Sedimentary pyrite formation: An update

Sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time. The amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter, dissolved sulfate, and reactive detrital iron minerals. Organic matter appears to be the major control on pyrite formation in normal (non-euxinic) terrigenous marine sediments where dissolved sulfate and iron minerals are abundant. By contrast, pyrite formation in non-marine, freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic-rich fresh water shales from marine shales. Under marine euxinic conditions sufficient H₂S is produced that the dominant control on pyrite formation is the availability of reactive iron minerals.Calculations, based on a sulfur isotope model, indicate that over Phanerozoic time the worldwide average organic carbon-to-pyrite sulfur ratio of sedimentary rocks has varied considerably. High $\text{C}\text{S}$ ratios during Permo-Carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time. Low $\text{C}\text{S}$ ratios, compared to today, during the early Paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean. The latter could have been due to lower oceanic oxygen levels and/or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time.     

Formation of pyrite in freshwater sediments: Implications for C/S ratios

The vertical distribution of pyrite, acid volatile sulphide (AVS), carbon, and total S (St) were determined directly in the sediments of three lakes of different trophic status. The results showed that freshwater pyrite formation reflects the redox status of the sediment or overlying waters. It appears to form chiefly in reducing sediments which are subject to oxidizing influences, by either a low turnover of organic carbon or periodic incursions of oxygen. Although there are high concentrations of AVS in the near-surface sediments of productive lakes, very little is diagenetically converted to pyrite.The feasibility of using sulphur ratios to diagnose whether rocks were formed in marine or freshwater environments is assessed. New values for FeS₂/FeS of 0.5-5 show that this ratio does not provide a reliable test. Values of C/Sp, where Sp represents pyrite sulphur, lie within the range of 160–700 and are much higher than previously measured ratios of C/S_t of 1–50. These new determinations show that, if pyrite sulphur is unequivocally measured, C/S ratios may be a more sensitive indicator of salinity than had been previously thought.     

Zur Systematik der Kohlenstoff-Schwefel-Eisen-Verhältnisse in Auftriebssedimenten

Abundances of organic carbon, sulfur, and reactive iron in sediments of three upwelling environments (Peru, Oman and Benguela) suggest that organic carbon/reduced sulfur ratios (C/S-ratios) in this category of marine sediments deviate considerably from previously established empirical ratios in normal marine sediments. To clarify the discrepancies, we investigated those components of the diagenetic system that limit the formation of pyrite: sulfate concentrations and reduction rates in pore waters, availability of reactive iron, and the quantity and quality of organic matter. All three limitations are evident in our sample pools. The results suggest that C/S-ratios in recent and fossil marine sediments rich in organic matter may be unsuitable as paleoenvironmental indicators.

     

An interpretation of carbon and sulfur relationships in Black Sea sediments as indicators of environments of deposition

Syngenetic iron sulfides in sediments are formed from dissolved sulfide resulting from sulfate reduction and catabolism of organic matter by anaerobic bacteria. It has been shown that in recent marine sediments deposited below oxygenated waters there is a constant relationship between reduced sulfur and organic carbon which is generally independent of the environment of deposition. Reexamination of data from recent sediments from euxinic marine environments (e.g., the Black Sea) also shows a linear relationship between carbon and sulfur, but the slope is variable and the line intercepts the S axis at a value between 1 and 2 percent S. It is proposed that the positive S intercept is due to watercolumn microbial reduction of sulfate using metabolizable small organic molecules and the sulfide formed is precipitated and accumulates at the sediment-water interface. The variation in slope and intercept of the C to S plots for several cores and for different stratigraphic zones for the Black Sea can be interpreted in relation to thickness of the aqueous sulfide layer or thinness of the oxygen containing layer and to deposition rate, but also may be influenced by availability of iron, and perhaps the type of organic matter (Leventhal, 1979).     

