Sunday, May 25, 2014

Salt of the Earth: The Source of Sodium and Chlorine in Our Oceans

As an adult, I sometimes forget to ask the obvious questions.  Kids, though, they have their heads on straight.  Living near the coast, I have the opportunity to visit the ocean pretty often.  Every once in a while I brave the frigid waters, and inevitably taste the salty sea, but I never really think about where it came from.  

Recently, I chatted with a friend about her grade school classroom.  She shared some of the science questions her students had asked.  "Why is the ocean saltier than a lake, even though it's bigger?"  I gave her the spiel: lakes and rivers have salt, but water moves through most lakes (and all rivers).  Most of that water, including salts, end up in the ocean.  Water has an easy route out of the ocean: evaporation.  Salt has no such egress.  It stays put.  Just like the ocean, some lakes and seas without outflows collect large amounts of salt. Therefore, we have Salt Lake in Utah, or the Dead Sea in Europe.  I imagined the students' next question: "Where did the salt come from in the first place?", and realized I didn't have a clue.

There are clues however.   The first is the quantity of elements in the Earth's crust.  The most common elements in sea salt vary greatly in their places on the list of most common elements in Earth's crust.  Sodium weighs in at number 6, while chlorine doesn't even make the top 20.  Sodium, therefore, is in everything.  Wikipedia lists 139 minerals, that are composed in part of the element.  When granite breaks down in water, a mineral group called feldspar releases its sodium to the water, and the water doesn't let go.  This weathering of feldspar, and other sodium minerals, would have delivered plenty of sodium to the Earth's oceans very early in their history.

Being rarer in the crust, it might seem that chlorine levels wouldn't be nearly as high in the sea.  In the ocean, chlorine content surpasses sodium as dominant element.  Why the strange ratios?  As it turns out chlorine doesn't play well with others.  A chlorine ion, which is a chlorine atom that has stolen another poor atom's electron, is large, at least relative to other common elements on Earth's surface.  The patterns these smaller ions create don't leave room for the hefty chlorine.  Chlorine elopes with a free hydrogen ion, and escapes out a volcanic vent, having never formed a rock mineral.  At the surface the hydrochloric acid splits, with the hydrogen joining oxygen to make water, and the chlorine dissolved in the ocean.  As chlorine atoms throughout geologic history jostled their way to the surface through volcanoes the oceans grew saltier.

Curiosity has always driven my study of geology.  But sometimes I forget the obvious.  The next time I visit the ocean and watch the surf gather on the shore, I'll be thinking of dissolving rock, belching volcanoes and the rivers that  bring their remnant salt downstream, my borrowed childlike wonder having been appeased.

Lorence G., Collins. "Time to Accumulate Chloride Ions in the World’s Oceans." Reports of the National Center for Science Education 26.5 (2006): n. pag. California State University Northridge. Web. 22 May 2014.

"How did the salt get into the oceans at the beginning of their formation?." UCSB Science Line sqtest. University of California, Santa Barbara, n.d. Web. 22 May 2014. <>.

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