Claudia Gutterres Vilela

Sea-level –and gas-hydrate–controlled catastrophic sediment failures of the Amazon Fan

The architecture and Quaternary history of the massive and highly structured Amazon Fan has been reconstructed using sediment recovered by Ocean Drilling Program Leg 155. Huge regional mass-transport deposits make up a significant component of the Amazon Fan. These massive mass-transport deposits each cover an area over 15,000 km{sup 2} (approximately the size of Jamaica), reach a maximum thickness of 200 m, and consist of {approx}50,000 Gt of sediment. Analysis of both benthic foraminiferal fauna and the sediments indicates that the mass-transport deposits originated at a water depth of between 200 and 600 m on the continental slope, which is at least 200 km lateral to and 1500 m above their present position. Each mass-failure event was formed by the catastrophic failure of the continental slope and has been dated and correlated with climate-induced changes in sea level. Two different mechanisms initiated these catastrophic slumps: (1) Rapid drops in sea level destabilized continental slope gas hydrate reservoirs, causing slope failure and the glacial mass-transport deposits, and (2) deglaciation of the Andes and the consequent flushing of Amazon River sediment to the continental slope caused over-burdening and the deglacial mass-transport deposits.

Benthic foraminifera distribution in high polluted sediments from Niterói Harbor (Guanabara Bay), Rio de Janeiro, Brazil

Dockyards and harbors are recognized as being important locations where sediment-associated pollutants can accumulate, which constitutes an environmental risk to aquatic life due to potential uptake and accumulation of heavy metals in the biota. The aim of this paper is to assess the concentrations and the effects of some heavy metals in the benthic foraminifera assemblage in Niterói Harbor. Low concentrations in the benthic foraminifera as well as the dominance of indicative species such as Ammonia tepida, Buliminella elegantissima and Bolivina lowmani can be associated with an environment under stress. In addition, the occurrence of test abnormalities among foraminifera may represent a useful biomarker for evaluating long-term environmental impacts in a coastal region.

Concentration and Bioavailability of Heavy Metals in Sediments from Niterói Harbour (Guanabara Bay/S.E. Brazil)

Abstract The coastal area of Niterói is marked by intensive naval activities and holds one of the countries main naval estates. The harbour and dockyards have been sited in the international literature as potential sources and sinks for accumulation of heavy metals and hence contaminated sediments. The aim of this paper is to assess the concentrations of heavy metals in Niterói Harbour and verify their bioavailability by determining the catalytic effects of the bacteria using esterase activity (EST) and electron transport system activity (ETSA). Samples were analysed for Ni, Zn, Pb, Cr and Cu and normalisation procedures were used to assess whether their concentrations represent background or contamination of the sediment. Heavy metal concentrations, especially for Cu, Zn and Pb were found to be much higher than natural background levels and the index of “geoaccumulation” shows moderate to extreme contamination. However, the absence of the inhibition of dehydrogenase activity indicated that the analysed heavy metals are not bioavailable in the EC50 values.

Benthic foraminifera distribution in a tourist lagoon in Rio de Janeiro, Brazil: a response to anthropogenic impacts.

Rodrigo de Freitas Lagoon, located in the Rio de Janeiro City, receives several types of polluted discharges. The knowledge of the sediment microfauna correlated with heavy metal and organic matter concentrations could supply important data about the conditions of the lagoon. The benthic foraminiferal assemblage presented larger diversity and more abundant samples in the lagoon entrance than in the inner area. The Ammonia tepida - Elphidium excavatum foraminiferal assemblage is characterized by dwarf, corroded and weak organisms. Agglutinated species were found only near the entrance. Low abundance values and sterility of five samples in the inner area (north/northeast) can be caused by high levels of heavy metals and organic matter. A. tepida shows negative correlation with increasing heavy metals values. PAHs and coprostanol high indexes, and the absence or low presence of microfauna in samples around the lagoon margin confirm illegal flows from gas stations and domestic sewage.

TAPHONOMY OF BENTHIC FORAMINIFERAL TESTS OF THE AMAZON SHELF

Benthic foraminifera of box core and piston core samples were collected from the Amazon middle to outer shelf. The assemblages and their taphonomic features confirmed a record of oceanographic changes from the early Holocene to the present. Foraminiferal data in conjunction with sedimentological, stratigraphic, seismic, and isotopic results point to three contrasting depositional environments. The first environment is that of a fluid mud with silt and organic laminations. A seismic discontinuity was observed and interpreted as an erosional surface between the fluid mud, and an underlying much denser mud. This fluid mud, deposited during the last 100 years, shows a very high sedimentation rate, with the erosional phase lasting for 700 years. The characteristic species of the dominant Ammonia beccarii Pseudononion atlanticum assemblage are opportunistic at the sediment-water interface. At deeper substrate intervals, the assemblage can be matched with that at the sediment-water interface, but they also show taphonomic effects of reworking and erosion. The second environment is characterized by relict sediment, which includes an older suite of foraminifera mixed with modern tests. This assemblage differs from the recent one by including large relict species dominated by Quinqueloculina bicostata, Q. lamarckiana, Amphistegina lessonii and Eponides repanda. Two hypotheses account for the relict assemblage: periods of low sea-level and mixing with younger sediments in deeper water. The third environment is in front of the Amazon River mouth. The cores taken here contain abnormal, large tests of Flintina bradyana. The strong currents and tides of the Amazon shelf could cause large tests to move along the seabed by bed-load transport processes. In living foraminifera, such tests may break and later regenerate abnormally. Such occurrences vary stratigraphically and geographically, demonstrating hydrodynamic variations in time and space.

