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Jens O. Herrle, Christoph Hemleben:
A new proxy to estimate orbitally-induced productivity cycles in mid-Cretaceous sediments
(Poster)

A high-resolution study, focusing on the distribution and absolute abundances of calcareous nannofossils, was carried out to improve our understanding of mid-Cretaceous orbital forcing of pale/dark sedimentary cycles and black shale sedimentation. The results of this study were compared to the distribution of benthic foraminifera, stable isotopes and magnetic susceptibility.

The studied sections of the Vocontian Basin are composed of rhythmically-alternating pale and dark marlstones with intercalated regional (Niveau Kilian) and supraregional (Niveau Paquier, Oceanic Anoxic Event - OAE - 1b) black shales of the Upper Aptian/Lower Albian.

A new nutrient index (NI), based on a principal component analysis of 279 samples from two sections, is defined by Zeugrhabdotus erectus and Discorhabdus rotatorius, interpreted as eutrophic indicators (Premoli-Silva et al., 1989), and Watznaueria barnesiae, interpreted as an oligotrophic indicator (Erba, 1992) ((Ze+Dr)/Wb+(Ze+Dr)*100). This index provides a distinct signal in contrast to abundance patterns of single taxa used in former studies.

The NI shows a strong cyclicity signal, steered by the precession. In the Upper Aptian, the pale layers indicate higher nutrient conditions of the surface-water, whereas the dark layers reflect lower nutrient conditions. The high frequency of productivity fluctuations can also be seen in the Upper Aptian black shales Niveau Kilian and Lower Albian Niveau Paquier. The pattern cannot be interpreted as a preservational effect, since the preservation of calcareous nannofossils is good in all beds. The dominance of the precession is typical for subtropical areas, and the Vocontian Basin is referred to this climatic zone in the mid-Cretaceous. Possibly, mechanisms of nutrient fluctuation reflect changes of intensity in monsoonal activity.

Based on the principal component analysis, one group of temperature-indicative species can be recognised. Warm-water conditions are characterised by a high abundance of Rhagodiscus asper, cool-water conditions by Seribiscutum spp. (Mutterlose, 1989) and Biscutum ellipticum. The distribution of temperature-related species shows cyclicity on a supraprecessional scale. Both black shales (Niveau Kilian, Niveau Paquier) indicate increasing temperature of the surface-water, becoming established at the beginning of the deposition, and a temperature decrease after the event. Oxygen isotope measurements of the same samples (bulk rock) support this climatic evolution. Furthermore, measurements of oxygen isotopes of planktic foraminifera of the Blake Nose Plateau indicate a similar temperature development as in SE France for OAE 1b of the Lower Albian. Although climatic conditions can be described as generally warm and balanced for the mid-Cretaceous, our studies show that it is characterised by temperature fluctuations on a small scale (<100ky).

References

Erba, E. 1992. Calcareous nannofossils distribution in pelagic rhythmic sediments (Aptian-Albian Piobbico Core, Central Italy). Rivista Italiana de Paleontologia e Stratigrafia, 97: 455-484.

Mutterlose, J. 1989. Temperature-controlled migration of calcareous nannofloras in the north-west European Aptian. In: J.A. Crux & S.E. van Heck (Eds). Nannofossils and their applications: 122-142.

Premoli-Silva, I., Erba, E. & Tornaghi, M.E. 1989. Paleoenvironmental signals and changes in surface fertility in mid-Cretaceous Corg-rich pelagic facies of the Fuccoid marls (Central Italy). Geobios, Memoire Speciale, 11: 225-236.

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

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