Ancient Marine Research Articles

Application of laser in-situ U-Pb dating to reconstruct the reservoir porosity evolution in the Cambrian Xiaoerbulake Formation, Tarim Basin 激光铀铅同位素定年技术在塔里木盆地肖尔布拉克组储层孔隙演化研究中的应用

The Laser Ablation (LA)Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) technique, co-developed by the CNPC Key Laboratory of Carbonate Reservoir and School of Earth and Environmental Sciences, The University of Queensland, has huge potential to overcome the difficulty in sampling and dating ultra-low concentration U-Pb minerals in ancient marine carbonates that are relatively smaller in diameter in diagenetic fabric, which was untenable by the conventional isotope dilution (ID) methods. The in-situ analysis by LA combined with MC-ICP-MS has been applied to the study on the diagenesis and porosity evolution of the Cambrian Xiaoerbulake reservoir, Tarim Basin.By dating of dolomitic and calcitic cements from filled vugs, matrix pores and fractures in different stages, we suggested that the burial diagenetic history of dolomite reservoir in the Cambrian Xiaoerblake Formation mainly features the filling of primary pores and vugs generated under karstification. The filling of vugs mainly occurred during the early Caledonian, while the filling of matrix pores mainly occurred in the middle Caledonian and Indosinian. Fractures activated during the Caledonian and Indosinian served as the channels for hydrothermal migration, providing the materials to cement the vugs and pores; while the residual vugs, pores and fractures with no filling of dolomite, calcite and siliceous cements were the main reservoir space. The diagenesis and porosity evolution history of dolomite reservoir in the Cambrian Xiaoerbulake Formation, Tarim Basin, was hence established. The understandings mentioned above are highly consistent with the tectonic-burial history and basin thermal history in the study area, which may be effective tools to date the ancient marine carbonate reservoir cements, to reconstruct diagenesis and porosity evolution history, and to evaluate effective pores before hydrocarbon migration in the ancient marine carbonates.

Behaviour of chromium and chromium isotopes during estuarine mixing in the Beaulieu Estuary, UK

Rivers are the principal source of chromium (Cr) to seawater and the Cr isotopic signatures of ancient marine sediments are widely considered to provide a record of the presence or absence of oxidative weathering processes on land. This assumes, however, that the δ53Cr value of river water is faithfully transferred to the oceans and is not modified in the estuarine mixing zone. To test this assumption we have determined the concentration and δ53Cr values of inorganic Cr (Cr(III) + Cr(VI)), and also Cr speciation for water samples collected within the estuarine mixing zone of the Beaulieu River, UK. The δ53Cr values of dissolved inorganic Cr ranged from −0.59 to 1.68‰, Cr(VI) concentrations from 0.39 to 1.83 nmolkg−1 and Cr(III) concentrations from 0.11 to 3.21 nmolkg−1. Both Cr(VI) concentrations and δ53Cr values increased linearly as a function of salinity, while Cr(III) concentrations decreased linearly with salinity. Thus δ53Cr, Cr(III) and Cr(VI) all showed conservative behaviour in the estuarine mixing zone, and the δ53Cr signature of Beaulieu River water was modified only by mixing between the river and seawater endmembers. The calculated average δ53Cr value of the river water endmember (−0.39 ± 0.08‰) was, however, lower than the range that has been observed in other rivers, which we attribute to input of organically-bound Cr(III) released by anoxic weathering processes. This is supported by the fact that Cr recovered by UV irradiation was found to have low δ53Cr values (−0.11 to −0.75‰). While input of Cr from anoxic weathering processes is unlikely to be an important source of Cr to the oceans today, this suggests that processes other than oxidative weathering may have an influence on the δ53Cr values of estuarine and coastal waters on the local scale. The δ53Cr value of the coastal seawater endmember (1.6 ± 0.4‰) was also higher than the range observed in the deep open ocean, due to in situ biogeochemical cycling of Cr. These factors need to be considered in the interpretation of marine sedimentary δ53Cr records.

