Marine Basin Research Articles

Palynostratigraphic, palynofacies, organic geochemical and palaeoenvironmental analysis of the Silurian Tanezzuft Formation in the Ghadames Basin of north-west Libya

Two palynofacies associations are documented from the Silurian Tanezzuft Formation in the Ghadames Basin. These are characteristic of the basal ‘Hot Shale’ and the overlying deposits, referred to here as the Cold Shale. The former reflects deposition in distal suboxic anoxic conditions and is dominated by highly oil-prone amorphous organic matter (AOM) typical of deposition in generally anoxic, restricted marine basins. Only a few acritarchs, prasinophyte algae and chitinozoans occur in association: virtually no spores or cryptospores were recorded. Thick-walled prasinophytes are most numerous in this part of the Tanezzuft Formation in both the Ghadames and Murzuq basins, suggesting enhanced surface water productivity. Deposition took place after the melting of the Late Ordovician ice sheets, which led to a major marine transgression. The palynofacies recorded from the overlying ‘Cold Shale’ deposits indicate deposition in distal shelf and basin conditions that were also relatively anoxic. They contain more palynomorphs, especially acritarchs, and generally less AOM. The phytoplankton assemblages are dominated by simple and thin-walled prasinophyte algae (leiospheres), suggesting dysoxic–anoxic conditions. Overall the middle and the upper parts of the Tanezzuft Formation are regarded as being deposited in distal dysoxic–anoxic shelf, distal dysoxic–oxic shelf and distal suboxic–anoxic basin respectively. Because chitinozoans are very rare, age determinations of the samples investigated are based mainly on acritarchs. The Hot Shale is dated as early-mid Rhuddanian (early Llandovery) whereas the rest of the formation is considered to late Rhuddanian–Telychian in age. As documented previously from other samples of the Tanezzuft Formation in both the Ghadames and Murzuq basins, the Hot Shale has a very high TOC content and excellent source potential for liquid hydrocarbons, whereas rest of the formation is less rich in organic matter with larger terrestrial and oxidized components and hence reduced potential for sourcing hydrocarbons (both oil and gas).

Emiliania huxleyi coccolith calcite mass modulation by morphological changes and ecology in the Mediterranean Sea.

To understand the response of marine calcifying organisms under high CO2 scenarios, it is critical to study their calcification patterns in the natural environment. This paper focuses on a major calcifying phytoplankton group, the coccolithophores, through the analysis of water samples collected along a W-E Mediterranean transect during two research cruises, in April 2011 (Meteor cruise M84/3) and May 2013 (MedSeA cruise 2013). The Mediterranean Sea is a marginal sea characterized by large biogeochemical gradients. Currently, it is undergoing both warming and ocean acidification, processes which are rapidly modifying species distribution and calcification. The species Emiliania huxleyi largely dominates the total coccolithophore production in present day oceans and marine basins, including the Mediterranean Sea. A series of morphometric measurements were performed on the coccoliths of this species to estimate their mass, length and calculate a calcification index (proxy for the size-normalized calcification degree). The most abundant morphotype of E. huxleyi in the Mediterranean Sea is Type A. Coccoliths of this morphotype were additionally analyzed based on scanning electron microscopy images: four calcification varieties were quantified, according to the relationship between slit length-tube width, and the state of the central area (open or closed). The average E. huxleyi coccolith mass along the Mediterranean oceanographic transect depended strongly on both the average coccolith length and calcification index. The variability in average coccolith length and calcification index across samples reflected oscillations in the relative abundance of the calcification varieties. We also demonstrated that the distribution of the calcification varieties followed the main environmental gradients (carbonate chemistry, salinity, temperature, nutrient concentrations). Hence, shifts in the distribution of the calcification varieties and of the average E. huxleyi coccolith mass are to be expected in the Mediterranean Sea under climate change. These physiological and ecological responses will modulate the net coccolithophore calcification and, ultimately, the regional carbonate export to the seafloor.

