Cátia Fernandes Barbosa

Geoacoustic character, sedimentology and chronology of a cross-shelf Holocene sediment deposit off Cabo Frio, Brazil (southwest Atlantic Ocean)

The Cabo Frio region in the state of Rio de Janeiro, southeast coast of Brazil, is characterized by a local coastal upwelling system and converging littoral sediment transport systems that are deflected offshore at Cabo Frio, as a consequence of which a thick cross-shelf sediment deposit has developed over time. To investigate the evolution of this muddy deposit, geophysical, sedimentological and geochemical data from four sediment cores (3.8–4.1 m in length) recovered in water depths between 88 and 141 m were analyzed. The high-resolution seismic data show variable sediment thicknesses ranging from 1 to 20 m, comprising two sedimentary units separated by a high-impedance layer at a depth of about 10 m below the seafloor at the coring sites. According to the available age datings, the upper sedimentary unit is late Pleistocene to Holocene in age, whereas the lower unit (not dated) must, by implication, be entirely Pleistocene in age. The boomer-seismic reflection signal can be divided into three echo-types, namely transparent (inner shelf), stratified (middle shelf) and reflective (outer shelf), each type seemingly related to the local sediment composition. The upper 4 m of the upper sedimentary unit is dominated by silty sediment on the middle shelf, and by upward-fining sediments (silty sand to sandy silt) on the inner and outer shelf. The downcore trends of P-wave velocity, gamma-ray density and acoustic impedance are largely similar, but generally reversed to those of water and organic carbon contents. Total organic carbon contents increase with decreasing mean grain size, periodic fluctuations suggesting temporal changes in the regional hydrodynamics and primary productivity fuelled by the local upwelling system. The reconstruction of sedimentation rates in the course of the Holocene is based on 35 AMS age datings of organic material recovered from variable downcore depths. These range from a maximum of 13.3 cm/decade near the base of the inner shelf core (7.73–7.70 ka BP) to generally very low values (<0.11 cm/century) over the last thousand years in all cores. Over the last 6 ka there appear to have been three distinct sedimentation peaks, one between 6 and 5 ka BP, another between 4 and 3 ka PB, and one around 1 ka BP. Due to different time intervals between dates, not every peak is equally well resolved in all four cores. Based on the similar sedimentology of the inner and outer shelf cores, an essentially identical sedimentation model is proposed to have been active in both cases, albeit at different times. Thus, already during the last glacial maximum, alongshore sediment transport was deflected offshore by a change in shoreline orientation caused by the Cabo Frio structural high. The source of terrigenous material was probably a barrier-island complex that was subsequently displaced landward in the course of sea-level rise until it stabilized some 6.5 ka BP along the modern coast.

PLANKTONIC FORAMINIFERAL VARIATIONS IN THE SOUTHWESTERN ATLANTIC SINCE THE LAST GLACIAL–INTERGLACIAL CYCLE

ABSTRACT Relative abundances of planktonic foraminiferal species from two cores from the subtropical, southwestern Atlantic indicate changes in oceanic conditions during the last glacial–interglacial cycle. During interglacial intervals (biozone X or Marine Isotopic Stage 5 (MIS-5) and biozone Z or MIS-1), the relative abundance of the intermediate water-dwelling species of menardiform plexus was high, whereas those of deep-dwelling species Globorotalia truncatulinoides and Globorotalia inflata were relatively low, suggesting high temperature conditions (≥22°C) and/or increased upper-water stratification during this period of time. In contrast, the absence of menardiform plexus and high abundances of cold-water species (G. truncatulinoides and G. inflata) during the glacial interval (biozone Y [MIS-4, MIS-3, and MIS-2]) suggest cold-water conditions (≤22°C) and/or a reduction of upper-water stratification. Two intervals of moderate temperature and/or low salinity during the last glaciation, however, are suggested by the increase in abundance of Pulleniatina plexus. Foraminiferal fauna have suggested a difference of 1 to 2°C between the last glacial interval and the late Holocene. Millennial-scale paleoceanographic events have been identified during the last interglacial interval (MIS-5), suggesting that warm conditions and/or a stratified water column were replaced by short intervals of cooler water and reduced upper-water stratification, as indicated by changes in abundances of menardiform plexus and G. truncatulinoides. The short interval during which menardiform plexus disappeared during the Holocene suggests a temperature decrease and can be related to the widespread stadial event in the Holocene at 8.2 ka.

Radiocarbon measurements of stromatolite heads and crusts at the Salgada Lagoon, Rio de Janeiro State, Brazil

Abstract In this work, we prepared and measured some stromatolite carbonate samples, from Salgada Lagoon, Rio de Janeiro, Brazil. Stromatolites are bio-sedimentary, laminated, carbonate structures produced by sedimentary, chemical and biological processes related to the development and growth of microbial benthic communities, mainly dominated by blue algae and cyanobacteria. These structures are present in the geological record in rocks older than 3.0 billion years and have been used to study the origin of primitive life and variations in past environmental conditions. Detailed AMS measurements were performed at PRIME Lab (Purdue Rare Isotope Measurement Laboratory, Purdue University, IN, USA).

AMS-14C measurements for the carbonate platform of the offshore Campos Basin, Rio de Janeiro State, Brazil

Abstract As part of our accelerator mass spectrometry (AMS) program in Brazil we prepared and measured some red algae carbonate crust samples from Campos Basin, Rio de Janeiro, Brazil. The measurements were performed at Purdue Rare Isotope Measurement Laboratory (PRIME Lab), Purdue University, IN, USA. This carbonate material is interlaminated with foraminiferal lime mud reflecting recurrent intervals of carbonate development, which might be linked to outer-shelf oceanographic circulation.

Methane release from the southern Brazilian margin during the last glacial

Seafloor methane release can significantly affect the global carbon cycle and climate. Appreciable quantities of methane are stored in continental margin sediments as shallow gas and hydrate deposits, and changes in pressure, temperature and/or bottom-currents can liberate significant amounts of this greenhouse gas. Understanding the spatial and temporal dynamics of marine methane deposits and their relationships to environmental change are critical for assessing past and future carbon cycle and climate change. Here we present foraminiferal stable carbon isotope and sediment mineralogy records suggesting for the first time that seafloor methane release occurred along the southern Brazilian margin during the last glacial period (40–20 cal ka BP). Our results show that shallow gas deposits on the southern Brazilian margin responded to glacial−interglacial paleoceanographic changes releasing methane due to the synergy of sea level lowstand, warmer bottom waters and vigorous bottom currents during the last glacial period. High sea level during the Holocene resulted in an upslope shift of the Brazil Current, cooling the bottom waters and reducing bottom current strength, reducing methane emissions from the southern Brazilian margin.