions (30~50°N and 30~40°S based on the available data) where severe rock weathering and active human interferences such as urbanization and agricultural irrigation are concentrated. This study highlights the urgency to protect river health from extreme changes in solute contents.

Mineral-scale variation in the trace metal and sulfur isotope composition of pyrite: implications for metal and sulfur sources in mafic VMS deposits

The link between metal enrichment and the addition of a magmatic volatile phase in volcanogenic massive sulfide deposits and actively forming seafloor massive sulfide deposits remains poorly characterized. This is especially true when considering how metal, sulfur and fluid flux change with time. In this study, we combine in situ sulfur isotope (δ34S; n = 31) measurements with trace metal chemistry of pyrite (n = 143) from the Mala VMS deposit, Troodos, Cyprus. The aim of our study is to assess the links between volatile influx and metal enrichment and establish how, or indeed if, this is preserved at the scale of individual mineral grains. We classify pyrite based on texture into colloform, granular, disseminated and massive varieties. The trace metal content of different pyrite textures is highly variable and relates to fluid temperature and secondary reworking that are influenced by the location of the sample within the mound. The sulfur isotope composition of pyrite at Mala ranges from − 17.1 to 7.5‰ (n = 31), with a range of − 10.9 to 2.5‰ within a single pyrite crystal. This variation is attributed to changes in the relative proportion of sulfur sourced from (i) SO2 disproportionation, (ii) thermochemical sulfate reduction, (iii) the leaching of igneous sulfur/sulfide and (iv) bacterial sulfate reduction. Our data shows that there is no correlation between δ34S values and the concentration of volatile elements (Te, Se) and Au in pyrite at Mala indicating that remobilization of trace metals occurred within the mound.

Chemical composition of rain at a regional site on the South African Highveld

Relatively limited data have been published on the chemical composition of wet deposition for South Africa, which is considered an important source region for atmospheric pollutants. Concentrations and wet deposition fluxes of ionic species determined in rain samples collected from December 2014 to April 2018 at a regional site, Welgegund, are presented, and contextualised by wet deposition composition in the north-eastern interior of South Africa. 89% of rain samples collected during the sampling period complied with the data quality objectives of the World Meteorological Organisation. The total ionic concentration of rainwater at Welgegund was similar to that at two regional sites located within proximity of industrial activities. The pH of rainwater (4.80) was comparable to that determined at two rural background sites, which indicated increased neutralisation. Similarly to the other sites located in the South African interior, SO42- was the most abundant species in rain, with concentrations thereof in the same order as SO42- levels determined at the two industrially influenced sites. Lower sulphur and nitrogen fluxes at Welgegund were attributed to lower average annual rainfall. The anthropogenic (industrial) source group had the largest contribution to wet deposition chemical composition, which signified the influence of major source regions in the South African interior that impact Welgegund. Relatively large contributions were also calculated from marine and crustal sources. The influence of agricultural activities was also evident, while biomass burning had the lowest contribution due to open biomass burning occurring mainly during the dry season.

The effect of CO2 concentration on DMSP production in Gephyrocapsa oceanica (Isochrysidales, Coccolithophyceae)

ABSTRACT Atmospheric CO2 is increasing at unprecedented rates, with modelling studies indicating this as a significant cause of continued warming of the planet. It has been suggested that if these conditions in some way lead to enhanced DMSP production by marine algae, then this compound, via the sulphate aerosols that result from its breakdown, might act to mitigate global warming by altering cloud albedo and so act as a natural thermostat. Among the major producers of DMSP in the oceans are the coccolithophores, with Gephyrocapsa oceanica being a common species that prefers warmer conditions than Emiliania huxleyi, and that arose in the Miocene when CO2 levels were higher than present. In this study we grew G. oceanica under CO2 concentrations equivalent to atmospheric values of 200, 390 and 1000 ppm, representative of glacial, near present and potential 2100 CE scenarios. As CO2 concentration increased, so did cell size, while both growth rates and intracellular DMSP concentrations decreased, consistent with the hypothesis that DMSP may act as part of an antioxidant mechanism within the cell. This led to a 50% decrease in net DMSP production under 1000 ppm CO2 relative to the 390 ppm treatment. While there are many variables in the open ocean environment that ultimately determine net DMSP production rates and flux of sulphur to the atmosphere, for the cellular response to increased atmospheric CO2 alone we conclude that elevated CO2 would reduce net DMSP production and, inasmuch as this reduces the production of sulphate aerosol in the atmosphere, reducing cloud albedo, then rather than acting to limit global warming, it will instead amplify it.

