An importance of diazotrophic cyanobacteria as a primary producer during Cretaceous Oceanic Anoxic Event 2

N. Ohkouchi (1) , Y. Kashiyama (1, 2) , J. Kuroda (1, 3) , N. O. Ogawa (1) , H. Kitazato (1)

1 IFREE - Institute for Research on Earth Evolution [Yokosuka]
2 Department of Earth and Planetary Sciences [Kobe]
3 Ocean Research Institute

In Livello Bonarelli black shale deposited during Cretaceous Oceanic Anoxic Event 2 (OAE-2, ca. 94 Ma), nitrogen isotopic compositions of bulk sediments are 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 cell was 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 was the highest concentration. Based on these evidence, we conclude that diazotrophic cyanobacteria were major primary producers during that time. The 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.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]; Interfaces continentales, environnement; Océan, Atmosphère; Sciences de la Terre

N. Ohkouchi, Y. Kashiyama, J. Kuroda, N. O. Ogawa, H. Kitazato. An importance of diazotrophic cyanobacteria as a primary producer during Cretaceous Oceanic Anoxic Event 2. Biogeosciences Discussions, 2006, 3 (3), pp.575-605. ⟨hal-00297807⟩