Global cellulose biomass, horizontal gene transfers and domain fusions drive microbial expansin evolution.

Plants must rearrange the network of complex carbohydrates in their cell walls during normal growth and development. To accomplish this, all plants depend on proteins called expansins that nonenzymatically loosen noncovalent bonding between cellulose microfibrils. Surprisingly, expansin genes have more recently been found in some bacteria and microbial eukaryotes, where their biological functions are largely unknown. Here, we reconstruct a comprehensive phylogeny of microbial expansin genes. We find these genes in all eukaryotic microorganisms that have structural cell wall cellulose, suggesting expansins evolved in ancient marine microorganisms long before the evolution of land plants. We also find expansins in an unexpectedly high diversity of bacteria and fungi that do not have cellulosic cell walls. These bacteria and fungi inhabit varied ecological contexts, mirroring the diversity of terrestrial and aquatic niches where plant and/or algal cellulosic cell walls are present. The microbial expansin phylogeny shows evidence of multiple horizontal gene transfer events within and between bacterial and eukaryotic microbial lineages, which may in part underlie their unusually broad phylogenetic distribution. Overall, expansins are unexpectedly widespread in bacteria and eukaryotes, and the contribution of these genes to microbial ecological interactions with plants and algae has probbaly been underappreciated.

Reconstructing the evolutionary history of nitrogenases: Evidence for ancestral molybdenum-cofactor utilization.

The nitrogenase metalloenzyme family, essential for supplying fixed nitrogen to the biosphere, is one of life's key biogeochemical innovations. The three forms of nitrogenase differ in their metal dependence, each binding either a FeMo‐, FeV‐, or FeFe‐cofactor where the reduction of dinitrogen takes place. The history of nitrogenase metal dependence has been of particular interest due to the possible implication that ancient marine metal availabilities have significantly constrained nitrogenase evolution over geologic time. Here, we reconstructed the evolutionary history of nitrogenases, and combined phylogenetic reconstruction, ancestral sequence inference, and structural homology modeling to evaluate the potential metal dependence of ancient nitrogenases. We find that active‐site sequence features can reliably distinguish extant Mo‐nitrogenases from V‐ and Fe‐nitrogenases and that inferred ancestral sequences at the deepest nodes of the phylogeny suggest these ancient proteins most resemble modern Mo‐nitrogenases. Taxa representing early‐branching nitrogenase lineages lack one or more biosynthetic nifE and nifN genes that both contribute to the assembly of the FeMo‐cofactor in studied organisms, suggesting that early Mo‐nitrogenases may have utilized an alternate and/or simplified pathway for cofactor biosynthesis. Our results underscore the profound impacts that protein‐level innovations likely had on shaping global biogeochemical cycles throughout the Precambrian, in contrast to organism‐level innovations that characterize the Phanerozoic Eon.

The draft genome of horseshoe crab Tachypleus tridentatus reveals its evolutionary scenario and well-developed innate immunity

BackgroundHorseshoe crabs are ancient marine arthropods with a long evolutionary history extending back approximately 450 million years, which may benefit from their innate immune systems. However, the genetic mechanisms underlying their abilities of distinguishing and defending against invading microbes are still unclear.ResultsHere, we describe the 2.06 Gbp genome assembly of Tachypleus tridentatus with 24,222 predicted protein-coding genes. Comparative genomics shows that T. tridentatus and the Atlantic horseshoe crab Limulus polyphemus have the most orthologues shared among two species, including genes involved in the immune-related JAK-STAT signalling pathway. Divergence time dating results show that the last common ancestor of Asian horseshoe crabs (including T. tridentatus and C. rotundicauda) and L. polyphemus appeared approximately 130 Mya (121–141), and the split of the two Asian horseshoe crabs was dated to approximately 63 Mya (57–69). Hox gene analysis suggests two clusters in both horseshoe crab assemblies. Surprisingly, selective analysis of immune-related gene families revealed the high expansion of conserved pattern recognition receptors. Genes involved in the IMD and JAK-STAT signal transduction pathways also exhibited a certain degree of expansion in both genomes. Intact coagulation cascade-related genes were present in the T. tridentatus genome with a higher number of coagulation factor genes. Moreover, most reported antibacterial peptides have been identified in T. tridentatus with their potentially effective antimicrobial sites.ConclusionsThe draft genome of T. tridentatus would provide important evidence for further clarifying the taxonomy and evolutionary relationship of Chelicerata. The expansion of conserved immune signalling pathway genes, coagulation factors and intact antimicrobial peptides in T. tridentatus constitutes its robust and effective innate immunity for self-defence in marine environments with an enormous number of invading pathogens and may affect the quality of the adaptive properties with regard to complicated marine environments.

