Brazilian Margin Research Articles

A data-model perspective on the Brazilian margin surface warming from the Last Glacial Maximum to the Holocene

The western South Atlantic along the Brazilian margin is an important region for the Atlantic Meridional Overturning Circulation (AMOC) because surface currents in this area transfer warm and salty waters from the Southern Hemisphere to the North Atlantic. Although the number of sea surface temperature (SST) reconstructions has grown in this region, it has been challenging to explain changes in different and sometimes proximal cores. To understand the SST evolution of the Brazilian margin from the Last Glacial Maximum (LGM) to the late Holocene, we present the first SST stack (BRSST stack) for this region. We compare the BRSST stack with the outputs of the transient climate model simulation TraCE-21ka. The BRSST stack shows an LGM cooling of 1.43 °C (from −1.31 to −1.55 °C, 2σ) relative to the late Holocene, followed by deglacial warming starting at ∼ 18.8 ka. TraCE-21ka simulates this early onset of the last deglaciation. Sensitivity experiments suggest that the input of meltwater from retreating ice sheets in the Northern Hemisphere triggered the post-LGM warming, which was subsequently sustained by increasing atmospheric CO2. Deglacial millennial-scale events of AMOC slowdown produced large-scale warming of the Brazilian margin not clearly distinguished by some previous studies. Analyzing our stack in its segregated components, i.e., the North Brazil Current (NBCSST stack) and Brazil Current (BCSST stack) – we noted in-phase warming at the onset of the last deglaciation, which is not found in TraCE-21ka simulations. We attributed this to underestimating the meltwater influence over the tropical Brazilian margin by the model. BRSST stack also presents episodes of abrupt cooling near periods of fast sea-level rise during the last deglaciation, which may be due to rapid AMOC reinvigoration or freshening of the Southern Ocean. Holocene climate resembles that recorded by other compilations, with no clear Holocene thermal maximum but presenting a cooling trend during the late Holocene possibly related to intensified volcanic activity. Our study indicates that the future human-induced SST change expected for the end of the 21st-century will overcome the background of natural climate variability of the last 22,000 years for the Brazilian margin.

Topological analysis of fault network in naturally fractured reservoirs: A case study from the pre-salt section of the Santos Basin, Brazil

The Santos Basin, located on the southeastern margin of Brazil, hosts giant hydrocarbon reserves of the pre-salt section that correspond to the carbonate sequences deposited during the syn-rift and transitional tectonic stages. The faults, which formed during the rift evolution, have proven to be crucial in controlling hydrocarbon accumulations and migration, not only as a pathway for fluid flow but as a factor controlling the deposition of sedimentary rocks. Thus, the topological analysis of fault networks is a key method to characterize the different fault sets, as it provides a way to describe the network arrangement of individual faults and their connectivity. In this work, the fault topology was analyzed in terms of nodes and branches for the main reservoir defined in the pre-salt section seismic units: Rift, Lower Barra Velha (Lower BVE), and Upper Barra Velha (Upper BVE). Four fault sets were characterized in these units with average orientations to NW-SE and E-W. The topology of this fault networks is characterized by single-phase rift network, featuring isolated systems with more than 50% of isolated branches and low connectivity (connections per branch being less than 0.5). The flow azimuth is NE-oriented, mostly oblique to the trend of the main faults, which suggests a low fault influence in the fluid flow. On the other hand, the paleotopographic highs developed during the Rift phase present the highest fault density and intensity, factors known to contribute to increased connectivity in reservoirs. A clear limitation of this type of study is the integration of the seismic-scale faults with high-frequency well log data, sparse but known to have great vertical resolution, and, in turn, assessing the vertical fluid flow, instead of lateral as is possible with this method. These limitations are to be known by one to apply this in other contexts. In this work, we seek to understand how the development of the rift fault system influences the generation of new fault sets, and fluid flow in a pre-salt carbonate reservoir.

