Limestone outcrops, fossil clams and calcium

Written February 16, 2014, published in late February, 2014

Waters that flow to the Pacific Ocean from the Coast Range, Cascades and Columbia River basin tend to be low in calcium. Columbia River waters are high in some heavy metals, several nutrients, any number of pollutants and several radioactive isotopes. These are not a surprise. With numerous dams, cities, thousands of acres of irrigated farmland, mining, and other activities upriver, and parent rocks from seafloor basalts and the Columbia Plateau flood basalts, opportunities for all these compounds and elements to enter the water is very good. But why so little calcium?

Calcium accumulates in oceans as calcium carbonate in shells when water conditions are basic, with a pH above 7.0, which is considered neutral. One of the three most common biopolymers on the planet (cellulose and chitin are the other two), calcium carbonates are formed by several groups of invertebrates into durable, protective outer walls to shield softer bodies.

Common invertebrate groups that make carbonate shells include mollusks, barnacles, corals and brachiopods. Vertebrates also use calcium carbonate to make bones which support bodies internally instead of protecting them externally. Calcium is important to plants, which use it to form healthy cell walls, and produce fruit. It's mobile in water, which is another way to say it washes out of the soil easily.

Mollusks are one of the most successful body types on the planet, and include snails, clams, oysters, limpets, slipper shells, and several shell-free forms. Species in this large and very diverse phylum live in almost all wet to damp habitats on earth, including deep in the ocean, in shallow saltwater and in freshwater, on land and trees. In some species, the external shell is reduced or absent, as with land-living slugs, as well as squid and octopus. More about land-living mollusks later, which include snails, jumping slugs, voracious garden-living slugs from around the world, and others.

Numerous small fossil clams can be seen as circles to ovals on this limestone rock, photographed at an old quarry on Bear River. The fossil clam in the middle of this image is 20 mm long. Photograph by Kathleen Sayce

Carbonates form masses of limestone when shells accumulate in channels, or grow together in large reefs, and then later are heated, subjected to pressure, and turn from individual shells into rocks. The more heat, pressure and time, the harder the rocks that form, going from fairly soft limestones to quite hard marble over millions of years. In geologic time, this area was a enclosed and increasingly shallow sea from around 55 mya (millions of years ago), to around 10 mya, as the Cascades and then Coast Range/Willapa Hills rose. There was a lot of volcanic activity, lava flows, flood basalts, ash falls, which buried the reefs and shell beds under layers of other rocks, often under very acidic conditions.

There's one thing calcium carbonates can't resist (sorry for the pun), and that is acid. Air or water with a pH of less than 7.0 is acidic, and gets more acidic as pH goes from 7 towards 1; if the pH is above 7, then it is alkaline. Acids break down the carbonates, release calcium and carbon dioxide in the carbonates, and dissolve the material, no matter if the carbonate is an oyster shell, a marble statue, or a limestone wall. When conditions are acidic, as is often the case during volcanic activities, or with high levels of pollution (think 'acid rain'), or with ocean acidification, then limestones or shells dissolve more quickly. All that volcanic activity in geologic time helped dissolve limestones when the reefs were young; the present climate of long wet winters with acidifying conditions also promotes dissolving.

A few outcrops remain in the Willapa Hills from those extensive reefs of millions of years ago. Tom Horning, geologist, who lives in Seaside, Oregon took me to see a small limestone outcrop on the Bear River a few months ago. I wanted to see the shells and distinctive color of the local limestone, which is a muddy yellow, very different from black-gray-brown basalts, the most common local rocks. We walked down to the river through a recent clearcut, and found the remains of a limestone quarry on the banks of Bear River. With Tom's help, I was able to photograph several patches of clam shells. This outcrop is less than one hundred feet wide along the river bank, and was mined decades ago to make cement. A bit of it still remains visible, a long gouge out of the hillside where the rocks are softer, yellowish in color, and if the moss and soil are scraped away, where fossil clams shells can be seen.

It's one thing to read a geology map and fossil books, and learn that extensive reefs were found in the shallow sea over our area in the Eocene to Miocene periods. Or to learn that oceans were so alkaline then that the water had a pH of 10 to 14, which is very alkaline. It's something else to hold a rock in your hand as a relict of those past eons. In generally acidic conditions on land or in water, limestones survive when they are protected from those acids, well elevated above fresh or saltwater, protected from rainfall by other sediments. In low calcium conditions today, we see the past, where a long parade of acids wore away those reefs and shell beds.

It matters now how much calcium is where on local lands and in waters, because our estuaries are acidifying along with the Pacific Ocean. Local scientists are studying how to boost calcium around shellfish beds, to protect young clams and oysters, and enhance growing conditions for fish and other species. We already know that upwelled water from the deep ocean is acidic and low in oxygen. If it's also low in calcium, then the mollusk larvae in our estuaries have a triple whammy to face in their first few days of life: low pH, not enough oxygen, not enough calcium to build or rebuild their new shells.

Willapa Bay's waters can oxygenate well with the tidal cycle, and if the water can be buffered with additional calcium, then the young oysters and clams may make it past the critical first couple of weeks, and have a chance of surviving. We can't solve ocean acidification for the Pacific Ocean, or the world. But perhaps we can buffer it for our shellfish industry. Finding the right way to do this starts with knowing all the local calcium sources.