Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 3 4 2006 10.5194/bg-3-467-2006 http://www.biogeosciences.net/3/467/2006/ http://www.biogeosciences.net/3/467/2006/bg-3-467-2006.html http://www.biogeosciences.net/3/467/2006/bg-3-467-2006.pdf 467 478 2006-10-26 The importance of diazotrophic cyanobacteria as primary producers during Cretaceous Oceanic Anoxic Event 2 N. Ohkouchi nohkouchi@jamstec.go.jp Y. Kashiyama J. Kuroda N O. Ogawa H. Kitazato Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho Yokosuka 237-0061, Japan Department of Earth and Planetary Sciences, University of Tokyo, Hongo Bunkyo-ku Tokyo133-0033, Japan Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku Tokyo 164-8639, Japan 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&permil;. 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&permil;, respectively, leading us to the estimation that the mean nitrogen isotopic composition of photoautotrophic cells were around +1&permil; 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. Altabet, M. 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