Modern Marine Sediments Research Articles

Sedimentary host phases of mercury (Hg) and implications for use of Hg as a volcanic proxy

Mercury (Hg) concentration enrichments have become a widely used proxy for volcanic inputs to sediments, especially for investigation of ancient large igneous province (LIP) eruptions. Its application for this purpose requires normalization to an element representing the dominant host phase of Hg–generally total organic carbon (TOC) for the organic fraction, but occasionally total sulfur (TS) or aluminum (Al) for the sulfide or clay fractions, respectively. Hg studies generally assume an organic matter host, but recent work has demonstrated that sulfide or clay fraction host phases are not uncommon, making it essential to determine formation-specific Hg host phases. Here, we investigate Hg concentrations and their relationships to TOC, TS, and Al in four modern marine settings (Black Sea, Japan Sea, Saanich Inlet, and Peru Margin) and six ancient marine formations (from Lower Cambrian to Lower Jurassic). Multiple regression analysis (MRA) shows that the organic fraction is the dominant host of Hg in all of the modern marine sediments examined here, as well as in many ancient marine units, although in some of the latter Hg resides primarily in the sulfide fraction (i.e., lower Cambrian and Upper Ordovician units) or partly in the clay mineral fraction (Middle Permian) of the sediment. A sulfide host phase is more likely in strongly euxinic depositional facies, as reflected in high TS concentrations (>1.0%) and TS/TOC ratios (>∼0.35). This study thus demonstrates the importance of determining Hg host phases in sediments prior to normalization and use of Hg as a volcanic proxy.

Aquatic palynomorphs from modern marine sediments in a reconnaissance transect across the Northwest Passage – Baffin Bay region

Abstract Surface sediments from 18 boxcores across the Canadian Arctic provide the first assessment of modern non-pollen, non-dinocyst palynomorph regional distribution patterns. Palynomorphs are abundant and well-preserved (~400–21,900 ind./g; average 5730 ind./g), comprising acritarchs, zoomorphs (invertebrate eggs, egg capsules, mouthparts), foraminiferal linings, tintinnid loricae, ciliate cysts, and freshwater algae. Acritarch P and Halodinium minor were the most abundant and widely distributed acritarchs (avg. ~410 and ~340 ind./g, respectively). Invertebrate eggs (avg. ~570 ind./g) and foraminiferal linings (avg. ~1600 ind./g) were prominent. Six tintinnid forms were identified, cf. Parafavella parumdentata (avg. ~40 ind./g; 10 stations) showing the widest distribution. Ciliate cysts included Strombidium? cyst A, a spherical, flask-shaped cell with short spines and a rounded base (avg. ~120 ind./g; 16 stations). Pediastrum sp. (avg. ~140 ind./g) and Botryococcus sp. (avg. ~1040 ind./g) were especially abundant in the west, likely delivered via rivers. Out of 14 environmental parameters only sedimentary biogenic silica (%Si(opal)) was significant in explaining palynomorph variation. At group level, tinitinnids showed the strongest positive correlation to %Si(opal). Individually, the tintinnid Acanthostomella? sp. 1, ciliate cyst Fusopsis? cyst A, and the acritarch Palaeostomocystis fritilla were almost solely and positively explained by %Si(opal). This suggests that %Si(opal) strongly determines palynomorph variability, particularly for microzooplankton, implying that siliceous organisms (diatoms) play an important role in shaping palynomorph content in the sedimentary record. Our data suggest that palynomorphs represent a valuable addition to organic-walled microfossils (dinocysts) routinely used in this region, offering a unique multiproxy glimpse into the ecosystem.

