Modern Black Sea Research Articles

Potential of antique and medieval cultural heritage in the space of geoecosystems of the Northern Black Sea area and the Black Sea shelf from the Danube to the Berezan estuary

The article examines the typification and gives a brief description of underwater archaeological sites of ancient and medieval cultural heritage, the main types of ancient shores, as well as the most significant objects studied so far in the space of the geoecosystems of the Northern Black Sea (GESNBS) and the marine geoecosystem (MGES) of the shelf from the Danube delta to the Berezan estuary and the island of Berezan. The results of research on archaeological sites located in this territory are presented. A preliminary reconstruction of the shores that existed in this part of the Northern Black Sea Coast has been made. Nowadays, these archaeological sites are partially or completely located in the areas of the Black Sea shelf and its basin subsystems of a lower level (bays, ducts, estuaries, deltas, etc.), mainly at the boundary of their aquatic (from the Latin aqua — water) (AQESUS) and geological (GEOESUS) subsystems of the MGES shelf. The main objective of this article, which the authors set while writing it, was to show the potential of antique and medieval cultural heritage, which was formed in the relevant ancient times in the space of paleogeoecosystems of the Northern Black Sea but is now preserved in the MGES space of the modern Black Sea shelf from the Danube River to the Berezan estuary. Among the main tasks of this article is a brief description of the already known archaeological, in particular underwater, sites of antique and medieval cultural heritage, some other interesting underwater archaeological artifacts, and their future imposition on the modern geological, geographical and bathymetric framework with reference to certain AQESUS and GEOESUS of MGES of the Black Sea. This is important not only for historical and archaeological reconstructions but also for solving the problems of in-depth knowledge of the causes and consequences of changes in the paleogeological and modern geoecological conditions of the Azov-Black Sea basin and GESNBS shelf, dynamics and directions of their development. Such research is also important both for the development of public culture, expansion of the historical consciousness of the population of Ukraine and for solving some economic problems in the development of communities in the Northern Black Sea region by attracting investment to the development of coastal and underwater tourism, predicting the impact of many geoecological processes inherent in this studied region on the peculiarities of the conditions of search, identification, use and preservation of the historical and archaeological potential of antique and medieval cultural heritage located in the space of GESNBS and MGES of the modern Black Sea shelf as their anthropogenic component, affecting the lives of residents of the region under consideration, from ancient times until the present.

Sr isotope-salinity modelling constraints on Quaternary Black Sea connectivity

Reconstruction of the Black Sea's connectivity history during the Quaternary is based on high-resolution isotope data and longer-term biotic records. These demonstrate that the Black Sea experienced rapidly changing hydrologic configurations involving, at different times, isolation from and/or connection with the Mediterranean and Caspian seas. Here, we simulate these connectivity scenarios using a numerical box model of Black Sea hydrography. This allows us to explore the magnitude of the different water fluxes and their consequences for Black Sea salinity and Sr isotope ratio; parameters that we can measure today and reconstruct in the past. We show that the model is able to replicate the present-day hydrography using new Sr isotopic data measured on modern Black Sea water (0.709143 ± 0.000008) and that the Black Sea has a homogenous Sr isotope ratio despite its strongly stratified water column. We then create a 1.2 Myr record of paired Sr isotope (ostracods) and palaeosalinity (dinoflagellate cysts) data, using samples from DSDP Site 379. We also provide the first constraints on the Sr isotope ratio of the Amu Darya River, a substantial fluvial system that previously fed the Caspian Sea and therefore influenced the Black Sea when the two basins were connected. Modelling the different connectivity scenarios and incorporating both new and previously published geochemical and biotic data indicates that influx from the Caspian Sea dominated Black Sea hydrography between 1.2 to around 0.5 Ma. In contrast, Mediterranean input has exerted increasing influence since 0.6 Ma, particularly during interglacial periods. The hypothesis that an interval of anomalously high Sr isotope ratios in the Black Sea during the late Saalian deglaciation (MIS 6) was caused by input from the Amu Darya is demonstrably unrealistic. Instead the high Sr ratios may have been driven by variations in glacial extent within the Dnieper catchment and the resultant erosion of Sr from granitic basement rocks. Our new combined model-data approach tests and refines interpretations of the glacio-hydrographic development of the Black Sea during the second half of the Quaternary.

