Ancient Sedimentary Environments Research Articles

Sedimentary Environment Reconstruction and Organic Matter Enrichment Mechanisms in Various Lithofacies of the Lacustrine Shale: A Case Study of the Da’anzhai Member, Central Sichuan Basin, China

Jurassic lacustrine shale in the Sichuan Basin is a focal exploration area in China, while the pronounced heterogeneity presents challenges, necessitating detailed research as a prerequisite. This study aims at the Jurassic Ziliujing Formation Da’anzhai shale in the central Sichuan Basin, systematically characterizing its geological features. Employing geochemical methods, we reconstructed the paleo-sedimentary environments and explored the mechanisms behind the organic matter enrichment. The findings reveal that the Da’anzhai shale exhibits three lithofacies: organic-rich argillaceous shale (ORA), organic-poor argillaceous shale (OPA), and organic-rich mixed shale (ORM). The sedimentary period was marked by a warm and humid climate, predominantly depositing in anoxic environments with freshwater to brackish conditions. The watershed areas that are provenance sources for the shale clastics are experiencing strong weathering. Significant differences in the sedimentary environments of various lithofacies’ shale were observed. Redox conditions and paleoclimate were identified as the primary factors controlling organic matter enrichment in the Da’anzhai shale of the study area. Salinity also played a role in organic matter enrichment, while terrigenous debris influx and paleo-productivity did not exert a significant controlling effect on organic matter enrichment. Utilizing the reconstructed ancient sedimentary environments, we developed sedimentary models for different lithofacies’ shale, contributing to a deeper understanding of lithofacies’ diversity and the mechanisms governing organic matter enrichment in lacustrine environments. This study provides new information for further understanding the response mechanism of lacustrine shales to the Toarcian Oceanic Anoxic Event in the Early Jurassic from the perspective of elemental geochemistry.

Biotic and abiotic processes in Ediacaran spheroid formation

Organic-rich shales from the uppermost Doushantuo Fm. (South China) record one of the most negative carbonate carbon isotopic excursions in Earth’s history, known as the Shuram excursion, and contain meter to micro-size spheroids. In this study, we use Raman and energy dispersive spectroscopy to identify and describe the most common diagenetic spheroids to refine our understanding of the profound perturbations of the carbon cycle and the evolution of pore fluid chemistry imprinted in the sedimentary Precambrian record, especially in the late Ediacaran. The presence of 13C-depleted carbonate concretions or organic matter (OM) enclosed by lenticular dolomitic structures within the host shale unit suggests OM remineralisation and anaerobic oxidation, resulting in authigenic carbonate precipitation during the earliest stages of sediment diagenesis. Other mineralogical features, however, point to high levels of primary production, such as apatite bands that host spheroidal microfossils with highly fluorescent quartz and OM within abiotic concretions. These observations highlight the importance of considering co-occurring biotic and abiotic processes in explaining the formation of diagenetic spheroids in ancient sedimentary environments. From an astrobiology perspective, the interplay of biotic and abiotic processes reflects the complexity of early life systems and the environments that may exist on other terrestrial planets. Understanding the signatures of biotic and abiotic interactions in the Doushantuo Fm. is crucial for identifying potential biosignatures in extraterrestrial materials, thereby enhancing our understanding of life’s universality and adaptability in diverse and extreme environments.

SERRA DA MIABA, POVOADO CURITUBA E VALE DOS MESTRES (SE), COMO EVIDÊNCIAS GEOLÓGICAS DE PERÍODOS GLACIAIS E INTERGLACIAIS DO PASSADO SERGIPANO

