João Wagner Alencar Castro

Sea-level fluctuations and coastal evolution in the state of Rio de Janeiro, southeastern Brazil

The present paper aims to investigate the relative sea-level and the coastal evolution during the Holocene in the Rio de Janeiro coastline, based on geological and biological indicators. Using topographic survey, excavation and coring, and 14C dating of these coastal deposits and beachrocks outcrops, we have reconstructed a sea-level curve for the Holocene. For the first time on the Brazilian coast it was identified a negative record of relative sea-level during Late Pleistocene and Early Holocene transition. After the transition, a relatively rapid increase of sea-level began. At approximately at 8500 cal yr BP, the sea-level was 0.5 m below the current level, was overtaken for the first time in the Holocene, at approximately 7500 cal yr BP. The maximum level of +2.5 m was reached between 4770 and 4490 cal yr BP. At the point of maximum transgression, the sea-level began a general behavior of lowering until the present. These results confirm other data already obtained elsewhere along the Atlantic coast of South America. The results of this study are consistent with previous researches and they help to refine the Holocene sea-level record along the Brazilian coast.

The Morphodynamics Behaviour of a Cross-Shore Sandbar in a Microtidal Environment, Anjos Cove, Arraial do Cabo, Rio de Janeiro – Brazil

The nearshore environment is a very dynamic and complex system with interaction of waves, currents and sediments movement on the inner continental shelf. Some of the most common morphological features are the cross-shore sandbars. The present work investigate the evolution of an offshore sandbar located off Anjos cove, Rio de Janeiro, within a time span of 55 years, based on 6 bathymetric surveys and the sediment distribution analysis. The bathymetric data were used to evaluate the morphodynamic evolution and to calculate the sediment volume changes. The morphology of studied sandbar area extents up to 2002 m, with a length of 1052 m and average depth between - 1.0 to - 2.0 m, and usually increase both volume and extension in the southwest direction. The difference in area and volume during the studied period was 310,851 m2 and 1157,772 m3 respectively. These results show that the cross-shore sandbar migrated preferable shoreward during moderate to severe storm conditions from the northeast quadrant. The northeast waves are responsible for the sediment transportation and deposition in the sandbar environment. These conditions explain the low rate, 1.72 m/years, of migration in the cross-shore sandbar studied. This rate is lower than usual rates reported to microtidal environments. The morphological feature studied has the same direction northeast - southwest of the dunefields located in the region of Arraial do Cabo and Cabo Frio. Along the years, if the same deposition condition is preserved, it will be expected the formation of a barrier island in the sandbar area.

Comments to Angulo et al. 2016 on “Sea-level fluctuations and coastal evolution in the state of Rio de Janeiro, southeastern - Brazil” by Castro et al. 2014

Interpretation of sea-level indicators is essential when studying paleo sea-level fluctuations during the Holocene. Sea-level indicators may have different origins, such as geological (beachrocks) and biological (vermetids and barnacles). In order to reconstruct paleo sea-level, it is necessary to attribute an indicative meaning to each sea-level indicator. This paper aims to discuss issues raised by Angulo et al. (2016) regarding to the sea-level fluctuations curve proposed by Castro et al. (2014) to the Rio de Janeiro State coast, Brazilian southeast. The key issue that deserves posing is that local or regional curves cannot be built based on large scale (global) RSL geophysical models even in places of steady crust like Brazil. Here, we put into question the relative sea-level fluctuation curve model proposed by Angulo et al. (2006, 2016) to the coast of Rio de Janeiro State and Pernambuco State. It is strengthened the proposal of using different origins indicators on RSL vertical variation, georeferenced by high precision altitude GPS, adjusted by Brazilian Geodetic System benchmarks, maintained by the Brazilian Institute of Geography and Statistics - IBGE. All issues regarding the curve drawn by Castro et al. (2014) are answered based on field data, laboratory analytical techniques, radiocarbon dating as well as relevant literature.

Holocene Shell Accumulations from the Cabo Frio Coastal Plain, Southeastern Brazil: Taxonomy, Taphonomy, Geochronology and Paleoenvironmental Implications

Abstract. The shell accumulations produced during the Holocene transgression are recorded along the entire coastal zone of South America and allow for the correlation of different areas. In order to obtain information about taxonomy, taphonomic processes and paleoecology, the mollusk faunas from six sites on the coastal plain of the Cabo Frio area, southeastern Brazil, were analyzed. These analyses were integrated with the sea level and paleoenvironmental changes in the area. The taxonomic analysis identified 42 mollusk species, of which 25 were bivalves, 15 gastropods and two scaphopods. The taphonomic analysis showed two types of assemblages, an autochthonous one, formed in anoxic hypersaline conditions, and an allochthonous one, characterized by a shallow marine environment. The radiocarbon dating of the shells revealed ages between 6260 and 5819 cal. years BP. The cluster analysis showed four faunal associations: Anomalocardia brasiliana, Tivela mactroides, Neritina virginea and Pitar fulminatus. The integration of mollusk associations and taphonomic signatures allowed the authors to distinguish two paleoenvironments: 1) a marine restricted one (6260–5950 cal. yearss BP) dominated by the Anomalocardia brasiliana association, with a low degree of abrasion and fragmentation suggesting a low-energy environment; and 2) a nearshore paleoenvironment (5950–5819 cal. years BP) dominated by the Tivela mactroides, the Neritina virginea/Olivella and the Pitar fulminatus associations, with a high degree of fragmentation and disarticulation denoting a high wave energy environment and marine currents.

A importância da preservação das acumulações bioclásticas da planície costeira do rio Una, municípios de Cabo Frio e Armação dos Búzios, RJ, Brasil

In the coastal plain of the Una river, county of Cabo Frio and Armacao dos Buzios, are found layers of shells of great lateral extent, related to the latest Holocene marine transgression that occurred approximately 5.000 years B.P. The aim of this study is to characterize the distribution of the bioclastic accumulations in this region, for palaeoenvironmental studies and identification of variation of relative sea level. In this area has been found so far six locations where outcrop bioclastic layers composed primarily of molluscs: outfall of the Una river; drainage channel of the Marina Porto Buzios; Taua Reserve; Condominium Portal de Buzios;Campos Novos; Araca Farm. The deposit Taua Reserve is a Site of Geology and Paleontology with description approved by SIGEP, and is considered one of the most important records during the Holocene marine transgression across the State of Rio de Janeiro. However, other deposits found in the region have special features that refer equal value, mainly due to differences regarding the taxonomic composition and habit of life and age of the mollusc deposits. Therefore, the preservation of these deposits is extremely important, because they are source material for taxonomic, palaeoenvironmental, taphonomic and geochronological studies. Because they are considered biological indicators of change in relative sea level, also have a great importance for the didactic and scientific research related to this topic. Comparative studies involving aspects of taxonomic, paleoecological, taphonomic and geochronological deposits in the coastal plain of the river Una are underway and results will contribute for understanding the processes palaeoenvironmental and change in relative sea level in the region of Cabo Frio.