Stable W and Mo isotopic evidence for increasing redox-potentials from the Paleoarchean towards the Paleoproterozoic deep ocean

The scavenging of dissolved trace metals such as chromium (Cr), molybdenum (Mo) and tungsten (W) and their authigenic enrichment in sedimentary archives mainly occur by particle shuttling that cause resolvable isotope fractionation. Because their scavenging is also dependent on the local marine redox potential and the overall marine chemical environment, the stable isotope composition of these elements in Archean sediments is widely used as paleoredox proxy. Tungsten, a new element in this tool box, is dissolved as the oxyanion tungstate (WO42−) at extremely low marine redox potentials and can thus help to reconstruct the earliest changes in the marine redox state.We tested the applicability of stable W isotopes as a new paleoredox proxy of the atmosphere–ocean system. We analyzed the δ186/184W of Precambrian igneous rocks to investigate the detrital background δ186/184W signature. This Precambrian igneous inventory (PII) shows δ186/184W values between −0.007 and +0.097‰, identical to the range of modern igneous crustal rocks, which indicates that the average Earth’s crust δ186/184W remained constant over billions of years. Furthermore, we present δ186/184W values of euxinic sediments from the Black Sea as a modern sedimentary analog, which we then compare with δ186/184W data of Archean and Proterozoic black shales (3.47–2.3 Ga). Modern Black Sea sapropels reveal crustal-like δ186/184W values between +0.050 and +0.071‰ suggesting limited authigenic enrichment of W in euxinic environments. Similarly, post-Great Oxidation Event shales (2.3 Ga) show crustal- or PII-like δ186/184W values indicating the deposition in euxinic or oxic environment. However, the Archean shale sample suites show elevated W concentrations and fractionated δ186/184W values up to +0.246‰, which we attribute to authigenic enrichment of heavy seawater W in a ferruginous setting. Thus, black shales δ186/184W values can help to distinguish between anoxic-ferruginous and euxinic depositional environments.Furthermore, we combine previously published Cr and Mo isotope data with new Cr and W isotope data to quantify the evolution of the marine redox state during the Archean. By combining the Eh-pH stability fields of W and Mo with the δ186/184W and δ98/95Mo values of Precambrian sediments, we suggest a 3-step evolution of the Precambrian ocean. (1) From 3.47 to 3.0 Ga, the Eh of a dominantly ferruginous ocean was between −0.4 V and −0.25 V, enabling the persistence of soluble WO42− but not MoO42− and thus only the authigenic enrichment of isotopically fractionated W. (2) Starting from 3.0 Ga, the Eh of some shallow-marine environments increased above −0.25 V, as indicated by δ98/95Mo values above the detrital background in shallow-marine sediments, while the Eh of the deep ocean still remained below −0.25 V indicated by the lack of authigenic Mo enrichment and fractionated δ98/95Mo values in deep-sea shales. (3) After 2.7 Ga elevated δ98/95Mo and δ186/184W values in deep-marine sediments suggest that the Eh of the ferruginous open-ocean now also increased above −0.25 V. We therefore suggest that the combination of δ98/95Mo and δ186/184W values of shales is a very promising tool to investigate earliest changes in marine redox conditions during the Archean.

Origin of the Oligocene manganese deposit at Obrochishte (Bulgaria): Insights from C, O, Fe, Sr, Nd, and Pb isotopes

The large manganese (Mn) deposit at Obrochishte (NE Bulgaria) is part of a cluster of similar Early Oligocene deposits located around present-day Black Sea. They collectively constitute the Earth’s second largest endowment of Mn, after the Kalahari Manganese Field in Africa. We have employed a battery of isotopic techniques (C, O, Fe, Sr, Nd, Pb) to help understand the genesis of this deposit. Carbon isotope data indicates that some sections of the Mn-ore layer have diagenetic MnCO3 mineralization, formed by reaction of Mn oxides with organic carbon (Corg), whereas other sections have MnCO3 precipitated directly from the seawater column. Oxygen isotopes show that the high-grade Mn mineralization had seawater as the fluid source, whereas some lower-grade sections had a mix of ground water and seawater as fluid sources. Sr and Nd isotope values of ore leachates also indicate that the Mn deposition occurred in normal Early Oligocene seawater. Nd and Pb isotope values suggest that the clastic host sediments were sourced from continental bedrock rather than younger arc volcanic rocks to the west. Iron isotope composition of the Mn ore implies deposition in a redox-stratified basin, similar to the modern Black Sea, with much of the Fe sequestered in deep, anoxic-euxinic water as sulfides. Similar to the modern Black Sea, most of the detrital Fe was transferred from shallow oxic sediments into deep, anoxic-euxinic water by an “iron shuttle” and remobilized Mn sequestered in the upper suboxic water layer. However, during the Oligocene, the “iron shuttle” operated intermittently due to the chemocline falling mostly below the shelf break, thereby limiting the efficiency of the shuttle mechanism. We propose a model for the Lower Oligocene strata in which intense weathering during the Eocene weathering phase produced a thick lateritic crust on the southern European continent. The drastic sea-level drop at the end of the Eocene initiated downcutting of streams through this weathered material, transferring Fe- and Mn-oxides to the redox-stratified Western Black Sea. Here, these oxides were partly or entirely dissolved in the suboxic (Mn-oxides partly, Fe-oxyhydroxides entirely dissolved) and anoxic-euxinic (Mn-oxides entirely dissolved, dissolved Fe2+ re-precipitated) water layers. Eventually, Fe was re-precipitated as sulfide in the deep anoxic-euxinic water, while Mn accumulated in the suboxic water layer. Transgression in the Early Oligocene brought this Mn-rich water onto the shallow shelf where it precipitated as Mn-oxide, then converted to Mn-carbonates during early diagenesis. Some Mn was also contributed by submarine groundwater discharge. Further transgression brought lower-oxygen water onto the shelf and Mn-carbonate precipitated directly from the water column. The findings from this work provide insights about the unique Oligocene geochemical event in the region that lead to the formation of the 2nd largest cluster of Mn deposits in the world.

