Sergio Rebello Dillenburg

Beach ridges, foredunes or transgressive dunefields? Definitions and an examination of the Torres to Tramandaí barrier system, Southern Brazil

Many prograded barriers and some dunefields in the world have been termed ‘beach ridge’ plains, but the actual genesis of the ‘ridges’ is often unknown. Use of the terms, berms, beach ridges and foredunes is also confusing in the literature because their definitions are highly variable and are commonly used interchangeably. Thus, the formation and definition of sand berms, beach ridges and foredunes is briefly reviewed. Beach ridges are re-defined as entirely wave formed deposits which are most commonly formed during high wave conditions and/or elevated water levels (e.g. storm surges). Foredunes are formed by aeolian sand deposition in vegetation on the backshore. Some dunefields in Brazil have been called beach ridge plains when they are, in fact, foredune plains, transgressive dunefields, or complex barriers (i.e. barriers comprising two types of dunes). The Holocene barrier extending from Torres to Tramandai in southern Brazil has been regarded as a beach ridge plain. The landforms of this Holocene barrier comprise wide, relatively linear, widely spaced (400–600m), shore parallel ridges on the landward half, and more closely spaced (80–400m), lobate and crescentic, discrete ridges on the seaward half. Low, rolling dunefields, sand sheets, nebkha fields and deflation plains occur between the ridges. The barrier is re-interpreted as a prograded, transgressive dunefield

Barrier evolution and placer formation at Bujuru southern Brazil

Abstract This paper demonstrates the link between the evolution of a coastal barrier in southern Brazil during the Late Holocene and the formation of a large volume of eolian disseminated heavy mineral deposits. Our data set is based on an earlier heavy mineral prospecting campaign (1991) and on 10 new shallow vibrocores, 2–5 m long. The model presented has three main steps of barrier evolution. The first step is the recycling of coastal plain deposits during the Postglacial Marine Transgression, which ended at 5.6 ka when heavy minerals were incorporated into beach and washover facies of a transgressive barrier. The second step is the shoreward retreat of the barrier, under a slow and small sea-level fall, during the last 5.6 ka. This second step eroded and recycled sediments from the Pleistocene substrate, which acted as an extra source of heavy minerals. Heavy minerals were concentrated in backshore deposits by wave action during barrier recession. The third step is the erosion and transport of backshore sands by onshore winds into an inter-barrier depression in the form of transgressive dune deposits. These deposits contain an average of 4.66±1.02% disseminated heavy minerals (1494 samples). This eolian placer deposit has started to form 1 ka ago and is still under formation.

Long- and Short-Term Coastal Erosion in Southern Brazil

ABSTRACT Rio Grande do Sul is the southernmost state in Brazil. Open sandy beaches dominate the 630-km long shoreline that is 76% still undeveloped. Less than 5% of the states population (totalling 9.7 million people) live in coastal cities. However, the coastal population is growing faster than the states average since 1990. Although intense erosion is widely accepted along the beaches of Conceição lighthouse and Hermenegildo, the extent of erosion along the Rio Grande do Sul shoreline is still a controversial issue. Discussions arise from the contrasting results presented by studies addressing coastal erosion in Rio Grande do Sul. Recent DGPS monitoring indicates that about 80% of the Rio Grande do Sul shoreline is eroding; wave refraction studies indicate that it is mainly stable, and long-term coastal evolution modelling reveals a predominantly prograding shore for the last 5 ka. This work critically evaluates published data on long- and short-term causes of coastal erosion in Rio Grande do Sul, in an attempt to highlight the unanswered questions that could minimize the debate. The analysis includes sea-level rise, concentration of wave energy due to large-scale coastal topography, sand deficit as the long-term causes of erosion, storm surges, concentration of wave energy due to small-scale submerged features, interference in the longshore sediment transport, and human activities as the short-term causes. Discrepancies in shoreline change results are a matter of the temporal scale in question and what are the causes that play a significant role in it. For coastal management purposes short-time events represent a far greater hazard than long-term trends. It is therefore reasonable to state that in order to support decision-making mechanisms in Rio Grande do Sul a better understanding of the relationship of storms, sand budget, and beach erosion is necessary.