Geochemical Characteristics of Late Ordovician Shales in the Upper Yangtze Platform, South China: Implications for Redox Environmental Evolution

Changes to the redox environment of seawater in the Late Ordovician affect the process of organic matter enrichment and biological evolution. However, the evolution of redox and its underlying causes remain unclear. This paper analyzed the vertical variability of main, trace elements and δ34Spy from a drill core section (well ZY5) in the Upper Yangtze Platform, and described the redox conditions, paleoproductivity and paleoclimate variability recorded in shale deposits of the P. pacificus zone and M. extraordinarius zone that accumulated during Wufeng Formation. The results showed that shale from well ZY5 in Late Ordovician was deposited under oxidized water environment, and there are more strongly reducing bottom water conditions of the M. extraordinarius zone compared with the P. pacificus zone. Excess silica (SiO2(exc)) and substitution index of paleoproductivity (Y) indicated that the P. pacificus zone had higher paleoproductivity whereas the M. extraordinarius zone was lower. The high productivity level controlled O2 release in the shallow water area as well as the oxidation degree of the P. pacificus zone. The decrease of productivity and the relatively stagnant water mass of the inner Yangtze Sea controlled the formation of relatively reduced water conditions in the M. extraordinarius zone. The chemical index of alteration (CIA) results suggested that palaeoclimatic conditions changed from warm and humid to cold and dry climate from the P. pacificus to the M. extraordinarius zones in the study area. A comparative analysis of the published Fe-S-C data for the Xiushan Datianba section showed that in the P. pacificus zone of the inner Yangtze Sea, warm and humid climate conditions drove high productivity, sulphate flux and low reactive iron flux, which promoted the expansion of oxic ocean-surface waters and mid-depth euxinic waters. In the M. extraordinarius zone, the cold and dry climate with significant uplift of the Xiang’e Submarine High led to the relative sea level decline, resulting in low productivity, sulfate flux and high reactive iron flux, which promoted the expansion of the mid-depth ferruginous waters and the shrinkage of oxic ocean-surface waters. The results offered new insights into the co-evolution of continents and oceans, and explained the role of continental weathering and uplift of the Xiang’e Submarine High in the exchange of sulfate flux and nutrients in the redox environment change of inner Yangtze Sea during the Late Ordovician.

Quadruple sulfur isotope biosignatures from terrestrial Mars analogue systems

In this study, we present quadruple sulfur isotope values (QSI: 32S,33S,34S,36S) measured in sediments from two sulfur-rich Mars analogue environments: i) the glacially-fed hydrothermal pools in Iceland (Kerlingarfjöll and Kverkfjöll), and ii) the Lost Hammer hypersaline spring from Axel Heiberg Island, Nunavut, Canada. The localities host different physical and geochemical characteristics, including aqueous geochemistry, volcanic input, temperature, pH and salinity. The δ34S values of sulfur compounds from the Lost Hammer hypersaline spring exhibit large fractionations typical of microbial sulfate reduction (MSR) with or without additional oxidative sulfur cycling and microbial sulfur disproportionation (MSD) (34εSO4-CRS from −49.5 to −43.5‰), contrary to the small S isotope fractionations reported for the Icelandic hydrothermal sites (34εSO4-CRS from −9.9 to −0.7‰). Lost Hammer minor S isotope values (Δ33S and Δ36S), interpreted within the context of a sulfur cycling box model, are consistent with a biogeochemical S cycle including both MSR and MSD. In contrast, the small range in δ34S values within the Iceland hydrothermal pools are consistent with a large volcanic H2S flux and minimal biological S cycling. The minor S isotope values recorded in the hydrothermal pools, however, indicate further biogeochemical sulfur cycling. Our results demonstrate that contrasting physical and chemical characteristics between sites support different microbial S cycling processes, as recorded in the QSI sedimentary values. The QSI data and the derived models support the strong potential for QSI values to be used as biosignatures in the search for life in Martian S-rich environments. These results also suggests that extreme, metabolic energy-limited environments with low abiotic sulfur fluxes could be more likely to produce unequivocal biological QSI signals than those with more moderate conditions or abundant available energy. This finding carries significant implications for targeting sites on Mars for in situ measurements or future sample return missions.

