Continental Platform Research Articles

Seismic forecasting by gapped wavelet transform for the Río de la Plata craton and adjacent continental platform

Historical and instrumental seismicity from 1845 to the present was compiled for the Pampean regions of Argentina and Uruguay and the adjacent continental platform. These data were analyzed using the Gapped Wavelet Transform (GWT) with the aim of investigating recurrence patterns in the time series.The GWT analysis reveals the existence of periods of seismic activity and periods of seismic silence within the considered area. A 22-year period was determined as the main pattern of seismic activity, during which earthquakes are registered only in 11 years. Based on these results, a probabilistic earthquake forecasting model was created using the Bayesian Machine Learning method, which suggests that the current seismically active period spans from 2011 to 2023 ± 1, with a cessation until 2034 ± 1, while the next seismically active period would begin in 2034 ± 2 ending in 2046 ± 2.These periods or recurrence patterns are interpreted as intervals during which stress accumulates throughout intracrustal faults in the studied area, being then released through various events within the established temporal interval. The accumulation and release mechanisms are still a subject of debate. However, the earthquakes distribution suggests a sublatitudinal arrangement that tends to confirm some correspondence with the controlling faulting of the Quilmes Trough and others transtensional regional lineaments.

Taxonomic and functional diversity of zooxanthellate corals and hydrocorals in Southwestern Atlantic reefs

The Southwestern Atlantic (SWA) harbors a relatively species poor but highly endemic coral assemblage due to historical processes, environmental and ecological drivers. Despite its low to moderate cover, corals still have a disproportionate contribution to ecosystem function and stability in this region. In the context of global change, it is imperative to know corals’ diversity and biogeographic patterns, yet a comprehensive approach is still missing for SWA corals. We integrated occurrence data from 21 sites and nine functional traits across 20 coral (scleractinian and hydrozoan) species to explore the taxonomic and functional diversity of coral assemblages in the SWA (1°N-27°S). We identified eight regions based on coral species composition, and then described their functional diversity using four metrics: functional richness (FRic), functional dispersion (FDis), functional evenness (FEve), and functional originality (FOri). Taxonomic and functional diversity peak between latitudes 13°S-20°S, decreasing with increasing distance from this diversity center, known as the Abrolhos Bank that harbors a wide continental platform. Our findings reveal a prevalent pattern of high functional redundancy across these eight regions (indicated by low functional originality), with species occupying the edges of the trait space (high functional evenness) and converging around few trait values (low functional dispersion). Such patterns resulted in low taxonomic and functional beta diversity and increased nestedness among regions caused by dispersal barriers and environmental filtering. The Southernmost region (24°-27°S) has the lowest taxonomic and functional diversity and comprises only two species that share similar traits, with these corals being: hermaphrodites, brooders and depth-tolerant, and having a wide corallite. As this region might become critical for corals in a future tropicalization scenario, tropical corals that share similar traits to those of the southernmost region can be more likely to thrive. Knowledge on taxonomic and functional diversity patterns can offer critical information to conservation by helping prioritizing areas with higher diversity and species with traits that enhance survival under climate change.

The Middle Stone Age of Atlantic Africa: A critical review

Evidence of early Homo sapiens populations at the Atlantic coast of Africa remains relatively poorly known in relation to other regions of the continent. Nevertheless, available data across the continent provides a good starting point for current and future research investigations. The many sites known, documented and studied contribute in an increasingly way to the global understanding of the human emergence, including evidence of human evolutionary and technological advances, specific adaptations to diverse environments, the diffusion of Homo species and how humans interacted with each other from the “Early Stone Age (ESA)” through to the Middle Stone Age (MSA) from northern and southern Africa to the West. The differences of knowledge between the Atlantic coast in regard to other regions might be attributed to a number of reasons including but not limited to the history of scientific interest, site formation processes or economic, institutional and political constraints. However, the region received a renewed attention and funds that, combined with new methods and techniques, has been allowing an increased training of new researchers and the acquisition of high-resolution archaeological, paleoenvironmental and chronological data. Together, these inputs will reduce the differences of knowledge between the Atlantic coast and the Northern, Southern and Eastern Africa regions. The African Atlantic Coast represents more than 40% of the continent's perimeter, covering all Africa's climate zones, the hot arid environments, mountainous regions, and tropical rainforest could become relevant barriers for human mobility, but the shallow continental platform, and the great number of river basins allowed mobility between north and south coastal biomes into the continental interiors. These may have provided predictable patchy clusters of resources allowing human populations to thrive, enabling greater mobility and consequent diffusion of cultural traits, resources, and DNA. In this paper we review the record about the prehistory, paleoenvironments and paleoanthropological visibility and potentiality of Atlantic Africa.

