Halogens: A Gaseous Peek into the Earth’s History

Understanding the historical prevalence of elements on Earth offers us insight into the processes that led to our planet’s development. One such group of elements is called halogens—nonmetallic elements occupying the seventeenth column of the periodic table. Over time, the amount of halogens present on the Earth’s surface has important implications and has been estimated to sit at about eighty percent—putting only twenty percent of these gasses currently within the mantle (below the surface). 

However, a pair of Yale researchers—graduate student Meng Guo and Professor Jun Korenga—found that this figure is a drastic overestimation. Attempting to validate previous estimates, the researchers instead found that almost ninety percent of stable halogens reside inside the present-day mantle! The error lies in the underlying assumptions of the prior calculations. The traditional method presumed that the ratio of elements in the crust and mantle have remained constant over time. When the Yale scientists abandoned this assumption, they realized that there is a mere ten percent of halogens on the surface of the present-day Earth.

 Guo and Korenga’s findings bear significant implications, not only for the history of halogen expulsion from and absorption into the mantle, but also for understanding the origins of life on Earth. The new budget suggests that the early Earth had high amounts of halogens on the surface that were gradually reabsorbed into the mantle over time. The significant takeaway relates to ancient seawater chemistry: a greater-than-expected presence of halogens would have affected the pH and alkalinity of the planet’s oceans. “There is debate about where life started. The ocean was proposed to fertilize the origin of life, but our results indicate that the ocean may have been too salty—three times saltier than the present-day ocean… It was more likely within inland freshwaters,” Guo said. To further this research, Guo intends to develop a simulated model allowing for further holistic analysis of the solid Earth.