Normal Marine Conditions Research Articles

THE DEMISE OF THE MESSINIAN SALINITY CRISIS AND THE BEGINNING OF THE ZANCLEAN IN THE NORTHERN MEDITERRANEAN BASIN: INSIGHT FROM HIGH-RESOLUTION BIOMAGNETOSTRATIGRAPHY (POLLENZO SECTION, NW ITALY)

The Messinian-Zanclean boundary in the Mediterranean basin marks the end of the Messinian salinity crisis (MSC) at approximately 5.33 Ma. The mechanism behind the return to normal marine conditions after the MSC are debated, with two main hypotheses proposed: an instantaneous reflooding of the Mediterranean at the base of the Zanclean, following its near-complete desiccation, or a gradual sea level rise in a non-desiccated basin during the late MSC phase (Lago-Mare). Our objectives are to refine the age model of the Pollenzo section in the Piedmont basin, Italy, and to elucidate environmental variability during this phase. We employ high-resolution biomagnetostratigraphic and cyclostratigraphic analyses, integrated with statistical multivariate and cluster analyses, and tie the results with other Italian Mediterranean reference sections of the basal Zanclean. The proposed age model is based on biostratigraphic markers (planktic foraminifera and calcareous nannofossils), alongside the identification of the base of the Thvera subchron. Although no astrochronological tuning was proposed, the influence of orbital parameters variation on calcareous plankton was noted, especially at tie of eccentricity maxima. Our results indicate a gradual restoration of open marine conditions after the MSC, spanning from 5.33 to 5.23 Ma. We observe a transition from a more marginal environment to more open marine conditions at the termination of MSC. Our findings challenge the notion of an abrupt transition at the end of the MSC, emphasizing the gradual nature of environmental change in the northernmost Mediterranean basin, from the late Messinian to the early Zanclean.

Toward a discrimination between reworked and in situ microfossil during the last phase of the Messinian Salinity Crisis: taphonomic insights from Maccarone section (Central Italy)

The paleoenvironmental change characterizing the final stage of the Messinian Salinity Crisis, referred to as Lago-Mare (LM) event is the core of a vibrant scientific debate. Two main scenarios have been proposed: an almost desiccated Mediterranean Basin filled by brackish water, or a rather deep basin characterized by normal marine conditions. The discrepancies between the two scenarios arose from the interpretation of the fossil record, represented simultaneously by brackish and marine taxa. Therefore, discriminating between reworked and in situ fossils represents a crucial step to disentangle the environmental conditions characterizing the LM interval. We present a micropaleontological approach to reconstruct the taphonomy involving micro/ nannofossils by analyzing samples from the Maccarone section (Italy), which records the Messinian-Zanclean transition. We show that during LM, the brackish ostracod assemblage is poorly preserved and associated with a high amount of terrigenous grains, suggesting reworking/displacing from marginal areas. The planktic foraminifer abundance is fluctuating and negatively correlated with terrigenous grains, likely suggesting intermittent conditions with normal salinity.

New data on the marine Upper Triassic palaeobiota from the Polzberg Konservat-Lagerstätte in Austria

Here we report new data on the Upper Triassic Polzberg Konservat-Lagerstätte in the Northern Calcarous Alps of Lower Austria. We examined new fossil material obtained from bed-by-bed collections of the well-laminated Reingraben Shales. Over 5290 new fossils of various marine taxa were collected within the fossiliferous layers from the Early Carnian (Julian 2 Ib, Austrotrachyceras austriacum Zone). The newly collected assemblage comprises ammonoids (Austrotrachyceras, Paratrachyceras, Carnites, Simonyceras), belemnoids (Phragmoteuthis, Lunzoteuthis), bivalves (Halobia, div. indet taxa), gastropods (caenogastropods/heterobranchs), thylacocephalan arthropods (Austriocaris, Atropicaris), crustaceans (the decapod Platychela and isopods Obtusotelson, Discosalaputium), branchiopods (Euestheria), polychaetes (Palaeoaphrodite sp. and an unidentified eunicid polychaete), acytinopterygians (Saurichthys, Polzbergia, Peltopleurus, Habroichthys), cartilaginous fishes (Acrodus), coelacanth fish (“Coelacanthus”), a lungfish (Tellerodus), and numerous conodont clusters (Mosherella). Bromalites (coprolites and regurgitalites) produced by piscivorous actinopterygians and durophagous fish accompany the Polzberg palaeobiota along with rare plant remains (Voltzia, div. indet plants). Based on new findings the palaeobiota characterises an intermittent colonisation by abundant benthic halobiid bivalves and a predator-dominated (fish, belemnoids) nektic community. The prerequisites for high-quality preservation—calm and oxygen-depleted conditions—prevailed at the sea floor of the Polzberg Konservat-Lagerstätte. Normal marine conditions prevail in the Reifling Basin, occasionally interrupted by freshwater influx. New in situ findings of benthic and nektic taxa highlight the great value of the unique Polzberg palaeobiota and the autochthonous deposition of the inhabitants within the palaeohabitat. The fauna and flora from the Polzberg Konservat-Lagerstätte, deposited during the Carnian Pluvial Episode or Carnian Wet Intermezzo, points to a carbonate platform decline followed by the deposition of laminated deposits in warmer and wetter conditions.

