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

Abstract

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 C 32 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 N 2 -fixation, a dominant process for these photoautotrophs when assimilating nitrogen. Furthermore, Ni-chelated C 32 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.

The evolution of Earth's biogeochemical nitrogen cycle

“…This hypothesis is supported by the canonical view that the final step in the oxygenation of Earth"s surface occurred in the Neoproterozoic (Section 2.1) and that therefore the Phanerozoic ocean was fully oxygenated with only brief interruptions during anoxic events that favored a temporary return to anaerobic nitrogen cycling (e.g. Junium and Arthur, 2007;Ohkouchi et al, 2006;Rau et al, 1987;Schoepfer et al, 2012;Sephton et al, 2002). However, it is possible that the redox state of the ocean was not stable in the early Phanerozoic, and this may have significantly affected nitrogen cycling.…”

Biochemical and physiological bases for the use of carbon and nitrogen isotopes in environmental and ecological studies

“…The biochemical processes of chlorophyll synthesis in the photoautotrophic cell have been studied intensively (e.g., Beale 1995). This strong research background is an advantage when interpreting the isotopic signatures of chlorophylls (Katase and Wada 1990; Ohkouchi et al 2006Ohkouchi et al , 2008, although the number of applications of this technique is still limited.…”

“…C and approximately 5‰ depleted in 15 N relative to the cell; Hayes et al 1989;Sachs et al 1999;Ohkouchi et al 2006). Nevertheless, the interpretation of the isotopic records of complex ecosystems can advance by separating processes, which is made possible by compound-specific isotope analyses.…”

Dominant eukaryotic export production during ocean anoxic events reflects the importance of recycled NH 4 +

“…We propose that new production during OAE 2 primarily was driven by direct NH 4 þ -assimilation supplemented by diazotrophy, whereas chemocline denitrification and anammox quantitatively consumed NO 3 − and NO 2 − . A marine nitrogen reservoir dominated by NH 4 þ , in combination with known kinetic isotope effects, could lead to eukaryotic biomass depleted in 15 N.…”