Modern Marine Sediments Research Articles

Formation of authigenic titania during the alteration of volcanic glasses in modern deep-sea environments

Titanium (Ti), a typical rock-forming element, is traditionally considered to be lithophile, incompatible, and fluid-immobile, and is recognized as a significant proxy for understanding the oceanic environments, such as the content of terrestrial materials in modern marine sediments. However, recent investigations have revealed the production of various titania minerals through the alteration of volcanic rocks. Considering the occurrence of abundant volcanic rocks (e.g., volcanoclastics, seamounts, and oceanic crusts) in the oceanic environments, the activities of Ti may have been underestimated. Our study provides novel findings on the occurrence and growth mechanism of authigenic titania in the completely altered volcanic glasses collected from Line and Marshall seamounts by using multiple microanalytical methods. Both specimens primarily consist of mixed-layer illite/smectite (I/S), phillipsite, and carbonate-bearing fluorapatite, revealing that the volcanic glasses underwent palagonization processes. Scanning electron micrographs reveal numerous spherical Ti-rich shells with thicknesses of 450–1000 nm, enveloping spherical mixed-layer I/S aggregates with diameters of dozens to hundreds of microns. From the center to the edge of the spherule-shell structures, the concentration of Ti significantly increased. Further analysis using transmission electron microscopy confirms the presence of authigenic Fe-bearing titania particles within these Ti-rich shells. These titania shells likely form as a result of the migration of dissolved Ti(IV) ions and titania nanoparticles, which fill, nucleate, and aggregate at the interfaces of microspherules originally present in the volcanic glasses, ultimately coalescing into micron-sized particles through oriented attachment. Additionally, numerous titania nanoparticles, with aggregate sizes of 4–32 nm, were observed in the pore spaces of the mixed-layer I/S aggregates, likely be those trapped during Ti migration. Brookite is the dominant titania phase in both the titania shells and the smectite pores, with rutile and anatase occasionally present in the titania shells, implying the continuous mineral phase evolution during the growth of titania particles. The occurrence and distribution of titania suggest short-distance migration of Ti during palagonization processes. These findings offer new insight into the brookite formation and the Ti mobilization in basic and oxygen-rich marine environments, which is crucial for understanding the marine geochemical behaviors of Ti.

Late Mesoproterozoic iron and manganese deposition on a deep-water carbonate platform (Penganga Group), Adilabad, India: Constraints from C, O and Sr isotopes

Abstract The Penganga Group in the Pranhita-Godavari Rift Valley of central India includes a rather unique example of Late Mesoproterozoic iron and manganese formations deposited in a deep-water below storm wave base distal carbonate platform environment. The carbonate rocks of the Penganga Group are mostly lime-mudstones that have been classified into a number of sedimentary cycles with intervals mostly distinguished by color, ranging from brown/pink and siliceous gray to steel-gray and black. At least two about one-meter-thick Mn-Fe formations occur in siliceous gray limestone of the distally steepened part of the Penganga carbonate platform. A number of northwest-southeast striking thrust faults repeat the succession in down dip sections bringing up distal profiles of the Penganga Group against proximal counterparts at the present exposure level. This contribution provides the first comprehensive record of stable C and O and radiogenic Sr isotopes for proximal and distal profiles, allowing for direct comparison and thus, providing context for the environmental conditions of Fe and Mn deposition in a Mesoproterozoic platform environment. Carbon and oxygen isotope compositions determined from the proximal and distal profiles of the platform yield trends that are attributed to variations in circulation pattern and cycles of warm and cold climatic stages across the platform. Secular trends of Sr-isotope compositions suggest a variation in continental input between cold (0.7065) and warm climatic stages (0.7149), similar to that observed for modern marine sediments. The sedimentation of siliceous deposits and ferromanganese deposits in the distal platform environment was likely favored by the onset of upwelling circulation during cold climatic stages. The moderately negative δ13CPDB values (-5.34‰ to -6.34‰) of the Mn-carbonates and variation in δ13Corg values 31.7‰ to -21.7‰ obtained for early diagenetic Mn-carbonate ovoids indicate oxidation of organic matter in the ferromanganese deposits by Mn-oxides during early diagenesis. We conclude that upwelling and platform-wide deposition of ferromanganese deposits on the Penganga carbonate platform may mark the transition from the stagnant Mid-Proterozoic ‘sulphidic’ ocean to a ‘respiring’ open ocean during the Late Mesoproterozoic Era.

