Degree Of Pyritization Research Articles

Arsenic fractionation in estuarine sediments: Does coastal eutrophication influence As behavior?

The Paranaguá Estuarine Complex (PEC) includes the naturally oligotrophic (NO) Mel Island which is surrounded by sea grasses, a naturally eutrophic (NE) Benito Inlet adjacent to mangrove wetlands and the highly impacted eutrophic (IE) Paranaguá Bay, home of one of Brazil’s largest ports. The results from this study indicate that reactive As and pyrite increase with sediment depth near Paranaguá port in the IE region. At the NE region, near a mangrove fringe, the reactive As, Fe, Mn and pyrite remained relatively high along the sediment column while near the sea grasses at NO the As contents were low. The degree of trace metal pyritization (DTMP) and the degree of pyritization (DOP) was highest at the IE site, slightly increasing with depth. These baseline results indicate that influence of trophic conditions and presence of marine vegetation may be directly related to As behavior in coastal systems.

Organic matter and pyritization relationship in recent sediments from a tropical and eutrophic bay

The degree of pyritization (DOP) and the extension of metals incorporation into pyrite was investigated at Guanabara Bay sediments. Maximum concentrations of total organic carbon (TOC), total sulfur, biopolymers and viable bacteria cells were observed in silted stations close to discharge points of sewage and minimum concentrations at sandy stations at the entrance of the bay. Pyrite iron concentrations (Fepy) was always lower than the reactive iron and Fepy were below the detection limit at sandy stations. The same trend was found to metals, which its degree of pyritization was Mn=Cu>As=Co>Ni>Cd>Zn≫Pb>Cr. The bay gathers all required factors to sulfate reduction and pyrite formation, once the C:S ratio express the reduced tendency conditions, almost half of the TOC present in its sediments is labile and both reactive sulfur and iron are available. However the degree of trace metals pyritization did not exceed 20%, consistent with the median DOP (29%).

Geochemistry of Early Frasnian (Late Devonian) pyrite-ammonoid level in the Kostomłoty Basin, Poland, and a new proxy parameter for assessing the relative amount of syngenetic and diagenetic pyrite

Pyrite geochemistry (isotope and trace element composition, degree of pyritization, S/Corg ratio) was used in context of selected lithogeochemical parameters (major and trace elements, including sulphur, organic carbon, and δ13C of carbonate carbon) to constrain fluctuations in depositional conditions during the Early to Middle Frasnian carbon isotopic perturbation (punctata Event) in the Kostomłoty Basin, Poland. Based on the ratio between the sum of oxyanionic elements and transition metals in pyrite, a new proxy parameter (index of syngenetic pyrite, ISYP) is proposed for assessing the relative amount of syngenetic pyrite in a sample. The distribution of the ISYP along the Kostomłoty – Małe Górki section (upper Szydłówek to the basal Kostomłoty beds) is in concert with conclusions inferred from paleoecologic data and other geochemical parameters (degree of pyritization, S/Corg, δ34Spyrite). According to these, the lower segment of the Szydłówek Beds was deposited in a normally oxygenated environment, but undergoing increasing primary productivity in surface water, as indicated by an increase in δ13Ccarb and in Cu/Zr ratio in bulk rock, which triggered the periodic deposition of sediments slightly enriched in organic matter, notably within the pyrite-ammonoid level (=Goniatite Level). Fluctuating, but in general high S/Corg ratios, DOPR values and ISYP values suggest that during this time – against the background of a generally dysoxic environment – shorter or longer lasting episodes of more restricted (anoxic and possibly even euxinic) bottom water conditions developed. Low sedimentation rates enabled a continuous and practically unlimited supply of sulphate during bacterial sulphate reduction (BSR), which in turn led to a strong depletion of pyrite sulphur in 34S in this interval (constantly around –29‰). In contrast, below and above the Goniatite Level, higher δ34S values (up to +3‰), are compatible with closed system conditions and higher sedimentation rates. The Styliolina Horizon is distinguished by the overall highest portion of syngenetic pyrite, which suggests that in this marker layer most of the pyrite precipitated from euxinic bottom waters.

