Pyrite accumulation and sulfate depletion as affected by root distribution in aJuncus (needle rush) salt marsh

This study was undertaken to investigate patterns of pyrite accumulation found in aJuncus roemerianus tidal marsh of north Florida. We speculate that the pattern of pyrite accumulation was caused mainly by the distribution of roots. Sediment cores from living stands (LS), dead stands (DS), and recently killed stands (RKS) were collected and analyzed for dissolved organic carbon (DOC), sulfate, chloride, Cr(II)-reducible sulfide, and biomass of roots and rhizomes. Living roots were distributed mainly in the upper 16 cm and concentrated 4–10 cm below the sediment surface. Pyrite accumulations were significantly different among the three types of sediment cores in the upper 16 cm sediment and follow the descending order of DS (615 μmol cm^?3)>RKS (547 μmol cm^?3)>LS (368 μmol cm^?3). Between 20 cm and 30 cm, pyrite contents approached values of 40 μmol cm^?3 and 55 μmol cm^?3 in all sediment cores. The degree of pyritization approaches 92–94% between 14 cm and 20 cm where pyrite accumulation was probably limited by available iron. Root distribution also affected the redistribution of iron through iron sulfide formation. Sulfate depletion in the upper 16 cm was higher in RKS (79.3 μmol cm^?3) than DS (49.1 μmol cm^?3). No significant sulfate depletion was found in LS. Sulfate reduction under DS was likely limited by the readily available substrates. Root distribution had a major influence on pyrite accumulation and sulfate reduction of the marsh, and its effects need to be addressed in studies of wetland sulfur dynamics.     

A cryptic sulfur cycle driven by iron in the methane zone of marine sediment (Aarhus Bay, Denmark)

Sulfate reduction and sulfur-iron geochemistry were studied in 5-6 m deep gravity cores of Holocene mud from Aarhus Bay (Denmark). A goal was to understand whether sulfate is generated by re-oxidation of sulfide throughout the sulfate and methane zones, which might explain the abundance of active sulfate reducers deep below the main sulfate zone. Sulfate penetrated down to 130 cm where methane started to build up and where the concentration of free sulfide peaked at 5.5 mM. Below this sulfate-methane transition, sulfide diffused downwards to a sulfidization front at 520 cm depth, below which dissolved iron, Fe²⁺, accumulated in the pore water. Sulfate reduction rates measured by ³⁵S-tracer incubations in the sulfate zone were high due to high concentrations of reactive organic matter. Within the sulfate-methane transition, sulfate reduction was distinctly stimulated by the anaerobic oxidation of methane. In the methane zone below, sulfate remained at positive “background” concentrations of <0.5 mM down to the sulfidization front. Sulfate reduction decreased steeply to rates which at 300-500 cm depth were 0.2-1 pmol SO₄²⁻ cm⁻³ d⁻¹, i.e., 4-5 orders of magnitude lower than rates measured near the sediment surface. The turn-over time of sulfate increased from 3 years at 12 cm depth to 100-1000 years down in the methane zone. Sulfate reduction in the methane zone accounted for only 0.1% of sulfate reduction in the entire sediment column and was apparently limited by the low pore water concentration of sulfate and the low availability of organic substrates. Amendment of the sediment with both sulfate and organic substrates immediately caused a 10- to 40-fold higher, “potential sulfate reduction” which showed that a physiologically intact community of sulfate reducing bacteria was present. The “background” sulfate concentration appears to be generated from the reaction of downwards diffusing sulfide with deeply buried Fe(III) species, such as poorly-reactive iron oxides or iron bound in reactive silicates. The oxidation of sulfide to sulfate in the sulfidic sediment may involve the formation of elemental sulfur and thiosulfate and their further disproportionation to sulfide and sulfate. The net reaction of sulfide and Fe(III) to form pyrite requires an additional oxidant, irrespective of the formation of sulfate. This could be CO₂ which is reduced with H₂ to methane. The methane subsequently diffuses upwards to become re-oxidized at the sulfate-methane transition and thereby removes excess reducing power and enables the formation of excess sulfate. We show here how the combination of these well-established sulfur-iron-carbon reactions may lead to the deep formation of sulfate and drive a cryptic sulfur cycle. The iron-rich post-glacial sediments underlying Holocene marine mud stimulate the strong sub-surface sulfide reoxidation observed in Aarhus Bay and are a result of the glacial to interglacial history of the Baltic Sea area. Yet, processes similar to the ones described here probably occur widespread in marine sediments, in particular along the ocean margins.     