Paleoenvironment, sequence stratigraphy, and sea level change in the upper Maastrichtian–Danian boundary interval in Gabal Aras, east Qena, Nile Valley, Egypt

The Cretaceous–Paleogene (K–Pg) boundary interval at Gabal Aras, Qena region (part of the southern Tethyan margin) was investigated in detail for calcareous nannofossil and foraminiferal (planktonic and benthonic) assemblages to characterize the surface and bottom sea water conditions. The K–Pg boundary is marked by a minor hiatus expressed by the absence of the lowermost Danian P0 and Pα planktonic foraminiferal zones. Benthonic foraminifera assemblages indicate moderately oxygenated bottom sea waters associated with oligo-mesotrophic conditions during early late Maastrichtian, and mesotrophic conditions during the latest Maastrichtian. The benthonic foraminiferal assemblage reflects a decrease in the food supply in the bottom sea water during the early Danian, while well-oxygenated and oligotrophic conditions dominated during late Danian. The late Maastrichtian surface water witnessed a general cooling trend that was interrupted by a warming episode that could correspond to the Late Maastrichtian Warming Event (LMWE). Acme of Thoracosphaera associated with increased abundance of Neobiscutum romeinii were observed within the upper part of NP1 Zone indicating stressful surface water conditions. The increase of the epifaunal foraminiferal taxa and the occurrence of Cruciplacolithus primus throughout NP2 and NP3 zones reflect a progressive return to normal marine conditions. In the framework of sequence stratigraphy, two third-order depositional sequences covering the K–Pg boundary interval were identified, interpreted, and correlated. The obtained sea-level curve was evaluated to assess whether it was controlled by the eustatic sea-level changes, regional tectonics, or both these factors.

Paleogeographic and sedimentary evolution of North Calabrian basins during the Messinian Salinity Crisis (South Italy)

The Rossano and Crotone basins in North Calabria represent key areas for the study of the Messinian Salinity Crisis (MSC) in the central Mediterranean. The integrated analysis of the outcropping successions coupled with well-borehole stratigraphies and seismic profiles permitted the stratigraphic and paleogeographic reconstruction of the MSC as follows:During the late Tortonian-early Messinian, open normal marine conditions typified the studied basins, as the deposition of deep-water claystones followed by diatomites (Tripoli Fm). The first basin restriction occurred during the early Messinian (5.96 Ma) when a relative sea-level still-stand triggered the formation of a carbonate platform system (Calcare di Base Fm − CdB). This latter was characterized by sabkha to shallow-water environments passing into a gentle slope to basin setting.A successive relative sea-level drop (ca. 5.60 Ma) induced a general exposure of the CdB platform systems and the creation of a marked erosional surface linkable with the Messinian Erosional Surface (MES). Consequently, progradational early LST forced regression wedges deposited and, as the reduction of open-sea connections, more severe restricted conditions took place. Later on, during the late LST, the massive deposition of salt dominated bodies brought to the sedimentary filling of the basins and to the consequent leveling of the paleobathymetric differences. Afterwards, a transgression phase (TST) inundated again the basins, causing firstly the sedimentation of clay dominated deposits, and successively, during a further sea-level still-stand (HST), the reestablishment of evaporitic conditions with deposition of widespread shallow-water sulphate deposits. Lastly, a new severe sea-level drop (LST) exposed again all the previous deposits and alluvial conglomerates deposited along incised paleovalleys above the HST sulphate deposits. This event marks the transition to the Lago-Mare phase that typifies the last stage of the MSC, before the base-Pliocene reflooding of the Mediterranean (ca. 5.33 Ma).

Environmental inferences based on benthic foraminifera in surface sediments from the Beagle Channel (Argentina): A multiproxy approach

Micropaleontological analysis was performed on 22 samples of recent marine sediments from the Beagle Channel (Tierra del Fuego, Argentina) between 10 and 100 m of depth. The most important foraminifera found in the channel were Cibicidoides dispars (d'Orbigny), Buccella peruviana (d'Orbigny), Cibicides aknerianus (d'Orbigny), Cibicides fletcheri Galloway and Wissler, and Neoeponides bradyi (Le Calvez), which inhabit in clayey–sandy marine sediments. The observed variations in diversity and abundance revealed shallow marine conditions, low-energy, and low to moderate salinity, in the west sector and a deeper environment with normal marine conditions, high-energy, highly‑oxygenated and cold seawater in the east sector of the channel. The study area is a complex scenario with numerous landforms that lead to the formation of micro-environments in which patchy distribution of foraminifera was recognized. This study based on foraminifera and the sedimentological analysis supports the previous palynological information from the channel, which also shows an increase in number and diversity of dinocyst species reflecting the eastward changes of the marine environmental conditions due to the influence of the Atlantic.