Exploring computed tomography in ichnological analysis of cores from modern marine sediments

Ichnological analysis is considered a very useful tool in several disciplines of Earth Sciences, including palaeoenvironmental studies and hydrocarbon exploration. Sediment cores provide excellent records, despite difficulties encountered during study runs due to specific core features. Previous studies using 2D images have proven the benefits of high-resolution image treatment in improving the visibility of ichnological features, but with limitations. 3D computed tomography (CT) techniques were applied to palaeoichnological studies in lithified cores and other disciplines of palaeontology to solve these limitations, but not used for ichnological studies in unconsolidated sediments due to the low density contrast between host sediment and trace fossils. In this study, a CT processing technique, previously tested in coral research, is applied to facilitate the characterisation of the ichnological signature of cores from modern marine soft sediments. This technique allows for the first time the isolation of burrows within these kinds of sediments and the differentiation of intervals based on burrow orientation. Data obtained from the technique are complemented with the ichnological information from conventional core description, thus providing a more complete characterisation of the trace fossil assemblage with additional ichnological properties such as burrow orientation and branching. This will improve palaeoenvironmental interpretations related to changes in energy or oxygenation, and the analysis of reservoir quality given the impact of burrows on porosity and permeability. Therefore, adopting CT to complement visual core description in the ichnological analysis of soft modern marine cores is a very informative approach.

An overview and brief description of common marine organic-walled dinoflagellate cyst taxa occurring in surface sediments of the Northern Hemisphere

Abstract Organic-walled resting cysts of planktonic dinoflagellates occur commonly in modern marine sediment where they represent, with rare exceptions, the only geologically preservable part of the life cycle. Although many species do not produce fossilizable resting cysts, upper Quaternary sediments contain a diverse cyst record that is used frequently for paleoenvironmental reconstruction and stratigraphic analysis. Reconstructions of past sea-surface conditions rely on an understanding of the distributions of dinoflagellate cysts in modern sediments linked to their respective environmental parameters, underpinned by sound taxonomy and standardized nomenclature. Stratigraphic studies additionally require knowledge of morphological details to distinguish extant from similar extinct taxa. Here, 51 dinoflagellate cyst species and two informal cyst morphotypes that are commonly encountered during routine palynological analysis of upper Quaternary marine sediments from the Northern Hemisphere are briefly described taxonomically and illustrated. In addition, their lowest stratigraphic occurrences are provided.

Boron paleosalinity proxy for deeply buried Paleozoic and Ediacaran fossils

Boron content is an indicator of paleosalinity for Mesozoic and Cenozoic clayey sediments, but is compromised by clay diagenesis in deeply-buried sediments of Paleozoic and Ediacaran age. This study of North American, Russian, and Australian, Paleozoic and Ediacaran fossils showed tight covariance of B (ppm) and K (wt%), confirming common wisdom that boron is carried largely by illlite-smectite clays. Ratios of B/K >40 μg/g distinguish modern marine and freshwater sediments, but this discrimination threshold declines to 21 μg/g for sequences with bituminous coal just below the anchimetamorphic zone, and to 5 μg/g for sequences within the anchimetamorphic zone toward greenschist facies. Marine and non-marine threshold B/K values can be predicted from Weaver and Kübler indices of illite crystallinity, as a measure of B depletion during deep burial and metamorphism. Deviations from that expectation (ΔB/K-Weaver and ΔB/K-Kübler) are positive in marine rocks, and negative in non-marine rocks. Trilobites, stromatolites and other marine fossils were correctly identified as marine by this proxy, and fossil land plants identified as non-marine, though in some cases by small margins because of declining B content with increased clay crystallinity. Cryogenian to Devonian problematic quilted fossils (Dickinsonia, Arumberia, Rangea, Pteridinium, Ernietta, Aspidella, Rutgersella, Protonympha) are indistinguishable from Devonian to Triassic plants using this proxy, and significantly different from trilobites, brachiopods, ammonites and other securely marine fossils.

Anthropogenic-induced acceleration of elemental burial rates in blue carbon repositories of the Arabian Gulf

