Paleoenvironments and geochemical signals from the late Barremian to the middle Aptian in a Tethyan marginal basin, northeast Spain: Implications for carbon sequestration in restricted basins

The hallmark of oceanic anoxic event 1a (OAE1a) (early Aptian ~125 Ma) corresponds to worldwide deposition of black shales with total organic carbon (TOC) content > 2% and a δ13C positive excursion up to ~5‰. OAE1a has been related to large igneous province volcanism and dissociation of methane hydrates during the Lower Cretaceous. However, the occurrence of atypical, coeval and diachronous organic-rich deposits associated with OAE1a, which are also characterized by positive spikes of the δ 13C in epicontinental to restricted marine environments of the Tethys Ocean, indicates localized responses decoupled from complex global forcing factors. ^ The present research is a high-resolution, multiproxy approach to assess the paleoenvironmental conditions that led to enhanced carbon sequestration from the late Barremian to the middle Aptian in a restricted, Tethyan marginal basin prior to and during OAE1a. I studied the lower 240 m of the El Pui section, Organyà Basin, Spanish Pyrenees. The basin developed as the result of extensional tectonism linked to the opening of the Atlantic Ocean. At the field scale the section consists of a sequence of alternating beds of cm – m-scale, medium-gray to grayish-black limestones and marlstones with TOC up to ~4%. ^ The results indicate that the lowest 85 m of the section, from latest Barremian -earliest Aptian, characterize a deepening phase of the basin concomitant with sustained riverine flux and intensified primary productivity. These changes induced a shift in the sedimentation pattern and decreased the oxygen levels in the water column through organic matter respiration and limited ventilation of the basin. ^ The upper 155 m comprising the earliest – late-early Aptian document the occurrence of OAE1a and its associated geochemical signatures (TOC up to 3% and a positive shift in δ13C of ~5‰). However, a low enrichment of redox-sensitive trace elements indicates that the basin did not achieve anoxic conditions. The results also suggest that a shallower-phase of the basin, coeval with platform progradation, may have increased ventilation of the basin at the same time that heightened sedimentation rates and additional input of organic matter from terrestrial sources increased the burial and preservation rate of TOC in the sediment.^

Chemical composition of Late Cenozoic volcanic and volcaniclastic deposits from the western Woodlark Basin area, SW Pacific

Tephra fallout layers and volcaniclastic deposits, derived from volcanic sources around and on the Papuan Peninsula, form a substantial part of the Woodlark Basin marine sedimentary succession. Sampling by the Ocean Drilling Program Leg 180 in the western Woodlark Basin provides the opportunity to document the distribution of the volcanically-derived components as well as to evaluate their chronology, chemistry, and isotope compositions in order to gain information on the volcanic sources and original magmatic systems. Glass shards selected from 57 volcanogenic layers within the sampled Pliocene-Pleistocene sedimentary sequence show predominantly rhyolitic compositions, with subordinate basaltic andesites, basaltic trachy-andesites, andesites, trachy-andesites, dacites, and phonolites. It was possible to correlate only a few of the volcanogenic layers between sites using geochemical and age information apparently because of the formation of strongly compartmentalised sedimentary realms on this actively rifting margin. In many cases it was possible to correlate Leg 180 volcanic components with their eruption source areas based on chemical and isotope compositions. Likely sources for a considerable number of the volcanogenic deposits are Moresby and Dawson Strait volcanoes (D'Entrecasteaux Islands region) for high-K calc-alkaline glasses. The Dawson Strait volcanoes appear to represent the source for five peralkaline tephra layers. One basaltic andesitic volcaniclastic layer shows affinities to basaltic andesites from the Woodlark spreading tip and Cheshire Seamount. For other layers, a clear identification of the sources proved impossible, although their isotope and chemical signatures suggest similarities to south-west Pacific subduction volcanism, e.g. New Britain and Tonga- Kermadec island arcs. Volcanic islands in the Trobriand Arc (for example, Woodlark Island Amphlett Islands and/or Egum Atoll) are probable sources for several volcaniclastic layers with ages between 1.5 to 3 Ma. The Lusancay Islands can be excluded as a source for the volcanogenic layers found during Leg 180. Generally, the volcanogenic layers indicate much calc-alkaline rhyolitic volcanism in eastern Papua since 3.8 Ma. Starting at 135 ka, however, peralkaline tephra layers appear. This geochemical change in source characteristics might reflect the onset of a change in geotectonic regime, from crustal subduction to spreading, affecting the D'Entrecasteaux Islands region. Initial 143Nd/144Nd ratios as low as 0.5121 and 0.5127 for two of the tephra layers are interpreted as indicating that D'Entrecasteaux Islands volcanism younger than 2.9 Ma occasionally interacted with the Late Archean basement, possibly reflecting the mobilisation of the deep continental crust during active rift propagation.