Properties of Ti(C,N)-based cermets reinforced with ZrO2 whiskers deposited via sulfate flux at high temperatures

Ti(C,N)-based cermets were prepared by vacuum hot-pressing sintering enhanced by the second phase of ZrO2 whiskers via sulfate flux. The effects of ZrO2 whiskers on the material properties of Ti(C,N)-based cermets at high temperature are discussed, including the room temperature microstructure, high-temperature oxidation resistance, and microstructure and mechanical properties at high temperature. The experimental result indicated that the addition of ZrO2 whiskers improves the bending strength, fracture toughness, and oxidation resistance of Ti(C,N)-based cermets at high temperature. At a temperature of 1000 °C, the hardness, flexural strength, fracture toughness and elasticity modulus of the cermets with 7.5 wt% ZrO2 whiskers were 9.32 GPa, 1032 MPa, 6.71 MPa m1/2 and 193.5 GPa, respectively. The high-temperature strength and toughness of Ti(C,N)-based cermets with ZrO2 whiskers were weakened by the softening of grain boundaries, the decrease in elastic modulus, and oxidation. Conversely, the high-temperature strength and toughness of Ti(C,N)-based cermets were increased by maintaining the rod crystal shape of ZrO2 whiskers at a high temperature, enhanced the elastic modulus of cermets through the whiskers, and improved oxidation resistance.

Photochemical Production and Photolysis of Acrylate in Seawater.

The marine organosulfur cycle has been studied intensively for over 30 years motivated by the hypothesis that dimethylsulfide (DMS) affects Earth's radiation balance and climate. The main source of DMS is from the enzymatic lysis of dimethylsulfoniopropionate (DMSP), the latter of which is a significant component of carbon, sulfur, and energy fluxes in the oceans. Acrylate is also produced during DMSP lysis, but unlike DMS or DMSP, very little is known about the marine acrylate cycle. Herein, a new source of acrylate was identified in seawater as a product formed from the photolysis of dissolved organic matter (DOM). Photochemical production rates varied from 1.6 to 5.0 pM (μmol quanta cm-2)-1, based on photon exposures determined from nitrite actinometry. A positive correlation (r = 0.87) was observed between acrylate photoproduction and the seawater absorption coefficient at 330 nm. Acrylate photoproduction was initiated by UV radiation, with UV-B and UV-A contributing approximately 32 and 68% to the total production, respectively. Acrylate did not photolyze in high-purity water or seawater at concentrations less than 100 nM. These findings improve our understanding of the role that sunlight plays in the marine acrylate cycle, a reactive form of DOM that significantly affects the carbon cycle and ecology of the upper ocean.

Coupling of dissolved organic carbon, sulfur and iron cycling in Black Sea sediments over the Holocene and the late Pleistocene: Insights from an empirical dynamic model