Reconstructing the paleoceanographic and redox conditions responsible for variations in uranium content in North American Devonian black shales

The uranium (U) content, and more recently, the ratio between 238 U and 235 U in black shales are commonly applied as a proxy to determine redox conditions and infer organic-richness. Uranium contents typically display a linear relationship with total organic carbon (TOC) in shales. This relationship is due to the processes and mechanisms responsible for the incorporation of U into the sediment during the deposition and remineralization of organic matter. This U/TOC relationship can vary, however, and some shales display uncharacteristically low U content despite having high TOC content, while others show large enrichments of U relative to TOC. Here we examine the U to TOC ratios and U-isotope compositions of three Upper Devonian-Lower Mississippian shales: the Woodford Shale, the Cleveland Shale, and the Bakken Shale, with two study sites in Oklahoma, one site in eastern Kentucky, and three sites in eastern Montana and western North Dakota, respectively. The U/TOC ratios of each shale are distinct from one another exhibiting average ratios ranging from 3 in the Cleveland Shale, to over 10 in the Bakken Shale. The distinct geochemical composition of the three shales suggests that, although lithologically similar, each study site represents a markedly different and dynamic depositional environment. The low average U/TOC (~3) along with the relatively high δ 238 U values (~0.03‰) of the Cleveland Shale core suggests deposition along the basin margin under normal marine conditions with periods of reduced bottom water oxygenation, likely due to fluctuations in the location of the pycnocline. The Woodford Shale on the other hand, shows higher U/TOC ratios (~4, George core, ~9, Poe core) and δ 238 U (~0.02‰ average, George core, ~0.06‰ average, Poe core), which suggests an unrestricted setting with intermittent euxinic conditions. In contrast, high U/TOC ratios (2–15), and very high δ 238 U values (up to 0.55‰) in the Bakken Shale cores indicate intense metal draw-down into sediments under sulfidic waters. The results show that when the U/TOC ratios and U-isotopic compositions of each studied shale are compared to modern anoxic basins and upwelling areas, it allows for an enhanced understanding of the paleoenvironmental conditions such as basin restriction and redox state of waters within the Late Devonian epicontinental seas of North America. • U/TOC ratios of Late Devonian shales vary due to basin restriction. • Bottom water euxinia does not require strong basin restriction. • δ 238 U values provide new insight into the paleogeography of N. American basins.

Exotic facies episodes of a carbonate platform: implications for middle and late Cambrian ecosystems and impact of bioturbation in the Alborz Basin, Iran

Abstract The sedimentological and ichnological data of the Mila Group in the Alborz Basin, northern Iran indicate that the appearance of exotic facies and resurgence of microbialites resulted from the development of extraordinary palaeoceanographic conditions with widespread environmental stress and enhanced precipitation of CaCO 3 . Inhospitable environmental conditions in the Mila Group platform led to a considerable reduction in ichnodiversity, bioturbation intensity and depth and size of burrows, which led to periods of dominating calcimicrobe ecology. The development of exotic or anachronistic facies started around the Cambrian Stage 4–Wuliuan boundary, probably simultaneous with the SPICE event, whereas the Furongian exotic facies period probably took place simultaneous with the HERB event. Biotic and physico-chemical changes in the ocean during these events may have led as well to the virtual absence of the mixed layer and infaunal ecospace utilization in the deposits of the Mila Group. Nevertheless, the local presence of metazoan-microbial build-ups that established between the two suggested exotic facies developments indicates that these periods started at the Cambrian Stage 4–Wuliuan boundary interval, but a fluctuation of abnormal palaeoceanographic conditions and ambient normal marine conditions took place during the Miaolingian and Furongian in the Alborz Basin as a representative of the northern Gondwanan margin.

Biostratigraphic ages and depositional environments of the upper Oligocene to lower Miocene Veldhoven Formation in the central Roer Valley Rift System (SE Netherlands-NE Belgium)