Large sulfur isotope fractionation during abiotic consumption of hydrogen sulfide after cessation of bacterial growth in batch culture experiments of sulfate reducing bacteria

Microbial sulfate reduction is a key process in the oceanic sulfur cycle and imparts a large sulfur isotope fractionation. The mechanism of sulfur isotope fractionation during microbial sulfate reduction has been studied from the geochemical and biochemical aspects since the 1950s. Recently, however, the large sulfur isotope fractionation, exceeding 47‰, has been observed in some pure culture experiments of sulfate reducing bacteria and the tentative understanding of microbial sulfur isotope fractionation has not well explained the mechanism. Here we quantified growth phase dependent sulfur isotope fractionation of a type sulfate reducer Desulfovibrio desulfuricans (DSM642) and revealed that the magnitude of isotope fractionation increased from -13.4 ± 3.6‰ in early exponential phase to -65.9 ± 21.0‰ in later exponential phase having clear negative correlation with cell specific sulfate reduction rate. Our results show the importance of cells growth phase and states that control cell specific sulfate reduction rate and sulfur isotope fractionation. Although microbial sulfate reduction likely continued to stationary phase of cells, microbial sulfur isotope fractionation could not be quantified because of the decreasing total dissolved sulfide (ΣH2S: H2S, HS−, and S2−) concentration after stationary phase. A non-microbial ΣH2S-consuming reaction is interpreted to have occurred during the stationary phase and this had large sulfur isotope fractionations, -10.5 ± 1.1‰ and -45.6 ± 12.4‰. Besides, the non-microbial ΣH2S-consuming reaction decreased Δ33S’ value of the ΣH2S. While the end-product of the non-microbial ΣH2S-consuming reaction in the stationary phase remains unidentified, we observed precipitates of some sulfide minerals and organic sulfur in the stationary phase media. Regardless of the end-product, the reaction increases the sulfur isotopic composition of dissolved ΣH2S, which can account for the high sulfur isotopic compositions of ΣH2S compared to pyrite and organic sulfur observed in some modern marine sediments.

High primary productivity in an Ediacaran shallow marine basin influenced by strong seasonal to perennial upwelling

AbstractA significant area of late Neoproterozoic–early Cambrian seafloor hosted a ferruginous to euxinic condition as a result of expanded primary productivity-driven pumping of organic matter into subsurface water column and weak water column mixing in the concomitant sea. However, the cause and extent of increased marine primary productivity during this time interval remain unknown. To estimate the primary productivity in a late Neoproterozoic sea, this study investigated the Sirbu Shale, Vindhyan Supergroup, for trace elements, organic carbon isotopes and total organic carbon (TOC). Among the trace elements, cadmium (Cd), known for extremely low concentration in crustal rocks but higher abundance in biogenic organic matter, was the key parameter in the palaeoproductivity estimation. The Cd enrichment in the Sirbu Shale samples is comparable to that in modern marine sediments of the oxygen minimum zones in Chilean margins, Arabian Sea and Gulf of California characterized by high primary productivity and seasonal upwelling. In terms of Cd enrichment, the lower section of the Sirbu Shale was deposited under suboxic conditions, while the upper section was deposited under a relatively less reducing condition. Cd/Mo ratios > 0.36 in the shale sample indicate that the palaeoproductivity was strongly influenced by the nutrient supply through sea-shelf upwelling. Using non-detrital enrichment of Cd in Sirbu Shale samples, we calculated that the TOC exported to the floor of Sirbu Shale palaeodepositional setting through primary productivity ranged from 0.71 to 10.16%.

Pyrites in a salt marsh-ria system: Quantification, morphology, and mobilization

Galician Rias are among the most productive ecosystems in the world. Consequently, the soils of their salt marshes and sediments of the intertidal flats show high organic matter contents, reactive Fe, and sulfate, which promote pyrite synthesis and accumulation, using sulfate for organic matter decomposition. This work studies the morphological variability and concentration of pyrites (individual crystals and framboids) in different geochemical environments found in the Ria de Ortigueira (salt marsh soils and bottom sediments in the inner, middle, and outer section), addressing their dynamics in the marsh-ria system in relation to the hydrodynamic characteristics defined by tides and river discharges. Framboidal pyrites were the dominant morphology in marsh soils and sediments in the middle and inner sections of the Ria, while isolated crystals dominated its outer section. The results showed that lower marsh soils (colonized by Spartina) are the most favorable environment for pyrite synthesis, showing high pyritic Fe concentrations and high degrees of pyritization, largely exceeding the values observed in sediments from Galician Rias and from most sedimentary environments worldwide. However, the amount of framboidal pyrites present in the lower marsh (SPE: 4–5 × 104 framboids) was clearly lower than in bottom sediments of the inner and middle part of the Ria de Ortigueira (∼2–7 × 106 framboids), mainly due to the fact that pyrites were found to form large framboids in lower salt marsh soils. Thus, the amount of framboidal pyrites does not seem to be a good indicator of redox conditions in modern marine sediments. Pyrite crystals found in the sediments of the Ria showed poorly defined vertices and facets, indicating their degradation and suggesting that a significant amount of the pyrites found in the middle and inner sections derive from marsh collapse. Finally, the output of framboidal pyrites towards the outer Ria de Ortigueira reflects the low intensity of residual flows in this Ria. Therefore, the pyrites observed in the outer section consisted only of isolated crystals, presumably formed in situ under low sulfate-reducing activity conditions.