An understanding of how the coupled cycles of carbon, iron and sulfur in sediments respond to environmental change throughout Earth history requires the reconstruction of biogeochemical processes over a range of spatial and temporal scales. In this study, sediment cores from the southwestern Black Sea were analyzed to gain insight into past changes in biogeochemical processes with particular focus on the cycling of dissolved organic carbon (DOC). The sediment consists of Late Pleistocene deposits of iron oxide-rich and organic-poor lacustrine sediments, a Holocene sapropel layer deposited after the inflow of saline Mediterranean seawater about 9300 yr BP, and overlying recent marine sediments. The porewaters displayed high concentrations of DOC, acetate, dissolved iron and an extended depth interval over which sulfate and methane were both present. The historical fluctuations of the fluxes of carbon, sulfur and iron species at the seafloor that led to these present-day geochemical profiles, and which cannot be easily interpreted from the measured data alone, were hindcasted with a reaction-transport model. The model suggests that the inflow of Mediterranean seawater impacted the rain rate and reactivity of organic matter reaching the sediments, which shifted the sedimentary redox regimes throughout the Holocene that now are reflected on different lithology units. Organic matter in the sapropel layer is apparently the main source of modern-day accumulations of DOC and acetate, both of which probably sustained subsurface microbial activity throughout the post-glacial period. The ratio between DOC and dissolved inorganic carbon (DIC) flux to the bottom water decreased from ∼40% before the inflow of Mediterranean water to ∼2% at the present day. We suggest that the coexistence of methanogenesis and sulfate reduction was associated with sulfate-reducing bacteria and methanogens sharing common substrates of acetate and lactate and utilizing non-competitive substrates such as methylated compounds in the sapropel layer and in the bottom of modern marine deposits. Intense sulfur and iron cycling mainly took place in the organic-poor freshwater deposits, today characterized by high concentrations of dissolved iron and methane. In contrast to previous studies in similar environments, anaerobic oxidation of methane coupled to the reduction of ferric iron was negligible. The results have broad implications for coastal environments that are currently experiencing deoxygenation and seawater intrusion and also for understanding the role of DOC in the sedimentary carbon cycle.

Coral-reef-derived dimethyl sulfide and the climatic impact of the loss of coral reefs

Abstract. Dimethyl sulfide (DMS) is a naturally occurring aerosol precursor gas which plays an important role in the global sulfur budget, aerosol formation and climate. While DMS is produced predominantly by phytoplankton, recent observational literature has suggested that corals and their symbionts produce a comparable amount of DMS, which is unaccounted for in models. It has further been hypothesised that the coral reef source of DMS may modulate regional climate. This hypothesis presents a particular concern given the current threat to coral reefs under anthropogenic climate change. In this paper, a global climate model with online chemistry and aerosol is used to explore the influence of coral-reef-derived DMS on atmospheric composition and climate. A simple representation of coral-reef-derived DMS is developed and added to a common DMS surface water climatology, resulting in an additional flux of 0.3 Tg yr−1 S, or 1.7 % of the global sulfur flux from DMS. By comparing the differences between both nudged and free-running ensemble simulations with and without coral-reef-derived DMS, the influence of coral-reef-derived DMS on regional climate is quantified. In the Maritime Continent–Australian region, where the highest density of coral reefs exists, a small decrease in nucleation- and Aitken-mode aerosol number concentration and mass is found when coral reef DMS emissions are removed from the system. However, these small responses are found to have no robust effect on regional climate via direct and indirect aerosol effects. This work emphasises the complexities of the aerosol–climate system, and the limitations of current modelling capabilities are highlighted, in particular surrounding convective responses to changes in aerosol. In conclusion, we find no robust evidence that coral-reef-derived DMS influences global and regional climate.

Organic sulfur fluxes and geomorphic control of sulfur isotope ratios in rivers

Pyrite oxidation plays a critical role in the relationship between weathering and climate, and its impact on the global carbon cycle has previously been constrained through inversion models utilizing observations of river sulfate (SO42−) and its 34S/32S isotope ratio (δ34SSO4). However, measurements from some rivers have suggested that δ34SSO4 can be substantially impacted by processes such as microbial sulfate reduction and/or sulfur assimilation and cycling, rather than simply reflecting a weighted mixture of lithologic sulfur sources. To study the prevalence and controls on SO42− transformations, we measured dissolved major element concentrations and δ34SSO4 in river water samples from throughout western Iceland. Our analyses focused on samples from a small catchment hosting the Efri Haukadalsá river, a system with relatively uniform and isotopically constrained basaltic bedrock. We also measured sediment δ34S and sulfur speciation using sulfur K-edge X-ray absorption spectroscopy on sediment and vegetation samples from this catchment. Values of dissolved δ34SSO4 in the Efri Haukadalsá ranged from 2.5‰ to 23.7‰ and had a linear relationship with Cl−/SO42− ratios, indicating that SO42− predominantly derived from basalt weathering and meteoric precipitation. The lower δ34SSO4 values were found in fluvial valleys with V-shaped cross sections, while higher values of δ34SSO4 occurred in U-shaped, glacially eroded valleys with thick alluvial fills blanketing the valley floor. Spectroscopic observations identified organic sulfur phases in suspended river sediment, floodplain deposits, and vegetation. Mass balance calculations quantified the organic sulfur flux as less than 10% of SO42− export, and sediment δ34S values were comparable to river δ34SSO4. We interpreted these isotopic and chemical patterns as reflecting differences in the availability of unweathered bedrock across the Efri Haukadalsá catchment, with V-shaped valleys having greater access to fresh sulfide-bearing minerals than alluviated U-shaped valleys; this interpretation is in contrast to one in which the elevated δ34SSO4 values reflect fractionation during sulfur transformations along alluvial reaches. These results validated the application of river inversion models for constraining weathering fluxes and affirmed that pyrite oxidation globally, even in the presence of river sulfur cycling, modulates the abundance of atmospheric carbon dioxide.

