SHELL MIDDENS: CaCO3 + CO2 + H2O → Ca (HCO3)2

This past weekend, I was exploring the western edge of central Vancouver Island, home to the Pacheedaht First Nation. Their traditional unceded territory is wonderous.  

The beaches are covered with bits of fir, cedar and arbutus worn smooth by the awesome west coast waves! I can see why they have made a home here for millennia. 

Those of you who live near the sea understand the compulsion to collect driftwood, unusual stones, fossils—and shells. They add a little something to our homes and gardens. 

With a strong love of natural objects, my own home boasts several stunning abalone shells conscripted into service as both spice dish, soap dish and the place I both store and display beautiful bits from nature.

As well as beautiful debris, shells also played an embalming role as they collect in shell middens from coastal communities. Having food “packaging” accumulate in vast heaps around towns and villages is hardly a modern phenomenon.

Many First Nations sites were inhabited continually for centuries. The discarded shells and scraps of bone from their food formed enormous mounds, called middens. Left over time, these unwanted dinner scraps transform through a quiet process of preservation.

Time and pressure leach the calcium carbonate, CaCO3, from the surrounding marine shells and help “embalm” bone and antler artifacts that would otherwise decay. Useful this, as antler makes for a fine sewing tool when worked into a needle. Much of what we know around the modification of natural objects into tools comes from this preservation.

Comox Beach, Kʼómoks First Nation / Photo: Kat Frank 

Calcium carbonate is a chemical compound that shares the typical properties of other carbonates. CaCO3 is common in rocks and shells and is a useful antacid for those of you with touchy stomachs. 

In prepping fossil specimens embedded in limestone, it is useful to know that it reacts with stronger acids, releasing carbon dioxide: CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)

For those of you wildly interested in the properties of CaCO3, may also find it interesting to note that calcium carbonate also releases carbon dioxide on when heated to greater than 840°C, to form calcium oxide or quicklime, reaction enthalpy 178 kJ / mole: CaCO3 → CaO + CO2.

Calcium carbonate reacts with water saturated with carbon dioxide to form the soluble calcium bicarbonate. Bone already contains calcium carbonate, as well as calcium phosphate, Ca2, but it is also made of protein, cells and living tissue.

Decaying bone acts as a sort of natural sponge that wicks in the calcium carbonate displaced from the shells. As protein decays inside the bone, it is replaced by the incoming calcium carbonate, making makes the bone harder and more durable.

The shells, beautiful in their own right, make the surrounding soil more alkaline, helping to preserve the bone and turning the dinner scraps into exquisite scientific specimens for future generations.

The lovely photo from Comox showing the many shells on the beach is by my beautiful cousin Kat Frank of the Kʼómoks First Nation—an amazing human being and, as you can see, a great photographer!