Методология комплексного мониторинга современного состояния фитопланктонного сообщества прибрежных вод Черного моря

Методология комплексного мониторинга современного состояния фитопланктонного сообщества прибрежных вод Черного моря

Beyond redox: Control of trace-metal enrichment in anoxic marine facies by watermass chemistry and sedimentation rate

Although redox conditions are the dominant control on authigenic enrichment of trace metals in marine sediments, other factors may be important within environments having relatively uniform redox characteristics, such as some anoxic silled basins. Notably, watermass chemistry (specifically, aqueous trace-metal concentrations) and sedimentation rate can also influence the authigenic accumulation of redox-sensitive trace metals such as molybdenum (Mo) and uranium (U) in the sediment, although these effects have received less attention than redox controls to date. Here, we (1) utilize a diffusion-reaction model to evaluate the effects of variations in watermass chemistry and sedimentation rate on authigenic trace-metal enrichment, and (2) present case studies of Mo and U enrichment in modern Black Sea sediments and North American Devonian-Carboniferous boundary (DCB) black shales that illustrate these influences. In both case studies, redox conditions were assessed using non-trace-metal-based proxies (i.e., C-S-Fe, FeT/Al, and Corg:P). Stations 6 and 7 of the modern Black Sea, at water depths of 380 and 1176 m, respectively, exhibit marked differences in authigenic Mo and U enrichment: median Mo/TOC is 13.2 at Station 6 (range 11.5–14.8) versus 5.7 at Station 7 (range 3.7–7.6), and median U/TOC is 2.6 at Station 6 (range 1.5–3.0) versus 1.3 at Station 7 (range 0.7–1.9) (note: units are ppm/% or 10−4, and ranges are 16th-84th percentiles). Given the nearly identical redox conditions and sedimentation rates at these two sites, the most likely cause of the >2× enrichment of Mo and U at Station 6 relative to Station 7 is differences in aqueous Mo and U concentrations, which decline steeply through the upper part of the Black Sea water column, demonstrating the influence of watermass chemistry on patterns of authigenic trace-metal enrichment in the sediment. For the DCB black shales, the median Mo/TOC is 22.2 in the Lower and Upper Bakken (range 15.8–27.0), 28.8 in the Sunbury (range 19.2–37.2), and 14.3 in the Cleveland (range 9.3–22.5), and the median U/TOC is 6.2 in the Lower and Upper Bakken (range 3.5–9.6), 2.6 in the Sunbury (range 1.7–3.4), and 1.2 in the Cleveland (range 0.7–1.7). Differences in Mo and U concentrations between these formations show no relationship to inferred aqueous trace-metal concentrations, Mn–Fe particulate shuttles, or paleoenvironmental redox conditions, but they broadly correlate with variation in sedimentation rates, providing evidence that sedimentation rates can measurably influence the degree of authigenic trace-metal enrichment of marine sediments.

Redox classification and calibration of redox thresholds in sedimentary systems

Existing redox classifications and the calibrations of elemental proxies to modern environmental redox scales are in need of re-evaluation. Here, we review environmental redox classifications, commonly used elemental redox proxies, and their intercalibration, and we propose a novel approach to improve the calibration of such proxies, using datasets from the modern Black Sea, Saanich Inlet, and California Margin as examples. Our approach is based on recognition of compound covariation patterns among pairs of elemental redox proxies within a redox framework based on three key thresholds: (1) the Re4+/Re3− couple near the suboxidized/subreduced boundary of the suboxic zone, (2) the U6+/U4+ couple in the middle of the subreduced zone, and (3) the SO42−/H2S couple at the suboxic/euxinic boundary. Within this framework, it is possible to determine the relative timing of onset and the degree of enrichment of other elemental redox proxies. Our analysis demonstrates that, even though some elements exhibit limited enrichment within the suboxic zone, the bulk of authigenic enrichment of the redox-sensitive elements considered in this study occurs within the euxinic zone. One important finding of our study is that the threshold value associated with a given elemental proxy can vary considerably between depositional systems. For this reason, it is inadvisable to transfer published threshold values (i.e., from earlier paleoredox studies) to completely different formations, and redox proxies must be internally calibrated for each individual paleodepositional system under investigation.