Abstract Discussions on the age and the depositional environments of the Veldhoven Formation and its members are persistent in Belgium and the Netherlands. Uncertainties on stratigraphy and the constructive process of sediment accumulation continue today as a result of lack of data on this succession within the Roer Valley Rift System. The present study provides new information on the bio- and lithostratigraphy and facies from two boreholes based on dinoflagellate cyst taxa. The results were correlated by gamma-ray logs towards other key boreholes in the region and show a good consistency for stratigraphy and depositional environments for the members of the Veldhoven Formation. After marginal to restricted marine conditions in the latest Rupelian (early Oligocene), the start of deposition of the Veldhoven Formation marked the transition towards a higher sea level, expressed by increased glauconite contents and gamma-ray values. The Voort Member in the lower part of the Veldhoven Formation has an early to late Chattian (Late Oligocene) age and comprises predominantly shallow marine (fluctuating restricted to open marine) conditions. The lithology in the lower part of this unit is often very clayey but is coarsening upward into sands. The superjacent Wintelre Member has a latest Chattian to early Aquitanian (early Miocene) age. This member is distinct by its clayey nature which is expressed by relatively high gamma-ray values. Earlier studies suggest a deeper marine facies for the Wintelre Member compared to the Someren and Voort members. However, the dinoflagellate cyst assemblages in this unit are mostly dominated by a single genus indicating a restricted marine setting, including salinities that deviate from normal marine conditions, most probably due to minor ventilation by narrow or lack of connection to the Atlantic Ocean. A glacio-eustatic sea-level fall around the Oligocene/Miocene boundary limited the sea coverage to the strongest subsiding areas, where deposition of the Wintelre Member is recorded, while non-deposition or erosion occurred in the surrounding highs, hence creating an isolated (sub)basin. The superjacent Someren Member was deposited during the late Aquitanian to middle Burdigalian and consists of shallow to open marine clayey fine sands. Increasing clay contents indicate a gradual development towards a higher sea level, which coincide with upward increasing gamma-ray values. The biostratigraphic results of this study suggest that no major hiatuses are present in the differentially subsiding blocks of the Roer Valley Rift System during the late Oligocene to early Miocene.

Archaeal tetraether lipids and their biphytane carbon isotope composition in sediments along an estuarine biogeochemical gradient

The stable carbon isotopic composition of Thaumarchaeota-produced crenarchaeol (δ13Ccren) is thought to reflect the δ13C of dissolved inorganic carbon (DIC) and is valuable in carbon cycle research. Recently, an isotope flux-balance model was proposed that δ13Ccren is dependent on passive CO2 uptake and a slow rate of intracellular conversion to HCO3−, which has been validated in open ocean environments. However, further studies are required in environments with larger ranges of aqueous CO2 and diverse archaeal communities to examine whether the δ13Ccren value could still be predicted by the model. Here, we report δ13C values of biphytanes chemically released from the archaeal tetraether lipids (i.e., GDGTs) in sediments along a transect exhibiting shifts in the archaeal community and showing gradients of salinity, pH, and CO2 in the Pearl River Estuary, South China. Methanogenic and methanotrophic Euryarchaeota are present in the upper estuary, although not significant, as revealed by the elevated relative abundances of GDGT-0 and higher values of methane index in suspended particulate matter (SPM) and surface sediments, as well as the more 13C-depleted C40:0, C40:1, and C40:2 biphytanes than the C40:3 biphytane released from sedimentary GDGTs. Toward the lower estuary and coastal sea, GDGT distributions in SPM and sediments tend to indicate archaeal communities under normal marine conditions. However, there are still differences in δ13C values between C40:0 and C40:3 biphytanes by up to 2.4‰ in sediments, although C40:1, C40:2, and C40:3 were similar in δ13C, likely due to the existence of marine group II Euryarchaeota. Along such a physicochemical and biogeochemical gradient, the values of sedimentary δ13C40:3 increase monotonically with the increase of mean annual pH and the decrease of mean annual CO2. This phenomenon resembles the reported δ13CGDGT profiles, along with the variations in pH and CO2, in water column in the open ocean, and hence consistent in direction with the isotope flux-balance model for Thaumarchaeota lipids. However, our estimated apparent biosynthetic isotope effect (εAr) of crenarchaeol, ranging from 15.2‰ to 19.7‰, is mostly lower than those estimated in the open ocean. With the constraints of thaumarchaeal growth rates in such a dynamic estuarine environment as well as those in the open ocean, we suggest slight reductions in catalytic efficiency and intracellular kinetic isotope effect of intracellular CO2 hydration relative to previously employed values. Such modifications would slightly change the sensitivities of εAr to archaeal growth rate and ambient CO2 concentration but might be more suitable for variable hydrology and community environments.

Brachiopods in early Mesozoic cryptic habitats: Continuous colonization, rapid adaptation, and wide geographic distribution

Late Triassic and early Jurassic dikes and fissures in the Dachstein Limestone in the Northern Calcareous Alps harbor mass occurrences of the rhynchonellide brachiopods Sulcirostra juvavica and Halorella amphitoma . To test recent hypotheses about their paleoecology, we characterized these habitats using petrography, carbon stable isotopes, and trace element patterns, and found no evidence for hydrocarbon seepage or hydrothermal venting. Thus the brachiopods lived under normal-marine conditions, in darkness and absence of local photosynthetic primary production, hence relying on the supply of limited and presumably small-sized food washed into the dikes and fissures. Because Halorella and Sulcirostra occur in dikes and fissures since the beginning of their stratigraphic ranges, these rhynchonellides are not relics of formerly widely distributed taxa, but instead are two genera that rapidly adapted to these habitats. Both Halorella and Sulcirostra occur also in late Triassic and early Jurassic deep-water settings such as deep-marine sills and hydrocarbon seeps, indicating that close phylogenetic relationships between submarine cave faunas and deep-sea faunas, as seen today, existed also in the early Mesozoic, albeit among very different taxa. Another analogy to the modern cave fauna is the wide but disjunct geographic distribution of Sulcirostra and Halorella , both found throughout the Tethys and Panthalassa oceans. Our findings support the view that submarine cave habitats were continuously colonized by new taxa throughout Earth's history.

