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 Hirnantian 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 O2
chemocline depth may have deepened during the Hirnantian 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 Hirnantian. 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.
Original language | English |
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Article number | 109321 |
Journal | Palaeogeography, Palaeoclimatology, Palaeoecology |
Volume | 534 |
Early online date | 12 Aug 2019 |
DOIs | |
Publication status | Published - 15 Nov 2019 |
Keywords
- Nitrogen isotopes
- Paleozoic nitrogen cycling
- Abiogenic carbonate storage
- Ammonium migration
- Nitrogen in clay
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Limitation of fixed nitrogen and deepening of the carbonate-compensation depth through the Hirnantian at Dob's Linn, Scotland
Koehler, M. (Creator), Stueeken, E. E. (Creator), Hillier, S. (Creator) & Prave, A. R. (Creator), Palaeogeography, Palaeoclimatology, Palaeoecology, 2019
DOI: 10.1016/j.palaeo.2019.109321
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