High climate model dependency of Pliocene Antarctic ice-sheet predictions

The mid-Pliocene warm period provides a natural laboratory to investigate the long-term response of the Earth’s ice-sheets and sea level in a warmer-than-present-day world. Proxy data suggest that during the warm Pliocene, portions of the Antarctic ice-sheets, including West Antarctica could have been lost. Ice-sheet modelling forced by Pliocene climate model outputs is an essential way to improve our understanding of ice-sheets during the Pliocene. However, uncertainty exists regarding the degree to which results are model-dependent. Using climatological forcing from an international climate modelling intercomparison project, we demonstrate the high dependency of Antarctic ice-sheet volume predictions on the climate model-based forcing used. In addition, the collapse of the vulnerable marine basins of Antarctica is dependent on the ice-sheet model used. These results demonstrate that great caution is required in order to avoid making unsound statements about the nature of the Pliocene Antarctic ice-sheet based on model results that do not account for structural uncertainty in both the climate and ice sheet models.

New 40Ar/39Ar, magnetostratigraphic and biostratigraphic constraints on the termination of the Badenian Salinity Crisis: Indications for tectonic improvement of basin interconnectivity in Southern Europe

Widespread evaporites were deposited in large parts of the Central Paratethys during the so-called Badenian Salinity Crisis (BSC). The adverse environmental conditions that accompanied the BSC triggered a demise in the basin's marine fauna, inducing the so-called middle-Badenian-extinction-event. While tectonic activity preconditioned the Central Paratethys for isolation, it has recently been shown that the BSC was eventually triggered by the base-level drop accompanying the Mi3b global cooling event, which terminated the Middle Miocene Climatic Optimum. Here, we provide new constraints on the termination of the BSC by 40Ar/39Ar dating of a volcanic ash layer, located in a marl succession several meters above the Badenian evaporites of the Slănic Syncline in the Romanian East Carpathians. The results reveal that the BSC ended before 13.32 ± 0.07 Ma. Comparison with previously obtained geochronological results in Poland constrains the duration of the BSC to 500 kyr, assuming evaporite deposition in the Central Paratethys occurred as one event. The obtained 40Ar/39Ar results are complemented with paleomagnetic and micropaleontological analyses. These reveal that the investigated post-BSC marl interval in the Slănic Syncline was deposited in a period of reversed polarity corresponding to C5AAr. This is in agreement with calcareous nannoplankton from the same interval that belong to the NN6 zone. Ostracod and foraminifera marker species are indicative of the middle part of the regional Badenian stage, traditionally known as the Wielician. The foraminiferal assemblage is nevertheless very similar to Serravallian assemblages from the Mediterranean, which suggests that, in addition to a connection between the Central Paratethys and the Eastern Paratethys, there was a marine connection with the Mediterranean following the BSC. A comparison with post-BSC successions in Ukraine and Poland illustrates that the BSC was terminated by a transgression that re-installed normal marine conditions in the Carpathian foredeep. This basin-wide transgression resulted from reconnection of the Carpathian Foreland Basin with the Mediterranean and Eastern Paratethys, which improved the exchange of water and fauna. Global eustacy cannot explain re-connection of these basins, because global sea level on average remained just as low after the BSC as it had been during the crisis. The improved interconnectivity between the basins must therefore have been primarily caused by tectonic modification of the interconnecting gateways. Geodynamics thus played a crucial role in the re-establishment of a flourishing marine environment.

Interaction of Biogeochemical Sulfur and Carbon Cycles in Marine Basins by the Example of the Black Sea

The interaction of biogeochemical sulfur and carbon cycles in water bodies has been studied by the different authors and methods, including the use of 35S and 14C radioactive tracers. The main reactions of sulfur and carbon interaction by the example of the Black Sea (sulfate reduction, methanogenesis, aerobic and anaerobic methane oxidation) are considered. Based on the experimental radioisotope data, the annual microbial production of H2S and CH4 in the water column of the Black Sea is estimated. The average annual production of CH4 is 3.8% of the annual production of H2S. The share of migration methane from the bottom sediments (seeps, mud volcanoes, etc.) is approximately 2% of the total dissolved methane pool.

