Interior Basin Research Articles

Deep-sea bottom currents influencing tracemaker community: An ichnological study from the NW Iberian margin

Marine macrobenthic tracemaker communities are controlled by limiting parameters (e.g., oxygen, organic matter availability and hydrodynamic energy, among others) which are determined by ocean/atmosphere dynamics affecting the sea bottom. Over the last few years, detailed ichnological studies have provided information about trace fossils content of contourites and the palaeoenvironmental conditions during deposition. However, there are still some open questions regarding ichnology in contourites, such as how the variation in the intensity of bottom currents could affect the macrobenthic community. Here, we present a trace fossils analysis, combining high-resolution images and CT data processing, of five gravity cores retrieved in the Galicia Interior Basin (GIB) from the NW Iberian margin. The ichnological record from contourites and other deep-sea deposits (i.e., pelagic, hemipelagic, turbiditic and ice rafted detritus layers) in the GIB over the last 60 ky is assessed. A trace fossil assemblage composed of 14 ichnotaxa occurs, mainly belonging to Zoophycos ichnofacies. Seven ichnofabrics were defined based on ichnological features. The distribution of these ichnofabrics is linked to depositional facies. In the contouritic intervals, ichnofabric alternations between Thalassinoides ichnofabric and Palaeophycus ichnofabric reveal changes in sedimentation rate, nutrient availability and substrate consistency. These variations in the ichnological content suggests intermittent deposition during contourite sedimentation in contrast with the continuous process traditionally considered in contourite facies models.

Intermittent Frontogenesis in the Alboran Sea

AbstractWe present a phenomenological description and dynamical analysis of the Alboran fronts using a realistic simulation at submesoscale resolution. The study is focused on east Alboran fronts emerging within relatively strong flows that separate from the Spanish coast into the basin interior. Despite modest lateral shifting associated with the position of the Alboran anticyclonic gyres and variations in intensity, these fronts present a similar structure and dynamical configuration as the climatological Almeria–Oran front. The statistical analysis of our solution shows that strained-induced frontogenesis is a recurrent submesoscale mechanism associated with these fronts, and the process is assessed in terms of the advective Lagrangian frontogenetic tendencies associated with buoyancy and velocity horizontal gradients. Intermittency in their strength and patterns is indicative of high variability in the occurrence of active frontogenesis in association with the secondary (overturning) circulation across the frontal gradient. As a result, we find many episodes with strong surface fronts that do not have much associated downwelling. Frontogenesis and the associated secondary circulation are further explored during two particular frontal events, both showing strong downwelling of (1) cm s−1 extending down into the pycnocline. A frontogenetic contribution of turbulent vertical momentum mixing to the secondary circulation is identified in the easternmost region during the cold season, when the dynamics are strongly influenced by the intrusion of the salty Northern Current. The background vertical velocity fields observed during the analyzed events indicate other currents in the submesoscale range, including tidal and topographic internal waves.

From the deep ocean to the coasts and estuaries through the shelf: linking coastal response to a deep blow-out

ABSTRACT # 685087As a marginal sea connected to neighboring basins through straits, the Gulf of Mexico (GoM) is dynamically and topographically complex. Physical processes are strongly influenced by the interaction of circulation in the GoM deep basin interior and in the surrounding shelf areas of diverse morphologies that include deltas, estuaries, barrier islands and marshes. This was particularly evident during the 2010 Deepwater Horizon (DwH) incident, a deep blow-out close to the Northern GoM shelves, over an area strongly affected by the brackish river plume originated from the Mississippi River Delta. The specific physical conditions are revisited, to illustrate the synergy between the evolution of the Loop Current – Florida Current system and the rapidly changing shelf and coastal currents under the influence of river runoff and winds. Each of these physical factors had been studied prior to the DwH incident, but their combined effects on hydrocarbon pathways were not known.Examples are given on what has been learned through research under the Gulf of Mexico Research Initiative (GoMRI) in the last 10 years. The focus is on transport processes in the GoM along the ocean continuum from the deep basin interior to the coastal and wetland areas, and their relevance for oil transport and fate. Post-DwH studies have advanced regarding methodologies and tools. These include multi-platform observations and data analyses, in tandem with high-resolution, data assimilative models for past simulations and predictions.Important new findings include the connectivity between remote coastal regions, as deep oceanic currents can facilitate the cross-marginal transport of materials not only locally, but regionally. This creates a broader and more challenging view for the management of coastal marine resources that should be integrated for preparedness and response. Two examples are presented on connectivity processes. First, advances in the understanding of transport rates and pathways from the Mississippi Delta to the Florida Keys. Second, new findings on how coastal circulation near Cuba influences the evolution of the Loop Current system and the oil fate from a potential oil spill in Cuban waters.The synthesis of the above findings aims to demonstrate how knowledge acquired during GoMRI can advise future planning of scientific research to aid preparedness and response not only for the GoM, but for many offshore areas of oil exploration. The goal is to advance the understanding and predictability of oil slick trajectories over pathways from the deep to the coastal environment and vice versa.