New detrital petrographic and thermochronologic constraints on the Late Cretaceous–Neogene erosional history of the equatorial margin of Brazil: Implications for the surface evolution of a complex rift margin

AbstractThe equatorial margin of Brazil is an example of a rift margin with a complex landscape, dominated by an escarpment perpendicular to the continental margin, which testifies to an equally complex rift and post‐rift surface and tectonic evolution. This has been the focus of a long debate on the driving mechanism for post‐rift tectonics and on the amount of exhumation. This study contributes to this debate with new petrographic and thermochronologic data on 152 samples from three basins, Pará‐Maranhão, Barreirinhas and Ceará, on the offshore continental platform. Our detrital record goes back to the rift time at ca. 100 Ma ago and outlines three major evolutionary phases of a changing landscape: a rift phase, with the erosion of a moderate rift escarpment, a Late Cretaceous‐Palaeogene post‐rift phase of major drainage reorganization and significant vertical erosion and a Late Oligocene‐to‐Recent post‐rift phase of moderate vertical erosion and river headwater migration. We estimate that along the equatorial margin of Brazil, over a large onshore area, exhumation since the Late Cretaceous has totalled locally up to 2–2.5 km and since the late Oligocene did not exceed 1 km.

Comparison of the Properties of a Statistical Functional from Measurements of the Magnetic Field in the Middle of the Russian Continental Platform and in a Seismically Active Region

Comparison of the Properties of a Statistical Functional from Measurements of the Magnetic Field in the Middle of the Russian Continental Platform and in a Seismically Active Region

Svalbard Composite Tectono-Sedimentary Element, Barents Sea

Abstract The Svalbard Composite Tectono-Sedimentary Element (SCTSE) is located on the northwestern corner of the Barents Shelf and comprises a Carboniferous–Pleistocene sedimentary succession. Due to Cenozoic uplift, the succession is subaerially exposed in the Svalbard archipelago. The oldest parts of the succession consist of Carboniferous–Permian mixed siliciclastic, carbonate and evaporite, and spiculitic sediments that developed during multiple phases of extension. The majority of the Mesozoic succession is composed of siliciclastic deposits formed in sag basins and continental platforms. Episodes of Late Jurassic and Early Cretaceous contraction are evident in the eastern part of the archipelago and in nearby offshore areas. Differential uplift related to the opening of the Amerasian Basin and the Cretaceous emplacement of the High Arctic Large Igneous Province created a major hiatus spanning from the Late Cretaceous and early Danian throughout the Svalbard CTSE. The West Spitsbergen Fold and Thrust Belt and the associated foreland basin in central Spitsbergen (Central Tertiary Basin) formed as a response to the Eurekan Orogeny and the progressive northward opening of the North Atlantic during the Paleogene. This event was followed by the formation of yet another major hiatus spanning the Oligocene–Pliocene. Multiple reservoir and source-rock units exposed in Svalbard provide analogues to the prolific offshore acreages in the SW Barents Sea, and are important for the de-risking of plays and prospects. However, the archipelago itself is regarded as a high-risk acreage for petroleum exploration. This is due to Paleogene contraction and late Neogene uplift of the western and central parts in particular. There is an absence of mature source rocks in the east, and the entire region is subjected to strict environmental protection.

