Authigenic Precipitation Research Articles

Late Ordovician ironstone and its relation to ocean redox instability, climate and glaciation

AbstractThe Upper Ordovician (Katian) Neda Formation, a phosphatic ironstone, records a widespread but short‐lived shift to ferruginous waters across a vast epicontinental area. Lithofacies and stratigraphic reappraisal indicate that Neda ironstone deposition occurred on a storm‐dominated ramp when coastal upwelling emplaced eutrophic ferruginous waters that mixed with oxygenated surface water. This stimulated primary productivity and precipitated Fe‐(oxyhydr)oxides in the water column that formed phosphorous and iron‐rich mud. Remobilization of iron beneath the seafloor led to the syndepositional authigenic precipitation of P and Fe minerals in the sediment, preferentially coating grains and forming granular ironstone in the top few decimetres of the sediment. The top of the Neda Formation is a pronounced unconformity punctuated by laterite that formed as sea level fell during the Hirnantian Glaciation. The transition from oxygenated to ferruginous conditions that led to ironstone deposition is interpreted to have been caused by an increase in the equator‐to‐pole temperature gradient and concomitant reorganization of thermohaline circulation during the Katian. This intensified upwelling off the Laurentian margin with upwelled waters transported into the midcontinent where ironstone accumulated through the Sebree Trough. The Neda ironstone's deposition coincident with, and potentially caused by, the same drivers as global oceanographic and biotic change during the Late Ordovician both adds greater insight into the major changes in the oceans preceding the Late Ordovician Mass Extinction and Hirnantian Glaciation, and also furthers an emerging model tying ironstone deposition throughout the Phanerozoic to major Earth system events.

A chemical weathering and paleoclimatic reconstruction of the early Cambrian environment of the Wyoming Craton from the Wind River Canyon, WY paleosol on the Great Unconformity

The weathering of continental rocks during the Precambrian-Cambrian transition contributed to the carving of the Great Unconformity and coincided with increasing biodiversity during the early Cambrian world. We use an early Cambrian paleosol exposed in the Wind River Canyon of Wyoming where it is buried by the overlying mid-Cambrian Flathead Sandstone to assess the paleo-weathering environment and constrain paleoclimate conditions on the Wyoming Craton in early Cambrian. The ∼ 3 m thick paleo-weathering profile is characterized by complete plagioclase weathering and moderate alkali feldspar weathering with maximum CIA (Chemical Index of Alteration) and PIA (Plagioclase Index of Alteration) reaching 69 and 98, respectively. Higher K concentrations toward the upper part of the profile associated with the authigenic precipitation of sericite clay minerals suggest that modest post-burial K-metasomatism may have overprinted the weathering profile. Our results suggest that the saprolite was developed under a mean annual paleoprecipitation and paleotemperature comparable to modern temperate environments. Geochemical characteristics of this paleosol formed during the final chapter of the Precambrian suggest intensive chemical weathering in the presence of elevated atmospheric CO2 concentration, forming a saprolite profile similar to other, previously described Cambrian and Precambrian-aged paleosols from the North American Craton.

A process-based geochemical framework for carbonate sediments during marine diagenesis