Recent alluvial fan developments in Muğla (SW Turkey)

The Mugla Polje (SW Turkey), which formed as a result of the Pleistocene activity of the Mugla Fault, is an example of a rift basin with two alluvial fans (the Mugla Fan, 3.33 km2 and the Dugerek Fan, 1.26 km2). This study focuses on the development of these fans through the evaluation of aerial photos, Google-Earth images, the measurements of sedimentary sections, lithological examinations, mineralogical discriminations, and in situ observations. Fractured Jurassic-Cretaceous limestone and upper Miocene-Pliocene clastics have been observed in drainage areas of both fans. In the drainage area of the Mugla Fan, where low strength upper Miocene-Pliocene clastics are abundant, the river incision is higher. As a result, higher amount of sediment supply led to formation of the larger Mugla Fan. Small scale tectonic activities and rainy periods led to the formation of poorly sorted, angular-subangular, and coarser-grained pebblestone-cobblestone deposits, whereas, tectonically quiescent and dry periods were represented by the finer-grained sediments at the apex and upper fan of both fans. They pass into the finer-grained deposits in the downdip direction. Recently, the Mugla and Dugerek fans surface has been covered by urbanization. Man-made channel was built to protect humans from floods, which starts from the apex and extends beyond the distal fan in Polje center. Thus, sediment and water required for fan development have been all cut. Both fan development were initially controlled by nature, such as tectonic activities and climate, and now by humans. Therefore, this study recommends the constant monitoring of the impact of human interference on fan development while regulating the feeding mechanism of fan basin interiors, natural balance of local environment, and urban lives.

Sedimentology and Miocene-Pliocene depositional evolution of the stream-dominated alluvial fan deposits at circum-Sultandağları region

Yalvaç and Ilgın basins surrounding the Sultandağları opened due to orogenic collapse in the early Miocene. The NW-SE trending Çakırçal and Akşehir Faults, forming boundaries with these basins, caused both opening of the basins and uplifting of the Sultandağları. During the Miocene-Pliocene period, alluvial fans fed from the Sultandağları developed on the basin margins, while lacustrine carbonate and clastic depositions occurred in the basin's interior. Alluvial fan sediments are laterally and vertically transitional with lacustrine sediments and alternate several times in the sequence. Alluvial fans, widespread in the region, consisting of "debris flow", "high density stream flood", "braided river" and "meandering river" deposits. Fan deposits that pass from the high-energy fluvial facies at the basin margin to the low-energy fluvial facies towards the basin interior have been interpreted as "stream-dominated alluvial fan deposits". Age data obtained from the fan deposits of the Yalvaç and the Ilgın basins show that the Sultandağları concurrently feeds the alluvial fans on both flanks. Alluvial fan deposition around the Sultandağları occurred under the control of tectonic and climatic processes. Tectonism led to the formation of basins and the creation of source areas. Simultaneously, the climate maintained the streams to be perennial by precipitation, thus proving a continuous sediment supply to the basins. Tectonism and climate-controlled base-level changes determined the quantity of sediments carried to the basin from the source area, causing regressive or transgressive developments.

