Free Astronomy Magazine January-February 2018

JANUARY-FEBRUARY 2018 by the fact that the mostly coastal location of oil fields and other non-renewable en- ergy sources at the end of the Cretaceous period did not differ much from that of today (as shown by maps reconstructed through estimates made by Kaiho). In this context, the Gulf of Mexico was and still is rich in hydrocarbons. In addition to the burning of hydrocar- bons, a primary role in the cooling of the climate was probably played by the sul- fates present in the rocks impacted by the asteroid. It is not easy to determine how much sulfates affected the mass extinction, but it has been calculated that by them- selves they may have contributed to low- ering the global temperature by 14 °C if there were high concentrations in the rocks (1-2% of the terrestrial surface). Here’s how Kaiho and Oshima summarize the scenario in Scientific Reports : “These results suggest that climate changes (in terms of extinction levels) can be esti- mated using stratospheric soot amounts. Soot from hydrocarbon-rich areas (approxi- mately 13%) including high sulfate areas limited to 1% of the Earth’s surface caused 8– ≥ 11 °C global cooling, 13– ≥ 17 °C cooling on land, a decrease in precipitation by ap- proximately 70– ≥ 85% on land, a decrease of approximately 5– ≥ 7 °C in seawater tem- perature at a 50-m water depth, and mass extinction marked by the extinction of di- nosaurs. [...] The Chicxulub impact occurred in a hydrocarbon-rich, sulfate-dominated area, and is a rare case of mass extinction being caused at such an impact site. [...] The probability of mass extinction at the K–Pg boundary was approximately 13% after the asteroid impacted Earth. The collapse of ecosystems with dinosaurs on land and large marine reptiles and ammonites in the sea at the top of the food chain was proba- bly due to soot with possible contributions by sulfate from the Chicxulub asteroid im- pact and led to the subsequent macroevo- lution and diversification of mammals. Therefore, the low probability of mass ex- tinction indicates the low probability of the subsequent macroevolution of mammals.” The conclusions reached by the two Japan- ese researchers through their computer sim- ulations make us reflect on how unlucky the dominant species was at the end of the Cre- taceous period and, from a different per- spective, what a great stroke of luck the Chicxulub event was for the mammals. How would life on Earth have evolved if that as- teroid was a little further ahead or a bit fur- ther back in its orbit and had slammed into our planet a few hundred kilometres away from where it fell? ! G lobal map showing the amount of or- ganic matter in sedimentary rocks ejected if the Chicxulub as- teroid hit various locations at the end of the Creta- ceous. Shaded areas correspond to 0–4 °C, 4–8 °C, 8–11 °C, and ≥ 11 °C cooling. Mass extinction could have been caused by 8–11 °C or more cooling when the aster- oid hit an orange or magenta area, which occupied approximately 13% of the Earth’s surface. The map is based on Courtillot et al.; thin lines indicate continen- tal crust shelf edges. [K. Kaiho & N. Oshima, Sci- entific Reports]