A significant proportion of marine calcium carbonate sediments are comprised of metastable minerals that are susceptible to diagenetic alterations during burial. These reactions can reset the geochemical signature of sediments and pore fluids and influence elemental cycling in the ocean. However, the timing and mechanisms by which these reactions take place are poorly constrained. This study uses cores drilled on the slope of the Great Bahama Bank to provide quantitative constraints on important diagenetic reactions; namely respiration-driven dissolution, authigenic carbonate mineral formation, and conversion of aragonite to low Mg calcite (LMC). We perform detailed mineralogical characterization using newly acquired, high resolution X-ray diffraction (XRD) data and over 1000 reanalyzed XRD scans, characterize sediments texturally and elementally using electron microprobe data, calculate pore fluid saturation state with respect to aragonite using a Pitzer ion interaction approach, and use pore fluid chemistry and experimental distribution coefficients to predict authigenic carbonate compositions. These data suggest that aerobic organic matter oxidation, enabled by the advection of oxygenated seawater throughout the upper ∼ 30 m interval of sediment, causes undersaturation and thus dissolution of biogenic high Mg calcite (HMC) and aragonite. Deeper, anaerobic organic matter oxidation takes over and causes supersaturation, promoting authigenic precipitation in the form of HMC, which in turn decreases pore fluid Mg/Ca and promotes aragonite conversion to LMC. One novel aspect of this study is the identification of three types of calcites using the newly acquired XRD and electron microprobe data. Based on their unique crystallographic characteristics and chemical compositions, these types of calcites are interpreted to represent pelagic biogenic LMC, bank-derived biogenic HMC, and authigenic HMC precipitated from pore fluids. Notably, the composition of the authigenic calcite matches that predicted to precipitate from pore fluids using an empirical Mg partition coefficient in calcite. Calcites that form authigenically and from aragonite via replacement are suggested to recrystallize with burial as evidenced by the decrease in Mg content and micro-strain. This process-based geochemical framework assigns diagenetic processes to a specific depth window within the sediment column and paves the way for a mechanistic understanding of carbonate diagenesis, one that is rooted in thermodynamic and kinetic bases.

Manganese‐Rich Sandstones as an Indicator of Ancient Oxic Lake Water Conditions in Gale Crater, Mars

AbstractManganese has been observed on Mars by the NASA Curiosity rover in a variety of contexts and is an important indicator of redox processes in hydrologic systems on Earth. Within the Murray formation, an ancient primarily fine‐grained lacustrine sedimentary deposit in Gale crater, Mars, have observed up to 45× enrichment in manganese and up to 1.5× enrichment in iron within coarser grained bedrock targets compared to the mean Murray sediment composition. This enrichment in manganese coincides with the transition between two stratigraphic units within the Murray: Sutton Island, interpreted as a lake margin environment, and Blunts Point, interpreted as a lake environment. On Earth, lacustrine environments are common locations of manganese precipitation due to highly oxidizing conditions in the lakes. Here, we explore three mechanisms for ferromanganese oxide precipitation at this location: authigenic precipitation from lake water along a lake shore, authigenic precipitation from reduced groundwater discharging through porous sands along a lake shore, and early diagenetic precipitation from groundwater through porous sands. All three scenarios require highly oxidizing conditions and we discuss oxidants that may be responsible for the oxidation and precipitation of manganese oxides. This work has important implications for the habitability of Mars to microbes that could have used Mn redox reactions, owing to its multiple redox states, as an energy source for metabolism.

Quantifying the characteristics of magnetic oil–water contacts in mature hydrocarbon reservoirs and their capacity for understanding hydrocarbon remigration

SUMMARY Increasing magnetization within mature hydrocarbon reservoirs provides a new technique in identifying oil–water contacts (OWCs) in cored wells with the potential to assess yield thereby reducing the need for further exploration. Authigenic precipitation of magnetic minerals at OWCs may also help locate palaeocontacts (PCs), where structural changes to the petroleum system have caused hydrocarbon remigration. This study determines the magnetic characteristics of magnetic enhancements at OWCs and possibly PCs in silliclastic and carbonate reservoirs at the Wytch Farm oil field, Wessex Basin, UK. Increases in saturation magnetization and susceptibility are observed at the OWC in 11 of the 12 analysed cored reservoirs owing to the increased presence of magnetite and vivianite. Geochemical analysis and shallow reservoirs suggest biogenic and inorganic mineral precipitation is extensive at the OWC depending on iron, sulphur and phosphorus availability. Similar magnetic characteristics have been observed in magnetic enhancements above the OWC in numerous wells which may represent OWCs before a basin-wide easterly tilt caused hydrocarbon remigration in the Cenozoic. Multiple magnetic enhancements above the OWC in westerly onshore wells, suggest this remigration may have occurred as numerous phases.

