Sand Month: Week 3, part 2: Carbonates

This is part of the third installation of my personal project to help people see sand differently, and communicate the science of one of earth's most ubiquitous and fascinating materials. This week is all about what sand does to influence life on earth. You can read the first part about sediment-bound nutrients here, or check out the posts from week 1 and week 2 at these links or on my instagram. 
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In this photo, the spiral shell of a small squid-like creature (an incredible animal called a Spirula, which is worthy of its own entire post) rests on the black sand of a beach in New Zealand. What is invisible in this photo is the deeply intertwined backstory that these two substances share. The role that sediments play in the existence of shells and other biogenic minerals is the subject of today’s entry in the Story of Sand: Carbonates.
To understand how sediments are linked to seashells, we need some background on how water interacts with gas, specifically carbon dioxide.

Gasses dissolve in water, which you know if you’ve ever opened a carbonated drink and watched gas that had been dissolved suddenly emerging as bubbles. When CO2 dissolves in water, it does something unique: it reacts with the H2O molecules to create a new set of compounds, which are carbonic acid (H2CO3, the one that drives chemical weathering) and two charged ions called bicarbonate (HCO3-) and carbonate (CO3 2-), formed when hydrogen atoms fall away from carbonic acid.
These molecular residents inhabit water nearly from the start; as a raindrop falls through the air it is already absorbing its fill of atmospheric CO2 and accumulating carbonic acid, bicarbonate, and carbonate. By the time rainwater hits the ground, it is primed to react with the rocks of the earth’s crust, dissolving them and balancing its negative carbonate and bicarbonate ions with positive mineral ions such as Calcium (Ca2+). 
This mineral water, now loaded up with calcium from rock and carbon from the atmosphere, coalesces into rivers and drains to the ocean. In the sea these minerals accumulate and get concentrated into an alkaline brine: the ocean’s saltiness that is the chemical legacy of earth’s eroded continents. 
At last, the stage is set for one of the most wondrous and consequential reactions that life on earth performs: biomineralization. 

​Biomineralization is the process by which organisms construct their own hard materials by shepherding minerals from their environment into new complex crystals, like an organic imitation of rock. This is the process behind most protective shells, supportive skeletons, or durable teeth.
In the ocean, the most common product of biomineralization reflects the most abundant building materials: in a world saturated with calcium from rock and bicarbonate from the air, nearly every shell, reef, and exoskeleton you can imagine is made from the combination of the two- a compound called calcium carbonate (CaCO3).
Calcium carbonate is a hard white solid that is familiar to you in many forms, from limestone caves and chalky cliffs to colorful seashells. At their core these materials are all essentially the same, and they are all made up of the two building blocks delivered to oceans by the chemical weathering of rock.
So it is that the two materials in this picture- the delicate shell of the spirula and the weathered remnants of broken rock- actually share a common origin story. Both are ultimately products of the creation, transportation, and transformation of sand.
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The next time you look at a seashell, I hope you can imagine its orderly crystals made up of  the calcium stripped from sediments, bound up with carbon that once floated through the air as CO2. 
Although the connection between sand and shells is fascinating, it is a tiny part of a much larger and more important picture. The carbonate system actually exerts an immense influence on life as we know it by driving the temperature of earth’s climate on a timescale of tens of millions of years. That story is coming in the next installment of the Story of Sand