# Signatures of acid rain at KT boundary

I read in Walter Alvarez' book T. Rex and the Crater of Doom that the Earth's collision with the large meteor leading to the K-T extinction catalyzed the reaction of atmospheric oxygen and nitrogen molecules to form nitric oxide, which in turn formed highly corrosive nitric acid when combined with water. In addition the impact is thought to have volatilized huge amounts of sulfur contained in anhydrite, which in turn formed sulfuric acid.

This question refers to a thickness of $$\pu{1.8m}$$ for the section where iridium could be detected in the K-T boundary, suggesting that the resolution may be too low to note the effects of acid rain following the collision.

Is there evidence in the geological record of the formation of these acid species thought to be associated with the collision, say from deposits and/or effects of the ensuing acid rain?

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• Indirectly there is evidence of a change in soil PH, there is a zone where fossils are lacking around the boundary, Many propose this is due to a drastic change in the soils around at the time. soils that would have preserved fossils suddenly shifted in PH and PH has a big effect in fossil preservation.
– John
Aug 13 '20 at 14:00
• @John Thanks for the comment. I imagine it is not simple to distinguish however between pH changes due to drastic alteration of the biosphere versus from deposition of massive amounts of sulfuric acid. Aug 14 '20 at 9:30

## 1 Answer

Estimated sulfur release 325 gigatonnes = 325,000 teragrams. The numbers in this diagram are in teragrams

so the release is $$\approx 1000\times$$ today's annual sulfur cycle.

I think most of the sulfur compounds would be washed into the ocean and then deposited into sediments. I can't find how much sulfur is currently in the oceans, this article says gigatons (because you can smell it). I also can't find how fast the increased sulfur would be deposited.

Another issue is that, unlike iridium, there is normally a lot of sulfur in marine deposits, so it is hard to distinguish what comes from Chicxulub.

• Excellent, thank you! The original article estimating volume of ejecta is agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017GL074879. I don't have access right now but the supporting info is also useful. I'll have to look to see if there is reference to the formation of nitric acid in the introduction or discussion. Feb 27 '19 at 7:29
• I'm sort of thinking out loud here. I see that the oceans are 3% sulfate so the increase in sulfur concentration is negligible in the oceans (1.332 billion cubic kilometers ). (Cubic kilometer $\approx$ gigatonne.) Feb 27 '19 at 17:58
• I am also just brainstorming out of curiosity. That does seem like "a drop in the ocean" once you dilute the sulfate in the sea against a background that is over 6 orders of magnitude larger. Sulfates are generally very soluble with some exceptions (Ba, Sr, Ag, Pb) says my solubility table. Maybe some unusual local deposits of these minerals formed. Maybe the fossil record shows evidence of sudden acid precipitation. The article you link to is a good lead... Feb 27 '19 at 21:59
• I was able to access the article here: hal.archives-ouvertes.fr/hal-01639659/document Feb 28 '19 at 8:58