Articles | Volume 11, issue 3
Clim. Past, 11, 495–508, 2015
Clim. Past, 11, 495–508, 2015

Research article 18 Mar 2015

Research article | 18 Mar 2015

Freshwater discharge controlled deposition of Cenomanian–Turonian black shales on the NW European epicontinental shelf (Wunstorf, northern Germany)

N. A. G. M. van Helmond1, A. Sluijs1, J. S. Sinninghe Damsté2,3, G.-J. Reichart2,3, S. Voigt4, J. Erbacher5, J. Pross6, and H. Brinkhuis1,3 N. A. G. M. van Helmond et al.
  • 1Marine Palynology and Paleoceanography, Laboratory of Palaeobotany and Palynology, Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, the Netherlands
  • 2Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, the Netherlands
  • 3NIOZ, Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, the Netherlands
  • 4Institute of Geosciences, Goethe University Frankfurt, Altenhöferallee 1, 60438 Frankfurt, Germany
  • 5Bundesanstalt für Geowissenschaften und Rohstoffe, P.O. Box 51 01 53, Alfred-Benz-Haus, Stilleweg 2, 30641 Hanover, Germany
  • 6Paleoenvironmental Dynamics Group, Institute of Earth Sciences, University of Heidelberg, Im Neuenheimer Feld 234, 69120 Heidelberg, Germany

Abstract. Global warming, changes in the hydrological cycle and enhanced marine primary productivity all have been invoked as having contributed to the occurrence of widespread ocean anoxia during the Cenomanian–Turonian oceanic anoxic event (OAE2; ~94 Ma), but disentangling these factors on a regional scale has remained problematic. In an attempt to separate these forcing factors, we generated palynological and organic geochemical records using a core spanning the OAE2 from Wunstorf, Lower Saxony Basin (LSB; northern Germany), which exhibits cyclic black shale–marl alternations related to the orbital precession cycle.

Despite the widely varying depositional conditions complicating the interpretation of the obtained records, TEX86H indicates that sea-surface temperature (SST) evolution in the LSB during OAE2 resembles that of previously studied sites throughout the proto-North Atlantic. Cooling during the so-called Plenus Cold Event interrupted black shale deposition during the early stages of OAE2. However, TEX86 does not vary significantly across black shale–marl alternations, suggesting that temperature variations did not force the formation of the cyclic black shale horizons. Relative (i.e., with respect to marine palynomorphs) and absolute abundances of pollen and spores are elevated during phases of black shale deposition, indicative of enhanced precipitation and run-off. High abundances of cysts from inferred heterotrophic and euryhaline dinoflagellates supports high run-off, which likely introduced additional nutrients to the epicontinental shelf resulting in elevated marine primary productivity.

We conclude that orbitally forced enhanced precipitation and run-off, in tandem with elevated marine primary productivity, were critical in cyclic black shale formation on the northern European epicontinental shelf and potentially for other OAE2 sections in the proto-Atlantic and Western Interior Seaway at similar latitudes as well.

Short summary
Based on the chemistry and microfossils preserved in sediments deposited in a shallow sea, in the current Lower Saxony region (NW Germany), we conclude that changes in Earth’s orbit around the Sun led to enhanced rainfall and organic matter production. The additional supply of organic matter, depleting oxygen upon degradation, and freshwater, inhibiting the mixing of oxygen-rich surface waters with deeper waters, caused the development of oxygen-poor waters about 94 million years ago.