Karen Badaraco Costa

Relationship between isotopic composition (Δ18O and Δ13C) and plaktonic foraminifera test size in core tops from the Brazilian Continental Margin

O tamanho de testa dos foraminiferos e uma importante fonte de variabilidade isotopica (δ18O e δ13C) em amostras de sedimento marinho comprometendo as interpretacoes paloeceanograficas. No presente estudo, avaliou-se a relacao entre o sinal isotopico medido em diferentes fracoes de tamanho de testa das especies planctonicas, Globigerinoides ruber (branca) e Globorotalia truncatulinoides (dextral) em amostras de topo de dois testemunhos localizados na Margem Continental Brasileira. Os valores de δ18O foram utilizados para estimar a profundidade de calcificacao de cada fracao de tamanho. Os desequilibrios nos valores de δ13C para cada fracao de tamanho foram estimados. Os valores de δ18O em G. ruber (branca) nao apresentaram tendencia com o tamanho sugerindo que calcifica dentro de um mesmo intervalo de profundidade (c.a. 100 m) durante a ontogenia. Os valores de δ18O em G. truncatulinoides (dextral) apresentaram aumento com o tamanho refletindo a migracao ontogenica em aguas da termoclina (250-400 m). Os valores e desequilibrios de δ13C aumentaram com o tamanho nas duas especies indicando o efeito da variacao nas taxas fisiologicas durante a ontogenia. Em G. ruber (white) os valores de δ13C dos maiores tamanhos (300µm e >355µm) refletem melhor os valores de δ13CDIC indicando que sao mais apropriados para utilizar nas reconstrucoes paleoceanograficas.

Stable isotope/test size relationship in Cibicidoides wuellerstorfi

C) measurements in foraminifera tests are widely used for paleoceanographic interpretations (WEFER et al., 1999; ROHLING; COOKE, 1999). This is attained since during calcification foraminifera registered in their tests the stable isotope signature and environmental conditions of seawater of their local of calcification in a somehow predictable way (ROHLING; COOKE, 1999). Foraminifera test size is an important source of stable isotope variability found in sediment samples. Some of this variability is associated to foraminifera physiological effects (e.g., symbiotic photosynthesis, respiration and calcification) which can vary along ontogeny and affect stable isotope measurements in foraminifera tests (ZEEBE et al., 1999). Regional environmental variables can also affect simultaneously both stable isotope values and foraminifera test size resulting in another source of test size/stable isotope variability (DEUSER et al., 1981). Although it is especially important in planktonic foraminifera (RAVELO; FAIRBANKS, 1995), some studies have found test size/stable isotope variation in benthic foraminifera (CORLISS et al., 2002). This test size/stable isotope variability can result in offsets from predicted values having important implications when choosing test size fractions for paleoceanographic interpretations (OPPO; FAIRBANKS, 1989). Cibicidoides wuellerstorfi is an epibenthic foraminifera species of great importance in stable isotopic paleoceanographic studies (MACKENSEN; LICARI, 2003). Nevertheless its paleoceanographic importance no stable isotope ( δ

Strontium isotope stratigraphy of the Pelotas Basin

Strontium isotope data were obtained from foraminifera shells of the Pelotas Basin Tertiary deposits to facilitate the refinement of the chronostratigraphic framework of this section. This represents the first approach to the acquisition of numerical ages for these strata. Strontium isotope stratigraphy allowed the identification of eight depositional hiatuses in the Eocene-Pliocene section, here classified as disconformities and a condensed section. The reconnaissance of depositional gaps based on confident age assignments represents an important advance considering the remarkably low chronostratigraphic resolution in the Cenozoic section of the Pelotas Basin. The recognition of hiatuses that match hiatuses is based on biostratigraphic data, as well as on global events. Furthermore, a substantial increase in the sedimentation rate of the upper Miocene section was identified. Paleotemperature and productivity trends were identified based on oxygen and carbon isotope data from the Oligocene-Miocene section, which are coherent with worldwide events, indicating the environmental conditions during sedimentation.