Search

A combined analysis of sedimentological, molecular, and microfossil proxies was used to determine paleoclimatic and paleoceanographic variations and assess sediment transport pathways along the last c.a. 6300 years in a sedimentary record collected on the inner shelf of the southwestern Atlantic Ocean. The combined use of terrestrial and marine proxies enables a more complete environmental perspective of the La Plata River plume influence on the southwestern Atlantic shelf, regarding the relationship between climate, productivity, as well as hydrodynamic. Variations in marine productivity, lipid biomarkers of terrestrial plants, diagnostic geochemical indices, and sea-surface temperature point to three phases within the core. The first interval (6300–5200 cal yr BP) was characterized by low marine and terrigenous organic matter input, with pulses of phytodetritus indicated by benthic foraminifera. These features suggested dry climatic conditions over the southeastern portion of South America, with the influence of sediments from the adjacent continent and, on a smaller scale, transported from the Argentinian shelf due to the high relative sea level. The second interval (5200–2800 cal yr BP) was distinguished by a slight increase in marine productivity and terrigenous input, indicating a transition period with an increase of total rainfall influenced by an intensified South American Monsoon System (SAMS) and the modern El Niño-Southern Oscillation (ENSO) phenomenon. These features, associated with the changes in wind patterns and the lower relative sea level, suggest the onset of the influence of the La Plata River (LPR) plume over the southern regions of the Brazilian continental shelf for the last 6300 cal yr BP. The last interval (2800–890 cal yr BP) had the highest contents of marine and terrigenous proxies, suggesting less climatic variability, as the long-term intensification of the SAMS and a further increase in the amplitude of the ENSO resulted in the modern establishment and dominant influence of the LPR plume on the core site. Variations in sea-surface temperature up to 3.3°C were related to the influence of the LPR plume on the shelf off southern Brazil. [ABSTRACT FROM AUTHOR]

Foraminiferal morphogroups were studied in 5 cores (ranging from 1.64 m to 4.50 m long) from a semi-confined bay, at depth between 2 m and 6 m. Except at the base of the longest cores where the sands are almost azoic with only rare typical shelf tests, the microfauna is characteristic of a coastal bay. The small sized tests found in the upper2 meters of the cores are mainly free-living, infaunal, detritivorous or herbivorous-detritivorous. Their morphology indicates a prevalence of low oxygen environments during the progressive filling of the bay during the Holocene transgression. An increase of the percentage of low oxygen and infaunal morphogroups occurs between 1.5 rn and 0.5 m. It corresponds to a change in bottom water circulation beginning about 5,000 years BP, and leading to a low oxygen period and probably to an increase of organic matter within the sediments. This change might result from an inversion of the dominant wave front direction dueto changes in atmospheric circulation. Such a phenomenon was reported in the Doce River mouth area (19 deg. N latitude), for example. Theperiod of reversed dominant wave front direction (5,100 to 3,900 years BP) observed in this region seems to be contemporaneous with the period of low oxygen conditions observed in Flamengo Bay. [ABSTRACT FROM AUTHOR]

Abstract: The benthic foraminiferal assemblages recovered from 41 surface sediment samples of the Campos Basin, southeastern Brazilian continental margin, were analyzed in order to understand their distribution patterns and ecological preferences. Living and dead specimens of benthic foraminifera were selected for identification and quantitative analysis from the 63 to 125 μm, the >125 μm and the combined >63 μm size fractions. Q-mode cluster and canonical correspondence analyses for all size fractions show the dissimilarity between the samples collected on the middle slope (Group 1: 750 to 1050 m water depth) and those collected on the lower slope (Group 2: 1350 to 1950 m water depth). Food supply, energy state (stability) at the benthic/pelagic boundary and the grain size of the substrate seem to be the most important environmental factors determining the distribution pattern of the benthic foraminiferal assemblages in the deep sea. The middle slope (between 750 and 1050 m water depth) is characterized by the dominance of different species of the genus Bolivina, Cassidulina laevigata and Globocassidulina subglobosa. The occurrence of these species in association with Cibicidoides kullenbergi, Epistominella exigua and Uvigerina proboscidea seems to be related to seasonal organic matter fluxes, relatively oxic bottom waters, strong bottom currents and sandy sediments. The lower slope (between 1650 and 1950 m water depth) is inhabited by a microfauna with different characteristics, composed of preferentially epifaunal or shallow infaunal deposit feeding species (e.g., Bolivina spp., Eponides weddellensis, Lenticulina cultrata) and suspension feeders that are adapted to oligotrophic conditions and high dissolved oxygen levels in the bottom waters (e.g., Rhabdammina spp., Rhizammina sp.). [Copyright &y& Elsevier]