How do extinction events kill so effectively?

“I am flying home from Europe in late August with nothing but a notebook and the 2011 Goldschmidt conference Geology giveaway issue to keep me occupied. Using the old-fashioned method of reading and writing on paper, I will blog my way through the compilation of highlighted geochemistry papers as time allows. These will then be posted via time delay to keep the blog moving while preventing paper burnout.”

The dinosaurs are still alive!

This was the conclusion reached by a group of my fellow undergrads way back in the Pliocene when I was in college. As an independent study project, they investigated all the possible effects of a giant meteorite impact (dust, fires, tsunami, etc), and concluded that none of these effects had the reach or duration to cause widespread global extinction.

Indeed, the actual kill mechanism is generally armwaved and/or hyperbolized (no sunlight for months, scorching acid rain, death from the skies!) under the circular reasoning that, “since everything died, these effects must have been lethal.” Understanding how entire niches get wiped out as actually rather tricky.

Enter Kump et al. (2005), who described a possible kill mechanism; poisoning from massive releases of H₂S gas from an oxygen-starved ocean.

In the absence of oxygen, bacteria will happily metabolize sugars by turning sulfate (SO₄^--) into sulphide (S^--), with the oxygen liberated form the sulfate used to burn sugar into CO₂ + H₂O. In the absence of iron or other base metals, this sulphide becomes H₂S in an aqueous system like the ocean. The ocean is full of sulfate; it is the second most common dissolved salt anion, after chloride.
So under oxygen-free conditions, generating significant amounts of H₂S is easy. Once this H₂S mixed with oxygen-rich water, it oxidizes back into salfate. Water with significant H₂S content is called “Euxinic”. While the modern ocean is well oxidized throughout all, but a few closed basins like the Black Sea, in the geologic past some or all of the deep water may have been euxinic.

In their study, Kump et al. (2005) do two things. First, they determine the conditions under which H₂S-bearing waters can upwell to the surface faster than oxygenated near-surface water can break down the H₂S. This is important because oxygen and hydrogen sulphide react easily in water, but if the H₂S exolves into the atmosphere, then it can co-exist metastably with O₂ gas in the air.

The second thing that Kump et al. (2005) do is to chemically model what happens to this H₂S once it gets into the atmosphere, how it is broken down,. and what other changes occur as a result.

Because H₂S and O₂ do not directly react under normal atmospheric conditions, H₂S oxidation in the atmosphere in generally performed by the OH and O radicals, which are in turn generated by the UV or radiological breakdown of H₂O and O₂ molecules. These are the same radicals that breakdown methane (CH₄), carbon monoxide (CO) and many other metastable gasses.

What Kump et al. (2005) find is that if the H₂S flux into the atmosphere exeeds the present flux by about a factor of 1000, then the H₂S accumulates after than the OH and O radicals can break it down. This leads to a step function increase in H₂S atmospheric lifetime and concentration, and a drop in O and OH abundance.

This depletion of O and OH, in turn reduced methane breakdown, so that methane concentrations and mean atmospheric lifetimes also increase. In addition, the lack of O means that ozone production is curtailed, so the ozone layer is reduced. The combination of reduced ozone protection and direct H2S toxicity is touted by Kump et al. as a highly effective kill mechanism, especially for land creatures and sea creatures in the near-surface waters.

Kump et al. then go on to show that there is evidence for anoxic waters reaching the surface during a number of Phanerozoic extinction events, and further hypothesize that the widespread euxinia in the Proterozoic inhibited the development of land life as a sort of “permanent extinction event” condition that persisted for most of Earth’s history, until mysteriously disappearing in the Cryogenean.

The H₂S-based kill mechanism (catchily coined as a “chemocline upward excursion”) is way outta my field of expertise. So I don’t know if there are reasons outside of my knowledge base to reject it out of hand. However, the nice thing about this paper is that it proposes a mechanism with specific, testable effects which we analysts can go looking for. While determining paleo-ozone and methane levels could be a bit tricky, the study of paleoeuxinity is a significant and ongoing field of study. I don’t know if this paper has withstood the test of time, but I suspect that it has inspired a whole slew of clever experiments. What more could we ask of the theoriticians?
Kump, L., Pavlov, A., & Arthur, M. (2005). Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia Geology, 33 (5) DOI: 10.1130/G21295.1