Reactive transport modelling of porewater geochemistry and sulfur isotope fractionation in organic carbon amended mine tailings

Field experiments previously conducted to assess organic carbon (OC) amendments for in situ biological treatment of tailings porewater at the Greens Creek Mine (Alaska, USA) showed sulfate reduction, metal-sulfide precipitation, and decreased fluxes of sulfate and dissolved metals. Here, we develop two reactive transport models using the reactive transport code MIN3P to simulate hydrogeochemical processes and δ34S–SO4 isotope fractionation over four years in test cells containing unamended (control) and amended (5 vol % OC) tailings. These models successfully simulate observed data including pH, SO4, δ34S–SO4, Ca, Fe, K, Mg, Mn, Si, and Zn. The models also indicate that dissolution of carbonate and, to a lesser extent, aluminosilicate minerals neutralize acidic porewater generated from sulfide mineral oxidation and sulfate reduction reactions. Application of a constant kinetic fractionation factor of 0.9820 to simulate measured δ34S–SO4 trends confirms that sulfate removal principally results from microbially-mediated sulfate reduction in conjunction with OC oxidation and subsequent metal-sulfide precipitation. Gypsum precipitation/dissolution and thiosulfate disproportionation have negligible effects on modelled porewater δ34S–SO4 signatures. Our simulations are consistent with the previous findings that metal-sulfide precipitation controls Fe and Zn attenuation in amended tailings and that coprecipitation reactions contribute to metal removal. Overall, these simulations demonstrate that coupled reactive transport modelling incorporating stable isotope fractionation can improve the understanding of hydrogeochemical and biogeochemical controls within in situ treatment systems, further illustrating the benefits and limitations of this technique for improving water quality of mine drainage.

Major ion chemistry of two cratonic rivers in the tropics: Weathering rates and their controlling factors

AbstractContinental weathering plays a dominant role in regulating the global carbon cycle, soil chemistry and nutrient supply to oceans. The CO2‐mediated silicate weathering acts as a major CO2 sink, whereas sulphuric acid‐mediated carbonate dissolution releases CO2 to the atmosphere–ocean system. In this study, dissolved major ions and silica concentrations of two tropical (Damodar and Subarnarekha) river systems from India have been measured to constrain the type and rate of chemical weathering for these basins. The total dissolved solids (TDS) of these rivers, a measure of total solute supply from all possible sources, are about 2–3 times higher than that of the global average for rivers. Mass balance calculations involving inverse modelling estimate that 63 ± 11% of total cations are derived from rock weathering, of which 27 ± 7% of cations are supplied through silicate weathering. The sulphide‐S concentrations are estimated by comparing the water chemistry of these two rivers with that of a nearby river (Brahmani) with similar lithology but no signatures of sulphide oxidation. The outflows of Damodar and Subarnarekha rivers receive 17% and 55% of SO4 through sulphide oxidation, respectively. The sulphide oxidation fluxes from the ore mining areas, such as upper Damodar (0.52 × 109 mol/yr) and lower Subarnarekha (0.66 × 109 mol/yr) basins, are disproportionally (~9 times) higher compared to their fractional areal coverage to the global drainage area. The corresponding CO2 release rate (2.84 × 104 mol/km2/yr) for the Damodar basin is lower by five times than its CO2 uptake rate (1.38 × 105 mol/km2/yr). The outcomes of this study underscore the dominance of sulphide oxidation in controlling the dissolved chemical (cationic and sulphur) fluxes.