Cyclonic eddies in the West Greenland Boundary Current System

AbstractThe boundary current system in the Labrador Sea plays an integral role in modulating convection in the interior basin. Four years of mooring data from the eastern Labrador Sea reveal persistent mesoscale variability in the West Greenland boundary current. Between 2014 and 2018, 197 mid-depth intensified cyclones were identified that passed the array near the 2000 m isobath. In this study, we quantify these features and show that they are the downstream manifestation of Denmark Strait Overflow Water (DSOW) cyclones. A composite cyclone is constructed revealing an average radius of 9 km, maximum azimuthal speed of 24 cm/s, and a core propagation velocity of 27 cm/s. The core propagation velocity is significantly smaller than upstream near Denmark Strait, allowing them to trap more water. The cyclones transport a 200-m thick lens of dense water at the bottom of the water column, and increase the transport of DSOW in the West Greenland boundary current by 17% relative to the background flow. Only a portion of the features generated at Denmark Strait make it to the Labrador Sea, implying that the remainder are shed into the interior Irminger Sea, are retroflected at Cape Farewell, or dissipate. A synoptic shipboard survey east of Cape Farewell, conducted in summer 2020, captured two of these features which shed further light on their structure and timing. This is the first time DSOW cyclones have been observed in the Labrador Sea—a discovery that could have important implications for interior stratification.

Volcanic events‐related hydrothermal dolomitisation and silicification controlled by intra‐cratonic strike‐slip fault systems: Insights from the northern slope of the Tazhong Uplift, Tarim Basin, China

AbstractOrdovician carbonate rocks on the northern slope of the Tazhong Uplift in the Tarim Basin are modified to varying degrees by hydrothermal fluids related to multiple tectonic‐thermal events, but the nature, origin and circulation mechanisms of such hydrothermal fluids remain debated. In this article, in situ rare‐earth elements and yttrium (REE + Y) concentrations, previously published geochemical datasets and petrographic investigations are integrated to develop a thermo‐tectonic evolution model to improve understanding of silicification and dolomitisation occurring in the interior of the craton basin. δ30Si values, REE + Y ratios and fluid mixing models using modern black smoker fluid and seawater indicate that magma‐sourced hydrothermal fluids, rather than basinal brine heated by magmatism, are responsible for the precipitation of cherts. Petrological and geochemical evidence suggests that deep‐seated, ultramafic‐derived hydrothermal fluids and/or magmatic fluids associated with Permian volcanism may have been involved in the formation of hydrothermal dolomites in the study area. Accompanying formation and reactivation of the strike‐slip faults on the northern slope of the Tazhong Uplift, small volumes of hydrothermal fluids might have been triggered by seismic pumping. The early formed strike‐slip fault systems may also have served as conduits for the subsequent large‐scale transport of deep‐seated, ultramafic‐derived thermal fluids and/or Permian magmatic fluids, along which fluids were transported upwards at high flow rates. Carbonation of ultramafic rocks through interaction with deep‐seated hydrothermal fluids rich in CO2 may have served as a magnesium and silica source for dolomitisation and silicification. Trans‐tensional troughs on NNE‐trending strike‐slip faults with positive flower structure are favourable locations for dolomitisation and silicification.

The geometry, sedimentary filling and depth estimate of the Mirandiba Basin, Pernambuco, Brazil: new insights about the depositional regional gap of rift phases in the interior basins

The Mirandiba Basin has a well-defined stratigraphic sequence and its sedimentation may be directly correlated with the Jatoba Basin and some other interior basins. The sedimentation began in intracratonic conditions with a Paleozoic sequence (Tacaratu/Inaja Formations), which acted as a substrate without any genetic relationship with the basin. The fluvial Tacaratu Formation emerges beyond the limits of this basin, where a remnant core named here as Poco do Ico was found wrapped around the basement to the west of the basin. Subsequently, there was a deposition in a shallow marine environment of the Inaja Formation (Devonian) formed by the intercalation of clay/siltstones and sandstones with ichnofossils. Pre-rift sedimentation occurs over the Paleozoic sequence, characterized in the basin by the lacustrine sediments of the Alianca Formation, represented by the intercalation of shales and calcarenites with desiccation cracks and fossil fragments. In spite of depositional gap described between the local Aratu and Alagoas Stages, the presence of Salvador Formation close to the north border fault may represent a rare rift phase record in the interior basins. The post-rift sequence is represented by the Marizal Formation, deposited during the Aptian. This formation is distinguished by the intercalation of fine sandstone, siltstone and conglomerate deposited in an alluvial fan environment. The pre-rift, rift and post-rift sedimentation are embedded in the depocenter of the basin, where the main NE-SWgraben stands out. A gravity survey was carried out, which obtained a negative, asymmetric Bouguer anomaly, with gradients added to the northwest, suggesting the existence of a half-graben. The result of the gravity 3D model indicates the existence of a main depocenter in the NE-SW direction with depths of up to 400 meters. These geophysical results support the tubular well location drilled 410 m in depth that was used in this work to better characterize the basin. The joint interpretation of gravity, aeromagnetic and structural data suggest that the half-graben framework of the basin developed by the evolution of a pull-apart extension system. The structural system that gave rise to the Mirandiba Basin has the maximum compression tensioner oriented to NE-SW and the distention to NW-SE. It has a depth of around 400 meters and the depressions formed by tectonic events were filled by Post-Tacaratu sedimentation.

