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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 3, issue 4 | Copyright

Special issue: SPOT-ON: Recent advances in the biogeochemistry of nitrogen...

Biogeosciences, 3, 467-478, 2006
https://doi.org/10.5194/bg-3-467-2006
© Author(s) 2006. This work is licensed under
the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.

  26 Oct 2006

26 Oct 2006

The importance of diazotrophic cyanobacteria as primary producers during Cretaceous Oceanic Anoxic Event 2

N. Ohkouchi1, Y. Kashiyama1,2, J. Kuroda1,3, N O. Ogawa1, and H. Kitazato1 N. Ohkouchi et al.
  • 1Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho Yokosuka 237-0061, Japan
  • 2Department of Earth and Planetary Sciences, University of Tokyo, Hongo Bunkyo-ku Tokyo133-0033, Japan
  • 3Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku Tokyo 164-8639, Japan

Abstract. In Livello Bonarelli black shale deposited during Cretaceous Oceanic Anoxic Event 2 (OAE-2, ca. 94 Ma), nitrogen isotopic compositions of bulk sediments are mostly in a narrow range from –2.7 to –0.7‰. We also determined molecular distribution and nitrogen isotopic compositions of geoporphyrins extracted from the black shale. The nitrogen isotopic compositions of C32 Ni deoxophylloerythroetioporphyrin (DPEP) and total Ni porphyrins are –3.5 and –3.3‰, respectively, leading us to the estimation that the mean nitrogen isotopic composition of photoautotrophic cells were around +1‰ during the formation of Bonarelli black shale. This value is suggestive of N2-fixation, a dominant process for these photoautotrophs when assimilating nitrogen. Furthermore, Ni-chelated C32 DPEP, derived mainly from chlorophyll a had the highest concentration. Based on this evidence, we conclude that diazotrophic cyanobacteria were major primary producers during that time. Cyanobacteria may be key photoautotrophs during the formation of black shale type sediments intermittently observed throughout the later half of the Earth's history, and hence may have played a crucial role in the evolution of geochemical cycles even in the later half of the Earth's history.

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