Abstract
Sulfur isotopic (delta (super 34) s) evidence for marine anoxia and links to Lower Ordovician (Tremadocian) trilobite extinctions
Abstracts with programs - Geological Society of America, Vol.47(7), p.62
Geological Society of America, 2015 annual meeting & exposition
2015
Abstract
The late Cambrian to Early Ordovician is well known for the occurrence of trilobite extinctions and in some cases associated positive carbon isotope excursions (delta 13C). This relationship is interpreted to record the expansion of anoxia onto the shelf that created conditions that initiated extinctions and caused delta (super 13) C excursions via increased organic burial. The largest of these excursions occurred during the late Cambrian where globally correlative delta (super 13) C excursions and in some cases sulfur (delta (super 34) S) excursions of varying magnitude immediately follow an extinction event. This relationship is consistent with the notion that anoxic conditions were global, but it remains unclear if anoxia persisted for younger Ordovician extinctions. Here we present new delta (super 34) S data measured from carbonate-associated sulfate from a Lower Ordovician carbonate succession in the Ibex area, UT and compare with two previously recognized trilobite extinctions. The older extinction occurs near the base of the North American Stairsian Stage where delta (super 34) S values increase from 28 ppm approximately 50 m below the extinction horizon, are depleted by approximately 15 ppm just below the extinction, and reach a peak at 48 ppm near the same interval with a previously recognized approximately 2 ppm positive delta (super 13) C excursion. Both excursions begin near a relative sea level lowstand within the uppermost House Limestone and continue to rise throughout lithologies that vary from bioturbated mudstone to intraclastic rudstone. delta (super 13) C returns to -1 ppm before delta (super 34) S returns to 33 ppm where trends remain throughout the Stairsian. The younger extinction occurs near the base of the Tulean Stage, but isotope trends decouple at this interval. Near the base Tulean extinction delta (super 13) C decreases from -1 to -2.3 ppm, whereas delta (super 34) S increases prior to the extinction interval from 35 to 43 ppm before returning to 32 ppm, similar to the older excursion. The cause of this decoupling remains unclear and if the timing represents local effects or global trends, but the link between the end of the extinctions and positive delta (super 34) S excursions is consistent with the view that anoxic conditions persisted throughout the Early Ordovician. The frequency and size of isotope excursions diminished throughout the Early Ordovician and end prior to some of the first pulses of biodiversification, suggesting that changes in ocean oxygenation permitted the Ordovician radiation.
Details
- Title: Subtitle
- Sulfur isotopic (delta (super 34) s) evidence for marine anoxia and links to Lower Ordovician (Tremadocian) trilobite extinctions
- Creators
- Cole T Edwards - Washington University in Saint Louis, Department of Earth and Planetary Sciences St. Louis, MO USA United StatesDavid A FikeMatthew R SaltzmanJonathan M AdrainStephen R Westrop
- Resource Type
- Abstract
- Publication Details
- Abstracts with programs - Geological Society of America, Vol.47(7), p.62
- Conference
- Geological Society of America, 2015 annual meeting & exposition
- Publisher
- Geological Society of America (GSA)
- ISSN
- 0016-7592
- Alternative title
- Geological Society of America, 2015 annual meeting & exposition
- Language
- English
- Date published
- 2015
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984240804702771
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