Modelling the 4D kinematics of extensional structures developed above discontinuous inclined ductile basal detachments

A series of scaled analogue models were used to a preliminary study on the influence of the basal detachment configuration on the evolution of extensional systems, developed above an inclined basal ductile detachment. Three different basal configurations were aimed to test extension above: i) a homogeneously distributed ductile detachment; ii) two sectors with a different distribution of the ductile detachment, separated by an abrupt, transversal boundary; ii) a laterally variable width of the ductile detachment, driven by an abrupt oblique boundary. Fault activity was monitored through a series of digital elevation models obtained from the laser scanning of the models' surface. Results of the models analysis show how the geometry, the distribution, the kinematic, and the timing of faulting is strongly controlled by the geometry and orientation of the ductile-brittle interface in respect to the extension direction. Faults close to the brittle-ductile interface develop following the interface geometry, whose effect is reduced moving away from it. In addition, we observe a major number of faults which accommodates lower amount of extension and are less closely spaced above longer ductile detachments. Our results can be compared with natural case-histories along continental passive margins, where extension occurs above a dipping salt detachment, with inhomogeneous distribution (e.g., the northern Gulf of Mexico, central Brazilian margin, Angola margin).

Obliquity Influence on Low‐Latitude Coastal Precipitation in Eastern Brazil During the Past ∼850 kyr

AbstractPaleoclimate records from tropical South America typically show precession‐paced variability in rainfall, caused by insolation‐driven changes in the South American Monsoon System, however this mechanism may not be responsible for hydroclimate change outside of the core monsoon domain, such as in the coastal zone of tropical eastern Brazil. Our findings are based on a ∼850 kyr‐long multiproxy record from a marine sediment core collected from the eastern Brazilian margin that represents the longest continuous record of South American hydroclimate to date. Utilizing the ln (K/Al) chemical weathering proxy from the core, we determine that past hydroclimate change in the coastal zone was primarily modulated by obliquity forcing. We demonstrate that high obliquity is associated with an increase in the boreal summer interhemispheric insolation contrast which decreases the zonality of the southern trade winds and reduces moisture advection to the coastal zone. Based on the long‐term coherence between the ln (K/Al) record and benthic δ13C records from the Atlantic during Marine Isotope Stages 16–13, we infer that an increase in the strength of the overturning circulation, caused by Northern Hemisphere high‐latitude forcing, may have produced surface cooling in the western tropical South Atlantic which led to reduced moisture advection to the coastal zone. We suggest that this mechanism may have also caused the amplification in millennial‐scale variability in the coastal hydroclimate system, which began after the decoupling of the coastal hydroclimate system from obliquity forcing.

Hafnium‑neodymium isotope evidence for enhanced weathering and uplift-climate interactions during the Late Cretaceous

The processes initiating the first cooling step of the last greenhouse-to-icehouse transition, from 90 million years ago (Ma) onward still remain enigmatic. While the combination of mountain uplift and continental weathering has been proposed as a major sink for atmospheric CO2 and a climate driver over geological timescales, this hypothesis is much debated and its potential importance in triggering the late Cretaceous global cooling is yet to be explored. In this work, we combined clay mineralogy, trace and major element concentrations, and a new proxy of silicate weathering intensity based on Nd and Hf isotopes (∆ɛHf(t)clay) to explore the potential links between the uplift of the brazilian margin, silicate weathering and climate evolution during the late Cretaceous. Our new ∆ɛHf(t)clay proxy data suggest - for the first time - that marked increase of silicate weathering intensity occurred in southeast Brazil during the late Cretaceous, from ~85 to 70 Ma, related to the tectonic uplift affecting the eastern South American margin at that time. Combined with clay mineralogical analyses, our Hf-Nd isotope data further suggests the existence of a relatively arid local climate during the Turonian-Santonian interval, during which physical disaggregation of rocks most likely prevailed, accompanying the uplift of the Brazilian margin. From the Santonian, we propose that the exposure of new high-elevation regions favored instead locally enhanced precipitations and more hydrolysing conditions, thereby promoting chemical weathering and atmospheric CO2 drawdown. Altogether, our multi-proxy investigation suggests that the uplift of the Brazilian margin could have contributed to the late Cretaceous cooling, potentially playing a key role in the onset of the last greenhouse-to-icehouse transition.