Short-term endproducts of sulfate reduction in a salt marsh: Formation of acid volatile sulfides,elemental sulfur,and pyrite

Rates of sulfate reduction, oxygen uptake and carbon dioxide production in sediments from a short Spartina alterniflora zone of Great Sippewissett Marsh were measured simultaneously during late summer. Surface sediments (0–2 cm) were dominated by aerobic metabolism which accounted for about 45% of the total carbon dioxide production over 0–15 cm. Rates of sulfate reduction agreed well with rates of total carbon dioxide production below 2 cm depth indicating that sulfate reduction was the primary pathway for sub-surface carbon metabolism. Sulfate reduction rates were determined using a radiotracer technique coupled with a chromous chloride digestion and carbon disulfide extraction of the sediment to determine the extent of formation of radiolabelled elemental sulfur and pyrite during shortterm (48 hr) incubations. In the surface 10 cm of the marsh sediments investigated, about 50% of the reduced radiosulfur was recovered as dissolved or acid volatile sulfides, 37% as carbon disulfide extractable sulfur, and only about 13% was recovered in a fraction operationally defined as pyrite. Correlations between the extent of sulfate depletion in the marsh sediments and the concentrations of dissolved and acid volatile sulfides supported the results of the radiotracer work. Our data suggest that sulfides and elemental sulfur may be major short-term end-products of sulfate reduction in salt marshes.     

Basic formation mechanisms of Lake Doroninskoye soda water,East Siberia,Russia

The primary scientific goal of studying salt lakes is to better understand the formation of small continental-type hydrogeochemical systems. Many scientists have attributed the metamorphism of the chemical composition of salt lakes to the evaporative concentration of water. However, the formation of soda water is inconsistent with this hypothesis. Thus, analyzing intrabasinal biochemical processes and water—rocks interactions during the evaporative concentration of water allows us to understand the major mechanisms of the formation and evolution of water compositions. Therefore, the aim of this paper is to identify the key processes involved in the formation of the chemical composition of the water in Lake Doroninskoye. An analysis of the distribution of major components shows that Na⁺, HCO₃ ^?, CO₃ ^2?, and Cl^? are dominant in this water. High concentrations of these elements are the result of evaporative water concentration. Calcium, magnesium, and potassium are not accumulated because the water is saturated in minerals containing these elements. The main barrier to the growth of the sulfate content of water is sulfate reduction. This process also contributes to the additional reproduction of carbon dioxide, which reacts with the products of the hydrolysis of aluminosilicates OH^? to form HCO₃ ^? and CO₃ ^2?, thus further contributing to the natural processes of soda formation.     

Epiphytic nitrogen fixation associated with standing dead shoots of smooth cordgrass, Spartina alterniflora

N₂ fixation associated with the epiphytic community on standing dead Spartina alterniflora shoots was examined in both a natural and transplanted salt marsh in North Carolina. Acetylene reduction (AR) assays were conducted over a 24-mo period to estimate N₂ fixation rates on standing dead stems and leaves. In the natural salt marsh, mean AR rates ranged from 0.5 nmol C₂H₄ cm^?2 h^?1 to 14 nmol C₂H₄ cm^?2 h^?1, while in the transplanted marsh mean AR rates ranged from 1 nmol C₂H₄ cm^?2 h^?1 to 33 nmol C₂H₄ cm^?2 h^?1. Diel AR activity of epiphytic communities in both marshes varied seasonally. Midday incubations yielded higher AR rates than nighttime incubations in the spring, while midday incubations in late summer and fall generally yielded AR rates equal to or lower than nighttime incubations. Desiccation during low tides occasionally repressed AR activity, although AR rates quickly rebounded with wetting. AR activity was localized in the epiphytic community, rather than in the underlying Spartina stem material. Based on the measured AR rates and the density of standing dead stems, the annual input of new N to the natural salt marsh via epiphytic N₂ fixation is estimated to be 2.6 g N m^?2 yr^?1. The estimate of annual input of new N to the transplanted marsh is 3.8 g N m^?2 yr^?1. These estimates should be added to previous estimates of N₂ fixation in marsh sediments to estimate the total contribution of new nitrogen to salt marsh nitrogen budgets.     