A Serra da Miaba, localizada na região centro-oeste do estado de Sergipe e o Vale dos Mestres, no extremo noroeste, compõem importantes pontos turísticos de Sergipe. Cada qual pode ser interpretado como a memória do ambiente de formação de rochas, de onde é possível extrair informações sobre climas glaciais e interglaciais na história geológica não só do Estado, mas de escala global. Contudo, apesar do tema ser muito debatido na literatura científica e de grande relevância para as discussões sobre aquecimento global, é pouco conhecido pelo grande público e por turistas. No sentido de suprir esta deficiência de informações e melhorar o conhecimento de guias e condutores de trilhas, principais agentes de comunicação com o grande público, a presente contribuição objetivou ilustrar dois exemplos de rochas que comprovam a existência de variações climáticas acompanhadas de mudanças na paisagem do passado sergipano. O primeiro exemplo está preservado na Formação Tacaratu, unidade de rochas aflorantes no Vale dos Mestres, interpretada como produto de rios ancestrais ao Rio São Francisco, capazes de transportar sedimentos por uma extensa rede de drenagens desde Sergipe até o Ceará, entre os períodos Siluriano e Devoniano, há 440-360 milhões de anos (Ma), quando a vegetação ainda era escassa sobre os continentes. No entorno do vale, próximo ao povoado Curituba, porém de idade mais jovem (Permo-carbonífera 360 – 250 Ma), estão preservadas as melhores evidências de abrasão e deformação por geleiras do Estado, em um período que a América do Sul e África estavam unidas e faziam parte de um supercontinente chamado Gondwana. Já as evidências da transição entre eventos glaciais e interglaciais de maior profundidade do tempo geológico em Sergipe, foram deixadas ao longo do Neoproterozoico (cerca de 800 Ma atrás), respectivamente, nas rochas das formações Ribeirópolis e Jacoca, localizadas na Serra da Miaba. Este registro estratigráfico, ou seja, do empilhamento das rochas, inclui Sergipe, assim como Bahia, Minas Gerais e o Mato Grosso, como uma das áreas influenciadas por processos glaciais de maiores proporções que se tem conhecimento na história do planeta.

Ancient Siliciclastic-Evaporites as Seen by Remote Sensing Instrumentation with Implications for the Rover-Scale Exploration of Sedimentary Environments on Mars.

Accurate interpretation of the martian sedimentary rock record-and by extension that planet's paleoenvironmental history and potential habitability-relies heavily on rover-based acquisition of textural and compositional data and researchers to properly interpret those data. However, the degree to which this type of remotely sensed information can be unambiguously resolved and accurately linked to geological processes in ancient sedimentary systems warrants further study. In this study, we characterize Mars-relevant siliciclastic-evaporite samples by traditional laboratory-based geological methods (thin section petrography, X-ray diffraction [XRD], backscattered electron imaging, microprobe chemical analyses) and remote sensing methods relevant to martian rover payloads (visible-near-mid infrared reflectance spectroscopy, X-ray fluorescence mapping, XRD). We assess each method's ability to resolve primary and secondary sedimentologic features necessary for the accurate interpretation of paleoenvironmental processes. While the most dominant textures and associated compositions (i.e., bedded gypsum evaporite) of the sample suite are readily identified by a combination of remote sensing techniques, equally important, although more subtle, components (i.e., interbedded windblown silt, meniscus cements) are not resolved unambiguously in bulk samples. However, rover-based techniques capable of coordinating spatially resolved compositional measurements with textural imaging reveal important features not readily detected using traditional assessments (i.e., subtle clay-organic associations, microscale diagenetic nodules). Our findings demonstrate the improved generational capacity of rovers to explore ancient sedimentary environments on Mars while also highlighting the complexities in extracting comprehensive paleoenvironmental information when limited to currently available rover-based techniques. Complete and accurate interpretation of ancient martian sedimentary environments, and by extension the habitability of those environments, likely requires sample return or in situ human exploration.

Mapping ancient sedimentary organic matter molecular structure at nanoscales using optical photothermal infrared spectroscopy