Composite micrite envelopes in the lagoon of Abu Dhabi and their application for the recognition of ancient firm- to hardgrounds

Micrite envelopes are common early diagenetic features in marine carbonates throughout the Phanerozoic. This paper documents a continuum of (sub)Recent, constructive and destructive (composite) micrite envelopes and discusses their application for the recognition of ancient marine firm- to hardgrounds. In the modern subtropical carbonate depositional environment of the Abu Dhabi coastal lagoon, micrite envelopes are actively forming and are directly assigned to a range of processes and depositional environments: (i) in the outer, high-energy lagoon, bioclasts and ooids typically contain empty microendolithic borings; (ii) in the unconsolidated sediments of the inner-to-middle lagoon, destructive micrite envelopes yield fragments of their host grains that result from boring/etching activity, and are bound and covered by biofilms; (iii) constructive envelopes are formed by high-Mg calcite micrite cements and coat destructive envelopes in the inner-to-middle lagoon, or may nucleate on aragonite needle cements in the outer lagoon. Micrite cements that form constructive envelopes are typical early marine diagenetic features and are among the main fabrics of firm- to hardgrounds. Calcitic skeletal grains, such as benthic foraminifera and coralline red algae, are less affected by microbioeroders compared to aragonitic ooids, gastropods and bivalves. High-Mg calcite constructive envelopes have a higher preservation potential relative to aragonite host particles and aragonite cements. Hence, the presence and recognition of constructive micrite envelopes in the rock record may be instrumental to identifying ancient aragonitic as well as unconventional firm- to hardgrounds that may otherwise be difficult to recognise.

Melting of recycled ancient crust responsible for the Gutenberg discontinuity

A discontinuity in the seismic velocity associated with the lithosphere-asthenosphere interface, known as the Gutenberg discontinuity, is enigmatic in its origin. While partial mantle melts are frequently suggested to explain this discontinuity, it is not well known which factors critically regulate the melt production. Here, we report geochemical evidence showing that the melt fractions in the lithosphere-asthenosphere boundary were enhanced not only by accumulation of compacted carbonated melts related to recycled ancient marine sediments, but also by partial melting of a pyroxene-rich mantle domain related to the recycled oceanic eclogite/pyroxenites. This conclusion is derived from the first set of Mg isotope data for a suite of young petit-spot basalts erupted on the northwest Pacific plate, where a clearly defined Gutenberg discontinuity exists. Our results reveal a specific linkage between the Gutenberg discontinuity beneath the normal oceanic regions and the recycling of ancient subducted crust and carbonate through the deep Earth.