Comparison of the Properties of a Statistical Functional from Measurements of the Magnetic Field in the Middle of the Russian Continental Platform and in a Seismically Active Region

This article investigates the properties of small-scale fluctuations of the probability density formeasurements of the X, Y, and Z-components of the Earth’s magnetic field that were carried out both in aseismically active region and in an area with a complete absence of seismic activity. In the first case, a muchgreater intensity of the occurrence of statistical phenomena considered as earthquake precursors was revealed.At an interval of several days before the earthquake with a magnitude of 5.3 in Metsavan (Armenia), whoseepicenter was located near the magnetic variation station in a seismically active region, a sharp increase in thefrequency of these phenomena was found. This effect was absent in the seismically quiet region

Geotraverse «Granite—Odesa—Kryvyi Rih—Pereschepine

The article examines the results of research carried out by the methods of reflected waves and common depth point (MRW-CDP) on the «Granit» geotraverse, which crosses the whole of Ukraine from the southwest to the northeast and passes through the territories of Odesa, Mykolaiv, Dnipropetrovsk, and Kharkiv regions, west of Belgorod region of Russia. In structural and tectonic terms, the geotraverse crosses large geological structures: the marginal part of the Scythian Plate (SP), the Ukrainian Shield (USH), the Dnieper-Donetsk Basin (DDB) and the southern slope of the Voronezh Crystalline Massif (VCM). The analysis of the features of the registered wave field and the completed structural constructions of the consolidated crust and upper mantle in the section of the «Granite» geotraverse shows a complex heterogeneous structure with certain spatially correlated regularities, against the background of which there are numerous inhomogeneities characteristic of both individual blocks and their individual intervals. The cross-sections of all the studied geostructures are characterized by a high level of saturation with non-extended, unevenly distributed reflective elements, which occupy positions from subhorizontal to steeply sloping in space. With all the diversity of the mutual location of both individual boundaries and their groups, a fairly strict correlation is established, which primarily provides information about the nature of stratification, deformations, and the stress state of the structures of the consolidated crust and upper mantle. In the modern structure of the crust, a significant role belongs to faults and tectonic plates, the vast majority of which are manifested as inclined zonal bodies with different orientations. Systems of faults and plates form a complex hierarchical interdependence. The «crust-mantle» transition zone is a complex, laterally variable area, with a thickness of 3 to 7 km, caused by systems of subhorizontal stratification, expressed by the concentration of reflective elements. The largest homogeneous and contrasting region of the transition is manifested in the structures of ancient consolidation, with a thickness of the crust of about 40 km. As a result of the research, more detailed information on the structure of individual tectonic elements was obtained, and some general principles of the structure of the transition zone from the ancient continental platform to the active folded belt were clarified.

Mantle gravitational anomalies in the regions of Eurasia, North America and the Atlantic

The construction of density models of the Earth's crust and upper mantle along the system of profiles crossing Eurasia and North America, the Atlantic Ocean, and transition zones between continents and oceans, begun by the works of the authors of recent years, is continued. In the crust, they are plotted by velocity sections along DSS profiles. The calculated effect of the section in anomalous densities in relation to the normal platform mantle (minus 870 mGl) corresponds to the zero observed field (Bouguer anomalies on land and Fay anomalies on sea). This value is established experimentally on the East European platform, where the geological and geophysical knowledge is the maximum. In the upper mantle, the authors' ideas about deep processes are used. They are associated with variants of heat and mass transfer under various endogenous regimes. Based on the results of these processes, temperature distribution that distinguish the densities of mantle rocks in each region from platform ones was determined. The gravitational effects calculated for such anomalous models are summarized with the crustal ones. Regions with all types of endogenous regimes were studied: platforms, zones of recent activation, geosynclines and rifts of different ages, mid-ocean ridge, oceanic depressions, marginal trench, island arc, back-arc depression. The total has compared with the observed fields. In all previously studied regions, the obtained differences do not exceed those explained by the errors of observations and calculations. Thus, the construction of density models of the tectonosphere becomes one of the methods for verifying the used schemes of deep processes. In the regions covered by recent activation and in the transition zones between continents and oceans, the differences between the experimental and calculated data turned out to be the maximum. Therefore, the work was continued using other DSS profiles, mainly on the continental platforms. The result generally confirmed the previous one, revealing additional modeling problems associated with velocity cross sections of the earth's crust.

