The Mitchell & Melchin comment on Bond & Grasby (B&G) paper is up now, so I can explain the issue(s). In short, there's a biostrat error that's retraction worthy. It not only undermines their warming argument, it undermines the whole LIP argument. 1/n https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G47946C.1/587311/COMMENT-Late-Ordovician-mass-extinction-caused-by
B&G argue for eruption-caused warming as the kill mechanism for the Late Ordovician Mass Extinction (LOME) based on correlation between the extinction and LIP geochem proxies (Hg, U, Mo) at Dob's Linn, Scotland. See yellow and red arrows. Fair enough. They seem to correlate. 2/n
To understand mistake, you've got to zoom into timeline/understand bit about graptolite biostrat. Base of Hirnantian Stage is defined by 1st appearance of graptolite M. extraordinarius. Pre-2003, it was thought to occur at Dob's Linn in cleverly named "Extraordinarius Band". 3/n
By beginning of Hirnantian, the mass extinction in graptolites is well underway, which agrees with where B&G place their 'LOME pulse 1' marker (grey band). Buuuuuut... here's the thing. Dob's Linn base of the Hirnantian *isn't* the Extraordinarius Band. Not since since 2003! 4/n
Melchin et al. (2003) IDed M. extraordinarius in the lower Anceps Band E *and* they IDed M. persculptus in the the "Extraordinarius Band". Melchin et al. is a bit obscure, but subsequent biostrat/extinction papers have accepted this revision. For example, Storch et al. (2011).5/n
Melchin et al. (2003) IDed M. extraordinarius in the lower Anceps Band E *and* they IDed M. persculptus in the the "Extraordinarius Band". Melchin et al. is a bit obscure, but subsequent biostrat/extinction papers have accepted this revision. For example, Storch et al. (2011).5/n
Melchin et al. (2003) IDed M. extraordinarius in the lower Anceps Band E *and* they IDed M. persculptus in the the "Extraordinarius Band". Melchin et al. is a bit obscure, but subsequent biostrat/extinction papers have accepted this revision. For example, Storch et al. (2011).5/n
This revision is even in the formal write up for the Hirnantian GSSP! (Chen et al., 2006). How it escaped B&G (and peer reviewers) is beyond me. 6/n http://www.episodes.org/journal/view.html?volume=29&number=3&spage=183&vmd=A
Why is this big deal? Because by base of the Hirnantian, the graptolite portion of the extinction is essentially over (communities are completely changed, most species extinct/extirpated), e.g., my thesis, Sheets et al (2016), Chen et al. (2005), Storch et al. (2011). 7/n
Why is this big deal? Because by base of the Hirnantian, the graptolite portion of the extinction is essentially over (communities are completely changed, most species extinct/extirpated), e.g., my thesis, Sheets et al (2016), Chen et al. (2005), Storch et al. (2011). 7/n
Why is this big deal? Because by base of the Hirnantian, the graptolite portion of the extinction is essentially over (communities are completely changed, most species extinct/extirpated), e.g., my thesis, Sheets et al (2016), Chen et al. (2005), Storch et al. (2011). 7/n
Why is this big deal? Because by base of the Hirnantian, the graptolite portion of the extinction is essentially over (communities are completely changed, most species extinct/extirpated), e.g., my thesis, Sheets et al (2016), Chen et al. (2005), Storch et al. (2011). 7/n
Why is this big deal? Because by base of the Hirnantian, the graptolite portion of the extinction is essentially over (communities are completely changed, most species extinct/extirpated), e.g., my thesis, Sheets et al (2016), Chen et al. (2005), Storch et al. (2011). 7/n
Going back to the B&G data with a correct timeline, you can see that *none* of their geochem proxies move till after the Hirnantian. That is, if the proxies really represent a LIP, the eruptions happen *long* after the graptolites are already nuked. 8/n
Going back to the B&G data with a correct timeline, you can see that *none* of their geochem proxies move till after the Hirnantian. That is, if the proxies really represent a LIP, the eruptions happen *long* after the graptolites are already nuked. 8/n
Going back to the B&G data with a correct timeline, you can see that *none* of their geochem proxies move till after the Hirnantian. That is, if the proxies really represent a LIP, the eruptions happen *long* after the graptolites are already nuked. 8/n
But LIP argument was already in some trouble. Shen et al. (2019) found in South China the Hg anomaly disappeared when sulphide % was controlled. B&G present well-constrained data... but that data is actually negative. I'd argue it presents a serious blow to LIP hypothesis. 9/n
I know don't much about benthic fauna (others can chime in), but this recent review correlates benthic extinctions before B&G's LIP evidence too. 10/n https://www.sciencedirect.com/science/article/pii/S0012825218305099#f0010
In addition to biostrat, B&G is also ignore biogeographic & paleoecological data. Again, I really only know graptolites, but their extinction corresponds w/ invasion of high latitude species tracking cold water masses/change in phytoplankton. Both point to cooling. 11/n
In addition to biostrat, B&G is also ignore biogeographic & paleoecological data. Again, I really only know graptolites, but their extinction corresponds w/ invasion of high latitude species tracking cold water masses/change in phytoplankton. Both point to cooling. 11/n
In addition to biostrat, B&G is also ignore biogeographic & paleoecological data. Again, I really only know graptolites, but their extinction corresponds w/ invasion of high latitude species tracking cold water masses/change in phytoplankton. Both point to cooling. 11/n
Paleogeographic extinction patterns (tropical species hit harder) pointing to cooling as a cause have been reported for other taxa too. None of this data is grappled with in B&G. That's an inexplicable omission. 12/n
All and all, I am honestly a bit baffled how this paper got into Geology. 13/n @geosociety @sethfinnegan1
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