Trends in temperature, salinity and productivity in the Vienna Basin (Austria) during the early and middle Miocene, based on foraminiferal ecology

The Neogene Vienna Basin (VB) is a major hydrocarbon province with a long history of exploration accumulating extensive stratigraphic and structural information from numerous seismic and drilling programs. Based on the quantitative analysis of hundreds of foraminiferal samples from 52 drillings, we present the first continuous reconstruction of paleoenvironmental evolution of the VB from the early Miocene to the middle Miocene spanning 6.4 million years. Our analyses comprise reconstructions of sea surface temperature (SST), bottom water temperature (BWT), salinity, trophic levels, stress indicators, mode of life, feeding preferences and diversity indices (Fisher α, dominance and equitability) based on the transfer function for foraminifers of Hohenegger (2005, Palaeogeogr. Palaeoclimatol. Palaeoecol. 217, 115–130) and applied statistics (e.g. box plots and ternary diagrams). Bottom water temperatures indicate a cooling during the early and middle Badenian (Langhian), which seemingly contradicts the global warming of the Middle Miocene Climatic Optimum (MMCO) and a subsequent warming, which contrasts the expected trend following the cooling of the Middle Miocene Climatic Transition. Both trends are discussed as result of bathymetric evolution of the VB and intense upwelling during the early and middle Badenian. Repetitive alternations of Orbulina-rich assemblages, indicating a warm and stratified waterbody, with globigerinid-dominated assemblages, indicating upwelling and high productivity, are discussed in the light of a yet undescribed waxing and waning of upwelling intensity on a decadal to millennial scale. The applied formula for SST-calculations seems to underestimate real temperatures. Nevertheless, it reveals a distinct warming of about 3 °C from the early to the middle Miocene. In contrast to previous interpretations, we document normal marine conditions from the Ottnangian to the Sarmatian (middle Burdigalian to Serravallian) with a slight increase in salinity during the late Sarmatian.

Foraminiferal micro-buildups (“reefs”) in the Wuchiapingian basin facies of the basal Zechstein carbonates in western Poland

Encrusting foraminifers locally can play an essential reef-forming role such as in the Carnian fossiliferous Hallstatt Limestones of Austria where a dense succession of hardgrounds occurs with numerous small buildups (up to 2 cm high) composed of sessile foraminifers. Similar foraminiferal micro-buildups occur in a 10-cm-thick bed in the basinal facies in the basal Zechstein (Upper Permian) strata in the Radlin 60 borehole, in western Poland. Foraminiferal micro-buildups of presumably columnar habit account for up to a half of the volume. The growth of columns was interrupted by hiatal surfaces, although usually the growth of columns was restored afterwards. In the upper part of the bed with foraminiferal micro-buildups, the foraminiferal encrustations become visibly less abundant, and in most cases, they have a shape of less regular masses. The bed with foraminiferal micro-buildups abounds in closely spaced discontinuity (hiatal) surfaces, occasionally encrusted by foraminifers. Tubular foraminifers in the bed with foraminiferal micro-buildups are accompanied by various encrusting organisms, possibly microbes; they are grouped under the name Palaeonubecularia. In the intercolumnar spaces, common Midiella sp. occur. During deposition, the conditions were mostly suboxic, and various types of ferruginous morphs, common in the lower part of the bed, suggest that dysoxic–anoxic interfaces produced various microaerophilic environments for iron-bacteria. The environmental conditions during the deposition of the upper part of the bed were very similar to those characterizing the starved basin environment in which the bioclastic-peloidal and oncoidal sediments in the Zechstein Limestone have been formed. In turn, the lower part of the bed registers the transition from normal (= not starved) marine conditions, with a clear chemocline, to the starved basin conditions. The deposition of the bed was coeval with the Kupferschiefer deposition based on the correlation of δ13C curves, which implies that the previous stratigraphical concept of basal Zechstein strata has to be re-evaluated.

