Sulfur and Chlorine in Late Cretaceous Deccan Magmas and Eruptive Gas Release

(kewl pic, huh?)

Large-volume pahoehoe lava flows erupted 67 to 65 million years ago, forming the Deccan Traps, India. The impact of these flood basalt eruptions on the global atmosphere and the coeval end-Cretaceous mass extinction has been uncertain. To assess the potential gas release from this volcanism, we measured sulfur and chlorine concentrations in rare glass inclusions inside crystals and on glassy selvages preserved within lavas. Concentrations range from ~1400 parts per million of S and 900 parts per million of Cl in inclusions down to a few hundred parts per million in the lava. These data indicate that eruptions of Deccan lavas could have released at most 0.103 weight % of S, yielding up to 5.4 teragrams of SO2 per cubic kilometer of lava. A more conservative estimate is 0.07 weight % of S and 0.04 weight % of Cl, yielding 3.5 teragrams of SO2 and 1 teragram of HCl for every cubic kilometer of lava erupted. The flows were very large in volume, and these results imply that huge amounts of S and Cl gases were released. The environmental impact from even individual eruptions during past flood basalt activity was probably severe.

hm. Another Vulcanists Strike Back Paper. Now the problem I have is not with the science wrt the geochemistry. It looks pretty good as far as I can tell. They did some very good work. This is the sort of science that needs to be done to test theories. That said, the only caveat is that they need to have it verified by another team. Like all scientific discoveries, it needs to be tested. otherwise, we would end up with a lot more fullerene at the PT Boundary embarrassments. That said, there needs to be more than merely sampling the lava flows from the Deccan Traps to back up this claim. There are two ways to test this idea - that out gassing from the Deccan Traps caused climate problems, namely erratic cooling, and acid rain.

The first one is to go through the isotope ratios from the time period that the Deccan Traps are supposed to be erupting. There are multiple isotopes that can be used for determining whether or not the Cretaceous had an erratic climate in terms of temperature changes. The most famous is the ratios of oxygen isotopes. It should be noted that there needs to be isotopic shifts that are chronogeologically specific to the time period in question that support their hypothesis. IIRC, there was no shift in the isotopes that in that time frame. At least the ones wrt climate. I'll verify tonight with my copy of Mass Extinctions and Their Aftermath (every extinction in there has a nice discussion of the isotope ratios and shifts thereof).

It should be noted that one of the traditional bits of evidence that the climate was cooling was that the fact that the Cretaceous inland seas receded (the "Oceans of Kansas"). If you look at a temperature graph across the KT Boundary there's not much shift at least under the current research. Furthermore, the recession of the inland seas now looks like it may be because of the expansion of the Atlantic...something that was not caused by any climate changes (though the Atlantic did change the climate!)

The second test is to check whether or not there is evidence of a sulfur induced "impact" globally (and I don't mean an asteroid smashing either). Originally, this was suggested as one of the kill mechanisms for the asteroid/comet impact theory: the Yucatan has strata that are very high in sulfur content and the vaporization of that rock would have dumped all that sulfur into the atmosphere and came back down as acid rain. Now, in this case, the vulcanists are stating the Deccan Traps could have done this as well since the evidence is insanely stacked against the KT Extinction being anything like the PT Extinction where the PT has been pretty much sown up as being caused volcanically (please see my post, Stop Dreaming). In either case, if there was acid rain, there ought to be evidence of either chemical weathering or ph-sensitive species die offs.

At this point I am unaware of anything that has been published about chemical weathering related to sulfuric acid at the KT boundary or the globally distributed sections chronospecifically associated with the Deccan Traps. I could be wrong and perhaps some readers could point me to papers that might have been done taht I haven't seen. I don't believe that this has been looked at yet. However, I could be very, very wrong. However, should there be evidence of it, it's not proof that either the impact or the eruptions would have been the cause. The specific ratios of sulfur isotopes will have to be examined and compared with the ones from the Yucatan and those that the team that produced this paper have resolved. Since the Yucatan's sources of sulfur ought to be different and laid down over a longer time period than during the Deccan Traps, there should be a different ratio. Whichever matches the that evidence at the KT boundary would probably be the source...and that would be evidence of which caused the die off.

On the other hand, there ought to be a lot of die offs of critters that are specifically sensitive to changes in water pH. Freshwater fish and amphibians ought to have been disproportionately whacked by anything that dumped tons of sulfuric acid into their habitats. That would cascade up the food chain and munge anything associated with freshwater. Yet...yet...this didn't happen as far as we can tell. If you were associated with the brownwater ecosystems you largely marched through the KT Extinction in better shape than anyone else. This would suggest that the brown water ecosystems continued to have nutrient influxes even when the terrestrial environment had been devastated and whatever caused the die-off on land didn't effect the brown water ecosystem. Or couldn't. That would probably rule out a sulfuric acid kill mechanism from either model.

So where does this leave us? Probably not much different than we were before. There has been some nice geochemical work about the outgassing of the Deccan Traps. This is scientifically good and useful research. In reading the paper and the extrapolations that the vulcanists involved are trying to connect the Deccan Traps with climatic and biotic effects that are quite contrary to the one extinction event that has been linked to vulcanism.

In fact, these effects have been proposed by the impactists. That thought is...interesting. hrm.

As a parting thought, I have to wonder if they could extract out information about the carbon isotopes frm that same source as they did for the chlorine and sulfur. Just a thought.