Oceanographical Geoquinica of sedimentary carbonates of the continental platform of the state of the Cearà and implications / Geoquimica de carbonatos sedimentares da plataforma continental do estado do Cearà e implicaÃÃes oceanogrÃficas

AUTOR(ES)
DATA DE PUBLICAÇÃO

2004

RESUMO

Eight foraminÃfera-rich sediment samples of the continental margin of Brazil, between the states of Cearà and Pernambuco and single foraminifera specimens (Amphistegina radiata, Peneropolis planatus and Globigenrinoides rubbe) of fifty sediment samples collected in the continental margin of the Ceara State were analysed for C and O- isotope. The objective were: investigate the mixtures of continental water in ocean; investigate and determining the possible δ18O of the seawater in the Ceara coast; to verify variations in the seawater temperature with depth and possible levels of apparent Oxygen consumption in the continental shelf of CearÃ. In a regional scale, sediment samples yield δ18O values between -1.3 and -0.6âPDB in samples of 31 and 490m of depth, respectively in coast of CearÃ. In coast of Rio Grande do Norte, between -1.3 and -0.7âPDB to sediments of 13 and 610m respectively. And between -0.7 and -0.4âPDB in 610 and 1145m of depth, respectively along the coast of Rio Grande do Norte and Pernambuco. These variations in δ18O values of sediments may reflect fluctuating seawater mass temperatures, reaching 21 to 20oC in samples with 31 and 490m respectively; 22 to 19oC in samples with 13 and 610m, and 19 to 18oC in samples of 610 and 1145m. Small diferences in calculate temperatures correspond to great diferences in depth (e.g. 1oC between 31 and 490m of depth at the CearÃ) may result of fact that variations in the temperature of the water masses along the Ceara area reflect the interaction of water currents with different calculated temperatures, reaching 20 to 28oC independent of depth. δ18O values in the foraminifera shells (singular species) in coast of Cearà may exibits important differences between samples collected very close one to each other (e.g. from -0.8 to -1.7âPDB at 14 and 16 m of depth respectively, in the Aracaà sector). These differences maybe the result of the interaction between continental waters and sea water in coastal sectors in continental shelf. A δ18O value of ~ 0.2âSMOW was estimated for the seawater of the continental platform in the Ceara State, and this value was obtained assuming the salinity of 35â. Regional variations in the δ13C values are higher in surface water than in deep waters (e.g. 2.5â PDB at 610m in the Rio Grande do Norte and 1.5â PDB at 10m of depth in the coast of Ceara. This may reflect that the sample of 610m depth (planktonic/bentonic: 24) contain more planktonic species than the sample of 10m depth (planktonic/bentonic: 0,11). This may reflect the enrichment in 13C of surface waters as the result of the 12C fixation in the living organism tissues, a process which turn the seawater more depleted in that isotope. A empiric relationship between the δ13C values and the apparent consumption of oxygen (AOU), the number of species in samples was also observed in continental margin of Cearà state. suggesting a close relationship between the δ13C and the organic matter decomposition in the bottom ocean (release of 12C to environment) and the respiratory function of the specimens (release of 13CO2 to environment). Different AUO levels were identified at different depths when comtrasting the δ13C values with depth. Finally, considering the composition of carbonate sediments (total sediment) of continental shelf of Pernambuco, were suggested the geochemistry of diadoxyc substitution of Ca++ for Mg++ in structure of carbonates. It were possible to detail a division in CaO/MgO rate around 15m depth, with greater values prevails in deep waters. This trend suggests that the substitution of Ca by Mg is more effective in shallow waters, or there is a differentiation of biota with predominance of Mg-calcite shells of bentonic species. In this continental shelf, observe that the Mn tenor decreases with depth, suggesting that input is continental pedogenetic and the mechanic dispersion to cause dilution in distal sediments. In Cearà shelf, despite is no great differences in species with depth, to notice that in hot waters (on to 23oC calculated) the calcites have more Mg than in deep waters, where the temperature calculates reach 20 to 23oC.

ASSUNTO(S)

waters oceanics geotecnica plataforma continental foraminÃferos plataforma continental Ãguas oceÃnicas foraminÃferos

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