The carbon and sulfur cycles and atmospheric oxygen from middle Permian to middle Triassic

The results of a theoretical isotope mass balance model are presented for the time dependence of burial and weathering-plus-degassing fluxes within the combined long-term carbon and sulfur cycles. Averaged data for oceanic δ¹³C and δ³⁴S were entered for every million years from 270 to 240 Ma (middle Permian to middle Triassic) to study general trends across the Permian-Triassic boundary. Results show a drop in the rate of global organic matter burial during the late Permian and a predominance of low values during the early-to-middle Triassic. This overall decrease with time is ascribed mainly to epochs of conversion of high biomass forests to low biomass herbaceous vegetation resulting in a decrease in the production of terrestrially derived organic debris. Additional contributions to lessened terrestrial carbon burial were increased aridity and a drop in sea level during the late Permian which led to smaller areas of low-lying coastal wetlands suitable for coal and peat deposition.Mirroring the drop in organic matter deposition was an increase in the burial of sedimentary pyrite, and a dramatic increase in the calculated global mean ratio of pyrite-S to organic-C. High S/C values resulted from an increase of deposition in marine euxinic basins combined with a decrease in the burial of low-pyrite associated terrestrial organic matter. The prediction of increased oceanic anoxia during the late Permian and early Triassic agrees with independent studies of the composition of sedimentary rocks.Weathering plus burial fluxes for organic carbon and pyrite sulfur were used to calculate changes in atmospheric oxygen. The striking result is a continuous drop in O₂ concentration from ∼30% to ∼13% over a twenty million year period. This drop was brought about mainly by a decrease in the burial of terrestrially derived organic matter. but with a possible contribution from the weathering of older organic matter on land. It must have exerted a considerable influence on animal evolution because of the role of O₂ in respiration. Some examples are the extinction of many vertebrates, loss of giant insects and amphibians, and the restriction of animals to low elevations. It is concluded that the extinction of plants may have contributed to the extinction of animals.     

Investigating pathways of diagenetic organic matter sulfurization using compound-specific sulfur isotope analysis

We present the results of compound-specific sulfur isotope analyses performed on organic sulfur compounds (OSCs) isolated from sediments deposited in the euxinic Cariaco Basin, Venezuela. Individual OSCs (sulfurized highly branched isoprenoids and malabaricatriene) have sulfur isotope compositions of ca. −15‰, which is ³⁴S enriched by 5-15‰ relative to coeval bulk organic and inorganic sulfur pools. These observed differences in the sulfur isotope composition of bulk organic sulfur in the kerogen and bitumen pools and individual OSCs demonstrate that there are multiple pathways of organic sulfur formation operating simultaneously in marine sediments. Comparison of our measured compound-specific sulfur isotope data with values predicted using simple isotopic mass balance assumptions suggests that the sulfurization process likely involves multiple sources of inorganic sulfur. Further, the isotopic composition of these various precursor inorganic sulfur species and the specific pathway of sulfur incorporation into organic matter (OM) impart distinct isotopic compositions to the resulting organic sulfur compounds. These data represent the first compound-specific sulfur isotope measurements made in marine sediments, and demonstrate the utility of compound-specific sulfur isotope analysis in identification of inorganic sulfur sources for OM sulfurization and tracking pathways of sulfur incorporation, which will lead to a more complete understanding of diagenetic sulfurization of OM.     

Pyrite-organic matter relationships: Currant Bush Limestone,Georgina Basin,Australia

The relationship between the TOC (total organic carbon) and S (pyritic sulphur) contents of some organic-rich (TOC 3–14%) silty limestones and calcareous siltstones from the Cambrian Currant Bush Limestone of the Georgina Basin, Australia, is similar to that of euxinic sediments, but could also indicate deposition in a non-saline environment. However, such conditions are not easily reconciled with the shallow equatorial epeiric sea in which the formation is considered to have been deposited. The relationship can alternatively be explained by the relatively low Fe content of the sediment being the main limiting factor in pyrite generation, coupled with a slightly increased degree of pyritization with increased TOC. The relationship between the TOC and S contents of organic-poor (TOC 0.2–2.4%) samples from the same formation is similar to that of normal marine sediments, but the Fe content is probably also the major controlling factor on pyrite generation. These interpretations demonstrate that in using TOC and S contents to aid in reconstruction of depositional environments, it is important to check whether Fe content could have been the limiting factor in the formation of pyrite, even in sediments with only moderate carbonate contents.     