Reconstructing marine redox conditions for the transition between Cambrian Series 2 and Cambrian Series 3, Kaili area, Yangtze Platform: Evidence from biogenic sulfur and degree of pyritization

The transition between Series 2 and Series 3 of the Cambrian Period was a time of environmental perturbations and biological innovations. A generally positive sulfur isotopic composition of sedimentary pyrite with δ34Spyrite values between +1.7 and +37.3‰ is consistent with a biological origin via reduction of strongly 34S enriched seawater sulfate. No substantial changes in the redox conditions of the water column are discernible from the DOP record, and the sulfide sulfur isotopic composition is independent of DOP, in particular an excursion towards less 34S enriched values immediately preceding the proposed level of the Cambrian Series 2 to Cambrian Series 3 transition. This level is also characterized by a negative carbonate carbon isotope excursion, believed to reflect the transgressive flooding of the shelf with basinal anoxic waters. Low DOP values would suggest that these might have been ferruginous rather than sulfidic.

Iron and sulfur geochemistry in semi-arid mangrove soils (Ceará, Brazil) in relation to seasonal changes and shrimp farming effluents

Iron and sulfur are key elements in the biogeochemistry of estuarine soils, in which Fe and sulfate reduction (SR) pathways are important for organic matter decomposition. In the semi-arid coast of NE Brazil, mangroves are characterized by large seasonal variations in weather and the presence of numerous shrimp farms. The objective was to determine the impacts of shrimp farm effluents on iron and sulfur geochemistry in mangrove soils under the semi-arid climate of NE Brazil. A seasonal study was made of two mangrove forest soils (SF, a mangrove forest that directly receives wastewater from shrimp ponds and CS, a control site). Pyrite Fe, oxyhydroxides Fe, acid volatile sulfide, degree of pyritization (DOP), pH, Eh, total organic carbon (TOC) and total S were determined. There was a clear decrease in pyritic Fe and DOP in the SF soils, which may be related to the anaerobic oxidation of pyrite coupled with nitrate reduction, or to the dominance of denitrification over SR. Lower TOC contents in the SF site suggest that below ground decomposition increased in response to eutrophication. The seasonal variations led to important changes in the semi-arid mangrove soils. During the dry period, both soils experienced oxidizing conditions with remarkable loss of reduced and oxidized forms of Fe, which may have important environmental implications as Fe is biolimiting for marine primary production. The data show that both factors (seasonal weather variations and shrimp effluents) play important roles in the geochemical processes that occur in these soils and, thus, may affect their functioning and maintenance.

Pyritization of trace metals in mangrove sediments

The present study investigates the levels of Mn, Zn, Ni, and Co pyritization in mangrove sediments along distinct sedimentary zones in Enseada das Gracas, a lagoon-type estuary located on the southeastern coast of Brazil. The coastal geology is characterized by intense interactions of trace metals, forming pyrite minerals. Specific orders of DOP (degree of pyritization) and DTMP (degree of trace-metal pyritization) are shown: supratidal flat < mangrove forest < mud flat. Distinct changes in content along the sediment profiles are noted, where a supratidal flat presented low levels of DOP and DTMP with little variance along the sedimentary depths. The mangrove forest showed relatively high values of DOP and DTMP in the lower depths, while the mud flat showed the highest levels of DOP and DTMP.

Early diagenesis of organic-matter-rich sediments in a ría environment: Organic matter sources, pyrites morphology and limitation of pyritization at depth

The subtidal sediments of the inner Ría de Vigo (NW Spain) are fine-grained, rich in organic matter, and accumulate at rates of around 3 mm/yr. Redoxomorphic pyrites formation was studied in four sediment cores. Four operationally-defined iron fractions were determined (HCl-extracted, silicate-bound, organic and pyritic), and the dimensions and morphology of pyrites (which mainly occurred as isolated framboids 3–5 μm in diameter) were studied by SEM. Sediment appearance and degree of pyritization (DOP) identified an oxic zone limited to the top few millimetres, a suboxic zone that in most cores was just a few decimetres thick (the exception was a core obtained in an area of strong turbulent currents), and an underlying anoxic zone with DOP values of around 60%. Pyritic iron increased at the expense of the HCl-extracted fraction in approximately the top metre, but no further pyrite formation occurred at greater depths. Pyrite formation in these lower levels was not limited by lack of either HCl-extracted iron or metabolizable organic matter, but apparently by lack of sulphate ion, the diffusion of which to these depths was probably prevented by the fine texture of the sediments. Our findings highlight the interest of studying transitional sedimentary environments in order to explore in detail the geochemical parameters controlling pyrite formation.