Paleoenvironmental insights from the deposition and diagenesis of Aptian pre-salt magnesium silicates from the Lula Field, Santos Basin, Brazil

ABSTRACT Since the discovery of giant Aptian pre-salt reservoirs in Brazilian margin basins, the study of lacustrine carbonates has drawn great attention from the scientific community. Comparatively, minor attention was given to the characterization and genesis of the Mg-silicates (e.g., stevensite, kerolite) which are commonly associated with these carbonates. A systematic petrological study was performed in the Aptian Barra Velha Formation (BVF) within distinct structural compartments of the giant Lula Field in the Santos Basin, in order to recognize the patterns of primary formation and diagenetic alteration of these Mg-silicates. Mg-silicates occur as peloids, ooids, intraclasts, and fine-grained laminated deposits, either mixed in variable proportions with other particles, such as carbonate bioclasts and volcanic rock fragments, or constituting specific intrabasinal deposits. In the BVF interval, clay peloids and laminated deposits are associated with spherulitic and fascicular calcite aggregates, as substrate and hosts for these precipitates. Ooids are interpreted as formed at the sediment–water interface by the nucleation of concentric envelopes on the surface of particles (heterogeneous nucleation), through repeated rolling under gentle wave and current action. Laminated deposits, interpreted as precipitated directly from the water column (homogeneous nucleation) in highly supersaturated and low-hydrodynamic-energy environments, constitute extensive deposits in the BVF. Peloids were probably formed in intermediate energy conditions. Some ooidal arenites show porosity from the dehydration and contraction, and/or the dissolution of ooids. In some rocks, these pores are filled with fibrous calcite, while the remaining Mg-silicates are replaced by dolomite, calcite, or silica. A similar diagenetic pattern occurs in the laminated deposits, where magnesite and dolomite fill shrinkage pores formed along their characteristic wavy laminae. Owing to their elevated solubility, most of the Mg-silicates were dissolved, or intensely replaced by calcite, dolomite, or silica. The detailed petrologic analysis indicates that the original volumes of Mg-silicates were substantially larger, and that their deposition was widespread in the basin, including on structurally high areas. The types and intensity of diagenetic alteration of the Mg-silicate deposits are distinct for each structural compartment, being more intense towards the highs and closer to the overlying evaporites, which imposed a strong influence on reservoir quality.

Chemical characterization of deep-sea corals from the continental slope of Santos Basin (southeastern Brazilian upper margin)

Carbonate mounds and pockmarks are geologically and ecologically important features distributed worldwide in the world’s oceans. In the present study, we present a chemical characterization of deep-sea scleractinian coral skeletons collected in these geomorphological areas at the southeastern continental margin of Brazil. Coral samples were collected from ten sampling stations during cruises aboard the R/V Alpha Crucis, in 2019. Three species of scleractinian corals were studied: Enallopsammia rostrata, Solenosmilia variabilis, and Desmophyllum pertusum. Stable isotopes of carbon and oxygen (δ13C and δ18O), metals, and phosphorus present in the coral carbonate skeletons were analyzed. Corals are recognized as archives of physical-chemical variations in the marine environment, and the Element/Ca ratios, δ13C, and δ18O allowed for the characterization of the studied areas. Chemical composition found in pockmark areas indicated affinity to terrigenous and particulate materials input (Ba/Ca, Fe/Ca, Mn/Ca, Li/Ca, and Mg/Ca). Greater availability of nutrients and anthropogenic materials (Pb/Ca, Cd/Ca, Zn/Ca, and P/Ca) is also likely to occur in this region, with some elemental ratios higher than those measured in other oceans. These mounds can act as barriers for metals from land flows. Also, corals benefit from a higher food supply due to stronger currents. The corals at the top of the Alpha Crucis Carbonate Ridge receive significant marine influence. Most coral samples have carbonate of aragonitic origin, except for a specimen of D. pertusum, which presented carbonate of biogenic calcite and aragonite. The results demonstrate the potential of scleractinian corals in the chemical characterization of the deep ocean and the need for further investigation of carbonate mound areas from the SW Atlantic.

