Diazotrophic cyanobacteria as the major photoautotrophs during mid-Cretaceous oceanic anoxic events: Nitrogen and carbon isotopic evidence from sedimentary porphyrin

We determined both the nitrogen and carbon isotopic compositions of nickel-chelated deoxophylloerythroetioporphyrin (Ni DPEP), a major sedimentary porphyrin extracted from the Livello Selli and Livello Bonarelli black shales deposited in the western Tethys Sea during mid-Cretaceous oceanic anoxic events (OAEs). Based on empirical isotopic relationships between the tetrapyrrole nuclei of chlorophylls and photoautotroph cells, we estimate that the mean nitrogen isotopic composition of the entire photoautotrophic communities during these periods ranged from −2‰ to +1‰. This result strongly suggests that N2-fixation was an important primary process in photoautotrophic production during these OAEs. The estimated carbon isotopic composition of the photoautotrophs was elevated (between −20‰ and −22‰) relative to typical Cretaceous examples, indicating as much as a 5‰ reduction in the magnitude of carbon isotopic fractionation associated with photosynthesis during OAEs in the western Tethys Sea. This anomaly can be well explained if cyanobacteria were the dominant producers because they commonly conduct β-carboxylation and/or active transport of carbon substrates, resulting in reduced carbon isotopic fractionation. We therefore conclude that diazotrophic cyanobacteria were the dominant components of primary production during OAE-1a and OAE-2 in the western Tethys Sea.