渔塘坝硒矿床多元素富集与沉积环境

对采自湖北西部渔塘坝二叠系独立硒矿区的硅质岩和页岩样品进行了元素测试（微量元素、总铁、总有机碳、总硫）,分析了该区域的多元素富集及沉积环境。该区域V、Cu、As、Cd、Tl、Se、Mo、V、 Sb、 U、Ag、 Cr、Ni相对富集, Ba、Hf、Ta、W、Pb、Th相对贫化或亏损。全硫—全铁（TS—TFe）的关系表明页岩及部分硅质岩的S/Fe比值均大于1.15,说明有过量的硫存在,可能为有机硫或为其他亲硫元素沉淀提供条件。TOC—TS—TFe三角图及V/(V + Ni)表明该区域的岩石形成于缺氧或静海相环境。U/Th、Zr—Cr、P—Y等微量元素地球化学参数表明该区域有热水沉积的存在。通过对微量元素可能的来源研究分析,该区域的微量元素可能存在多种来源:热水来源、海水沉积、峨眉山玄武岩。     

Sulphur,organic carbon and iron relationships in estuarine and freshwater sediments: effects of sedimentation rate

Concentrations of S, organic C and Fe were investigated in profiles of sediments from two estuarine systems in the SW of Western Australia. In marine-affected sediments, inorganic S dominates total S and concentrations of total S correlate with Fe and not with organic C. In freshwater sediments, organic S dominates total S and concentrations of total S correlate with organic C and not with Fe. Molar Fe/S ratios in the estuarine sediments decrease with increasing salinity and approach unity for marine conditions. Net accumulation rates of S in sediments were estimated with a numerical computer model, calibrated with published data on profiles of marine sediments for diffusion of SO²⁻₄, sedimentation rates and distributions of S. Measured depth-integrated reduction rates of SO²⁻₄ in the marine-affected estuarine sediments approach those obtained for Fe-limited marine conditions at similar rates of sedimentation. Measured concentrations of inorganic S in anoxic freshwater sediments fit a numerically calculated relationship between inorganic S and sedimentation rate.     

Sulphur Enrichment in Organic Matter of Eastern Mediterranean Sapropels: A Study of Sulphur Isotope Partitioning

Sulphur isotope compositions and S/C ratios of organic matter were analysed in detail by combustion-isotope ratio monitoring mass spectrometry (C-irmMS) in eastern Mediterranean sediments containing three sapropels of different ages and with different organic carbon contents (sapropel S1 in core UM26, formed from 5–9 ka ago with a maximum organic carbon content of 2.3 wt%; sapropel 967 from ODP Site 160-967C, with an age of 1.8 Ma and a maximum organic carbon content of 7.4 wt%; and sapropel 969 from ODP Site 160-969E, with an age of 2.9 Ma and a maximum organic carbon content of 23.5 wt%). Sulphur isotopic compositions (34S) of the organic matter ranged from -29.5 to +15.8 and the atomic S/C ratio was 0.005 to 0.038. The organic sulphur in the sediments is a mixture of sulphur derived from (1) incorporation of 34S-depleted inorganic reduced sulphur produced by dissimilatory microbial sulphate reduction; and (2) biosynthetic sulphur with an isotopic signature close to seawater sulphate. The calculated biosynthetic fraction of organic sulphur in non-sapropelic sediments ranges from 68–87%. The biosynthetic fraction of the organic sulphur of the sapropels (60–22%) decreases with increasing organic carbon content of the sapropels. We propose that uptake of reduced sulphur into organic matter predominantly took place within sapropels where pyrite formation was iron-limited and thus an excess of dissolved sulphide was present for certain periods of time. Simultaneously, sulphide escaped into the bottom water and into sediments below the sapropels where pyrite formation occurred.     

The Neoproterozoic oxygenation event: Environmental perturbations and biogeochemical cycling