Elucidating the molecular structure of sedimentary organic matter (SOM) is key to understanding petroleum generation processes, as well as ancient sedimentary environments. SOM structure is primarily controlled by biogenic source material (e.g., marine vs. terrigenous), depositional conditions, and subsurface thermal history. Additional factors, e.g., strain, may also impact the molecular structure of SOM. Multiple spatially resolved approaches exist for in situ evaluation of SOM, including Raman and infrared spectroscopies, as well as mass spectrometric methods. While these methods have enabled increased understanding of the occurrence and distribution of SOM functional groups, they suffer from disadvantages including low spatial resolution (infrared spectroscopy), limited molecular information (Raman spectroscopy), and sample destruction (mass spectrometric methods). Recent technological advances have resulted in infrared spectrometers capable of breaking the Abbe diffraction limit, greatly increasing the spatial resolutions accessible for an infrared measurement.Here we utilize optical photothermal infrared spectroscopy (O-PTIR) to record maps of functional group distributions at 500 nm spatial resolution in Tasmanites (algal microfossils) from the Upper Devonian Ohio Shale. These data allow for discrimination between Tasmanites, adjacent SOM, and fine-grained minerals. Additionally, functional group distributions within Tasmanites were found to be generally homogenous, although slight variations exist between the body and fold apices (zones of greatest deformation) which may indicate strain-induced chemical reactions. The data presented here represent the first application of O-PTIR to study SOM, highlighting the promise of this analytical approach for future studies evaluating the molecular composition of geologic materials at sub-micron scales.

Cold seep activity in the early Cambrian: Evidence from the world-class shale-hosted Tianzhu barite deposit, South China

Barite (BaSO4) is widely formed in modern and ancient sedimentary environments by the interaction of barium- and sulfate-bearing fluids from a variety of sources. A word-class barite deposit, hosted by lower Cambrian black shale in the Tianzhu area of the Yangtze block, is a well-known stratiform barite ore deposits. Its origin remains highly debated. A variety of genetic models have been proposed, including biological enrichment, cold seep and submarine exhalative hydrothermal activity, which have differing implications for paleo-oceanic environmental interpretation and resource exploration. We conducted a detailed sedimentological, mineralogical and geochemical study of the Tianzhu barite deposit. It dominated by anhedral barite with variable carbonate-pyrite-hyalophane associations and siliciclastic admixtures. These typically display porous and pore-filling structures, as well as dendritic and rosette-like growth forms, indicating in situ precipitation near the sediment-water interface. Based on vertical variation in sedimentary and mineralogical features, four vertically successive Units (I–IV) can be recognized. Barite samples in Units I and III display 87Sr/86Sr ratios within a very narrow range (0.7084 to 0.7085), nearly identical with coeval seawater values; while others display elevated ratios (0.7086 to 0.7279) that likely reflect contributions from modified porewater and/or siliciclastic admixture. Barites also display seawater-like and higher δ34Sbrt values (32.3 ‰ to 48.6 ‰) with respect to coeval seawater sulfate, and δ34Sbrt-δ18Obrt co-variation trends extending from seawater sulfate to higher δ-values. Co-occurring authigenic dolomites display moderately negative δ13Ccarb values (−18.3 ‰ to −7.2 ‰). These features suggest precipitation at the sulfate-methane transition zone (SMTZ) near the sediment-water interface. Overall, these characteristics of the Tianzhu barite deposit most closely resemble features associated with barite enrichment in modern and ancient cold seeps. This genetic model differs from previous interpretations, and does not require a restricted basin with seawater sulfate-depletion. This study has implications for understanding of oceanic barium‑sulfur cycling and environmental change in the early Cambrian, and formation of shale-hosted stratiform barite deposit in the geological record.

Contribution of deep-sourced carbon from hydrocarbon seeps to sedimentary organic carbon: Evidence from radiocarbon and stable isotope geochemistry

Sulfate-driven anaerobic oxidation of methane (AOM) limits the release of methane from marine sediments and promotes the formation of carbonates close to the seafloor in seepage areas along continental margins. It has been established that hydrocarbon seeps are a source of methane, dissolved inorganic carbon, and dissolved organic carbon to marine environments. However, questions remain about the contribution of deep-sourced carbon from hydrocarbon seeps to the sedimentary organic carbon pool. In this study, we analyzed carbon quantity, radiocarbon content (as percent modern carbon, pMC), stable carbon isotopic compositions (as δ13C) of organic matter enclosed within seep carbonates from the Gulf of Mexico and the South China Sea to assess if sediment organic matter may be used as a proxy for methane seepage intensity. The δ13C values of organic matter (δ13Corg) exhibited a large range from −81.4‰ to −23.9‰. Radiocarbon contents of the carbonate-bound organic matter in seep carbonates ranged from 6% to 28% pMC, suggesting organic matter of the carbonates is a mixture of marine particulate organic matter (δ13C = −22‰ VPDB and 90% modern carbon) and biomass resulting from methane oxidation (assumed to have 0% modern carbon). Assuming constant productivity in the marine photic zone, it is proposed that seepage intensity and duration are the most important factors controlling the contribution of methane-derived carbon to the sedimentary column. This study reinforces the potential for using δ13C values of organic carbon to discern methane-rich environments in ancient sedimentary environments where authigenic carbonate is not present and to constrain the record of AOM through Earth history.