Sulfate enrichments in estuarine waters of North Carolina

Sulfate concentrations in the mesohaline surface waters of the Pamlico and Neuse River estuaries were enriched, relative to the conservative seawater ratio with Cl^? (0.0517 (M)), by 5 to 43% between late winter and early summer. During this period, sulfate concentrations increased to a maximum excess of 3.5 mM in the bottom waters (0.5 m deep) through intermittent periods of both very low river flow and bottom water anoxia. The calculated net sulfate production rate for this period was 18 mmol per m² per d in the bottom waters. By late summer, the excess sulfate (an average of 12 mol per m²) had been removed from the water column, presumably due to SO₄ ^?2 reduction in anoxic bottom waters and sediments. Qualitative laboratory experiments with slurries of mud exposed to excess O₂ and treated with inhibitors of cytochrome oxidase showed that it was possible to produce the excess SO₄ ^?2 under these conditions via biochemical (not chemical) oxidation of pyride that occurs within the top 5 cm of mud (ca. 100 mmol pyrite-S per g dry mud). Whether the in situ substrate for SO₄ ^?2 production was pyrite, S⁰, or S⁺², is unknown, but the predominance of aqua regia extractable pyrite-Fe and the accumulation of excess SO₄ ^?2 in slurries with insufficient other sources of oxidizable S, indicates that it may be an important substrate for biochemical production of SO₄ ^?2.     

Sulfate reduction and porewater chemistry in a gulf coast<Emphasis Type="Italic">Juncus roemerianus</Emphasis> (Needlerush) marsh

Sulfate reduction rates were measured over the course of a year in the sediments of aJuncus roemerianus marsh located in coastal Alabama. Sulfate reduction rates were typically highest in the surface 0–2 cm and at depths corresponding to peak belowground biomass of the plants. The highest volume-based sulfate reduction rate measured was 1,350 μmol liter-sediment⁻¹ d⁻¹ in September 1995. Areal sulfate reduction rates (integrated to 20 cm depth) were strongly correlated to sediment temperature and varied seasonally from 15.2 mmol SO ₄ ²⁻ m⁻² d⁻¹ in January 1995 to 117 mmol SO ₄ ²⁻ m⁻² d⁻¹ in late August 1995. Despite high sulfate reduction rates porewater dissolved sulfide concentrations were low (<73 μM), indicating rapid sulfide oxidation or precipitation. Sulfate depletion data indicated that net oxidation of sediment sulfides occurred in March through May, following a period of infrequent tidal flooding and during a period of high plant production. Porewater Fe(II) reached very high levels (maximum of 969 μM; mean for all dates was 160 μM), particularly during periods of high sulfate reduction. The annual sulfate reduction rate integrated over the upper 20 cm of sediment was 22.0 mol SO ₄ ²⁻ m⁻² yr⁻¹, which is among the highest rates measured in a wetland ecosystem. Based on literature values of net primary production inJ. roemerianus marshes, we estimate that an amount equivalent to 16% to 90% of the annual belowground production may be remineralized through sulfate reduction.     

The Impact of Sediment and Carbon Fluxes on the Biogeochemistry of Methane and Sulfur in Littoral Baltic Sea Sediments (Himmerfjärden, Sweden)

Three sediment stations in Himmerfjärden estuary (Baltic Sea, Sweden) were sampled in May 2009 and June 2010 to test how low salinity (5–7 ‰), high primary productivity partially induced by nutrient input from an upstream waste water treatment plant, and high overall sedimentation rates impact the sedimentary cycling of methane and sulfur. Rates of sediment accumulation determined using ²¹⁰Pb_excess and ¹³⁷Cs were very high (0.65–0.95 cm?year^?1), as were the corresponding rates of organic matter accumulation (8.9–9.5 mol C?m^?2?year^?1) at all three sites. Dissolved sulfate penetrated <20 cm below the sediment surface. Although measured rates of bicarbonate methanogenesis integrated over 1 m depth were low (0.96–1.09 mol?m^?2?year^?1), methane concentrations increased to >2 mmol?L^?1 below the sulfate–methane transition. A steep gradient of methane through the entire sulfate zone led to upward (diffusive and bio-irrigative) fluxes of 0.32 to 0.78 mol?m^?2?year^?1 methane to the sediment–water interface. Areal rates of sulfate reduction (1.46–1.92 mol?m^?2?year^?1) integrated over the upper 0–14 cm of sediment appeared to be limited by the restricted diffusive supply of sulfate, low bio-irrigation (α?=?2.8–3.1 year^?1), and limited residence time of the sedimentary organic carbon in the sulfate zone. A large fraction of reduced sulfur as pyrite and organic-bound sulfur was buried and thus escaped reoxidation in the surface sediment. The presence of ferrous iron in the pore water (with concentrations up to 110 μM) suggests that iron reduction plays an important role in surface sediments, as well as in sediment layers deep below the sulfate–methane transition. We conclude that high rates of sediment accumulation and shallow sulfate penetration are the master variables for biogeochemistry of methane and sulfur cycling; in particular, they may significantly allow for release of methane into the water column in the Himmerfjärden estuary.     

