José Maria Landim Dominguez

Controls on Quaternary coastal evolution of the East-Northeastern coast of Brazil : roles of sea-level history, trade winds and climate

Abstract East-northeastern Brazil has a wave-dominated, micro- to meso-tidal coast, lying entirely within the southern Atlantic trade wind belt. Integration of geologic mapping, radiocarbon dating and vibracoring data shows that the Quaternary coastal evolution of this area was controlled by three major factors: (1) sea-level history; (2) trade winds; and (3) climate change. Sea-level history. Along the east-northeastern coast of Brazil, relative sea level has fallen approximately 5 m during the last 5000 y. Correlation of this sea-level history with the evolution of beach-ridge, lagoonal and coastal plain deposits shows that: (1) sea-level rise favours the formation of barrier island—lagoonal systems and the construction of intralagoonal deltas; (2) sea-level lowering is not conductive to barrier island formation. Rather, lagoons and bays become emergent and beach-ridge plains rapidly prograde. Trade winds. Sediment dispersal systems along the coastal zone of east-northeastern Brazil have been highly persistent since Pleistocene time, as deduced from beach-ridge orientation. This persistence results from the fact that sediment dispersal in wave-dominated settings is ultimately controlled by atmospheric circulation which, for the east-northeastern coast of Brazil is associated with the South Atlantic high-pressure cell. The remarkable stability of this cell through time, has allowed the accumulation of extensive beach-ridge plains at the longshore drift sinks located along the coast. Climate change. Effects of Quaternary climate changes on coastal sedimentation are twofold. Climate changes may affect rainfall patterns, thus exerting an important control on coastal dune development. Along the coast of northeastern Brazil, active coastal dunes are only present in those areas in which at least four consecutive dry months occur during the year. Mapping of these areas has shown that dune development during the Holocene has been episodic, these episodes being probably controlled by variations in rainfall patterns associated with climate changes. Secondly, despite its overall stability, the position of the high-pressure cell has experienced small shifts in position during the Holocene in response to climate changes. Changes in wind direction associated with these shifts have induced modifications in the coastal dispersion system, which are recorded in the strandplains as small truncations in the beach-ridge alignments. These results have important implications in understanding accumulation of ancient sandstone shoreline sequences.

A 21 000 cal years paleoclimatic record from Caçó Lake, northern Brazil: evidence from sedimentary and pollen analyses

Abstract Sedimentological studies including seismic profiles, mineralogy and organic geochemistry on two cores from the center and margin of Caco Lake, Maranhao State, northern Brazil, revealed variable climatic and environmental conditions during the last 21 cal kyr BP. Between 21 and 17 cal kyr BP, during the Late Glacial Maximum, regional climate was predominantly dry, interrupted by short humid phases, as reflected by a succession of very thin layers of sand and organic matter. The Late Pleistocene climate was relatively wet as is suggested by rapid lake-level rise and forest expansion. The Late Pleistocene humid climate differed significantly from present conditions. We suggest that Late Glacial humid conditions were the consequence of intensification of the Inter-Tropical Convergence Zone or shifts of its position, resulting in Antarctic cold-front occurrences. The abrupt climatic changes during this period were marked by siderite deposition into Caco Lake, which appears to be related to regional hydrologic changes linked to global/Northern Hemisphere events. The Holocene was characterized by lower moisture availability and a distinct dry period until 7 cal kyr BP, in response to South American insolation conditions.

Patterns of sediment dispersion coastwise the State of Bahia - Brazil

Using the average directions of the main wave-fronts which approach the coast of Bahia State - coinciding with that of the main wind occurring in the area - and of their periods, we define a wave climate model based on the construction of refraction diagrams. The resulting model of sediment transport was able to reproduce, in a general way, the sediment dispersion patterns furnished by geomorphic indicators of the littoral drift. These dispersion patterns control the generation of different types of sediment accumulations and of coastal stretches under erosion. We demonstrate that the presence of the Abrolhos and Corumbaú Point coral reefs is an important factor controlling the sediment dispersion patterns, since them act as a large protection against the waves action.

The tides and tidal circulation of Todos os Santos Bay, Northeast Brazil: a general characterization

The Todos os Santos Bay, located on northeastern Brazil, is the second largest coastal bay in the country, after Sao Marcos Bay. The three main drainage basins convey an average of 120 m3/s of freshwater towards the bay, the majority of it, however, held by a dam in Paraguacu River since 1985. The original average freshwater inflow was two orders of magnitude smaller than the estimated tidal discharge through the main bay entrance, and the oceanographic characteristics of the bay, as indicated by bay salinity measurements, are clearly marine. The tides are semi-diurnal, and are amplified up the bay by a factor of 1.5. Shallow water constituents become more important as the tide propagates along Paraguacu Channel and Paraguacu River, where they generate time asymmetries that change between spring and neap tides. Currents in the bay are mainly bi-directional, and are stronger during the ebbing tide in most of the bay. Offshore, relatively strong tidal currents appear to be felt in a radius of about 10 km, where they are superimposed on winds driven currents orientated to the southwest.

