The Treasure of the Sierra Nevada

Treasure, and where it’s found.

In the old John Huston movie, it was “The Treasure of the Sierra Madre”.  That’s in Mexico, and the treasure was gold.  The Sierra Nevada, in California, has also produced its share of gold, but that’s not the treasure I’m thinking of.  Gold was found mostly in the western foothills, in what’s now known locally as “Gold Country”.  What I’m thinking of is over the other side of the crest.  It’s about as far from gold as one can imagine.  In fact, nearly everyone regards it as barren and worthless desert soil.

Nonetheless, treasure I call it, and treasure it is — if we can work out an economical way to make use of it.  Unlike gold, a perennial lure for human folly, this treasure could be at least a partial remedy for a different sort of human folly — the consequences of our addiction to the “black gold” of fossil fuels.  This treasure has the potential to help stop the rise of atmospheric CO₂ and reverse the acidification of ocean waters that is already taking a toll on ocean life.

The carbon cycle

In the natural carbon cycle, active volcanoes pour CO₂ into the atmosphere.  The long term average rate at which they do so is only on the order of 1% of the rate at which CO₂ is now being emitted by our burning of fossil fuels.  Over countless millions of years, however, that trickle would have accumulated into a catastrophically thick CO₂ atmosphere of the sort found on Venus.  Were there not a natural counter-process for taking CO₂ out of the atmosphere, the earth might be as hot and lifeless as our unfortunate sister planet.

That counter-process is the chemical weathering of silicate rocks.  Carbon dioxide in the atmosphere dissolves slightly in rain drops. The dissolved CO₂ forms carbonic acid and makes the rain drops weakly acidic.  The acid reacts with silicate groups on the surfaces of newly exposed rocks, yielding silicon dioxide, water and bicarbonate minerals.  The bicarbonates are soluble and carried away in the runoff.  The modified minerals left behind on the rock surface tend to be soft and easily abraded by particles of waterborne or windborne sand.  Tiny particles abraded from weathered rock surfaces form clay.  Their removal exposes fresh rock surface, enabling the weathering process to continue.

In the normal course of things, the dissolved bicarbonates are carried to the sea.  There they take up residence as “dissolved inorganic carbon” (DIC).  A fraction of the singly charged bicarbonate ions, chemically HCO₃-, lose their hydrogen atom to become doubly charged carbonate ions, CO₃²-. As carbonate ions, they may hook up with calcium ions to form the calcium carbonate shells of corals, shellfish, or the important microscopic organisms known “calcareous phytoplankton”.

Cycle diverted

That, as I said, is the normal course for chemical weathering of rocks.  In the eastern Sierras, however, the products of rock weathering never made it to the sea.  Due to a geological quirk in the uplift of the Sierras, a large basin region was formed — the Great Basin, covering most of the current state of Nevada, plus parts of western Utah and southeast Oregon.  Runoff from the eastern slope of the uplifted Sierras flowed into the basin and evaporated. The products of weathering carried by that runoff never reached the sea.

The extent of the Great Basin and the impact of the geological processes that formed it can be seen in the map at the right. It shows the extent of ancient Lake Lohontan as it existed at the end of the last glaciation. That was 10 to 15 thousand years ago, around the time when humans are believed to have first moved into North America. The basin itself was formed millions of years earlier, in the epoch when the modern Sierra Nevada were uplifted. The parallel north-south ranges of low mountains shown on the map were created when the underlying continental crust was stretched in an east-west direction and thinned. Over the thousands of millennia since the basin was formed, the sinks between the ranges have filled in. They are mainly filled with the products of weathering of the eastern Sierra Nevada. The area occupied by Lake Lohontan in this map has been dry lake bed for a few thousand years, but the clay and mineral deposits below are ancient.  They go down for thousands of feet.

What’s it all about?

So what’s the significance of all that material left behind when the runoff waters evaporated? What does it have to do with helping to stop the rise of atmospheric CO₂? The answer can be given in one word:  carbonates! The clay and mineral deposits of the desert playas are rich in carbonates.

So what, you ask? Aye, there’s the rub. Because while the above answer was easy to state, it’s not so easy to explain. It has to do with the rather complex chemistry of the “dissolved inorganic carbon” mentioned earlier.

Readers who are up for the full technical monte can find a concise presentation of it on this Utah State University web page. Those not interested in a chemistry lesson can scroll to the bottom line section below. For the “interested layperson” in between, I’ll try here for a short “Cliff’s Notes” version of equilibrium chemistry 101, as applied to ocean uptake of CO₂.

Equilibrium reactions for DIC

What we’re dealing with are five related species of carbon dioxide and carbonate ion molecules. These are:

1. Gaseous CO₂ in the atmosphere;
2. CO₂ dissolved in the ocean;
3. Carbonic acid, H₂CO₃, formed when dissolved CO₂ molecules join with water molecules;
4. Bicarbonate ion, HCO₃-, formed when a carbonic acid molecule loses a hydrogen ion;
5. Carbonate ion, CO₃²-, formed when a bicarbonate ion loses its remaining hydrogen ion.

