Geochemical records reveal
protracted and differential marine redox change associated with Late
Ordovician climate and mass extinctions
Nevin P.
Kozik1*, Benjamin C. Gill2, Jeremy
D. Owens1, Timothy W. Lyons3, and
Seth A. Young1
1 Department of Earth, Ocean, and Atmospheric Science
and National High Magnetic Field Laboratory, Florida State University,
Tallahassee, Florida, 32306, USA
2 Department of Geosciences, Virginia Polytechnic
Institute and State University, Blacksburg, Virginia, USA
3 Department of Earth and Planetary Sciences,
University of California, Riverside, CA, USA
Corresponding author: Nevin Kozik
(nkozik@fsu.edu)
Key Points:
- Newly paired iodine and sulfur geochemistry highlight the role of
shelf anoxia during the Late Ordovician Mass Extinction.
- Iodine records show pervasive local anoxia, while sulfur records
reveal waning then waxing sulfidic conditions in Late Ordovician
oceans.
- A combination of reducing marine conditions, climatic cooling, and
glacioeustacy caused the first mass extinction in the Phanerozoic.
Abstract
The Ordovician (Hirnantian; 445
Ma) hosts the second most severe mass extinction in Earth history,
coinciding with Gondwanan glaciation and a growing body of geochemical
evidence for marine anoxia. It remains unclear whether global cooling,
expanded oxygen-deficiency, or a combination drove the Late Ordovician
Mass Extinction (LOME). Here, we present new paired iodine and sulfur
isotope geochemical data from three globally distributed carbonate
successions to constrain changes in local and global marine redox
conditions. Iodine records suggest locally anoxic conditions were
potentially pervasive on shallow carbonate shelves, while sulfur
isotopes suggest a reduction in global euxinic (anoxic and sulfidic)
conditions. Late Katian sulfate-sulfur isotope data show a large
negative excursion that initiated during elevated sea level and
continued through peak Hirnantian glaciation. Geochemical box modeling
suggests a combination of decreasing pyrite burial and increasing
weathering are required to drive the observed negative excursion. This
reduction of pyrite burial suggests a ~3% decrease of
global seafloor euxinia during the Late Ordovician. The sulfur datasets
spanning the late Hirnantian–early Silurian provide further evidence
that this trend was followed by increases in euxinia which coincided
with eustatic sea-level rise during subsequent deglaciation. A
persistence of shelf anoxia against a backdrop of waning then waxing
global euxinia was linked to the two LOME pulses. These results place
important constraints on both local and global marine redox conditions
throughout the Late Ordovician and suggest that non-sulfidic shelfal
anoxia—along with glacioeustatic sea level and climatic cooling—were
important factors leading to the LOME.