Heavy metals in the bottom sediments of the Tendrivska bay

The article describes several research results of geoecological direction, which included field sampling of bottom sediments, soils, coastal sediments at different sites within the territory of the Black Sea Biosphere Reserve NAS of Ukraine, in particular, the Tendrivska and Yahorlitska bays. The complex of the laboratory works included determination of chemical, microelement, particle size distribution composition of samples and study of their material characteristics on a scanning electron microscope. The study of the ecological state of the Reserve’s bottom sediments, as well as comparison of pollutant content in sediments of various parts of the protected water area was based, in particular, on determining the concentrations a few heavy metals (copper, nickel, zinc, lead, tin). The choice of research objects was determined by the peculiarities of the organization of agricultural activities within the arable lands bordering the boundaries of this reserve. The results obtained showed a significant impact of the geomorphological structure of the area and hydrodynamic characteristics of certain areas of the water area within it on the distribution of the material component of bottom sediments. The mineral and particle-size composition of bottom sediments should be considered the main factor in the uneven distribution of microelement concentrations, which in turn is determined by the location of zones of coastal erosion and sediment accumulation. Insignificant relative excesses of the copper content in the surface layer of the bottom sediments of the water area, fixed in the northern part of the Tendrivska Bay, can be connected with the discharge of fresh water from the network of canals, which is part of the agro complexes for rice cultivation, as well as “tailings” of water from the water supply systems of the Krasnoznam’yanska irrigation system. Studies have also shown that the content of most heavy metals in the upper layer of bottom sediments in the Black Sea Biosphere Reserve is lower than the average figures for modern Black Sea bottom sediments presented in literature sources.

A 10-fold decline in the deep Eastern Mediterranean thermohaline overturning circulation during the last interglacial period

Present-day Mediterranean deep-waters are well oxygenated, but the episodic formation of organic-rich sediments (sapropels) indicates that this pattern was frequently perturbed in the past. Both high export productivity and disruption of the thermohaline circulation, leading to reduced deep-water ventilation, have been proposed to account for sapropel deposition and anoxia. The last interglacial sapropel S5 is considered one of the most strongly developed. Here, we apply the redox-sensitive Mo and U (elemental and isotope) systems to quantify the intensity of anoxic deep-water conditions in the Eastern Mediterranean Sea from ODP core 967 (2550 mbsl). Both U and Mo show strong authigenic enrichment, coupled to progressive increase in δ98Moauth (+1.2–1.8‰ to +2.0–2.3‰) and decrease in δ238Uauth (+0.10‰ to −0.15‰) from the beginning to the end of S5, suggesting increasing water column euxinia and removal fluxes of Mo and U. Based on modern euxinic basins, we show that sedimentary δ238Uauth can be used to derive estimates of water column U depletion and, ultimately, deep-water renewal rates. These principles are first tested on the modern Black Sea, which yields calculated deep-water renewal times of 830+690/−500 yrs, in good agreement with independent estimates. Applying these principles to the end of S5 suggests bottom-water U depletion of ∼50% and deep-water renewal times of 1030+820/−520 yrs. The significantly slower deep-water renewal rates in the Eastern Mediterranean Sea compared to today (∼100 yrs) would have played an important role in the formation of sapropel S5 and are consistent with the proposed suppression of overturning during the last interglacial, due to increased stratification resulting from higher riverine freshwater input under enhanced monsoon forcing.

Coupled sulfur, iron and molybdenum isotope data from black shales of the Teplá-Barrandian unit argue against deep ocean oxygenation during the Ediacaran

The Earth’s atmosphere and hydrosphere changed from an Archean anoxic to a modern oxygenated world in two major steps, the Paleoproterozoic Great Oxidation Event (2.4–2.3 billion years ago) and the Neoproterozoic Oxidation Event (0.8–0.5 billion years ago). Both events had a strong influence on the availability of redox sensitive and bio-essential metals within the ocean and are, thus, strongly linked to fundamental biological innovations and diversification. Biological diversification during the Precambrian–Cambrian transition between 555 and 540 million years ago may have been driven by ocean–atmosphere oxygenation. The exact timing and the extent of (deep) ocean oxygenation within this time period remains unresolved though.Here we present major and trace element compositions as well as Mo, S and Fe isotopic data of organic-rich black shales from the Teplá-Barrandian unit, Czech Republic. New in situ zircon U–Pb ages provide a maximum depositional age of 559.8±3.8 million years. Black shales with strong metal enrichment show low δ56Fe values due to the dominance of authigenic pyrite-Fe with δ56Fe values around −0.6‰ over detrital Fe with δ56Fe values around 0.1‰. Samples with lower authigenic metal enrichment show relatively low Mo/TOC ratios and increasing δ34S values, which is interpreted to reflect basinal restriction and longer seawater renewal times. In analogy to the modern Black Sea, the accompanied depletion of basinal Moaq due to near quantitative Mo removal might have led to the preservation of the seawater δ98Mo in the respective black shales. Our best estimate for this seawater Mo isotopic composition <560 million years ago is +1.2‰ in δ98Mo, which is nearly identical to seawater δ98Mo values inferred from Mid-Proterozoic black shales. The lack of higher δ98Mo values in black shales (and seawater) argues against contemporaneous Mn oxide formation in well oxygenated deep sea settings, which would preferentially adsorb isotopically light Mo leaving behind an isotopically heavy ocean. By contrast, the deep ocean might have remained ferruginous with the hydrothermal Fe still outbalancing surficial oxygen production. Our results therefore contribute to a growing data set, which suggests limited deep water oxygenation during major biological innovations in the late Ediacaran period.