The reactions which convert any of these species to the ones above or below it on the list are equilibrium reactions; they are reversible and can run in either direction. In chemical terms, that’s expressed by the following set of equilibrium reactions:

CO₂(g) + H₂O(l) ⇌ CO₂ · H₂O(aq); // dissolution of CO₂ in water

CO₂(aq) + H₂O ⇌ H₂CO₃(aq) ⇌ H⁺ + HCO₃^–(aq) ⇌ 2H⁺ + CO₃²¹(aq)

(For those not familiar with the notations, “(g)” indicates that the reactant is a gas, “(l)” that it is a liquid, and “(aq)” that it’s present in an aqueous solution — i.e., dissolved in water. They’re often omitted, such things presumably being obvious to chemists. The “·” means associated, but not chemically bonded. The equilibrium reaction symbol “⇌” indicates that the reactions run both ways — reaction left to right, counter reaction right to left.)

It’s a basic principle of chemistry that the speed of any reaction is proportional to the product of the concentrations (well, technically, the “activities”, which are not quite the same) of the reactants. When a system is in equilibrium, the reactions haven’t actually stopped; it’s just that the reactants have achieved a balance of concentrations, such that the speed of any given reaction is equal to that of its counter reaction. That leaves the net concentrations of the reactants unchanged.

If we increase the concentration of one of the reactants in an equilibrium system — say by adding more of a particular reagent — then the effect is to increase the speed of the reactions (or counter reactions) that consume that reactant. The effect ripples through the system until a new equilibrium is achieved. The concentration of other reactants will increase if they are products of the reactions that consume the added reactant. It will decrease if they are co-reactants consumed along with the added reactant. That may be hard to follow, so let’s consider a specific example.

Say we dump a load of soluble carbonates into a volume of seawater. We thereby increase the concentration of carbonate ions, CO₃^2-. That increases the speed of the above reaction that consumes carbonate ions, namely:

2H⁺ + CO₃^2- ⇀ H⁺ + HCO₃^–
But that reaction also consumes H⁺ ions, so their concentration is reduced. The seawater is made more alkaline, with a higher pH.

The effects don’t stop there. The reduced concentration of H⁺ ions slows the reactions that consume H⁺ ions and accelerates those that produce them. So:

H⁺ + HCO₃^–(aq) ⇀ H₂CO₃ (aq)

is slowed, while its counter reaction is accelerated. The net result is to reduce the concentration of H₂CO₃ (carbonic acid). But then that increases the speed of the reaction that produces H₂CO₃ and consumes dissolved CO₂:

CO₂(aq) + H₂O ⇀ H₂CO₃(aq)

Which — ta da! — enables more CO₂ to be absorbed from the atmosphere.

Bottom line

So there you  have it. When the activity coefficients for all the reactants are plugged in and the equilibrium equations worked through, the bottom line is that for every ten carbonate ions from soluble carbonates added to seawater, roughly nine molecules of CO₂ will be taken up from the atmosphere.  And because of the diverted transport of Sierra Nevada weathering products to the sea, there are a whole lot of soluble carbonates held in the Great Basin. The Treasure of the Sierra Nevada.

How much is “a whole lot”? Fair question, but not quite the one we need to ask. Remember, we’re talking about an accumulation over geological time. Several million years. That’s a short interval in the context of earth’s total history, but it’s still geological time. I can’t put an exact figure on the volume of accumulated material, but it would be measured in thousands of cubic miles. Only a fraction of that is soluble carbonates, but we’re still looking at trillions of tons. That’s more than enough, if it were all carried to the sea, to sequester not only 100% of our fossil carbon emissions for the next couple of centuries, but to reduce atmospheric CO₂ below pre-industrial levels. If fact, it would easily bring on the next ice age.

To move that much material, however, we’d be rolling back the landscape of Nevada by three to five million years. Thousands of feet of excavation over half the state. Not going to happen. So the question we need to ask is how much of the material (if any) could we reasonably be able to move to the sea, and would it be enough to make a difference?

I’ll leave the details and justification for another article, but my answer is that yes, it would be practical (cost-effective) to transport billions of tons of material annually from the Great Basin to the Pacific. The effects of doing so would be environmentally positive.

The transport would have to be by salt water canal barge, and the energy cost would be negative. I.e., it would generate more energy than it would consume. It would have interesting collateral benefits as well, but I’m getting ahead of myself. For the full story, tune in for “Cruising to Vegas”.

Acknowledgement

David Newell, a semi-retired PE experienced in mining and environmental engineering, first turned me on to the possibility of exploiting material from the Great Basin for climate change mitigation. I recommend his paper, “Living on the Edge of Vision: a blueprint for thriving on earth”. Well written and well researched, it provided inspiration and much of the background material for this article. A draft can be found online here, at Academia.edu.

Photo Credit: Randy Lemoine via Flickr