The chlorine abundance of Earth: Implications for a habitable planet

• High pressure/temperature experiments demonstrate that Cl is not sequestered into the core.
• The heavy halogens are depleted on Earth by a factor of 10 relative to chondritic-based estimates.
• A low volatilization temperature for Cl is refuted.
• Cl loss on Earth is explained by ‘collisional erosion’ during planetary growth.
• Without early loss of Cl during Earth accretion, development of life would have been seriously impeded.

The Cl, Br and I contents of Earth are depleted by a factor of 10 relative to predicted values from chondritic and solar abundances. Possible explanations for the apparent discrepancy include (1) unrecognized sequestration of Cl in the core, (2) a much higher nebular volatility than normally presumed or (3) a preferential loss of the heavy halogens during planetary accretion. We tested the first assumption by conducting high pressure–temperature equilibration experiments between silicate and metal. At 15 GPa and 1900 °C, the DCl(metal–silicate) value for Cl is less than 0.007, indicating that the core is not a significant reservoir for Cl. The concentration of Cl in all chondritic classes follows a depletion trend very similar to that of Na and Mn, arguing against a low condensation temperature for Cl. Instead, we propose that the depletion of the heavy halogens is due to their unique hydrophilic behavior. Almost half of Earth's Cl and Br inventory resides in the ocean and evaporites, demonstrating the unique affinity for aqueous solutions for these elements. During planetary accretion, there would have been a strong sequestration of halogens into the crustal reservoir. ‘Collisional erosion’ during planetary accretion provides a mechanism that would uniquely strip the heavy halogens out of an accreting Earth. Had such loss not occurred, the salinity of the oceans would be 10× the present value, and complex life would probably never have evolved.

Graphical AbstractIllustration of the extreme depletion of Cl, Br and I relative to other lithophile elements.The cause of this depletion is explained by the extreme hydrophilic behavior of the heavy halogens followed by loss during collisional erosion during the planetary accretion stage of Earth.Figure optionsDownload high-quality image (112 K)Download as PowerPoint slide