Modern Black Sea oceanography applied to the end-Permian extinction event

The modern Black Sea has a mixed upper layer in the top 150–200 m of the water column, below which the water is anoxic, separated from the mixed layer by a redox boundary. There is limited vertical movement of water. Pyrite framboids form in the water column of the anoxic zone, then have been traditionally interpreted to sink immediately and accumulate in the sediments of the Black Sea. Thus the occurrence of framboids in sediments in the rock record is widely interpreted to indicate poorly oxygenated to anoxic conditions in ancient environments. However, in the Permian–Triassic boundary (PTB) microbialites of South China, which formed in shallow marine conditions in contact with the atmosphere, the published occurrence of framboids is inconsistent with abundant gastropod and ostracod shells in the microbialite. Furthermore, in the modern Black Sea, (a) framboids may be suspended, attached to organic matter in the water column, thus not settle to the sea floor immediately after formation; and (b) the redox zone is an unstable complex area subject to rapid vertical water movement including occasional upwelling. The model presented here supposes that upwelling through the redox zone can lead to upward transport of suspended pyrite framboids into the mixed layer. Advective circulation could then draw suspended framboids onto the shelf to be deposited in oxygenated sediments. In the Permian–Triassic transition, if framboids were up-welled from below the redox boundary and mixed with oxygenated waters, sediment deposited in these conditions could provide a mixed signal for potentially misleading interpretations of low oxygen conditions. However, stratigraphic sampling resolution of post-extinction microbialites is currently insufficient to demonstrate possible separation of framboid-bearing layers from those where framboids are absent.Profound differences between microbialite constructors and sequences between the western and eastern Tethys demonstrate barriers to migration of microbial organisms. However, framboid occurrences in both areas indicate upwelling and emphasize vertical movement of water from the lower to upper ocean, yet the mixed layer advective motion may not have been as effective as in modern oceans. In the modern Black Sea, such advection is highly effective in water mixing, and provides an interesting contrast with the PTB times.

Sedimentary and faunal signatures of the post-glacial marine drowning of the Pontocaspian Gemlik “lake” (Sea of Marmara)

A Late Pleistocene (Neoeuxinian) Pontocaspian fauna is documented from sediments cored at̴ ∼75 m present water depth in the shallow-silled (∼50 m) Gulf of Gemlik, Sea of Marmara. As determined by AMS-14C dating of Pontocaspian gastropods (15,860 ± 90 BP uncal.), the age of such fauna is consistent with known episodes of Caspian overflow into the Black Sea Basin (Early Hvalynian floods). The fossil mollusc fauna is relatively diverse (12 non-marine taxa), made up of neritid and hydrobioid gastropods, and dreissenid and lymnocardiine bivalves. The presence of hitherto poorly known Pontocaspian taxa in the Sea of Marmara, further documented by the record in Gulf of Gemlik, seems to document that the Neoeuxinian Sea extended south of the Bosporus in the latest Pleistocene. Further, they provide insight into the origin of the modern Black Sea Pontocaspian taxa. The Gemlik Neoeuxinian lacustrine-brackish fauna under scrutiny is the westernmost known expansion of the Pontocaspian domain during the Late Pleistocene. This distinct biogeographic faunal affinity, with all its eastbound implications in terms of landscape and seaway connections, came to an end with the abrupt marine inundation that accompanied the postglacial sea-level rise from the Mediterranean Sea.

A modern framework for the interpretation of 238U/235U in studies of ancient ocean redox