Along the past century, the Arabian Gulf has experienced a continuous and fast coastal development leading to increase the human pressures on the marine environment. The present study attempts to describe the historical changes of trace elements in the sediments of vegetated coastal habitats in the western Arabian Gulf. 210Pb–dated sediment cores collected from seagrass, mangrove and saltmarsh habitats were analyzed to evaluate historical variations in concentrations and burial rates of 20 trace elements (Al, As, Ba, Ca, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Na, Ni, P, Pb, S, Sr, V and Zn). The highest correlations (Spearman correlation coefficients ≥0.51) were found between crustal elements (Al, Fe, Co, Cr, K, Na, Mg, Mn, Ni, V, and P), suggesting a common crustal source in the Gulf. The increased concentrations of these crustal elements in modern marine sediments of the Arabian Gulf seem to be linked to increased mineral dust deposition in the area. Over the last century, both elemental concentrations and burial rates increased by factors of 1–9 and 1–15, respectively, with a remarkably fast increase occurring in the past six decades (~1960 – early 2000). This is most likely due to an increase in anthropogenic pressures along the Gulf coast. Our study demonstrates that sediments in vegetated coastal habitats provide long-term archives of trace elements concentrations and burial rates reflecting human activities in the Arabian Gulf.

Atlanticodinium striaticonulum n. gen., n. sp., a widespread extant dinoflagellate cyst from the late Cenozoic, and its comparison with Atlanticodinium janduchenei (Head et al., 1989) n. comb.

Abstract Atlanticodinium n. gen. accommodates small- to medium-sized proximate dinoflagellate cysts with a spherical or subspherical central body having a nonfibrous wall with a smooth to finely ornamented surface bearing numerous, short, hollow, unbranched, nontabular, nonfibrous processes. The archeopyle is precingular, its margins having well defined angles. Atlanticodinium striaticonulum n. gen., n. sp. has a IP archeopyle, and distinctive short, distally closed processes with internal striations. This species has a confirmed Lower Pleistocene record, possibly extending to at least upper Lower Pliocene, and occurs widely in modern marine sediments. Atlanticodinium janduchenei (Head et al., 1989) n. comb., here reassigned from the genus Operculodinium Wall, 1967, has smooth-shafted processes that can be distally open or closed, as confirmed by re-examination of the holotype and paratype from the Upper Miocene of the Labrador Sea. Atlanticodinium janduchenei has a reported stratigraphic range of Lower Miocene to Lower Pleistocene, but this requires reevaluation. Atlanticodinium striaticonulum has been widely mistaken for Atlanticodinium janduchenei in modern sediments. Atlanticodinium striaticonulum has a tropical to temperate modern distribution, and is particularly associated with offshore to oceanic settings where conditions are oligotrophic.

Reevaluating the carbon sink due to sedimentary carbonate formation in modern marine sediments

Previous attempts to quantify the amount of sedimentary carbonate precipitation in modern marine sediments have been derived from the flux of calcium into the sediments due to diffusion under assumed steady state, the application of Fick's first law, and then extrapolation of these site-specific rates to the global ocean sediment column. This approach is limited, however, as much of the ocean floor has not been sampled. We take a machine learning approach to update and refine the estimate of the amount of sedimentary carbonate precipitation, as well as define whether sedimentary carbonate precipitation is driven by organoclastic microbial sulfate reduction or anaerobic methane oxidation. We identify areas where there is sedimentary carbonate formation using machine learning, based upon oceanic physical and chemical properties including bathymetry, temperature, water depth, distance from shore, and tracers of primary production, and data from the global ODP/IODP database. Our results suggest that the total amount of sedimentary carbonate formation is much lower than previous estimates, at 1.35±0.5×1011 molC/yr. We suggest that this rate is a lower estimate and discuss why machine-learning approaches may always produce lower-bound estimates of global processes. Our calculations suggest that the formation of sedimentary carbonate today is mainly driven by anaerobic methane oxidation (77%), with the remainder attributed to organoclastic sulfate reduction. We use our machine-learning results to speculate the impact that sedimentary carbonate precipitation may have had on the carbon isotope composition of the surface dissolved inorganic carbon reservoir over Earth history.