Formation of 35S-labelled elemental sulfur and pyrite in coastal marine sediments (Limfjorden and Kysing Fjord,Denmark) during short-term 35SO42− reduction measurements

Analyses of the specific products of ³⁵SO₄^2? reduction measurements were made in marine sediments in Denmark. We injected tracer quantities of ³⁵SO₄^2? into cores, incubated the cores, and assayed for ³⁵S-labelled acid volatile sulfides. Additionally, we assayed for ³⁵S-labelled elemental sulfur by extraction with CS₂, and for ³⁵S-labelled pyrite by reduction with chromium (II). We separately determined that elemental sulfur which formed in situ and that which formed by oxidation during the acid distillation of acid volatile sulfides.In subtidal sediments in Limfjorden, ³⁵S-labelled elemental sulfur and pyrite make up 14–32% of the reduced sulfur formed in short-term (0–48 hours) ³⁵SO₄^2? reduction experiments, at all depths studied (0–15 cm). Labelled elemental sulfur which formed in situ during the incubations at depths below 1 cm made up a fairly constant 5–11% of the total labelled reduced sulfur, from 0–1 cm, it made up 27%. An additional small amount (1–2% from 1–15 cm and 5% from 0–1 cm) of labelled elemental sulfur was formed during the acid-distillation step in our assay for labelled acid-volatile sulndes. Pyrite contained 4–13% of the total labelled reduced sulfur at all depths. Rates of sulfate reduction in Limfjorden were linear over the period 0–48 hours, and ³⁵S-pyrite made up a nearly constant percentage of the ³⁵S-labelled reduced sulfur formed over this time period.Estimates of sulfate reduction rates for Limfjorden which do not include elemental sulfur and pyrite as products are 19% too low. At Kysing Fjord, estimates of sulfate reduction which do not include elemental sulfur and pyrite are 24% to 32% too low. Thus, while previously published data on sulfate reduction in similar environments are probably low, they are not greatly in error.     

Methane and carbon dioxide flux from a macrotidal salt marsh,Bay of Fundy,New Brunswick

Fluxes of methane (CH₄) and carbon dioxide (CO₂) to the atmosphere at 52 sites within a salt marsh were measured by a dark static chamber technique from mid July to mid September. Mean CH₄ fluxes ranged from 0.2 mg m^?2 d^?1 to 11.0 mg m^?2 d^?1, with an overall average of 1.6 mg m^?2 d^?1. Flux of CH₄ was inversely correlated (r²=0.23, p = 0.001) with salinity of the upper porewater at the site, suggesting the dominant role of SO₄ ^2? in inhibiting methanogenesis in salt-marsh sediments. The combination of salinity and water table position was able to explain only 29% of the variance in CH₄ emission. Mean soil flux of CO₂ ranged from 0.3 g m^?2 d^?1 to 3.7 g m^?2 d^?1, with an overall average of 2.5 g m^?2 d^?1; it was correlated with aboveground biomass (positive, r²=0.38, p = 0.001) and position of the water table (negative, r² = 0.55, p = 0.001). The combination of biomass and water table position accounted for 63% of the variance in CO₂ flux. There were high variations in gas flux within the six plant communities. The sequences were CH₄: upland edge > panne > pool > middle marsh > low marsh > high marsh, and CO₂: middle marsh > low marsh > upland edge > high marsh > panne > pool. Compared to other salt-marsh systems, this Bay of Fundy marsh emits small amounts of CH₄ and CO₂.     