An alternative model for the role of salt depositional configuration and preexisting salt structures in the evolution of the Southern Pyrenees, Spain

Salt depositional configuration exerted a fundamental control on both extensional and contractional deformation in the south-central Pyrenees. We divide the area into three domains and suggest that these domains are inherited from the original configuration of the Triassic Keuper salt basin. The Northern and Southern Domains are salt-diapir provinces with complex Mesozoic growth histories, whereas the Central Domain separating them has no diapirs and simpler Mesozoic isopachs. We use these patterns to interpret the Northern and Southern Domains as transported salt basins that were originally separated by the Central Horst, on which salt was thinner.During Dogger to mid-Cenomanian rifting, the Southern Salt Basin was an interior basin, isolated from deformation in the rift axis, whereas the Northern Salt Basin hosted an extensional breakaway at the updip end of the axial rift. During Pyrenean shortening, regions with preexisting salt structures were preferentially reactivated, whereas the stronger Central Horst remained relatively rigid. Eventually the sedimentary cover of the Southern Pyrenees was translated southwards and the connection with the original basement structure was lost.We tested our hypotheses using physical models. We confirmed that shortening a network of preexisting salt pillows could generate structures like the dome-and-basin pattern observed in the Southern Domain, and that it is possible to shorten salt basins on either side of a central horst while leaving sediments above the horst relatively undeformed.

Mixed siliciclastic–carbonate sedimentation in an evolving epicontinental sea: Aptian record of marginal marine settings in the interior basins of north‐eastern Brazil

AbstractThe Aptian evaporitic transitional sequence (sag phase) of the Brazilian marginal basins records the first connection between the Central and South Atlantic oceans in the equatorial area. During this phase, lacustrine carbonate reservoirs and giant salt deposits developed in the southern basins of Santos and Campos, forming world‐class petroleum reservoirs. The sag succession is also well‐preserved in the interior rift basins of north‐eastern Brazil, where upper Aptian strata are represented by a continuous section. This contribution presents an interpretation of the evolution of a third‐order depositional sequence comprising lacustrine limestones, marine siliciclastic facies and evaporite deposits of the intracontinental Araripe Basin. The Lowstand Systems Tract is characterized by fluvial deposits filling accommodation generated during the syn‐rift phase. The overlying deposits comprise marine facies with preservation of dinoflagellates and marine ichnotaxa, evidencing marine deposition in bayhead delta settings. The transgressive surface is represented by limestones, including the Konservat–Lagerstätte laminites, onlapping the basement rocks. Above, tide‐dominated bay deposits with distinct subtidal, intertidal and supratidal portions are recorded. The Maximum Flooding Surface is a dark shale below foreshore to shoreface facies, marking the beginning of the Highstand Systems Tract, culminating with deposition of evaporites (gypsum and anhydrite). The stratigraphic architecture and palaeocurrents measured in different facies associations suggest that marine waters reached the basin from the south. The data unequivocally records late Aptian marine ingressions of an incipient South Atlantic Ocean over the interior basins of north‐eastern Brazil, while lacustrine conditions were still prevailing in the Pre‐Salt sag units of the Santos and Campos basins.