Palygorskite in Indian tropical soils: a clay mineral of pedogenic, geogenic, or climate-induced origin – a mechanistic review

Abstract Most clay minerals are characterized by a platy morphology. By contrast, palygorskite has a fibrous morphology and is structurally distinct from the typical 1:1 and 2:1 layer structures. Diverse opinions exist on the origin of palygorskite in soils. Many authors suggest that palygorskite forms after smectite. Others favor its authigenesis during pedogenic processes or its inheritance from the parent material. This review provides a critical synthesis on the origin of palygorskite in the semi-arid-tropical (SAT) Vertisols and arid calcic soils of the Thar Desert of India. It also highlights the specific genetic pathway for the presence of palygorskite in the soils. The ubiquitous association of smectite with palygorskite is inadequate to explain the formation at the expense of smectite, because at pH 8.2 and above the smectite structure is subjected to dissolution to create soluble Si and Al, and the recrystallization of the soluble Si and Al to form palygorskite may not be possible in the Vertisols of the Indian SAT environment. Thus, mildly to moderate alkaline pedochemcial environments of the SAT Vertisols do not favor authigenic precipitation of the palygorskite in such soils. This review shows that the presence of palygorskite in the SAT Vertisols is due to its inheritance from the exhumed inter-trappean beds, infra-trappean beds, and bole beds. This view on the genesis of the palygorskite is also justified by its presence in weakly developed calcic soils of the Thar Desert as detrital flux from the adjoining marine sedimentary rocks.

Methanogen-mediated dolomite precipitation in an early Permian lake in northwestern China

Abstract Microbes are known to mediate dolomite precipitation in laboratory experiments; however, the linkage of specific microbes to ancient dolomites remains poorly constrained due to scarce diagnostic biogeochemical signatures and mineralized microbial relics in the rock record. Here, we report the occurrence of methanogen-mediated dolomite in the Lower Permian lacustrine Lucaogou Formation in northwestern China. The clumped isotope (Δ47) temperature provides direct evidence of a low-temperature origin (typically <40 °C). The extremely positive δ26MgDSM3 (up to +0.44‰) and δ13CVPDB (up to +19‰) values in the dolomite indicate authigenic precipitation in methanogenic lake sediments. Micron-sized spheroidal bodies and filamentous and sheetlike structures are interpreted as mineralized coccoid methanogenic archaea and extracellular polymeric substances (EPSs), respectively. Dolomite nanoglobules (primarily 40–100 nm in diameter) are interpreted as mineralized viruses attached to the archaea and EPSs and between the cells. A combination of geochemical and microscale evidence confirms the microbial origin of the dolomite induced by methanogens and their associated bacteriophages. Furthermore, dolomite nanoglobules initially nucleated on the surfaces of methanogen cells, EPSs, and viruses and then merged into larger aggregates. The formation of microbial dolomite is characterized by a metabolic incubation, heterogeneous nucleation, and aggregative growth pathway. These findings provide valuable clues to decipher the biosignatures of these particular ancient dolomites.

An investigation into the characteristics of reactive silicon pools of coastal marine sediments

Many studies use traditional alkaline leach digestions to quantify biogenic silica (bSiO2) in sediments. Studies from river-plume systems demonstrate this methodology precludes information on other operational Si reactive pools, missing potentially valuable information on early diagenetic products. We build on previous studies by broadly examining the reactive Si pools in Mississippi River plume-influenced coastal sediments. The magnitude of Si released among operational pools across the region revealed large spatial variations in the conservation of detrital materials. Contrary to other analyses using different approaches, we suggest the availability of detrital sourced components (i.e. Al, Fe and Mn) may be a factor limiting authigenesis and resulting Si storage capacity in coastal sediments by this chemical mechanism. Overall, these data suggest authigenic precipitates may be forming as a function of bSiO2 accumulation, thereby implying that bSiO2 acts as the primary substrate for formation of authigenic products. The amount of authigenic Si products, relative to bSiO2, appears to be larger in this region than other systems studied to date, and this may be due to regional eutrophication which has led to an increase in sediment bSiO2 compounding the substrate available for authigenic precipitates. Thus, regions which have all experienced increased sediment bSiO2 accumulation coincident with eutrophication, may be primed for substantial authigenic product formation and increased Si sequestration.