The abundance and isotope composition of redox sensitive elements in ancient sediments are increasingly used to understand the past ocean's geochemical state and the oxygenation history of the Earth. The redox transition of uranium (U) from soluble U+6 to relatively insoluble U+4 and its subsequent incorporation into reduced sediments has been used to deduce the redox state of the oceans in the past. Furthermore, recent analytical improvements have revealed significant 238U/235U fractionation during this redox transition, offering the potential for U isotopes to act as a redox proxy. However, the development of U isotopes as a geochemical tracer requires that U isotope systematics associated with redox changes, are well-characterized.This study focuses on U isotopes in recent sediments from the two largest modern anoxic ocean basins, the Black Sea and the Cariaco Basin, with the aim of advancing our understanding of the U isotope systematics in reducing marine environments. These anoxic sediments have high U accumulation rates and high 238U/235U ratios relative to seawater, in general agreement with a process that accumulates reduced U with a heavy isotopic composition. Using Al and Ca concentrations to correct for detrital and biogenic carbonate-bound U, we estimate the reduced authigenic U accumulated in the sediments and its 238U/235U.These results highlight the importance of isotopic mass balance constraints during diffusive transport and reaction of U from seawater and through pore-water, affecting the observed 238U/235U in sediments. Using these constraints, the average percentages of U depletion from top to bottom of the water column can be estimated, assuming batch-removal of U into anoxic sediments in a restricted basin. Using this framework, 238U/235U in modern anoxic sediments from the Black Sea imply U depletions in the water column of ∼30%, which is close to the observed ∼40% U depletion in the modern Black Sea water column at these depths. Similar U depletion in the water column is estimated from anoxic sediment samples of the Cariaco Basin. These recent anoxic sediments provide a basis for interpreting authigenic 238U/235U in ancient sediments. In particular, such analyses may offer insights, based on mass balance relationships, into whether particular ancient sediments were deposited in an open ocean or restricted basin. As such, this approach may provide key insight into the controls on local versus ocean-scale redox and, in that light, constraints the capacity of other proxies to capture global signals for anoxia/euxinia.

Re-Os age and depositional environment for black shales from the Cambrian-Ordovician boundary, Green Point, western Newfoundland

Chemical and isotopic signatures for black shales serve as potential proxies for reconstruction of paleoenvironmental conditions. Here we bring Rock-Eval, major and trace element and Re-Os isotopic data together to examine the environmental record at the Cambrian-Ordovician Global Stratotype Section and Point (GSSP) at Green Point in western Newfoundland, Canada. The Green Point shales are oil mature and contain Type II organic material of marine origin. A Re-Os isochron for the shales provides the first radiometric age for shale deposition at the GSSP at 484 ± 16 Ma, with an initial 187Os/188Os ratio of 0.74 ± 0.05 (Model 3 age; MSWD = 21; n = 13; 2σ uncertainties). Factor analysis of the geochemical data set shows association of most trace elements with total organic carbon (TOC) and S contents, ensuring an authigenic origin for most elements and hence, their validity for evaluating the paleo-redox state. Relatively high-enrichment factors for redox-sensitive elements (e.g., Re, U, and Mo) compared to average shale, but comparatively low enrichment compared to modern Black Sea sediments, suggest deposition in anoxic, but not euxinic waters. Comparison of Lower Ordovician shale geochemistry data sets at a global scale leads us to suggest that anoxic conditions and warm oceanic regimes were restricted to the margins of Laurentia and Baltica, whereas depositional basins with colder waters (e.g., Avalonia and Gondwana) were less reducing. These outcomes underscore the important role of paleogeography in regulating oceanic conditions and marine life.

Holocene ostracod and foraminiferal assemblages of the Romanian Black Sea shelf

Qualitative and quantitative study of the ostracod and foraminiferal assemblages were performed on two cores, around 0.30 m depth bsf in thickness, collected from the Romanian Black Sea shelf. The lithology of the studied cores is characterised by grey and black muds, topped by a centimetre thick fluffy mud layer. One of the cores, MN 103_09, is placed on the Romanian inner shelf, at water depth of 17.80 m. The sediments of this core contain very few and small bivalve fragments and no gastropods. In this core, the ostracod and foraminiferal assemblages are influenced by a quite high-energy environment, due to the Sulina arm of the Danube Delta. This feature is mirrored in the poor preservation and low abundance of the species, as well as the mixed Caspian and Mediterranean character of the ostracod assemblages. The other studied core, MN 103_04, is situated on the Romanian outer shelf, at a water depth of 78 m. Its deposits commonly contain gastropods and bivalves, as well as microfauna displaying a low diversity and a very high abundance; the character of the ostracod assemblages is a Mediterranean one. Overall, the identified microfaunas of the two investigated cores reflect the environmental conditions of modern and/or latest Holocene sea-floor assemblages and their relationships to salinity, sediment input and water depth. Even the depositional time of the 30 cm cored interval in the two studied cores is quite different, i.e., possibly several hundreds of years, up to 1000 years, on the inner shelf and several thousands (from 3000 BP to the Present), in the outer shelf, the results are indicative for modern Black Sea conditions. A stable marine environment with a salinity around 17–18‰, comparable with the present one, could be assumed for the whole cored interval of the Romanian shelf. Besides, the age structure, i.e., the ratio between adults and juvenile taxa of ostracod population in the outer shelf indicates a moderate to high energy autochthonous thanatocoenosis, with some post-mortem disturbance by currents. By contrast, the ratio between adults and juvenile ostracod taxa in the inner shelf mirrored a taphocoenosis with post-mortem disturbance, sorting and transport. This assemblage, dominated mainly by disarticulated valves of juveniles, does not represent the environment in which the ostracods lived.

