NITROGEN ISOTOPES IN KUKERSITE AND BLACK SHALE IMPLYING ORDOVICIAN-SILURIAN SEAWATER REDOX CONDITIONS

Abstract

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 δ 15 N -0.3‰ on average. The average δ 15 N of kukersite oil shale is +7.4‰. The positive δ 15 N 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.