A laboratory investigation on the effect of crushability on the interface parameters of Hormoz carbonated soil

Carbonated sands, also referred to as calcareous sediments, are a common type of soil in marine environments such as the West Australia Continental Platform, South China Sea, Persian Gulf and Oman sea and many parts of the world. During pile driving in such soils, imposed shear and volumetric strain to particles, crushing occurs which leads to change in the particle size distribution as well as interface properties of the soil-pile system. In this study, two types of sands (siliceous Firoozkooh-161 and carbonate Hormoz sand) and three types of materials (steel, wood and concrete) have been selected and were investigated using a direct shear and a tall oedometer apparatus for the prediction of the sand-pile interaction properties considering particle crushing. The smoothness and roughness of materials as well as soil relative density, and shearing rate on the interface properties were examined. Based on the results, smooth steel, wood at the direction of vessels, rough steel, smooth concrete, wood at the right angle to vessels, and rough concrete represent the lowest to the highest interface shear strength, respectively. Results revealed, increasing soil relative density leads to an increase in the interface friction angle. The results also emphasized that increasing the vertical stress leads to increasing in the breakage index both dense and loose states, however, the amount of particle increases with density. Particle crushing also decreases significantly when soil interact with smooth surfaces. Furthermore, increasing the shearing rate enhances the shear strength and reduces particle crushing. It also investigated that Hormoz sand compared to Firoozkooh sand exhibits a higher level of particle breakage due to the shape and texture of carbonated particles. Finally based on the achievements of this research, the limit values for skin friction bearing capacity of piles driven in carbonate and siliceous sands proposed in ARGEMA and API approach were examined.

Multiscale distribution patterns of pelagic rafts of sargasso (Sargassum spp.) in the Mexican Caribbean (2014–2020)

As the biomass of pelagic Sargassum spp. increased across the North Atlantic equatorial recirculation region from 2011 onwards, massive rafts of sargasso appeared in the Western Caribbean in 2015, 2018, 2019, and 2020. These events raised concerns regarding their negative consequences on the environment, local income, and human wellbeing. As adequate monitoring and analysis tools are needed for designing in-water and on-beach control strategies to reduce potential negative impacts, more robust and spatially explicit information is needed in order to improve sargasso management and focus restoration efforts. In this paper, we offer a spatiotemporal multiscale description of sargasso distribution and dynamics for 2014–2020 in the Mexican Caribbean: (1) for the entire region (millions of km2); (2) at the local scale (thousands of km2) evaluating the dynamics inside the reef lagoon at Puerto Morelos, Mexico; and finally, (3) specific beach observations (hundreds of km2) derived from data on beach cleaning volumes. Fifteen areas in the Mexican Caribbean, with different sargasso dispersions and on-shore accumulations, were evaluated. The areas around Tulum, Solidaridad, and Puerto Morelos have the most extreme and most frequent episodes but also exhibited the greatest seasonal variability. Extreme sargasso presence can occur in the Western Caribbean in any season, albeit with increasing coverage and recurrence in the summer. Images from a coastal video monitoring station at Puerto Morelos showed that massive sargasso beaching was associated with low energy conditions (Hs< 0.25 m, wind speed<4 m/s, neap tide), while non-accumulation of sargasso on the beach occurred under high energy conditions (Hs >0.4 m, wind speed = 8 m/s, spring tide). Time-series analyses of sargasso beaching showed different periods of historic maximum sargasso coverage over July–October 2018 and others in January–February 2019. Wind and wave regimes influenced sargasso in distinct ways, depending on the coastal section, probably related to coastline morphology, oceanic regime, or the extent of the continental platform. This work presents the longest systematic time series (2014-2020) of high resolution satellite detected sargasso in Mexico. Spatial and temporal patterns are proposed as fundamental steps for managing sargasso accumulations.

The Seabed Makes the Dolphins: Physiographic Features Shape the Size and Structure of the Bottlenose Dolphin Geographical Units

The common bottlenose dolphin (Tursiops truncatus) is a cosmopolitan delphinid, regularly present in the Mediterranean Sea. According to previous studies, this dolphin tends to form resident geographical units scattered on the continental shelf. We investigated how the physiographic characteristics of the area of residence, with special reference to the size and shape of the continental shelf, affect the home range and the group size of the local units. We analysed and compared data collected between 2004–2016 by 15 research groups operating in different study areas of the Mediterranean Sea: the Alboran Sea, in the South-Western Mediterranean, the Gulf of Lion and the Pelagos Sanctuary for the marine mammals, in the North-Western Mediterranean, and the Gulf of Ambracia, in the North-Central Mediterranean Sea. We have found that in areas characterised by a wide continental platform, dolphins have wider home ranges and aggregate into larger groups. In areas characterized by a narrow continental platform, dolphins show much smaller home ranges and aggregate into smaller groups. The results obtained from this collective research effort highlight the importance of data sharing to improve our scientific knowledge in the field of cetaceans and beyond.