Quaternary Ostracoda and Foraminifera from the Pelotas Basin, southernmost Brazil: Assemblage variation in gas-hydrate bearing sediments

Late Quaternary ostracod and foraminifera assemblages from five piston cores in Pelotas Basin, southern Brazilian margin, were analyzed in this paper. The study revealed a diverse bathyal fauna of ostracods, with abundance of the genera Apatihowella, Cytheropteron and Cytherella, interspersed with peaks of allochthonous neritic faunas. Benthic foraminifera are represented mainly by the genera Bolivina, Bulimina, Cassidulina, Epistominella, Globobulimina, Globocassidulina, Nonion, Oridorsalis and Uvigerina. Biostratigraphic analysis of planktic foraminifera positioned the samples in Pleistocene-Holocene interval. The ostracod genera Paracytherois, Cytheropteron, Cytherella, Macropyxis and the species Krithe hunti, Krithe reversa, Henryhowella asperrima, Eucytherura fossapunctata, Rimacytheropteron longipunctatum, Apatihowella besnardi and Apatihowella convexa, and also the benthic foraminifera Bolivina, Bulimina, Nonion, Nonionellina, Oridorsalis, Uvigerina, Epistominella and Cassidulina are related to environments of gas hydrates and associated with a chemosynthetic community in eutrophic-mesotrophic environments.

Andean fingerprint on placer sands from the southern Brazilian coast

On the passive margin of southern Brazil, where the availability of sediments and coastal system conditions were adequate for forming a Ti–Zr-bearing placer in the Holocene, could there be a source of detrital contribution other than the Precambrian South American cratons? To answer that, detrital zircon U–Pb–Hf analyses were performed using the LA-ICP-MS method. The U–Pb age pattern distribution of 866 zircon grains showed that 35.3% of the grains belong to the Neoproterozoic, which covers the Brasiliano orogenic belts. However, 9.1% of the total zircons analyzed (up to 12% in some samples) correspond to grains younger than 50 Ma, restricted to the Andean orogeny in southern South America. The Hf signature of the zircons stands out by pointing to five significant and distinct groups interpreted as coming from the Andean (0–50 Ma), Gondwanides (230–380 Ma), Famatinian (380–500 Ma), Brasiliano (850–541 Ma), and Grenville (900–1300 Ma) orogens, confirming an Andean fingerprint as source rocks in addition to the sediments originating from the craton. This Andean source is related to the distal sediment contribution from the La Plata River system and Argentina rivers that discharge in the Atlantic Coast. Based on a statistical and Hf isotope approach, we defined that around 50% of the zircons grains were transported from the Argentina coast and La Plata River by the littoral drift for more than 1000 km to their final sink in the southern Brazilian coast.

Depositional and diagenetic model of the Aptian potash-bearing Loémé evaporites in onshore Congo

The evaporites of the Aptian Loémé Formation occur along the stable structural domain of onshore Congo as horizontally well-stratified depositional halite interbedded with Mg-poor potash minerals including carnallite, bischofite, tachyhydrite and sylvite. Eleven correlative depositional recharge-to-evaporation cycles are composed of suites of organic-prone shale followed by primary halite and carnallite beds, sometimes punctuated by a bischofite-tachyhydrite end-member.A sedimentological and petrographical analysis performed on cores and thin sections allowed an interpretation of nine facies associations, named as FA1 to FA9, relying upon the occurrence of bands of fluid inclusions, grain-size, dissolution features, detrital sediments, minute inclusions and sedimentary structures. A depositional model reconciles the spatial distribution of primary textures, hydrology, brine saturation and palaeo-depth during a standard regressive evaporation cycle. Shallow burial eo-diagenesis led to compaction-driven dissolution and recrystallization into fine-grained halite and carnallite laminites but poorly affected the overall preservation of primary deposits.Measurements and profile analysis of bromine trace element concentrations reveal a palaeo-salinity increase of the parent brine for cycle II to base IX caused by a relative brine level fall and a gradual confinement of the salt basin, likely associated with an excess of saline elements inflow from hydrothermal vents. Similar depositional textures and bromine concentration variations are observed in the lower cycles of the Aptian evaporites from the counter-part Sergipe Basin in the northern part of the Brazilian margin, testifying of a progressive brine salinity rise due to a palaeogeographic confinement along the northern segment of the South Atlantic Ocean.In Congo, the sylvite displays horizontal “pseudo-stratifications” that unconformably overlay the crests of anticlines and propagate along flanks as a sharp-based caprock. The flushing and transformation of depositional carnallite – halite into secondary sylvite – halite is interpreted as having been caused by down-stepping incremental dissolutions sourced by younger pervasive connate to phreatic waters. In Sergipe, the sylvite is interpreted as secondary but further results from syndepositional dissolution of carnallite and tachyhydrite due to a regional North Atlantic Ocean ingression.