Potassium isotope signatures in modern marine sediments: Insights into early diagenesis

The sedimentary fluxes controlling potassium (K) budget of the oceans and the isotope composition of K (δ41K) potentially yield valuable information about the global K cycling. However, at present sedimentary diagenesis in the oceans is one of the least well-known components of the seawater K budget. This study presents a dataset of the modern (<50 cm) sediments from a range of hydrographic regimes including continental margin settings (the Peru Margin, California Borderland, and Mexican Margin) and deep-sea environments (the Equatorial Pacific). We determine the K isotope compositions and origins of K in the sediments and compare them with the results of elemental abundances and spectroscopic analyses. The wide variability in sedimentary K/Al and Rb/K ratios, K-phase distributions, and K isotope compositions support the interplay between continental weathering, which provides clastic inputs, and sedimentary diagenesis, which constitutes an important sink of seawater K. The net result of these processes potentially alters the K elemental and isotopic budgets in sediment strata of various marine environments. We demonstrate that K is dominantly hosted by illite, glauconite, and feldspars, and regional K-phase partitioning modulates marine sedimentary K budget. For example, considerable amounts of K hosted in illite (up to 90%) primarily of terrestrial origins increase Rb/K ratios, producing low δ41K in California Borderland sediments (−0.57 to −0.40‰), the Pescadero Slope from the Mexican Margin (−0.42 to −0.39‰), and Equatorial Pacific deep-sea basins (−0.56 to −0.44‰). In contrast, glauconite authigenesis causes decreases in the Rb/K ratio and relatively high δ41K at a site along the Peru Margin (−0.35 to −0.29‰), as well as at two other sites along the Mexican Margin (−0.39 to −0.27‰). Despite the presence of feldspar-K (<20%), δ41K in the sediments reflects dominant contributions of detrital illite (δ41Killite ∼ −0.56‰) and authigenic glauconite (δ41Kglauconite ∼ −0.18‰). We conclude that the formation of glauconite in the sediments takes up isotopically light K from seawater and serves as an important sink of seawater K (including K in seawater-derived porewater) (∼1 to 16, average ∼6 TgK⋅yr−1) during early diagenesis, especially on continental margins.

A global reassessment of the controls on iron speciation in modern sediments and sedimentary rocks: A dominant role for diagenesis

The speciation of iron in sediments and sedimentary rocks is a widely used proxy for the chemistry and oxidation state of ancient water bodies. Specifically, the fraction of reactive iron out of the total iron (FeHR/FeT) and the fraction of pyrite iron out of the reactive iron pool (FePYR/FeHR) are thought to constrain the oxidation state and the presence of sulfide in the water column, respectively. This approach was developed and tested against modern core-top sediments, but application to sedimentary rocks requires consideration of the effects of diagenesis and lithification on iron speciation. Furthermore, the effects of deep burial, metamorphism, and late-stage alteration during exhumation or sampling (e.g., oxidative weathering) have not been systematically explored. To bridge this gap, we combined new data from four sediment cores (n = 54) with an extensive literature compilation of modern sediments (2936 measurements from 316 cores) and ancient sedimentary rocks (12,173 measurements spanning the Neoarchean to Quaternary). The modern data include both surface and buried sediments, allowing an investigation of the effects of diagenesis on iron speciation. Depending on the thresholds used to distinguish oxic from anoxic environments and ferruginous from euxinic environments, interpretation of the modern sedimentary iron speciation data within the existing framework yields incorrect environmental classifications up to ≈70% of the time. In modern sediments, diagenesis is the main reason that iron speciation does not represent the chemistry and oxidation state of the water column. We find that iron speciation correlates with porewater chemistry and that it changes with progressive burial along three distinctive FeHR/FeT–FePYR/FeHR arrays, each of which represents a different set of diagenetic processes. We suggest that similarly to modern sediments, stratigraphic variation in iron speciation in sedimentary rocks primarily reflects progressive burial diagenesis or variation in depositional conditions rather than temporal variation in water-column chemistry and oxidation state. Indeed, analysis of the geologic iron speciation data reveals no statistically significant trends in either FeHR/FeT or FePYR/FeHR from the Archean to the present day. The diagenetic FeHR/FeT–FePYR/FeHR arrays that we identified in modern marine sediments suggest that under certain conditions, iron speciation analyses may be used to constrain FeHR/FeT in the local sediment source(s). Hence, we suggest that iron speciation data, together with complementary petrographic, mineralogical and geochemical constraints, may be used to constrain the local iron source(s) and early and late diagenetic processes, but rarely the chemistry or oxidation state of ancient water columns.