Flexural‐isostatic reconstruction of the Western Mediterranean during the Messinian Salinity Crisis: Implications for water level and basin connectivity

AbstractDuring the Messinian Salinity Crisis (MSC, 5.97–5.33 Ma), thick evaporites were deposited in the Mediterranean Sea associated with major margin erosion. This has been interpreted by most authors as resulting from water level drop by evaporation but its timing, amplitude and variations between subbasins are poorly constrained due to uncertainty in post‐Messinian vertical motions and lack of a clear time‐correlation between the marginal basin and offshore records. The Balearic Promontory and surrounding basins exemplify a range of responses to this event, from margin erosion to up to a kilometre thick Messinian units in the abyssal areas containing the majority of the MSC halite. The Balearic Promontory contains unique patches of halite with thickness up to 325 m at intermediate depths that provide valuable information on water level during the stage of halite deposition. We compile seismic markers potentially indicating ancient shorelines during the drawdown phase: the first is marked by the transition from the MES to UU based on seismic data. The second is the limit between the bottom erosion surface (BES) and abyssal halite deposits. We restore these shorelines to their original depth accounting for flexural isostasy and sediment compaction. The best‐fitting scenario involves a water level drop of ca. 1,100 ± 100 m for the Upper unit level and 1,500 ± 100 m for the BES level. According to our results, halite deposition began in the Central Mallorca Depression at 1,300–1,500 m depth, perched hundreds of metres above the deep basins, which were at 1,500–1,800 m (Valencia Basin) and >2,900 m (Algerian Basin). The hypothesis that erosion surfaces were formed subaerially during the drawdown phase is consistent with a model of halite deposition before/during the water level drop of at least 1,000 m, followed by the deposition of the Upper unit until the MSC is terminated by the reinstatement of normal marine conditions.

The late Paleozoic Ice Age along the southwestern margin of Gondwana: Facies models, age constraints, correlation and sequence stratigraphic framework

The southwestern margin of South America offers a complete record of the Late Paleozoic Ice Age (LPIA) that affected the Gondwana supercontinent. The tripartite division of LPIA glacial episodes has been refined with the help of new radiometric dates and biostratigraphic (flora and fauna) zonations in recent years to five shorter-lived discrete events: 1. Latest Devonian-earliest Tournaisian, 2. Tournaisian, 3. Visean, 4. Serpukhovian-Early Bashkirian, 5. late Pennsylvanian-earliest Permian. These glacial events are capped by postglacial transgressive deposits with marine fauna. The unbalanced preservation potential of the glacial deposits, skewed toward the glaciomarine sediments, provides an uneven stratigraphic record with few cases of continental glacial sedimentation, confined to the Serpukhovian-Early Bashkirian event, and numerous examples of glacial sedimentation in marine environments. Glacial sedimentation in marine settings has been grouped in two main facies associations: a valley-glacier-retreat (fjord) facies association and a submarine-retreat (glaciomarine apron) facies association in open-marine areas.Transitional facies, correspondent to those formed by the flooding of valleys during postglacial transgressions, are widely distributed along the Protoprecordillera in western Argentina, where paleofjord successions are well exposed particularly in western Paganzo Basin, and mapped in subsurface in the Tarija basin. A general paleofjord model includes (from base to top) the following stages: (i) Incision of paleovalley and deposition of subglacial diamictites in ice contact deltas, (ii) Early Transgressive stage characterized by resedimentation of subglacial material by subaqueous sediment gravity flows and slumps in proglacial settings, (iii) Maximum flooding (late transgressive stage) dominated by black shales or laminated mudstones related to a marine incursion that flooded valleys; normal marine or brackish conditions may dominate this stage and (iv) Highstand: progradation of a fluvial-deltaic system including in some cases Gilbert-type deltas. In glaciomarine apron environments, the basal facies includes massive clast-supported conglomerates, with few striated and polished clasts, followed by fining-upward successions including thinly bedded diamictites with ice-rafted debris (IRD) and locally contorted sandstone masses in diamictite beds, indicative of mass-emplacement mechanisms. The presence of inter- and intratill pavements suggests glacial advance/retreat fluctuations along the basin margins.Deglaciation sequences, reflecting deposition mainly during the retreat of ice sheets, ice caps and alpine glaciers and successive deglaciation, can be used as operational tools for the analysis of glacial successions in southwestern Gondwana. They are characterized as rather simple upward-fining successions in open marine settings as exemplified in most of the Calingasta-Uspallata Basin,Claromecó Basin (and adjacent Ventana foldbelt, VFB) and central portions of the Paraná and Karoo basins. In more proximal areas (i.e., paleofjords) this vertical trend is commonly punctuated with deltaic wedges fed by nearby provenance areas.The late Paleozoic glacial-related successions of southwestern Gondwana exhibit a common tripartite motif, equivalent to second-order sequences with estimated durations of 10–80 Myr. The lower section corresponds to glacial and glacially-influenced diamictites; the middle interval is initiated with postglacial transgressions. The lower and middle intervals correspond to the deglaciation sequence as described and identified in several basins of Gondwana. Finally, the upper term includes coastal progradation, followed in some places by continentalization, accompanied in many sectors by increasing aridization. Examples of second-order sequences can be identified in the thick late Paleozoic successions of the Paraná and Karoo basins and in the VFB. Thinner second order sequences can be identified in the Calingasta-Uspallata, Rio Blanco, Paganzo and San Rafael basins. In the Paganzo and San Rafael basins the middle interval is also punctuated by short lived marine ingressions. The basal sequence boundary is commonly an abrasion surface (glacial erosion surface, GES) developed on bedrock. Deglaciation sequences are assigned to third order sequences made up, when present, of a thin lowstand system tract (LST) of subglacial deposits followed upward by thick glaciomarine and glacially influenced sediments. These facies are part of a thick transgressive systems tract (TST) that culminates with marine shales that reflect interglacial or postglacial conditions during ice retreat. Thus, the deglaciation sequences are proposed to be third order sequences made up of LST-TST or exclusively TST.