Stable isotope geochemistry and diagenetic mineralization associated with the Tono sandstone-type uranium deposit in Japan

The Tono sandstone-type uranium mine area, middle Honsyu, Japan is composed of Miocene lacustrine sedimentary rocks in the lower part (18–22 Ma) and marine facies in the upper part (15–16 Ma). Calcite and pyrite occur as dominant diagenetic alteration products in these Neogene sedimentary rocks. The characteristics of calcite and pyrite differ significantly between lacustrine and marine facies. Abundant pyrite, calcite, organic matter, and small amounts of marcasite or pyrrhotite occur in the lacustrine facies, whereas small amounts of calcite and framboidal pyrite, organic matter and no marcasite or pyrrhotite are found within the marine units. The δ¹³C values of calcite in the lacustrine deposits are low (−19 to −6‰ PDB) but those in marine formation are high (−11 to +3‰). This implies that the contribution of marine carbonate is larger in upper marine sedimentary rocks, and carbon in calcite in the lower lacustrine formation was derived both from oxidation of organic matter and from dissolved marine inorganic carbon. The δ³⁴S values of framboidal pyrite in the upper marine formation are low (−14 to −8‰ CDT), indicating a small extent of bacterial seawater sulfate reduction, whereas those of euhedral-subhedral pyrite in the lower lignite-bearing arkose sandstone are high (+10 to +43‰), implying a large extent of closed-system bacterial seawater sulfate reduction. The δ³⁴S and δ¹³C data which deviate from a negative correlation line toward higher δ¹³C values suggest methanogenic CO₂ production. During diagenesis of the lacustrine unit, large amounts of euhedral-subhedral pyrite were formed, facilitated by extensive bacterial reduction of seawater sulfate with concomitant oxidation of organic matter, and by hydrolysis reactions of organic matter, producing CH₄ and CO₂. Uranium minerals (coffinite and uraninite) were also formed at this stage by the reduction of U⁶⁺ to U⁴⁺. The conditions of diagenetic alteration within the lacustrine deposits and uranium mineralization is characterized by low Eh in which nearly equal concentrations of CH₄ and HCO₃ ⁻ existed and reduced sulfur species (H₂S, HS⁻) are predominant among aqueous sulfur species, whereas diagenetic alteration of the marine formations was characterized by a predominance of SO₄ ²⁻ among dissolved sulfur species. Modern groundwater in the lacustrine formation has a low Eh value (−335 mV). Estimated and measured low Eh values of modern and ancient interstitial waters in lacustrine environments indicate that a reducing environment in which U⁴⁺ is stable has been maintained since precipitation of uranium minerals. Received: 9 February 1996 / Accepted: 11 April 1997     

Thermochemical Sulfate Reduction in the Tazhong District,Tarim Basin,Northeast China: Evidence from Formation Water and Natural Gas Geochemistry

<正>Systematic analyses of the formation water and natural gas geochemistry in the Central Uplift of the Tarim Basin(CUTB) show that gas invasion at the late stage is accompanied by an increase of the contents of H_2S and CO_2 in natural gas,by the forming of the high total dissolved solids formation water,by an increase of the content of HCO_3~-,relative to Cl~-,by an increase of the 2nd family ions(Ca~(2+),Mg~(2+),Sr~(2+) and Ba~(2+)) and by a decrease of the content of SO_4~(2-),relative to Cl~-.The above phenomena can be explained only by way of thermochemical sulfate reduction(TSR).TSR often occurs in the transition zone of oil and water and is often described in the following reaction formula:ΣCH+CaSO_4+H-_2O→H_2S+CO_2+CaCO_3.(1) Dissolved SO_4~(2-) in the formation water is consumed in the above reaction,when H_2S and CO_2 are generated,resulting in a decrease of SO_4~(2-) in the formation water and an increase of both H_2S and CO_2 in the natural gas.If formation water exists, the generated CO_2 will go on reacting with the carbonate to form bicarbonate,which can be dissolved in the formation water,thus resulting in the enrichment of Ca~(2+) and HCO_3~-.The above reaction can be described by the following equation:CO_2+H_2O+CaCO_3→Ca~(2+)+2HCO_3~-.The stratigraphic temperatures of the Cambrian and lower Ordovician in CUTB exceeded 120℃,which is the minimum for TSR to occur.At the same time,dolomitization,which might be a direct result of TSR,has been found in both the Cambrian and the lower Ordovician.The above evidence indicates that TSR is in an active reaction,providing a novel way to reevaluate the exploration potentials of natural gas in this district.     

