Nitrogen isotopes in kukersite and black shale implying Ordovician—Silurian seawater redox conditions

For the first time data on nitrogen isotopes from the Ordovician-Silurian sedimentary rocks of the Baltic Basin are reported. Supplementary samples come from several regions worldwide. The data reveal the existence of different primary bioproductivity pathways in the Ordovician-Silurian. During the formation of black shale surface waters were oxygen-poor and maintained N2-fixing primary production indicated by δ15N -0.3‰ on average. The average δ15N of kukersite oil shale is +7.4‰. The positive δ15N values are in accordance with the formation of kukersite in oxic waters, showing that Gloeocapsomorpha prisca was a nitrate-using not N2-fixing cyanobacterium-like organism. The black shale samples from the deep shelf suggest that seawater, including the photic zone, often suffered from deficiency of oxygen.