Integrity of benthic assemblages along the arctic estuarine-coastal system

This study was carried out in the Yenisei estuary and in the adjacent part of the Kara Sea shelf. We analyzed and compared the distribution patterns of the different benthic taxa and size classes (micro-, meio-, macro- and megabenthos) along a pronounced gradient of environmental factors. Four ecological zones were distinguished based on certain environmental parameters, from the upper riverine area to the mouth of the estuary and the Kara Sea slope, which had normal marine conditions. The distribution of the metazoan assemblages generally corresponded to the divisions of these zones and represented a series of species assemblages with coincident boundaries that changed successively along the estuarine gradient. For all three size classes of metazoan benthos, we detected three main species assemblages: brackish-water, transitional and marine. Each taxon showed, however, its own particular spatial pattern and, therefore, has a set of environmental variables that is responsible for its fine-scale distribution. The distribution of unicellular organisms showed a worse match with the division of the estuary into environmental zones. Foraminifers established a single association in most of the estuary but were absent in the upper desalinated part. Diatom algae were patchily distributed, with a number of local communities and many rare species.We conclude that the estuarine habitat appears to drive the distribution of the various benthic taxa in taxa-specific ways. The distribution of size classes (i.e., mega-, macro-, or meiofauna) that combine several high-level taxa demonstrates a better match with the multivariate environmental zonation than the distribution of the separate taxa. The role of different benthic taxa and size classes in biomonitoring studies is discussed.

Winding down the Chicxulub impact: The transition between impact and normal marine sedimentation near ground zero

The Chicxulub impact led to the formation of a ~ 200-km wide by ~1-km deep crater on México's Yucatán Peninsula. Over a period of hours after the impact the ocean re-entered and covered the impact basin beneath several hundred meters of water. A suite of impactites were deposited across the crater during crater formation, and by the resurge, tsunami and seiche events that followed. International Ocean Discovery Program/International Continental Scientific Drilling Program Expedition 364 drilled into the peak ring of the Chicxulub crater, and recovered ~130 m of impact deposits and a 75-cm thick, fine-grained, carbonate-rich “Transitional Unit”, above which normal marine sedimentation resumed. Here, we describe the results of analyses of the uppermost impact breccia (suevite) and the Transitional Unit, which suggests a gradual waning of energy recorded by this local K-Pg boundary sequence.The dominant depositional motif in the upper suevite and the Transitional Unit is of rapid sedimentation characterized by graded bedding, local cross bedding, and evidence of oscillatory currents. The lower Transitional Unit records the change from deposition of dominantly sand-sized to mainly silt to clay sized material with impact debris that decreases in both grain size and abundance upward. The middle part of the Transitional Unit is interrupted by a 20 cm thick soft sediment slump overlain by graded and oscillatory current cross-laminated beds. The uppermost Transitional Unit is also soft sediment deformed, contains trace fossils, and an increasing abundance of planktic foraminifer and calcareous nannoplankton survivors. The Transitional Unit, as with similar deposits in other marine target impact craters, records the final phases of impact-related sedimentation prior to resumption of normal marine conditions. Petrographic and stable isotopic analyses of carbon from organic matter provide insight into post-impact processes. δ13Corg values are between terrestrial and marine end members with fluctuations of 1–3‰.Timing of deposition of the Transitional Unit is complicated to ascertain. The repetitive normally graded laminae, both below and above the soft sediment deformed interval, record rapid deposition from currents driven by tsunami and seiches, processes that likely operated for weeks to potentially years post-impact due to subsequent continental margin collapse events. Highly siderophile element-enrichment at the top of the unit is likely from fine-grained ejecta that circulated in the atmosphere for several years prior to settling. The Transitional Unit is thus an exquisite record of the final phases of impact-related sedimentation related to one of the most consequential events in Earth history.