Uranium isotope fractionation suggests oxidative uranium mobilization at 2.50 Ga

Geochemical data from late Archean sedimentary rocks point to photosynthetic O2 production and at least intermittent occurrences of locally mild oxidative weathering and surface ocean oxygenation (“oxygen oases”) prior to the early Paleoproterozoic Great Oxidation Event. For example, distinctive authigenic enrichments of Mo and Re in euxinic (anoxic and sulfidic) black shales are best explained by the oxidative mobilization of these metals from crustal sulfide minerals and their accumulation as oxyanions in seawater. In contrast, it is not clear if low U enrichments in the same shales reflect negligible oxidation of U from the upper crust or a very small oceanic U inventory that was derived from oxidative U mobilization. Here, we report U isotope data for the 2.50Ga Mt. McRae Shale (Hamersley basin, Western Australia), which provides a more sensitive test for the presence or absence of authigenic U compared to U concentrations and enrichment factors normalized to average shale or upper crustal compositions (that may not be representative of the local detrital composition). We find instances where the U isotope composition in the upper Mt. McRae Shale (δ238U=−0.2 to 0.0‰ relative to standard SRM950a) is isotopically heavier than average upper crust (δ238U=−0.31±0.14 [2SD] based on granitoids and basalts). The high δ238U values point to U isotope fractionation in the late Archean marine environment and hence indicate the presence of a small amount of dissolved U in seawater and authigenic U in the Mt. McRae Shale. Volume-dependent equilibrium isotope fractionation during the reduction of dissolved UVI to insoluble UIV, like that observed in the modern Black Sea, may explain the high δ238U signatures if U removal from bottom waters was not quantitative. Alternatively, quantitative U removal would require that late Archean seawater had high δ238U, which could have arisen from the preferential sequestration of 235U to Fe (oxyhydr)oxide minerals elsewhere in the Hamersley basin. The supracrustal δ238U signatures are associated with some of the highest Mo and Re enrichments in the Mt. McRae Shale as well as distinctive Mo, S and N isotope signatures that are indicative of mild environmental oxygenation. Hence, our findings suggest that small amounts of U were oxidatively mobilized from the upper crust at 2.50Ga. Unlike Mo and Re, however, U oxidation may not have occurred on land. Instead, we hypothesize that oxidative U mobilization occurred by submarine weathering in an oxygen oasis.

Sulfate reduction below the sulfate–methane transition in Black Sea sediments

A sudden increase in salinity about 7000 years ago caused a shift in the deposited sediments of the Black Sea from limnic to brackish-marine. Due to the development of an anoxic deep water basin and a relatively high sulfate concentration, organic matter is mineralized primarily through sulfate reduction in modern Black Sea sediments. Earlier studies showed that sulfate-reducing bacteria are abundant within the limnic sub-surface sediment in spite of extremely low concentrations of sulfate and organic carbon. A main objective of the present study was therefore to understand the depth distribution of sulfate reduction across the different sediment units, even deep below the sulfate–methane transition. Our study combined experimental measurements of sulfate reduction using 35S radiotracer with analyses of sulfur and iron geochemistry in pore water and sediment. Potential sulfate reduction rates were measured with 35S in sediment samples that were amended with sulfate and organic substrates and incubated in time-series up to 48 h. Sulfate reduction could thereby be detected and quantified at depths where concentrations of sulfate were otherwise too low to enable calculation of the rates. The results demonstrate that sulfate-reducing bacteria are active several meters below the sulfate–methane transition in Black Sea sediments. The cryptic sulfate reduction below the sulfate–methane transition may be driven by sulfate produced from re-oxidation of reduced sulfur species with oxidized iron minerals buried in the deep limnic sediment.

Molybdenum isotope, multiple sulfur isotope, and redox-sensitive element behavior in early Pleistocene Mediterranean sapropels

Organic-rich sediments (sapropels) deposited in the Mediterranean are presumed to have formed during periods of increased productivity, and/or deep water oxygen depletion, possibly including the development of sulfidic conditions (euxinia). Geochemical redox proxies (Re, Mo, Mo isotopes, V, Fe/Al, and multiple S isotopes) in 8 sapropels from the Pleistocene confirm water column euxinic conditions of varying intensity during sapropel deposition. These same proxies indicate an oxic origin for hemipelagic sediments deposited between sapropel-forming episodes. In one intensively sampled sapropel, deposited between 1.450 and 1.458 Ma, changing concentrations of organic carbon, Ba, Re, Mo, V, and Fe/Al track one another closely, reflecting coupling between water column euxinia and biological productivity. Multiple S isotope data from this sapropel suggest that the redox interface where oxidative sulfur cycling occurred was present in the sediments during hemipelagic sedimentation, but moved into the water column during sapropel deposition. Molybdenum isotopes of these 8 sapropels encompass a range of values (δ 98Mo = + 0.2 to + 1.7), but are all 98Mo-depleted relative to seawater (δ 98Mo = + 2.3‰), suggesting that quantitative removal of Mo did not occur. This finding contrasts with modern Black Sea sediments. In general, Re/Mo ratios in sapropels are greater than in modern seawater, implying that the water column was not sufficiently sulfidic during sapropel-forming episodes to induce complete removal of both these elements. Surprisingly, the heaviest δ 98Mo values are found within hemipelagic sediments. Very few of the hemipelagic samples preserve the negative δ 98Mo values commonly associated with modern oxic marine sediments. Many of the hemipelagic samples also contained higher concentrations of Re and Mo than are common in oxic sediments. These features may be attributable to diffusion from the sapropels of a 98Mo-enriched component into the hemipelagic sediments.