Carbon and sulfur isotopic compositions of Early Cambrian black shales, NW Hunan, China: Implications for the Paleoceanographic sedimentary environment

In order to better understand the paleoceanographic sedimentary environment of the Lower Cambrian black shales extensively distributed in South China, outcropped along the present southern margin of the Yangtze Platform with a width of ca. 200–400 km and a length of more than 1500 km, we present new paired δ13C data on carbonates (δ13Ccarb) and associated organic carbon (δ13Corg) and δ34Spy data on sedimentary pyrite in black shales from three sections (Ganziping, Shancha and Xiaohekou) located in NW Hunan, China. In these sections, a total of 82 Lower Cambrian black shale samples have δ13Ccarb values ranging from −4.0‰ to 1.7‰ with an average value of −2.1‰, and δ13Corg values between −34.9‰ and −28.8‰, averaging −31.9‰. The δ34Spy values of 16 separated sedimentary pyrite samples from the black shales vary between +10.2‰ and +28.7‰ with an average value of +19.5‰, presenting a small isotope fractionation between seawater sulfate and sedimentary sulfide. The model calculation based on credible data from the paired analyses for δ13Ccarb and δ13Corg of 11 black shale samples shows a high CO2 concentration in the Early Cambrian atmosphere, about 20 times higher than pre-industrial revolution values, consistent with previous global predictions. The small sulfur isotope fractionation between seawater sulfate and sedimentary sulfide in black shales, only 15.5‰ on average, implies a low sulfate level in the Early Cambrian seawater around 1 mmol. In combination with a high degree of pyritization (DOP) in the black shales, it is suggested that sulfidic deep-ocean water could have lingered up to the earliest Cambrian in this area. The black shale deposition is envisaged in a stratified marine basin, with a surface euphotic and oxygenated water layer and sulfidic deeper water, controlled by a continental margin rift.

Sequestration of metals through association with pyrite in subtidal sediments of the Nanpaishui Estuary on the Western Bank of the Bohai Sea, China

Measurements were made of the degree of trace metal pyritization (DTMP) and ancillary characteristics of four undisturbed sediment cores collected from the subtidal zone of the Nanpaishui Estuary on the Western Bank of the Bohai Sea, a seriously polluted inland sea in northeastern China. The remarkably low concentrations of organic carbon (<0.72%) in these sediments likely constrained sulfate reduction rates, and the low concentrations of acid volatile sulfide (AVS) (<12.5μmolg−1) limited the sequestration of metals through association with pyrite. The most consistent cause of inter-station differences and depth variations in the degree of pyritization was differences in pyrite metal concentrations rather than reactive metal concentrations. Reactive metal concentrations were in several cases negatively correlated with pore water concentrations, consistent with a dissolution/precipitation mechanism. The relationship between pore water metal concentrations and DTMPs was evidenced by a qualitative similarity of the inter-station variability of these same parameters.

Prevailing oxic environments in the Pacific Ocean during the mid-Cretaceous Oceanic Anoxic Event 2

The occurrence of Oceanic Anoxic Event 2 (OAE2) 94 million years ago is considered to be one of the largest carbon cycle perturbations in the Earth's history. The marked increase in the spatial extent of the anoxic conditions in the world's oceans associated with OAE2 resulted in the mass accumulation of organic-rich sediments. Although extensive oceanographic studies of OAE2 have been undertaken in the Atlantic Ocean, the Tethys Sea, and the epicontinental seas of Europe and America, little is known about OAE2 in the Pacific Ocean. Here, we present high-resolution carbon-isotope and degree of pyritization (DOP) data from marine sequences that formed along the continental margins of North America and Asia below the northeastern and northwestern Pacific Ocean. The predominance of low DOP values in these areas revealed that the continental margins of the Pacific Ocean were oxic for most of the OAE2 interval.

Iron speciation in cherts from the Laobao Formation, South China: Implications for anoxic and ferruginous deep-water conditions

Cherts in the Laobao Formation in the Silikou section, South China were deposited in terminal Ediacaran deep-water. The speciations of iron in cherts, such as pyrite Fe (FeP), HCl-extractable Fe (FeH), and total Fe (FeT), and their interrelationships, are proxies for distinguishing oxic, anoxic but ferruginous, and sulfidic conditions, which is an effective method for studying the terminal Ediacaran ocean environment. For chert samples from the Laobao Formation, pyrite concentrations are generally low (<0.56%), degree of pyritization (DOP) ranges from <0.01 to 0.78 and most are <0.45; (FeP+FeH)/FeT ratios are between 0.27 and 0.84 and most exceed 0.38, and most of FeT/Al ratios range from 0.55 to 5.87 (averaging 1.1). Relatively high content of highly reactive Fe and low content of pyrite indicate a scarcity of H2S and Fe-enriched deep-water. These features suggested that the deposition setting of Laobao cherts was anoxic and ferruginous. Terminal Ediacaran deep-water in the South China Basin was most likely anoxic and ferruginous, which played a key role in the emergence and evolution of early multicellular life.

