After a short hiatus while my brother was visiting me from England (woohoo!) we're back with the second-to-last post in the Story of Sand. For those just joining us, this is part of a personal science communication project I'm doing to help people see sand differently, and communicate the science of one of earth's most ubiquitous and fascinating materials.
This one builds on last week's post on Carbonates to explain how sediment weathering works together with living creatures to control earth's climate. You can read the other posts in this series at these links: Week 1, Week 2, and Week 3.
In this photo, the landscape of southern Thailand is dotted with massive limestone towers- remnants of a colossal reef system that once covered much of Southeast Asia. What goes unseen within these sheer masses of calcium carbonate are the billions of tons of carbon that they store, which have been transformed from atmospheric CO2 into rock through a cycle that governs Earth’s climate on a timescale of tens of millions of years. That cycle is the subject of this penultimate installation of my Story of Sand project.
Throughout earth’s history, its climate has swung back and forth many times. Worlds where mile-thick ice sheets cover continents give way to worlds where alligators bask beneath palm trees at the north pole. These swings are driven in large part by how much carbon dioxide is in the atmosphere and the magnitude of the greenhouse effect it creates. This, in turn, is dictated by how carbon moves between the earth and the air, and whether it is being emitted or stored away.
In this era of human-induced climate change, it is easy to assume that most of that exchange occurs in the world of biology, but in the long term it is actually the world of geology that does the heavy lifting. The amount of carbon stored in the lithosphere dwarfs the carbon in the biosphere and atmosphere combined by a factor of about 5000.
The story of how this carbon moves between air and rock is intertwined with the story of sand: in the long run, the single greatest force that draws carbon out of the atmosphere is the weathering of silicate bedrock into sediments. When rock is chemically weathered, the minerals swept up by water neutralize carbonic acid and stabilize it, locking atmospheric CO2 into an aqueous form. Once it travels through rivers and arrives in the ocean, the stage is set for some earth-altering alchemy that can store it away for millions of years.
Calcium carbonate is easy to make in the ocean- seawater is saturated with calcium (Ca+) and bicarbonate (HCO3-) ions, and the abundance of alkaline salt makes the world’s oceans ~10 times less acidic than fresh water, which encourages dissolved minerals to ]un-dissolve. In these conditions, calcium carbonate (CaCO3) can form spontaneously: two dissolved ions joining to form a neutral fleck of limestone. Although this reaction can happen spontaneously, for it to reach earth-altering proportions it needs something to drive the environment to be far more basic than it typically is. For that, we need life.
We started talking about the wonder of biomineralization in last week’s post, but made little mention of just how widespread and impactful of a process it is. Living things play a role in almost every pathway that carbon takes from gas to rock: a collaboration between the worlds of biology and geology that dramatically alters both.
One such pathway is the “intentional” creation of shells and skeletons by way of intracellular enzymes. Another venue is almost accidental- the result of a chemical quirk of performing photosynthesis underwater. Just as the addition of CO2 makes water acidic, the subtraction of CO2 makes it basic. As the ocean’s photosynthesizers suck CO2 from their vicinity, they create basic microclimates where CaCO3 can easily precipitate . Between these two processes, living things throughout the world’s oceans are creating calcium carbonate whether they mean to or not, and they have been since the dawn of life on earth.
This all adds up over time. Limestone mud accumulates on seagrass, and is waved off, the skeletons of coral grow up over the millenia into massive reefs, drifting phytoplankton die and contribute their minuscule shells into thickening drifts. Where the water is warm and the sunlight plentiful, these sources accumulate into mountainous deposits with enough mass to sink the seafloor into earth’s mantle like an overloaded ship. In several places on earth today, these massive accumulations of carbonate rock form their own landmasses. We call one of these The Bahamas, which sit on a stack of carbonates some 5 *miles* thick. Others have long since been transformed and uplifted to become landscapes like the one in this photo.
It is in these rocky storehouses that the vast majority of earth’s carbon resides, and the flux and in and out of them that largely determines earth’s climate in the long term. Through this process, the Story of Sand is woven into another dominant aspect of life as we know it.
Carbon will remain in its calcium carbonate form for many millions of years, but how does it return? Limestone that is uplifted into dramatic mountains and massive cave systems releases its carbon as it is re-dissolved by the same chemical weathering that birthed it. The limestone that is not uplifted back into land is buried or subducted and thrust back into the magma of the earth’s mantle, where its carbon vaporizes and is erupted from vents and volcanoes. The carbon molecules that are released thus come full circle: awaiting an errant raindrop to begin the cycle anew.