Multidimensional Scaling (MDS): A quantitative approximation of zircon ages to sedimentary provenance with some examples from Mexico

During the last decades, provenance studies have proven to be an invaluable tool to evaluate modern and ancient sedimentary environments and to reconstruct the paleogeography and evolution of different tectonic settings. Whereas in the past provenance studies were mainly based on qualitative comparisons of whole-rock sandstone detrital modes, the advance and implementation of microanalytical techniques driven by the community during the last ~15 years permit the quantification of detrital-mineral ages and the isotopic characterization by in situ analysis. The combination of ages, isotopes, and mineral chemistry of individual detrital components provides a better understanding and reconstruction of source-to-sink systems. However, the introduction of these largely accessible microanalytical techniques has produced a large amount of data that requires proper management to be objectively interpreted. While a quantitative comparison between the age distributions of detrital sample pairs can be easily performed by Kolmogorov-Smirnov (K–S) statistics, comparing multi-samples data sets requires a more complex statistical approach. One of those is Multidimensional Scaling analysis (MDS), which allows, by using K–S statistics, to compare the dissimilarity between two or more samples. To compare among samples, a dissimilarities matrix, based on the stress from the ideal fit, is constructed. Dissimilarities are graphically represented in a “map” that tends to group more similar samples, pulling apart those that are more dissimilar. We contribute with three examples from the Mexican geology, showing how MDS can be used to evaluate in a more objective way the provenance of clastic rocks. We show that the MDS is more suitable than the visual comparison of probability density plots and kernel density estimations in marking the similarities and differences between the available samples; thus, it is a more suitable approach to reconstruct more rigorously the evolution of source-to-sink systems. Particularly, we examine fluvial to deep-marine Triassic strata, Paleozoic–lower Mesozoic metasedimentary rocks of the Ayú and Acatlán complexes, and Upper Jurassic–Cretaceous successions of the Pacific continental margin. The MDS challenges the scenario in which all Triassic submarine fan deposits of Mexico were part of a giant single fan developed along the Mexican Pacific margin, and supports the idea that the western margin of Pangea was drained by different fluvial systems that supplied distinct submarine fans. Applied to the Paleozoic–lower Mesozoic metasedimentary rocks of southern Mexico, this approach shows major dissimilarities between the Ayú and the Acatlán complexes, supporting the idea that these are likely two different tectonic complexes. Finally, the MDS suggests that two distinct and independent provenance domains were established in Mexico during the development of the Upper Jurassic–Early Cretaceous Arperos back-arc basin, and that such a detrital signature compartmentalization was lost by the end of Early Cretaceous time with the advent of compressive tectonics and the development of orogenic belts.

A prominent isotopic fingerprint of nitrogen uptake by anaerobic methanotrophic archaea

Marine sediments represent the largest reservoir of methane on Earth. Anaerobic oxidation of methane (AOM) mediated by anaerobic methanotrophic archaea (ANME) and associated sulfate-reducing bacteria is the main process controlling methane emission from marine sediments. Recent laboratory studies have revealed that ANME or AOM consortia can mediate nitrogen (N) fixation and ammonium assimilation. These processes are known to preferentially utilize 14N rather than the 15N isotope. However, geochemical fingerprints of microbial nitrogen uptake in marine sediments are lacking, limiting our understanding of its role in modern and ancient sedimentary environments. Here we report prominent negative excursions of bulk sediment δ15N values (up to −5‰) at sulfate-methane transitions (SMTs) where ANME show peak abundances in sediments at four sites of the Haima seeps of the South China Sea. Such δ15N excursions are restricted to SMTs, suggesting that 15N depletion results from ANME-mediated nitrogen uptake. We posit that this process is ubiquitous at the SMT – a reaction front in coastal and continental margin sediments worldwide – resulting in similar isotope patterns. Negative δ15N excursions are consequently a promising proxy to track AOM in the geological record and to better constrain carbon and nitrogen cycling in ancient sedimentary environments.