Isotopic evidence for Fe cycling and repartitioning in ancient oxygen-deficient settings: Examples from black shales of the mid-to-late Devonian Appalachian basin

Abstract We have measured iron (Fe) isotope compositions of bulk samples and chemically extracted pyrite in two black shale units: the Oatka Creek Formation (OCF) and the younger Geneseo Formation (GF) deposited in the Appalachian basin during the mid-to-late Devonian. The depositional redox conditions for these units are well established through multiple paleoproxies, including degree of pyritization (DOP) and ratios of total Fe to Al (Fe T /Al), suggesting that both deposits reflect oxygen-deficient environments, but that euxinia (anoxia with hydrogen sulfide in the bottom waters) was more frequent and persistent during deposition of the OCF. Iron isotopes show systematic variations that are consistent with the inferred water column redox conditions. Samples from the OCF yield low and variable bulk Fe isotope compositions (− 0.44‰ to 0.03‰ in δ 56 Fe relative to average igneous rocks) that are inversely correlated with Fe T /Al, whereas bulk δ 56 Fe values ( δ 56 Fe T ) of the GF fall in a narrower range (− 0.09‰ to 0.12‰). δ 56 Fe values of pyrite ( δ 56 Fe Py ) display good correlation with δ 56 Fe T in the OCF, but no such correlation is observed in the GF. The Fe isotope data and other paleo-redox indicators, when viewed collectively, point to a benthic Fe source on the shelf and shelf-to-basin transfer that operated during deposition of the OCF, similar to what has been observed in the modern Black Sea. For the first time this study confirms the strength of Fe isotopes in delineating this Fe enrichment mechanism in the ancient geological record and emphasizes the utility of the Fe isotope proxy for fingerprinting and quantifying ancient biogeochemical cycling of Fe.

Re–Os and Mo isotope systematics of black shales from the Middle Proterozoic Velkerri and Wollogorang Formations, McArthur Basin, northern Australia

Joint application of the Mo isotope paleoredox proxy and Re–Os deposition-age geochronometer to euxinic black shales has potential for tracing the evolution of ocean redox chemistry over geological time. Here, we report new Re–Os and Mo isotope data for the Mesoproterozoic Velkerri Formation (Roper Group) and Paleoproterozoic Wollogorang Formation (Tawallah Group), McArthur Basin, northern Australia. New Re–Os ages of 1361 ± 21 Ma (2 σ, n = 14, mean square of weighted deviates [MSWD] = 1.3, Model 1) and 1417 ± 29 Ma (2 σ, n = 12, MSWD = 1.3, Model 1) constrain the depositional age of the Velkerri Formation and its contained biomarkers, as well as acritarchs and microfossils from the Roper Group. Black shales from the upper Velkerri Formation have high Mo abundances (105–119 ppm) and degree of pyritization [DOP] values (0.90–0.92) implying quantitative conversion of molybdate (MoO 4 2−) to thiomolybdate (MoS 4 2−) in overlying bottom waters. The average δ 97/95Mo (0.72 ± 0.10‰, 2 σ, n = 6) of these shales is consistent with previous data, but represents a significantly more precise determination for global seawater δ 97/95Mo at 1.4 Ga. This value is lighter than present-day seawater by ∼0.85‰ and reflects expanded strongly euxinic deep ocean conditions ([H 2S] aq > 11 μM) relative to oxic, suboxic, and weakly/intermittently euxinic ([H 2S] aq < 11 μM) marine deposition in the 1.4 Ga oceans. Mass-balance modelling suggests Mo removal into strongly euxinic and oxic sediments may have comprised 30–70% and less than 15%, respectively, of the oceanic Mo sink at 1.4 Ga as opposed to 5% and 35% today, respectively. The Re–Os radioisotope system in organic-rich shales serves as a test for post-depositional alteration that could affect the usefulness of paleoredox tracers such as Mo stable isotopes. Re–Os isotope data for the Wollogorang Formation black shales are scattered and yield a highly imprecise date of 1359 ± 150 Ma (2 σ, n = 21, MSWD = 85, Model 3). This age is younger than U–Pb zircon ages from interbedded tuffs that constrain the age of deposition at ca. 1730 Ma. In conjunction with previous petrological, geochemical, and paleomagnetic data, the Re–Os isotope data suggest hydrothermal fluid flow through the Wollogorang Formation, possibly associated with formation of the ca. 1640 Ma McArthur River Pb–Zn–Ag sedimentary exhalative deposit, resulted in post-depositional mobilization of Re and Os. Based on the degree of deviation of the Re–Os data from a 1730 Ma reference line, open-system behavior of Re and Os was greatest near the base of the black shale unit. Wollogorang Formation black shales are enriched in Mo (41–58 ppm), but are characterized by variable δ 97/95Mo (0.3–0.8‰) and DOP (0.57–0.92). The lightest δ 97/95Mo values occur near the base of the black shale unit. Based on the Re–Os systematics, hydrothermal fluids have probably overprinted the authigenic δ 97/95Mo signature in those shales. However, the heaviest δ 97/95Mo values in the Wollogorang Formation come from stratigraphically higher shales, and are similar to those observed for the Velkerri Formation, and thus may reflect seawater δ 97/95Mo at 1.73 Ga.

