Biogenic silica and silicic acid benthic fluxes of a North-eastern Atlantic Abyssal Locality

Within the framework of the EU-funded BENGAL programme, the effects of seasonality on biogenic silica early diagenesis have been studied at the Porcupine Abyssal Plain (PAP), an abyssal locality located in the northeast Atlantic Ocean. Nine cruises were carried out between August 1996 and August 1998. Silicic acid (DSi) increased downward from 46.2 to 213 µM (mean of 27 profiles). Biogenic silica (BSi) decreased from ca. 2% near the sediment-water interface to <1% at depth. Benthic silicic acid fluxes as measured from benthic chambers were close to those estimated from non-linear DSi porewater gradients. Some 90% of the dissolution occurred within the top 5.5 cm of the sediment column, rather than at the sediment-water interface and the annual DSi efflux was close to 0.057 mol Si/m**2/yr. Biogenic silica accumulation was close to 0.008 mol Si/m**2/yr and the annual opal delivery reconstructed from sedimentary fluxes, assuming steady state, was 0.065 mol Si/m**2/yr. This is in good agreement with the mean annual opal flux determined from sediment trap samples, averaged over the last decade (0.062 mol Si/m**2/yr). Thus ca. 12% of the opal flux delivered to the seafloor get preserved in the sediments. A simple comparison between the sedimentation rate and the dissolution rate in the uppermost 5.5 cm of the sediment column suggests that there should be no accumulation of opal in PAP sediments. However, by combining the BENGAL high sampling frequency with our experimental results on BSi dissolution, we conclude that non-steady state processes associated with the seasonal deposition of fresh biogenic particles may well play a fundamental role in the preservation of BSi in these sediments. This comes about though the way seasonal variability affects the quality of the biogenic matter reaching the seafloor. Hence it influences the intrinsic dissolution properties of the opal at the seafloor and also the part played by non-local mixing events by ensuring the rapid transport of BSi particles deep into the sediment to where saturation is reached.

Diário da Constituinte [gravação de vídeo] : [programa n. 075]

Os relatores das comissões irão elaborar um novo projeto que substituirá os anteprojetos das subcomissões. Geraldo Campos (PMDB-DF), presidente da Subcomissão dos Direitos dos Trabalhadores e Servidores Públicos, espera assegurar os avanços que a sociedade reclama. A Comissão do Sistema Tributário Orçamento e Finanças reuniu-se para alterar o seu calendário. A Comissão da Organização do Estado da Assembleia Nacional Constituinte (ANC) suspendeu duas sessões para que o senador José Richa (PMDB-PR) entregue o substitutivo mais cedo. A Comissão da Família, da Educação, Cultura e Esporte, da Ciência e Tecnologia e da Comunicação discute temas relativos a questão do menor carente e dos idosos. Artur da Távola (PMDB-RJ), relator, diz que a proposta elaborada trará modernização e democratizará as relações sociais. O Instituto Tancredo Neves promove seminário sobre as Constituições estrangeiras que deram certo. Lúcio Alcântara (PFL-CE) diz que a experiência será útil. Marco Maciel (PFL-PE) observa os modelos e mecanismos usados. Três emendas populares chegam ao Congresso. Mauro Benevides (PMDB-CE) diz que a Constituição deve ser o reflexo da manifestação popular.