This marks the final post in my Story of Sand project, thank you all for following along!
In the past several weeks, I’ve tried to share some of the rich detail that exists in a handful of sand by way of diving into its endo-story (the little worlds contained within it) and its exo-story (the role they play in the world around them).
In this final post, I want to speak to one final principle that, to me, makes the story of sand one of the most profound and interesting things on earth. My favorite thing about the story of sand is that it is cyclical: sand is its own marvelous material, but it's also just a transition state: the interim form that rock takes between its reincarnations. A handful of sand is a million unique stories of a landscape being disassembled and rearranged, and also a tiny sample of the raw materials for land that is yet to be. To hold it is to hold the ashes of the past and the seed of the future-- fragments of a once and future world.
This picture encapsulates that cyclical nature. In it, the Colorado river snakes through the bottom of the grand canyon, carrying both the remnants of the Rocky Mountains and recycled sediments of the Colorado Plateau to the pacific. Within those muddy waters, sediments on their inaugural journey mingle with others that have been through the wringer many times. Those that are swept up from the canyon walls today may be moving for the first time in 500 million years, roused from their temporary rest to rejoin the procession downward. Sand might travel through this cycle many times and in many ways- but ultimately it will all undergo the same fate: being buried so deeply that it is subsumed back into the earth, to be melted and reformed anew. With this final transformation, the stage is set for the Story of Sand to play out once again, and give its gifts to some future world.
This photo also speaks to the profound impact of that cycle on past worlds. On the canyon’s inner rim, Maddie sits atop Cambrian-era sandstone laid down just as the first complex life exploded into Earth's oceans. Directly below that is the Vishnu schist, a gnarled and jagged metamorphic bedrock more than a billion years older. The missing rock and time between these two layers is a gap in the rock record known globally as the Great Unconformity, which represents a period of intense continental erosion with profound consequences. By stocking the ancient oceans with the raw materials for biomineralization, the formation of the Great Unconformity appears to have helped trigger the evolutionary arms race that led to complex life on earth. What goes unseen between those layers may be the single most dramatic impact that sediments have had on life as we know it.
Just as it has since the dawn of complex life on earth, the story of sand continues to underlie our everyday lives in more ways than most people can fathom. In diving into these stories, I hope I’ve helped bring out some of the wondrous depth that exists within a handful of this incredible, ubiquitous, utterly mundane stuff. And I hope next time you’re at the beach, or the grand canyon, or just thinking about the ground beneath your feet, you think, “Thanks, sand.” 😉
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.