Biogeochemical In Situ Barriers in the Aquifers near Uranium Sludge Storages

The long-term operation of uranium sludge storages causes serious problems: it contaminates the neighboring aquifers with dangerous substances (uranium, nitrate, ammonium, and sulfate). To purify the aquifers can be costly and time-consuming; therefore, it is important to use the potential of in situ conditions, e.g., the aboriginal microflora and its ability to biologically remediate water reservoirs. In this work, we study the geological, geochemical, and microbiological characteristics of groundwater contaminated by uranium sludge storages resulting from the production cycles of four Russian chemical plants. All of the sites under consideration were extremely contaminated with nitrate (up to 15 g/L); in each case, we used denitrifying bacteria as a dominant group of microorganisms for purification. Our laboratory studies showed that microbial stimulation of water samples by milk whey promotes O2 and nitrate removal; this, in turn, started the cycle of anaerobic processes of authigenic precipitation caused by the reduction of iron and sulfate in the system. Thus, a mineral geochemical barrier preventing uranium immobilization formed. As a result, the uranium of the liquid phase decreased about 92–98% after 3–6 months (decomposition time depends on the nitrate concentration in the groundwater probe). The resulting amorphous biogenic phases contain sulfur, iron, phosphorus, and uranium.

Mineralogical characteristics and genesis mechanism of glauconite in the meso-neoproterozoic and cambrian strata in north China

Glauconite is a special mineral indicator of sedimentary facies and is widely developed in the Meso-Neoproterozoic and Cambrian strata in North China. A systematic study on the occurrence characteristics and genetic mechanism of glauconite is of great geological significance for the reconstruction of paleogeographic environments in North China. The results show that glauconites are well-developed in the Meso-Neoproterozoic Tieling, Xiamaling, Changlongshan, and Jingeryu formations and the Cambrian Xuzhuang Formation, with granular, cemented, fecal pellet, and halo - rimmed forms. The oxide components of glauconite mainly include K2O, Si2O, Al2O3, TFeO, MgO, etc. Moreover, the study area mainly developed low mature to mature glauconite based on the analysis of K2O content, color, SEM energy spectrum and XRD, and the maturity of glauconite in the Longshan and Jiguanshan areas is lower than that in the Jizhou area. The K, Fe, Mg, and Al ions source of glauconite is sufficient, and the parent rocks also provide sufficient substrate favorable for the formation of glauconite in the study area. Glauconites with different substrate components indicate different formation mechanisms. The glauconite with quartz and feldspar grains as the substrate is of a pseudomorphic replacement genesis, while the glauconite with biotite as the substrate is of a layer lattice theory genesis, and cemented glauconite is of an authigenic precipitation genesis. Furthermore, the study revealed that glauconites with fecal pellets and stromatolites as substrate are a combination of biogenesis and verdissement or authigenic precipitation, and halo - rimmed glauconite is of a combination of pseudomorphic replacement and authigenic precipitation. According to the findings, the evolution of K2O-TFeO in Precambrian glauconite is not only controlled by a specific substrate, but also by specific chemical conditions. Because of the differences in composition of solubility of the substrate, the pH conditions of the pore water required for glauconization are different. The sedimentary environment and substrate type affect the form and genetic mechanism of glauconite, but the varied pH conditions suitable for the substrate and the sub-oxide environment near the Fe oxidation-reduction interface are the necessary conditions for glauconization. As a result, glauconite can be used as a signature mineral for the palaeo-redox facies. The study provides insights into the formation mechanism of different glauconite in the process of sedimentation or diagenesis, and the scientific basis for the restoration of diagenesis conditions and the paleogeographic environment in North China.

Authigenic Formation of Clay Minerals in the Abyssal North Pacific

AbstractPresent estimates of the biogeochemical cycles of calcium, strontium, and potassium in the ocean reveal large imbalances between known input and output fluxes. Using pore fluid, incubation, and solid sediment data from North Pacific multi‐corer cores we show that, contrary to the common paradigm, the top centimeters of abyssal sediments can be an active site of authigenic precipitation of clay minerals. In this region, clay authigenesis is the dominant sink for potassium and strontium and consumes nearly all calcium released from benthic dissolution of calcium carbonates. These observations support the idea that clay authigenesis occurring over broad regions of the world ocean may be a major buffer for ocean chemistry on the time scale of the ocean overturning circulation, and key to the long‐term stability of Earth's climate.