Foraminiferal communities of a mid-Holocene reef: Isla Colón, Caribbean Panama

The distribution of mid-Holocene, tropical benthic foraminiferal assemblages, including species' proportions, diversity, dominance and wall type, were analyzed for their correspondence to marine habitats and invertebrate facies. Benthic foraminifera are useful for paleoenvironmental reconstructions because the modern ecology of many species found as Quaternary fossils is known. Samples were collected from trenches in ~30,000 m2 of an excavated Acropora cervicornis-dominated, mid-Holocene reef with an age of ~6 kyr on Isla Colón (Colon island), bordering Almirante Bay in Bocas del Toro, Caribbean Panama. Bulk sediment samples were collected from a maximum depth of ~7 m below modern mean sea level and classified in the field into five invertebrate biofacies: 1) A. cervicornis-dominated reef, 2) molluscan mud, 3) Porites-Agaricia reef, 4) mixed coral, and 5) Lobatus-dominated seagrass. Sediment carbon and grain size analyses, a cluster analysis performed on the relative abundance of species per sample, and the Fisher's alpha diversity index were used to compare sample similarity and environmental variables to determine habitat relationships. Most samples contained high total inorganic carbon and poorly sorted, medium-coarse sediments. Principle component analysis of sediment grain size and carbon values did not show a clear correspondence among samples, habitat type or location of trenches. Foraminiferal assemblages in A. cervicornis and other reefal samples (categories 1, 3 and 4, above) had the greatest diversity and did not distinguish between the three reefal types, suggesting similar, normal marine conditions and/or mixing of coral fragments. Molluscan mud samples with high total organic carbon content were least diverse with dominant Ammonia and Elphidium taxa, though foraminifera in other molluscan mud samples showed a transition to proximal reef corals. Seagrass samples were differentiated from molluscan mud samples and had similar diversities and species assemblages to the Porites-Agaricia samples, and taxa known to be temporary grazers and living temporarily attached to seagrass blades are present in relatively greater amounts in these samples.Based on the distribution of foraminiferal species across this mid-Holocene coral reef, we conclude that it was a patch reef that included high-organic and seagrass facies similar to those of modern Almirante Bay. Results from this study can be compared to modern foraminiferal studies to investigate whether the modern habitats of BDT are significantly different from the pristine reefs of the mid-Holocene.

Widespread seagrass meadows during the Early Miocene (Burdigalian) in southwestern Australia paralleled modern seagrass distributions

The Trealla Limestone, which forms a veneer of foraminiferal-rich limestone and covers over 4000 km2 of the tectonically stable Southern Carnarvon Platform, contains the oldest evidence for seagrass meadows known in Western Australia. It belongs to Lower Tf1 Letter Stage (based on larger benthic foraminifers) of the Burdigalian, within interval 19–16 Ma, and is the only Miocene unit on the platform. The Trealla Limestone here is coeval to the deeper water lower part of the much thicker Trealla Limestone in the Exmouth Sub-basin to the northwest. A seagrass-meadow depositional environment is indicated by most of the studied microfacies based on comparisons with modern sediment composition and foraminiferal assemblages found in innermost shelf seagrass banks between 23°S and 32°S on the present Western Australian coast. Some levels in the Trealla Limestone on the platform (e.g. with abundant miliolid foraminifers, and absence of coral debris) probably were deposited in and around dense seagrass meadows similar to those present today in the metahaline (40–55 ppt salinity) reaches of Shark Bay; whereas other levels (e.g. coral floatstone) were deposited under normal-marine conditions. Based on comparisons with modern carbonate sediment and biota in the region, the biogenic composition of the Trealla Limestone on the platform suggests winter minimum sea-surface temperatures within the range of 17–22 °C. The Nullabor Limestone in the Eucla Basin, at 10° further south on the southern Australian margin, is similar, at least in part, in composition and age to the Trealla Limestone on the Southern Carnarvon Platform. Both may have been deposited during a Burdigalian sea-level rise, and during part of the Miocene climate optimum recognized elsewhere. Sediment composition and foraminiferal assemblages (including epiphytic types) found in the Trealla and Nullabor limestones are similar to those in the modern seagrass banks of the region. This suggests that seagrasses similar to the extensive Posidonia and Amphibolus stands, which are dominant today between 25°S and 35°S along the Western Australian coast, were widespread during the Burdigalian.

Rétablissement total des conditions marines après la phase "Lago Mare" du Messinien supérieur de la Mer Méditerranée dans les régions de Licodia Eubea et Villafranca Tirrena (Sicile orientale)

After the Upper Messinian Mediterranean Lago-Mare phase, at the end of the Messinian salinity crisis, the Mediterranean basin was characterized by a return to normal marine conditions starting with the Lower Pliocene (Zanclean). In this period, remarkably thick layers of calcareous pelitic sediments, very rich in planktonic foraminifers and nannofossils, were widely deposited in the Mediterranean basin. The calcareous pelitic deposits of Sicily (Trubi Formation), as well as in other Mediterranean regions, were deposited conformably on the pre-Pliocene substrate, which is mainly represented by Messinian evaporites or by the post-evaporitic Lago-Mare facies. We have analysed samples taken from the lower portion of the Trubi Formation (Zanclean), just above the Messinian facies, collected from Licodia Eubea and Villafranca Tirrena areas (eastern Sicily). Micropalaentological assemblages consist of benthic and planktonic foraminifers and a typical deep-water ostracod fauna. The microfossil association and the stratigraphy of the Messinian-Pliocene transitional layers, supported by calcareous nannoplankton data, demonstrate not only rapid sea-level rise in this sector of the Mediterranean region, but also that this event, occurred without producing erosional features, unlike at other sites (e.g., Strait of Gibraltar).