New Facies Model and Carbon Isotope Stratigraphy for an Ediacaran Carbonate Platform From South America (Tamengo Formation—Corumbá Group, SW Brazil)

The Ediacaran is a period characterized by the diversification of early animals and extensive neritic carbonate deposits. These deposits are still not well understood in terms of facies and carbon isotope composition (δ13C). In this study we focus on the Tamengo Formation, in southwestern Brazil, which constitutes one of the most continuous and well-preserved sedimentary record of the late Ediacaran in South America. We present new detailed lithofacies and stable isotopes data from two representative sections (Corcal and Laginha) and revise the paleoenvironmental and stratigraphic interpretation of the Tamengo Formation. The Corcal section consists of neritic deposits including shallow-water limestone beds, alternated with shale and subordinate marl beds. These facies yield specimens of the Ediacaran fossils Cloudina lucianoi and Corumbella werneri. On the other hand, the Laginha section shows more heterogeneous facies, such as impure carbonates, breccias, marls, and subordinate mudstone beds, as well as no evidence of Corumbella werneri. The stable carbon isotope record is also different between the two sections, despite belonging to the same unit. The Corcal section displays higher and more homogeneous δ13C values, consistent with those of Ediacaran successions worldwide. The Laginha section, instead, displays more variable δ13C values, which suggest the influence of local and post depositional processes. The difference between the two sections was attributed to the different distance from the shore. We propose that the difference is due to topographic variations of the continental platform, which, at the Laginha site, was steeper and controlled by extensional faults. Therefore, the Corcal section is a better reference for the Tamengo Formation, whereas the Laginha is more particular and influenced by local factors. Besides, the lithofacies associations of the Tamengo Formation are like those of the Doushantuo and Dengying formatios, in South China, with no significant biogenic carbonate buildups, and different from those of other important Ediacaran units, such as the Nama Group in Nmibia and the Buah Formation in Oman. Our work highlights the complexity and heterogeneity of Ediacaran carbonate platforms and of their carbon isotopic composition. In addition, we characterize the Corcal section as a possible reference for the Ediacaran in South America.

Social justice driving open access publishing: an African perspective

The OA movement is generally considered to have been founded for the truly philanthropic purpose of promoting equity and inclusivity in access to scholarship. For Africans, this meant the opening of the research ecosystem to marginalized research communities who could then freely make use of shared research to aid in the socio-economic development and emancipation of the continent. However, this philanthropic purpose has been deviated from, leading instead to the disenfranchisement of the African research community. Through systemic inequalities embedded in the scholarly ecosystem, the publishing landscape has been northernised, with research from the global north sitting at the very top of the knowledge hierarchy to the exclusion of Africa and other parts of the global south. For this reason, progressive open access practices and policies need to be adopted, with an emphasis on social justice as an impetus, to enhance the sharing and recognition of African scholarship, while also bridging the ‘research-exchange’ divide that exists between the global south and north. Furthermore, advocates of open access must collaborate to create equal opportunities for African voices to participate in the scholarly landscape through the creation and dissemination of global south research. Thusly, the continental platform was developed by the University of Cape Town. This platform was developed around the concept of a tenant model to act as a contributor to social justice-driven open access advocacy, and as a disruptor of the unjust knowledge hierarchies that exist.