Natural attenuation processes in two water reservoirs receiving acid mine drainage

Characteristics of water profiles and sulphide formation processes in sediments were studied in two water reservoirs affected by acid mine drainage in order to investigate the mechanisms controlling the physical and chemical processes that, under favourable conditions, act to reduce the toxicity, mobility and concentration of metals and metalloids in the water column. Water columns and pore-waters from sediments were analysed for Fe species, trace elements (As, Cd, Co, Cu, Mn, Ni, Pb, Zn, Cr), sulphide, sulphate and bicarbonate. Inorganic reduced sulphur compounds (acid volatile sulphur, pyrite sulphur and elemental sulphur) and reactive Fe were determined in the sediments. A sequential extraction was also performed. Both reservoirs behave like holomictic and monomictic lakes, with a summer thermal stratification that disappears during winter. pH values between 4 and 7 can be observed along the water columns. Pore-water concentrations of up to 25 mg/l of Fe, 4 mg/l of Al, 1.3 mg/l of Zn, 170 µg/l of Pb, 11 µg/l of As, etc. have been found. The results suggest that toxic elements such as Cu, Zn, Co, Pb, Cr, As, etc. are mainly found in the bioavailable fraction which is the most hazardous for the environment. The calculated degree of sulphidization (DOS) and degree of pyritization (DOP) values indicates that removal of trace elements from anoxic pore-waters occurs by coprecipitation and/or adsorption on newly formed Fe sulphides (framboidal pyrite), attenuating the contamination. However oxidation of the sediments during turnover periods also occurs, which releases toxic elements back into the water column.

Mississippian Barnett Formation, Fort Worth Basin, Texas: Bulk geochemical inferences and Mo–TOC constraints on the severity of hydrographic restriction

Chemostratigraphic evidence from the Barnett Formation (Texas, USA) elucidates the restricted nature of the depositional environment in the Fort Worth Basin during the early progression of the late Paleozoic Ouachita Orogeny. In accord with recent lithostratigraphic and petrographic studies, stratigraphic bulk geochemical analyses reveal that the environment of deposition was anoxic to euxinic, sediment-starved, with relatively high rates of organic matter accumulation. Using an environmental proxy developed from the sediment geochemistry of modern anoxic silled basins, the stratigraphic concentrations of total organic carbon (TOC) and molybdenum (Mo) in the Barnett Formation reveal a high degree of subpycnoclinal water mass restriction and an extended timescale of deep-water renewal of at least 8 × 10 3 years and potentially as long as 2 × 10 4 years. An integrated assessment of elemental concentrations, the degree of pyritization, and TOC–S–Fe relationships reveals that severe Fe limitation controlled pyrite formation and the large excess of sulfide in the overlying water column. Mineralogical and elemental constraints suggest that, following sulfate reduction, less easily bio-extractable Fe(III) was bio-reduced under methanogenic conditions, consequently liberating Fe(II) which was subsequently incorporated into pore-water-formed dolomite (i.e., organogenic dolomite). The preserved Mo–TOC, TOC–S–Fe, and Fe-bearing mineral relationships collectively indicate that the protracted turnover rate and persistent sediment starvation resulted in a biologically inhospitable environment that limited the microbial consumption rate of organic carbon.