Comparison of Multiple Surface Ocean Wind Products with Buoy Data over Blue Amazon (Brazilian Continental Margin)

Remote sensing data for space-time characterization of wind fields in extensive oceanic areas have been shown to be increasingly useful. Orbital sensors, such as radar scatterometers, provide data on ocean surface wind speed and direction with spatial and temporal resolutions suitable for multiple applications and air-sea studies. Even considering the relevant role of orbital scatterometers to estimate ocean surface wind vectors on a regional and global scale, the products must be validated regionally. Six different ocean surface wind datasets, including advanced scatterometer (ASCAT-A and ASCAT-B products) estimates, numerical modelling simulations (BRAMS), reanalysis (ERA5), and a blended product (CCMP), were compared statistically with in situ measurements obtained by anemometers installed in fifteen moored buoys in the Brazilian margin (8 buoys in oceanic and 7 in shelf waters) to analyze which dataset best represents the wind field in this region. The operational ASCAT wind products presented the lowest differences in wind speed and direction from the in situ data (0.77 ms−1 < RMSEspd < 1.59 ms−1, 0.75 < Rspd < 0.96, −0.68 ms−1 < biasspd < 0.38 ms−1, and 12.7° < RMSEdir < 46.8°). CCMP and ERA5 products also performed well in the statistical comparison with the in situ data (0.81 ms−1 < RMSEspd < 1.87 ms−1, 0.76 < Rspd < 0.91, −1.21 ms−1 < biasspd < 0.19 ms−1, and 13.7° < RMSEdir < 46.3°). The BRAMS model was the one with the worst performance (RMSEspd > 1.04 m·s−1, Rspd < 0.87). For regions with a higher wind variability, as in the southern Brazilian continental margin, wind direction estimation by the wind products is more susceptible to errors (RMSEdir > 42.4°). The results here presented can be used for climatological studies and for the estimation of the potential wind power generation in the Brazilian margin, especially considering the lack of availability or representativeness of regional data for this type of application.

Marlim R3D phase 3: The marine magnetotelluric regional model and associated data set

Synthetic data provided by earth models are essential to investigate several geologic problems. Marlim R3D (MR3D) is an open-source realistic earth modeling project for electromagnetic simulations of the postsalt reservoirs of the Brazilian offshore margin. In phase 3, we have conducted a 3D marine magnetotelluric (MMT) study with the finite-difference method to generate the synthetic magnetotelluric (MT) data set for the MR3D earth model. To that end, we upscaled the original controlled-source electromagnetic model to preserve all local-scale features, such as the thin-layer turbidite reservoirs, and to include several geologic regional features, such as the coastline, land topography, basement rocks representing the continental crust, and mantle rocks. Then, we simulated an MMT survey with 500 receivers evenly spaced at 1 km intervals along the rugged seafloor of the MR3D model. To accurately represent the MMT model with a 329 × 329 × 200 km volume, we have produced a mesh with 161 × 136 × 242 cells (approximately 5.3 million cells). We computed the full MT and tipper tensor at 25 periods in the time range of 1–10,000 s. The data set, the model, and companion material are freely distributed for research or commercial use under the Creative Commons License at the Zenodo platform.

INTERPRETAÇÃO SÍSMICA E MODELAGEM 3D ESTRUTURAL NA BACIA DE BARREIRINHAS (MARANHÃO, BRASIL)

Discoveries of hydrocarbons in the basins of the African Equatorial Margin and Guinea Gulf stimulated the exploratory interest in the basins of the Brazilian Equatorial Margin, for being together before the Continental Drift. This interest emerges because both African and South American equatorial margin are considered analogous. The Barreirinhas Basin is a member of the Brazilian equatorial margin. The objective of this work is to present the results obtained through the seismic interpretation and structural 3D modeling, in the context of gravitational tectonics, in an area covered by 3D seismic data, in the Barreirinhas Basin. The compressional domain of an extensive-compressive system was mapped. In this study, were identified reverse faults, thrust faults and fault-related folding like fault-bend and fault-propagation fold that can be accompanied by backthrust features in deep to ultra-deep waters. The 3D structural model allowed the representation of the geometric variations present in the study area. The new information will be important for the identification and evaluation of structures with greater potential for hydrocarbon accumulations and can help in future studies to characterize the reservoir, contributing to the evolution of knowledge of the equatorial margin, especially in the Barreirinhas Basin.