A Changing Arctic Ocean: How Measured and Modeled 129I Distributions Indicate Fundamental Shifts in Circulation Between 1994 and 2015

Abstract129I measurements on samples collected during GEOTRACES oceanographic missions in the Arctic Ocean in 2015 have provided the first synoptic 129I sections across the Eurasian, Canada, and Makarov Basins. During the 1990s, increased discharges of 129I from European nuclear fuel reprocessing plants produced a large, tracer spike whose passage through the Arctic Ocean has been followed by 129I time series measurements over the past 25 years. Elevated 129I levels measured over the Lomonosov and Alpha‐Mendeleyev Ridges in 2015 were associated with tracer labeled, Atlantic‐origin water bathymetrically steered by the ridge systems through the central Arctic while lower 129I levels were evident in the more poorly ventilated basin interiors. 129I levels of 200–400 × 107 at/l measured in intermediate waters had increased by a factor of 10 in 2015 compared to 1994–1996 owing to the circulation of the 1990s, 129I input spike. Comparisons of 129I distributions between the mid‐1990s and 2015 delineate large scale circulation changes that occurred during the shift from a positive Arctic Oscillation and a cyclonic circulation regime in the mid‐1990s to anticyclonic circulation in 2015. The latter is characterized by a broadened Beaufort Gyre in the upper ocean, a weakened boundary current and partial mid‐depth, AW flow reversal in the southern Canada Basin. Tracer 129I simulations using the applied circulation model, NAOSIM, agree with both historical 129I results and recent GEOTRACES data sets, thereby supporting the present interpretation of the relationship of changes in arctic circulation to shifts in climate indices revealed by tracer 129I distributions.

A Review of 50 Years of Study of Hydrology, Wetland Dynamics, Aquatic Metabolism, Water Quality and Trophic Status, and Nutrient Biogeochemistry in the Barataria Basin, Mississippi Delta—System Functioning, Human Impacts and Restoration Approaches

Here we review an extensive series of studies of Barataria Basin, an economically and ecologically important coastal basin of the Mississippi Delta. Human activity has greatly altered the hydrology of the basin by decreasing riverine inflows from leveeing of the river and its distributaries, increasing runoff with high nutrient concentrations from agricultural fields, and channelization of wetlands of the basin interior that has altered flow paths to often bypass wetlands. This has resulted in degraded water quality in the upper basin and wetland loss in the lower basin. Trophic state analysis found the upper basin to be eutrophic and the lower basin to be mesotrophic. Gross aquatic primary production (GAPP) was highest in the upper basin, lowest in the mid basin, and intermediate in the lower basin. Forested wetlands in the upper basin have degraded over the past several decades due to increased periods of flooding, while there has been massive loss of emergent wetlands in the lower basin due to increasing water levels and pervasive alteration of hydrology. Restoration will entail reconnection of waterways with surrounding wetlands in the upper basin, and implementation of river sediment diversions, marsh creation using dredged sediments and barrier island restoration. Findings from this review are discussed in terms of the functioning of deltas globally.

An extreme climate gradient-induced ecological regionalization in the Upper Cretaceous Western Interior Basin of North America

Abstract The Upper Cretaceous Western Interior Basin of North America provides a unique laboratory for constraining the effects of spatial climate patterns on the macroevolution and spatiotemporal distribution of biological communities across geologic timescales. Previous studies suggested that Western Interior Basin terrestrial ecosystems were divided into distinct southern and northern communities, and that this provincialism was maintained by a putative climate barrier at ∼50°N paleolatitude; however, this climate barrier hypothesis has yet to be tested. We present mean annual temperature (MAT) spatial interpolations for the Western Interior Basin that confirm the presence of a distinct terrestrial climate barrier in the form of a MAT transition zone between 48°N and 58°N paleolatitude during the final 15 m.y. of the Cretaceous. This transition zone was characterized by steep latitudinal temperature gradients and divided the Western Interior Basin into warm southern and cool northern biomes. Similarity analyses of new compilations of fossil pollen and leaf records from the Western Interior Basin suggest that the biogeographical distribution of primary producers in the Western Interior Basin was heavily influenced by the presence of this temperature transition zone, which in turn may have impacted the distribution of the entire trophic system across western North America.