Magnetic Fingerprints of Modern Sediments in the South China Sea Resulting From Source‐to‐Sink Processes

AbstractMore than 650 Mt/year of fluvial sediment are delivered from continental regions into the South China Sea (SCS). Previous studies have shown that the composition of the magnetic fraction of riverine sediments drained into the SCS is significantly variable from north to south. On the basis of this evidence, we now examine a full set of magnetic properties for a number of core tops taken at water depth of mostly between 800 and 3,500 m. Room temperature magnetic parameters and thermal spectra are used to obtain information about the concentration and mineralogical magnetic composition. Spatial changes are observed in the relative proportion of magnetite and hematite with an increase of the latter toward the south, similarly to the observation on land. However, the N‐S contrast is much weaker in marine core tops than in river sediments, because of the role played by the shelf in partly trapping river‐borne sediments, in particular in the southern SCS. In part, sediments also reach the continental slope and the deep basins, being transported and mixed by surface and deepwater currents, which yield the magnetite‐hematite mixing in the south. For the first time, we characterize a wide spectrum of magnetic properties of modern marine sediment in the SCS. Our results give important insights into the modern pathways of sediment particles, depicting the source‐to‐sink processes that affect the terrigenous sediment load.

Early Cambrian origin of the shelf sediment mixed layer

The mixed layer of modern oceans is a zone of fully homogenized sediment resulting from bioturbation. The mixed layer is host to complex biogeochemical cycles that directly impact ecosystem functioning, affecting ocean productivity and marine biodiversity. The timing of origin of the mixed layer has been controversial, with estimates ranging from Cambrian to Silurian, hindering our understanding of biogeochemical cycling and ecosystem dynamics in deep time. Here we report evidence from the Global Stratotype Section and Point (GSSP) of the basal Cambrian in the Burin Peninsula of Newfoundland, Canada, showing that a well-developed mixed layer of similar structure to that of modern marine sediments was established in shallow marine settings by the early Cambrian (approximately 529 million years ago). These findings imply that the benthos significantly contributed to establishing new biogeochemical cycles during the Cambrian explosion.

Copper isotope signatures in modern marine sediments

The development of metal stable isotopes as tools in paleoceanography requires a thorough understanding of their modern marine cycling. To date, no Cu isotope data has been published for modern sediments deposited under low oxygen conditions. We present data encompassing a broad spectrum of hydrographic and redox regimes, including continental margin and euxinic (sulphide-containing) settings. Taken together with previously published data from oxic settings, these data indicate that the modern oceanic sink for Cu has a surprisingly homogeneous isotopic composition of about +0.3‰ (δ65Cu, relative to NIST SRM976). We suggest that this signature reflects one of two specific water-column processes: (1) an equilibrium isotope fractionation between soluble, isotopically heavy, Cu complexed to strong organic ligands and an isotopically light pool sorbed to particles that deliver Cu to the sediment, or (2) an equilibrium isotope fractionation between the same isotopically heavy ligand-bound pool and the particle reactive free Cu2+ species, with the latter being scavenged by particulates and thereby delivered to the sediment. An output flux of about +0.3‰ into sediments is isotopically light relative to the known inputs to the ocean (at around +0.6‰) and the seawater value of +0.6 to +0.9‰, suggesting the presence of an as yet unidentified isotopically light source of Cu to the oceans. We hypothesize that this source may be hydrothermal, or may result from the partial dissolution of continentally derived particles.

Volatile earliest Triassic sulfur cycle: A consequence of persistent low seawater sulfate concentrations and a high sulfur cycle turnover rate?

