8th International Nannoplankton Association Conference


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Fabrizio Tremolada, Elisabetta Erba:
Late Valanginian and Early Aptian nannoconid crisis: a phytoplankton response to large igneous events?

Quantitative analysis of calcareous nannofloras was carried out on sections from the Tethyan area, to highlight major changes in abundance and composition in the Valanginian and Aptian time-intervals.

Nannofossils were quantified in smear-slides and thin-sections. On each smear-slide, at least 300 specimens were counted, whereas all the specimens present in 1mm2 were counted in each thin-section. Fluctuations in nannofossil total abundance, diversity and abundance of single taxa reflect palaeoenvironmental changes in the surface-waters. The sequence of nannofossil events is reproducible world-wide, suggesting that the variations in abundance, composition and diversity of nannofloras were provoked by global changes.

The most marked fluctuations, in both absolute and relative abundance, concern the group of calcareous nannoplankton named nannoconids. Lower Cretaceous limestones from the Tethyan area are often almost entirely composed of nannoconids. The narrow-canal forms dominate the assemblages from the Valanginian to Barremian, whereas the wide-canal specimens are scarcely represented during the Valanginian stage; they increase in abundance in the uppermost Barremian and outnumber the narrow-canal nannoconids beginning in Magnetic Chron M0. Nannoconids experienced two major crises during the Late Valanginian and Early Aptian (prior to OAE1a deposition), followed by an interval virtually barren of nannoconids. The Early Aptian 'nannoconid crisis' is preceded by a coccolith speciation and is marked by an increase in absolute and relative abundance of A. infracretacea, normal- and large-sized specimens, and R. terebrodentarius, normal- and large-sized specimens. The nannofossil assemblages of the Late Valanginian were almost entirely dominated by W. barnesiae, with a relative increase of Diazomatolithus spp.

Nannoconid crises and fluctuations in abundance, and coccolithophorid speciation, seem to be controlled by many palaeoenvironmental factors. The nannoconid crises probably were induced by changes in nutrient-content caused by Early Cretaceous volcanic activity connected with the emplacement of the Paranà Plateau (Late Valanginian) and Ontong Java and Manihiki Plateaus (Early Aptian). These volcanic and tectonic events provoked excessive CO2 levels in the atmosphere, favouring warm and humid conditions that induced an accelerated transfer of nutrients from the continents to the oceans increasing the fertility of surface-waters. Also, a nutrification event might have been directly caused by hydrothermal processes connected with the igneous activity.


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 [Division of Micropalaeontology] [Department of Geosciences] [Bremen University]

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