Earth’s Chlorine Abundance Fine-Tuned for Life

Earth’s Chlorine Abundance Fine-Tuned for Life

What do chlorine and Rodney Dangerfield have in common? They both “don’t get no respect.” As a recent research study indicates,1 Earth is uniquely endowed in many overlooked ways with a fine-tuned abundance of chlorine and its many compounds.

Scientists recognize chlorine and thousands of known organic chlorine compounds as “vital poisons.” If any one of them exists at too high a concentration, it proves toxic and even deadly to life. On the other hand, if any one of them exists at too low a concentration, it also proves deadly. Each one must exist at a just-right level.

A good example is sodium chloride, otherwise known as table salt. All known organisms need such salt in small quantities. It is crucial for metabolism, for maintaining essential fluid and pH balances, and for electrical signaling in nervous systems. Too much or too little salt in the diet causes muscle cramps, dizziness, electrolyte disturbances, neurological malfunctions, and/or death.

For several decades, astronomers have recognized that Earth possesses a superabundance of chlorine. Compared to magnesium and iron, Earth’s crust contains about three times as much chlorine as the rest of the Milky Way Galaxy. Earth’s oceans add nearly an equal quantity of chlorine as exists in the crust. However, relative to the proportion of chlorine in chondritic meteorites (remnants of the raw material from which Earth formed) and in the Sun, Earth is depleted by a factor of ten. It is similarly depleted in bromine and iodine.

This “missing” chlorine has perplexed geophysicists and geochemists. Until now, the only proposed solution was that chlorine somehow was dragged into the deep interior of Earth by the metals that form Earth’s core. In the new study, researchers Zachary Sharp and David Draper performed a lab experiment that ruled out the possibility that chlorine can dissolve in iron (the core’s predominant metal) under the pressure and temperature conditions of Earth’s core.

So, where is the missing chlorine? Sharp and Draper propose that Earth’s unique bombardment history explains how Earth lost so much chlorine.

When Earth was 30–50 million years old (it is now 4.57 billion years old), a planet nearly twice the mass of Mars collided with Earth at low velocity. This collision caused Earth to lose about 99 percent of its primordial water and atmosphere and much of its light elements, and led to the formation of the Moon. Later, when Earth was about 700 million years old, the Late Heavy Bombardment, an event during which most of the Kuiper and Main belts of asteroids and comets were destabilized, pummeled the entire inner solar system. As the most massive of the inner solar system planets, Earth suffered the most damage.

Sharp and Draper point out that Mars has more than twice the concentration of chlorine as does Earth. Earth’s more intense bombardments would explain the difference. Furthermore, the bombardments explain why both Mars and Earth contain a much lower proportion of chlorine than both the Sun and chondritic meteorites.

Minus the intense impacts experienced during its infancy and youth, Earth’s present-day oceans would be too salty to support animals with body sizes exceeding a centimeter. The high ocean salinity would reduce precipitation. Less rain implies less land erosion and, consequently, fewer nutrients being recycled. Meanwhile, the high concentration of chlorides in the continental landmasses would rule out the possibility of advanced land life.


Crustal Abundance Relative to Magnesium
carbon* 1,200 times less
nitrogen* 2,400 times less
fluorine* 50 times more
sodium* 20 times more
aluminum 40 times more
phosphorus* 4 times more
sulfur* 60 times less
chlorine* 3 times more
potassium* 90 times more
calcium 20 times more
titanium 65 times more
vanadium* 9 times more
chromium* 5 times less
manganese* 3 times more
nickel* 20 times less
cobalt* 6 times less
copper* 21 times more
zinc* 6 times more
arsenic* 5 times more
selenium* 30 times less
yttrium 50 times more
zirconium 130 times more
niobium 170 times more
molybdenum* 5 times more
silver 3 times more
tin* 3 times more
antinomy 10 times more
iodine* 4 times more
gold 5 times less
lead 170 times more
uranium 340 times more
thorium 610 times more

This table indicates how much more or less abundant, by mass,
relative to magnesium’s abundance (which ranks eighth in Earth’s
crust), each of these thirty-two elements in Earth’s crust is compared
to its abundance in the rest of the Milky Way Galaxy. Asterisks denote
elements critical for advanced life.

Chlorine is just one of Earth’s exceptional elemental abundances. Twenty must exist at fine-tuned abundance levels for advanced life to be possible. An additional twelve must have fine-tuned abundance levels for global civilization to be possible. See table.

Such divergent abundance levels demonstrate that Earth clearly is not an accident of nature. The psalmist declares, “The earth is the Lord’s and everything in it” (Psalm 24:1). The next time we reach for
the salt shaker, we can give chlorine its proper respect and extend gratitude for the Creator’s provision.

  1. Z. D. Sharp and D. S. Draper, “The Chlorine Abundance of Earth: Implications for a Habitable Planet,” Earth and Planetary Science Letters 369–70 (May 2013): 71–77.