Marine biodiversity decreases and ecosystem destruction during the end-Permian mass extinction (EPME) have been linked to widespread marine euxinic conditions. Changes in the biogeochemical sulfur cycle, microbial sulfate reduction (MSR), and marine dissolved sulfate concentrations during the Permian-Triassic transition can provide insights into the role of ocean chemistry change in the largest mass extinction in Earth history. In this study, we constrain marine dissolved sulfate concentrations using the MSR-trend method of Algeo et al. [Algeo, T.J., Luo, G.M., Song, H.Y., Lyons, T.W., Canfield, D.E., 2015. Reconstruction of secular variation in seawater sulfate concentrations. Biogeosciences 12, 2131–2151] on sulfur (S) isotope records from Iran (the Kuh-e-Ali Bashi and Zal sections) and Hungary (the Bálvány North and Bálvány East sections). This empirically derived transfer function is based on the S isotope fractionation between sulfate and sulfide associated with MSR in natural aquatic environments. This fractionation is proxied by the difference in S isotope compositions between chromium-reducible sulfur (CRS) and carbonate-associated sulfate (CAS), i.e., Δ34SCAS-CRS. We show that, despite region-specific redox conditions, Δ34SCAS-CRS exhibits a nearly invariant value of 15–16‰ in both study sections. By comparing our record with a Δ34Ssulfate-sulfide density distribution for modern marine sediments, we deduce that porewater Rayleigh distillation, carbonate diagenesis, and other effects are unlikely to have appreciably altered the S isotope offset between CRS and CAS in the study sections. In addition, differences in sedimentary regimes and organic carbon (OC) fluxes between the Iranian and Hungarian sections exclude major influence of the electron donor on MSR-S isotope fractionation and point to a more universal control, i.e., contemporaneous seawater sulfate concentration.The MSR-trend transfer function yielded estimates of seawater sulfate of 0.6–2.8mM for the latest Permian to earliest Triassic, suggesting a balanced oceanic S-cycle with equal S inputs and outputs and no major changes in sulfate concentrations during this interval. However, a secular trend toward heavier δ34SCAS (by >5‰) in the earliest Triassic can be explained only by increasing the turnover rate of the S-cycle (by ca. one order of magnitude) and a concomitant change in terrestrial S sources in a box model experiment. Exposure of evaporite deposits having a high δ34S may account for the source change, with a possible role for the Siberian Traps volcanism by magmatic remobilization of Cambrian rock salt. A high sulfur cycle turnover rate would have left the ocean system vulnerable to development of widespread euxinic conditions, posing a sustained threat to marine life during the Early Triassic.

Carbon and Phosphorus Cycling in Arabian Sea Sediments across the Oxygen Minimum Zone

Several studies have focused on carbon, oxygen, and phosphorus dynamics across the modern oxygen minimum zone (OMZ) to constrain how signals of modern systems get “locked in” upon burial. In this study, a sequential phosphorus fractionation technique was applied to surficial and sub-surface sediments from stations at depths spanning the OMZ on the Pakistan margin of the Arabian Sea in order to test the oxygen-carbon-phosphorus connection in modern marine sediments. Some early diagenetic loss of phosphorus compared to organic carbon was observed, but a significant portion of the released phosphorus was retained by uptake on oxyhydroxides and by the formation of an authigenic phosphorus-bearing phase. This process is unaffected by station location relative to the OMZ, and results in an effective organic carbon-to-reactive-phosphorus sediment ratio that is close to the average observed for open-ocean sediments, regardless of bottom water oxygen content.

A Review of Research Progress in the Genesis of Colloform Pyrite and Its Environmental Indications

AbstractColloform pyrite is a special form of nano‐micro polycrystalline aggregation growth, for which a suitable term is “aggregates of nano‐micro crystals”. This kind of colloform texture is observed in various geological bodies, such as ancient sedimentary rocks, modern marine and lake sediments, various types of ore deposits, and modern seafloor hydrothermal vents. This paper summarizes the latest developments and research into the definition, formation mechanisms, and environmental indications of colloform pyrite. There appears to be three main formation mechanisms of colloform pyrite: pseudomorphic replacement; biogenic precipitation; and inorganic precipitation. The morphology, particle size, trace element content and preferential growth orientations of colloform pyrite microcrystals can be important indicators for sedimentary environments, hydrothermal activity, and ore‐forming processes. We suggest that the microscopic features of nano‐micro crystals in colloform pyrite and their aggregation growth patterns need further investigation. The relationships between formation mechanisms of colloform pyrite, organic activity and depositional environments require further exploration. To reveal the nature of nano‐micro grain aggregation growth in colloform pyrite and analyse its growth environment and evolutionary history, it is supposed to apply nanoscientific and nanotechnological methods, further integrate consideration of macroscopic geological backgrounds and microscopic mineral growth phenomena, combine high–resolution imaging systems and in situ quantitative microanalysis methods and constitute a mergence of earth science, thermodynamics and kinetics, life science, material science, and chemistry in the study.

Sulfur Isotope Effects of Dissimilatory Sulfite Reductase.

