Abstract

The late Ordovician is characterized by dramatic changes in global climate concurrent with a major mass extinction and possible changes in ocean redox. To further refine our understanding of these events, we present nitrogen and carbon isotope and abundance data from the Ordovician-Silurian (O-S) Global Boundary Stratotype Section and Point at Dob's Linn, Scotland. We show that this section experienced post-depositional ammonium migration from the organic-rich to the organic-poor horizons. However, our data suggest that isotopic fractionations from ammonium substitution into illitic clay minerals are small and can be corrected. Reconstructed primary nitrogen isotope ratios indicate that unlike in tropical continental shelf sections that were transiently enriched in nitrate during the Himantian glaciation, the sub-tropical continental slope setting at Dob's Linn experienced persistent limitation of fixed nitrogen across the O-S boundary. Shallow subpolar settings appear to be the only environment that shows persistent nitrate availability at that time. This pattern suggests that spatial trends in marine nitrate concentrations - which are observed in the modern ocean as a result of latitudinal temperature gradients - were already established during the Paleozoic. While the average marine O-2 chemocline depth may have deepened during the Himantian glaciation, it probably did not lead to global ventilation of the deep ocean, which may have been delayed until the Carboniferous. Furthermore, carbonate data from this and other sections suggest a deepening of the carbonate compensation depth (CCD) during the Himantian. This observation indicates that Pacific-style responses of the CCD to glacial/interglacial periods were operational across the O-S boundary, and that the expansion of abiotic carbonate deposition and preservation beyond the shelf break could have in-part mediated changes to surface CO2 during these extreme changes in climate.

Keywords

Nitrogen isotopes; Paleozoic nitrogen cycling; Abiogenic carbonate storage; Ammonium migration; Nitrogen in clay

Published in

Palaeogeography, Palaeoclimatology, Palaeoecology
2019, Volume: 534, article number: 109321
Publisher: ELSEVIER

SLU Authors

• Hillier, Stephen

  • Department of Soil and Environment, Swedish University of Agricultural Sciences
    
  • The James Hutton Institute

UKÄ Subject classification

Climate Research


Publication identifier

DOI: https://doi.org/10.1016/j.palaeo.2019.109321

Permanent link to this page (URI)

https://res.slu.se/id/publ/103860