Convergence of Daily GRACE Solutions and Models of Submonthly Ocean Bottom Pressure Variability

AbstractKnowledge of submonthly variability in ocean bottom pressure (pb) is an essential element in space‐geodetic analyses and global gravity field research. Estimates of these mass changes are typically drawn from numerical ocean models and, more recently, GRACE (Gravity Recovery and Climate Experiment) series at daily sampling. However, the quality of pb fields from either source has been difficult to assess and reservations persist as to the dependence of regularized GRACE solutions on their oceanographic priors. Here, we make headway on the subject by comparing two daily satellite gravimetry products (years 2007–2009) both with each other and with pb output from a diverse mix of ocean models, complemented by insights from bottom pressure gauges. Emphasis is given to large spatial scales and periods <60 days. Satellite‐based mass changes are in good agreement over basin interiors and point to excess pb signals (∼2 cm root‐mean‐square error) over Southern Ocean abyssal plains in the present GRACE de‐aliasing model. These and other imperfections in baroclinic models are especially apparent at periods <10 days, although none of the GRACE series presents a realistic ground truth on time scales of a few days. A barotropic model simulation with parameterized topographic wave drag is most commensurate with the GRACE fields over the entire submonthly band, allowing for first‐order inferences about error and noise in the gravimetric mass changes. Estimated pb errors vary with signal magnitude and location but are generally low enough (0.5–1.5 cm) to judge model skill in dynamically active regions.

Heterogeneous snowpack response and snow drought occurrence across river basins of northwestern North America under 1.0\xb0C to 4.0\xb0C global warming

Anthropogenic climate change is affecting the snowpack freshwater storage, with socioeconomic and ecological impacts. We present an assessment of maximum snow water equivalent (SWEmax) change in large river basins of the northwestern North America region using the Canadian Regional Climate Model 50-member ensemble under 1.0 °C to 4.0 °C global warming thresholds above the pre-industrial period. The projections indicate steep SWEmax decline in the warmer coastal/southern basins (i.e., Skeena, Fraser and Columbia), moderate decline in the milder interior basins (i.e., Peace, Athabasca and Saskatchewan), and either a small increase or decrease in the colder northern basins (i.e., Yukon, Peel, and Liard). A key factor for these spatial differences is the proximity of winter mean temperature to the freeze/melt threshold, with larger SWEmax declines for the basins closer to the threshold. Using the random forests machine-learning model, we find that the SWEmax change is primarily temperature controlled, especially for warmer basins. Further, under a categorical framework of below-normal SWEmax defined as snow drought (SD), we find that above-normal temperature and precipitation are the dominant conditions for SD occurrences under higher global warming thresholds. This implies a limited capacity of precipitation increase to compensate the temperature-driven snowpack decline. Additionally, the frequency and severity of SD occurrences are projected to be most extreme in the southern basins where current water demands are highest. Overall, the results of this study, including insights on snowpack changes, their climatic controls, and the framework for SD classification, are applicable for basins spanning a range of hydro-climatological regimes.

Age of the Post-rift Sequence I from the Araripe Basin, Lower Cretaceous, NE Brazil: implications for spatio-temporal correlation

A robust biostratigraphic zonation based on microfossils supports the stratigraphic framework and correlation of the interior basins of the Lower Cretaceous of NE Brazil. This zonation has also allowed correlations with coeval sections in the Brazilian marginal basins and in the Gabon and Congo basins (central-west Africa). These records, consisting mainly of non-marine sediments, were a great challenge with regard to the correlation with the International Chronostratigraphic Chart. Therefore, local stages were used, the most recent being the Alagoas local Brazilian Stage, with which the Post-rift Sequence I of the Araripe Basin is related. Regarding lithostratigraphy, this sequence includes the Rio da Batateira (Barbalha for some authors) and Santana formations, the last one with the famous Crato, Ipubi, and Romualdo members, from the base to the top. Although currently there is a consensus on the age of the Alagoas local Brazilian Stage in the Araripe Basin, recently a new age for at least part of the Post-rift Sequence I was proposed. This new proposal, based on isotopic analysis of Re-Os, arose as a panacea to correlate the Rio da Batateira Formation and the Crato and Ipubi members with the international stages. Surprisingly, their authors, although on the one hand, they seem to underestimate biostratigraphic results, on the other they seek to support their proposal from microfossils studied by previous authors, but they do so in an inappropriate way, leading readers to misinterpret their results. Therefore, this paper presents a critical review on the age of the Alagoas local Brazilian Stage in the Araripe Basin and nearby basins, refuting a Barremian age for part of the Post-rift Sequence I. Keywords: Alagoas local Brazilian Stage, biostratigraphy, ostracods, palynomorphs, radiometric ages.