Covariation between molybdenum and uranium isotopes in reducing marine sediments

The isotopic compositions of redox-sensitive elements such as uranium (U) and molybdenum (Mo) have been extensively used to track the evolution of redox conditions on Earth's surface. However, additional investigation of the behaviors of U and Mo in modern marine sediments—particularly in reducing sediments underlying oxic seawater—can bolster understanding and improve application of these systems as biogeochemical tracers. Here we present elemental and isotopic Mo and U data from reducing sediments from various sites in the Bering Sea where the extent of bioturbation is well characterized. Total Mo concentrations and total organic carbon contents (TOC) are positively correlated, as are authigenic Mo isotopic compositions and TOC, indicating the presence of authigenic Mo enrichments in these sediments. Similar to what has previously been observed in euxinic settings, we observe a positive correlation between authigenic U and Mo concentrations, and a negative correlation between the isotopic compositions of authigenic U and Mo (δ238Uauth and δ98Moauth). These patterns likely reflect close associations of U and Mo reduction to organic carbon degradation. Observed variation in δ98Moauth values could reflect variations in Mo removal efficiency or Mo isotopic fractionation between particles and seawater related to porewater H2S levels. Conversely, we interpret variation in δ238Uauth values in these Bering Sea samples to be related to varying extents of U reduction and removal, coupled with anaerobic carbon degradation. There is no significant correlation between the extent of bioturbation and Mo or U isotope values, suggesting that bioturbation would likely not have a significant influence on sedimentary Mo and U records. Further, the array of U and Mo isotopic data from these sediments is distinct from those observed in anoxic sediments below euxinic seawater—suggesting that a combination of U and Mo isotopic compositions could help to more accurately resolve the redox state of depositional environments from past intervals of Earth's history.

Uranium isotope reconstruction of ocean deoxygenation during OAE 2 hampered by uncertainties in fractionation factors and local U-cycling

A δ238U record of changing ocean anoxia during OAE 2 is reconstructed using seawater derived U in pelagic marine sediments in the Portland #1 core in the south-central region of the Western Interior Seaway of North America. The peak negative excursion of 1.4‰ in authigenic sedimentary δ238U values is consistent with expansion of marine anoxia during the event, but the size of the shift is much larger than the negative excursions recorded in two other published records: (1) a pelagic chalk succession at Eastbourne (0.3‰) in the Paris-Anglo basin, and a black shale succession at Demerara Rise (0.15‰) in the proto-North Atlantic Ocean. By incorporating the estimated fractionation factors of processes governing removal of U into modern marine sediments, proxy data on pore fluid oxygen penetration into the sediment, faunal information on circulation in the Western Interior Seaway, and an ocean U box model, we identify and discuss the complications that hampered our effort to find a consensus value for the change in the δ238U value of the ocean U reservoir during OAE 2. These include: (1) strong local controls governing post-depositional uptake of seawater-derived U isotopes in marine sedimentary successions, and (2) local U cycling effects in epicontinental seas, which can work together or alone to mask the true change in the δ238U value of the ocean U reservoir during ocean anoxic events. Correcting the Portland record for stratigraphic changes in fractionation factors reduced the magnitude of the excursion to 0.9‰. Using an ocean U box model, this corresponds to 21% of the area of the seafloor covered by anoxic bottom waters during OAE 2. However, since the study setting represents an epicontinental sea that was located thousands of kilometers from the open ocean, we cannot exclude the possibility that the shift in seawater δ238U values was amplified by local U cycling effects, or that smaller negative excursions recorded elsewhere in the proto-North Atlantic Ocean were diminished by fractionation factor uncertainty and/or local U cycling in those settings.

Co-variation systematics of uranium and molybdenum isotopes reveal pathways for descent into euxinia in Mediterranean sapropels

The elemental concentrations and isotope compositions of molybdenum (Mo) and uranium (U) are commonly used for the reconstruction of past global and local redox conditions, and recent studies using both elements have revealed the potential of their paired application. However, such studies have generally focused either on modern marine sediments or on relatively low-resolution reconstructions of deep-time paleo-redox conditions. Here, we present high-resolution profiles (every 0.2-0.3 kyr) of Mo and U elemental and isotope compositions for anoxic organic-rich sediments of Eastern Mediterranean sapropels S5 and S7. The new Mo-U data reveal the processes leading to descent into basinal euxinia in more precise and systematic detail than lower resolution datasets focused on either Mo or U only.During the intensification from anoxic non-sulfidic to persistently euxinic conditions, δ98Mo and δ238U covariation systematics exhibit two stages. We identify the Mo-U isotope signature of the early transition from anoxic non-sulfidic to mildly euxinic conditions in the first stage, characterized by a rise in U isotope ratios (from −0.3‰ to +0.2‰ ±0.05‰) controlled by the depth within the sediment of the uranium reduction-accumulation front. As the water column turns persistently euxinic, δ98Mo values approach the seawater value for both sapropels, but δ238U evolves to different final values in the two sapropels. We interpret these differences as reflecting different redox potentials and/or different degrees of restriction of these two sapropel events, and the more gradual response of U sequestration to redox variation in comparison with threshold behavior of Mo. The findings presented here show temporal patterns in δ98Mo and δ238U on relatively short timescales that suggest the combined use of these proxies at high resolution allows detailed reconstruction of local redox and hydrographic conditions.

