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The story of ocean alkalinity is a story of the contrariness of acid rain and basic rock erosion, knitted together.
Up in the atmosphere, carbon dioxide gas dissolves in raindrops, becoming carbonic acid, abbreviated here as HH-carbonate. The H’s (hydrogens) are loose, and loose H’s define acids.
The acid rains down on rocks, including limestones, which are made of calcium-carbonate. The seeds of Mary’s garden are sown: acid rain separates the calcium-carbonate into calcium and carbonate, which wash into the oceans. Carbonate is a base. It can bond to H instead of calcium, and H removal defines bases. The ocean, a vibrant chowder of chemistry, has become basically basic in part from the slow inflow of extra carbonate streaming in from continents.
Some acid rain, of course, falls directly on the oceans, loose H’s and all. There, carbonates gobble up some of the H’s, forming an intermediate, H-carbonate. H-carbonate is the kingpin of the ocean’s buffering system, which has kept seawater at that surprisingly stable base-to-acid ratio of 10-to-1 for thousands of years.
A buffer is like a big hat in a candy factory: candies come down a conveyor belt and must be wrapped. An employee, Lucy, has a big pocket in her hat, big enough to hold several handfuls of candies. If the conveyor delivers candies too slowly, she can pull some from the hat and add them to the conveyor to keep her candy wrapping rate constant. If the conveyor runs too fast, she can toss some of the candies into her hat to wrap later. The hat buffers the supply of candies. In the same way, the ocean’s base-to-acid ratio has been stable because of H-carbonate, which can either lose its H like an acid or steal one like a base. It neutralizes both acids and bases. If there are lots of H’s around, H-carbonate will suck up H. If there are not as many, H-carbonate will shed H.
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But buffering that is beneficial for acid stability is not good for cockles. There are no happy clams.
Cockle shells are made of calcium-carbonate.
Atmospheric carbon dioxide held steady at 0.025% to 0.028% for 10,000 years, during human history from the Neolithic Period to the Industrial Age, as measured in ice core bubbles in Antarctica. Over the last 200 years, it climbed to 0.041%. Extra acid raining into the ocean pushed an H onto some of the plain carbonate, doggedly maintaining the base-acid balance.
More acid thus corresponds to less available carbonate for cockles to shell. When the candies are coming thick and fast, there are no spare wrappers! And a less-alkaline ocean is less hospitable to shells already cast, dissolving them away to keep up the base-to-acid ratio.
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More carbon dioxide and acid rain may gradually increase erosion of rocks, and perhaps increase the flow of basic carbonate into the oceans to sop up some acid and strap up the cockleshells. The circle of chemistry may soon ring silver bells for the cockles in the contrary ocean garden.
Joy Moses-Hall, Reflector, 28 May 2022. Full article.