Environmental Redox Changes of the Ancient Sea in the Yangtze Area during the Ordo‐Silurian Transition

Abstract: Extensive organic‐matter (OM) rich facies (black shales) occur in the Ordo‐Silurian boundary successions in the Yangtze area, South China. To investigate the redox changes of the Yangtze Sea during the Ordo‐Silurian transition, two OM sections (Wangjiawan in Yichang, Hubei Province, and Sanjiaguan in Zhangjiajie, Hunan Province) straddling the Ordo‐Silurian boundary are studied. The measurements finished in this study include contents of the total organic carbon (TOC), pyrite sulphur, and different species of Fe, including dithionite‐extractable Fe (FeD), pyrite Fe (FeP), HCI‐extractable Fe (FeH), and total Fe (FeT), in black shales, as well as other redox proxies, such as the S/C ratio, the ratio between highly reactive Fe (FeHR = FeD + FeP) and FeT, and the Fep/(FeP + FeH) ratio, known as the degree of pyritization (DOP). In the Wangjiawan section, the Middle Ashgill sediments have high FeHR/FeT ratios (0.20‐0.77; avg. 0.45), high DOP values (0.21‐0.72; avg. 0.54), and a relatively constant sulfur content independent of the organic carbon content. By the contrast, the mid‐early Hirnantian deposits generally have low FeHR/FeT ratios (0.10‐0.35; avg. 0.21), low DOP values (0.11‐0.40; avg. 0.28), and S/C values are clustering on the normal marine value (S/C = 0.36). The late Hirnantian and early Rhuddanian deposits, similar to those of the Middle Ashgill deposits, are characterized by high FeHR/FeT ratios (0.32‐0.49; avg. 0.41), high DOP values (0.46‐0.68; avg. 0.53) and fairly constant sulfur contents. These data suggest the occurrences of marine anoxia on the Yangtze Sea shelf during intervals of the Mid Ashgill, Late Hirnantian and Early Rhuddanian, and ventilated and oxygenated marine conditions during the mid‐early Hirnantian time. The mid‐early Hirnantian ventilated event was concomitant with the global glacial period, likely resulted from the glacio‐eustatic sea‐level fall and subsequent circulation of cold, dense oxygenated waters upon the shelf seabed. Accordingly, the abrupt change from oxygenated to anoxic marine waters from the late Hirnantian and the early Rhuddanian were resulted from the post‐glacial rise of eustatic sea level. Combined with the data from Sanjiaguan section near the subemergent highland, salinity stratification of water columns are strongly evidenced by very low S/C ratios (0.00‐0.08, avg. 0.02), low FeHR/FeT ratios (0.10‐0.37, avg. 0.27), low DOP values (0.01‐0.27, avg. 0.10), and TOC contents (0.72%‐4.27%, avg. 2.55%). Under this circumstance, the anoxic water columns could have formed beneath the halocline, above which desalinized waters formed. In the Wangjiawan section, TOC contents are generally high (0.94‐9.32%, avg. 4.44%), but low (0.35%‐2.12%, avg. 1.29%) in the mid‐early Hirnantian that is coincident with the oceanic oxic stage, together with relation of the organic contents to the stratal thickness, suggesting that the accumulation of the organic matter was mainly controlled by the oxygen levels of the water columns; on the other hand, productivity and depositional rate may also have played a role in the organic accumulation and preservation.

Effects of bioturbation by root and crab activity on iron and sulfur biogeochemistry in mangrove substrate