The precise interpretation of environmental sulfur isotope records requires a quantitative understanding of the biochemical controls on sulfur isotope fractionation by the principle isotope-fractionating process within the S cycle, microbial sulfate reduction (MSR). Here we provide the only direct observation of the major (34S/32S) and minor (33S/32S, 36S/32S) sulfur isotope fractionations imparted by a central enzyme in the energy metabolism of sulfate reducers, dissimilatory sulfite reductase (DsrAB). Results from in vitro sulfite reduction experiments allow us to calculate the in vitro DsrAB isotope effect in 34S/32S (hereafter, ) to be 15.3 ± 2‰, 2σ. The accompanying minor isotope effect in 33S, described as , is calculated to be 0.5150 ± 0.0012, 2σ. These observations facilitate a rigorous evaluation of the isotopic fractionation associated with the dissimilatory MSR pathway, as well as of the environmental variables that govern the overall magnitude of fractionation by natural communities of sulfate reducers. The isotope effect induced by DsrAB upon sulfite reduction is a factor of 0.3–0.6 times prior indirect estimates, which have ranged from 25 to 53‰ in 34εDsrAB. The minor isotope fractionation observed from DsrAB is consistent with a kinetic or equilibrium effect. Our in vitro constraints on the magnitude of is similar to the median value of experimental observations compiled from all known published work, where 34εr−p = 16.1‰ (r–p indicates reactant vs. product, n = 648). This value closely matches those of MSR operating at high sulfate reduction rates in both laboratory chemostat experiments ( 17.3 ± 1.5‰, 2σ) and in modern marine sediments ( 17.3 ± 3.8‰). Targeting the direct isotopic consequences of a specific enzymatic processes is a fundamental step toward a biochemical foundation for reinterpreting the biogeochemical and geobiological sulfur isotope records in modern and ancient environments.

Determination of the stable iron isotopic composition of sequentially leached iron phases in marine sediments

Reactive iron (oxyhydr)oxide minerals preferentially undergo early diagenetic redox cycling which can result in the production of dissolved Fe(II), the adsorption of Fe(II) onto particle surfaces, and the formation of authigenic Fe minerals. The partitioning of iron in sediments has traditionally been studied by applying sequential extractions that target operationally-defined iron phases. Here, we complement an existing sequential leaching method by developing a sample processing protocol for δ56Fe analysis, which we subsequently use to study Fe phase-specific fractionation related to dissimilatory iron reduction in a modern marine sediment. Carbonate-Fe was extracted by acetate, easily reducible oxides (e.g. ferrihydrite and lepidocrocite) by hydroxylamine–HCl, reducible oxides (e.g. goethite and hematite) by dithionite–citrate, and magnetite by ammonium oxalate. Subsequently, the samples were repeatedly oxidized, heated and purified via Fe precipitation and column chromatography. The method was applied to surface sediments collected from the North Sea, south of the island of Helgoland. The acetate-soluble fraction (targeting siderite and ankerite) showed a pronounced downcore δ56Fe trend. This iron pool was most depleted in 56Fe close to the sediment–water interface, similar to trends observed for pore-water Fe(II). We interpret this pool as surface-reduced Fe(II), rather than siderite or ankerite, that was open to electron and atom exchange with the oxide surface. Common extractions using 0.5M HCl or Na-dithionite alone may not resolve such trends, as they dissolve iron from isotopically distinct pools leading to a mixed signal. Na-dithionite leaching alone, for example, targets the sum of reducible Fe oxides that potentially differ in their isotopic fingerprint. Hence, the development of a sequential extraction Fe isotope protocol provides a new opportunity for detailed study of the behavior of iron in a wide range of environmental settings.