Geochemical signatures of transgressive shale intervals from the 811 Ma Fifteenmile Group in Yukon, Canada: Disentangling sedimentary redox cycling from weathering alteration

Despite iron and trace element proxies informing much of our insight into Earth’s oxygenation history, the processes that controlled their accumulation and distribution in ancient sedimentary environments are not fully understood. Furthermore, deciphering between primary, depositional signals and oxidative weathering alteration poses a substantial challenge to reconstructing Earth’s redox history using its sedimentary record. Here, we present a multi-proxy geochemical investigation of three transgressive shale intervals from the ca. 811 Ma Reefal assemblage (upper Fifteenmile Group) in Yukon, Canada to help resolve these issues. Systematic stratigraphic trends from low to high FeHR/FeT and FeT/Al compositions across shale intervals represent shoaling of a discrete redoxcline that separated oxygenated surface waters from anoxic, ferruginous deep waters during marine transgression. In some cases, these trends are followed by a symmetric fall, which represents a full “transgressive-regressive cycle” of the redoxcline. The high proportion of glauconite to total illite indicate short-term oscillating redox conditions as the redoxcline migrated across the sediment-water interface during relative sea-level rise. These conditions invigorated dissimilatory iron reduction (DIR) within anoxic sediment pore waters, which released Fe(II) into the overlying water column where it was oxidized to iron oxy(hydr)oxide and shuttled downslope. High rates of DIR within sediment replete with highly reactive iron oxy(hydr)oxide and organic matter removed isotopically light, bio-available iron, rendering the residual sediment depleted in FeT/Al relative to the detrital baseline and enriched in 56Fe. The low iron content of the detrital flux to the basin (Fe/Al ∼ 0.3) rendered the sediment susceptible to changes in its bulk δ56Fe composition by modification of its authigenic components alone and enabled fractionation from DIR to drive bulk-rock δ56Fe values up to 0.8‰. Anomalously high concentrations of redox-sensitive elements in three samples (up to 5792 ppm Cr, 586 ppm Mn, 726 ppm Mo, and 3509 ppm Ni) and their relative distribution patterns show similarities to co-enrichment due to particulate shuttling within a weakly restricted basin. However, SEM-EDS element maps and large-area BSE image mosaics show the remains of framboidal pyrite “ghosts” with secondary enrichment of redox-sensitive elements. Thus, these geochemical trends in redox-sensitive trace metals most likely reflect mobilization and re-accumulation following pyrite dissolution during outcrop weathering. The low primary pyrite content and local transformation of pyrite iron to iron oxy(hdr)oxide suggest that δ56Fe and FeHR/FeT data were unaltered despite indications of pyrite weathering. Ultimately, this dataset elucidates key aspects of sedimentary redox cycling directly preceding evolutionary milestones and a major perturbation to the global carbon cycle and also provides a template for evaluating the effects of outcrop weathering on commonly used sedimentary redox proxies.

Grain size analysis and characterization of sedimentary environment along the Atlantic Coast, Kenitra (Morocco)

Grain size, sorting, skewness, and kurtosis studies are of great importance in order to texturally characterize the sedimentary environments. These parameters are useful in order to characterize the dynamic beach environment, which is of potential interest for research on natural resources and to interpret analogous ancient sedimentary environments. The purpose of this paper is to investigate the characteristics and distribution of sediments along Kenitra coast (Morocco) for first time using size grain analysis. Grain size analysis aims to determine physical properties and textural parameters such as mean, sorting, skewness and kurtosis. Samplings were done at different locations on the beaches, which represent low tide line, high tide line, foot dune and dune areas. The observation from this study shows an abundance of the medium sand to fine sand which indicates the prevalence of comparatively moderate-to low-energy condition in the Kenitra area. Linear Discriminant function (LDA) analysis of the samples indicates an aeolian, shallow marine deposition environment and less influence of fluvial (7%) process. Analysis of sediments using Passega’s C-M diagram (C = one percentile in microns and M = medium in microns) of the Kenitra coast suggests that sediment deposition is mainly by rolling and graded suspension.