Solid-phase analysis was undertaken to obtain information on the effects of bioturbation (activity of plants and macrofauna) on the distribution of different forms of iron and sulfur in a mangrove ecosystem in Brazil. A seasonal study involving four compartments from a mangrove forest (UN: an unvegetated mudflat; SP: a low intertidal zone colonized by Spartina alterniflora; RH: monospecific forest of Rhizophora mangle and; AV: monospecific forest of Avicennia schauerianna) was carried out. Pyrite-Fe, oxyhydroxides-Fe, acid volatile sulfide (AVS), degree of pyritization (DOP), pH and Eh were determined. The results show an intense effect of mangrove vegetation on soil redox processes. Higher live root densities (56–132 g m − 2 ) in surface layers (0–10 cm) from vegetated sites led to suboxic/oxic conditions (129–374 mV), pyrite oxidation, and acidic (3.8–5.5) conditions, showing the role of plant activity on iron sulfide biogeochemistry. The combination of more oxidizing conditions and pyrite oxidation process also favored oxyhydroxide precipitation, as indicated by higher values of oxyhydroxides-Fe found in superficial layers. In contrast, vegetated zones presented higher values of pyrite-Fe than UN, indicating that root exudades may stimulate the activity of sulfate-reducing bacteria. The results show a clear seasonal variation in AVS and pyritic fractions. In summer, the elevated temperatures cause higher evapotranspiration rates and plant activity which intensified the oxidation of iron sulfides. Thus, at all sites the AVS and pyrite-Fe values were substantially lower during summer (especially in superficial layers). A clear seasonal pattern was also observed in the unvegetated mudflat compartment. The lowest pyrite-Fe, DOP and AVS values, observed in summer, appear to be related to increased crab activity ( Uca spp), probably because of more intense foraging and reproductive activity.

Trace element diagenesis in pyrite-rich sediments of the Achterwasser lagoon, SW Baltic Sea

The early diagenesis of trace elements (V, Cr, Co, Cu, Zn, As, Cd, Ba, U) in anoxic sediments of the Achterwasser, a shallow lagoon in the non-tidal Oder estuary in the Baltic Sea, was investigated in the context of pyrite formation. The dissolved major redox parameters show a two-tier distribution with transient signals in the occasionally re-suspended fluid mud layer (FM) and a permanently established diagenetic sequence in the sediment below. Intense microbial respiration leads to rapid depletion of O 2 within the uppermost mm of the FM. The reduction zones of Mn, Fe and sulfate overlap in the FM and in the permanently anoxic sediment section which appears to be a typical feature of estuarine sediments, under low-sulfate conditions. Degrees of pyritization (DOP) range from 50% in the FM to remarkably high values > 90% at 50 cm depth. Pyrite formation at the sediment surface is attributed to the reaction of Fe-monosulfides with intermediate sulfur species via the polysulfide pathway. By contrast, intense pyritization in the permanently anoxic sediment below is attributed to mineral growth via adsorption of aqueous Fe-sulfide complexes onto pyrite crystals which had originally formed in the surface layer. The studied trace elements show differential behavior patterns which are closely coupled to the diagenetic processes described above: (i) Zn, Cu and Cd are liberated from organic matter in the thin oxic layer of the sediment and diffuse both upwards across the sediment/water boundary and downwards to be trapped as monosulfides, (ii) V, Cr, Co and As are released during reductive dissolution of Mn- and Fe-oxyhydroxides, (iii) U removal from pore water occurs concomitantly to Fe reduction in the FM and is attributed to reduction of U(VI) to U(IV), (iv) the Ba distribution is controlled by reductive dissolution of authigenic barite in the sulfate reduction zone coupled with upward diffusion and re-precipitation. The incorporation of trace elements into pyrite is most intense for Co, Mn and As, intermediate for Cu and Cr and little to negligible for U, Zn, Cd, V and Ba. The observed trend is largely in agreement with previous studies and may be explained with differing rates for ligand exchange. Slow and fast ligand exchange and thus precipitation kinetics are also displayed by downcore increasing (Mn, Cr, Co and As) or constantly low (Zn, Cu, Cd) pore water concentrations. The downward increasing degrees of trace metal pyritization (DTMP) for Co, Cu, Zn and As are, in analogy to pyrite growth, assigned to adsorption of sulfide complexes or As oxyanions onto preexisting pyrite minerals.

Reconstructing marine redox conditions for the Early Cambrian Yangtze Platform: Evidence from biogenic sulphur and organic carbon isotopes