The oxygen content of the Earth's surface environment is thought to have increased in two broad steps: the Great Oxygenation Event (GOE) around the Archean–Proterozoic boundary and the Neoproterozoic Oxygenation Event (NOE), during which oxygen possibly accumulated to the levels required to support animal life and ventilate the deep oceans. Although the concept of the GOE is widely accepted, the NOE is less well constrained and its timing and extent remain the subjects of debate. We review available evidence for the NOE against the background of major climatic perturbations, tectonic upheaval related to the break-up of the supercontinent Rodinia and reassembly into Gondwana, and, most importantly, major biological innovations exemplified by the Ediacarian Biota and the Cambrian ‘Explosion’.Geochemical lines of evidence for the NOE include perturbations to the biogeochemical cycling of carbon. Generally high δ¹³C values are possibly indicative of increased organic carbon burial and the release of oxidative power to the Earth's surface environment after c. 800 Ma. A demonstrably global and primary record of extremely negative δ¹³C values after about 580 Ma strongly suggests the oxidation of a large dissolved organic carbon pool (DOC), the culmination of which around c. 550 Ma coincided with an abrupt diversification of Ediacaran macrobiota. Increasing ⁸⁷Sr/⁸⁶Sr ratios toward the Neoproterozoic–Cambrian transition indicates enhanced continental weathering which may have fuelled higher organic production and burial during the later Neoproterozoic.Evidence for enhanced oxidative recycling is given by the increase in sulfur isotope fractionation between sulfide and sulfate, exceeding the range usually attained by sulfate reduction alone, reflecting an increasing importance of the oxidative part in the sulfur cycle. S/C ratios attained a maximum during the Precambrian–Cambrian transition, further indicating higher sulfate concentrations in the ocean and a transition from dominantly pyrite burial to sulfate burial after the Neoproterozoic. Strong evidence for the oxygenation of the deep marine environment has emerged through elemental approaches over the past few years which were able to show significant increases in redox-sensitive trace-metal (notably Mo) enrichment in marine sediments not only during the GOE but even more pronounced during the inferred NOE. In addition to past studies involving Mo enrichment, which has been extended and further substantiated in the current review, we present new compilations of V and U concentrations in black shales throughout Earth history that confirm such a rise and further support the NOE. With regard to ocean ventilation, we also review other sedimentary redox indicators, such as iron speciation, molybdenum isotopes and the more ambiguous REE patterns. Although the timing and extent of the NOE remain the subjects of debate and speculation, we consider the record of redox-sensitive trace-metals and C and S contents in black shales to indicate delayed ocean ventilation later in the Cambrian on a global scale with regard to rising oxygen levels in the atmosphere which likely rose during the Late Neoproterozoic.     

Sulfur geochemistry of Jurassic high-sulfur coals from Egypt

Jurassic high-sulfur coals from the Maghara area in Egypt were analyzed for the abundance and isotopic composition of different forms of sulfur. Analyses indicated that the sulfur occurs in the form of organic, pyrite, and sulfate forms. Pyrite sulfur represents the major fraction, while sulfate sulfur is minor and could be formed during sample preparation for the analyses.The δ³⁴S CDT values of the organic sulfur are positive ranging between 1.0‰ and 13.5‰ with an average of 9.1‰. Pyrite δ³⁴S values are also positive ranging between 1.5‰ and 15.4‰ with an average of 6.6‰. The high δ³⁴S values of the organic sulfur in the Maghara coals suggest a freshwater origin of the organic components of these coals. The lack of correlation between pyrite and organic sulfur isotopes implies different incorporation mechanisms of sulfur. The high-sulfur contents along with the positive and high δ³⁴S values suggest a marine origin of pyrite sulfur and support the geological interpretation of marine invasion after the peat formation that was responsible for the incorporation of the pyrite sulfur.The occurrence of pyrite as euhedral crystals as well as the high and positive δ³⁴S values of the pyrite sulfur indicates the formation of pyrite during diagenesis as a result of marine water invasion of the preexisting peat in a brackish coastal plain environment.     

Occurrence and distribution of sulfur in peat-forming environments of southern Florida

Cores and surface samples of peats from the Everglades—Mangrove region of Southern Florida were analyzed for total sulfur and pyritic sulfur. These values were compared with the petrographic-botanical components of the peats as determined from point-counts of oriented microtome sections. Pyrite occurs as individual euhedral crystals, loosely packed framboids, and lenses or crusts of minute crystals. Framboids and minute crystals are often associated with organic matter and sometimes with bacteria and fungi. Pyrite tends to selectively occur in void spaces in or between peat tissues. In samples containing very small amounts of pyrite, framboidal pyrite is the prevalent form.Marine to brackish peats contain the highest pyrite and total sulfur contents, with brackish peats generally containing more pyrite than marine peats. Pyrite tends to be lower in all peats within 30 cm of the surface, whether marine or brackish.Burial of freshwater peats beneath marine or brackish peats tends to increase the total sulfur and pyritic sulfur in the underlying peats. Burial beneath brackish-water, clay-rich deposits (such as splays) tends to increase total sulfur and pyritic sulfur in the underlying deposits more than burial beneath less clay-rich deposits.     