Chemistry and sulfur isotope investigation of industrial wastewater contamination into groundwater aquifers, Piteå County, N. Sweden

The possible contamination of a groundwater system with industrial wastewater originating from a paper mill factory has been investigated in Piteå, N. Sweden. Six samples were collected from the wastewater in the waste dump and twelve samples from the adjacent groundwater were analyzed for chemistry and sulfur isotopes. The industrial wastewater is a saline water consisting mainly of Na–HCO₃–SO₄, having a high pH and showing δ³⁴S values between 7‰ and 9‰ affected by bacterial sulfate reduction. The groundwaters are relatively dilute, dominated by Na⁺, Ca²⁺ and HCO₃⁻, but with varying concentrations as exemplified by sulfate with concentrations varying between 3 and 69 mg L^− 1 while the δ³⁴S values range from − 0.5‰ to 14.3‰. The data suggest that the main S sources in the waters are the bedrock sulfides and/or atmospheric deposition, which, sometimes, are overlapped by bacterial sulfate reduction. Contamination from the waste dump does not occur.     

Nutrient transport in a riverine-influenced,tidal freshwater bayou in Louisiana

Transport of ammonium (NH₄ ⁺), nitrate + nitrite (NO₃ ^?), total Kjeldahl nitrogen (TKN), soluble reactive phosphate (SRP), and total suspended solids (TSS) was measured in a freshwater tidal bayou located in a marsh system near the mouth of the Atchafalaya River in Louisiana. Sampling was conducted six times over one year and was timed to assess effects of seasonal variation in river flow and mean sea level of the Gulf of Mexico on material fluxes. Net fluxes of all materials were large and ebb directed in all seasons except fall, when net transport was 2 to 3 orders-of-magnitude smaller than in any other season. These results demonstrate that riverine forcing was the primary influence on materials transport in all seasons except fall when tidal forcing was most important. The range of net fluxes (g s^?1) for each nutrient was as follows (a negative sign indicates a net export toward the Gulf): NO₃ ^?, ?0.006 to ?6.69; TKN, 0.09 to ?10.41; NH₄ ⁺, ?0.02 to ?1.36; SRP, ?0.001 to ?0.53; TSS, ?2 to ?81. Analysis of nutrient concentrations indicated the marsh/aquatic system removed NO₃ ^?, SRP, and TSS from the water column from late spring through early fall and released NH₄ ⁺ and TKN in summer. The results of this study show that net materials export per unit cross section channel area increased as riverine influence increased.     

The microbiological formation of carbonate concretions in the Upper Lias of NE England

Carbonate concretions from the Jet Rock (Upper Lias, Lower Jurassic) of NE England grew in uncompacted sediment, close to the sediment surface. Microbiological activity created isolated microenvironments in which dissolved carbonate and sulphide species were produced more rapidly than they could be dispersed by diffusion, so establishing the localised supersaturation of calcite and metastable iron sulphides. Precipitation of these minerals in the microenvironment formed a single concretion.Mass-balance calculations demonstrate that at least two different microbiological processes participated in concretionary growth. The early growth stages had an unidentifiable microbiological source of carbonate which declined in importance relative to sulphate reduction as growth proceeded. It is suggested that the diffusion of dissolved organic material was important in sustaining microbiological activity.Mineralogical zonations in the concretions result from changes in the chemistry of the microenvironment due to variations in the rates of addition/removal Ca²⁺, Fe²⁺, HCO^?₃ and HS^? by microbiological activity, the crystallization of authigenic minerals and diffusion between the microenvironment and surrounding pore waters. Such changes are of only local significance and the resulting mineralogical zonations in a concretion cannot be used to deduce successive stages of diagenesis in the whole sediment.     

