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

Abstract

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 14 C dating of these coastal deposits and beachrocks outcrops, we have reconstructed a sealevel 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 sealevel 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.

Holocene paleo-sea level changes along the coast of Rio de Janeiro, southern Brazil: Comment on Castro et al. (2014)

“…As a result, their final paleo-sea level curve is not a relative curve but a curve showing the altitude of the paleosea level indicators (vermetids and beachrock shells). Castro et al (2014) postulate that one of the greatest challenges in generating an RSL curve is defining the altitude of the biological and geological samples in reference to "zero" (i.e., the current mean sea-level). It has been well known since the seminal papers of Hartt (1870) and Branner (1902Branner ( , 1904 that paleo-sea level reconstructions must consider the vertical distance between a given indicator and its present homologous one.…”

“…Martin et al 1982, Van de Plassche 1986, Pirazzoli 1996. Castro et al (2014) also postulate that several investigations conducted on the South American coasts regarding relative sea-level variations have not clearly or precisely defined the reference level of the samples used in the vertical plane. This is true because it is not necessary.…”

“…To use the beachrock, or any coastal sedimentary deposit, as an indication of paleo-sea levels demands knowledge of its sedimentary facies association. No information about the beachrock facies is provided by Castro et al (2014). The authors only inform that the sedimentological and stratigraphic characteristics of the beach environment, which include a tabular morphology with smooth dips below >3° and a micro-tidal regime with variations of <1.0 m on the coast of the state of Rio de Janeiro, confer accuracy to these rocks as indicators of the mean sea-level.…”

“…Stoddart and Cann 1965, Hopley 1986, Turner 2005, Vousdoukas et al 2007. One of the samples used by Castro et al (2014) to build their curve corresponds to mollusk shell fragments from Jaconé beachrock described and dated by Mansur et al (2011). After analyzing the beachrock facies, Mansur et al (2011) concluded that the dated shells fragments were deposited at the lower part of the beach face at a time when sea-level was a little lower than the present one.…”

“…This works aims to comment and provide alternative interpretations to the data used by Castro et al (2014) to discuss the behavior of the paleo-sea level at Rio de Janeiro coast during the Mid to Late Holocene. Castro et al (2014) paper presents new relevant information to aid with Holocene relative sealevel (RSL) reconstructions along the coast of Rio de Janeiro. The data set is extensive (29 listed indicators, out of which 14 were used in the sea level curve) and from different sources.…”