Anabar–Lena Composite Tectono-Sedimentary Element, northern East Siberia

Abstract The Anabar–Lena Composite Tectono-Sedimentary Element (AL CTSE) is located in northern East Siberia extending for c. 700 km along the Laptev Sea coast between the Khatanga Bay and Lena River delta. The AL CTSE consists of rocks from the Mesoproterozoic to Late Cretaceous in age with the total thickness reaching 14 km. It evolved through the following tectonic settings: (1) Meso-Early Neoproterozoic intracratonic basin; (2) Ediacaran–Early Devonian passive margin; (3) Middle Devonian–Early Carboniferous rift; (4) late Early Carboniferous–latest Jurassic passive margin; (5) Permian foreland basin; (6) Triassic to Jurassic continental platform basin; and (7) latest Jurassic–earliest Late Cretaceous foreland basin. Proterozoic and lower–middle Paleozoic successions are composed mainly of carbonate rocks, while siliciclastic rocks dominate upper Paleozoic and Mesozoic sections. Several petroleum systems are assumed in the AL CTSE. Permian source rocks and Triassic sandstone reservoirs are the most important play elements. The presence of several mature source rock units and abundant oil and gas shows (both in wells and in outcrops), including the giant Olenek Bitumen Field, suggests that further exploration in this area may result in economic discoveries.

T-S and hydrodynamical structures within the deltaic regions and continental platforms adjacent to two northeastern Brazilian rivers

The delta-front estuaries of large rivers are crucial interfaces between lands and oceans, being considered hotspots of vulnerability to the impact of climate changes due to several causes: sea level rise; changes in the river discharges driven by climatic alterations in the hydrographic basins; high human concentration and other anthropic factors. This study focuses on the hydrodynamics/circulation and T-S structure within the shelf regions and continental platform adjacent to two among the major rivers of Northeastern Brazil: the Sao Francisco and Parnaiba. We aim to qualitatively and quantitatively describe the physical dynamics typical of these coastal regions and try to relate their main alterations to the effects of local mechanisms as tidal forcing. The choice of these deltas was motivated by the great potential that such, biodiverse regions have in acting as important proxies of environmental modifications affecting the Brazilian tropical zone, allowing to investigate the role of geographical variability in response to these changes. A series of regional, modelling simulations was realized, at increasingly higher resolutions, in order to assess the mean and seasonal spatio-temporal variability of physical properties (T-S distribution, salt transport, hydrodynamical features of the local circulation). The identification and analysis of the major scales of variability was also used to evaluate the performance and applicability of our numerical tool, the Regional Ocean Modelling System, in the study areas. Preliminary results on mesoscales processes indicate that both deltas are modulated by an intense turbulent activity. The continental platform near the Sao Francisco estuary is marked by the presence of subsurface-intensified vortices, and the one in proximity of the Parnaiba mouth is characterized by the appearance of small vortices, fine meanders and filaments probably triggered by non-linear interactions between the river plume and the North Brazilian Current rings.

Reservoir characterization of fluvio-deltaic sandstone packages in the framework of depositional environment and diagenesis, the south Caspian Sea basin

The Middle Pliocene Fasila and Balakhany Suites are important petroleum-bearing reservoirs in the South Caspian Basin. There are four lithofacies associated with the interbedded sandprone reservoir intervals. The mineral constituents, sedimentary structures and depositional setting of the productive interval are interpreted as river-dominated depositional setting, including conglomeratic facies relating to a channel floor sub-environment and sandstone facies associated with the upper parts of river channels. The drainage basin of the Volga River is considered as the provenance for these deposits derived from basement rock outcrops to the north, which belongs to the Russian Precambrian semi-oceanic continental platform. Reservoir quality is assessed using the core-based porosity and permeability data in relation to the sedimentary and diagenetic processes influencing each lithofacies. Lithofacies 1 and 2 have good reservoir quality, while lithofacies 3 and 4 have poorer reservoir quality. Reservoir quality is controlled by both depositional setting (grain size and texture) and diagenetic imprints associated mainly with compaction, cementation and neumorphism. The role of clay minerals in the diagenesis of the sandstones with increasing burial depth is revealed by SEM and XRD analysis. Clay minerals progressively alter to illite, smectite and chlorite with increasing burial depth. This alteration leads to the occlusion of pore throats and degrading reservoir permeability.