Calculation and inducement of lacuna in the Mid-Permian Maokou Fm of Sichuan Basin

Erosion of carbonate strata not only can reflect the karst landform, but also can reveal the characteristics of carbonate karst reservoirs indirectly. In Sichuan Basin, the quantity and root causes of lacuna in the top strata of the Maokou Fm are unclear. In this paper, the features of such stratigraphic lacuna were analyzed qualitatively. Then, in accordance with the spectral transformation of GR logs and the characteristics of cycle in the Maokou Fm penetrated by four typical wells, the Milankovich cycles of the logs were identified. Then they were used for quantitative calculation of erosion quantity and for highlighting the root causes for the stratigraphic lacuna. According to the research results, the Mao-4 Member is residual in the Yibin–Ya'an–Jiangyou area and the Shizhu area, but missing in the other parts of Sichuan Basin with a stratigraphic lacuna intensity gradually increasing from the southern part to the central part and to the northern parts of the basin. Besides, the stratigraphic lacuna of the Maokou Fm varies from 0 to 200 m – for example, 0–60 m in the southwestern and northeastern parts of the basin, and 140–200 m in the southern, central and northern parts of the basin. Moreover, formation denudation occurred at the end of the Maokou Period was induced predominantly by erosion generated as the sea level fell during the glacial age, especially in the northern part of the basin. Also, the Mid-Permian karst landform inherited the sedimentary features of being high in the southwest and low in the northeast. In the areas from the southwestern to central and to the northern Sichuan Basin, the karst landform transits from erosion highlands to karst upper and lower slopes, coinciding well with the sedimentary features of the Wujiaping Period. In conclusion, the Milankovich cycles can be used to calculate the stratigraphic erosion in carbonate formations efficiently and accurately. The technique can be extended to the restoration of stratigraphic erosion in other marine basins.

Interaction Biogeochemical Sulfur and Carbon Cycles in Marine Basins (as an Example The Black Sea)

Interaction Biogeochemical Sulfur and Carbon Cycles in Marine Basins (as an Example The Black Sea)

Sedimentation Conditions and Provenance Composition of the Upper Jurassic–Lower Cretaceous Deposits of the Upper Pegtymel Depression, Chukotka Terrane

The deposits of the Imlekin Formation of the Upper Pegtymel depression were formed in the southern margin of the Chukotka Terrane synchronously with accretion of the Kulpolney island arc to the Chukotka–Arctic Alaska microplate. The sedimentological structural features of the Upper Jurassic–Lower Cretaceous sections indicate their accumulation within various parts of submarine fans of a shallow marine basin. The submarine slump origin of the boulder–pebble (tillite-like) conglomerates is substantiated. The petrographic, geochemical, and geochronological methods identified two dominant provenances: Triassic clastic rocks and Volgian suprasubduction volcanic rocks. The Middle Volgian–Early Berriasian age of the Imlekin Formation is specified. The geodynamic model proposed explains the change of the sources and direction of the sediment transport in the Volgian.

A depositional model for deep‐lacustrine, partially confined, turbidite fans: Early Cretaceous, North Falkland Basin

AbstractThis paper presents a model of facies distribution within a set of early Cretaceous, deep‐lacustrine, partially confined turbidite fans (Sea Lion Fan, Sea Lion North Fan and Otter Fan) in the North Falkland Basin, South Atlantic. As a whole, ancient deep‐lacustrine turbidite systems are under‐represented in the literature when compared with those documented in marine basins. Lacustrine turbidite systems can form extensive, good quality hydrocarbon reservoirs, making the understanding of such systems crucial to exploration within lacustrine basins. An integrated analysis of seismic cross‐sections, seismic amplitude extraction maps and 455 m of core has enabled the identification of a series of turbidite fans. The deposits of these fans have been separated into lobe axis, lobe fringe and lobe distal fringe settings. Seismic architectures, observed in the seismic amplitude extraction maps, are interpreted to represent geologically associated heterogeneities, including: feeder systems, terminal mouth lobes, flow deflection, sinuous lobe axis deposits, flow constriction and stranded lobe fringe areas. When found in combination, these architectures suggest ‘partial confinement’ of a system, something that appears to be a key feature in the lacustrine turbidite setting of the North Falkland Basin. Partial confinement of a system occurs when depositionally generated topography controls the flow‐pathway and deposition of subsequent turbidite fan deposits. The term ‘partial confinement’ provides an expression for categorising a system whose depositional boundaries are unconfined by the margins of the basin, yet exhibit evidence of internal confinement, primarily controlled by depositional topography. Understanding the controls that dictate partial confinement; and the resultant distribution of sand‐prone facies within deep‐lacustrine turbidite fans, is important, particularly considering their recent rise as hydrocarbon reservoirs in rift and failed‐rift settings.