Enhanced framboidal pyrite formation related to anaerobic oxidation of methane in the sulfate-methane transition zone of the northern South China Sea

Many studies have confirmed that size distributions of framboidal pyrite can be an effective indicator of bottom-water redox conditions in modern as well as ancient sedimentary environments. However, one environment in which production of framboidal pyrite has not been sufficiently studied to date is the sulfate-methane transition zone (SMTZ), in which anaerobic oxidation of methane (AOM) is coupled with microbial sulfate reduction (MSR) to enhance iron sulfide mineral precipitation (e.g., FeS2, FeS, and Fe3S4). Here, we document for the first time size distributions for pyrite framboids from the SMTZ, based on data from two sites in the methane hydrate-bearing region of the northern South China Sea. On the basis of framboid size, pyrite concentration, and sulfur isotope data, we propose new insights into the formation process of authigenic pyrite framboids within the SMTZ. We conclude that (1) Enhanced anaerobic oxidation of methane (AOM) not only plays a dominant role in the accumulation of 34S-enriched pyrite but also is responsible for formation of highly variable and sometimes exceptionally large pyrite framboids in the SMTZ; (2) Most framboids occur in ‘framboid clusters’ which commonly exhibit rod-like shapes, secondary overgrowths, heavier δ34S values, and unusual size distributions (e.g., mean size>20μm and standard deviation>3.0μm) in the SMTZ; (3) Pyrite framboids formed in the SMTZ, which have characteristics different from those formed in the sulfate reduction zone (SRZ), do not comment on redox conditions of the overlying water column; and (4) Framboid occurrences with similar characteristics in ancient marine deposits may be considered indicators of enhanced AOM and mark the former position of the SMTZ in the paleo-marine system. In addition, significant quantities of elemental sulfur were observed in the SMTZ, possibly related to anaerobic oxidation of hydrogen sulfide and fluctuations of the SMTZ.

Horizon volumes with interpreted constraints

Horizons are geologically significant surfaces that can be extracted from seismic images. Color coding of horizons based on amplitude or other attributes can help reveal ancient sedimentary environments and structural features. Extracted horizons are also used for building structure models and stratigraphic interpretations. We propose two methods for constructing seismic horizons aligned with reflectors in a 3D seismic image. The first method generates horizons one at a time; the second method generates an entire volume of horizons at once by first computing a relative geologic time volume from seismic normal vectors. Rather than gradually building a horizon by extending one or more seed points to a surface along seismic reflectors, both of our methods generate whole horizons at once by solving partial differential equations derived from seismic normal vectors. The most significant new aspect of both methods is the ability to specify, perhaps interactively during interpretation, a small number of control points that may be scattered throughout a 3D seismic image. Experiments revealed that with our method, control points enable the extraction of more accurate horizons from seismic images in which noise, unconformities, and faults are apparent. These points represent constraints that we implemented as preconditioners in the conjugate gradient method used to construct horizons.

In Wave Tanks, Microbial Fragments Wrangle Sands into Wrinkle Structures

Wrinkle structures, found in ancient sedimentary environments along the surfaces of sandy beds, can be formed experimentally by adding microbial fragments to sand layers within wave tanks, according to Giulio Mariotti of the Massachusetts Institute of Technology (MIT) in Cambridge and his collaborators there and at Smith College in Northampton, Mass. They say that wrinkle structures in natural sediments are “morphological biosignatures” of microbial fragments—inscribed at the sediment–water interface in wave-dominated environments, and not beneath larger microbial mats as previously thought. Those wrinkle structures likely formed early during the evolution of animals, possibly while they were grazing on microbial fragments. Details appeared 31 August 2014 in Nature Geoscience Letters (doi:10.1038/ngeo2229).