Chapter 3 Geological History of Coastal Lagoons

This paper discusses the geological processes controlling the origin and evolution of coastal lagoons. The study of a lagoon is intimately connected with the barrier island enclosing it-one cannot exist without the other. Three main factors control the origin and maintenance of sandy barriers and thus determine the ultimate fate of coastal lagoons: (1) sea-level history; (2) shoreface dynamics; and (3) tidal range. The history of infilling of coastal lagoons is controlled primarily by rates of sediment supply relative to rates of sea-level rise. In areas of high relative sediment supply, such as on mesotidal coasts or where rivers empty directly into lagoons, the lagoon floor is rapidly brought to the intertidal level. Thus, sediments deposited in the intertidal zone will dominate the sedimentary record of the lagoons. In areas of low sediment supply, lagoons will be characterized by unobstructed water bodies and the lagoonal sedimentary record will be dominated by sediments deposited in sub-tidal environments. The effects of sea-level history and rates of sediment supply on the evolution and infilling of ‘fossil’ coastal lagoons along the eastern coast of Brazil are documented and discussed for three major scenarios: (1) lagoons formed on a low-relief coastal plain during a sea-level rise; (2) lagoons formed on a low-relief coastal plain during a sea-level drop; and (3) lagoons formed on a high-relief coastal plain during a sea-level rise.

Quaternary evolution of the Caravelas strandplain - Southern Bahia State - Brazil

An evolutionary model is proposed for the Caravelas strandplain. The model encompasses integration of: (i) mapping of Quaternary deposits, (ii) cartography of beach-ridge alignments and their truncations, (iii) relative sea-level history, (iv) development history of the Abrolhos coral reefs, (v) vibra-coring and (vi) C 14 dating of Quaternary deposits. Seven major evolutionary stages were identified. These stages show that the strandplain has had its Quaternary evolution strongly controlled by relative sea-level changes. In addition, the development of the Abrolhos coral reefs has also played an important role in dispersion and accumulation of sediments along the coastline, causing localized inversion in longshore sediment transport.

Wave Refraction, River Damming, and Episodes of Severe Shoreline Erosion: The São Francisco River Mouth, Northeastern Brazil

Abstract The São Francisco River mouth is undergoing a severe erosional process that has caused the destruction of a village and the partial immersion of a lighthouse constructed in 1856. The present work comprises an historic time evaluation of this phenomenon based on the analysis of the sediment dispersion patterns caused by waves along three distinct shoreline traces. For this, we define a wave climate model based on the construction of refraction diagrams, which are assumed to be valid for the three shorelines. The resulting sediment transport patterns obtained by numerical modeling from the refraction diagrams—taking into account the angle of approach and the wave height along the coast—allow us to establish a first-order correlation between coastal dynamics and erosion phenomenon. The results also suggest that a permanent reduction in the solid river discharge, in response to the construction of large dams, probably will have two direct consequences in the river mouth region: (i) a chronic shoreline erosion downdrift of the mouth and (ii) a progressive deflection of the mouth in the downdrift direction.

A reevaluation of the late quaternary sedimentation in Todos os Santos Bay (BA), Brazil

Todos os Santos Bay is a large ( 1000 km2), structurally controlled tidal bay in northeast Brazil. Three main drainage basins debouch into the bay, providing a mean freshwater discharge of 200 m3/s (prior to 1985), or less than 1% of the spring tidal discharge through the bay mouth. Based on the result of several sedimentological studies performed in the 1970s, five surface sedimentary facies were identified inside the bay, namely i) transgressive siliciclastic marine sand facies; ii) transgressive bay sand-mud facies; iii) a transgressive carbonate marine sand facies; iv) regressive bay-mud facies, and v) regressive fluvial sand facies. The spatial distribution of these facies would follow, somewhat closely, the hydrodynamic-energy distribution inside the bay. Seismic profiles along the bay bottom indicate the existence of several paleochannels, 5-10 m deep, blanketed at least by three different sedimentary units. The topmost sedimentary unit, 5-20 m thick, appears to be associated with the regressive bay-mud facies, and assuming that it was laid down within the last 5000 years, sedimentation rates for the central and northeastern part of the bay would average at 2,4 mm/y.

Integrated assessment of mangrove sediments in the Camamu Bay (Bahia, Brazil)

Camamu Bay, an Environmentally Protected Area, may be affected by the pressures of tourism and oil exploration in the adjacent continental platform. The current quality of the mangrove sediments was evaluated by porewater bioassays using embryos of Crassostrea rhizophorae and by an analysis of benthic macrofauna and its relationships with organic compounds, trace metals and bioavailability. Porewater toxicity varied from low to moderate in the majority of the samples, and polychaetes dominated the benthos. The Grande Island sampling station (Station 1) presented more sandy sediments, differentiated macrobenthic assemblages and the highest metal concentrations in relation to other stations and guideline values, and it was the only station that indicated a possible bioavailability of metals. The origin of the metals (mainly barium) is most likely associated with the barite ore deposits located in the Grande and Pequena islands. These results may be useful for future assessment of the impact of oil exploration in the coastal region.

Is Ocypode quadrata (Fabricius, 1787) a useful tool for exposed sandy beaches management in Bahia State (Northeast Brazil)?

Universidade Federal da Bahia - Instituto de Geociencias landim@ufba.br Despite the existing proposals for sustainable use of the Brazilian coast, there are few mechanisms for quantification and qualification of anthropogenic impacts. For instance, the Projeto Orla (MMA, 2006) defines typologies of coastline classification, taking into account aspects of the landscape such as originality, human population density and levels of occupation. Nevertheless, the effects of urbanization on resident biota are not considered in this classification. Furthermore, there is a need for studies to validate the use of biological indicators that take into account the spatial and temporal variation of the biota, so as to arrive at a complete assessment of the urbanization impacts on exposed sandy beach. The genus