Geochemical expressions of anoxic conditions in Nordåsvannet,a land-locked fjord in western Norway

The Nordåsvannet fjord in western Norway is a modern semi-enclosed basin suitable for studying sedimentary cycles as they occur under anoxic bottom conditions. It is characterized by strongly anoxic conditions in the water column and bottom sediments. Diagenetic pyrite formation occurs in the sediments, and syngenetic pyrite is formed in the lower water column. Organic matter burial in the fjord exceeds that of other environments with normal marine or upwelling conditions. This is due to the better preservation of organic matter. Organic matter composition appears to have changed over time with higher fractions of terrigenous organic matter being present in the most recent sediments. This may be a result of increased input of terrigenous organic matter, possibly due to sewage supply to the fjord over the last decades. Organic C and CaCO₃ contents of the sediments do not appear to reflect a productivity signal. Calcium carbonate content is influenced by chemogenic calcite formation. Biogenic opal content appears to reflect a productivity signal, but different degrees of dissolution may obscure its clear recognition.     

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

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）, HCl-extractable Fe （FeH）, and total Fe （FeT）, in black shales, as well as other redox proxies, such as the SIC 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 SIC values are clustering on the normal marine value （SIC = 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     

Influence of a turbidite deposit on the extent of pyritization of iron,manganese and trace metals in sediments from the Guaymas Basin,Gulf of California (Mexico)

A core collected in the Guaymas Basin contained an organic-poor, Mn oxide-rich and (relatively) Fe oxide-rich turbidite layer that affected the distribution of Fe, Mn, C, S and trace metals. Results indicate that sediments not influenced by the turbidite layer achieved a 100% degree of pyritization and, by extension, that pyrite production is Fe-limited in these sediments. In contrast, the mud slide layer apparently supplied enough reactive Fe to transfer essentially 98% of the total S present at the base of the turbidite (17–19 cm) to the pyrite reservoir. C/S ratios showed rapid decreases with depth, from a high of 38 close to the sediment-water interface, to minimum values of 2.8 at the lower limit of the turbidite layer, a ratio equal to the average C/S value of normal marine modern sediments, where concentrations of organic C and pyrite supposedly have attained quasi-steady values. A significant part of the reactive Mn was associated with carbonates (41±12%) and, to a much lower degree, with pyrite (2.7±1.2%). The turbidite layer is currently showing a depletion of Mn relative to the host sediment. It is possible that Mn, a major metal constituent in these sediments, was initially present in high concentrations in the mud slide, but was eventually mobilized and transferred either to the water column or to the sediments immediately below the turbidite layer. Metals associated with this element probably followed the same path, affecting their incorporation into pyrite. The turbidite layer apparently affected the distribution of most of the trace metals associated with pyrite, except maybe Cd, Pb and, to a certain, extent Cr. However, Cu, Cr, Zn, Ni and Co were all found to be highly pyritized (>80%) in the sediments of the Guaymas Basin.     

Stable Sulfur Isotopic Evidence for Historical Changes of Sulfur Cycling in Estuarine Sediments from Northern Florida

Data on abundance and isotopic composition of porewater and sedimentary sulfur species are reported for relatively uncontaminated and highly contaminated fine-grained anoxic sediments of St. Andrew Bay, Florida. A strong contrast in amount and composition of sedimentary organic matter at the two sites allows a comparative study of the historical effects of increased organic loading on sulfur cycling and sulfur isotopic fractionation. In the contaminated sediments, an increase in organic loading caused increased sedimentary carbon/sulfur ratios and resulted in higher rates of bacterial sulfate reduction, but a lower efficiency of sulfide oxidation. These differences are well reflected in the isotopic composition of dissolved sulfate, sulfide, and sedimentary pyrite. Concentration and isotopic profiles of dissolved sulfate, organic carbon, and total sulfur suggest that the anaerobic decomposition of organic matter is most active in the upper 8cm but proceeds at very slow rates below this depth. The rapid formation of more than 90% of pyrite in the uppermost 2 cm which corresponds to about 3 years of sediment deposition allows the use of pyrite isotopic composition for tracing changing diagenetic conditions. Sediment profiles of the sulfur isotopic composition of pyrite reflect present-day higher rates of bacterial sulfate reduction and lower rates of sulfide oxidation, and record a profound change in the diagenetic cycling of sulfur in the contaminated sediments coincident with urban and industrial development of the St. Andrew Bay area.     