Carbon- and oxygen-stable isotopic signatures of methanogenesis, temperature, and water column stratification in Holocene siderite varves

Manganoan siderite ([Fe,Mn]CO3) occurs in abundance of up to 19% (dry weight) as the sole endogenic carbonate within a succession of Holocene, organic-rich, varved sediments from freshwater Otter Lake (OL), Michigan. Radiocarbon dating and varve counts from a 7-m piston core constrain periods of major siderite accumulation to sediments older than 1200calyr BP. Sediment petrography suggests that siderite was a seasonal precipitate confined to the summer layer of the varve couplet. Bulk-sediment chemistry reveals cycles in abundance of manganese and aluminum coincident with centennial-scale cycles of siderite accumulation. Siderite δ13C and δ18O are enriched when the mineral is abundant and depleted when it is least abundant. Samples with high abundance of siderite precipitated in oxygen isotopic equilibrium with modern lake water, but are δ13C enriched relative to modern waters. Samples from intervals of low siderite abundance are δ13C and δ18O depleted relative to equilibrium with modern lake water. These data suggest that abundant siderite precipitation occurred when the OL water column was ferruginous (iron meromictic), allowing for enhanced ferrous iron concentrations and dissolved inorganic carbon (DIC) enriched in δ13C below the chemocline, where methanogenesis in waters and sediments influenced DIC composition. Seasonal siderite precipitation was triggered by water column alkalinity fluctuations driven by summer calcite dissolution. Manganese substitution in siderite lowered kinetic barriers to low-temperature mineral precipitation. Climate changes and basin filling influenced lake stratification and the rate of siderite precipitation. The siderite occurrence reported here displays remarkable similarity to its occurrence in Paleogene maar lake deposits, suggesting that further studies of Holocene lacustrine siderites may provide insight into ancient sedimentary systems and environments.

Microbial shaping of sedimentary wrinkle structures

Wrinkle structures in ancient sedimentary environments are enigmatic. Wave-tank experiments suggest that wrinkle structures are shaped by microbial mat fragments that are moved by waves over sandy-bed surfaces, and thus are morphological biosignatures.

Effects of Metabolism and Physiology on the Production of Okenone and Bacteriochlorophyll a in Purple Sulfur Bacteria

Purple sulfur bacteria (PSB) are important photoautotrophs inhabiting chemoclines in euxinic and meromictic lakes. These organisms are the only producers of the carotenoid, okenone, a compound that has been targeted as a biomarker for photic zone euxinia, particularly in ancient sedimentary environments. Although the natural occurrence and geochemistry of this compound has been studied previously, this is the first systematic and comprehensive report on the microbial physiology of okenone production in pure cultures. Four strains/species: Marichromatium purpuratum DSMZ 1591, Marichromatium purpuratum DSMZ 1711, Thiocapsa marina DSMZ 5653, and FGL21 (isolated from Fayetteville Green Lake, New York) were chosen because they produce okenone and Bacteriochlorophyll a (Bchl a). We developed a new, in vivo technique for the quantification of okenone allowing for more rapid and accurate quantification. The ratio of okenone to Bchl a differs among species and strains of PSB, varying from 0.463 ± 0.002 to 0.864 ± 0.002. Photoheterotrophically grown PSB have statistically significant, lowered okenone:Bchl a ratios, decreasing from 0.784 ± 0.009 under autotrophic metabolism to 0.681 ± 0.002, which we interpret to indicate a decreased requirement for okenone when PSB are provided with a complex (> C1) carbon source. The variation in okenone production raises the question on whether okenone expression is constitutive or inducible. The broader implication is that concentrations of okenone in sediments are dependent on metabolism and species composition, and not solely on PSB cell density.

