Title
Geoacoustic character, sedimentology and chronology of a cross-shelf holocene sediment deposit off cabo frio, Brazil (southwest Atlantic Ocean)
Date Issued
22 April 2014
Access level
metadata only access
Resource Type
journal article
Author(s)
Neto A.
Abuchacra R.
Barbosa C.
Figueiredo A.
Gomes M.
Belem A.
Capilla R.
Albuquerque A.
Universidade Federal Fluminense
Publisher(s)
Springer Verlag
Abstract
The Cabo Frio region in the state of Rio de Janeiro, southeast coast of Brazil, is characterized by a local coastal upwelling system and converging littoral sediment transport systems that are deflected offshore at Cabo Frio, as a consequence of which a thick cross-shelf sediment deposit has developed over time. To investigate the evolution of this muddy deposit, geophysical, sedimentological and geochemical data from four sediment cores (3.8–4.1 m in length) recovered in water depths between 88 and 141 m were analyzed. The high-resolution seismic data show variable sediment thicknesses ranging from 1 to 20 m, comprising two sedimentary units separated by a high-impedance layer at a depth of about 10 m below the seafloor at the coring sites. According to the available age datings, the upper sedimentary unit is late Pleistocene to Holocene in age, whereas the lower unit (not dated) must, by implication, be entirely Pleistocene in age. The boomer-seismic reflection signal can be divided into three echo-types, namely transparent (inner shelf), stratified (middle shelf) and reflective (outer shelf), each type seemingly related to the local sediment composition. The upper 4 m of the upper sedimentary unit is dominated by silty sediment on the middle shelf, and by upward-fining sediments (silty sand to sandy silt) on the inner and outer shelf. The downcore trends of P-wave velocity, gamma-ray density and acoustic impedance are largely similar, but generally reversed to those of water and organic carbon contents. Total organic carbon contents increase with decreasing mean grain size, periodic fluctuations suggesting temporal changes in the regional hydrodynamics and primary productivity fuelled by the local upwelling system. The reconstruction of sedimentation rates in the course of the Holocene is based on 35 AMS age datings of organic material recovered from variable downcore depths. These range from a maximum of 13.3 cm/decade near the base of the inner shelf core (7.73–7.70 ka BP) to generally very low values (0.11 cm/century) over the last thousand years in all cores. Over the last 6 ka there appear to have been three distinct sedimentation peaks, one between 6 and 5 ka BP, another between 4 and 3 ka PB, and one around 1 ka BP. Due to different time intervals between dates, not every peak is equally well resolved in all four cores. Based on the similar sedimentology of the inner and outer shelf cores, an essentially identical sedimentation model is proposed to have been active in both cases, albeit at different times. Thus, already during the last glacial maximum, alongshore sediment transport was deflected offshore by a change in shoreline orientation caused by the Cabo Frio structural high. The source of terrigenous material was probably a barrier-island complex that was subsequently displaced landward in the course of sealevel rise until it stabilized some 6.5 ka BP along the modern coast.
Start page
297
End page
314
Volume
34
Issue
4
Language
English
OCDE Knowledge area
Geoquímica, Geofísica
Scopus EID
2-s2.0-85027918788
Source
Geo-Marine Letters
ISSN of the container
02760460
Sponsor(s)
This work was funded by the Geochemistry Network of PETROBRAS/CENPES and the Brazilian National Petroleum and the Biofuels Agency (ANP). We thank Dr. Marcio Gurgel for collecting the samples. We also wish to acknowledge constructive reviews by R. Gyllencreutz and T. Mulder as well as additional comments by the editors, all of which helped improve the final paper.
Sources of information:
Directorio de Producción Científica
Scopus