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Anais da Academia Brasileira de Ciências

versão impressa ISSN 0001-3765versão On-line ISSN 1678-2690

An. Acad. Bras. Ciênc. v.74 n.2 Rio de Janeiro jun. 2002

http://dx.doi.org/10.1590/S0001-37652002000200025 

MIDDLE CRETACEOUS PALYNOLOGICAL ASSEMBLAGES RICH IN COPEPOD EGGS: THEIR PALEOECOLOGICAL SIGNIFICANCE

MITSURU ARAI

PETROBRAS / CENPES, 21949-900 Rio de Janeiro, RJ, Brazil.

Presented by ALEXANDER W.A. KELLNER

 

Animalian palynomorphs, of the zoomorph category and considered to be copepod eggs, have been observed, often very abundant, during routine palynological investigations at PETROBRAS laboratories. Palynological assemblages exceptionally rich in these zoomorphs have been recorded recently in samples from middle Cretaceous sequences of several Brazilian basins. In some samples, copepod eggs attain proportions as high as 90% of the total assemblage, and in a few cases even reaching 100%. As copepods constitute heterotrophic zooplankton, grazing mainly on phytoplankton, one would infer that phytoplankton productivity would have been concomitantly high in order to sustain the copepod population. But, paradoxically, the assemblages very rich in copepod eggs are lacking organic-walled phytoplanktonic fossils. This may well have resulted from the copepod population undergoing a very sudden and rapid increase, thus depleting and ultimately exhausting the food supply. This could also account for the fact that the assemblages are poor even in continental palynomorphs (e.g., pollen and spores); hungry copepods are known to consume pollen grains, which are rejected under normal conditions.

Multidisciplinary analyses, involving sedimentology, geochemistry and micropaleontology, indicate that the assemblages abnormally rich in copepod eggs were probably produced by a combination of the following factors: (1) marine environment; (2) high plankton productivity; (3) low sedimentation rate; (4) low proportion of dissolved oxygen in the water; and (5) restricted basin with poorly ventilated bottom conditions. — ( December 20, 2001 ).

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