Imaging Early Oceanic Crust spreading in the Equatorial Atlantic Ocean: Insights from the MAGIC wide-angle experiment

During the MAGIC (Margins of brAzil, Ghana, and Ivory Coast) experiment, five combined wide-angle, and reflection seismic profiles were acquired in the Pará-Maranhão/Barreirinhas/Ceará basins northern Brazil. This is a pull-apart passive margin, with two strike-slip borders. The equipment deployed includes 143 sea-bottom seismometers (OBS), a 4.5-km seismic streamer, and a 7587-in3 airgun array. In this paper, we focus on the distal parts of three profiles, and one entire transverse NW-SE profile, located on the presumed Cretaceous oceanic crust.Forward modelling of these wide-angle data sets reveals an E-W lateral evolution of the oceanic crust spreading initiation with: 1) just after the so-called intermediate domain, 60 km-wide domain that consists of a 5-km-thick crystalline crust. The basement presents two layers characterized by high acoustic velocity. This domain is bounded to the NW by a NW-SE volcanic line (Volcano Alignment), and 2) a 5-km-thick oceanic crust consisting of two layers characterized by “normal velocities” spanning between the two main fracture zones that fringe the Pará-Maranhão-Barreirinhas-Ceará segment. Despite a similar thickness, these two sub-domains present different velocity distribution in their two layers. They are both overlain by 5.5 km of sedimentary deposits. Forward wide-angle modelling confirms that the seafloor spreading process was progressive, with firstly the emplacement of a proto-oceanic crust, and then a thin oceanic crust. The “proto-oceanic” crust presents a similar seismic velocity with the intermediate domain interpreted as exhumed lower continental crust except for the lower part where the intruded lower crust gives place to a very sharp Moho at the base of the proto-oceanic domain. By contrast, the thin oceanic crust domain has a lower velocity structure in its upper layer, that is interpreted as basalt and is absent in the proto-oceanic crust. This eastward evolution, as also observed in the Provençal Basin, and the Santos Basin, suggests the involvement of the lower continental crust in the first steps of seafloor spreading.

Proxies for paleo-oxygenation: A downcore comparison between benthic foraminiferal surface porosity and I/Ca

Benthic foraminiferal surface porosity (the mean percentage of surface area covered by pores; higher porosity: lower oxygenation) and iodine to calcium ratio (I/Ca, higher I/Ca: higher oxygenation) are both promising paleoceanographic proxies that will advance through testing in down-core studies. Here we report the first down-core comparison (~45 kyr) of these proxies for a core from the southern Brazilian margin (26°40.22′ S, 46°26.46′ W, 475 m water depth). Both proxies are most sensitive to low-O2 conditions (<50 μmol/kg), and not well-constrained at higher O2 concentrations. Porosity values are generally low (<15%) and I/Ca ranges between ~4 and ~ 6 μmol/mol throughout the core. The two proxies are overall consistent, suggesting that bottom-water oxygen concentrations at the site remained above 50 μmol/kg during the last 45 kyr. Several non-O2 factors (e.g., iodate reduction rates, water mass mixing, temperature, foraminiferal shell robustness) could influence the proxies and require further investigation.

Morphostructure, emplacement and duration of the Abrolhos Magmatic Province: A geophysical analysis of the largest post-breakup magmatism of the South-Eastern Brazilian margin