Espessura da zona de transição do manto sob a Província Borborema, NE Brasil, através de conversões Ps telessísmicas

The mantle transition zone is defined by two seismic discontinuities, nominally at 410 and 660 km depth, which result from transformations in the mineral olivine. The topography of these discontinuities provides information about lateral temperature changes in the transition zone. In this work, P-to-S conversions from teleseismic events recorded at 32 broadband stations in the Borborema Province were used to determine the transition zone thickness beneath this region and to investigate whether there are lateral temperature changes within this depth range. For this analysis, stacking and migration of receiver functions was performed. In the Borborema Province, geophysical studies have revealed a geoid anomaly which could reflect the presence of a thermal anomaly related to the origin of intraplate volcanism and uplift that marked the evolution of the Province in the Cenozoic. Several models have been proposed to explain these phenomena, which include those invoking the presence of a deep-seated mantle plume and those invoking shallower sources, such as small-scale convection cells. The results of this work show that no thermal anomalies are present at transition zone depths, as significant variations in the transition zone thickness were not observed. However, regions of depressed topography for both discontinuities (410 and 660 km) that approximately overlap in space were identified, suggesting that lower-thanaverage, lateral variations in seismic velocity above 410 km depth may exist below the the Borborema Province. This is consistent with the presence of a thermally-induced, low-density body independently inferred from analysis of geoid anomalies. Therefore, the magma source responsible for the Cenozoic intraplate volcanism and related uplift in the Province, is likely to be confined above the upper mantle transition zone.

Estudo da estrutura subsuperficial da província Borborema com correlação de ruído sísmico

Ambient seismic noise has traditionally been considered as an unwanted perturbation in seismic data acquisition that "contaminates" the clean recording of earthquakes. Over the last decade, however, it has been demonstrated that consistent information about the subsurface structure can be extracted from cross-correlation of ambient seismic noise. In this context, the rules are reversed: the ambient seismic noise becomes the desired seismic signal, while earthquakes become the unwanted perturbation that needs to be removed. At periods lower than 30 s, the spectrum of ambient seismic noise is dominated by microseism, which originates from distant atmospheric perturbations over the oceans. The microsseism is the most continuous seismic signal and can be classified as primary – when observed in the range 10-20 s – and secondary – when observed in the range 5-10 s. The Green‘s function of the propagating medium between two receivers (seismic stations) can be reconstructed by cross-correlating seismic noise simultaneously recorded at the receivers. The reconstruction of the Green‘s function is generally proportional to the surface-wave portion of the seismic wavefield, as microsseismic energy travels mostly as surface-waves. In this work, 194 Green‘s functions obtained from stacking of one month of daily cross-correlations of ambient seismic noise recorded in the vertical component of several pairs of broadband seismic stations in Northeast Brazil are presented. The daily cross-correlations were stacked using a timefrequency, phase-weighted scheme that enhances weak coherent signals by reducing incoherent noise. The cross-correlations show that, as expected, the emerged signal is dominated by Rayleigh waves, with dispersion velocities being reliably measured for periods ranging between 5 and 20 s. Both permanent stations from a monitoring seismic network and temporary stations from past passive experiments in the region are considered, resulting in a combined network of 33 stations separated by distances between 60 and 1311 km, approximately. The Rayleigh-wave, dispersion velocity measurements are then used to develop tomographic images of group velocity variation for the Borborema Province of Northeast Brazil. The tomographic maps allow to satisfactorily map buried structural features in the region. At short periods (~5 s) the images reflect shallow crustal structure, clearly delineating intra-continental and marginal sedimentary basins, as well as portions of important shear zones traversing the Borborema Province. At longer periods (10 – 20 s) the images are sensitive to deeper structure in the upper crust, and most of the shallower anomalies fade away. Interestingly, some of them do persist. The deep anomalies do not correlate with either the location of Cenozoic volcanism and uplift - which marked the evolution of the Borborema Province in the Cenozoic - or available maps of surface heat-flow, and the origin of the deep anomalies remains enigmatic.