Basinal hydrographic and redox controls on selenium enrichment and isotopic composition in Paleozoic black shales

Mass-dependent variations in selenium stable isotope ratios have recently been developed as a paleo-redox proxy. Since the reduction of selenium oxyanions occurs at a relatively high redox potential, this system holds promise for probing conditions relevant to the evolution and diversification of eukaryotic and animal life, which required substantial dissolved oxygen levels. Although several studies have identified selenium isotopic variability during oxygenation events in Earth’s distant past, we still have only a broad understanding of the mechanisms controlling this isotopic variability. This currently limits the robust interpretation of selenium isotope variability to first-order mechanisms driving large-magnitude changes. Here, we explore selenium isotope variability within and among Paleozoic black shales deposited on the North American craton that have been well-studied using a variety of other paleo-environmental proxies. Using this combined dataset, we attempt to unravel the controls on selenium abundance and isotope ratios in organic-rich ancient marine sedimentary rocks. We find that in the Late Pennsylvanian units, an estuarine nutrient trap on the Midcontinent Shelf enabled vigorous selenium recycling, leading to very high concentrations in sediments and enrichment of heavy isotopes in the aqueous selenium reservoir. In contrast, we find that among the Late Devonian units, differences in local basinal hydrography led to a gradient in selenium abundance and isotopic fractionation, with the more restricted basins depleting their selenium reservoirs and causing enrichment of heavy isotopes in the residual aqueous reservoir. In both of these case studies, the additional context provided by complementary paleo-environmental proxies was critical for distinguishing between possible drivers of selenium isotopic variability. When extending such studies to other paleo-environmental settings, we suggest that the continued use of complementary datasets will enable the most robust use of the selenium paleo-redox proxy. Moreover, further development of techniques for high-precision and phase-specific selenium isotope measurements will greatly improve the ability to deduce subtle redox fluctuations with this proxy.

Peculiarities of the Structure and Locomotor Function of the Tail in Sauropterygia

Among ancient and modern marine reptiles, several structural types of the locomotor apparatus were or are present, supporting different styles of swimming. Ichthyosaurs, mosasaurs, saltwater crocodiles, and representatives of many other groups swam or swim with horizontal undulations of the body primarily using the tail with a vertical caudal fin. Sea turtles with a reduced tail and their body completely immobilized by the shell use only limbs transformed into flippers for swimming. Sauropterygians had a substantially immobilized trunk and a horizontal caudal fin. They used a unique type of subaquatic locomotion with the leading role of two pairs of powerful flippers and vertical undulations of the tail, partial analogs of which are found in sea turtles, sirens, cetaceans, and some semi-aquatic mammals.

Laser ablation in situ U-Pb dating and its application to diagenesis-porosity evolution of carbonate reservoirs

Abstract Through the development and calibration of a reference material which is 209.8 Ma old using a newly-developed Laser Ablation (LA) Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) technique, we successfully overcome the difficulty in sampling and dating ultra-low U-Pb ancient marine carbonates, which was previously untenable by isotope dilution (ID) methods. We developed the LA-MC-ICP-MS in situ U-Pb dating technique for ancient marine carbonates for the study of diagenesis-porosity evolution history in Sinian Dengying Formation, Sichuan Basin. By systematically dating of dolomitic cements from vugs, matrix pores and fractures, we found that the burial and diagenetic process of dolomite reservoirs in Sinian Dengying Formation was characterized by progressive filling-up of primary pores and epigenic dissolution vugs. The filling of vugs happened in three stages, early Caledonian, late Hercynian-Indosinian and Yanshanian-Himalayan, while the filling of matrix pores mainly took place in early Caledonian. The unfilled residual vugs, pores and fractures constitute the main reservoir sapce. Based on the above knowledge, we established the diagenesis-porosity evolution history of the dolomite reservoir in Sinian Dengying Formation, Sichuan Basin. These findings are highly consistent with the tectonic-burial and basin thermal histories of the study area. Our study confirmed the reliability of this in situ U-Pb dating technique, which provides an effective way for the investigation of diagenesis-porosity evolution history and evaluation of porosity in ancient marine carbonate reservoirs before hydrocarbon migration.