Pore-Water Geochemical Gradients of Sulfate, Calcium, Magnesium, and Iodide Correlated With Underlying Gas Hydrate Potential: A Case Study of the Shenhu Area, South China Sea

Geochemical profiles in pore water of marine sediments have been considered as, important indicators of gas hydrate occurrence. In the gas hydrate area around the world, the decrease of sulfate, calcium, and magnesium concentrations with depth mainly results directly or indirectly from the anaerobic oxidation of methane (AOM). The ubiquitous abnormally high concentration gradients of iodide in the research area reflect the large methane-generating potential of the area. Thus, we explore the feasibility of using gradients of sulfate, iodide, and authigenic carbonate precipitation as indicators for gas hydrate in the regional exploration of gas hydrate formation. We test the criterion in the gas hydrate zone in the South China Sea (SCS) for the recognition of gas hydrate by using the gradients of sulfate, calcium plus magnesium, and iodide. Contour maps of pore-water gradients from expeditions in the study area are used to correlate the key gradients to underlying gas hydrate occurrence. The results show that the largest potential gas hydrate indicated by the contour maps of the indicators are well consistent with the discovery of GMGS1 and GMGS3 expedition. It implies the possible applicability of this geochemical method in gas hydrate exploration. Also, we identify a promising area in the South China Sea for future gas hydrate investigations. It is the first collective application of the gradients of sulfate, calcium plus magnesium, and iodide to a gas hydrate terrane, especially to a large area of the SCS. We believe that the result of this research will benefit the future exploration of gas hydrate and will arouse a lot of interest from other researchers.

Application of Chemical Sequence Stratigraphy to the Prediction of Shale Gas Sweet Spots in the Wufeng and Lower Longmaxi Formations within the Upper Yangtze Region

Effective shale gas exploration is hindered by the need for obtaining high-resolution correlations between shale strata and the need for classifying shale facies. To address these issues, chemostratigraphy, sequence stratigraphy, and shale gas geology methods were integrated to develop a new method known as “chemical sequence stratigraphy,” which was successfully applied to the Wufeng–Lower Longmaxi Formations in the upper Yangtze region. Well Huadi 1 was used as a case study, and detailed data were acquired. Multivariate statistical analyses were applied to three defined indices having different genetic significance, namely: terrigenous input intensity (TII), authigenic precipitation intensity (API), and organic matter adsorption and reduction intensity (OARI). By analyzing the trends of these three indices, the Wufeng–Lower Longmaxi Formations were divided into five fourth-order chemical sequences (from bottom to top): LCW, MCL1-1, MCL1-2, MCL1-3, and MCL1-4. The geochemical facies were named and classified using the chemical sequence stratigraphic framework. The enrichment factor (EF) transformation of elements was conducted to determine whether an element is rich or deficient. The results showed that the favorable geochemical facies in the well were EF-Al deficient, EF-Ca rich, and EF-V rich. The organic matter content and rock brittle strength were then used as chemical parameters, and it was predicted that the LCW and MCL1-1 chemical sequences most likely comprised shale gas sweet spots. This conclusion is consistent with the drilling results and indicates that our proposed method is effective and reliable. This method is further applied to the Changning Shuanghe section, the Shizhu Liutang section, and sections in the Xindi 1 well in the upper Yangtze region. The comparative study of these four sections showed that LCW and MCL1-1 are the key chemical sequences for shale gas exploration and development in the Wufeng–Lower Longmaxi Formations within the Upper Yangtze region.