Extreme eolian delivery of reactive iron to late Paleozoic icehouse seas

The biogeochemical impacts of iron-rich dust to the oceans are known for Earth’s recent record but unexplored for deep time, despite recognition of large ancient dust fluxes, particularly during the late Paleozoic. We report a unique Fe relationship for Upper Pennsylvanian mudrock of eolian origin that records lowstand (glacial) conditions within a carbonate buildup of western equatorial Pangaea (western United States) well removed from other detrital inputs. Here, reactive Fe unambiguously linked to dust is enriched without a corresponding increase in total Fe. More broadly, data from thick coeval loess deposits of western equatorial Pangaea show the same marked enrichment in reactive Fe. This enrichment—atypical compared to modern marine, fluvial, glacial, loess, and soil sediments—suggests an enhancement of the reactivity of the internal Fe pool that increased the bioavailability of the Fe for marine primary production. Regardless of the mechanism behind this enhancement, our data in combination with other evidence for high dust fluxes imply delivery of extraordinarily large amounts of biogeochemically reactive Fe to glacial-stage late Paleozoic seas, and modeling of this indicates major impacts on carbon cycling and attendant climatic feedbacks.

Geochemistry of sediments in the modern Aral Basin

The study presents the first geochemical data on the bottom sediments from the modern desiccating Aral Sea, which allowed a direct comparison between the compositions of the sediments collected during the pre- and postdesiccation periods. This study revealed the differences and similarities in the major and trace element composition of the sediments collected from the former bottom of the Large Aral Sea, the present-day sea bottom, the desiccated sea bottom, and the Amu Darya suspended sediments with respect to the average shale composition. The euxinic sediments from the western deep-water depression are characterized by high concentrations of U, Mo, and organic matter. The precipitation of evaporite salts occurs at the shallowest depths and in the western depression. The salt deposits exposed by the shrinking sea tend to be desalinated due to intense weathering. The mercury detected at high levels in the river suspended sediments of the former Amu Darya discharge was found to be absent in the modern marine sediments due to reductions in the river flow and the possible migration to the atmosphere. Many other trace elements, besides uranium, may also accumulate in seawater.

Some aspects of excellent marine source rock formation: implications on enrichment regularity of organic matter in continental margin basins

Through the analysis of ocean organisms, the distribution characteristics and enrichment of organic matters in modern marine sediments and ancient marine strata, this paper shows that the main factors influencing the formation of excellent marine source rocks are the paleoclimate, biologic productivity, terrestrial organic matter, oxidation–reduction environment, sedimentation rate, and the type of the basin. In addition to those factors, high biologic productivity or high content of terrestrial organic matter input is a requirement for the enrichment of the organic matter in a marine environment. Reducing environment was favorable for organic matter accumulation and preservation in depositing and early diagenesis stage, which is an important element for the formation of high-quality marine source rocks. Paleoclimate also influences the marine source rocks formation, as humid subtropical and tropical climates are the most favorable regimes for the formation of marine source rocks. Wind transports some vascular plant materials into the marine environment. Furthermore, upwellings driven by steady wind can cause high biologic productivity, thus forming organic-C-rich mud. Suitable sedimentation rate is beneficial for marine organic matter accumulation. Moreover, the type of the basin also plays an important role in the development of marine source rocks. Silled basins with a positive water balance often act as nutrient traps, thus enhancing both productivity and organic matter preservations, while in open oceans, organic matter enrichment in sediments has just been found in the oxygen minimum layers.

A NOVEL APPLICATION OF DIGITAL IMAGE TREATMENT BY QUANTITATIVE PIXEL ANALYSIS TO TRACE FOSSIL RESEARCH IN MARINE CORES

ABSTRACT Ichnological analysis in cores has grown substantially in the past two decades because of its importance in fields such as paleoecology, biostratigraphy, or reservoir characterization. Yet core analysis entails some added difficulties in comparison to ichnological outcrop study, especially in modern marine sediments. Quantitative pixel analysis conducted on high-resolution images is used to facilitate the study of bioturbation in marine cores from site U1385 of IODP Expedition 339 in the Gulf of Cadiz. Lateral and vertical variation of pixel values obtained from the infilling trace fossils and from the host sediment are revealed to be highly useful to: (1) differentiate between trace fossils and biodeformational structures, including the host sediment; (2) discriminate between ichnotaxa (i.e., Palaeophycus vs. Planolites); (3) differentiate between passively and actively infilled structures (e.g., Thalassinoides vs. Planolites), and (4) characterize the horizon of bioturbation and thus evaluate t...