Paleomagnetic constraints on the early Miocene closure of the southern Neo-Tethys (Van region; East Anatolia): Inferences for the timing of Eurasia-Arabia collision

Oligocene-Miocene convergence of the Eurasian and Arabian plates resulted in (i) the gradual closure of the Neo-Tethys Ocean that formed an open marine connection between the Indian Ocean and the proto-Mediterranean until the early Miocene and (ii) Eurasia-Arabia continental collision. Remnants of the Neo-Tethys basin are found scattered over eastern Anatolia. The Van region of SE Anatolia contains a unique stratigraphic succession (Van Formation) of this ancient marine corridor, showing a gradual transition from deep-marine marls to continental clastics and shallow marine deposits. This formation is considered a key unit for the late stage evolution of the Neo-Tethys Ocean as it contains one of the youngest marine deposits of the southern Neo-Tethys branch in SE Anatolia. Here, we present new magnetostratigraphic and Anisotropy of Magnetic Susceptibility (AMS) data to better constrain the timing of the marine-continental transition and the style of deformation in the Van region. The Van Formation was sampled in ~2-m stratigraphic resolution, with ~350 paleomagnetic cores drilled in stratigraphic order. These cores were analyzed with thermal and alternating field demagnetization, resulting in a magnetic polarity pattern that could straightforwardly be correlated to the standard Geomagnetic Polarity Time Scale (GPTS). The base of the section has an age of ~19.5 Ma, the marine-continental transition is dated at 18.8 Ma, and the top of the succession has an age of ~16.8 Ma. The AMS data show a conspicuous change from extensional to contractional patterns, coinciding with the end of open marine environments, at an age of ~19 Ma. We hypothesize that the closure of the marine basin and the concomitant change in stress regime in the Van region are related to the onset of Eurasia-Arabia collision and the terminal subduction of the Neo-Tethys oceanic lithosphere.

Marine and freshwater taxa: some numerical trends

Most of the freshwater fauna originates from ancient or recent marine ancestors. In this study, we considered only completely aquatic non-parasitic animals, counting 25 phyla, 77 classes, 363 orders for a total that should include 236,070 species. We divided these taxa into three categories: exclusively marine, marine and freshwater, and exclusively freshwater. By doing so, we obtained three distribution curves which could reflect the marine species’ mode of invasion into continental waters. The lack of planktonic stages in the benthic fauna of inland waters, in addition to what we know about the effects of the impoundment of epicontinental seas following marine regressions, lead us to think that the main invasion mode into inland waters is more linked to the sea level fluctuations of the past than to slow and “voluntary” ascents of rivers by marine elements.

Sargassum: the atypical irruption of an ancient ecosystem

Sargassum constitutes an ancient marine ecosystem that circulates clockwise on the Atlantic Ocean. Upon 2011, the pelagic seaweed which is the main component of sargassum started to reach beaches on 19 Caribbean countries, with environmental, health and economic impacts that need to be addressed urgently.