Transboundary air pollution reduction rapidly reflected in stream water chemistry in forested catchment on the sea of Japan coast in central Japan

Since the 1980s, Japan has been affected by transboundary air pollution from the Asian continent due to northwesterly seasonal winds in the winter. However, after peaks in 2006 and 2011/2012, sulfur dioxide (SO2) and nitrogen oxide (NOx) emissions began decreasing, respectively. To assess the biogeochemical response of forest ecosystems to the changing atmospheric deposition, flux observations of rainfall outside forest canopies (RF), throughfall and stemflow (TF + SF), and stream water (SW) chemistry were conducted in a Japanese cedar plantation in Kajikawa (KJK) on the Sea of Japan coast in central Japan. Fluxes of S and N by RF and TF + SF at KJK significantly decreased during the observation period (2002–2018). The non-sea salt sulfate (SO42−) flux by TF + SF peaked in 2006/2007 and decreased thereafter, which likely reflected the S emission trends. Accordingly, SO42− concentrations in SW at KJK decreased with increasing alkalinity. Similar decreases in SO42− concentrations beginning in 2006, 2007, and 2008 were observed at eight monitoring lakes in Japan. Thus, the SW chemistry at KJK and possibly that of Japanese lakes were sensitive to atmospheric S deposition reductions. However, as of late, the net S export at KJK has been increasing, exhibiting budget discrepancies in 2014/2015 and 2017/2018. Sulfur isotopic analysis suggested that the S deposited from the atmosphere was retained/cycled in the forest before it was gradually discharged into SW. Thus, a slightly weakened reaction of the output due to the internal cycle and residual effects of previous years may have caused the S budget discrepancies. The nitrate (NO3−) concentration in the SW at KJK has also been continuously increasing. N deposition and other factors, such as the maturation of the planted trees, may play an important role in NO3− leaching in plantations.

Rainfall partitioning and associated nitrate and sulfate fluxes along a slope gradient in a subtropical broadleaved forest

Forest canopies create a high spatial variability of both canopy interception (Ei) and acid deposition. However, spatial characteristics of these hydrochemical fluxes along a slope gradient in forest watersheds remain unclear. Here we monitored bulk precipitation (BP), throughfall (TF), stemflow (SF), and associated nitrate (NO3−) and sulfate (SO42−) in an upper slope plot (UP) and a lower slope plot (LP) in a subtropical broadleaved forest, Southeast China. Results showed that annual rates of TF, SF and Ei to BP in UP were 85.2%, 2.6%, and 12.2%, respectively. Corresponding values in LP were 88.6%, 2.2%, and 9.2%. There were no significant differences among these rainfall partitioning components between UP and LP. Additionally, annual volume weighted mean concentrations of NO3−-N in TF and SF were significantly lower than in BP, whereas no significant differences were found for SO42−-S among BP, TF and SF. Unlike annual TF NO3−-N fluxes were similar in UP and LP, annual TF SO42−-S fluxes significantly increased along the UP to LP. Net TF + SF (NTF) fluxes were negative for NO3−-N and SO42−-S at the two slope plots, illustrating retained/uptake in the canopy. Dormant season NTF S fluxes were close to zero, indicating that dry deposition does not appear to play a major role, whereas exchange with foliage surfaces should be the dominant factor controlling the spatial patterns of TF N and S fluxes in the watershed. This study can contribute to the evaluation of watershed-scale water and acidifying input into the soil during canopy passage, and further consequent effects on forest ecosystems.

Assessing critical load exceedances and ecosystem impacts of anthropogenic nitrogen and sulphur deposition at unmanaged forested catchments in Europe