Glauconite authigenesis during the warm climatic events of Paleogene: Case studies from shallow marine sections of Western India

Glauconite forms abundantly within the Paleogene warm climatic intervals. However, the role of warm climate on glauconitization is yet to be explored. Glauconitic shales are ubiquitous in transgressive shallow marine deposits of Cambay, Kutch, Jaisalmer, and Barmer basins at the western margin of India. Although the glauconite is most abundant in upper Paleocene-lower Eocene sedimentary deposits in these basins, it also occurs within the middle Eocene and upper Oligocene successions. High-resolution biostratigraphic data, integrated with carbon isotope signatures, demarcate the Paleogene hyperthermal events and reveal an exceptionally high abundance of glauconite corresponding to the warming events. Glauconite occurs as pellets and infillings within the pores of bioclasts, although they differ in chemical composition. The glauconite pellets show variable K2O content, ranging from 4 to 8 wt% with moderately high Fe2O3 (>20 wt%), representing the entire maturation spectrum. Glauconite marking the Paleocene-Eocene transitional sediments is distinctive by the high Al2O3 content (>10 wt%), while those within the middle Eocene and Late Oligocene show considerably low Al2O3 (<8 wt%). The glauconitic shales often show minor bioturbation and are rich in rectilinear benthic foraminifera, indicative of the oxygen-depleted bottom-water conditions. The unusual composition of upper Paleocene-lower Eocene glauconites relates to their formation within kaolinite substrates during the extremely warm climatic interval. Contrary to this, during the middle Eocene and Late Oligocene, the waning phase of Paleogene warm climatic conditions, glauconite formed by the initial authigenic precipitation of Fe-smectite/ Fe-Al-smectite and its subsequent maturation. The warm climatic condition enhanced the precipitation and runoff, which supplied enhanced nutrients including K, Fe, Al, Si, and Mg into the shallow marine environment, facilitating prolific organic growth and enriching the seawater with cations. The decomposition of organic matter might have resulted in an oxygen-depleted bottom water condition, which was suitable for the mobility and fixation of iron into the glauconite structure. The glauconite formed abundantly during hyperthermal events because of the convergence of favorable factors such as rapid transgression, reduced sedimentation rate, warm seawater condition, enhanced continental weathering, and enhanced supply of nutrients favoring dysoxic shallow shelves. However, rapid and extreme hyperthermal events such as PETM inhibits glauconite formation.

Research on Diagenetic Evolution and Hydrocarbon Accumulation Periods of Chang 8 Reservoir in Zhenjing Area of Ordos Basin

The Mesozoic Chang 8 Section in the Zhenjing area is a typical low permeability-tight sand reservoir and is regarded as the most important set of paybeds in the study area. Guided by the principles of basic geological theory, the diagenetic evolution process and hydrocarbon accumulation periods of the Chang 8 reservoir in the study area were determined through various techniques. More specifically, core observation, scanning electron microscopy (SEM), X-ray diffraction (XRD), and vitrinite reflectance experiments were performed in combination with systematic studies on rock pyrolysis and the thermal evolutionary history of basins, the illite-dating method, and so on. The Chang 8 reservoir is dominated by feldspar lithic and lithic feldspar sandstones. Quartz, feldspar, and lithic fragments are the major clastic constituents. In clay minerals, the chlorite content is the highest, followed by illite/smectite formation and kaolinite, while the illite content is the lowest. The major diagenesis effect of the Chang 8 reservoir includes compaction, cementation, dissolution, metasomatism, and rupturing. The assumed diagenetic sequence is the following: mechanical composition → early sedimentation of chlorite clay mineral membrane → early cementation of sparry calcite → authigenic kaolinite precipitation → secondary production and amplification of quartz → dissolution of carbonate cement → dissolution of feldspar → late cementation of minerals such as ferrocalcite. Now, the study area is in Stage A in the middle diagenetic period. Through the inclusion of temperature measurements, in conjunction with illite dating and thermal evolutionary history analysis technology in basins, the Chang 8 reservoir of this study was determined as the phase-I continuous accumulation process and the reservoir formation epoch was 105~125 Ma, which was assigned to the Middle Early Cretaceous Epoch.