Intracellular nitrate in sediments of an oxygen-deficient marine basin is linked to pelagic diatoms.

Intracellular nitrate is an important electron acceptor in oxygen-deficient aquatic environments, either for the nitrate-storing microbes themselves, or for ambient microbial communities through nitrate leakage. This study links the spatial distribution of intracellular nitrate with the abundance and identity of nitrate-storing microbes in sediments of the Bornholm Basin, an environmental showcase for severe hypoxia. Intracellular nitrate (up to 270 nmol cm-3 sediment) was detected at all 18 stations along a 35-km transect through the basin and typically extended as deep as 1.6 cm into the sediment. Intracellular nitrate contents were particularly high at stations where chlorophyll contents suggested high settling rates of pelagic primary production. The depth distribution of intracellular nitrate matched that of the diatom-specific photopigment fucoxanthin in the upper 1.6 cm and calculations support that diatoms are the major nitrate-storing microbes in these sediments. In contrast, other known nitrate-storing microbes, such as sulfide-oxidizing bacteria and foraminifers, played only a minor role, if any. Strikingly, 18S rRNA gene sequencing revealed that the majority of the diatoms in the sediment were pelagic species. We conclude that intracellular nitrate stored by pelagic diatoms is transported to the seafloor by settling phytoplankton blooms, implying a so far overlooked 'biological nitrate pump'.

A review of the Villafranchian fossiliferous sites of Latium in the framework of the geodynamic setting and paleogeographic evolution of the Tyrrhenian Sea margin of central Italy

In the present study we provide a paleontological and chronostratigraphic review of the Villafranchian fossiliferous sites of Latium, revising the biochronologic attribution based on their framing within the geodynamic and paleogeographic evolutionary picture for this region. Aimed at this scope, we reconstruct the sedimentary and structural history of the Early Pleistocene marine basins through the review and the regional correlation of published stratigraphic sections and borehole data. Moreover, we combine the chronostratigraphic constraints provided in this study to the near-coast deposits of Gelasian-Santernian age (2.58–1.5 Ma) with the results of a recent geomorphologic study of this area, allowing us to reconstruct a suite of terraced paleo-surfaces correlated with marine isotopic stages 21 through 5. By doing so, we provide further age constraints to the sedimentary and tectonic processes acting on the Tyrrhenian Sea margin in Quaternary times, highlighting a possible different paleogeographic evolution of the southern coastal area where the Middle-Villafranchian type-section of Coste San Giacomo site is located, with respect to the northern sector.

Assembly and breakup of the Nuna supercontinent: Geodynamic constraints from 1800 to 1600 Ma sedimentary basins and basaltic magmatism in northern Australia

Basin formation from 1800 to 1640 Ma in northern Australia took place in a backarc extensional upper plate setting accompanied by lithospheric thinning and voluminous but episodic basaltic magmatism. West-dipping subduction combined with oceanward retreat of an associated magmatic arc drove these processes and commenced after the separation of Laurentia, precluding any connection with eastern Australia from 1800 Ma until basin closure and re-accretion of the arc ca. 1650–1640 Ma. Early rift-related basaltic magmatism generated km-thick piles of interstratified 1785–1775 Ma shallow-water sediment and tholeiitic lava whose low- and high-Ti compositions bear some resemblance to basalts erupted during the earlier stages of Gondwana breakup. Later basalts evolved in a western Pacific-style marginal marine basin and exhibit compositions consistent with melting of a subcontinental mantle source that had either been metasomatically altered during flat-slab subduction or contaminated with crustal material during an earlier subduction-related event. From 1700 to 1670 Ma, the crust and underlying lithospheric mantle rapidly thinned, and basaltic magmatism increasingly occurred in a turbidite-dominated deep marine environment with concomitant changes towards more subdued subduction-related signatures in the east and less enrichment in incompatible elements. Coincidently, granites emplaced from 1780 to 1670 Ma at deeper crustal levels were extensionally unroofed along with their country rocks. Backarc extension concluded no later than 1655 Ma by which time MORB-like magmas were being sourced directly from the asthenosphere and northern Australia had transitioned from an active to passive continental margin lying along the inboard side of the marginal sea. With ongoing subduction and consumption of oceanic crust to the east, Laurentia began to encroach upon both the arc and Australian margin, resulting initially in collapse of the backarc basin and then continent–continent collision by ca. 1620 Ma. These events represent an important milestone in assembly of the Nuna supercontinent and mark the end of one complete Wilson cycle from the time that the two continents first separated until their re-amalgamation 150 Myr later. Timeframes for the opening and closure of ocean basins in southern Laurentia are similar, raising the possibility that the same convergent margin and backarc extensional processes may once have extended along strike into ancestral North America.