Estrutura crustal e mantélica da província Borborema através de funções do receptor e dispersão de ondas superficiais

The Borborema Province, located in northeastern Brazil, has a basement of Precambrian age and a tectonic framework structured at the Neoproterozoic (740-560 Ma). After separation between South America and Africa during the Mesozoic, a rift system was formed, giving rise to a number of marginal and inland basins in the Province. After continental breakup, episodes of volcanism and uplift characterized the evolution of the Province. Plateau uplift was initially related to magmatic underplating of mafic material at the base of the crust, perhaps related to the generation of young continental plugs (45-7 Ma) along the Macau-Queimadas Alignment (MQA), due to a small-scale convection at the continental edge. The goal of this study is to investigate the causes of intra-plate uplift and its relationship to MQA volcanism, by using broadband seismology and integrating our results with independent geophysical and geological studies in the Borborema Province. The investigation of the deep structure of the Province with broadband seismic data includes receiver functions and surface-wave dispersion tomography. Both the receiver functions and surface-wave dispersion tomography are methods that use teleseismic events and allow to develop estimates of crustal parameters such as crustal thickness, Vp/Vs ratio, and S-velocity structure. The seismograms used for the receiver function work were obtained from 52 stations in Northeast Brazil: 16 broadband stations from the RSISNE network (Rede Sismográfica do Nordeste do Brasil), and 21 short-period and 6 broadband stations from the INCT-ET network (Instituto Nacional de Ciência e Tecnologia – Estudos Tectônicos). These results add signifi- cantly to previous datasets collected at individual stations in the Province, which include station RCBR (GSN - Global Seismic Network), stations CAUB and AGBL (Brazilian Lithosphere Seismic Project IAG/USP), and 6 other broadband stations that were part of the Projeto Milênio - Estudos geofísicos e tectônicos na Província Borborema/CNPq. For the surface-wave vii tomography, seismograms recorde at 22 broadband stations were utilized: 16 broadband stations from the RSISNE network and 6 broadband stations from the Milênio project. The new constraints developed in this work include: (i) estimates of crustal thickness and bulk Vp/Vs ratio for each station using receiver functions; (ii) new measurements of surfassewave group velocity, which were integrated to existing measurementes from a continental-scale tomography for South America, and (iii) S-wave velocity models (1D) at various locations in the Borborema Province, developed through the simultaneous inversion of receiver functions and surface-wave dispersion velocities. The results display S-wave velocity structure down to the base of the crust that are consistent with the presence of a 5-7.5 km thick mafic layer. The mafic layer was observed only in the southern portion of the Plateau and absent in its northern portion. Another important observation is that our models divide the plateau into a region of thin crust (northern Plateau) and a region of thick crust (southern Plateau), confirming results from independent refraction surveys and receiver function analyses. Existing models of plateau uplift, nonetheless, cannot explain all the new observations. It is proposed that during the Brazilian orogeny a layer of preexisting mafic material was delaminated, as a whole or in part, from the original Brasiliano crust. Partial delamination would have happened in the southern portion of the plateau, where independent studies found evidence of a more resistant rheology. During Mesozoic rifting, thinning of the crust around the southern Plateau would have formed the marginal basins and the Sertaneja depression, which would have included the northern part of the Plateau. In the Cenozoic, uplift of the northern Plateau would have occurred, resulting in a northern Plateau without mafic material at the base of the crust and a southern Plateau with partially delaminated mafic layer.

Geochemical and isotopic composition of Bransfield Basin Lavas collected during SONNE cruise SO155

Bransfield Basin is an actively extending marginal basin separating the inactive South Shetland arc from the northern Antarctic Peninsula. Rift-related volcanism is widespread throughout the central Bransfield Basin, but the wider eastern Bransfield Basin was previously unsampled. Lavas recovered from the eastern subbasin form three distinct groups: (1) Bransfield Group has moderate large-ion lithophile element (LILE) enrichment relative to normal mid-ocean ridge basalt (NMORB), (2) Gibbs Group has strong LILE enrichment and is restricted to a relic seamount interpreted as part of the South Shetland arc, and (3) fresh alkali basalt was recovered from the NE part of the basin near Spanish Rise. The subduction-related component in Bransfield and Gibbs Group lavas is a LILE-rich fluid with radiogenic Sr, Nd, and Pb isotope compositions derived predominantly from subducting sediment. These lavas can be modeled as melts from Pacific MORB source mantle contaminated by up to 5% of the subduction-related component. They further reveal that Pacific mantle, rather than South Atlantic mantle, has underlain Bransfield Basin since 3 Ma. Magma productivity decreases abruptly east of Bridgeman Rise, and lavas with the least subduction component outcrop at that end. Both the eastward decrease in subduction component and occurrence of young alkali basalts require that subduction-modified mantle generated during the lifetime of the South Shetland arc has been progressively removed from NE to SW. This is inconsistent with previous models suggesting continued slow subduction at the South Shetland Trench but instead favors models in which the South Scotia Ridge fault has propagated westward since 3 Ma generating transtension across the basin.