ЭВОЛЮЦИЯ ГИДРОГЕОХИМИЧЕСКОГО ПОЛЯ СЕВЕРНЫХ И АРКТИЧЕСКИХ РАЙОНОВ ЗАПАДНО-СИБИРСКОГО ОСАДОЧНОГО БАССЕЙНА В МЕЗОЗОЕ

Актуальность исследования состоит в выявлении фундаментальной роли палеогидрогеологических факторов в эволюции гидрогеохимического поля северных и арктических районов Западно-Сибирского осадочного бассейна в мезозойскую эру. Впервые предложен механизм, объясняющий их вклад в формирование гидрогеохимических аномалий в оксфордском резервуаре (верхнеюрском водоносном комплексе) изучаемого региона, где широко развита гидрогеохимическая инверсия. Цель: выявить особенности состава сингенетичных вод, захораниваемых одновременно с осадком на элизионных и инфильтрационных этапах развития осадочного бассейна на основе установленной цикличности гидрогеологической истории северных и арктических районов Западно-Сибирского осадочного бассейна; выполнить сравнительных анализ состава современных и сингенетичных вод и установить природу гидрогеохимических аномалий. Методы. Использованы методические приемы А.А. Карцева, С.Б. Вагина, Е.А. Баскова и других исследователей по палеогидрогеологическим реконструкциям осадочных бассейнов, выполнена периодизация гидрогеологической истории с выделением основных гидрогеологических циклов и инфильтрационных и элизионных этапов. На основе методики восстановления солевого состава вод древних морских и озерно-аллювиальных бассейнов, палеогеографических карт на юрский и меловой периоды, составленных сотрудниками ИНГГ СО РАН, информационного банка данных лаборатории гидрогеологии осадочных бассейнов Сибири ИНГГ СО РАН по арктическим районам Западно-Сибирского осадочного бассейна проведены палеогидрогеохимические реконструкции юрско-мелового разреза. Результаты. Приводятся результаты палеогидрогеологических реконструкций северных и арктических районов Западно-Сибирского осадочного бассейна. Установлено, что в гидрогеологическом разрезе исследуемого региона доминируют седиментогенные воды, в некоторых случаях разбавленные инфильтрогенными, проникшими при регрессии морского бассейна. Сравнительный анализ захороненных сингенетичных вод оксфордского бассейна и данных по современной гидрогеохимии подземных вод выявил ряд отрицательных и положительных аномалий. Отрицательные аномалии закономерно трассируют границу глинизации оксфордского регионального резервуара и связаны с развитием процессов элизионного водообмена. Геостатическая (литостатическая) водонапорная система на глубинах 2‒2,5 км переходит в термодегидратационную, что сопровождается аномально высокими пластовыми давлениями и появлением в водонапорной системе пресных химически-связанных вод. Большинство положительных аномалий приурочены к структурам Южно-Надымской и Предъенисейской мегамоноклиз, где широко проявлены процессы вертикальной разгрузки подземных вод нижне-среднеюрского и палеозойского водоносных комплексов в зонах тектонических нарушений. Системе «вода–порода–газ–органическое вещество» принадлежит ключевая роль в процессах формирования химического состава подземных вод нефтегазоносных отложений.

Oceanic efflux of ancient marine dissolved organic carbon in primary marine aerosol.

Breaking waves produce bubble plumes that burst at the sea surface, injecting primary marine aerosol (PMA) highly enriched with marine organic carbon (OC) into the atmosphere. It is widely assumed that this OC is modern, produced by present-day biological activity, even though nearly all marine OC is thousands of years old, produced by biological activity long ago. We used natural abundance radiocarbon (14C) measurements to show that 19 to 40% of the OC associated with freshly produced PMA was refractory dissolved OC (RDOC). Globally, this process removes 2 to 20 Tg of RDOC from the oceans annually, comparable to other RDOC losses. This process represents a major removal pathway for old OC from the sea, with important implications for oceanic and atmospheric biogeochemistry, the global carbon cycle, and climate.

Geological conditions of natural gas accumulation and new exploration areas in the Mesoproterozoic to Lower Paleozoic of Ordos Basin, NW China

Based on field outcrop investigation, interpretation and analysis of drilling and seismic data, and consulting on a large number of previous research results, the characteristics of ancient marine hydrocarbon source rocks, favorable reservoir facies belts, hydrocarbon migration direction and reservoir-forming law in the Ordos Basin have been studied from the viewpoints of North China Craton breakup and Qilian-Qinling oceanic basin opening and closing. Four main results are obtained: (1) Controlled by deep-water shelf-rift, there are three suites of source rocks in the Ordos Basin and its periphery: Mesoproterozoic, Lower Cambrian and Middle-Upper Ordovician. (2) Controlled by littoral environment, paleo-uplift and platform margin, four types of reservoirs are developed in the area: Mesoproterozoic– Lower Cambrian littoral shallow sea quartz sandstone, Middle-Upper Cambrian–Ordovician weathering crust and dolomitized reservoir, and Ordovician L-shape platform margin reef and beach bodies. (3) Reservoir-forming assemblages vary greatly in the study area, with “upper generation and lower storage” as the main pattern in the platform, followed by “self-generation and self-storage”. There are both “upper generation and lower storage” and “self-generation and self-storage” in the platform margin zone. In addition, in the case of communication between deep-large faults and the Changchengian system paleo-rift trough, there may also exist a “lower generation and upper reservoir” combination between the platform and the margin. (4) There are four new exploration fields including Qingyang paleo-uplift pre-Carboniferous weathering crust, L-shape platform margin zone in southwestern margin of the basin, Ordovician subsalt assemblage in central and eastern parts of the basin, and Mesoproterozoic–Cambrian. Among them, pre-Carboniferous weathering crust and L-shape platform margin facies zone are more realistic replacement areas, and Ordovician subsalt assemblage and the Proterozoic-Cambrian have certain potential and are worth exploring.