Bedrock Erosion Surfaces Record Former East Antarctic Ice Sheet Extent

AbstractEast Antarctica hosts large subglacial basins into which the East Antarctic Ice Sheet (EAIS) likely retreated during past warmer climates. However, the extent of retreat remains poorly constrained, making quantifying past and predicted future contributions to global sea level rise from these marine basins challenging. Geomorphological analysis and flexural modeling within the Wilkes Subglacial Basin are used to reconstruct the ice margin during warm intervals of the Oligocene‐Miocene. Flat‐lying bedrock plateaus are indicative of an ice sheet margin positioned >400–500 km inland of the modern grounding zone for extended periods of the Oligocene‐Miocene, equivalent to a 2‐m rise in global sea level. Our findings imply that if major EAIS retreat occurs in the future, isostatic rebound will enable the plateau surfaces to act as seeding points for extensive ice rises, thus limiting extensive ice margin retreat of the scale seen during the early EAIS.

The active submarine NW termination of the South Aegean Active Volcanic Arc: The Submarine Pausanias Volcanic Field

Methana peninsula shows the longest recorded volcanic history at the western end of the South Aegean Active Volcanic Arc, including volcanic products from the Upper Pliocene to recent times. The volcanic rocks comprise widespread dacite domes and andesite lava flows from several small volcanic centers and are only imprecisely dated. In this paper, the integrated analysis of swath bathymetry, side scan sonar data, and high resolution seismic reflection profiles correlated with core samples, has allowed detailed mapping, characterization and precise chronological identification of the Pausanias submarine volcanic field activity offshore northern Methana. Six volcanic cones or domes are recognized, typically 1–3 km in diameter, some elongated NE–SW and some with a small central crater. On their flanks, the acoustically reflective volcanic rocks pass laterally into incoherent transparent seismic facies interpreted as volcaniclastic deposits, possibly including hyaloclastites, that interfinger with the regional basin sediments. A sea-bottom hummocky field, is interpreted as volcanic avalanche and appears to be the submarine continuation of the volcaniclastic apron of northern Methana peninsula. A robust chronostratigraphic framework has been established, based on the recognition of shoreline progradational units and their connection with Quaternary eustatic sea level cycles. Relative dating of the different phases of submarine volcanic activity during the Upper Quaternary has been achieved by correlating the imaged volcaniclastic flows, interlayered within the chronostratigraphically dated sediments. Dating by stratigraphic position, relative to 2D imaged eustatic sea level clinoform wedges appears to be more precise than radiometric methods on land. Three main submarine Volcanic Events (VE) are recognized: VE3 at ~450 ka, a less precisely dated interval at 200–130 ka (VE2), and VE1 at ~14 ka. Based on chronostratigraphic constraints, subsidence rates of 0.16 (±0.008) m/ka in-between Marine Isotopic Stages 6 and 12 and 0.19 (±0.009) m/ka in-between Marine Isotopic Stages 12 and 16 were estimated for the marine basin north of Methana. The morphological similarity to the onshore volcanoes of Methana Peninsula implies that magmatic constructive processes were dominant, regardless of whether in air or in water. The Upper Quaternary submarine volcanic rocks of Methana differ from those known from stratovolcanoes elsewhere in the Mediterranean, (e.g. Kos-Nisyros, Stromboli) and in other volcanic arcs (e.g., Montserrat, St Vincent), in the submarine development of domes or small cones, the paucity of volcano flank failure deposits and the lack of explosive events. Pausanias volcanic products date the onset of NE–SW faulting as well as the following tectonic phase of E-W striking faults, possibly related to basin inversion, caused by a major rifting phase that also affected most of the South Aegean Arc and the adjacent Gulfs of Corinth and Argolikos.