About terraces on the shore of the Teriberskaya Bay of the Barents Sea (the Kola Peninsula)

The paper considers the results of the study of modern terraces on the coast of the Lodeyny Peninsula in the Barents Sea. According to field observations, decoding of space and geodetic maps and volumetric modeling, signs of three terraces have been identified. Two terraces having a rear seam, a cliff, a brow and horizontal platforms have been studied in more detail. The rear seams are located at 7 and 30 m, and the edges at 10 and 40 m above the sea level. The third terrace is confined to the level of 40-60 m, and has not been studied in detail. According to estimates, the first two terraces were formed about 3,300 and 6,000 years ago respectively. The 1st terrace characterizes the period of slow land rise by 10 m, at the speed up to 3 mm/year, which began about 3,300 years ago and continues to the present. It is characterized by a uniform sea retreat of about 15 mm/year. The erosion products of this terrace are common in the lateral zone and represent modern marine sediments, including beaches forming a new terrace. The 2nd terrace characterizes to the dynamics of rapid land rise by 30 m at a speed of about 11 mm/year, which occurred in the period from 6,000 to 3,300 years ago. The erosion products of this terrace are most common on the coast and are found at a distance of 100 m to 2-3 km from the shore. It is assumed that the speed of movement of the coastline during the retreat of the sea was different and reached up to 900 mm/year. According to the authors, preliminary data indicate the instability of the territory and the manifestation of block movements, which may have a negative impact on the development of infrastructure.

Why do calcareous microfossils preferentially occur in tuffaceous sandstones of the Santa Marta formation (James Ross Island, Antarctica)?

The Santa Marta Formation (James Ross Island, Antarctica) presents one of the most complete Santonian-Campanian high-latitude marine records in the Southern Hemisphere. Recent micropaleontological studies pointed to an unusual preferential preservation of calcareous microfossils, such as foraminifera and nannofossils, in tuffaceous sandstone levels of the Santa Marta Formation. Here we aim to characterize depositional and early diagenetic conditions that influenced preservation of calcareous microfossils within the succession of the Santa Marta Formation (Alpha Member). We used X-ray fluorescence (FRX)-derived elemental ratios, as well as carbonate (CaCO3), total organic carbon (TOC), and total sulfur (TS) contents to characterize sediment properties, as well as X-ray diffraction (XRD)-derived mineralogical analysis. Elemental ratios suggest hydrothermal influence during diagenesis throughout the stratigraphic succession, as indicated by (Fe + Mn)/Ti values > 20. This interpretation is supported by the occurrence of typical low-temperature hydrothermal minerals, such as garronite and heulandite. Contribution of volcanic ashes in the tuffaceous sandstone levels is evidenced by high values of the log (Zr/Ti) ratio, and the presence of volcanogenic minerals. Since stratigraphic levels characterized by the highest abundances and richnesses of calcareous microfossils also present high log (Zr/Ti) values and CaCO3 contents, we hypothesize that the input and subsequent alteration of volcanogenic material (e.g., ashes) acted as a buffer, increasing porewater alkalinity during early diagenesis. Such a chemical mechanism was already described for modern marine sediments deposited under the influence of significant input of volcanogenic materials, and can explain the preferential preservation of calcareous microfossils in tuffaceous sandstones of the Upper Cretaceous Santa Marta Formation.

Quantifying the Seawater Sulfate Concentration in the Cambrian Ocean

Although the earliest animals might have evolved in certain “sweet spots” in the last 10 million years of Ediacaran (550–541 Ma), the Cambrian explosion requires sufficiently high levels of oxygen (O2) in the atmosphere and diverse habitable niches in the substantively oxygenated seafloor. However, previous studies indicate that the marine redox landscape was temporally oscillatory and spatially heterogeneous, suggesting the decoupling of atmospheric oxygenation and oceanic oxidation. The seawater sulfate concentration is controlled by both the atmospheric O2 level and the marine redox condition, with sulfide oxidation in continents as the major source, and sulfate reduction and pyrite burial as the major sink of seawater sulfate. It is thus important to quantify the sulfate concentration on the eve of the Cambrian explosion. In this study, we measured the pyrite contents and pyrite sulfur isotopes of black shale samples from the Yurtus Formation (Cambrian Series 2) in the Tarim Block, northwestern China. A numerical model is developed to calculate the seawater sulfate concentration using the pyrite content and pyrite sulfur isotope data. We first calibrate some key parameters based on observations from modern marine sediments. Then, the Monte Carlo simulation is applied to reduce the uncertainty raised by loosely confined parameters. Based on the geochemical data from both Tarim and Yangtze blocks, the modeling results indicate the seawater sulfate concentration of 8.9–14 mM, suggesting the seawater sulfate concentration was already 30–50% of the present level (28 mM). High seawater sulfate concentration might be attributed to the enhanced terrestrial sulfate input and widespread ocean oxygenation on the eve of the Cambrian explosion.