Anthropogenic emissions of nitrogen (N) and sulphur (S) compounds and their long-range transport have caused widespread negative impacts on different ecosystems. Critical loads (CLs) are deposition thresholds used to describe the sensitivity of ecosystems to atmospheric deposition. The CL methodology has been a key science-based tool for assessing the environmental consequences of air pollution. We computed CLs for eutrophication and acidification using a European long-term dataset of intensively studied forested ecosystem sites (n = 17) in northern and central Europe. The sites belong to the ICP IM and eLTER networks. The link between the site-specific calculations and time-series of CL exceedances and measured site data was evaluated using long-term measurements (1990–2017) for bulk deposition, throughfall and runoff water chemistry. Novel techniques for presenting exceedances of CLs and their temporal development were also developed. Concentrations and fluxes of sulphate, total inorganic nitrogen (TIN) and acidity in deposition substantially decreased at the sites. Decreases in S deposition resulted in statistically significant decreased concentrations and fluxes of sulphate in runoff and decreasing trends of TIN in runoff were more common than increasing trends. The temporal developments of the exceedance of the CLs indicated the more effective reductions of S deposition compared to N at the sites. There was a relation between calculated exceedance of the CLs and measured runoff water concentrations and fluxes, and most sites with higher CL exceedances showed larger decreases in both TIN and H+ concentrations and fluxes. Sites with higher cumulative exceedance of eutrophication CLs (averaged over 3 and 30 years) generally showed higher TIN concentrations in runoff. The results provided evidence on the link between CL exceedances and empirical impacts, increasing confidence in the methodology used for the European-scale CL calculations. The results also confirm that emission abatement actions are having their intended effects on CL exceedances and ecosystem impacts.

The impact of diffusive transport of methane on pore-water and sediment geochemistry constrained by authigenic enrichments of carbon, sulfur, and trace elements: A case study from the Shenhu area of the South China Sea

Sulfate-driven anaerobic oxidation of methane (SD-AOM) controls methane release from marine sediments to the ocean. A variety of authigenic precipitates within sediments has been widely used to identify past occurrences of SD-AOM. However, the lack of a systematic evaluation of the formation conditions of these precipitates sometimes impedes the recognition of SD-AOM in past environments characterized by diffusive transport of methane. Carbon, sulfur, and trace element geochemistry of pore-water and sediments was investigated at a site affected by upward methane diffusion in the Shenhu area of the South China Sea. Here, the sulfate-methane transition zone (SMTZ) is located ~7.6 m below the seafloor based on sulfate and methane concentrations. The slope of δ34S vs. δ18O values of sulfate is consistent with diffusive transport of methane. Concentration and isotope profiles of pore-water species point to diffusive rather than advective transport of solutes. Enhanced sulfate reduction inferred from δ34S values of sulfate agrees with the local occurrence of relatively abundant, 34S-enriched iron sulfide minerals. The time required to produce the observed authigenic iron sulfides around the SMTZ is estimated to be ~1.1 ka based on the amount of iron sulfide minerals and the present diffusive flux of sulfate. No enrichment of authigenic carbonate, barium (Ba), and molybdenum (Mo) was detected in the studied sediments. This observation is consistent with the calculation that the current fluxes of pore-water calcium, magnesium, Ba, and Mo are too low to allow for authigenic enrichments. The low fluxes are largely controlled by diffusion, which facilitates the formation of 34S-enriched iron sulfide minerals. The observed enrichment patterns – unlike those of sediments affected by advective seepage – are expected to be prevalent in modern and ancient continental-margin sediments, and may contribute to the identification of past methane-rich zones and overlying SD-AOM zones as sinks for methane in the geological record.

Diffuse sources dominate the sulfate load into Finnish surface waters

Sulfate (SO42−) affects the cycling of ecologically important substances, such as carbon, nutrients and metals, but the contribution of anthropogenic activities in sulfate load entering aquatic systems is poorly known. We calculate specific sulfate loads for land cover types, atmospheric deposition and point sources, and then estimate the source-specific flux of sulfate to Finnish surface waters. The largest sulfate flux, entering mostly the Baltic Sea, originates from agricultural fields on acid sulfate soil (24% of total flux). Forests on mineral soil, which cover 67% of the country's surface area, form the second largest source (21%). Additionally, agricultural fields on non-acid soil cause noticeable sulfate flux (16%). Pulp and paper mills were the key point sources (20%) for sulfate. We find that anthropogenic activities contribute to elevated sulfate levels in waters potentially affecting the cycling of nutrients, metals and formation of greenhouse gases in naturally sulfate-poor freshwaters. Based on these findings, sulfate should be systematically included in monitoring and pollution control measures.