Infection by Nanophyetus salmincola and Toxic Contaminant Exposure in Out-migrating Steelhead from Puget Sound, Washington: Implications for Early Marine Survival.

Out-migrating steelhead Oncorhynchus mykiss from four Puget Sound rivers and associated marine basins of Puget Sound in Washington State were examined for the parasite, Nanophyetus salmincola in 2014 to determine whether recent trends in reduced marine survival are associated with the presence of this pathogen. A subset of steelhead from three of these river-marine basin combinations was analyzed for the presence of persistent organic pollutants (POPs) to assess whether exposure to these contaminants is a contributing factor to their reduced marine survival. The prevalence and parasite load of N. salmincola were significantly higher in fish from central and southern Puget Sound than in fish from river systems in northern Puget Sound. The proportion of steelhead samples with concentrations of POPs higher than adverse effects thresholds (AETs) or concentrations known to cause adverse effects was also greater in fish from the central and southern regions of Puget Sound than in those from the northern region. Polybrominated diphenyl ether concentrations associated with increased disease susceptibility were observed in 10% and 40% of the steelhead sampled from central and southern Puget Sound regions, respectively, but in none of the fish sampled from the northern region. The AET for polychlorinated biphenyls was exceeded in steelhead collected from marine habitats: 25% of the samples from the marine basins in the central and southern regions of Puget Sound and 17% of samples from northern Puget Sound region. Both N. salmincola and POP levels suggest there are adverse health effects on out-migrating steelhead from one southern and one central Puget Sound river that have lower early marine survival than those from a river system in northern Puget Sound.

Geochemistry of the Upper Triassic black mudstones in the Qiangtang Basin, Tibet: Implications for paleoenvironment, provenance, and tectonic setting

The Qiangtang Basin is the largest Mesozoic marine basin in the Tibetan Plateau. The Upper Triassic black mudstones are among the most significant hydrocarbon source rocks in this basin. Here, we present geochemical data for the Upper Triassic black mudstones to determine their paleoenvironment conditions, provenance, and tectonic setting. To achieve these, 30 black mudstones formed in various sedimentary environments were collected from the Zangxiahe, Zana, and Bagong formations. The results show that the total REE concentrations of mudstones from these formations range from 169 to 214 ppm, 204 to 220 ppm, and 141 to 194 ppm, respectively. All samples have chondrite-normalized REE patterns with enrichment of LREE, depletion of HREE and negative Eu and Ce anomalies. Specifically, mudstones from the Bagong Formation exhibit higher negative Eu anomalies and lower REE contents than those from the Zangxiahe and Zana formations.Mudstones from the Zangxiahe and Zana formations with low Sr/Ba and Sr/Cu ratios indicate the humid climate, whereas the high Sr/Ba and Sr/Cu ratios of rocks from the Bagong Formation suggest the arid climate. The low U/Th, (Cu + Mo)/Zn, V/Cr and Ni/Co ratios of rocks from the Zangxiahe, Zana, and Bagong formations are indicators of oxidized conditions. The bivariate diagrams (TiO2 vs. Al2O3, TiO2 vs. Zr, La/Th vs. Hf, and Co/Th vs. La/Sc) reveal that mudstones from the Zangxiahe and Zana formations were potentially derived from intermediate igneous rocks, whereas mudstones from the Bagong Formation were probably sourced from felsic igneous rocks. Their source rocks are mostly deposited in the collisional setting. REE of mudstones from the Zangxiahe, Zana, and Bagong formations were possibly originated from terrigenous detritus, with minor non-terrigenous contributions into the Zana samples. The REE contents of these mudstones are controlled mainly by terrigenous detrital minerals, rather than by the paleoclimate, paleoredox conditions, or organic matter. However, calcite minerals could dilute REE. Therefore, the REE contents of the Bagong Formation mudstones are significant lower than those of the Zangxiahe and Zana formations mudstones.