In order to constrain marine redox conditions for the sedimentary environments immediately preceding the rapid diversification of metazoans carbon and sulphur isotopes, as well as carbon, sulphur and iron abundances, were measured in shales, cherts, and carbonates from two Lower Cambrian (Nemakit-Daldynian to Atdabanian) sections belonging to a shallow shelf and deeper water basinal environment on the Yangtze Platform, South China. A largely anoxic and most probably sulphidic deeper water body for most of the Nemakit-Daldynian and lower Tommotian is supported by positive δ 34S values for sulphide, as well as high degree of pyritization (DOP) and total organic carbon (TOC) values. Additionally, an isotope fractionation between organic and carbonate carbon of > 32‰ suggests that sulphate reducing and sulphide oxidizing bacteria flourished and added to the biomass. Furthermore a positive correlation between δ 34S of sulphide and δ 13C of organic and possibly inorganic carbon is observed. In the lower Tommotian, low δ 13C values for organic and inorganic carbon together with lower δ 34S values for pyrite, if compared to underlying and overlying samples, have been measured. This is indicative of bacterial sulphate reduction taking place not only in the interstitial waters but also in an anoxic, possibly euxinic water column, and thus leading together with chemoautotrophic bacteria to recycling of organic matter, superimposed on the temporal evolution of the global carbon cycle. In the upper Tommotian and Atdabanian, decreasing DOP and TOC and high δ 34S values for pyrite and organic sulphur suggest that the water column became oxic and that bacterial sulphate reduction occurred primarily in the sediment, where sulphate limitation was soon established. Organic carbon and carbonate carbon isotope ratios increase, reflecting open marine conditions and oxic photosynthesis dominating carbon cycling in the shelf environment. In the basinal deeper water section, anoxic conditions were still present or re-established during the upper Tommotian. In the lower Atdabanian, δ 34S is variable, but mostly high in the deeper section, with DOP indicating an oscillation between oxic and dysoxic conditions.

Variations of Fatty Acids During the Sulphidization Process in the Authie Bay Sediments (8 pp)

Background, Aim and Scope The sulphidization process in relatively clean sediments sampled in a mudflat of the Authie estuary (located in Northern France) has been studied by coupling geochemical expertise and the use of fatty acids (FAs) as biochemical markers. Materials and Methods: Three sediment cores have been sampled in September 2003, November 2003 and May 2004, and cut every 2 cm in the field under nitrogen atmosphere so as to prevent any oxidation of reduced species. In the solid phase, reduced sulphur compounds, e.g. AVS (Acid Volatile Sulphides) and CRS (Chromium Reducible Sulphur) [including also the calculation of the degree of sulphidization (DOS) and the degree of pyritization (DOP)], and fatty acids have been carried out. Eh, pH, metal species (mostly iron and manganese), dissolved S(-II) and sulphate have also been determined in the porewaters. Results: The sediment cores display a lot of differences due to the high sedimentation rate and the seasonal evolution as well. The presence of Mn2+, Fe2+, S(-II) and the decrease of the redox potential and the concentration of sulphates clearly indicate early diagenetic transformations promoted by the bacterial activity. Acid Volatile Sulphides are produced in the first cm and are stabilized with depth. A rapid decrease of FAs concentrations in September and May has also been pointed out owing to a rapid consumption of the labile organic matter. Several categories of FAs have been separated and most of them belong here to the saturated and monounsaturated groups. In the saturated group, branched chain FAs, iso and anteiso C15:0 are predominant and represent the bacterial imprint in the sediments. Maximum proportions are observed between 5 and 10 cm in September, and between 13 and 17 cm in November and May. Discussion: As sulphate concentrations remain high in the porewater, the limitation of the sulphidization process in our sediments must be due to a lack of labile organic matter input. The presence of pyrite in our sediment is bound to its formation at the water-sediment interface, where partial reoxidation may take place. However, at deeper depths, pyritization processes does not continue any more. Presence of maximum, dissolved S(-II) concentrations have been observed, simultaneously with maximum proportion relative to total FAs of iso and anteiso C15:0, and, in September, with an increase in proportions of C18:1ω7. This indicates the presence of sulphate-reducing bacterial activity at the time when the sediments were sampled. However, no close correspondence between bacterial FAs concentrations and S(-II) concentrations has been found. Conclusions: In each core, the sulphidization process is not complete, and this is probably due to the lack of biodegradable organic matter, which appears as the limiting factor from a qualitative point of view. S(-II) production in porewaters is linked with the activity of sulphate-reducing bacteria. Seasonal effects have also been pointed out and, especially, a more important input of diatom organic matter in May when compared to September and November. Recommendations and Perspectives: Fatty acid analyses represent an original and a useful tool for a better understanding of an early diagenetic process in the first cm of the sediments. More studies should be carried out associating inorganic chemical parameters and chemical biomarkers for pointing out stronger and more reproducible relations. Moreover, the use of microcosms in our group is on the way to take into account the kinetics of the organic matter degradation during the early diagenesis.