New on Matter: How Earth Escaped Becoming A Dead Sea Planet

My “Matter” column this week at the New York Times is a biography of the element chlorine. You may know it from roles in such classics as The Swimming Pool or Table Salt. But chlorine has had a long, weird history on Earth. Recent studies suggest that chlorine levels were ten times higher on our planet when it formed. If it still had that much chlorine today, we probably never would have existed. Check out my column for the full story.

3 thoughts on “New on Matter: How Earth Escaped Becoming A Dead Sea Planet

  1. So you’re saying Earth’s extra chlorine was eliminated by asteroids splashing away a lot of the water in which a lot of the chlorine was dissolved? But that would not lower the concentration of chlorine in the water. It doesn’t sound right to me.

  2. The clue is in the abstract: ” 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.”

    So since Cl and Br would be concentrated to the crust, erosion of it and its water would preferentially remove Cl & Br. Asteroids would replenish the volatile supply including a 10x today concentration of Cl & Br. But as long as the crustal concentration of Cl and Br was higher than 10x there would be a net loss.

    Afterwards the crust would again deposit evaporites which would take the oceans down to towards today’s saline concentrations. And I would assume plate tectonics, which got started before the Great Oxygenation Event, would help take the ocean further. There are IIRC signs that the early oceans, certainly as regards sulfur, was laced with stuff not seen today (Fe, S, Cl, Br). But there is also a crystal encapsulated water sample from ~ 3.1 Ga bp which, if I remember correctly, tells us that at that time ocean salines were rather as today (except at ~ 40 degC and oxygen free).

  3. The collisional erosion mechanism would be generic for accreting terrestrials in planetary disks, so it augurs well for habitability.

    On the other hand the paper adds concerns about disks having more of saline building compounds than ours. As they say, rain could be scarce. (And further there are papers that say ~ 80 % of today’s water circulation is due to life, plant evaporation, bacterial rain drop seeds and what not.)

    Saline planets may as well be ocean planets for as much use continents could be for life.

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