Validation of an intermediate‐complexity model for simulating marine biogeochemistry under anoxic conditions in the modern Black Sea

We test the ability of a new 1‐D intermediate‐complexity box model (ICBM) that includes process‐based C, N, P, O, and S biogeochemistry to simulate profiles and fluxes of biogeochemically reactive species across a wide range of ocean redox states. The ICBM was developed to simulate whole ocean processes for paleoceanographic applications and has been tested with data from the modern global ocean. Here we adapt the circulation submodel of the ICBM to simulate water mass exchange and eddy diffusion processes in the Black Sea but make only very minor changes to the biogeochemical submodel. We force the model with estimated natural and anthropogenic inputs of tracers and nutrients to the Black Sea and compare the results of the simulations to modern observations. Ventilation of the Black Sea is modeled by depth‐dependent entrainment of Cold Intermediate Layer water into Bosphorus plume water and subsequent intrusion into deep layers. The simulated profiles of circulation tracers θ, salinity, CFC‐12, and radiocarbon agree well with available data, suggesting that the model does a reasonable job of representing physical exchange. Vertical profiles of biogeochemically active components are in good overall agreement with observations. The lack of trace metal (Mn and Fe) cycling in the model results in some discrepancies between the simulated profiles and observation across the suboxic zone; however, the overall redox balance is not sensitive to this difference. We compare modeled basin‐wide biogeochemical fluxes to available estimates, but in a number of cases uncertainties in modern budgets limit our ability to test the model rigorously. In agreement with earlier work we find that fixed N losses via thiodenitrification are likely a major pathway in the Black Sea N cycle. Overall, the same biogeochemical submodel used to simulate the modern global ocean appears to perform well in simulating Black Sea processes without requiring significant modification. The ability of a single model to perform across a wide range of redox states is an important prerequisite for applying the ICBM to deep time paleoceanographic problems. The model source code is available as MATLAB™ 7 m‐files provided as auxiliary material.

An intermediate‐complexity model for simulating marine biogeochemistry in deep time: Validation against the modern global ocean

We present a new high‐resolution 1‐D intermediate‐complexity box model (ICBM) of ocean biogeochemical processes for paleoceanographic applications. The model contains 79 reservoirs in three regions that should be generally applicable throughout much of Earth history: (1) a stratified gyre region, (2) a high‐latitude convective region, and (3) an upwelling region analogous to those found associated with eastern boundary currents. Transport processes are modeled as exchange fluxes between boxes and by eddy diffusion terms. Significant improvement in the representation of middepth oxygen budgets was achieved by implementing nonlocal mixing between the high‐latitude surface and gyre thermocline reservoirs. The biogeochemical submodel simulates coupled C, N, P, O, and S systematics with explicit representation of microbial populations, using a process‐based approach. Primary production follows Redfield stoichiometry, while water column remineralization is depth‐ and redox couple–dependent. Settling particulate organic matter is incorporated into a benthic submodel that accounts for burial and remineralization. The C/P ratio of burial depends on bottom water oxygen. Denitrification takes place both by classical and anammox pathways. The ICBM was tested against modern oceanographic observations from the Global Ocean Data Analysis Project, Joint Global Ocean Flux Study, and other databases. Comparisons of model output with circulation tracers including θ, salinity, CFC‐12, and radiocarbon permit a test of the physical exchange scheme. Vertical profiles of biogeochemically reactive components in each of the three regions are in good agreement with observations. Under modern conditions the upwelling zone displays a pronounced oxygen minimum zone and water column denitrification, while these are not present in the high‐latitude or gyre regions. Model‐generated global fluxes also compare well to independent estimates of primary production, burial, and phosphorous and nitrogen cycling. The ICBM appears to adequately simulate the long‐term (kyr) evolution of several biogeochemical cycles and improves on previous box models in several important ways. In a companion paper, the model's performance under euxinic conditions is tested against modern Black Sea data. The simple and adaptable structure of the model should make it applicable to a wide range of paleoceanographic problems. The model source code is available in MATLABTM 7 m‐files provided as auxiliary material.