Sulfur aerosols form when sulfur gas reacts with oxygen and water in the atmosphere to form tiny droplets. These droplets are so small that they can remain suspended in the air for a year or more, until they eventually settle onto the ground or are washed out by rain. While they are in the atmosphere, sulfur aerosols can interact with sunlight, scattering and absorbing the sun’s warming rays, and preventing some sunlight from reaching Earth’s surface. On average, this has a cooling effect while the aerosols are aloft—though some regions may grow temporarily warmer (and of course, the cooling can be counteracted by other processes, such as the greenhouse effect). For example, sulfur from the Laki eruption in Iceland in 1783 likely caused many disruptions of climate in Europe at the time (e.g., Thordarson et al., 1996). Since sulfur aerosols are essentially droplets of sulfuric acid, they can also cause problems when they leave the atmosphere. Thus the two main stresses scientists have investigated related to sulfur release are climate perturbations and acid deposition.
Most of the sulfur in the stratosphere (an upper region of the atmosphere) today comes from volcanic eruptions. However, Earth’s history reveals other sources of stratospheric sulfur as well. The Chicxulub asteroid struck in a location with abundant sulfur stored in sedimentary rocks, instantly liberating billions of tons of sulfur into the atmosphere (e.g., Pope et al., 1994; Toon et al. 1997) – and of course, the Deccan Traps were erupting at the same time, releasing even more sulfur. Consequently, in the end-Cretaceous mass extinction, environmental stresses caused by sulfur aerosols may have been a particularly important ‘kill mechanism’ (so called to distinguish them from the proposed ‘trigger mechanisms,’ like volcanism and the Chicxulub asteroid, which triggered these stresses).
So how did sulfur release from the Deccan Traps and from the Chicxulub impact affect the environment during the end-Cretaceous mass extinction? Recent modeling has shown that sulfur aerosols from flood basalt volcanism may trigger episodic cooling and selective stress from acid mists, but also emphasizes that these stresses are not globally uniform and that prolonged sulfur emissions are required to cause a biotic crisis (Schmidt et al., 2015). This modeling has also shown that volcanic sulfur release—even though it can create a potent acid—probably did not contribute much to ocean acidification. This may not be true for sulfur liberated by the Chicxulub impact, which happened much faster (Ohno et al., 2014). Rates matter in geology!
To compare the effects of sulfur from the Deccan Traps and the Chicxulub impact, and also to examine the combined effects of both events at once, our team will investigate two lines of evidence. The first is the Deccan Traps volcanic rocks themselves, which hold clues to sulfur release 66 million years ago. These clues take the form of melt inclusions, which are tiny specks of magma that were trapped inside crystals and that in some cases never had a chance to release their sulfur and other gases to the atmosphere. The second line of evidence comes from a global climate model (GCM for short), a complex computer program that can be used to predict how different changes in earth systems affect the atmosphere. Colleagues at the National Center for Atmospheric Research (NCAR) led by Dr. Clay Tabor have been adapting one of the GCMs that we use to understand present and future climate to investigate the effects of the Chicxulub impact. We are excited to work with this team, including our UK collaborator, Dr. Anja Schmidt, to include Deccan volcanism in this modeling.
- Ohno, S., T. Kadona, K., Kurosawa, T. Hamura, T. Sakaiya, K. Shigemori, … and S. Sugita, 2014, Production of sulphate-rich vapour during the Chicxulub impact and implications for ocean acidification, Nature Geoscience, 7, 279.
- Pope, K. O., K. H. Baines, A. C. Ocampo, and B. A. Ivanov, 1994, Impact winter and the Cretaceous/Tertiary extinctions: Results of a Chicxulub asteroid impact model, Earth and Planetary Science Letters, 128, 719-725.
- Schmidt, A., R. A. Skeffington, T. Thordarson, S. Self, P. M. Forster, A. Rap, … and K. S. Carslaw, 2016, Selective environmental stress from sulphur emitted by continental flood basalt eruptions, Nature Geoscience, 9, 77-82.
- Schulte, P., L. Alegret, I. Arenillas, J. A. Arz, P. J. Barton, P. R. Bown, … and P. S. Willumsen, 2010, The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary, Science, 327, 1214-1218.
- Self, S., S. Blake, K. Sharma, M. Widdowson, and S. Sephton, 2008, Sulfur and chlorine in Late Cretaceous Deccan magmas and eruptive gas release, Science, 319, 1654-1657.
- Thordarson, T., S. Self, N. Óskarsson, and T. Hulsebosch, 1996, Sulfur, chlorine, and fluorine degassing and atmospheric loading by the 1783–1784 AD Laki (Skaftár Fires) eruption in Iceland, Bulletin of Volcanology, 58, 205-225.
- Toon, O. B., K. Zahnle, D. Morrison, R. P. Turco, and C. Covey, 1997, Environmental perturbations caused by the impacts of asteroids and comets, Reviews of Geophysics, 35, 41-78.