O vector açoriano na Política Externa Portuguesa | The azorean vector in the Portuguese Foreign Policy

O arquipélago dos Açores influi sobremaneira, desde há muito, na diplomacia portuguesa. Em 1871, já Andrade de Corvo defendera, no seu livro Perigos, que a aproximação de Portugal aos Estados Unidos deveria ser uma das prioridades da política externa portuguesa, dado o posicionamento geoestratégico dos Açores. Esta antevisão mostrou-se certeira no século passado, durante as duas guerras mundiais, com a instalação de bases militares norte-americanas nestas Ilhas. Desde então, os Açores tornaram-se imprescindíveis para a projecção do poder militar norte-americano no Atlântico e foram a principal razão para o convite endereçado a Portugal para aderir à NATO. Se o término da Guerra-Fria, a valorização do Pacífico e o desenvolvimento tecnológico colocaram sérios desafios à relevância do Arquipélago no sistema internacional, é mister que o alargamento da Plataforma Continental Portuguesa, só por si, dado o posicionamento geoestratégico das Ilhas, de novo lhes confira prevalência no âmbito da política externa portuguesa.

Nitrogen Isotope Discrepancy Between Primary Producers and Sediments in an Anoxic and Alkaline Lake

Nitrogen isotope compositions (δ15N) in sedimentary rocks are extensively used to investigate the biogeochemical nitrogen cycle through geological times. This use relies on the observation that, in modern continental platforms and anoxic basins, surface sediments faithfully record the δ15N of primary producers, assuming that it was similar in the past. Over Earth’s history, however, surface environments experienced profound changes, including the transition of ammonium-dominated to nitrate-dominated waters and the transition from exclusively microbial ecosystems to ecosystems including multicellularity, which make modern environments significantly different compared to earlier ones, potentially invalidating the fundamental assumption that surface sediments faithfully record the δ15N of primary producers. In order to improve our understanding of the nitrogen isotopic information contained in the early Earth’s sedimentary rock record, we investigate here the nitrogen isotope systematics in a microbial, nitrate free and ammonium-rich modern system, the Dziani Dzaha Lake. In this modern system, the δ15N of the reduced dissolved inorganic nitrogen (i.e., NH4+ and NH3) in the water column is close to ∼7‰ . δ15N of suspended particulate matter (SPM) show a similar average value in surface waters (i.e., where SPM is massively composed of active primary producers), but increases up to 14‰ in the deeper part of the water column during periods when it is enriched in dissolved reduced species (i.e., CH4, H2S/HS− and NH4+/NH3). Surface sediments δ15N, with values comprised between 10 and 14 ‰, seem to preferentially record these positive isotopic signatures, rather than those of active primary producers. We propose here that the observed isotopic pattern is mainly linked to the assimilation of ammonium strongly enriched in 15N by isotope exchange with ammonia under basic conditions. Although ammonium assimilation seems here to be responsible for a significant isotopic enrichment due to the basic conditions, in neutral anoxic environments inhabited by similar microbial ecosystems, this process may also significantly impact the δ15N of primary producers towards more negative values. This would have strong implications for our interpretation of the Precambrian sedimentary record as this finding challenges one the fundamental hypotheses underlying the use of sedimentary δ15N in paleo-oceanographic reconstructions, i.e. that surface sediments faithfully record the δ15N of active primary producers in the photic zone.

Discovery of Disulfane (H2S2) in Fluid Inclusions in Rubies from Yuanjiang, China, and Its Implications

Raman spectra of fluid inclusions in gem rubies from Yuanjiang deposit (China) within the Ailao Shan-Red River (ASRR) metamorphic belt showed the presence of compounds such as CO2, COS, CH4, H2S, and elemental sulfur (S8), accompanied by two bands at approximately 2499 and 2570 cm−1. These two frequencies could be assigned to the vibrations of disulfane (H2S2). This is the second case of the sulfane-bearing fluid inclusions in geological samples reported, followed by the first in quartzite from Bastar Craton of India. The H2S2 was likely in situ enclosed by the host rubies rather than a reaction product that formed during the cooling of H2S and S8, suggesting sulfanes are stable at elevated temperatures (e.g., >600 °C). By comparing the lithologies and metamorphic conditions of these two sulfane-bearing cases (Bastar and Yuanjiang), it is suggested that amphibolite facies metamorphism of sedimentary sequence that deposited in a continental platform setting might favor the generation of sulfanes. Sulfanes may play an important role in the mobilization of Cr that is essential for ruby crystallization.