Sedimentary pyrite in carbonaceous shales of the Mamfe Cretaceous basin, SW Cameroon: Morphologies, composition, pyrite framboid size frequency distribution, and formation pathways

Sedimentary pyrite morphologies, spatial distribution relative to their spectral compositions were studied in carbonaceous shales (Fc) of the Mamfe Cretaceous Basin (MCB) in SW Cameroon. Lithofacies analysis of Fc reveals laminated (Fcl), oolitic (Fco), marlstone (Fcm), algal mat (Fca) and pelletoidal (Fcp) pyritized microfacies. Results of petrographic, scanning electron microscope (SEM), backscattered electron imaging (BSEI) microscopic study coupled with energy dispersive atomic x-ray (EDAX) reveal micron-sized, densely packed framboidal and non-framboidal morphologies. Diameter measurement of 519 framboids exhibit a monomodal distribution with a narrow size-range of 0.50–14.0 μm and mean diameter range of 2.0–6.5 μm, accounting for 70.4–95.3% of all framboids measured. Non-framboidal morphologies include discrete euhedral, etched or zoned crystals, including core-sheath cell-like microstructures. Euhedral morphologies show a close spatial relationship with syngenetic mineral suite (calcite, microdolomite, chalcedony and evaporites). Meanwhile, degraded morphologies with moldic pores are mineralized in Zn and appear recrystallized, etched, in-filled by silica and clays or rimmed by ferroan carbonates and oxides. A three transitory pathways model suggests that morphological and compositional variations in pyrites are linked to authigenic precipitation, co-precipitation of Fe and Zn which depict sulphate reduction conditions under saline and carbonate lacustrine settings, followed by alteration processes. Pyritized core-sheath replicas in microstructures reminiscent of cells are indicative of a microbial paleoecosystem and could play an intrinsic role in biogeochemical recycling of S and Fe and the timing of fossilization which precedes the decay of microorganisms. Rimmed pyrite morphologies are potential indicators of local redox paleoenvironmental settings.

Stable tungsten isotope systematics on the Earth’s surface

Stable tungsten isotopes (δ186/184W) show great potential for tracing the cycling of materials among geological reservoirs. This work investigates the behavior of stable W isotopes on the Earth’s surface by presenting new δ186/184W data of granitic rocks, loess, river water, fluvial sediments, regolith, estuarine seawater, and marine sediments. The investigated granites document significant W isotopic heterogeneity (−0.08 to 0.16‰) of the upper continental crust (UCC). In contrast, the eolian loess sourced from the vast crustal surface shows a high degree of homogeneity with an average δ186/184W of 0.01 ± 0.01‰ (mean ± 2 × standard error). The δ186/184W of river water (0.17 to 0.71‰) is significantly higher than that of the corresponding bedrock, which is likely to be caused by preferential uptake of light W isotopes by the authigenic precipitates. The δ186/184W and W concentration of river water also show a correlation with the lithological sources of weathering solute, the extrapolation of which results in a global riverine W flux of 17 × 106 mol yr−1 and a W flux-weighted average δ186/184W of 0.37‰. Data from the Yangtze estuary likely suggest a contribution of benthic W reflux from estuarine sediments to the ocean. However, benthic reflux, as well as hydrothermal W inputs, is insignificant compared to riverine input. The recalculated oceanic residence time of W is ∼4 kyrs, which is much shorter than previously thought. Sinks of W in the ocean are mainly associated with non-euxinic sediments, where the enrichment of W correlates with that of Mo. Constraints from the oceanic Mo budget give a W sink of 19 × 106 mol yr−1 in non-euxinic sediments, which nearly balances the riverine input. A simple box model shows that the steady-state δ186/184W of seawater (which can be achieved on time scales more than several times the residence time of W in seawater) is mainly controlled by the δ186/184W of riverine input and the partition of W into different sinks with varying fractionation factors.

ARTIST’S IRON-BASED NATURAL EARTH PIGMENTS OF TUSCANY (MONTE AMIATA VOLCANO, ITALY)

ABSTRACT Among the artist’s iron-based natural earth pigments, the so-called terra di Siena (raw sienna), terra di Siena bruciata (burnt sienna) and terra d’ombra (umber) have been among the yellow-brown and reddish-brown earth pigments most widely used by Italian and European painters since the Renaissance. We present the history of discovery, designation, and production of these famous pigments, their geological, lithological, and geochemical characterization, and the recognition of their genesis and places of origin, based on new geological field surveys, and on the critical analysis of textual documents and rock sample collections assembled during the eighteenth and nineteenth centuries. In particular, the terra di Siena earth pigment exclusively originated at, and was extracted from, late Pleistocene paleo-lake basins surrounding the Monte Amiata volcano. This earth pigment consists of primarily lacustrine sediments composed of hydrated iron oxide (limonite/goethite) produced by biochemical authigenic precipitation from fresh waters rich in metal solutes.