Possibilities and restrictions of heavy-mineral analysis for the reconstruction of sedimentary environments and source areas

Abstract The possible reconstruction of ancient sedimentary environments on the basis of heavy-minerals assemblages is presented by means of discriminant lithogeodynamic diagrams that compare modern and ancient sedimentary environments. This is exemplified by Mesozoic-Cenozoic deposits recovered from ODP cores obtained from the Philippine and Japan Seas, the Japan Trench and the North Atlantic, as well as by deposits from folded areas onshore. On the basis of the comparative analysis, it can be deduced that the main tendencies in mineral assemblages of modern deposits that depend on the structural-tectonic conditions, are fairly well preserved in Cenozoic deposits (including the deposits recovered by ODP drilling). On the other hand, the environmental reconstruction of folded and faulted pre-Cenozoic continental areas on the basis of their heavy-mineral assemblages, by comparing them with supposed modern analogs, is not always possible with much certainty. The main reasons may be either a considerable change in the composition of the initial (primary) mineral assemblages as a result of intralayer solution or the absence, at the time of deposition, of geodynamic environments that closely resembled modern ones.

Isotopic Constraints on Biogeochemical Cycling of Fe

Cycling of redox-sensitive elements such as Fe is affected by not only ambient Eh-pH conditions, but also by a significant biomass that may derive energy through changes in redox state (e.g., Nealson 1983; Lovely et al. 1987; Myers and Nealson 1988; Ghiorse 1989). The evidence now seems overwhelming that biological processing of redox-sensitive metals is likely to be the rule in surface- and near-surface environments, rather than the exception. The Fe redox cycle of the Earth fundamentally begins with tectonic processes, where “juvenile” crust (high-temperature metamorphic and igneous rocks) that contains Fe which is largely in the divalent state is continuously exposed on the surface. If the surface is oxidizing, which is likely for the Earth over at least the last two billion years (e.g., Holland 1984), exposure of large quantities of Fe(II) at the surface represents a tremendous redox disequilibrium. Oxidation of Fe(II) early in Earth’s history may have occurred through increases in ambient O2 contents through photosynthesis (e.g., Cloud 1965, 1968), UV-photo oxidation (e.g., Braterman and Cairns-Smith 1987), or anaerobic photosynthetic Fe(II) oxidation (e.g., Hartman 1984; Widdel et al. 1993; Ehrenreich and Widdel 1994). Iron oxides produced by oxidation of Fe(II) represent an important sink for Fe released by terrestrial weathering processes, which will generally be quite reactive. In turn, dissimilatory microbial reduction of ferric oxides, coupled to oxidation of organic carbon and/or H2, is an important process by which Fe(III) is reduced in both modern and ancient sedimentary environments (Lovley 1991; Nealson and Saffarini 1994). Recent microbiological evidence (Vargas et al. 1998), together with a wealth of geochemical information, suggests that microbial Fe(III) reduction may have been one of the earliest forms of respiration on Earth. It therefore seems inescapable that biological redox cycling of Fe has occurred for at least several billion years of Earth's history.

Accumulation of a ferric mineral in the biofilm of Montacuta ferruginosa (Mollusca, Bivalvia). Biomineralization, bioaccumulation, and inference of paleoenvironments

Infrared absorption spectra of the amorphous and phosphorus-rich ferric mineral associated with the biofilm of Montacuta ferruginosa, a marine bivalve, were obtained by applying the KBr disc method. Phosphate absorption bands due to PO stretching vibrations were observed at 1100 and 1020 cm −1. This result, as well as the similarity of spectra with other well known (bio)minerals, suggests that the mineral is an amorphous iron oxyhydroxide gel with phosphate sorbed on its surface rather than a pure ferric phosphate. It is suggested that phosphate ions are of microbial origin because phosphate-producing enzymes were detected in vivo. Apart from iron, the mineral phase is devoid of other heavy metals. Despite its similarity with other abiotically precipitated phases, it is argued that the mineral is the result of in situ microbial biomineralization processes in the biofilm and not the result of a simple bioaccumulation process. This is supported by microscopic observations. A geological implication is that the simple presence of such iron minerals in ancient sedimentary environments, where microbes have not been fossilized, might be considered as an indicator of microorganisms performing biomineralization. By comparison with the present-day environment of M. ferruginosa, precise paleoenvironmental conditions may be inferred.