A critical evaluation of coastal erosion in Rio Grande do Sul, Southern Brazil

Evidences of coastal erosion in Rio Grande do Sul have been obtained by three methods: (a) analysis of the long-term morphodynamics and stratigraphy of coastal barriers, (b) annual shoreline mapping using the Differential Global Positioning System (DGPS), and (c) local beach profile measurements. The first method reflects coastal erosion as continuity of the geological evolution in the last 5 ka, taking place mainly along the southern half of gentle coastal projections. The second method represents a shorter temporal scale and indicates that approximately 80% of the coast is eroding. Beach profiling has been measured in very few places that are distant from each other since the early 1990s; consequently, their results reflect local and very short time shoreline behavior. A critical evaluation of published data addressing coastal erosion in Rio Grande do Sul strongly suggests that short and long term negative balance on the sediment budget is the main cause of erosion along this coastline.

Seasonal and Interannual Influences on the Patterns of Shoreline Changes in Rio Grande do Sul, Southern Brazil

Abstract Changes in shoreline positions along the Rio Grande do Sul coast, southern Brazil, are analyzed to determine the spatial and temporal variability of shoreline movements at a regional scale. Using a kinematic Differential Global Positioning System (DGPS) survey method, the 618-km long shoreline of Rio Grande do Sul has been mapped on five occasions between November 1997 and April 2002. These data show that the coastline responds differently to hydrodynamic and meteorological forcing along the three major coastal sectors. The observed differences occur especially in the magnitude of changes, in the time interval in which the shoreline returns to a previous position, and in the length scale of dominant shoreline changes. Here changes in shoreline position are examined with respect to grain size, shoreline orientation, storms, El Nino–Southern Oscillation (ENSO) events, and gradients of longshore sediment transport. Data analyses and modeling indicate that the alongshore variability in the patterns of shoreline change through time is closely related to shoreline orientation and the associated gradients of longshore sediment transport. Results indicate also that variability in wave energy and storminess influence seasonal changes, whereas ENSO events influence interannual changes. This study demonstrates the importance of understanding the drivers of shoreline change at a regional scale and has applications in studies concerned with coastal engineering and shoreline response to climate change.

The strike-fed sandy coast of Southern Brazil

Abstract The Holocene barrier along the southern Brazilian coast is most likely the most continuous barrier structure in the world. This coast is characterized by a Holocene barrier extending for 760 km, interrupted by only six discontinuities, which are associated with narrow lagoonal inlets and river mouths. This coast is marked by a huge availability of sand, a gently sloping continental shelf, and incident waves of moderate to high energy. Together, these factors constitute ideal conditions for the formation of coastal barriers. During periods of low sea-levels, the continental shelf in front of the La Plata estuary was fed by immature fluvial sands delivered by the Paraná River, which were transported to the southern Brazilian coast by a predominantly northward littoral drift. Based on forecasts of sea-level rise, it will take decades or centuries for change to occur in the progradation of the regressive barriers along the coastal embayments. During the last centuries and millennia (Middle to Late Holocene), the evolution of this coast has been controlled by the variations in wave energy along the coast.

Wave Energy and Longshore Sediment Transport Gradients Controlling Barrier Evolution in Rio Grande do Sul, Brazil

Abstract This paper aims to verify how wave energy and longshore sediment transport rates could have influenced the evolution of the Rio Grande do Sul Holocene barriers in the last 5000 y, assuming that wave climate conditions did not change much during the Middle and Late Holocene. Calculations of wave energy based on visual observations and longshore sediment transport show that both wave energy and sediment transport decrease as the coastline becomes concave (embayed) and increase as the coastline becomes convex (projected). These variations alongshore create a positive and negative imbalance, respectively, in the sediment budget. The long-term operation of these processes has produced progradational barriers in embayments and retrogradational barriers along projections. In the transition zones between embayed to convex coastlines, neither depositional nor erosional processes predominate, creating a sediment balance and producing aggradational barriers.

Detrital Minerals of Modern Beach Sediments in Southern Brazil: A Provenance Study Based on the Chemistry of Zircon