The Abrolhos Magmatic Province (AMP), located in the Ocean Continent Transition (OCT) of the Brazilian South Atlantic rifted margin, is spatially extensive covering an area larger than 63,000 km2 and was emplaced in the Early Cenozoic, approximately 40 Ma after rifted margin formation in Early Cretaceous. The explanation for the origin of the AMP is that it forms part of a hotspot track that initiated at ~85 Ma in the Poxoréu region, an igneous province located ~1500 km to the west of the AMP, within Brazilian continental lithosphere and is presently located under Trindade and Martin Vaz Islands at the Vitoria-Trindade seamount chain (VTC). The aim of this work is to investigate the morphostructure, duration and spatial distribution of the AMP, exploring its significance for the margin evolution and possible origins. The magnetic, reflexion seismic and well data analysis show that the AMP consists of an assembly of large volcanic edifices formed during the Paleogene, each with individual diameters in excess of 50 km, sizes varying from 280 to 2100 km2, and formed by multiple volcanic events. The igneous bodies are mainly buried at different depths (from ~2 Km to 8 km below seafloor), while some emerge to form the Abrolhos Archipelago. The spatial arrangement of the igneous edifices of the AMP suggest that in its central part are the main feeders for the outer and shallower bodies at the borders which have a radial distribution. Their distribution seems to partially follow preexisting structural trends like Precambrian and Mesozoic weakness zones, represented by the onshore Paramirin Aulacogen, the Araçuaí and Ribeira belts, and the offshore rifting structures. These observations suggest an influence of lithosphere inheritance on the emplacement of the AMP. The tectono-stratigraphic architecture indicate margin uplift at Early Cenozoic related to the emplacement of the AMP, creating an elevated area. Mapped hydrothermal vents indicate AMP magmatic activity continued into the Neogene, and connect Abrolhos volcanism with that in the VTC located 650 km to the east. This and reported low S-wave velocity from tomographic model and anomalous dynamic topography suggest a distinct mantle thermal structure under the AMC and VTC. However, the spatial extent and duration of the AMP are inconsistent with a fixed hotspot mechanism and linear trail.

Environmental controls on holocene reef development along the eastern brazilian margin

Environmental controls on holocene reef development along the eastern brazilian margin

Soft-sediment deformation in the Sergipe-Alagoas Basin, Brazil: Implications for paleoseismicity in intraplate areas

Several studies have cited soft-sediment deformation as evidence of fault reactivation and paleoseismicity in intraplate coastal settings. However, a consensus has not been reached on the extent of paleoseismic activity in several regions and the link with continental-scale faults. This research aims to determine the origin of the deformation in soft-sediments, both in the Neogene Barreiras Formation and in Pleistocene sediments of the Sergipe-Alagoas Basin, on the continental margin of northeastern Brazil. The region shows a historical record since 1808 of earthquakes with maximum modified Mercalli intensity of VII and a 40-year instrumental record of events with maximum body-wave magnitude (mb) of 5.2. The brevity of these records leads to large uncertainties in estimates of fault slip rate, recurrence, and seismic hazard. Geologic studies of seismically induced soft-sediment deformation structures and their relationship to seismically active faults offer the opportunity to reduce these uncertainties. We present detailed descriptions of the deformation structures with emphasis on morphology, ductile or brittle character, size, and distribution of the features. In addition, we integrate geologic data with seismic reflection data, thus allowing for correlation between the normal faults at the seismic scale and the soft-sediment deformation structures at the outcrop scale. Faults associated with seismites at the outcrop scale exhibit the same strike/dip and kinematics and are spatially related to the major faults identified at the seismic-section scale. Regarding ductile features, the most remarkable are convoluted folds associated with a deformed muddy layer, giant sinking structures, and complex folds. Concerning brittle structures, the most striking are clastic dikes, mixed layers, autoclastic breccias, and fault breccias. The alternating records in the Neogene sediments indicate clustered seismic activity, combined with tectonic quiescence periods. In Pleistocene sediments, the size, frequency, lateral continuity, and spatial distribution of tens of kilometers of the deformation structures indicate a period of frequent earthquakes with body-wave magnitudes equal to or greater than 6. The recognition of seismic activity and brittle reactivation of preexisting faults during the Neogene-Quaternary has far-reaching implications for seismic hazards in intraplate settings.

Deep structure of the Pará-Maranhão/Barreirinhas passive margin in the equatorial Atlantic (NE Brazil)