安徽马鞍山市当涂地区地下水水化学特征及演化机制

【研究目的】 了解长江中下游平原地区地下水流系统并深入分析其地下水水化学特征及其演化机制。【研究方法】 综合马鞍山市当涂地区的水文地质条件、水动力场等,基于研究区水化学基本特征,运用多元统计分析、水化学图件、离子比值和反向水文地球化学模拟等方法对该地区浅层地下水水化学演化进行分析。【研究结果】 结果表明：（1）研究区地下水主要为低矿化度偏碱性水,地下水组分中阳离子以Ca²⁺和Mg²⁺为主,阴离子以HCO₃^-和SO₄^2-为主。（2）研究区地下水水化学类型主要可分为7类,其中松散岩类孔隙含水岩组和碎屑岩类孔隙裂隙含水岩组的水化学类型主要为HCO₃^-Ca型、HCO₃^-Ca·Na型、HCO₃·Cl-Ca·Na型以及HCO₃^-Ca·Mg型;基岩类裂隙含水岩组的化学类型主要为HCO₃·SO₄-Ca·Mg型和SO₄·HCO₃^-Ca·Mg型。（3）研究区浅层地下水水样超标率为46%,总体水质较差,超标率较高的组分依次为Mn、高锰酸盐指数（COD_Mn）、硝酸盐（以N计）、Fe、As、氨氮（以N计）等。（4）研究区地下水的化学组分主要受到岩石风化作用的控制;此外,还存在Na-Ca的正向阳离子交替吸附作用。反向水文地球化学模拟结果进一步定量论证了水岩相互作用对本区浅层地下水组分的形成和演化起着主导作用。【结论】 研究区地下水主要为低矿化度偏碱性,主要可分为松散岩类孔隙水、碎屑岩类孔隙裂隙水和基岩类裂隙水。主要离子比例和反向水文地球化学模拟揭示了本区浅层地下水化学组分主要是地下水溶滤方解石、白云石等碳酸盐矿物、石英、长石等硅酸盐矿物,高岭土等黏土矿物以及岩盐、石膏等达到过饱和之后形成的。     

Geochemistry of Diagenesis of Organogenic Sediments: An Example of Small Lakes in Southern West Siberia and Western Baikal Area

Organogenic sediments (sapropels) in lakes are characterized by a reduced type of diagenesis, during which organic compounds are decomposed, the chemical composition of the pore waters is modified, and authigenic minerals (first of all, pyrite) are formed. Pyrolysis data indicate that organic matter undergoes radical transformatons already in the uppermost sapropel layers, and the composition of this organic matter is principally different from the composition of the organic matter of the its producers. The sapropels contain kerogen, whose macromolecular structure starts to develop during the very early stages of diagenesis, in the horizon of unconsolidated sediment (0–5 cm). The main role in the diagenetic transformations of organic matter in sediments is played by various physiological groups of microorganisms, first of all, heterotrophic, which amonifying, and sulfate-reducing bacteria. SO₄^2? and Fe²⁺ concentrations in the pore waters of the sediments are determined to decrease (because of bacterial sulfate reduction), while concentrations of reduced Fe and S species (pyrite) in the solid phase of the sediment, conversely, increase. Comparative analysis shows that, unlike sapropels in lakes in the Baikal area, sapropels in southern West Siberia are affected by more active sulfate reduction, which can depend on both the composition of the organic matter and the SO₄^2? concentration in the pore waters.     