Abstract The trace element content of zircon grains from Holocene beach sands from the Rio Grande do Sul state in southern Brazil was investigated in order to discuss their probable primary source. Zircon grains from 10 samples were separated and analysed by laser ablation inductively coupled plasma mass spectrometer (LA ICP-MS) for rare earth elements (REE), niobium (Nb), uranium (U), thorium (Th), yttrium (Y), hafnium (Hf), and tantalum (Ta) and by microprobe and scanning electron microscopy (SEM) for silicon (Si), zirconium (Zr), and phosphorus (P). The results obtained are similar to those suggested by the zircon classification developed by Belousova et al. (2002, Igneous zircon: trace element composition as an indicator on source rock type. Contributions to Mineralogy and Petrology, 143, 602–622), nevertheless, additional information and a more detailed characterisation of zircon sources were yielded. The probable geotectonic setting and geochemical affinity of zircon igneous sources were predicted mainly based on thorium/uranium (Th/U), yttrium/holmium (Y/Ho), and niobium/tantalum (Nb/Ta) ratios; lanthanum (La) and niobium (Nb) contents; and REE patterns of zircons. The statistical groups identified by multivariate analysis were an additional useful tool for identifying compositional groups of zircons. Zircon grains from the studied sediments are probably derived from an association of subalkaline to alkaline granitic rocks with their mafic counterparts and metamorphic rocks of amphibolite to granulite facies. The metamorphic sources were more important in the southern sector of the studied coastal segment, whilst in the northern sector zircons from A-type granites are more abundant. The so-called Pelotas Batholith, situated in the eastern part of the Sul-rio-grandense Shield, which is composed mainly of a postcollisional granitic association of Neoproterozoic age and Paleo to Neoproterozoic high-grade metamorphic sequences, is proposed as the probable source of the studied zircons. The compositional variations of zircons from the southern to the northern sector are coherent with the hypothesis that the studied sediments came from relatively near continental source areas, probably transported by several small streams and rivers during phases of sea-level low stands. The chemical composition of zircons, particularly trace element contents and ratios, can be a powerful tool for determining the primary sources of detrital zircons.

Algal Palynomorphs Response to Environmental Changes in the Tramandai Lagoon, Southern Brazil, and Climatic Oscillations in the 20th Century

Abstract This article presents an evaluation of the importance of algal palynomorphs for palaeoenvironmental and climatic reconstructions. The palynomorphs were identified in sediments formed during the 20th century in the Tramandaí Lagoon (southern Brazil). The sediments sample corresponds to the 1905–97 period as dated by 210Pb. Five algal palynomorph zones (IA–VA) were defined based on Botryococcus ∶ and Pediastrum frequency of occurrence ratio as well as on the presence of other freshwater algal palynomorphs. The sensitiveness of algal Chlorophyta palynomorphs to environmental variables, such as temperature and salinity, reveals that these palynomorph zones are related to five major stages of the Tramandaí Lagoons evolution, reflecting variations in the regional hydrological cycle. The last identified stage (VA) is characterized by a notable increase in marine algae and a decrease in freshwater algal palynomorphs. This marine algal palynomorph enrichment was caused by an increase in sea water influx as a result of the construction of an artificial channel connecting the lagoon with the Atlantic Ocean. The obtained results on algal palynomorph frequencies of occurrence were compared with available records of water-level measurements from the coastal Mirim Lagoon, situated relatively close to the Tramandaí Lagoon, and to the Southern Oscillation Index of the Australian Bureau of Meteorology. Wetter/drier periods of the past century as revealed by algal palynomorph frequencies generally coincide with low-frequency water-level oscillations in the Mirim Lagoon and are probably related to interannual variations of rainfall in the region, driven by the variability of the El Niño–Southern Oscillation phenomena.

The sea-level highstand correlated to marine isotope stage (MIS) 7 in the coastal plain of the state of Rio Grande do Sul, Brazil.

The coastal plain of the state of Rio Grande do Sul, in southern Brazil, includes four barrier-lagoon depositional systems formed by successive Quaternary sea-level highstands that were correlated to marine isotope stages (MIS) 11, 9, 5 and 1, despite the scarcity of absolute ages. This study describes a sea-level highstand older than MIS 5, based on the stratigraphy, ages and fossils of the shallow marine facies found in coastal barrier (Barrier II). This facies outcrops along the banks of Chuí Creek, it is composed of fine, well-sorted quartz sand and contains ichnofossils Ophiomorpha nodosa and Rosselia sp., and molluscan shells. The sedimentary record indicates coastal aggradation followed by sea-level fall and progradation of the coastline. Thermoluminescence (TL) and electron spin resonance (ESR) ages from sediments and fossil shells point to an age of ∼220 ka for the end of this marine transgression, thus correlating it to MIS 7 (substage 7e). Altimetric data point to a maximum amplitude of about 10 meters above present-day mean sea-level, but tectonic processes may be involved. Paleoceanographic conditions at the time of the highstand and correlations with other deposits in the Brazilian coasts are also discussed.