Estratigrafia da Bacia do Araripe: estado da arte, revisão crítica e resultados novos

A Bacia do Araripe constitui-se em bacia rifte originada pelo rompimento do Supercontinente Gondwana, conhecida como a bacia interior de registro sedimentar mais completo na Região Nordeste do Brasil. O manuscrito apresenta o estado da arte da estratigrafia da Bacia do Araripe enfocando sua evolução no tempo, nas sequências estratigráficas com base em superfícies de discordâncias, em novos resultados e faz ainda revisão crítica das classificações estratigráficas propostas com vistas à elaboração de classificação integrada. As primeiras propostas estratigráficas foram elaboradas de forma muito abrangente e simplificada. Com o tempo, foram realizados estudos mais minuciosos corroborando uma melhor e mais refinada compartimentação dos pacotes sedimentares presentes. A década de 1980 foi imprescindível para as produções científicas inerentes à geologia da bacia em função do interesse no potencial petrolífero das bacias brasileiras. Não obstante, ainda hoje a bacia é motivadora de inúmeras pesquisas determinantes para um melhor entendimento da geologia local e regional. A Bacia do Araripe tem cinco sequências maiores: Paleozoica, representada pela Formação Cariri; Início de Rifte, constituída pela Formação Brejo Santo e parte inferior da Formação Missão Velha; Rifte, formada pela parte superior da Formação Missão Velha e por toda a Formação Abaiara; e Pós-Rifte, separada em duas sequências — pós-rifte I, constituída pelas formações Barbalha, Crato, Ipubi e Romualdo, e pós-rifte II, caracterizada pelas formações Araripina e Exu. Novos resultados estão incorporados às formações Missão Velha e Abaiara, cuja distinção é demonstrada aqui; também com relação às formações Barbalha e Cariri, novos elementos foram reunidos. Este trabalho foca a viabilidade dos dados, as interpretações propostas conforme as normas estratigráficas e a proposição de uma classificação estratigráfica sequencial.

Genesis and Decay of Mesoscale Baroclinic Eddies in the Seasonally Ice-Covered Interior Arctic Ocean

AbstractObservations of ocean currents in the Arctic interior show a curious, and hitherto unexplained, vertical and temporal distribution of mesoscale activity. A marked seasonal cycle is found close to the surface: strong eddy activity during summer, observed from both satellites and moorings, is followed by very quiet winters. In contrast, subsurface eddies persist all year long within the deeper halocline and below. Informed by baroclinic instability analysis, we explore the origin and evolution of mesoscale eddies in the seasonally ice-covered interior Arctic Ocean. We find that the surface seasonal cycle is controlled by friction with sea ice, dissipating existing eddies and preventing the growth of new ones. In contrast, subsurface eddies, enabled by interior potential vorticity gradients and shielded by a strong stratification at a depth of approximately 50 m, can grow independently of the presence of sea ice. A high-resolution pan-Arctic ocean model confirms that the interior Arctic basin is baroclinically unstable all year long at depth. We address possible implications for the transport of water masses between the margins and the interior of the Arctic basin, and for climate models’ ability to capture the fundamental difference in mesoscale activity between ice-covered and ice-free regions.