Evaluation of high-frequency paleoenvironmental variation using an optimized cyclostratigraphic framework: Example for C-S-Fe analysis of Devonian-Mississippian black shales (Central Appalachian Basin, U.S.A.)

Stratigraphic successions are rarely sampled at sufficiently dense spacing to permit evaluation of paleoenvironmental variation at the shortest fundamental frequency operative in the depositional system of interest, often because that frequency remains undetermined. Here, we provide an example of a sampling strategy optimized for recovery of high-frequency paleoenvironmental signals, based on (1) full-core analysis using a continuous survey tool such as X-radiography, followed by (2) fine sampling of selected small-scale cycles that record significant short-term paleoenvironmental variation. We show that this strategy results in recovery of high-frequency paleoenvironmental signals that are completely missed by low-density sampling protocols, permitting key insights into depositional and early diagenetic processes that may otherwise by overlooked.The present case study is based on C-S-Fe systematics of the Cleveland and Sunbury shales, which belong to the Upper Devonian-lowermost Carboniferous black shale succession of the Central Appalachian Basin in eastern North America. In these units, the fundamental scale of paleoenvironmental variation is recorded by pervasive decimeter-scale (~5- to 20-cm-thick) compositional cycles that are readily visible in outcrop, in which high-organic layers weather out as resistant ledges. We identified these cycles in drillcores using X-radiography, which proved to be an effective proxy for compositional variation owing to control of bulk density primarily by total organic carbon (TOC) content. We then sampled selected dm-scale cycles (n = 21) at a centimeter spacing, generating short, high-resolution, multiproxy chemostratigraphic records including TOC, sulfur and Fe concentrations, Fepy/FeHR (a redox proxy), δ34Spyrite, and organic maceral content. This analysis yielded the novel insight that all proxies related to authigenic iron sulfides (e.g., Stotal and Spyrite, Fetotal and Fepyrite, Fepy/FeHR, and δ34Spyrite) covary negatively with TOC within the dm-scale cycles.We interpret the unusual pattern of negative TOC-pyrite covariation in the Cleveland and Sunbury shales to have resulted from differential distributions of labile organic matter (OM), which drove H2S production via microbial sulfate reduction (MSR), and reactive Fe, which controlled pyrite formation. Labile OM was concentrated in high-TOC layers whereas reactive Fe was concentrated in low-TOC layers (possibly as Fe-oxyhydroxide coatings on silt grains), as a consequence of which part of the H2S generated in the high-TOC layers diffused upward into the low-TOC layers before becoming fixed as pyrite. This process is evidenced by several contrasts between high-TOC and low-TOC layers: (1) higher Spyrite concentrations and Fepy/FeHR values in the latter, (2) concentration of pyrite in the lower part of low-TOC layers, immediately above high-TOC layers, and (3) higher δ34Spyrite values in the latter, reflecting late diagenetic formation of the H2S that diffused upward into the low-TOC layers. To our knowledge, this process has not been documented previously in ancient marine shales, probably owing to insufficiently high-resolution analysis in earlier studies.