Predominant microbial iron reduction in sediment in early Cambrian sulfidic oceans

Microbial sulfate reduction (MSR) and microbial iron reduction (MIR) are two major anaerobic metabolic pathways, and account for the majority of anaerobic organic matter degradation in modern marine sediments. Because it is thermodynamically more favorable, the zone of porewater Fe2+ accumulation usually overlies the zone of porewater H2S accumulation in modern marine sediments. The sequence of redox reactions is collectively known as the ‘redox latter’, which has been used as a yardstick in reconstructing the marine redox landscape in Earth's history. To understand the redox profile in the early Cambrian oceans, here we analyzed syndepositional and diagenetic pyrite (FeS2) in carbonate concretions hosted in the black shale of the early Cambrian Shuijingtuo Formation in the Yangtze Block, South China. Syndepositional pyrite laminae, disseminated pyrite and diagenetic pyrite aggregates in the periphery of carbonate concretions have nearly identical sulfur isotope compositions, which cannot be resolved by quantitative reduction of seawater sulfate. Instead, a homogeneous H2S source from sulfidic (H2S-enriched) seawater might be the major sulfur source, and pyrite precipitation was sustained by H2S diffusion from sulfidic seawater. In addition, the homogeneous pyrite sulfur isotopes also imply negligible MSR in sediment porewater, reflecting low sulfate concentration in sulfidic seawater. On the other hand, the enrichment of Fe2+ in carbonate concretions implies deposition in ferruginous (Fe2+-enriched) porewater, resulting from active MIR that reduced detrital iron oxides to Fe2+. Thus, the early Cambrian sulfidic ocean might be characterized by extensive MSR in seawater and MIR in sediment porewater, which differs from the modern sulfidic ocean, such as the Black Sea, showing predominantly MSR in sediment. The development of such a reverse redox profile might be attributed to the terrestrial supply of particulate iron oxides that passed through the thermodynamically unstable sulfidic water column and low seawater sulfate concentration in sulfidic seawater that prohibited MSR in sediment. Finally, our study indicates that the redox profile might be diverse in the early Cambrian oceans.

Mantle melting models of the Kızıldağ ophiolite in SE Turkey: Two types of partial melting processes in the oceanic upper mantle of southern Neo-Tethys

Many ophiolites in southern Neo-Tethys contain complete lithologic sequences, with the dikes and lavas commonly having highly variable geochemical compositions. In this study, we examine the sheeted dikes and layered and isotropic gabbros of the Kızıldağ ophiolite (southern Turkey) and suggest that the compositional variations of the rocks are likely related to melting models of the mantle. The sheeted dikes and gabbros of the ophiolite have 206Pb/204Pb(90 Ma) of 17.765–19.361, 207Pb/204Pb(90 Ma) of 15.570–15.665 and 208Pb/204Pb(90 Ma) of 37.812–39.269, straddling the fields of Indian Ocean MORB and modern marine sediments. The sheeted dikes have low rare earth element (REE) concentrations and LaN/YbN ratios (e.g., Yb < 2.5 ppm and LaN/YbN of 0.33 to 0.62 on average), comparable with those of forearc basalt (FAB), except for a few samples whose trace element concentrations and patterns are similar to those of boninite. All the studied samples have positive anomalies of large ion lithophile elements (LILE) (e.g., Rb, Ba, Th, U and Sr) on a N-MORB-normalized trace element diagram and show considerable scatter on the plot of Th/Yb vs. Nb/Yb, which is attributed to involvement of slab-derived melts/fluids in a protoarc-forearc setting. Thermodynamic modelling results suggest that the parental magmas of the sheeted dikes were combined melts derived by both polybaric continuous melting (<10%) and flux-melting (<25%) of the mantle wedge during subduction initiation. In contrast, clinopyroxene and plagioclase in the layered and isotropic gabbros have compositions similar to those of boninite. The gabbro compositions can be simulated by batch crystallization of the boninitic melts, and the relatively high trace element concentrations of some isotropic gabbros likely reflect a high fraction of interstitial liquid in the crystal mush. Thus, we propose that the compositional variations of the sheeted dikes reflect two types of mantle melting, and that the gabbros were produced by subsequent crystallization within a crystal mush having variable fractions of interstitial liquid. Not only can such complex magmatic processes explain the observed compositional variations of dikes and lavas of the Kızıldağ ophiolite, but may be appliable to other ophiolites in southern Neo-Tethys, such as the Lycian, Pozantı-Karsantı and Troodos massifs.