Ecology of Quaternary benthic foraminiferal assemblages on the Amazon shelf, northern Brazil

The distribution of benthic foraminiferal microfauna recovered from two offshore transects on the Amazon shelf was studied. Eight box-core samples were collected along the inner, middle, and outer parts of the continental shelf. Each box-core sample was divided into 10 subsamples, of 1- to 5-cm intervals, in order to investigate the foraminiferal microhabitat preferences. The variations of salinity, oxygen, lithology, and sediment accumulation rates were used to evaluate ecological patterns. Four benthic foraminiferal assemblages identified in the study area characterize a record of oceanographic changes from early Holocene to the present. An abundant assemblage, with opportunistic species, was found near the river mouth. Two assemblages were identified northwest of the river mouth, with dominant epifaunal and infaunal species, respectively. A relict assemblage was encountered northwest of the river mouth.

Mid- to Late-Holocene estuarine infilling processes studied by radiocarbon dates, high resolution seismic and biofacies at Vitoria Bay, Espirito Santo, Southeastern Brazil

Vitoria Bay is a 20 km long estuary, morphologically narrow, with a microtidal regime and, as other modern estuaries, was formed during the last post-glacial transgression. The estuarine bed morphology is characterised by a main natural channel limited by tidal flats with developed mangroves. Original radiocarbon dates were obtained for the site. Five radiocarbon ages ranging from 1,010 to 7,240 years BP were obtained from two sedimentary cores, which represent a 5 m thick stratigraphic sequence. The results indicate that, until about 4,000 cal. yrs BP, environmental conditions in Vitoria Bay were still of an open bay, with a free and wide connection with marine waters. During the last 4,000 yrs, the bay has experienced a major regression phase, by becoming more restricted in terms of seawater circulation and probably increasing tidal energy. Three main stratigraphic surfaces were recognised, which limit trangressive, trangressive/highstand and regressive facies. The present channel morphology represents a tidal scouring surface or a tidal diastem, which erodes and truncates regressive facies bedding. Foraminiferal biofacies, which change from marine to brackish and mangrove tidal-flat environments, support the seismic stratigraphic interpretation. Absence of mangrove biofacies at one of the two cores is also an indication of modern tidal ravinement.

Palynological analysis of a sediment core obtained in Guanabara Bay, Rio de Janeiro, Brazil

The sediment of a core collected about 2 km north of Paquetá Island, Guanabara Bay, was submitted to pollen analysis, in order to recognize the dynamics of the regional vegetation, and the anthropic influence. Radiocarbon dating of a sample next to the bottom of the core indicates an age of 4.210+/-40 14C yrs B.P (calibrated age). It was possible to establish four palynological zones. Starting at the bottom of the core, a decrease in palynomorph concentration, the presence of degraded pollen grains and spores, and the predominance of ombrophilous forest pollen grains were observed in the basal portion of Zone I. These data may indicate the presence of an exuberant Atlantic Forest, dominated by a marine regressive event. The concentration of well-preserved palynomorphs increased in Zone II, with the predominance of the ombrophilous forest vegetation also, and an expressive increase of hygrophytes, indicating more humid environmental conditions. Palynomorph concentration decreased again toward the top of Zone III, and the field vegetation was predominant. In the upper Zone IV occurred a strong decrease in pollen and spore concentration, with predominance of field vegetation also, and the appearance of exotic pollen grains, showing the anthropic influence at this time.

A mid-Holocene vegetational and anthropogenic record in the Guanabara Bay region, Rio de Janeiro State, SE Brazil, assessed by palynological and charcoal data

Abstract Palynological and charcoal fragment analyses of Guanabara Bay sediments, as well as radiocarbon dating, were carried out on one 220 cm long sedimentary core collected from the northeast sector of the bay, near the São Gonçalo coast. This study aims at recognising and explaining the environmental history of this region during the mid-Holocene, and to identify the anthropogenic influences on it. The palynological data indicate the predominance of ombrophilous forest vegetation in the Guanabara Bay Hydrographic Basin at c. 6500 calendar years bp. During this period (pollen zone 1), the concentrations of pollen grains and spores may still have been controlled by the Holocene Maximum Transgressive Event. After a phase of low concentrations of palynomorphs, there was a considerable increase in the accumulation patterns of pollen and spore grains (pollen zone 2). This increase can probably be related to the retreat of the sea level event after the Holocene Maximum Transgressive Event. The 75 cm of the core top (pollen zone 3) provide clear evidence of human influence in the area, which is inferred from the significant reduction in ombrophilous forest pollen grains, the significant increase in herbaceous pollen grains, the presence of exotic pollen types (Eucalyptus and Pinus), and high concentration of carbonaceous particles.

Evolução Paleogeográfica da Planície Costeira da Praia Vermelha, entrada da Baía de Guanabara, Rio de Janeiro, por meio de registros palinológicos

Peat sediments were observed during high tide occasions in the Praia Vermelha beach, urban area of the Rio de Janeiro city. They were studied to investigate possible environmental variations during the Quaternary Period. Palynological analysis was carried out comprising a time interval from ca. 4.520 A 80 to 4.270 A 60 years B.P. During this time the results obtained indicate that a restinga forest vegetation was sequentially dominated by a restinga bog vegetation. A great number of spores presenting corrosion/degradation damage, typical characteristics of exposition to the air, indicate that low quantity of water was disposable occasionally.