Hydrochemistry of Salt Lakes of the Qinghai-Tibet Plateau, China

The authors have carried out scientific investigations of salt lakes on the Qinghai-Tibet Plateau since 1956 and collected 550 hydrochemical data from various types of salt lakes. On that basis, combined with the tectonic characteristics of the plateau, the hydrochemical characteristics of the salt lakes of the plateau are discussed. The salinity of the lakes of the plateau is closely related to the natural environment of lake evolution, especially the climatic conditions. According to the available data and interpretation of satellite images, the salinity of the lakes of the plateau has a general trend of decreasing from north and northwest to south and southeast, broadly showing synchronous variations with the annual precipitation and aridity (annual evaporation/annual precipitation) of the modern plateau. The pH values of the plateau salt lakes are related to both hydrochemical types and salinities of the lake waters, i.e., the pH values tend to decrease from the carbonate type → sodium sulfate subtype → magnesium sulfate subtype → chloride type; on the other hand, a negative correlation is observed between the pH and salinities of the lakes. Geoscientists and biological limnologists generally use main ions in salt lakes as the basis for the hydrochemical classification of salt lakes. The common ions in salt lakes are Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl^? SO₄ ^2?, CO₃ ^2?, and HCO₃ ^?. In this paper, the Kurnakov-Valyashko classification is used to divide the salt lakes into the chloride type, magnesium sulfate subtype, sodium sulfate subtype and carbonate type, and then according to different total alkalinities (K _C = Na₂CO₃ + NaHCO₃/total salt × 100%) and different saline mineral assemblages, the carbonate type is further divided into three subtypes, namely, strong carbonate subtype, moderate carbonate subtype and weak carbonate subtypes. According to the aforesaid hydrochemical classifications, a complete and meticulous hydrochemical classification of the salt lakes of the plateau has been made and then a clear understanding of the characteristics of N–S hydrochemical zoning and E-W hydrochemical differentiation has been obtained. The plateau is divided into four zones and one area. There is a genetic association between certain saline minerals and specific salt lake hydrochemical types: the representative mineral assemblages of the carbonate type of salt lake is borax (tincalconite) and borax-zabuyelite (L₂CO₃) and alkali carbonate-mirabilite; the representative mineral assemblages of the sodium sulfate subtype are mirabilite (thenardite)-halite and magnesium borate (kurnakovite, inderite etc.)-ulexite-mirabilite; the representative mineral assemblages of the magnesium sulfate subtype are magnesium sulfate (epsomite, bloedite)-halite, magnesium borate-mirabilite, and mirabilite-schoenite-halite, as well as large amount of gypsum; The representative mineral assemblages of the chloride type are carnallite-bischofite-halite and carnallite-halite, with antarcticite in a few individual salt lakes. The above-mentioned salt lake mineral assemblages of various types on the plateau have features of cold-phase assemblages. Mirabilite and its associated cold-phase saline minerals are important indicators for the study of paleoclimate changes of the plateau. A total of 59 elements have been detected in lake waters of the plateau now, of which the concentrations of Na, K, Mg, Ca, and Cl, and SO₄ ^2?, CO₃ ^2?, and HCO₃ ^? ions are highest, but, compared with the hydrochemical compositions of other salt lake regions, the plateau salt lakes, especially those in the southern Qiangtang carbonate type subzone (I₂), contain high concentrations of Li, B, K, Cs, and Rb, and there are also As, U, Th, Br, Sr, and Nd positive anomalies in some lakes. In the plateau lake waters, B is intimately associated with Li, Cs, K and Rb and its concentration shows a general positive correlation with increasing salinity of the lake waters. The highest positive anomalies of B, Li, Cs, and K center on the Ngangla Ringco Lake area in the western segment of the southern Qiangtang carbonate type subzone (I₂) and coincide with Miocene volcanic-sedimentary rocks and high-value areas of B, Li, and Cs of the plateau. This strongly demonstrates that special elements such as B, Li, and Cs on the plateau were related to deep sources. Based on recent voluminous geophysical study and geochemical study of volcanic rocks, their origin had close genetic relation to anatectic magmatism resulting from India–Eurasia continent–continent collision, and B–Li (-Ce) salt lakes in the Cordillera Plateau of South America just originated on active continental margins, both of which indicate that global specific tectonically active belts are the main cause for the high abundances of B, Li, and Cs (K and Rb) in natural water and mineralization of these elements.     

Carbon Cycling and the Coupling Between Proton and Electron Transfer Reactions in Aquatic Sediments in Lake Champlain

We used fine-scale porewater profiles and rate measurements together with a multiple component transport–reaction model to investigate carbon degradation pathways and the coupling between electron and proton transfer reactions in Lake Champlain sediments. We measured porewater profiles of O₂, Mn²⁺, Fe²⁺, HS⁻, pH and pCO₂ at mm resolution by microelectrodes, and profiles of NO₃ ⁻, SO₄ ²⁻, NH₄ ⁺, total inorganic carbon (DIC) and total alkalinity (TA) at cm resolution using standard wet chemical techniques. In addition, sediment–water fluxes of oxygen, DIC, nitrate, ammonium and N₂ were measured. Rates of gross and net sulfate reduction were also measured in the sediments. It is shown that organic matter (OM) decomposes via six pathways: oxic respiration (35.2%), denitrification (10.4%), MnO₂ reduction (3.6%), FeOOH reduction (9.6%), sulfate reduction (14.9%), and methanogenesis (26.4%). In the lake sediments, about half of the benthic O₂ flux is used for aerobic respiration, and the rest is used for the regeneration of other electron acceptors produced during the above diagenetic reactions. There is a strong coupling between O₂ usage and Mn²⁺ oxidation. MnO₂ is also an important player in Fe and S cycles and in pH and TA balance. Although nitrate concentrations in the overlying water were low, denitrification becomes a quantitatively important pathway for OM decomposition due to the oxidation of NH₄ ⁺ to NO₃ ⁻. Finally, despite its low concentration in freshwater, sulfate is an important electron acceptor due to its high efficiency of internal cycling. This paper also discusses quantitatively the relationship between redox reactions and the porewater pH values. It is demonstrated here that pH and pCO₂ are sensitive variables that reflect various oxidation and precipitation reactions in porewater, while DIC and TA profiles provide effective constraints on the rates of various diagenetic reactions.