Controls on the cadmium isotope composition of modern marine sediments

Continental margin sediments have been identified as the dominant sink in the marine budget of cadmium (Cd). The isotopic composition of this important output flux is, however, unknown. Here we present, with measurements on the Argentine continental margin, the first observational constraints on the isotopic composition of Cd in modern marine oxic and sub-oxic sediments. We identify two main removal mechanisms of Cd; in organic material, and by sulfide formation. Surface margin sediments (0–0.5 cm), with dissolved O2 below detection from ∼0.5 cm, are isotopically lighter than overlying oxygenated waters. A mass balance for these surface sediments indicates that Cd is present dominantly as organically-bound particulate Cd. In sub-surface sediments, Cd concentrations increase in the zone of nitrate reduction, and attain similar isotopic compositions as the water that overlies the sediment (i.e. ∼0.35‰ in deep waters). These observations are consistent with a downward diffusive flux of seawater Cd and redox-driven quantitative removal of that Cd during sulfide precipitation. In combination, these two routes of Cd removal lead to burial of isotopically light organic Cd in margin sub-oxic sediments that enables the global isotopic Cd budget to be balanced.

Controls on the Precipitation of Carbonate Minerals Within Marine Sediments

The vast majority of carbonate minerals in modern marine sediments are biogenic, derived from the skeletal remains of organisms living in the ocean. However, carbonate minerals can also precipitate abiotically within marine sediments, and this carbonate mineral precipitation within sediments has been suggested as a third major, and isotopically distinct, sink in the global carbon cycle, particularly important earlier in Earth history. Here we present a global compilation of pore fluid data and compare the sulfate, calcium, phosphate and magnesium concentrations with pore fluid alkalinity to explore the emerging relationships and explore what drives carbonate mineral precipitation in sediments. Our data compilation shows that the gradient of pore fluid sulfate concentrations correlates strongly with the gradient of alkalinity as well as with the gradient of calcium, and that these correlations improve dramatically in sediments where methane is present. We also note that sedimentary pore fluids that are high in phosphate concentration are also high in alkalinity, which may indicate suppression of carbonate mineral precipitation in the presence of sedimentary phosphate. Our data can be used to highlight sediments where both dolomite formation and dolomitization of previously deposited calcium carbonate minerals is occurring. We explore how carbonate mineral saturation state changes as a function of calcium concentrations, alkalinity, and pH, and suggest a reason why calcium concentrations are never fully depleted in sedimentary pore fluids. We conclude that carbonate minerals precipitate in sediments with methane, where the anaerobic oxidation of this methane helps promote particularly high saturation states for carbonate minerals.

Copper isotope evidence of particulate shuttle dynamics in the Late Pennsylvanian North American Midcontinent Sea, with implications for glacio-eustatic magnitude

The North American Midcontinent Sea (NAMS) covered a vast area during Late Pennsylvanian glacio-eustatic highstands, providing a laboratory for analysis of the internal watermass dynamics of large cratonic seas (of which few exist in the Recent). In this study, a novel proxy, copper (Cu) isotopes, was used to gain a better understanding of NAMS watermass dynamics. We analyzed the 63-cm-thick Stark Shale (Dennis Formation, Missourian Stage, Upper Pennsylvanian) in the Iowa Riverton core (IRC) at a centimeter scale in order to reconstruct secular variations in Cu-isotopic compositions (δ65Cu) and other geochemical proxies. The gray shale facies yielded δ65Cu of +0.02 ± 0.06‰ (2σ, n = 14), similar to detrital Cu in modern marine sediments (δ65Cu = +0.08 ± 0.20‰; 2σ, n = 42). In contrast, the black shale facies, in which the proportion of authigenic Cu hosted by the organic fraction is 50–100% of total Cu content, exhibits heavier δ65Cuauth values, mostly between +0.09 and +0.43‰. In modern marine systems, one of the main processes leading to 65Cu-enriched sediment compositions is adsorption of aqueous Cu onto Fe-Mn particulates. The black shale facies of the Stark Shale exhibits four peaks in Mo/U ratios that are indicative of an active particulate shuttle because of the tendency of Mo to adsorb more strongly than U to Fe-Mn particulates. The Sr/Ba ratio, a paleosalinity proxy, shows correlated variations, with high values (indicative of more fully marine conditions) linked to low Mo/U and light δ65Cu, and low values (indicative of brackish conditions) linked to high Mo/U and heavy δ65Cu. These considerations suggest that the flux of isotopically heavy Cu to the sediment–water interface via Fe-Mn cycling was enhanced during brackish intervals, with subsequent reductive dissolution of Fe-Mn particulates allowing transfer of hydrogenous Cu to the organic fraction of the sediment. Control of δ65Cu by Fe-Mn cycling is supported by the results of a sequential extraction experiment showing that organic δ65Cu is positively correlated with Mo/U. These findings provide evidence of large fluctuations in watermass salinity and sea-level elevation within the NAMS at timescales of ∼104 yr during core shale deposition, as previously inferred but not conclusively demonstrated until now. The dynamics of particulate shuttle operation suggest that sea-level fluctuations must have had amplitudes of at least a few tens of meters. These fluctuations ended during the final stages of black shale deposition, probably owing to overall sea-level fall and permanent shallowing of the study site into the oxic, low-salinity surface layer of the NAMS.

