RESUMEN
Calcareous microplankton shells form the dominant components of ancient and modern pelagic sea-floor carbonates and are widely used in palaeoenvironmental reconstructions. The efficacy of these applications, however, is dependent upon minimal geochemical alteration during diagenesis, but these modifying processes are poorly understood. Here we report on new biomineralization architectures of previously unsuspected complexity in calcareous cell-wall coverings of extinct dinoflagellates (pithonellids) from a Tanzanian microfossil-lagerstätte. These Cretaceous 'calcispheres' have previously been considered biomineralogically unremarkable but our new observations show that the true nature of these tests has been masked by recrystallization. The pristine Tanzanian fossils are formed from fibre-like crystallites and show archeopyles and exquisitely constructed opercula, demonstrating the dinoflagellate affinity of pithonellids, which has long been uncertain. The interwoven fibre-like structures provide strength and flexibility enhancing the protective function of these tests. The low-density wall fabrics may represent specific adaptation for oceanic encystment life cycles, preventing the cells from rapid sinking.
Asunto(s)
Dinoflagelados/química , Fósiles , Minerales/química , Preservación Biológica , Esmalte Dental/ultraestructura , Dinoflagelados/ultraestructura , Tanzanía , Factores de TiempoRESUMEN
A comparative cathodoluminescence (CL) spectroscopic study of extraordinarily well-preserved versus diagenetically altered Turonian (â¼92 Ma before present) calcitic and aragonitic microfossils was performed to document the cathodoluminescence characteristics of two common Cretaceous carbonate producers, foraminifera and calcareous dinoflagellates. Unaltered specimens reveal a conspicuous peak in the blue CL band at ≈ 400 nm that has rarely been previously reported for biogenic carbonates. We interpret this luminescence as an indicative feature of the primary bio-mineralized shells of calcareous dinoflagellates and foraminifera. Orange luminescence as the second important CL emission band (≈ 620 nm) in calcite generally increases with diagenetic cement overgrowth and recrystallization but can also be present in unaltered material. Thus, orange CL of biogenic calcite is not an unequivocal diagenetic indicator. Accordingly, spectroscopic investigation of both the ≈ 400 and ≈ 620 nm peaks represents a more objective criterion to evaluate the degree of diagenetic alteration. The ratio of relative intensities of the blue CL versus orange CL can provide a semiquantitative measure with relative intensity ratios blue:orange >2 occurring in the least diagenetically altered microfossils. Comparison of unaltered specimens of separate species reveals elemental differences that potentially indicate species-specific biomineralization or habitats.
Asunto(s)
Carbonato de Calcio/química , Dinoflagelados/química , Foraminíferos/química , Fósiles , Minerales/química , Análisis Espectral/métodos , Color , Dinoflagelados/ultraestructura , Foraminíferos/ultraestructura , Luminiscencia , Mediciones Luminiscentes/métodos , Óxido de Magnesio/química , Compuestos de Manganeso/química , Microscopía Electrónica de Rastreo , Óxidos/química , Especificidad de la Especie , Propiedades de Superficie , Factores de TiempoRESUMEN
The course of the biotic recovery after the impact-related disruption of photosynthesis and mass extinction event at the Cretaceous-Paleogene boundary has been intensely debated. The resurgence of marine primary production in the aftermath remains poorly constrained because of the paucity of fossil records tracing primary producers that lack skeletons. Here we present a high-resolution record of geochemical variation in the remarkably thick Fiskeler (also known as the Fish Clay) boundary layer at Kulstirenden, Denmark. Converging evidence from the stable isotopes of carbon and nitrogen and abundances of algal steranes and bacterial hopanes indicates that algal primary productivity was strongly reduced for only a brief period of possibly less than a century after the impact, followed by a rapid resurgence of carbon fixation and ecological reorganization.