Geochronology of late Albian–Cenomanian strata in the U.S. Western Interior

Abstract Since the publication of 40Ar/39Ar dates from Cretaceous bentonites in the Western Interior Basin by J.D. Obradovich in 1993 and in Japan by J.D. Obradovich and colleagues in 2002, improvements in the 40Ar/39Ar method have included a shift to astronomically calibrated ages for standard minerals and development of a new generation of multi-collector mass spectrometers. Thus, the 40Ar/39Ar chronometer can yield results that are synchronous with U-Pb zircon dates and astrochronologic age models for Cretaceous strata. Ages determined by Obradovich have ± 2σ analytical uncertainties of ± 400 ka (excluding J value or systematic contributions) that have been used to discriminate stratigraphic events at ca. 1 Ma resolution. From among several dozen sanidine samples, 32 of which were dated by Obradovich in 1993, we present new multi-collector 40Ar/39Ar ages that reduce the average analytical uncertainties by nearly an order of magnitude. These new ages (where the uncertainties also include the contribution of the neutron fluence J value) include: Topmost Bentonite, Mowry Shale, Kaycee, Wyoming, USA, 97.52 ± 0.09 Ma Clay Spur Bentonite, Mowry Shale, Casper, Wyoming, 98.17 ± 0.11 Ma Arrow Creek Bentonite, Colorado Shale, Montana, USA, 99.12 ± 0.14 Ma Upper Newcastle Sandstone, Black Hills, Wyoming, 99.49 ± 0.07 Ma Middle Newcastle Sandstone, Black Hills, Wyoming, 99.58 ± 0.12 Ma Shell Creek Shale, Bighorn Basin, Crow Reservation, Wyoming, 99.62 ± 0.07 Ma Shell Creek Shale, Bighorn Basin, Greybull, Wyoming, 99.67 ± 0.13 Ma Shell Creek Shale, Bighorn Basin, Lander, Montana, 100.07 ± 0.07 Ma Muddy Sandstone, Wind River Basin, Wyoming, 101.23 ± 0.09 Ma Thermopolis Shale, Bighorn Basin, Wyoming, 101.36 ± 0.11 Ma Vaughn Member, Blackleaf Formation, Sweetgrass Arch, Montana, 102.68 ± 0.07 Ma Taft Hill Member, Blackleaf Formation, Sweetgrass Arch, Montana, 103.08 ± 0.11 Ma Base of the Skull Creek Shale, Black Hills, Wyoming, 104.87 ± 0.10 Ma Thermopolis Shale, Bighorn Basin, Wyoming, 106.37 ± 0.11 Ma A new U-Pb zircon age of 104.69 ± 0.07 Ma from the Skull Creek Shale at Dinosaur Ridge, Colorado, USA, is close to the new 40Ar/39Ar age of the Skull Creek Shale in the Black Hills, Wyoming, but 5 m.y. is missing in the unconformity between the Skull Creek Shale of the Black Hills and the overlying Newcastle Sandstone. Considering the average total uncertainties that include decay constant and standard age or tracer composition for the 40Ar/39Ar (± 0.19 Ma) and the U-Pb (± 0.13 Ma) ages does not alter this finding. Moreover, the lower Thermopolis Shale in the Bighorn Basin is 1.5 Ma older than the Skull Creek Shale in the Black Hills. The 100.07 ± 0.07 Ma Shell Creek Bentonite in Montana is close to the Albian–Cenomanian boundary age of 100.2 ± 0.2 Ma of Obradovich and colleagues from Hokkaido, Japan, and 100.5 ± 0.5 Ma adopted in the 2012 geological time scale of J.G. Ogg and L.A. Hinnov. Our findings indicate that correlations based on similarity of lithology, without independent radioisotopic ages or detailed biostratigraphic constraints, can be problematic or invalid. There is much more time missing in unconformities than has been previously recognized in these important, petroleum-bearing reservoir strata.

Observed Transport Variability of the Atlantic Subtropical Cells and Their Connection to Tropical Sea Surface Temperature Variability

AbstractThe shallow meridional overturning cells of the Atlantic Ocean, the subtropical cells (STCs), consist of poleward Ekman transport at the surface, subduction in the subtropics, equatorward flow at thermocline level and upwelling along the equator and at the eastern boundary. In this study, we provide the first observational estimate of transport variability associated with the horizontal branches of the Atlantic STCs in both hemispheres based on Argo float data and supplemented by reanalysis products. Thermocline layer transport convergence and surface layer transport divergence between 10°N and 10°S are dominated by seasonal variability. Meridional thermocline layer transport anomalies at the western boundary and in the interior basin are anti‐correlated and partially compensate each other at all resolved time scales. It is suggested that the seesaw‐like relation is forced by the large‐scale off‐equatorial wind stress changes through low‐baroclinic‐mode Rossby wave adjustment. We further show that anomalies of the thermocline layer interior transport convergence modulate sea surface temperature (SST) variability in the upwelling regions along the equator and at the eastern boundary at time scales longer than 5 years. Phases of weaker (stronger) interior transport are associated with phases of higher (lower) equatorial SST. At these time scales, STC transport variability is forced by off‐equatorial wind stress changes, especially by those in the southern hemisphere. At shorter time scales, equatorial SST anomalies are, instead, mainly forced by local changes of zonal wind stress.