The paleo-lacustrine diatomaceous deposits of Monte Amiata volcano (Tuscany, Italy) and the Ezio Tongiorgi paleontological collection in the Museum of Natural History of the University of Pisa

At the foothillof Monte Amiata volcano (southern Tuscany, Italy), small extinct lake basins of late Pleistocene age are documented. These lake basins were characterized by the deposition of two very different types of sediment: a) derived from the authigenic precipitation of iron oxides (goethite) and exploited as earth pigments; b) biogenic siliceous sediment composed of fossil diatoms and named diatomaceous earth or diatomite. The lacustrine sediments of Mount Amiata volcano were widely exploited for various applications since ancient times. Literary documents begin in the 16th century, with the descriptions of Cesalpino, Gesner, Agricola, and Imperato. Specific references to the diatomites of Monte Amiata are quoted in the 17th century by Boccone and Bonanno. The quarrying activity was described by Micheli in 1733. During the 18th and 19th centuries, the diatomaceous earths of Monte Amiata are part of the important geological collections of Micheli, Targioni Tozzetti, Baldassarri, Campani, and Tommi. A particular significance has the collection of botanic and ichthyologic fossils collected by Ezio Tongiorgi, and now preserved in the Museum of Natural History of the University of Pisa sited at the Charterhouse of Pisa in the Calci village. These paleontological samples preserve the biological and physical testimonies of the environmental and climatic changes of the late Pleistocene and are now particularly valuable because they are the only remaining evidence of the diatomaceous lacustrine deposits of the paleo-lakes of Monte Amiata. For these reasons, they represent geological materials with a fundamental cultural value.

Reconstructing 1200 years of Hydroclimate Variability in the Southern Margins of the Arabian Desert: Insights From a Paleo-Lake in Southern Yemen

The climate of the Arabian Desert is not well documented during the past two millennia due to the scarcity of continuous and well-dated terrestrial archives in the region. Reliable interpretation from the climatic records from this region are pivotal for identifying periodicities of inter-annual to multi-decadal variability and trends driven by shifts in position of the Intertropical Convergence Zone (ITCZ) and the strength of the monsoons. A high-resolution multiproxy approach is presented for a ∼3.3 m composite core, GBW, from a karst lake located in Ghayl ba Wazir, southern Yemen. Sedimentary proxies, including particle size distribution, coupled with magnetic susceptibility (MS) and geochemistry (XRF), provide a comprehensive picture of sediment depositional changes that may be linked to climate and environmental variability over the southern Arabian Desert. The chronology of the GBW core is provided by five radiocarbon (14C) dates from terrestrial macrofossils (wood and twigs) extracted from sediment samples and indicates the core extends to ∼900 CE. Our data indicates generally wetter conditions from 930 to 1400 CE corresponding to the “Medieval climate anomaly (MCA)” followed by arid phases during 1,410–1700 CE coinciding with the “Little Ice Age (LIA)”. Evidence for a drier LIA include high authigenic calcium precipitation [Ca/(Al + Fe + Ti)], decreased TOC/TIC values, and gypsum precipitation, whereas the wetter MCA is characterized by higher detrital element ratios (Ti/Al, K/Al, Rb/Sr), and increased TOC/TIC and deposition of finer sediments (EM1). Furthermore, end-member mixing analyses (EMMA) derived from the grain-size distribution (EM2 and EM3) corroborates the deposition of coarser silt sediment through wind erosion and production of carbonate sand during the LIA concurrently with low lake levels under generally dry conditions. Aridity during the LIA is consistent with evidence and theory for weakened boreal summer monsoons during intervals of northern hemisphere cooling.