Eclogite-facies metamorphism in impure marble from north Qaidam orogenic belt: Geodynamic implications for early Paleozoic continental-arc collision

In the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, impure marble and interbedded eclogite represent a particular sedimentary provenance and tectonic setting, which have important implications for a controversial problem - the dynamic evolution of early Paleozoic subduction-collision complexes. In this contribution, detailed field work, mineral chemistry, and whole-rock geochemistry are presented for impure marble to provide the first direct evidence for the recycling of carbonate sediments under ultrahigh-pressures during subduction and collision in the Yuka terrane, in the North Qaidam UHP metamorphic belt. According to conventional geothermobarometry, pre-peak subduction to 0.8–1.3GPa/485–569°C was followed by peak UHP metamorphism at 2.5–3.3GPa/567–754°C and cooling to amphibolite facies conditions at 0.6–0.7GPa/571–589°C. U-Pb dating of zircons from impure marble reveals a large group with ages ranging from 441 to 458Ma (peak at ~450Ma), a smaller group ranging from 770 to 1000Ma (peak at 780Ma), and minor >1.8Ga zircon aged ca. 430Ma UHP metamorphism. The youngest detrital zircons suggest a maximum depositional age of ca. 442Ma and a burial rate of ca. 1.0–1.1cm/yr when combined with P–T conditions and UHP metamorphic age. The REE and trace element patterns of impure marble with positive Sr and U anomalies, negative high field strength elements (Nb, Ta, Zr, Hf, and Ti), and Ce anomalies imply that the marble had a marine limestone precursor. Impure marble intercalated with micaschist and eclogite was similar to limestone and siltstone protoliths deposited in continental fore-arc or arc setting with basic volcanic activity. Therefore, the Yuka terrane most likely evolved in a continental island arc setting during the Paleozoic. These data suggest that metasediments were derived from a mixture of Proterozoic continental crust and juvenile early Paleozoic oceanic and/or island arc crust. In addition, their protoliths were likely deposited in a terrigenous-dominated forearc marine basin rather than an intracontinental basin environment, further evidence that some continental arc volcanic rock may have been the source of eclogite in the North Qaidam. These sediments, formed in a forearc basin close to the Qaidam Block to the north, were transported in the subduction zone to 100–110km depth with UHP metamorphism prior to exhumation. Meanwhile, the new results suggest that subduction erosion occurred along the active continental margin during the Qaidam Block with north-dipping subduction, indicating that the North Qaidam UHP metamorphic belt may have formed during continental-arc collision.

Paleoenvironmental reconstruction by means of palynofacies and lithofacies analyses: An example from the Upper Triassic subsurface succession of the Hyblean Plateau Petroleum System (SE Sicily, Italy)

A combined palynofacies and lithofacies analysis was carried out on two borehole successions (Streppenosa 1 and Bimmisca 1) from the Hyblean Plateau Petroleum System (SE Sicily, Italy). It was found that both the wells penetrated the most important source and seal rocks of the Sicilian region (Noto and Streppenosa formations), previously assigned to the Late Triassic-Early Jurassic, deposited within a carbonate platform-basin system. Based on new palynological data, the organic rich succession (Noto Formation and Upper Streppenosa Member) can now be entirely assigned to the Rhaetian, thus constraining its deposition to a time interval characterized by increasing global humidity and seasonality. The integrated palynofacies and lithofacies data enabled characterization of the timing of the drowning phases of the carbonate platform-basin system as being controlled by relative sea level changes mostly triggered by the Triassic extensional tectonic activity. During the first phase of the relative sea-level rise, clayey and organic-rich sediments were deposited only in the deepest portion of the basin. As the sea level continued to rise, the entire system drowned completely and suboxic-anoxic basinal sediments were deposited across the whole Hyblean region, onlapping the shallow-water facies. In the meantime increasing global humidity contributed to an increased freshwater input in the marine depositional system as documented by the presence of fern spores and clay. It caused water stratification and subsequent anoxia at marine basins, favoring the preservation of sedimentary organic matter. This atmospheric change could be related to the degassing of the Central Atlantic Magmatic Province.

Pigment carbon and nitrogen isotopic signatures in euxinic basins.