Black shales and massive sulfide deposits: causal or casual relationships? Insights from Rammelsberg, Tharsis, and Draa Sfar

Black shales and massive sulfides represent reduced lithofacies that require isolation from oxic environments to be preserved. This, together with the sedimentary affinity of both lithofacies, can explain their common concurrence in the geologic record. The present study is based on the comparison of Rammelsberg in Germany, Tharsis in Spain, and Draa Sfar in Morocco, three massive sulfide deposits closely associated with black shales that are distributed along the European and North African Variscan orogen. The study entails geochemical, biostratigraphic, and stratigraphic analyses of the black shale sequences hosting the three deposits and mineralogical and textural analyses of the sulfides. All three deposits were formed in immature, tectonically unstable basins within an active continental margin or continental magmatic arc. Their stratigraphic records consist of a sequence of black shales enclosing massive sulfides and variable proportions of bimodal volcanic and subvolcanic rocks. The major differences among the three deposits concern the size, composition, and mineralogy. Regarding age, they are diachronous and younger southward: Rammelsberg is middle Eifelian, Tharsis latest Famennian, and Draa Sfar late Viséan. The study of redox conditions of the paleoenvironment using organic and inorganic proxies highlights similarities and significant differences among the three ore-hosting basins during massive sulfide and black shale deposition. The black shales generally display low C_org and high S_tot contents. At Rammelsberg, the S_tot/C_tot ratios provide values typical for normal Middle Devonian marine environments, which suggests that the original reactive organic C is now fixed in carbonates. At Tharsis, most of the samples have C_org >1 and S_tot/C_org values equivalent to those of Devonian?CCarboniferous normal marine sediments. However, some pyritic hanging-wall samples have C_org <1 and S_tot up to 5?wt.%, suggesting the epigenetic addition of HS^?. The S_tot/C_org ratio for the Draa Sfar samples resembles that of Middle Carboniferous normal marine environments. Geochemical inorganic proxies used to define the environmental conditions include the enrichment factors of U (U_EF) and Mo (Mo_EF) together with V/Cr and V/(V?+?Ni) ratios. Footwall shales at Filón Norte (Tharsis) show positive and eventually elevated U_EF and Mo_EF values, which suggests anoxic conditions, whereas at Rammelsberg and Draa Sfar oxic bottom water is indicated. The relations V/Cr and V/(V + Ni) in all three cases point to a redox boundary near the sediment?Cwater interface, although at Tharsis some samples indicate anoxic/euxinic conditions (i.e., V/(V + Ni) >0.9). Regarding the environmental conditions of the source areas, feldspar illitization and selective depletion in Na and Ca occurred at the three studies sites. Available sulfur isotopic data from the Rammelsberg and Tharsis sulfide ore indicate that biogenic reduction of marine sulfate was a major sulfur source during massive sulfide generation. Nevertheless, a hydrothermal sulfur source has also been detected. At Rammelsberg, this is indicated from the polymetallic sulfides that replace sedimentary and diagenetic pyrite. At Tharsis, the bacteriogenic sulfur signature is also restricted to sulfide with less evolved textures, whereas a hydrothermal source is more evident in sulfides showing evidence of recrystallization. Both geochemical and isotopic data suggest that the bacteriogenic reduction process was inhibited by rapid burial. The sedimentation rates calculated for Rammelsberg, Tharsis, and Draa Sfar were in the range 7?C13, 8?C14, and 19?C27?cm/ka, respectively. Continuous sedimentation of black shale favored the isolation of the massive sulfides and organic material from bottom waters and hence favored their preservation. Accordingly, the relationships between black shales and massive sulfides are considered to be casual. Nevertheless, the tectono-sedimentary evolution of each basin controlled the deposition of both black shales and massive sulfides and the parameters that favored their coeval deposition.     