Sedimentary vanadium isotope signatures in low oxygen marine conditions

It has been hypothesized that vanadium (V) isotopes have the potential to track sedimentary redox conditions due to multiple valence states occurring in nature, which might induce variable V isotope fractionation as a function of sedimentary redox state. These characteristica could make V isotopes a useful paleo-redox proxy. However, in order to understand the mechanisms driving V isotope fractionation, it is crucial to build a framework for the depositional and post-depositional controls on sedimentary V isotope records from a diverse set of sedimentary environments. This study, for the first time, investigates the V isotope variations of modern marine sediments deposited under a range of redox environments. Our results document that changes in local redox conditions impart a significant isotopic fractionation from seawater as recorded in the local sedimentary V isotopic signature. Importantly, there is a significant difference between the V isotope composition of sediments deposited in the open ocean setting with oxygen-deficient bottom waters compared to less reducing environments, whereby oxic sediments (benthic oxygen contents > 10 μΜ) exhibit Δoxic = −1.1 ± 0.3‰ and anoxic sediments exhibit Δanoxic = −0.7 ± 0.2‰. Combined with previous studies on seawater particulate and sediment pore fluid analysis, our results indicate that V is mainly delivered and enriched in anoxic sediments through settling particulates. Authigenic V isotope compositions in marine sediments are likely controlled by isotope fractionation between V species bound to particulates and dissolved in seawater, which likely varies with the speciation and adsorption properties of V that are strongly controlled by local redox conditions. In addition, the euxinic Cariaco Basin sediments exhibit distinctive Δeuxinic = −0.4 ± 0.2‰, which is likely influenced by the relationship between the seawater V removal rate and the seawater renewal rate. Our results highlight the direct link between authigenic marine sedimentary V isotope compositions and the overlying local redox conditions. This investigation of V isotopes in modern marine environments provides an initial framework for the utilization of V isotopes to reconstruct ancient redox fluctuations, which has the potential to track subtle redox variations of local oxygen-deficient to low oxygen environments.

Early diagenesis of iron and sulfur in Bornholm Basin sediments: The role of near-surface pyrite formation

Pyrite formation in marine sedimentary environments plays a key role in the global biogeochemical cycles of carbon, sulfur and iron, regulating Earth’s surface redox balance over geological time scales. The sulfur isotopic composition of pyrite is one of the major geochemical tools for investigating early diagenetic processes in modern marine sediments and substantive changes to the Earth’s surface environment in ancient sedimentary rocks. We studied sulfur–iron diagenesis and the sulfur isotopic evolution in sediments of the Bornholm Basin, southwestern Baltic Sea, to track the formation of pyrite in the near-surface sediments. Pyrite accumulation is observed with depth over the uppermost 100 cm before the extent of pyritization of the highly reactive iron pool (Fepy/FeHR) stays constant at ca. 0.9, suggesting that the use of a single iron-speciation parameter as a proxy for anoxic and sulfidic conditions needs to be supported by other independent indicators in sedimentary records. Stable sulfur isotopic analysis demonstrates that the bulk pools of elemental sulfur and iron monosulfide do not exchange isotopes completely with aqueous sulfide. We suggest that the reactions with polysulfide and aqueous sulfide are probably restricted to the surface of the solid-phase sulfur and iron-sulfur aggregates. Although pyrite is growing throughout the uppermost sediment column, the pyrite at depth has a sulfur isotopic composition similar to that of pyrite formed near the sediment surface. To understand the isotopic discrepancy between pyrite and aqueous sulfide in the deeper sediments, we developed a simple diagenetic model, which reproduces the observed sulfur isotopic composition of pyrite well. Our results suggest that much of the pyrite is rapidly formed near the sediment–water interface, and its δ34S is not as influenced by the 34S-enriched pool of aqueous sulfide in the deeper part of the sediment, allowing 32S-enriched pyrite to be preserved in deeper sediments. This near-surface diagenesis and the associated isotopic pattern are possibly of relevance for many marine sediments with high organic matter content, and high aqueous sulfide but low reactive iron availability. Moreover, our sulfur isotopic data demonstrate that extremely slow pyritization is ongoing in the deep lacustrine clay sediments. These results have implications for the interpretation of sulfur–iron geochemical data in both modern and ancient settings as well as for improving reconstructions of ancient depositional environments and a better understanding of the marine sulfur cycle throughout Earth’s history.