The carbon and nitrogen isotopic signatures of chloropigments and porphyrins from the sediments of redox-stratified lakes and marine basins reveal details of past biogeochemical nutrient cycling. Such interpretations are strengthened by modern calibration studies, and here, we report on the C and N isotopic composition of pigments and nutrients in the water column and surface sediment of redox-stratified Fayetteville Green Lake (FGL; New York). We also report δ13 C and δ15 N values for pyropheophytin a (Pphe a) and bacteriochlorophyll e (Bchl e) deposited in the Black Sea during its transition to a redox-stratified basin ca. 7.8 ka. We propose a model for evolving nutrient cycling in the Black Sea from 7.8 to 6.4 ka, informed by the new pigment data from FGL. The seasonal study of water column nutrients and pigments at FGL revealed population dynamics in surface and deep waters that were also captured in the sediments. Biomass was greatest near the chemocline, where cyanobacteria, purple sulfur bacteria (PSB), and green sulfur bacteria (GSB) had seasonally variable populations. Bulk organic matter in the surface sediment, however, was derived mainly from the oxygenated surface waters. Surface sediment pigment δ13 C and δ15 N values indicate intact chlorophyll a (Chl a) was derived from near the chemocline, but its degradation product pheophytin a (Phe a) was derived primarily from surface waters. Bacteriopheophytin a (Bphe a) and Bchl e in the sediments came from chemocline populations of PSB and GSB, respectively. The distinctive δ13 C and δ15 N values for Chl a, Phe a, and Bphe a in the surface sediment are inputs to an isotopic mixing model that shows their decomposition to a common porphyrin derivative can produce non-specific sedimentary isotope signatures. This model serves as a caveat for paleobiogeochemical interpretations in basins that had diverse populations near a shallow chemocline.

Hydrological factors of formation of oxygen regime of inland seas

Introduction. The regularities of the formation and dynamics of the oxygen regime in inland seas - the Black Sea and Caspian, under the influence of natural and anthropogenic factors are estimated.
 Purpose. One of the features of the Black Sea is the absence of dissolved oxygen in the water below 200 meters depth. A comparison is made between the hydrological conditions of the Black Sea and the Caspian, where the depths are also quite large, but the intensity of vertical exchange is different. In addition, it is necessary to distinguish the reasons for the formation of oxygen deficient in the seas. They can be natural and human-made origin as well. The paper presents an analysis of the cause-and-effect patterns of the development of pelagial hypoxia as the examples of the Caspian and the Black Sea.
 Methods. The work is the review of the problem literature that allows an assessment of the cur-rent state of gas exchange of pelagic and abyssal marine basins.
 Results. Three main formation factors of the features of the hydrological structure and processes responsible for the intensity of gas exchange in the pelagic zone are identified. So, in the Black Sea, saline waters come with the Lower Bosporus Current and flow down the slope, filling the deep layers of the Black Sea basin, forming a stable vertical stratification density, that limiting vertical gas exchange. The second factor is contributing to oxygen saturation of the lower layers during the process of winter vertical circulation that is mainly expressed in the northwestern part.
 The third factor is the regime shift of 1976-1978 in the Black Sea as a decrease of the winter temperature and salinity in the surface layer that led to increasing of static stability. According to actively developed convection processes, covering the entire body of the Caspian Sea, in contrast to the Black Sea, there is an intensive exchange between the upper and the deep layers and the intensity of convection depends on the temperature regime of the year. In the Caspian Sea, the regime shift of 1976-1978 led to a twofold increase in the static stability of water below 100 m, the almost complete cessation of the ventilation of the deep waters of the (the process the slope cascading) and the extreme decrease in the concentrations of dissolved oxygen.
 Conclusion. In the Black Sea, the formation of a stable of the density vertical stratification is due to the intrusion of saline dense waters of the Lower Bosporus Current, and the winter vertical circulation is expressed only in the northwestern part of the sea, which generally limits vertical gas exchange with the deep water. In the Caspian Sea, convective mixing plays a main role in the for-mation of the hydrological structure of water and the ventilation of the bottom layers. In cold winters in the northern Caspian, strong cooling, as well as salinity during ice formation, creates the conditions for the formation of waters with a density that allows them to flow down the slopes of the bottom along isopycnic surfaces and aerating the deep layers of the sea.