三水盆地古近系(土布)心组黑色页岩中黄铁矿的形成及其控制因素

黄铁矿是富有机质沉积的特征矿物。根据TOC/S、TOC/DOP、S/Fe关系以及S TOC Fe多重线性回归分析结果对三水盆地古近系〖HT5”,6”〗土〖KG-*3〗布〖HT5”SS〗心组红岗段黑色页岩中沉积黄铁矿的形成及其控制因素进行了分析。土布心组红岗段黑色页岩的黄铁矿有成岩黄铁矿和同生黄铁矿两种成因组分。红岗段下部（亚段A）有机碳含量普遍较低,底部水体以弱氧化条件为主,硫酸盐还原作用发生于沉积物/水界面以下,黄铁矿为成岩成因,其形成主要受有机质的限制。红岗段中上部（亚段B和C）的沉积条件变化频繁,其有机碳含量变化幅度大。富有机质（TOC>4%）岩层形成于缺氧的底部水体条件下。水体中可含H2S,碎屑铁矿物在埋藏之前即与之在水体中反应形成同生黄铁矿。这一过程不受有机质的限制,而是受活性铁与H2S接触时间的限制。同时,由于大量淡水输入导致硫酸盐浓度的降低,从而对硫化物形成有一定的限制作用。对于低有机质（TOC<4%）样品,黄铁矿由同生和成岩组分组成。其中以成岩黄铁矿为主,其形成过程主要受有机质限制,而同生黄铁矿受铁矿物与H2S接触时间的限制。     

A critical look at iron paleoredox proxies: New insights from modern euxinic marine basins

Enrichments in reactive iron occur under euxinic marine conditions, that is, where dissolved sulfide is present in the water column. These enrichments result primarily from the export of remobilized iron from the oxic shelf, which is scavenged from the euxinic water column during syngenetic pyrite formation and deposited in the underlying sediments. Strongly elevated ratios of highly reactive iron to total iron (Fe_HR/Fe_T) and total iron to aluminum (Fe_T/Al) and high degrees of pyritization (DOP) are each products of this enrichment process. These paleoredox proxies are among the most faithful recorders of ancient euxinia.Contrary to previous arguments, iron enrichment is decoupled from biogenic sediment inputs, but it does appear to be a uniquely euxinic phenomenon. In other words, we can rule out a major contribution from preferential physical transport of Fe_HR-rich detrital sediment to the deep basin, which could also operate under oxic conditions. Furthermore, enrichment via the shuttling of iron remobilized from oxic shelves appears to be limited by inefficient transport and trapping processes in deep oxic basins. Elevated Fe_T/Al ratios in the euxinic sediments also cannot be a product of internal enhancement of the reactivity of the detrital iron pool without net Fe_HR addition. These conclusions are supported by observations in the modern Black Sea, Orca Basin, and Effingham Inlet.Fe_T/Al ratios are unambiguous recorders of paleoredox even in sediments that have experienced high degrees of metamorphic alteration. However, this study suggests that high siliciclastic accumulation rates can swamp the enrichment mechanism, resulting in only intermediate DOP values for euxinic sediments and Fe_T/Al ratios that mimic the oxic shelf. Such dilution effects are well expressed in Black Sea basinal turbidites and rapidly accumulating muds on euxinic basin margins. Under conditions of persistent euxinia, varying extents of Fe_HR enrichment can illuminate spatial and temporal gradients in siliciclastic sedimentation. The magnitude of enrichment is a function of the source (shelf) to sink (ocean basin) areal ratio, suggesting that iron proxies can also record ocean-scale paleoenvironmental properties through muted enrichments at times of very widespread euxinia. For the first time, manganese data are interpreted in light of the redox shuttle model. As for the iron data, the Black Sea, Orca Basin, and Effingham Inlet show enrichments in total manganese in the deep euxinic basin, suggesting export from the suboxic porewaters of the oxic shelf and scavenging and burial in the basin. The Black Sea data reveal iron and manganese enrichment across the broad, deep euxinic basin, suggesting efficient lateral transport and deep-water mixing tied to the physical properties of the water column.