Metaphorically, the salt of the ocean is the metabolic waste of the sun, for the sun cycles the water that erodes, weathers and dissolves rocks, carrying salt to accumulate in the oceans. In a similar metaphorical spirit, tidal marshes are the children of the moon, for it is the moon that washes the salty tides back and forth over the margins of the land, making it suitable for only a few specially adapted plants. Were it not for the moon, the ocean-land boundary would be a narrow band instead of a sea of grasses and arborating tidal creeks up to many miles wide.
In mountains, increases in elevation of hundreds of meters change vegetation zones from humid or desert lowland, through forest to alpine tundra, but here in our coastal salt marshes on the Gulf of Mexico, elevation increases of mere centimeters can change the vegetation radically. Traversing the marsh from its seaward margin, the land rises too little to be perceptible, but the marsh grass gets shorter and sparser, interspersed with different plants until it gives way to a barren, sandy strip a few dozen meters wide, beyond which rises a forest of pine, juniper, coyote brush and sabal palm.
This salt barren is too dry and salty for marsh grass, too wet and salty for pines and palms, flooded only on extreme tides. It is home to saltwort whose succulent leaves, true to its name, are salty to taste, borne like little strings of sausages on a stick. In the relentless sun, they protect their photo systems with red pigment, and form straggling crowds at the seaward edges of the barrens. When the barrens are flooded, little fishes flit through waters only a centimeter or two deep, swirling through the clusters of saltwort as snails make furrows in the sand with their pointed spiral shells, and fiddler crabs push pellets of sand out of their burrows.
Permanent as they may appear by the measure of our lives, marshes are restless wanderers in time and space, driven inland by rising sea level or chasing the sea as it recedes. During the ice ages, sea level was 100 meters lower, and the Gulf Coast marshes were many miles seaward from their present location. Prehistoric campsites have been found underwater, miles from the current shore. But even at any one instant and averaged to a tideless dead calm, the sea is not really level, but rather bumpy with higher and lower areas where the density of underlying rock makes gravity lower or higher, and the water rises up or is pressed down as the weight of air on its surface changes with atmospheric pressure.
Along the Gulf Coast, slow subsidence and slumping of the land adds to the sea level rise caused by the warming of the oceans and melting of polar ice. Standing in the middle of the marsh at the St. Marks National Wildlife Refuge, the signs of the sea's gradual rise are visible in the islands of pine, juniper and palms with their halo of salt barren. The islands are only slightly higher than the surrounding marsh, and a few centimeters of sea level rise can mean their extinction. Newly formed coastal land is colonized first by grasses and bushes such as coyote brush, later by juniper, sabal palm and finally slash and longleaf pine. For 30 years I watched a couple of islands as the sea rose and succession went backwards --- first the pines died, then the palms and junipers and finally the coyote brush, and the islands were erased, leaving only eroded juniper and pine stumps in the middle of salt barrens, washed by high tides and surrounded by clumps of saltwort.
But being at the edge of the sea brings up a vexing question. Rains have carried salt to the sea since water first condensed on the young, hot earth several billion years ago, and only the water is recycled through sun and rain. Why is the ocean not a saturated solution of salt, rather than having an average salinity of only about 3.5%? Is there something that removes salt from the ocean? Salt has a vapor pressure close to zero, so it does not evaporate.
The clue lies in the huge deposits of salt in sedimentary rocks found and mined all over the world. Most of these were formed through the evaporation of shallow seas, increasing their salinity until salt precipitated or the sea dried completely. The weight of repeated cycles of salt and sediment depressed the surface, making room for more sediment and salt, creating deposits thousands of feet thick over millions of years. These salty layers are a source of the salt we use for many purposes beyond food.
The mineral salt (sodium chloride) is called halite, and is less dense than most sedimentary rocks. Salt layers buried under denser sedimentary layers are unstable, and the salt slowly floats upward through the denser rock, forming salt domes, whose importance is that as they float upwards, they form traps for accumulating oil and gas around the margins of the domes. We once visited Avery Island, one of the many salt domes along the Louisiana and Texas coast (in addition to being a wildlife refuge, Avery Island is the source of Tabasco Sauce). The circular dome is conspicuous on Google Earth, but contrary to what you might expect, its upward movement does not create high topography because, of course, the salt dissolves and is carried away by water in this high rainfall region.
When ground water percolates through salt deposits, it can form salt springs. We visited one such site in Peru, where a warm salty spring has been tapped to produce salt by evaporating the water in hundreds of terraced pools. High elevation and dry air speed the evaporation of the water to create a dazzling white checkerboard of a salty landscape. The terraces have been exploited for centuries and are passed down in families from generation to generation.
What could be more ordinary than salt? There are oceans of it and thick buried layers of it. Its cycles are driven by the sun and carried on rain, spanning all of earth since the earth, oceans, and rains were young. Because life arose in a salty sea, salt has forever been an essential component of all living things, even after those marine ancestors colonized the land. Today, wherever the salty sea meets the land, ancient life that began four billion years ago looks upon the Johnny-come-late-lies that have lived on the land for only a billion years. But all are the beings of salt.
I wake on the first day of 2024 to your meditation and scientific observations of salt and learned so much about what I usually measure for recipes or sprinkle on foods. I know someone whose death is held at bay by consuming no salt. Your work as always is both a probe of life on earth and a philosophical way of looking at it.
Walter, your wonderful essays serve as artful meditations that can lead one to further explore the intriguing subjects you present. Today's riff on salt reminded me of the remnants of large rusting iron fire pots we encountered in the marshes of the St. Marks Wildlife Refuge, which you explained were used to evaporate salt as part of the civil war effort. Your essay also prompted me to read about the importance of salt licks as sources of essential minerals for wildlife and farm animals. As a kid visiting the family farm, I was fascinated watching holstein cows in pasture licking out graceful curves in big pink salt blocks, but I don't recall thinking much about the purpose of blocks until I dug into the internet today. As always, thanks for sharing your remarkably wide-ranging and deeply penetrating curiosity. It really is infectious.