The Pará-Maranhão/Barreirinhas margin, North Brazil, is a pull-apart passive margin, with two strike-slip borders, formed during the opening of the Equatorial Atlantic Ocean during Cretaceous time. Its geometry and evolution are speculative due to the lack of information on the crustal structure and the crustal nature. We present here the E-W profiles of the MAGIC (Margins of brAzil, Ghana and Ivory Coast) deep seismic experiment, a joint project between French and Brazilian universities, research institutes and the industry. Fifty-six Ocean Bottom Seismometers (OBS) and a 4.5 seismic streamer were deployed at sea along 2 of the 5 MAGIC profiles. One profile was extended onshore by installing 8 land stations. We perform forward modelling through combined interpretation of the multichannel seismic and of the main reflected and refracted of these phases recorded by the OBSs. The final P-wave velocity models reveal distinct structural domains from onshore Brazil towards the Atlantic Ocean characterized by variations of the crustal thicknesses and velocities: (1) an unthinned continental crust below the São Luís Craton, where the crust is 33 km thick, (2) a 60 km wide necking domain below the Ilha de Santana Platform; (3) offshore, east of the continental slope, a 10 km-thick deep sedimentary basin underlain by a 5 km thick crust with velocity of 6.2–6.9 km/s that we interpret as an exhumed lower continental crust, on the top of an Anomalous Velocity Layer (AVL) probably made of intrusions of mantle-derived melts into the lower continental crust, or a mixture of them; (4) eastwards, the limit of the previous domain is marked by NW-SE aligned volcanoes and the disappearance of the AVL. The sedimentary succession becomes thinner (6 km) overlaying a proto-oceanic crust characterized by seismic velocities higher than “normal” oceanic crust in its upper part, but in continuity with the velocity described in the previous domain; (5) followed by a more characteristic but thin oceanic crust.The middle/lower continental crust seems not only to have a crucial role in the genesis of the passive margin but also to be involved in the genesis of the first oceanic crust. The passage to a typical oceanic crust seems to have occurred progressively by steps: first in the deeper layer by the setup of more and more intrusions of mantle-derived melts at the base of the crust or mixture of exhumed lower crust and mantle, producing a domain of proto-oceanic crust, then by the emplacement of an upper 1-2 km-thick layer with typical oceanic characteristics.

Millennial-Scale Environmental Variability in Late Quaternary Deep-Sea Sediments from the Demerara Rise, NE Coast of South America

We carried out a rock magnetic study of two deep-sea gravity cores from the Demerara Rise, NE South America. Our previous studies provided radiocarbon and paleomagnetic chronologies for these cores. This study presents detailed rock magnetic measurements on these cores in order to characterize the rock magnetic mineralogy and grain size as indicators of the overall clastic fraction. We measured the magnetic susceptibility, anhysteretic remanence, and isothermal remanence and demagnetized the remanences at several alternating field demagnetization levels. The magnetic intensities estimate the magnetic material concentration (and indirectly the overall clastic fraction) in the cores. Ratios of rock magnetic parameters indicate the relative grain size of the magnetic material (and indirectly the overall clastic grain size). Rock magnetic intensity parameters and rock magnetic ratios both vary systematically and synchronously over the last 30,000 years in both cores. There is a multi-millennial-scale cyclicity, with intervals of high magnetic intensity (high magnetic and clastic content) with low magnetic ratios (coarser magnetic and clastic grain size), alternating in sequence with intervals of low magnetic intensity with high magnetic ratios (finer grain size). There is also a higher-frequency millennial-scale variability in intensity superposed on the multi-millennial-scale variability. There are nine (A–I) multi-millennial-scale intervals in the cores. Intervals A, C, E, G, and I have high magnetic and clastic content with coarser overall magnetic and clastic grain size and are likely intervals of enhanced rainfall and runoff from the NE South American margin to the coastal ocean. In contrast, intervals B, D, F, and H have lower clastic flux with finer overall grain size, probably indicating lower continental rainfall and runoff. During the Holocene, high rainfall and runoff intervals can be related to cooler times and low rainfall and runoff to warmer times. The opposite pattern existed during the Pleistocene, with higher rainfall and runoff during interstadial conditions and lower rainfall and runoff during stadial conditions. We noted a similar pattern of Pleistocene multi-millennial-scale variability in a transect of deep-sea sediment cores along the NE Brazilian margin, from the Cariaco Basin (~10 N) to the NE Brazilian margin (~1° N–4° S). However the NW part of this transect (Cariaco Basin, Demerara Rise, Amazon Fan) has an out-of-phase relationship with the SE part of the transect (NE Brazilian margin) between warm–cold and wet–dry conditions. One possible cause of the high–low rainfall and runoff patterns might be oscillation of the Intertropical Convergence Zone (ITCZ), with higher rainfall and runoff associated with a more southerly average position of the ITCZ and lower rainfall and runoff associated with a more northerly average position of the ITCZ.