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Fabienne Giraud, Stéphane Reboulet, François Baudin, Davide Olivero, Anne-Marie Aucour:
Characterisation and significance of calcareous nannoplankton variations during an 'anoxic' event: the Breistroffer Level (Upper Albian, SE France)
(Talk)

During the Late Cretaceous anoxic events, calcareous nannoplankton record significant variations in the relative abundance of some taxa ('blooms' or very low abundances: Thomsen, 1989; Erba, 1994). According to the considered anoxic episode, the evolution model of the calcareous nannoplankton differs. The Early Aptian and Late Albian correspond to periods of significant radiation of the nannofossils. These radiations precede a global anoxic event with no significant change in the speciation rate (Bralower et al., 1993; Erba, 1994). In contrast, during the Early Albian, a significant reduction in the diversity, renewal and diversification rates of the calcareous nannoplankton is recorded during a major anoxic event (Bralower et al., 1993). A recent study (Bréheret, 1997) shows significant fluctuations in the planktonic productivity, during a similar event. These results emphasise the importance of the ecological strategies of calcareous nannofossils in the structuring of the communities in periods of major 'crises' for many groups. Thus, it appears necessary to confront the response of the calcareous nannoplankton from various anoxic events by detailed quantitative analysis.

Late Cretaceous subpelagic deposits of the Vocontian Basin (SE France) present numerous levels enriched in organic carbon (Bréheret & Crumière, 1989). This study is focused on the Breistroffer Level (Upper Albian, dispar Ammonite Zone), well studied by Bréheret (1983, 1997). This work is integrated as part of a multifield project concerning the evolution analysis of the populations between, and in correspondence with, anoxic crises of different scales and intensities. The Breistroffer Level, about 30m thick, is composed of nine laminated horizons with TOC from 1 to 2%, intercalated in marly deposits. On a global scale, this episode seems not well represented. The benthic microfauna assemblages occurred in the laminated horizons indicating suboxic or hypoxic conditions (Bréheret, 1997).

The 50 samples studied, including the Breistroffer Level itself and lower and upper sediments, were selected for calcareous nannofossils, ammonoids, ichnofossils and geochemistry (organic carbon and calcium carbonate) studies. To approach the total abundance of nannofossil species, we estimate, per sample, the density of particles in a dozen random fields of view (Pittet & Mattioli, oral comm.). The first observations carried out on 30 samples show an abundant, highly-diversified (more than 80 species) and well-preserved calcareous nannoflora. The nannofossil assemblage composition is relatively stable through the studied interval. Abundant species include Biscutum ellipticum, Watznaueria barnesiae, Discorhabdus rotatorius, Zeugrhabdotus spp. Tranolithus orionatus, and Rhagodiscus spp. Very rare nannoconids are observed. The fluctuations in abundance of abundant species display some rhythmicities below, in and above the Breistroffer Level, with opposite trends for the two dominant species, Biscutum ellipticum and Watznaueria barnesiae. Comparison of the fluctuations in total and relative abundance of Watznaueria barnesiae show that W. barnesiae is constant and its fluctuations reflect changes in other species. In this interval, CaCO₃ content fluctuates between 30% and 60%; the TOC is about 1.0%, and the HI lower than 150 at the base and the top of the Breistroffer Level, characterised by the highest values. Nannofossil assemblages with the highest abundance, highest diversification and best preservation appear more frequent with decreasing CaCO₃ content and increasing TOC content.

References

Bralower, T.J., Sliter, W.V., Arthur, M.A., Leckie, R.M., Allard, D. & Schlanger, S.O. 1993. Geophysical Monograph, 77.

Bréheret, J.G. 1983. Bull. Mus. nat. Hist. nat. Paris, C5: 113-159.

Bréheret, J.G., Soc. géol. Nord, 25, 614 p. (1997).

Bréheret, J.G. & Crumière, J.P. 1989. Geobios, 11: 205-210.

Erba, E. 1994. Paleoceanography, 9: 483-501.

Thomsen, E. 1989. Marine Micropaleontology, 15.

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