Development of an apatite oxygen paleobarometer: Experimental characterization of Sm3+-substituted apatite fluorescence as a function of oxygen availability

Global, apatite-bearing phosphorites represent unique biogeochemical periods coincident with major transitions in biological evolution and, particularly, marine oxygenation. However, current understanding of such oxygenation is limited by qualitative and not uncommonly contrasting interpretations of the marine redox conditions evidenced by sedimentary isotope and trace element abundances. A potentially novel measure of ancient marine oxygenation has previously been indicated by fluorescence signatures detected in Sm3+-substituted microfossil-preserving apatite from Early Cambrian phosphorites, consistent with experimental studies demonstrating an oxygen-dependent mechanism for Sm3+ incorporation. Quantitative calibration of these signatures would enable the interpretation of oxygen concentrations during sedimentary apatite formation. In the experiments described, heat-promoted substitution of Sm3+ in apatite powder pellets has been conducted under gaseous oxygen concentrations ranging from 0 to 20.9%. The resulting fluorescence signals provide a comparison to those observed in naturally occurring phosphorites that should reflect the levels of oxygen in the immediate precipitating environment. These analyses suggest that a metric describing the relative spectral characteristics derived from end-member oxic and anoxic experiments, here described as an apatite oxygen paleobarometer (AOP), robustly correlates with gaseous oxygen concentrations. The application of this quantifiable metric to phosphorite apatite is expected to provide a fundamentally new dataset by which to probe ancient marine redox conditions, particularly during the Neoproterozoic-Cambrian phosphogenic event and diversification of early metazoans.

Development of a modified SEDEX phosphorus speciation method for ancient rocks and modern iron-rich sediments

We report the development of a modified method for evaluating different reservoirs of sedimentary phosphorus (P) in ancient marine sedimentary rocks and in modern Fe-rich sediments. Utilising the existing SEDEX scheme for P partitioning in modern sediments, we initially demonstrate limitations in the application of the original scheme to sediments and rocks containing crystalline hematite and magnetite. We tested additional extractions for these crystalline Fe phases, using both synthetic minerals, and modern and ancient sediments. The addition of 6 h oxalate and 6 h citrate-dithionate-acetate extractions considerably enhanced the total recovery of synthetic magnetite and hematite to 88.7 ± 1.1% and 76.9 ± 3.8%, respectively. In addition, application of the 6 h oxalate extraction to synthetic P-containing magnetite recovered 93.9 ± 1.7% of the Fe present and 88.2 ± 12.8% of the co-precipitated P. Based upon these results we developed a modified SEDEX extraction scheme. The modified scheme was applied to modern Fe-rich sediments from Golfo Dulce, Costa Rica, which resulted in 16% higher Fe-bound P recovery. Application of the scheme to a variety of ancient marine rocks increased the recovery of Fe-bound P by up to 22%. We also highlight the potential for authigenic carbonate fluorapatite to convert to more crystalline apatite in ancient rocks during deep burial and metamorphism. We suggest that in such systems minimum and maximum estimates of the total reactive P pool may be calculated with and without the inclusion of crystalline P. It is noted that the application of the revised method may have important implications for understanding the cycling of P in ancient marine environments.

Sources of paleomagnetic signal in iron-bearing marine sedimentary deposits

This paper presents the results of interpreting the change in the magnetic signal of ferruginous marine sedimentary rocks using the example of the Cretaceous-Paleogene Bakchar deposit (Western Siberia). Anomalous magnetic susceptibility (MS) values are determined by two factors: (a) a diagenetic process accompanied by the formation of pyrrhotite, greigite and siderite in ironstones, (b) an increase detrital input with deposition of magnetite and ilmenite in the siltstones on the background of climate moistening and intense weathering. It is shown that increased MS values in iron-bearing marine sedimentary deposits can be associated with the formation of ferrimagnetic sulfides, which indicates to the methane emission through ancient marine bottom sediments.